# Trans-Concerted Addition to Alkynes: the case of Ynamide Silylzincation

Frédéric Guégan, Fabrice Chemla, Franck Ferreira, Hélène Gérard, Alejandro

Perez-Luna, Stéphanie Halbert

► To cite this version:

Frédéric Guégan, Fabrice Chemla, Franck Ferreira, Hélène Gérard, Alejandro Perez-Luna, et al. Trans-Concerted Addition to Alkynes: the case of Ynamide Silylzincation. Chemistry - A European Journal, 2024, 30 (43), 10.1002/chem.202401751. [DOI: 10.1002/chem.202401751](https://doi.org/10.1002/chem.202401751)

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# Trans-Concerted Addition to Alkynes: the case of Ynamide Silylzincation

Frédéric Guégan,[a, c] Fabrice Chemla,[b] Franck Ferreira,[b] Hélène Gérard,[a] Alejandro Perez-Luna,[*b] and Stéphanie Halbert*[a]

An original concerted antarafacial mechanism for the addition of diorganosilyl-zinc reagents across the C–C triple bond of ynamides is computationally investigated using DFT calculations. This concerted mechanism, leading to a *trans*-product in only one step, results in the formation of a Si–C and a Zn–C σ-bond on opposite sides of the π-system. We demonstrate that the mechanism going through a η²-vinyl intermediate and the proposal of a radical chain pathway are energetically unsustainable.

The retained concerted antarafacial pathway is tested on experimental selectivities: the regioselectivity, in favor of the silyl β-addition in ynamide, and stereoselectivity, which is *cis*-with (Me₂PhSi)₂Zn but *trans*- with [(Me₃Si)₃Si]₂Zn, are well reproduced by DFT calculations. The regio- and stereoselectivity are discussed using the activation strain model and a chemical bonding analysis.

## Introduction

Alkyne silylmetalation, i.e. the 1,2-addition of silicon–metal bonds across the carbon–carbon triple bond of alkynes, represents a valuable method for the construction of multi-substituted alkenes, and considerable efforts have been devoted to develop procedures allowing for high levels of regio- and stereocontrol.[1] The majority of the reported alkyne silylmetalation reactions occur through the insertion of the carbon–carbon triple bond into a silicon–transition-metal bond.[2] The addition of Si–Zn bonds across terminal alkyl-, aryl-substituted alkynes was achieved from di[3]- or monoanion[4]-type zincates, as well as from silyl-zinc species generated in-situ from silylboranes[5] Such addition processes are *cis*-stereoselective and lead (primarily) to silylmetalation products with the newly formed bonds in *cis* configuration.

Recently, as part of our efforts to develop silylzincation in the quest for the elusive *trans* stereoselectivity,[6] we uncovered a remarkable dichotomy in the reactivity of disilylzinc reagents. Indeed, both (Me₂PhSi)₂Zn and [(Me₃Si)₃Si]₂Zn achieve the β-

regioselective silylzincation of ynamide in the absence of any transition metal (Scheme 1, middle), but the stereoselectivity varied with the silyl group: Me₂PhSi gave the *cis*- and (Me₃Si)₃Si

<figure><img src="image_3.png" /><figcaption>------ Stereodivergent silylzincations of ynamides⁷------</figcaption></figure>

<figure><img src="image_4.png" /></figure>

This work : DFT calculations

<figure><img src="image_5.png" /></figure>

Mechanistic investigations on polar and radical additions and the regio- and stereoselectivity

**Scheme 1.** State of art of the stereodivergent silylzincation of arylacetylenes and terminal ynamides with (Me₂PhSi)₂Zn or [(Me₃Si)₃Si]₂Zn reagents.

<footnote>[a] Dr. F. Guégan, H. Gérard, Dr. S. Halbert
Sorbonne Université, CNRS, Laboratoire de Chimie Théorique, LCT
F-75005 Paris, France
E-mail: stephanie.halbert@sorbonne-universite.fr</footnote>

<footnote>[b] F. Chemla, F. Ferreira, Dr. A. Perez-Luna
Sorbonne Université, CNRS, Institut Parisien de Chimie Moléculaire, IPCM
F-75005 Paris, France
E-mail: alejandro.perez_luna@sorbonne-universite.fr</footnote>

<footnote>[c] Dr. F. Guégan
Present Address: Institut de Chimie des Milieux et matériaux de Poitiers,
CNRS, Université de Poitiers,
1 rue Marcel Doré, 86073 Poitiers, France</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202401751</footnote>

<footnote>© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_6.png" /></figure>

gave the *trans*-product.[7,8] Owing to the capacity of tris(trimethylsilyl)silyl to support radical chain mechanisms,[9] we ascribed the stereodivergent behavior to a competition between the established polar *cis*-addition pathway in the case of (Me₂PhSi)₂Zn and a radical chain mechanism for [(Me₃Si)₃Si]₂Zn analogous to oxygen initiated radical alkylzinc group-transfer processes.[10,11] A similar stereodivergent behaviour was also found in the case of terminal arylacetylenes (Scheme 1, top).[12] In this case, we found that [(Me₃Si)₃Si]₂Zn reagent delivers the *trans*-silylzination products, contrary to (Me₂PhSi)ZnMe reagent which reacts with *cis* stereoselectivity through a polar addition mechanism (supported by DFT study).[5] In our report, experimental evidence for the radical behaviour of this reagent was provided and the theoretical mechanistic study on radical pathway supports the observed *trans*-stereoselectivity. Here, however, the silylzination of aryl-substituted acetylenes only proceeded in the presence of added Et₂Zn, by contrast with the reaction of ynamides (Scheme 1, middle). Intrigued by this observation and given that Et₂Zn might serve as radical initiator, we decided to investigate computationally the mechanisms of addition of disilylzinc reagents across terminal ynamides (Scheme 1, bottom).

In the literature, only two examples described a combined experimental and computational study on the silylzination of C–C multiple bonds (phenylallene or terminal alkyne) using (PhMe₂Si)ZnR reagent (R=Me, tBu).[5,13] In these studies, the silylmetalation takes place directly from the two reactants through distorted transition state (TS), in which the silyl and zinc groups are in the same side of the C–C bond, corresponding to a *cis*-silylmetalation. In addition, several theoretical studies of *cis*-silylmetalation in hydrosilylation reactions have been extensively examined in transition-metal-catalysis (Pt, Rh, Co, Fe, Ir).[14,15,16] Different mechanisms were proposed, from the known Chalk-Harrod mechanism[15] to the modified one[16] and both imply the formation of a π-complex with the coordination of metal into the unsaturated bond. In the case of zinc(II) metal, the formation of alkene-zinc complexes is extremely rare as the π-back-bonding from a filled zinc d-orbital into the π*-orbital of the olefin is weak due to the very high energy of zinc d orbitals.[17] For *trans*-silylmetalation, the *trans*-hydrosilylation products are generally believed to be derived from initial *cis*-silylmetalation followed by olefin isomerization.[18–20] In the case of cationic Ru-catalyzed hydrosilylation of terminal alkynes, it was proposed that the *trans*-addition stereochemistry results from the formation of metallocyclopropene intermediates.[21–22] More recently, this intermediate was proposed for the *trans*-selective carbosilylation of terminal alkyne with nickel catalyst.[23]

In this paper, we investigate the mechanism of silylzination of ynamides using DFT calculations. For the [(Me₃Si)₃Si]₂Zn reagent, the originally proposed radical pathway leading to the *trans*-isomer is first explored. For the *cis*-product, the traditional *cis*-silylmetalation, considered as polar mechanism, is next investigated. For the *trans*-product formation, we consider two pathways: an isomerization step from *cis*- to *trans*-product by analogy with the previous studies[21] and an alternative pathway leading in only one step to the *trans*-product. The most

favorable pathway is discriminated on energetic aspects, in particular the activation barriers, and supported by the evaluation of selectivity. For this purpose, we test the most favorable mechanism for the regio- (α or β-additions) and stereoselectivity (relative to the silyl groups [(Me₃Si)₃Si]₂Zn vs. [(Me₂PhSi)₂Zn] and compare the computed selectivity with the experimental data. The stereo- and regioselectivity is finally discussed using the *activation strain* model and a chemical bonding analysis.

## Results and Discussion

The radical silylzination mechanism is first explored (Figure 1, geometric and energetic patterns are gathered in Table S1) using the following models (Scheme 1, bottom): ynamide 1 and [(Me₃Si)₃Si]₂Zn. The mechanism proposed is a chain mechanism involving the Si(SiMe₃)₃ radical, which is regenerated in the product formation step. The initiation step is not discussed here for reasons which will appear later. The first step of the radical propagation is the addition of the (Me₃Si)₃Si radical across the C≡C triple bond in **1**, with an activation barrier calculated at 14.2 kcal mol⁻¹. The corresponding transition state **TS**<sub>add</sub> is early, indicated by the large Si–C<sub>β</sub> distance of 2.82 Å (vs. 1.92 Å in the formed Si–C<sub>β</sub> bond in **I**<sub>rad-trans</sub>), and the slightly bent geometry of the alkyne (N–C<sub>α</sub>–C<sub>β</sub> angle of 169° in **TS**<sub>add</sub> vs. 177° in ynamide 1). The resulting vinyl radical **I**<sub>rad-trans</sub> featuring a *trans* bending at C<sub>α</sub> (N–C<sub>α</sub>–C<sub>β</sub> of 138°) is stable with a relative Gibbs Energy of –7.1 kcal mol⁻¹. The isomerization from *trans*- (I<sub>rad-trans</sub>) to *cis*- vinyl radical (I<sub>rad-cis</sub>) is accessible through a transition state **TS**<sub>rad-isom</sub> characterized by a linear N–C<sub>α</sub>–C<sub>β</sub> angle of 178°. This isomerization is reversible with a low activation barrier of 4.9 kcal mol⁻¹ between I<sub>rad-trans</sub> and **TS**<sub>rad-isom</sub> and slightly in favor of *cis* vinyl radical I<sub>rad-cis</sub> (more stable by only 4 kcal mol⁻¹ over I<sub>rad-trans</sub>). The electron in I<sub>rad-cis</sub> and I<sub>rad-trans</sub> stabilized by the nitrogen atom[24] of the oxazolidinone substituent, is localized on C<sub>α</sub> as represented by the spin density in Figure S1.

The second step results from the reaction of *cis* or *trans* vinyl radical I<sub>rad</sub> with [(Me₃Si)₃Si]₂Zn. As previously found for 2-ethynylanisole,[12] it also occurs in a single step with ynamide 1. The coordination of [(Me₃Si)₃Si]₂Zn to I<sub>rad</sub> is associated with a cleavage of one of the Zn–Si bonds leading to the regeneration of the (Me₃Si)₃Si radical and the formation of the silylzination products P. The formation of products P<sub>cis</sub> and P<sub>trans</sub> relative to the separated reactants, ynamide 1 and (Me₃Si)₃Si radical, is highly exergonic (–29.8 and –32.8 kcal mol⁻¹, respectively). In addition, this homolytic substitution is the rate-determining step as the activation barrier is the highest of the mechanism. It is also stereoselective as the *trans*-product P<sub>trans</sub> is kinetically and thermodynamically favored over the *cis*-product P<sub>cis</sub> by 9.4 and 3 kcal mol⁻¹, respectively, in line with the experimental observations. The stereodivergence originates thus in the second step, namely the radical chain transfer at the Zn center.

The structures of the two stereodetermining transition states **TS**<sub>rad-cis</sub> and **TS**<sub>rad-trans</sub> are represented in Figure 2 (geometrical parameters are available in Table S1). The approach of [(Me₃Si)₃Si]₂Zn to vinyl radical I is associated with large Zn–C<sub>α</sub>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 1.** Gibbs energy profiles (in kcal mol⁻¹) for the radical cis- (red) and trans- (blue) silylzincation of ynamide 1 by [(Me₃Si)₃Si]₂Zn (with respect to separated reactants, 1 and (Me₃Si)₂Si radical).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 2.** Structures of the radical transition states TS<sub>rad-cis</sub> (left, cis-silylzincation, 41.9 kcal mol<sup>-1</sup> Gibbs energy barrier) and TS<sub>rad-trans</sub> (right, trans-silylzincation, 28.5 kcal mol<sup>-1</sup> Gibbs energy barrier). Angles are given in degrees, distances in Å; most methyl groups are omitted for clarity.</figcaption></figure>
<figure><img src="image_10.png" /></figure>

distances of 4.18 and 4.01 Å for TS<sub>rad-cis</sub> and TS<sub>rad-trans</sub>, respectively. The Zn–C<sub>α</sub> bonds are thus very far to be formed as they are more than 2 Å larger than in the products (2.01 and 1.99 Å in P<sub>cis</sub> and P<sub>trans</sub>). This is corroborated with the slightly elongated Zn–Si bond in the two TS (2.42 and 2.40 Å in TS<sub>rad-cis</sub> and TS<sub>rad-trans</sub> vs. 2.34 Å in free [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn) which indicated the beginning of the (Me<sub>3</sub>Si)<sub>2</sub>Si radical release. In addition, the bending of (Me<sub>3</sub>Si)<sub>3</sub>Si<sub>2</sub>Zn is noticeably comparable between TS<sub>rad-cis</sub> and TS<sub>rad-trans</sub> (Si–Zn–Si angle of 151 and 156°, respectively vs. 180° in free [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn). A slight difference is noted for the N–C<sub>α</sub>–C<sub>β</sub> angle, which is more constrained for cis TS with an angle of 124° compared to 137° for the *trans* one, close to 138° in I<sub>rad-trans</sub> or I<sub>rad-cis</sub>. The preference for the *trans* pathway by almost 10 kcal mol<sup>-1</sup> is attributed to the large distortion energy for cis pathway (19 vs. 9.6 kcal mol<sup>-1</sup> for cis and *trans*-pathway, respectively, Table S2), following the *activation strain* model.<sup>[25-27]</sup>

The polar mechanisms are next investigated inspired by previous studies.[21,22] The *cis*-product is first formed by the well-known *cis*-silylmetalation[5,28,29] and the *trans*-product is obtained through an isomerization step occurring *via* a metal-

locyclopropene intermediate. The formation of the *cis*-product P*cis* (in red in Figure 3, geometrical parameters are given in Table S3) is highly exergonic (−29.8 kcal mol⁻¹) and thus irreversible. It is associated with a high activation barrier of 32.6 kcal mol⁻¹, nevertheless lower than that computed for the *cis* radical mechanism (41.9 kcal mol⁻¹, see Figure 1). This is in good agreement with the introductory statement that *cis*-addition is related to a polar mechanism. The *cis* transition state TS*polar-cis* involves a suprafacial process[30] wherein the Si–Cβ and Zn–Cα σ bonds are formed on the same side of the reacting π-system as evidenced in Figure 4, by a quasi-planar dihedral angle of 32° between Zn–Si and Cβ–Cα fragments.

The formation of the *trans*-product is envisioned *via* the isomerization step of the *cis*-product through TS<sub>polar-isom</sub> computed at 22.9 kcal mol<sup>-1</sup> relative to the separated reactants. This TS (Figure 4) is associated with the Hβ rotation from *cis* to *trans* to the zinc moiety, indicated by the orthogonal Hβ position out of Zn–Cα–Cβ plane (dihedral angle of 101° between Zn–Cα and Cβ–Hβ bonds). The shorter Zn–Cβ bond of 2.03 Å and Zn–Cα bond of 2.10 Å, as well as the longer Cα–Cβ distance of 1.47 Å, intermediate between a double and simple bond, are character-

<figure><img src="image_11.png" /><figcaption>**Figure 3.** Gibbs energy profiles (in kcal mol⁻¹) for the polar *cis*- (red) and *trans*- (blue) silylzincation of ynamide 1 by [(Me₃Si)₃Si]₂Zn (with respect to separated reactants, 1 and [(Me₃Si)₃Si]₂Zn).</figcaption></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 4.** Structures of the polar transition states TS<sub>polar-cis</sub> (left, cis-silylzincation, 32.6 kcal mol<sup>-1</sup> Gibbs energy barrier), TS<sub>polar-trans</sub> (center, trans-silylzincation, 24.4 kcal mol<sup>-1</sup> Gibbs energy barrier) and TS<sub>polar-isom</sub> (right, isomerization from cis- to trans-products, 52.7 kcal mol<sup>-1</sup> Gibbs energy barrier). Angles are given in degrees, distances in Å; most methyl groups are omitted for clarity.</figcaption></figure>

istic of metallocyclopropene intermediate.[31] Indeed, this structure is similar to the structure reported by Trost and coworkers,[21] optimized as a minimum, for hydrosilylation of terminal alkynes by cationic ruthenium catalysts. Our attempt to locate a metallocyclopropene intermediate as a local minimum was not successful probably due to low ability of zinc(II) metal to react as a π-back-bonders. As a conclusion, the formation of *trans*-product P<sub>trans</sub> from isomerization of *cis*-product P<sub>cis</sub> through this peculiar TS<sub>polar-isom</sub> is not kinetically accessible considering the high activation barrier (52.7 kcal mol<sup>-1</sup> between P<sub>cis</sub> and TS<sub>polar-isom</sub> vs. 28.5 kcal mol<sup>-1</sup> for *trans* radical pathway, see Figure 1).

To our surprise, we also located a transition state leading directly to the *trans*-product in only one step from reactants (in blue in Figure 3, geometries are given in Figure 4 and Table S2). This transition state TS<sub>polar-trans</sub> is associated with an antarafacial process[30,32] where Si–C<sub>β</sub> and Zn–C<sub>α</sub> bonds are formed from opposite sides of the π-system as indicated by the Zn–Si bond twisted over the C<sub>α</sub>–C<sub>β</sub> bond (C<sub>β</sub>–C<sub>α</sub>–Zn–Si = 46°). This transition state also appears slightly more product-like than the *cis* one, with shorter Si–C<sub>β</sub> (2.67 in TS<sub>polar-trans</sub> vs. 2.79 Å in TS<sub>polar-cis</sub>) despite similar Zn–Si bonds (2.51 vs. 2.50 Å). More interesting, this antarafacial mechanism is an asynchronous process as the major geometrical change around the transition state is associated with the Si–C<sub>β</sub> bond formation (Figure S2). The Zn–C<sub>α</sub> is formed during large geometrical rearrangement described by a relaxation to reach the *trans*-product. The activation barrier of this concerted mechanism is computed at 24.4 kcal mol<sup>-1</sup>, lower than that calculated for the radical pathway (28.5 kcal mol<sup>-1</sup> between I<sub>rad-trans</sub> and TS<sub>rad-trans</sub>, see Figure 1). As the energies of polar and radical species are possibly sensitive to the methodology used, the *trans*-activation barrier was calculated for polar and radical chain mechanism for various DFT functionals and basis sets as described in the Computational Details. This test

confirms the lowest barrier for concerted mechanism over the radical pathway. Hence this computational finding suggests that the *trans*-silylzincation of terminal ynamides by reaction with [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn proceeds through a concerted polar mechanism rather than a radical chain one. The consistency of the experimental observations regarding stereo- and regioselectivity with this new mechanism is then investigated.

As reminder, the stereoselectivity of the silylzincation of terminal ynamides is dependent on the silyl group of the silyl zinc reagent: (Me<sub>2</sub>PhSi)<sub>2</sub>Zn gives the *cis* whereas [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn gives the *trans* product. The competition between the proposed supra- and antarafacial pathways is thus explored for (Me<sub>2</sub>PhSi)<sub>2</sub>Zn (Table 1, Table S4 and Figure S3-S4) and [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn (Figure 3, Table 1). For (Me<sub>2</sub>PhSi)<sub>2</sub>Zn, the highly exergonic reaction (–56.1 and –50.8 kcal mol<sup>-1</sup>, respectively for *cis* and *trans* products, noted P<sub>C-cis</sub> and P<sub>C-trans</sub>) is thermodynamically and kinetically favored for the *cis* pathway with a very small *cis* activation barrier (8.6 vs. 9.9 kcal mol<sup>-1</sup> for TS<sub>C-cis</sub> and TS<sub>C-trans</sub>, respectively).[33] As previously discussed, a reverse selectivity is computed for [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn in favor of the *trans* product (–32.8 kcal mol<sup>-1</sup> vs. –29.8 kcal mol<sup>-1</sup> for P<sub>polar-trans</sub> and P<sub>polar-cis</sub>, respectively and 24.4 vs. 32.6 kcal mol<sup>-1</sup> for TS<sub>polar</sub>.

<table><caption>**Table 1.** Gibbs energy for the transition states (TS) and products (P) (in kcal mol<sup>-1</sup>, with respect to the separated reactants) for the concerted mechanism.</caption><thead><tr><th rowspan="2">Regioselectivity</th><th rowspan="2">Si<sub>2</sub>Zn</th><th colspan="2"><i>cis</i></th><th colspan="2"><i>trans</i></th></tr><tr><th>TS</th><th>P</th><th>TS</th><th>P</th></tr></thead><tbody><tr><td rowspan="2">C<sub>β</sub></td><td>[(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn</td><td>32.6</td><td>-29.8</td><td><b>24.4</b></td><td><b>-32.8</b></td></tr><tr><td>(Me<sub>2</sub>PhSi)<sub>2</sub>Zn</td><td><b>8.6</b></td><td><b>-56.1</b></td><td>9.9</td><td>-50.8</td></tr><tr><td rowspan="2">C<sub>α</sub></td><td>[(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn</td><td>40.9</td><td>-15.6</td><td>33.7</td><td>-28.0</td></tr><tr><td>(Me<sub>2</sub>PhSi)<sub>2</sub>Zn</td><td>10.2</td><td>-43.3</td><td>14.5</td><td>-48.0</td></tr></tbody></table>
<figure><img src="image_14.png" /></figure>

trans and TS<sub>polar-cis</sub>, respectively). The proposed polar mechanism is thus in line with experimentally observed stereoselectivity dependence on the silyl group.

The experimental observed regioselectivity in favor of the β-isomer ($C_β$–Si bond formed) is next examined. The results for the α-addition of [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn or (Me<sub>2</sub>PhSi)<sub>2</sub>Zn with 1 are given in Table 1 (Tables S5-S6 and Figures S5-S6). For the observed *trans*-mechanism, the β-addition is kinetically and thermodynamically favored by 9.3 (between TS<sub>polar-trans</sub> and TS<sub>α-trans</sub>) and 4.8 kcal mol<sup>-1</sup> (between P<sub>polar-trans</sub> and P<sub>α-trans</sub>), respectively, over the α-addition (the results for the cis-addition are 8.3 and 14.6 kcal mol<sup>-1</sup>, respectively). The computed regioselectivity for the concerted mechanisms is in accordance with the experimental results, independently of the silyl group used. Hence, the retained mechanism leading either to the cis- or trans-products in only one step successfully reproduces also the experimental regioselectivity.

To understand the difference of stereo- and regioselectivity, we have performed the decomposition of the activation energy $ΔΔE^+$ into distortion ($⊗ΔE^+$<sub>distortion</sub>) and interaction ($⊗ΔE^+$<sub>interaction</sub>) energies following the fragment-based approach named *activation strain model* (Figure S7).<sup>[25-27]</sup> The results of this activation strain analysis are given in Table 2 for all transition states. In addition, we have performed this analysis along reaction coordinates (Si–$C_β$, Zn–$C_α$ and $C_α$–$C_β$) in the transition state (Figures S8-S10).<sup>[26]</sup> For the [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn species, the larger activation energy of 12.7 kcal mol<sup>-1</sup> obtained for TS<sub>polar-cis</sub> is essentially due to the distortion energy (38.0 and 32.1 kcal mol<sup>-1</sup> for TS<sub>polar-cis</sub> and TS<sub>polar-trans</sub>, respectively, see Table 2), as the interaction energies are quite similar (–25.3 and –27.7 kcal mol<sup>-1</sup> for TS<sub>polar-cis</sub> and TS<sub>polar-trans</sub>, respectively). This observation remains valid all along the reaction coordinate (Figure S9a). For a smaller substituent, like SiMe<sub>2</sub>Ph, which should induce less steric hindrance, the distortion and interaction energies are very different than [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn: for the cis-addition, the high interaction energy (–49.0 kcal mol<sup>-1</sup> for TS<sub>c-cis</sub>) is compensated by high strain energy (41.1 kcal mol<sup>-1</sup> for TS<sub>c-cis</sub>) leading to a small activation barrier. This is confirmed all along the reaction path (Figure S9c). We can thus propose that the cis-pathway for [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn is disfavoured because of steric hindrance of (Me<sub>3</sub>Si)<sub>3</sub>Si, which forces ynamide to undergo

<table><caption>**Table 2.** Activation, Interaction and Distortion Energies (in kcal mol<sup>-1</sup>) for TS<sub>polar-cis</sub>, TS<sub>polar-trans</sub>, TS<sub>c-cis</sub>, TS<sub>c-trans</sub>, TS<sub>α-cis</sub> and TS<sub>α-trans</sub> transition states using the *activation strain model*. The distortion energy $⊗ΔE^+$<sub>distortion</sub> is decomposed into the distortion energies of the two fragments: diorganosilyl zinc $⊗ΔE^+$<sub>ZnSi2</sub> and 1 ($⊗Δ^+E_1$).</caption><thead><tr><th></th><th>Activation</th><th>Interaction</th><th>Distortion</th><th></th></tr></thead><tbody><tr><td>TS<sub>polar-cis</sub></td><td>12.7</td><td>-25.3</td><td>38.0</td><td>21.5</td></tr><tr><td>TS<sub>polar-trans</sub></td><td>4.4</td><td>-27.7</td><td>32.1</td><td>21.3</td></tr><tr><td>TS<sub>c-cis</sub></td><td>-7.9</td><td>-49.0</td><td>41.1</td><td>28.9</td></tr><tr><td>TS<sub>c-trans</sub></td><td>-5.5</td><td>-28.8</td><td>23.3</td><td>17.8</td></tr><tr><td>TS<sub>α-cis</sub></td><td>21.0</td><td>-37.7</td><td>58.7</td><td>37.7</td></tr><tr><td>TS<sub>α-trans</sub></td><td>13.6</td><td>-30.5</td><td>44.1</td><td>29.6</td></tr></tbody></table>

a larger distortion in order to welcome the addition of the disilyl zinc reagent on the same side of the π system.

The distortion interaction analysis was also performed for [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn to understand the regioselectivity in favor of β-addition. As already described for this species, the dominant contribution to the selectivity (difference in $⊗ΔE^+$) is the distortion energy. For the (preferred) *trans*-pathway, it is more than 10 kcal mol<sup>-1</sup> higher for α-addition (44.1 kcal mol<sup>-1</sup> for TS<sub>α-trans</sub>, Table 2) over β-addition (32.1 kcal mol<sup>-1</sup> for TS<sub>polar-trans</sub>), in contrast to a difference smaller than 3 kcal mol<sup>-1</sup> in interaction energy. This higher $⊗ΔE^+$<sub>distortion</sub> is principally caused by the distortion energy of ynamide, higher for TS<sub>α-trans</sub> (29.6 kcal mol<sup>-1</sup> vs. 21.3 kcal mol<sup>-1</sup> for TS<sub>polar-trans</sub>, see Figure S10). We propose thus that the (Me<sub>3</sub>Si)<sub>3</sub>Si addition into $C_α$ in ynamide is disfavored because of the steric hindrance at $C_α$ substituted by oxazolidinone group.

This regioselectivity in favor of β-addition of diorganosilyl-zinc reagents on C≡C triple bond of terminal alkyne can be considered as a reverse polarity process considering the polarization of ynamide. Indeed, the electron donating ability of the nitrogen atom of oxazolidinone group enables the keteniminium resonance structure of ynamides, rendering $C_α$ as the electrophilic site and $C_β$ as the nucleophilic center. This is in accordance with the atomic charges calculated in 1 (Figure 5, Table S7): as expected, the charges on $C_α$ and $C_β$ are 0.15 and –0.27e, respectively, corresponding well to a nucleophilic site on $C_β$ under charge control.<sup>[34]</sup> In [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn, the Zn–Si bond is polarized and the corresponding charge on Zn and Si are calculated at 1.16 and –0.89 (Figure 5, Table S7). From the point of view of charges, the silyl addition on ynamide occurs between two nucleophilic partners. The polar mechanism is generally associated with zwitterionic or ion-pair intermediates. In the present case, the polar term used in our previous studies to contrast with the radical mechanism, is not appropriate considering the low polarization of the C≡C bond during the

<figure><img src="image_15.png" /><figcaption>**Figure 5.** Atomic charges obtained through the Natural Population Analysis method and selected Molecular Orbitals (isovalue: 0.04 a.u., H are omitted for clarity) for 1 and [(Me<sub>3</sub>Si)<sub>3</sub>Si]<sub>2</sub>Zn.</figcaption></figure>
<figure><img src="image_16.png" /></figure>

process (Table S7). The two proposed antara- (*trans*) and suprafacial (*cis*) additions are thus better viewed as “closed shell” concerted mechanisms. Taking account the HOMO molecular orbitals of TS<sub>polar-trans</sub>, given in Figure 6, the contribution of orbital fragments associated with the σ<sub>Zn–Si</sub> Zn–Si bond of [(Me<sub>3</sub>Si)<sub>3</sub>Si]₂Zn (corresponding to its HOMO shown in Figure 5) and one of the π*<sub>C–C</sub> C<sub>α</sub>=C<sub>β</sub> bond (corresponding to its LUMO given in Figure 5) corresponds to a nucleophilic attack of silyl group to the electrophilic ynamide. This corroborates the Natural Bonding Orbital (NBO) analysis in the TS structure, in which a Lone Pair (LP) of the reactive silyl group interacts with the π*<sub>C–C</sub> ynamide from a donor-acceptor interactions scheme (E = 52,28 kcal mol<sup>-1</sup>).<sup>[35]</sup> The silyl addition is thus described, under an orbital control, as a nucleophilic attack of silyl group to the electrophilic ynamide

It is also known that the oxazolidinone group on ynamide
can reverse the regioselectivity in favor of the β-addition
through a metal-carbonyl chelation.[³⁶] In our study, we find this
secondary interaction between the Zn center and the oxygen
atom of the carbonyl group from oxazolidinone[³⁵⁻³⁶] only for
trans-additions, described by a short Zn–O distance of 2.19 Å in
β-addition TS<sub>polar-trans</sub> (Figure 4, Table S3) and 2.16 Å in α-
addition TS<sub>α-trans</sub> (Table S5). As this Zn–O distance is present in
the two regioselective TS, we can conclude that a Zn–Carbonyl
chelation is not a factor that changes the regioselectivity of this
reaction. We evaluate the Zn–O interaction energy at
8.5 kcal mol<sup>-1</sup> (Figure S11, Table S8) by comparing the energies
of the two trans transition states (TS<sub>polar-trans</sub> vs. TS<sub>polar-trans-conf</sub>) in
which the conformations of oxazolidinone group are different.
In TS<sub>polar-trans-conf</sub> the oxygen of carbonyl group is pointed at the
opposite of the zinc moiety to avoid a Zn–O secondary
interaction. The trans transition state without Zn–O interaction
(TS<sub>polar-trans-conf</sub>) is computed at 32.9 kcal mol<sup>-1</sup> and competes
with the stereoselective cis-transition state (32.6 kcal mol<sup>-1</sup> for
TS<sub>polar-cis</sub>, see Figure S11). We can thus propose that this Zn–O
interaction is not innocent as it strongly stabilizes TS<sub>polar-trans</sub>

<figure><img src="image_17.png" /><figcaption>**Figure 6.** Highest Occupied Molecular Orbital of TS<sub>polar-trans</sub> (isovalue: 0.03 a.u., H are omitted for clarity).</figcaption></figure>

(24.4 kcal mol⁻¹) over TS<sub>polar-cis</sub> leading thus to a stereoselectivity
in favor of the *trans*-product. This Zn–O interaction is not
present for TS<sub>polar-cis</sub> (3.90 Å) as the suprafacial process position-
ing the zinc center too far from the carbonyl function to be
accessible. The Zn–O metal-carbonyl chelation can thus reverse
the stereoselectivity of this reaction.

Conclusions

In conclusion, we show that the addition of diorganosilyl-zinc
reagents across terminal ynamides can take place through a
one-step concerted addition with a lower activation barrier
than that of the originally proposed radical pathway[⁷] and that
of a possible *cis* to *trans* isomerization process.[¹⁸,¹⁹,²¹,²²] This
concerted mechanism occurs either in a suprafacial or antar-
afacial manner, leading respectively to a *cis*- or *trans*-addition.
The antarafacial mechanism resulting in the formation of a Si–C
and a Zn–C σ-bond on opposite sides of the π-system is
compatible with the observed regio- and stereo-selectivities. As
revealed by the *activation strain* analysis, the *trans*-stereo-
selectivity for [(Me₃Si)₃Si]₂Zn is partially due to the steric effect
of the silyl group and is assisted by the oxazolidinone group. In
the same way, the regioselectivity is explained by the bulkiness
of ynamide carbon substituted by oxazolidinone group. Our
new findings thus call for analyzing the effect of substituents in
terminal alkyne and characterizing the bonding scheme of
these concerted transition states along the reaction coordinate.
Most importantly, beyond ynamide silylzincation chemistry, we
believe that this new *trans*-concerted mechanism can be
relevant to a large variety of addition mechanisms to
carbon–carbon multiple bonds involving late transition metal
complexes, either in replacement or in competition with the
isomerization mechanisms proposed in the literature.[¹⁴–¹⁶,¹⁸–²³]

**Computational Details**

The calculations are carried out with Gaussian16 package.[38] All
geometries are calculated using the hybrid B3PW91[39] functional
and the 6-31G(d,p) basis set[40] for all atoms. The geometry
optimizations are performed without any constraint and the nature
of the extrema are verified by analytical calculations of frequencies.
The connection between TS and minima is confirmed by carrying
out a small displacement along the reaction coordinate in each
direction and optimizing geometry starting from these structures.
Two reactant complexes are located for *cis* and *trans*-pathways with
(Me₂PhSi)₂Zn and [(Me₃Si)₃Si]₂Zn and are not added in the Gibbs
Energy profile, as they do not change the activation barrier for *cis*-
and *trans*-additions. Their structures and energies are given in
Figure S12. The Gibbs free energies are calculated assuming an
ideal gas, unscaled harmonic frequencies, and the rigid rotor
approximation in the standard conditions (P = 1 atm and T = 298 K).
Density-based Solvation Model (SMD)[41] single point calculations
are performed using experimental solvents: n-hexane for
[(Me₃Si)₃Si]₂Zn and diethylether for (Me₃Si)₂Zn. The results of these
single points calculations are given in the Supporting Information
(Tables S9 and S10) and confirm that the *trans*-addition is favored
over the *cis*-one. To investigate the competition of polar and radical
chain mechanisms, a series of functionals and basis sets, detailed in
<figure><img src="image_18.png" /></figure>

<table><caption>**Table 3.** Gibbs energies of activation barrier (in kcal mol⁻¹) for trans-silylzincation of [(Me₃Si)₃Si]₂Zn in ynamide 1 in polar (Figure 3, relative to separated reactants) and radical chain mechanisms (Figure 1, with respect to I<sub>rad-trans</sub>) using a series of DFT functional and basis set.</caption><thead><tr><th>DFT functional/basis set</th><th>ΔG<sup>+</sup>(TS<sub>polar-trans</sub>)</th><th>ΔG<sup>+</sup>(TS<sub>rad-trans</sub>)</th></tr></thead><tbody><tr><td>B3PW91 6-31G(d,p)</td><td>24.4</td><td>28.5</td></tr><tr><td>PW91 6-31G(d,p)</td><td>10.4</td><td>19.3</td></tr><tr><td>CAM B3LYP 6-31G(d,p)</td><td>23.8</td><td>23.6</td></tr><tr><td>B3PW91 def2svp</td><td>30.5</td><td>30.8</td></tr><tr><td>B3PW91 6-31G(d,p), SDD ECP for Zn</td><td>31.2</td><td>32.5</td></tr><tr><td>B3PW91 6-311G(d,p)</td><td>32.3</td><td>33.0</td></tr></tbody></table>

Supporting Information, are used and the results are summarized in Table 3 (Tables S11–S13 for more details). The *trans* activation barrier of polar pathway is lower than that calculated for radical pathway, except for CAM–B3LYP functional. For this last one, the energy of radical *trans*-addition is more favorable by only 0.2 kcal mol⁻¹ than the polar pathway. Considering this low energetic difference which is in the range of error of DFT calculations, we are confident in our results. The Natural Population Analysis (NPA) is used to evaluate the atomic charges[42] using the NBO package[43] available in Gaussian16.

## Supporting Information

Additional computational data, cartesian coordinates of all intermediates and transition states and additional references [44–49] are provided in the Supporting Information.

## Acknowledgements

This work was supported by LabEx MiChem of French state funds managed by the ANR within the Investissements d'Avenir programme under reference ANR-11-IDEX-0004-02.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords:** trans-concerted addition · silylzincation · radical · antarafacial · DFT

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Manuscript received: May 3, 2024

Accepted manuscript online: May 29, 2024

Version of record online: July 11, 2024