# Pd/S cooperativity: from stoichiometric activation of Si-H bonds to catalytic hydrosilylation

Arnaud Clerc, Marceline Humbert, Sonia Mallet-Ladeira, Karinne Miqueu, Julien Monot, Blanca Martín-Vaca, Didier Bourissou

► To cite this version:

Arnaud Clerc, Marceline Humbert, Sonia Mallet-Ladeira, Karinne Miqueu, Julien Monot, et al., Pd/S cooperativity: from stoichiometric activation of Si-H bonds to catalytic hydrosilylation. ChemistryEurope, 2024, 3 (1), pp.e202400079. 10.1002/ceur.202400079. hal-05150792

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# Pd/S cooperativity: from stoichiometric activation of Si–H bonds to catalytic hydrosilylation

Arnaud Clerc<sup>+</sup>,<sup>[a]</sup> Marceline Humbert<sup>+</sup>,<sup>[a]</sup> Sonia Mallet-Ladeira,<sup>[b]</sup> Karinne Miqueu,<sup>[c]</sup> Julien Monot,<sup>[a]</sup> Blanca Martín-Vaca,<sup>*</sup>[a] and Didier Bourissou<sup>*</sup>[a]

With the aim to explore metal-ligand cooperation across Pd–S linkages, we designed a quinoline-based PNS tridentate ligand and prepared Pd(II) complexes thereof. Optimal solubility and stability of the dicationic complex [(PNS)Pd(II)]<sub>2</sub><sup>2+</sup> IV<sub>2</sub> were achieved with B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub><sup>−</sup> as counter-anion. It adopts a thiolate-bridged dimeric structure with a (PdS)<sub>2</sub> core, but readily activates hydrosilanes across the Pd–S linkage. The resulting

mononuclear Pd(II) hydride [(PNS<sup>SIR3</sup>)Pd(II)H]<sup>+</sup> VI was characterized by multi-nuclear NMR spectroscopy and the Si–H bond coordination/activation was studied computationally. Complex IV<sub>2</sub> was found to efficiently catalyze the hydrosilylation of alkynes, internal as well as terminal, with good stereo and regioselectivity.

## Introduction

Metal-ligand cooperativity (MLC) undoubtedly stands as one of the major breakthroughs in contemporary homogeneous catalysis.[1] Here, the ligand directly participates to the key bond cleavage/formation steps and the metal usually remains at the same oxidation state over the whole catalytic cycle. Historically, the concept has been mainly studied and exploited with group 8 and 9 metals (Ru, Rh and Ir in particular) for hydrogenation/dehydrogenation transformations, although it is in principle very general. Varying the metal and the framework of the non-innocent ligand beyond the iconic systems introduced by Shvo, Noyori and Milstein currently attracts much attention. It is highly desirable to develop further the approach, both in terms of catalytic performance and scope of achievable transformations.[2–11]

In this regard, besides the well-established cyclopentadiene, amido and pyridine-derived ligands, thiolates have

recently emerged as powerful non-innocent motifs prone to MLC.[12,13] In particular, the groups of Ohki, Tatsumi and Oestreich have obtained impressive results with a Ru(II) complex featuring the terphenyl thiolate DmpS (Figure 1a). Ru/S cooperativity has been shown to enable the activation of E–H bonds (E = Si, Sn, B), opening the way to a range of catalytic dehydrogenative C–E couplings (such as the silylation of indoles).[14–18]

As for the metal variation, our group has worked extensively on Pd and achieved MLC catalysis using a non-innocent SCS pincer ligand derived from indene. These Pd(II) complexes

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Metal-ligand cooperative activation of silanes across M–S bonds and ensuing catalytic transformations.</figcaption></figure>

[a] A. Clerc,\* M. Humbert,\* Dr. J. Monot, Prof. B. Martín-Vaca, Dr. D. Bourissou
Laboratoire Hétérochimie Fondamentale et Appliquée (LHFA, UMR 5069)
CNRS/Université Paul Sabatier
118 Route de Narbonne, 31062 Toulouse Cedex 09 (France)
E-mail: blanca-maria.martin-vaca@univ-tlse3.fr
didier.bourissou@univ-tlse3.fr

[b] S. Mallet-Ladeira
Institut de Chimie de Toulouse (FR2599)
118 Route de Narbonne, 31062 Toulouse Cedex 09 (France)

[c] Dr. K. Miqueu
CNRS/Université de Pau et des Pays de l'Adour
E2S-UPPA
Institut des Sciences Analytiques et Physico-Chimie pour l'Environnement et
les Matériaux (IPREM, UMR 5254)
Hélioparc, 2 Avenue du Président Angot, 64053 Pau Cedex 09 (France)

[\*] These authors contributed equally to this work.

[ ] Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ceur.202400079

[c] © 2024 The Author(s). ChemistryEurope published by Chemistry Europe and 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.
<figure><img src="image_4.png" /></figure>

showed great performance in a number of C–O, C–N and C–C bond-forming cycloisomerization reactions.[19]

In this work, we questioned the possibility to make Pd and S to cooperate for H–E bond activation and MLC catalysis. We hoped to uncover a reactivity profile complementary to the Ru/ S system of Ohki, Tatsumi and Oestreich. Inspired by the PNF framework studied about 10 years ago by Vigalok et al.,[20] we designed a PNS tridentate ligand with a quinoline core.[21] As described hereafter, the corresponding cationic Pd(II) complex adopts a thiolate-bridged dimeric structure, which activates Si–H bonds thanks to Pd/S cooperativity (Figure 1b). The hydrogen atom and silyl moiety can then be readily transferred to π-CC bonds. The system is amenable to hydrosilylation catalysis. It works with internal as well as terminal alkynes, it shows good stereo and regioselectivity.

<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Reaction of methyl iodide with complex IV'₂.</figcaption></figure>

## Results and Discussion

### Synthesis of the PNS pro-ligand and derived Pd(II) complexes

The PNS<sup>tBu</sup> pro-ligand L1 (Scheme 1) was prepared in 3 steps from o-fluoro aniline (Scheme S1). First, the quinoline scaffold was formed by cyclisation with crotonaldehyde. Then, the tert-butyl thioether group was installed upon nucleophilic aromatic substitution and finally, the phosphine sidearm was introduced by deprotonation of the 2-methyl position and electrophilic trapping with di(iso-propyl)chlorophosphine. The procedure was optimized to avoid the need for column chromatography and enable ready access to compound L1 in multi-gram scale (39% yield over 3 steps). From there, various Pd(II) pincer complexes were prepared (Figure 2). Upon reaction of L1 with PdCl₂(PhCN)₂, the cationic complex [[PNS<sup>tBu</sup>)PdCl]Cl I was first obtained in almost quantitative yield (95%). The tert-butyl group at sulfur was then removed by treatment with potassium tert-butylate at 75°C to give the neutral complex [[PNS)PdCl] II. A similar transformation was incidentally observed by Milstein et al. when reacting a pyridine-based PNS Ru(II) complex with NaH.[22] The last step was to open a coordination site at Pd by chloride abstraction. Here, the choice of the reagent turned to be crucial. A preliminary test with AgSbF₆ enabled to generate the targeted (PNS)Pd⁺ cationic fragment IV, isolated and characterized in fact as the dicationic dimer [[PNS)Pd]₂ IV₂ featuring two thiolate-Pd bridging interactions.[23] However, the very poor solubility in organic solvents prevented further reactivity studies and catalytic applications. Borate salts were

<figure><img src="image_6.png" /><figcaption>**Figure 2.** Synthesis of the key PNS Pd(II) complex IV₂, key NMR data (δ¹H refers to the CH₃P signal), molecular structures of complexes I, II and III' as determined by X-ray diffraction (for clarity, the hydrogen and fluorine atoms, the CH₃ groups of the 'Pr substituents, the solvent molecules and the Cl / BFxyl₄ counter-anions for I / III' are omitted; ellipsoids are drawn at 50% probability). For X-ray diffraction analysis of IV₂ (with X=SbF₆), see Ref. [22].</figcaption></figure>
<figure><img src="image_7.png" /></figure>

considered as possible alternatives. Using NaBPh₄, the S-bridged heterodimer III was obtained as the result of Ph group transfer from the borate counter-anion to the electrophilic Pd(II) center and quenching of the targeted (PNS)Pd⁺ complex by S bridging (Figure 2).[24] Using NaBFxyl₄ (Fxyl = 3,5-bis(trifluoromethyl)phenyl), the desired cationic complex IV'₂ could be obtained and characterized at room temperature but B→Pd aryl group transfer occurred upon heating to 80 °C to give the S-bridged heterodimer III'. The best option turned to be KBArF₄ with pentafluorophenyl groups at boron. Here, the [[(PNS)Pd]₂]²⁺ complex IV₂ could be obtained in good yield (89%) and it showed high thermal stability (no change was noticed by ³¹P NMR spectroscopy after 4 days of heating at 120 °C in o-dichlorobenzene).

Complexes I, II, III/III' and IV₂ were thoroughly characterized by multi-nuclear NMR spectroscopy, mass spectrometry and X-ray diffraction analyses (XRD).[25,26] Only the most salient features are reported and discussed here (Figure 2). Spectroscopically, the coordination of the PNS ligand is clearly apparent from the shifts of the ³¹P, ¹⁵N (as determined by ¹H–¹⁵N HSQC experiments) and ¹H (CH₂P) resonance signals compared to the proligand L1. The pattern of the ¹H (CH₂P) signals turns to be very indicative of the symmetry and nuclearity of the complex. Found as a doublet (with JₚH coupling) for the mononuclear complexes I and II (due most likely to rapid inversion at the S atom), it splits into two doublets of doublets (AB system with JHH and JPH couplings) for the S-bridged dinuclear complex IV₂. The situation is different for III and III', for which two doublets with only JₚH coupling are observed (one *per* non-equivalent metal fragment), in agreement with dissymmetric dinuclear structures with only one bridging S atom. Besides confirming connectivity and tridentate PNS coordination, the XRD structures show that all complexes adopt square-planar geometries at Pd, with τ₄ values ranging from 0.09 to 0.17.[27] Complex IV₂ adopts a dimeric structure with two bridging S atoms connecting the (PNS)Pd⁺ fragments. The most noticeable

geometric variation in the coordination sphere of Pd is found in the Pd–S bond length. From 2.407(1) Å in complex I, it decreases to 2.3265(4)-2.3572(4) Å in III', to 2.3149(6)-2.3657(17) Å in IV₂, and to 2.305(3) Å in II, consistent with the strengthening of the Pd–S interaction in the series thioether < bridging thiolate < terminal thiolate.

## Pd/S Cooperative Activation of Mel and Hydrosilanes

To probe the feasibility of small molecule activation across the Pd–S bond, the dicationic thiolate complex was reacted with methyl iodide (5 equiv, 7 h, 80 °C). The thioether iodo complex V was thereby obtained in 95% yield (Scheme 1). The ¹H NMR spectrum shows diagnostic signals for a monomeric species (the CH₂P moiety resonates as a doublet at δ 4.04 ppm) and for the Me–S linkage (doublet at δ 3.08 ppm, JₚH 3.8 Hz showing correlation with the aromatic H atom ortho to S in NOESY). We then turned to dihydrogen, but no reaction was observed under different conditions (–78 to 50 °C, 2 bars) (Scheme 1).

Better results were obtained with hydrosilanes (Figure 3).[28] Indeed, complex IV₂ was found to smoothly and cleanly react with PhMe₂SiH (2 equiv) within 7 h at room temperature to afford complex VI (95% spectroscopic yield). Addition of the hydride to Pd and silylation of the thiolate moiety was unambiguously established by multi-nuclear NMR spectroscopy.[29] The H atom at Pd resonates as a doublet at δ –12.7 ppm with JₚH 6.5 Hz coupling. Compared to the hydrosilane, the ²⁹Si NMR signal shifts downfield by about 40 ppm and shows small Jₛip coupling (1.6 Hz). These data are diagnostic in their own and very much resemble those reported by Ohki and Oestreich upon Si–H activation by the Ru thiolate complex.[17] Complex IV₂ reacts similarly with Et₃SiH (5 equiv) to give VI' (1 h, 100% spectroscopic yield). Attempts to obtain crystals of VI/VI' suitable for XRD systematically led to the

<figure><img src="image_8.png" /><figcaption>**Figure 3.** Reaction of PhMe₂SiH with complex IV₂, key NMR features of the ensuing complex VI.</figcaption></figure>
<figure><img src="image_9.png" /></figure>

backformation of the starting complex IV₂, suggesting reversi-
bility of the Si–H activation across the Pd–S bond. This
hypothesis was further substantiated by a cross-over experi-
ment (see Figure S1).[26] Following activation of PhMe₂SiH and
formation of VI, the addition of Et₃SiH was found to result in an
about 80/20 mixture of VI and VI'.[26]

To shed light into the Pd/S cooperation and draw compar-
ison with the Ru/S complex,[17,30] the Si-H activation step was
studied computationally. Calculations were performed at the
B3PW91-D3(BJ)/SDD + f(Pd),6-31G** (other atoms) level of
theory, without taking into account counter-anion. All sta-
tionary points involved were fully optimized in the gas phase
and in solvent (DCM: Dichloromethane), by means of SMD
model.[31]

Inspection of the molecular orbitals of the monomeric metal
complex (PNS)Pd⁺ indicated the presence of both a low-lying
vacant orbital at Pd and a high-lying occupied orbital at S,
found as the LUMO and HOMO, respectively (Figure 4a). Upon
reaction with PhMe₂SiH (Figure 4b), an η¹-hydrosilane complex

forms first (a side-on η²-hydrosilane form was also located on
the Potential Energy Surface +3.7 kcal/mol higher in energy,
Figure S3).[26] End-on coordination is apparent from the short
Pd–H distance (1.605 Å) vs long Pd···Si one (2.583 Å) and from
the wide Pd–H–Si bond angle (102.9°). The σ(Si–H) bond is
significantly elongated at 1.697 Å (vs 1.495 Å in the free
silane).[32] Note that the splitting of the dicationic (PdS) dimer
IV₂ to give the monocationic silane complex is energetically
accessible (ΔG +0.7 kcal/mol including solvent effects).[26] The
dicationic nature of IV₂ is likely to facilitate the splitting of the
dimer and the formation of the η¹-hydrosilane complex.
Activation of the Si–H bond from the end-on σ-complex
proceeds in a concerted manner. The activation barrier is low at
4.5 kcal/mol and the reaction is exergonic by 10.7 kcal/mol. In
the corresponding transition state, the silyl group atom
approaches the S atom (about perpendicular from the Pd
coordination plane) and the S–Si bond starts to form (2.558 vs
2.250 Å in the addition product). The Si–H bond is much
elongated compared to the silane complex (2.505 Å) while the
Pd–H is shortened (from 1.605 Å in the silane complex to
1.545 Å). Intrinsic Bond Orbital (IBO) analyses give insight into
the Si–H bond coordination and activation (Figure 4b).[33]

A 3-center orbital accounts for the Si–H–Pd interaction in the η¹-hydrosilane complex. It evolves into a covalent Pd–H bond in the addition product. In turn, the lone pair at S evolves into a polarized covalent σ(S–Si) bond. At the transition state, the S atom starts to capture the silyl group, while the H atom is essentially transferred to Pd, with only weak residual interaction towards Si. Thus, the Pd thiolate complex IV shows some similarities with the Ru thiolate complex<sup>[17]</sup> in the Si–H bond activation step. Both involve hydrosilane precomplexes and proceed by concerted addition across the M–S linkage.

Pd/S Cooperative Catalytic Hydrosilylation

The facile activation of hydrosilanes across the Pd–S bond of IV raises the question of possible catalysis. We first considered the dehydrogenative silylation of indoles and the hydrosilylation of pyridines, by comparison with the Ru thiolate complex of Ohki and Oestreich (Scheme 2).[15,16] In our hands, using 2.5 mol% of

<figure><img src="image_10.png" /><figcaption>**Figure 4.** (a) Frontier molecular orbitals (cutoff: 0.05) computed for the (PNS)Pd⁺ complex, ΔE(HOMO-LUMO) 2.21 eV; (b) Reaction profile computed for the Pd/S cooperative activation of PhMe₂Si–H with selected Intrinsic Bond Orbitals (IBOs). For clarity, the hydrogen atoms are omitted, except that at Si/Pd. Calculations performed at the SMD(DCM)-B3PW91-D3(BJ)/SDD + f(Pd),6-31G** (other atoms) level of theory. Main distances are in Å and bond angles in °. For the IBOs, the fractions of electrons in the doubly occupied orbitals assigned to each atom are given.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Scheme 2.** Preliminary catalytic tests of dehydrogenative coupling / hydrosilylation with complex IV₂.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

complex IV₂, no reaction was observed between PhMe₂SiH and N-methyl indole (50°C, 16 h) or pyridine (70°C, 72 h). Conversely, the thiolate Pd complex showed high catalytic activity towards acetophenone.[34] Within 10 min at room temperature, ethylbenzene was obtained in 95% yield using 2 equiv. of PhMe₂SiH.[35] Styrene was also rapidly and efficiently hydrosilylated. The reaction was achieved in 1 h at room temperature and occurred with complete selectivity for the β-addition product. Diphenylacetylene was then tested as model alkyne. Pleasingly, complex IV₂ displayed here also high activity (95% yield within 1 h at room temperature) as well as high selectivity for anti addition (95/5). The obtained vinyl silanes have been unambiguously authenticated by comparison of the ¹H NMR signals of the vinylic proton with literature data (δ 7.47 ppm for the anti addition product, δ 6.87 ppm for the syn addition product).[26,36] According to this preliminary catalytic survey, the Pd thiolate complex IV₂ is catalytically efficient for H–Si addition to π-CC multiple bonds but its reactivity profile is complementary to that of the previously reported Ru thiolate complex. The good result obtained with diphenylacetylene and the high synthetic interest of vinyl silanes prompted us to investigate in more detail the hydrosilylation of alkynes with this Pd/S cooperative system.

First, we questioned the relevance of the Si–H activation product to the catalytic transformation. Following the formation of complex VI by reaction of IV₂ with 5 equiv. of PhMe₂SiH, the addition of 5 equiv. of diphenylacetylene led to rapid and quantitative consumption of the substrates. The anti-hydrosilylation product was obtained in 75% yield. We then sought to optimize the reaction conditions. The same catalytic results were obtained using preformed Pd thiolate complex IV₂ or preparing it in situ (from II and KBArF₄). Lowering the catalytic loading is also possible: 95% yield and 95% anti selectivity was equally achieved within 1 h using only 1 mol% of Pd. With the dialkyl alkyne EtCCEt, the reaction is more demanding but heating at 70 °C for 20 h with 5 mol% Pd affords the hydrosilylation product in 98% yield. Strikingly, syn not anti addition largely prevails in this case (90/10).[37] At this point, control reactions were performed with KBArF₄ but without Pd complex. For diphenylacetylene as well as hex-3-yne, very little silane consumption was observed even after prolonged heating at 70 °C (8% after 22 h) and only traces of the hydrosilylation products were detected (< 8%). The impact of the solvent was then assessed on the hydrosilylation of hex-3-yne, which is more demanding and was thus expected to enable better discrimination (Table S1). The reaction worked very well in dichloromethane, 1,2-dichloroethane, o-dichlorobenzene: >85% yield and >94% syn selectivity. Chloroform and diethylether proved also suitable, although giving slightly reduced syn selectivities (85%), while a significantly lower yield (40%) was obtained in THF.

The reaction reached completion within 1 h at 25 °C (< 10 min at 70 °C). Mono addition of the silane largely prevailed with only ca. 10% of multiple Si–H addition (a test with 3 equiv. of PhSiH₃ did not prevent poly addition) and high selectivity for syn addition was here also observed (entry 4). Conversely, the reactivity of the silane decreased when the silicon was sterically

congested, as in ¹BuMe₂SiH. Here, the hydrosilylation of hex-3-yne was slow and plateaued at 22% yield after 72 h at 70 °C. It is noteworthy, however, that the stereoselectivity was reversed compared with PhMe₂SiH, *anti* addition largely prevailing with ¹BuMe₂SiH (entry 5). With the more reactive diphenylacetylene substrate, ¹BuMe₂SiH reacted slowly but cleanly, and the corresponding hydrosilylation product was obtained in 86% yield after 30 h at 25 °C, with 90/10 selectivity for the *anti* addition (entry 8), as with PhMe₂SiH.

Different alkyne substrates were then tested to assess the feasibility and efficiency/selectivity of the reaction towards aryl- as well as alkyl- and silyl-substituted alkynes, towards internal as well as terminal alkynes (Table 1, entries 9–14). As silane, we used PhMe₂SiH as the best compromise in reactivity and selectivity. Accordingly, the internal disymmetric alkynes PhC≡CMe and PhC≡CCy could be efficiently hydrosilylated. In both cases, the reaction proceeded within hours at 50 °C and 5 mol% Pd, with ~75/25 selectivity for the β addition product (the silyl group being introduced in the β position to the Ph substituent) and >70/30 selectivity in favor of the syn addition (entries 9 and 10). With the internal silyl-substituted alkyne PhC≡CSiMe₂Ph, the hydrosilylation worked at 25 °C and gave exclusively the β addition product (entry 11). The Pd complex IV₂ proved also competent to hydrosilylate terminal alkynes.

Both phenylacetylene and hex-1-yne reacted within a few hours at 70 °C using 5 mol% Pd, with complete selectivity for β addition of the silane (entries 12–14). As for internal alkynes, the phenyl-substituted substrate reacted significantly faster (full conversion is essentially achieved within 10 min) and the ensuing vinyl silane isomerized over time so that syn selectivity increased from 60/40 to 100/0 prolonging heating for 6 hours. Of note, no trace of dehydrogenative silylation of the terminal alkynes was detected, but a secondary product resulting from chain-walking of the C=C bond was authenticated for the ¹Bu-substituted alkyne. Finally, the screening was extended to an allene, PhCH=C=CH₂ (entry 15). The (PNS)Pd complex IV₂ also efficiently catalyzed its hydrosilylation (96% yield after 24 h at 50 °C) and gave the terminal vinyl silane 3ha as very major product. The synthetic complementarity with the hydrosilylation of the alkyne PhC≡CMe leading to the internal vinyl silane 3ca is noteworthy.

Complex IV₂ thus appears as a general catalyst for the hydrosilylation of alkynes (and allenes), internal as well as terminal, with aryl as well as alkyl and silyl substituents. The switch of stereoselectivity observed upon hydrosilylation of diphenylacetylene and hex-3-yne from *anti* to *syn* addition suggests that the (PNS)Pd catalyst can operate *via* different paths, and that the prevailing path depends on the substrate. Cooperative activation of the Si–H bond across the Pd–S linkage first gives the experimental observed complex VI. From there, the reaction may proceed by an inner-sphere path involving a π-alkyne complex (Figure 5a). Outer-sphere paths involving concerted or stepwise transfers of the hydrogen atom and silyl group without coordination of the alkyne to Pd are also conceivable (Figure 5b) and could account for the formation of the *anti* addition product.[38,39]
<figure><img src="image_13.png" /></figure>

## 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:** Homogeneous catalysis • Hydrosilylation • Metal/ligand cooperation • Palladium • Pincer ligand • Sulfur

<figure><img src="image_14.png" /><figcaption>**Figure 5.** Catalytic cycles envisioned to account for the hydrosilylation of alkynes reported in this work.</figcaption></figure>

efficiently catalyze the hydrosilylation of alkynes. It works with internal as well as terminal substrates, with aryl, alkyl and silyl substituents. Good to high regio and stereo-selectivities are obtained. β-Addition of the silyl group always prevails, and syn or anti addition of the silane is observed.

The difference in reactivity between the (PNS)Pd complex and the previously reported (DmpS)Ru species, *ie* hydrosilylation versus dehydrogenative coupling, is noteworthy and synthetically valuable. Future work will aim to further develop MLC catalysis with group 10 metals and thiolate non-innocent ligands, with special interest for complexes that display reactivity profiles complementary to those of known systems. In particular, fine modulation of the PNS pincer ligand, seeking to disfavor dimerization of the (PNS)Pd fragment and thereby to increase its reactivity, is currently under investigation.

## Acknowledgements

Financial support from the Centre National de la Recherche Scientifique, the Université de Toulouse, and the Agence Nationale de la Recherche (ANR AAPG2020 CE07 MLC Photos project) is gratefully acknowledged. A.C. thanks MESRI (Ministère de l'Enseignement Supérieur, de la Recherche et de l'Innovation) for his PhD fellowship.

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[6] For selected recent examples with Nickel, see: a) D. V. Gutsulyak, W. E. Piers, J. Borau-Garcia, M. Parvez, *J. Am. Chem. Soc.* **2013**, *135*, 11776-11779; b) M. L. G. Sansores-Paredes, M. Lutz, M.-E. Moret, *Nat. Chem.* **2024**, *16*, 417-425.

[7] For selected recent examples with Palladium, see: a) A. Scharf, I. Goldberg, A. Vigalok, *J. Am. Chem. Soc.* **2013**, *135*, 967-970; b) V. Salamanca, A. Toledo, A. C. Albéniz, *J. Am. Chem. Soc.* **2018**, *140*, 17851-17856; c) C. Pinilla, V. Salamanca, A. Lledós, A. C. Albéniz, *ACS Catal.* **2022**, *12*, 14527-14532; d) S. Fernández-Moyano, V. Salamanca, A. C. Albéniz, *Chem. Sci.* **2023**, *14*, 6688-6694.

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[9] For a recent example with Magnesium, see: Y. Liang, J. Luo, Y. Diskin-Posner, D. Milstein, *J. Am. Chem. Soc.* **2023**, *145*, 9164-9175.

[10] For a recent example with Calcium, see: Y. Liang, I. Efremenko, Y. Diskin-Posner, L. Avram, D. Milstein, *Angew. Chem. Int. Ed.* **2024**, *63*, e202401702.

[11] For a recent example with Iridium, see: K. Iizuka, Y. Nakajima, K. Sato, *Dalton Trans.* **2022**, *51*, 12781-12785.

[12] L. Omann, C. D. Königs, H. F. T. Klare, M. Oestreich, *Acc. Chem. Res.* **2017**, *50*, 1258-1269.

[13] For a recent review dealing with thiols as transient cooperative ligands, see: J. Luo, M. Montag, D. Milstein, *Acc. Chem. Res.* **2024**, *57*, 1709-1721.
<figure><img src="image_15.png" /></figure>

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[25] Deposition numbers 2346512 (for I), 2346513 (for II), and 2346514 (for III') contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

[26] See the ESI.

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[31] For details and a benchmark of functionals, see Supporting Information.

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[35] With only 1 equiv. of hydrosilane, ethylbenzene was also obtained and the conversion of acetophenone was limited to < 50%.

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[37] The syn and anti products were here also authenticated unequivocally by comparing the vinylic ¹H NMR signals with literature data, see Ref. [36a] and C. Xu, B. Huang, T. Yan, M. Cai, *Green Chem.* **2018**, *20*, 391–397.

[38] Stepwise outer-sphere mechanisms have been occasionally proposed for transition metal-catalyzed hydrosilylation of alkynes, see: a) M. Iglesias, P. J. Sanz Miguel, V. Polo, F. J. Fernandez-Alvarez, J. J. Perez-Torrente, L. A. Oro, *Chem. Eur. J.* **2013**, *19*, 17559–17566; b) M. Iglesias, F. J. Fernandez-Alvarez, L. A. Oro, *Coord. Chem. Rev.* **2019**, *386*, 240–266; c) R. Puerta-Oteo, J. Munarriz, V. Polo, M. V. Jiménez, J. J. Pérez-Torrente, *ACS Catal.* **2020**, *10*, 7367–7380.

[39] Ionic stepwise mechanisms are also frequently invoked to account for Lewis-acid catalyzed hydrosilylation of alkenes and alkynes, see for examples: a) M. Rubin, T. Schwier, V. Gevorgyan, *J. Org. Chem.* **2002**, *67*, 1936–1940; b) M. Pérez, L. J. Hounjet, C. B. Caputo, R. Dobrovetsky, D. W. Stephan, *J. Am. Chem. Soc.* **2013**, *135*, 18308–18310; c) A. Simonneau, M. Oestreich, *Angew. Chem. Int. Ed.* **2013**, *52*, 11905–11907.

Version of record online: November 12, 2024