# Stereoselective Synthesis of 4-Hydroxy-1-Silyl-1-Allenylboranes and Access to 2-Silylethynyl-1,3-Diols

Ichrak El Haj Brahim, Islem Ishak Dridi, Raoudha Abderrahim, Fabrice

Chemla, Franck Ferreira, Olivier Jackowski, Alejandro Perez-Luna

<figure><img src="image_1.png" /></figure>

► To cite this version:

Ichrak El Haj Brahim, Islem Ishak Dridi, Raoudha Abderrahim, Fabrice Chemla, Franck Ferreira, et al.. Stereoselective Synthesis of 4-Hydroxy-1-Silyl-1-Allenylboranes and Access to 2-Silylethynyl-1,3-Diols. Asian Journal of Organic Chemistry, 2024, 13 (5), pp.e202400042. 10.1002/ajoc.202400042. hal-04737222

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# Stereoselective Synthesis of 4-Hydroxy-1-Silyl-1-Allenylboranes and Access to 2-Silylethynyl-1,3-Diols

Ichrak El Haj Brahim,[a, b] Islem Ishak Dridi,[a, b] Raoudha Abderrahim,[b] Fabrice Chemla,[a] Franck Ferreira,[*a] Olivier Jackowski,[*a] and Alejandro Pérez-Luna[a]

The highly stereoselective gem-silylboration of chiral 2-substituted 1-ethynylepoxides is reported herein. The reaction involves first the deprotonation of the epoxides in the acetylenic position followed by transmetallation with Et₃SiBpin. The transient silylborane ate-complexes generated undergo the stereoselective 1,2-migration of the Et₃Si-group to the sp-hybridized terminus carbon of the ethynyl-moiety in a stereo-

specific anti-addition. The moisture-sensitive and thermally unstable 4-hydroxy-1-triethylsilyl-1-allenyl(pinacolato)boranes thus obtained react with aldehydes through an S<sub>E</sub>2'-mode of addition, affording highly functionalized 2-triethysilethynyl-1,3-diols with a good diastereoselectivity which is rationalized by a Yamamoto-Houk type transition state model.

## Introduction

Unlike allenylboranes 1[1] and allenylsilanes 2,[2] the synthesis and use of which are well-documented, only three methods of preparation of 1-silyl-1-allenylboranes 3 have been reported to date in the literature (Figure 1).

In the early 1980's, Wang first showed that chiral 1-silyl-1-allenylboranes were obtained through the deprotonation of 1-trimethylsilylhex-1-yne 4 in the propargylic position followed by the transmetallation with B-MeO-9-BBN/BF₃·OEt₂ to provide 1-trimethylsilyl-1-allenylborane 5 (Scheme 1, top).[3] However, due to a rapid metallotropic equilibrium with the propargylic borane, this species could not be isolated and was immediately used in reaction with carbonyl derivatives to provide α-allenic alcohols 6 as major products via a favored S<sub>E</sub>2-pathway. On the other hand, homopropargylic amines 7 were predominantly obtained through the reaction of 5 with imines via a favored S<sub>E</sub>2'-pathway. Nevertheless, in both cases no information was given on the diastereochemical outcome of the reaction.

More recently, Shimizu developed a more general method of preparation of 1-silyl-1-allenylboranes 9 based on the gem-

<figure><img src="image_4.png" /><figcaption>Figure 1. Allenylboranes, allenylsilanes and 1-silyl-1-allenylboranes.</figcaption></figure>

silylboration of chiral propargylic electrophiles 8 (Scheme 1, middle).[4-7] The method involves the deprotonation of 8 in the acetylenic position and the subsequent transmetallation with the use of Suginome's silylborane PhMe₂SiBpin.[8] The protocol was amenable to asymmetric synthesis of metallotropically stable 1-phenyldimethylsilyl-1-allenylboranes 9 upon central-to-axial chirality transfer from secondary mesylates resulting from anti-1,2-migration of the silyl moiety. Even though the yields were good-to-excellent, the chirality transfer was modestly selective and the 1-silyl-1-allenylboranes 9 were obtained in only ~70% ee. Importantly however, the reaction of these species with cyclohexanecarboxaldehyde was shown to proceed via an S<sub>E</sub>2'-pathway, giving the two homopropargylic alcohols 10 with a high diastereoselectivity.

Finally, very recently in 2021, Miura and Shishido reported the heterogeneous gold-catalyzed S<sub>N</sub>1'-type gem-silylboration of propargylic methyl carbonate 11 with B₂pin₂ providing 1-trimethylsilyl-1-allenyl(pinacolato)borane 12, albeit in achiral series (Scheme 1, bottom).[9]

In this article, we disclose a new approach to accomplish with high diastereoselectivity the gem-silylboration of chiral 2-substituted 1-ethynylepoxides, thereby giving a stereoselective access to 4-hydroxy-1-silyl-1-allenylboranes (Scheme 2).[10] The subsequent reaction of the allenylborane unit species with aldehydes is also shown to afford highly functionalized 2-silylethynyl-1,3-diols via a diastereoselective S<sub>E</sub>2'-pathway.[11]

<footnote>[a] I. El Haj Brahim, I. Ishak Dridi, Prof. F. Chemla, Prof. F. Ferreira, Dr. O. Jackowski, Dr. A. Pérez-Luna Sorbonne Université, CNRS Institut Parisien de Chimie Moléculaire 4 place Jussieu, 75005 Paris (France) E-mail: franck.ferreira@sorbonne-universite.fr olivier.jackowski@sorbonne-universite.fr</footnote>

<footnote>[b] I. El Haj Brahim, I. Ishak Dridi, Prof. R. Abderrahim Université de Carthage, Faculté des Sciences de Bizerte Laboratoire de Physique des Matériaux Lamellaires et Nanomatériaux Hybrides Zarzouna, 7021 Bizerte (Tunisie)</footnote>

<footnote>Supporting information for this article is available on the WWW under https://doi.org/10.1002/ajoc.202400042</footnote>

<footnote>© 2024 The Authors. Asian Journal of Organic Chemistry 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>
Wang's work

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

Shimizu's work

<figure><img src="image_6.png" /></figure>

Miura and Shishido's work

<figure><img src="image_7.png" /><figcaption>**Scheme 1.** Previous works. [Correction added on 30 April 2024, after first online publication: Scheme 1 caption was updated in this version.]</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Scheme 2.** This work.</figcaption></figure>

## Results and Discussion

We initiated our study by investigating the reaction of racemic 2-substituted 1-ethynylepoxide *trans*-13a as a model substrate which was readily prepared with a high 96:4 *trans:cis* ratio (see

SI). In the first attempt, 1,2 equivalents of *n*BuLi (1.6 M in hexanes) were added dropwise to a THF solution of *trans*-13a at -78 °C followed, after 15 min of stirring, by the addition of 1,2 equivalents of Suginome's silylborane PhMe₂SiBpin. After an additional 1 h of stirring, the mixture was quenched with an aqueous NH₃/NH₄Cl solution (Scheme 3). Pleasantly, under these conditions, the formation of the desired product 14a was observed (42% yield), accompanied by the two proto-deborylated compounds 15a and 16a (47% yield) and enyne 17a (3% yield). Very interestingly, product 14a was obtained with an excellent diastereoselectivity (dr = 94:6) suggesting a remarkable central-to-axial chirality transfer. Both compounds 15a and 16a were assumed to result from hydrolysis of product 14a by proto-deborylation with water following S<sub>E</sub>2- and S<sub>E</sub>2'-processes, respectively.[12] In addition, product 14a was found to be thermally unstable since after 4 h of standing at rt or 12 h at 5 °C, it underwent complete degradation into enyne 17a. It also showed instability upon purification over both silica gel (SiO₂) or neutral alumina (Al₂O₃), again leading to enyne 17a.

This preliminary result was encouraging since it showed that compounds 15a, 16a and 17a arose from the expected product 14a, suggesting that gem-silylborylation efficiently occurred. We therefore undertook an intensive study to define the optimal reaction conditions (see SI). Several other lithiated bases (*n*BuLi, 2.5 M in hexanes or 3.2 M in pentane; *t*BuLi, 1.7 M in pentane; LDA, 1.7 M in pentane or 2.0 M in hexanes; MeLi, 1.5 M in Et₂O or 3.2 M in DME), at temperatures down to 0 °C in coordinating or non-coordinating solvents (THF/Et₂O mixtures, Et₂O, MTBE and pentane) were tested with PhMe₂SiBpin or Et₃SiBpin by varying both the order of addition and the stoichiometry of the reagents. A large number of quenching conditions were also examined with emphasis on the use of a minimum of the proton source to avoid, or even prevent, the proto-deborylation of 14a (solid Na₂HPO₄ and NaH₂PO₄·H₃PO₄, 3.6 M in THF; HCl, 4 M in Et₂O and glacial AcOH). All these parameters were shown to have a great influence on the

<figure><img src="image_9.png" /><figcaption>**Scheme 3.** Preliminary study. [a] Yield calculated by ¹H NMR spectroscopy using biphenyl as internal standard on the crude material. [b] Dr determined by ¹H NMR spectroscopy on the crude material.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Table 1.** Stereoselective synthesis of 4-hydroxy-1-triethylsilyl-1-allenyl(pinacolato)boranes (aS*,S*)-14. [a]</figcaption></figure>

[a] Reactions run with 0.5 mmol of 2-subsitized 1-ethynylepoxides *trans*-13. [b] Conversions and yields calculated by ¹H NMR spectroscopy on the crude materials using biphenyl as internal standard. [c] Dr determined by ¹H NMR spectroscopy on the crude materials. [d] With an unidentified side-product.

chemical outcome of the reaction. We then found that the best result was obtained when adding one equivalent of *n*BuLi (3.6 M in pentane) or MeLi (1.6 M in Et₂O) to a 1:2 mixture of epoxide *trans*-13a and Et₃SiBpin[13] (the use of commercially available PhMe₂SiBpin led to difficult-to-reproduce results) in THF at -78°C for 45 min, and quenching with one equivalent of glacial acetic acid from -78°C to 0°C, over a 20 min-period (Table 1). Furthermore, in order to avoid any contact with water, the work-up was limited to drying over anhydrous Na₂SO₄ and filtration over cotton followed by concentration of the filtrate in vacuo. By following this procedure, product (aS*,S*)-14a was obtained in 90% yield with a high 94:6 dr. Except for traces of enyne 17a (<3% yield), no other side-products were observed.

The optimal conditions in hands, we next extended the method to other epoxides *trans*-13 (Table 1). With epoxides *trans*-13b-d, bearing primary or secondary alkyl substituents, the expected products 14b-d were obtained in good yields (60-85%) and with an excellent diastereoselectivity (dr up to 89:11) corresponding to a high central-to-axial chirality transfer. In all these cases, enynes 17 were formed in small amounts (≤ 7% yield). Conversely, these conditions proved to be ineffective with the bulky 2-tert-butyl-substituted and the 2-phenyl-substituted epoxides *trans*-13e and 13f for which only traces of 14e and 14f were detected by ¹H NMR analysis.[14] The relative (aS*,S*)-configuration of products 14a-e was assigned on the basis of the gem-silylboration taking place via the 1,2-migration of Et₃Si in an anti-manner, as reported by Shimizu.[4,15] Worthy of note, in all cases, the crude material was rapidly used in order to limit its thermal decomposition.[16]

The reaction was next studied with epoxide cis-13a which was easily prepared with an 85:15 cis:trans ratio (see SI). With this epoxide, product 14a with the (aR*,S*)-configuration was obtained in lower 40% conversion and yield, albeit with a

diastereomeric ratio of 90:10, suggesting again an excellent central-to-axial chirality transfer (Scheme 4).

We next envisioned to develop a straightforward access to highly functionalized chiral 2-triethysilethynyl-1,3-diols 18 for which no synthesis had been reported yet. All our initial efforts to obtain product 18aa by trapping alkoxide intermediate Li-14a in situ with benzaldehyde failed (Table 2). We finally manage to engage (aS*,S*)-14a, obtained following the work-up described above and without purification, in the reaction with benzaldehyde. Worthy of note, in all cases the reaction provided only two out of the four possible diastereomers. When the reaction was run at room temperature in THF, Et₂O, benzene or pentane, moderate yields (≤ 78%) and poor diastereoselectivity (dr ≤ 60:40:0:0) were obtained (Table 2, entries 1-4). Despite the complete consumption of (aS*,S*)-14a, in these four solvents, considerable degradation was observed. Using CH₂Cl₂ did not improve the yield but led to an increased diastereoselectivity, up to 77:23:0:0 (Table 1, entry 5). The best result was finally obtained in CHCl₃ with which product 18aa was isolated in 69% yield with a satisfactory diastereoselectivity (dr = 80:20:0:0), within 4 h at room temperature (Table 2, entry 6). Lowering the temperature resulted in a slightly

<figure><img src="image_11.png" /><figcaption>**Scheme 4.** Stereospecific gem-silylboration. [a] Conversion and yield calculated by ¹H NMR spectroscopy using biphenyl as internal standard. [b] Dr determined by ¹H NMR spectroscopy on the crude material. [Correction added on 30 April 2024, after first online publication: Scheme 4 caption was updated in this version.]</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Table 2.** Preliminary study for access to 2-triethysilylethynyl-1,3-diols.[a]</figcaption></figure>

<table><thead><tr><th>Entry</th><th>Solvent</th><th>T (°C)</th><th>t (h)<sup>[b]</sup></th><th>18aa (%)</th><th>dr</th></tr></thead><tbody><tr><td>1</td><td>THF</td><td>rt</td><td>48</td><td>54<sup>[c,d]</sup></td><td>53:47:0:0<sup>[g]</sup></td></tr><tr><td>2</td><td>Et<sub>2</sub>O</td><td>rt</td><td>6</td><td>55<sup>[c,d]</sup></td><td>55:45:0:0<sup>[g]</sup></td></tr><tr><td>3</td><td>benzene</td><td>rt</td><td>48</td><td>78<sup>[c,d]</sup></td><td>60:40:0:0<sup>[g]</sup></td></tr><tr><td>4</td><td>pentane</td><td>rt</td><td>4</td><td>72<sup>[c,d]</sup></td><td>50:50:0:0<sup>[g]</sup></td></tr><tr><td>5</td><td>CH<sub>2</sub>Cl<sub>2</sub></td><td>rt</td><td>48</td><td>54<sup>[c,d]</sup></td><td>77:23:0:0<sup>[g]</sup></td></tr><tr><td>6</td><td>CHCl<sub>3</sub></td><td>rt</td><td>4</td><td>69<sup>[e]</sup> (93<sup>[c]</sup>)</td><td>80:20:0:0<sup>[h]</sup></td></tr><tr><td>7</td><td>CHCl<sub>3</sub></td><td>0</td><td>24</td><td>80<sup>[e]</sup></td><td>82:18:0:0<sup>[g]</sup></td></tr><tr><td>8</td><td>CHCl<sub>3</sub></td><td>-20</td><td>48</td><td>42<sup>[e,f]</sup></td><td>85:15:0:0<sup>[h]</sup></td></tr></tbody></table>

[a] Reactions run with 0.25 – 0.5 mmol of *trans*-13a. [b] Times required for the complete consumption of *trans*-13a by TLC. [c] Yields calculated by ¹H NMR spectroscopy using biphenyl as internal standard. [d] Important degradation. [e] Isolated yields. [f] Important formation of enyne 17a. [g] Dr determined by ¹H NMR spectroscopy on the crude materials. [h] Dr determined by ¹H NMR spectroscopy on isolated 18aa.

improved diastereoselectivity but at the expense of the yield and with a high proportion of enyne **17a** at –20 °C (Table 2, entries 7 and 8).

The best reaction conditions were next applied to other aldehydes. In each case, two equivalents of the aldehyde were employed at room temperature within 4 h in CHCl₃ (Table 3). Acetaldehyde and cyclohexanecarboxaldehyde led to products **18ab** and **18ac** in good isolated yields (68% and 63%) with excellent diastereomeric ratios (dr = 83:17:0:0 and 96:4:0:0). Crotonaldehyde and cinnamaldehyde also provided products **18ad** and **18ae** in satisfactory isolated yields (59% and 71%), albeit with a significantly lower diastereoselectivity (dr = 63:37:0:0). By contrast, the reaction was unsuccessful with pivaldehyde, providing only enyne **17a** without any trace of the desired product **18af**. The same lack of reactivity was also observed with ketones. By slowing down the reaction, the steric hindrance of pivaldehyde and ketones probably gave more time to degradation of (aS*,S*)-14a into enyne **17a** to occur.

All products **18** were fully characterized by ¹H and ¹³C NMR analysis and HRMS. Moreover, the relative (*anti*,*syn*)-configuration of the (*O*,*C*,*O*)-stereotriad of these products could be unambiguously deduced from the single crystal X-ray analysis of **18ac** (Figure 2).[17] On the other hand, the relative configuration of the minor diastereomers could not be assigned.

The relative (*anti*,*syn*)-configuration of 2-triethysilylethynyl-1,3-diols **18** was rationalized by the S<sub>e</sub>2'-nucleophilic addition of (aS*,S*)-14 to aldehydes occurring through chair-like Yamamoto-Houk[18,19] type transition state model M1 (Scheme 5). In this model, the electrophilicity of the aldehyde is enhanced by the coordination of the boron atom of (aS*,S*)-14 on the oxygen of

<figure><img src="image_13.png" /><figcaption>**Table 3.** Stereoselective synthesis of 2-triethysilylethynyl-1,3-diols (*anti*,*syn*)-18.[a]</figcaption></figure>

[a] Reactions carried out with 0.25 – 0.5 mmol of epoxide *trans*-13a. [b] Isolated yields over two steps. [c] Dr determined by ¹H NMR spectroscopy on isolated products (*anti*,*syn*)-18aa-*ae*. [d] Degradation into enyne 17a.
<figure><img src="image_14.png" /><figcaption>**Figure 2.** Single crystal X-ray analysis of (anti,syn)-18ac. Hydrogen atoms have been omitted for clarity.</figcaption></figure>

the carbonyl function. In addition, (aS*,S*)-14 adopts a conformation such that the hydrogen of the α-allenic stereocenter eclipses the allenyl moiety while the electron-donating substituent R¹ reinforces the nucleophilicity of (aS*,S*)-14 by the σ(=C-C)/π(=C-C) overlap. In order to minimize steric interactions, the nucleophilic attack thus takes place on the *re*-face of the aldehyde and opposite to the R¹ substituent. The analogous competitive model M2, in which the nucleophilic addition of (aS*,S*)-14 takes place on the *si*-face of the aldehyde, is more sterically demanding because of non-bonding interactions

between the R² substituent of the aldehyde and the γ-substituent of (aS*,S*)-14. Model M2 is therefore less favorable than model M1 and can be invoked in the formation of the minor diastereomers of products 18, for which the relative (anti,anti)-configuration can reasonably be assigned. The two other possible Yamamoto-Houk type transition state models, M3 and M4, which correspond to the nucleophilic addition of (aR*,S*)-14 on the *si*- and *re*-face of the aldehyde respectively, lead to the two remaining (*syn*,*syn*)- and (*syn*,*anti*)-diastereomers. From these four possible models, the formation of products 18 with the relative (anti,syn)-configuration allows us to confirm the relative (aS*,S*)-configuration of products 14 and to assert with certainty that the *gem*-silylboration of 2-substituted 1-ethynylepoxides *trans*-13 proceeds by an *anti*-S<sub>N</sub>i through the 1,2-migration of the silyl group.

## Conclusions

To sum up, we have unveiled the highly stereoselective *gem*-silylboration of chiral 2-substituted 1-ethynylepoxides by deprotonation and subsequent transmetallation with Et<sub>3</sub>SiBpin. The reaction was shown to proceed through an *anti*-S<sub>N</sub>i with the 1,2-migration of the Et<sub>3</sub>Si-group from the in situ generated silylborane ate complexes and gave 4-hydroxy-1-triethylsilyl-1-allenyl(pinacolato)boranes in a remarkable stereoselective and stereospecific manner. These chiral *gem*-bimetallic species further reacted with aldehydes to afford highly functionalized

<figure><img src="image_15.png" /><figcaption>**Scheme 5.** Rationalization of the stereoselectivity.</figcaption></figure>
2-triethysilylethynyl-1,3-diol s with a good diastereoselectivity. Similar S<sub>E</sub>2'-substitution reactions with other electrophiles are currently being investigated in our group.

**Supporting Information**

The authors have cited additional references within the Supporting Information. [Correction added on 30 April 2024, after first online publication: Citation for references 20-24 were removed in this version.]

**Experimental Section**

**Representative procedure for the synthesis of compounds 14.** Under argon atmosphere, to a solution of epoxide 13 (0.50 mmol) and Et<sub>3</sub>SiBpin (242 mg, 1.00 mmol) in THF (5 mL), nBuLi (3.2 M in pentane from the dilution of an 11 M commercial solution, 0.16 mL, 0.50 mmol) or MeLi (1.6 M in Et<sub>2</sub>O, 0.31 mL, 0.50 mmol) was added dropwise at -78 °C. The solution was stirred for 45 min at -78 °C at which time Et<sub>2</sub>O (5 mL) and glacial AcOH (29 µL, 1.00 mmol) were successively added. The resulting mixture was gently warmed to 0 °C, over a 20 min-period. It was then dried over anhydrous Na<sub>2</sub>SO<sub>4</sub>, filtered through cotton and concentrated in vacuo to afford 14, as a pale-yellow oil, which was immediately used without purification or stored in the fridge before use as soon as possible.

**Representative procedure for the synthesis of compounds (anti,syn)-18.** Under argon atmosphere, to a 0.25 M solution of freshly prepared crude (aS*,S*)-14 in CHCl<sub>3</sub>, the aldehyde (2 equiv) was added dropwise at room temperature. The mixture was stirred for 4 h and hydrolyzed with a 0.1 M aqueous HCl solution (5 mL). After addition of CH<sub>2</sub>Cl<sub>2</sub> (5 mL), the layers were separated and the aqueous one was extracted with CH<sub>2</sub>Cl<sub>2</sub> (3×). The combined organic layers were successively washed with brine and water, dried over anhydrous Na<sub>2</sub>SO<sub>4</sub> and concentrated in vacuo. Purification by flash column chromatography over silica gel (pentane/Et<sub>2</sub>O = 100/0 to 40/60 gradient, v/v) afforded (anti,syn)-18.

**Acknowledgements**

The authors thank the Université de Carthage for financial support to I. El Haj Brahim and I. Ishak Dridi. The authors also thank Geoffrey Gontard from IPCM for the determination of the X-ray structure of 2-ethynyl-1,3-diol (anti,syn)-18ac.

**Conflict of Interests**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords:** allenylboranes · allenylsilanes · silylboration · stereoselectivity · synthesis

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[3] a) K. K. Wang, S. S. Nikam, C. D. Ho, *J. Org. Chem.* **1983**, 48, 5376; b) S. S. Nikam, K. K. Wang, *J. Org. Chem.* **1985**, 50, 2193.

[4] M. Shimizu, T. Kurahashi, H. Kitagawa, T. Hiyama, *Org. Lett.* **2003**, 5, 225.

[5] For selected examples of gem-silylboration of alkynes, see: a) R. Fritzemeier, W. L. Santos, *Chem. Eur. J.* **2017**, 23, 15524; b) Y. Gu, Y. Duan, Y. Shen, R. Martin, *Angew. Chem. Int. Ed.* **2020**, 59, 2061.

[6] For selected examples of gem-silylboration of vinyllithium and allyllithium carbенoids, see: a) M. Shimizu, H. Kitagawa, T. Kurahashi, *Angew. Chem. Int. Ed.* **2001**, 40, 4283; b) M. Shimizu, T. Kurahashi, H. Kitagawa, K. Shimono, T. Hiyama, *J. Organomet. Chem.* **2003**, 686, 286; c) M. Shimizu, T. Hiyama, *Polyfunctional Silicon Organometallics for Organic Synthesis in Handbook of Functionalized Organometallics* (Ed: P. Knochel), Wiley-VCH, Weinheim, 2005, pp. 1731–202; d) E. La Cascia, A. B. Cuenca, E. Fernández, *Chem. Eur. J.* **2016**, 22, 18737.

[7] For a review on silylboration of unsaturated organic compounds, see: M. Suginome, T. Ohmura, *Transition Metal-Catalyzed Element-Boryl Additions to Unsaturated Organic Compounds in Boronic Acids: Preparation and Applications in Organic Synthesis*, Medicine and Material, Vol. 33 (Ed: D. G. Hall), Wiley-VCH, Weinheim, 2011, pp. 171–212.

[8] For recent reviews on the synthesis and use of silylborane reagents, see: a) M. Oestreich, E. Hartmann, M. Mewald, *Chem. Rev.* **2013**, 113, 402; b) J.-J. Feng, W. Mao, L. Zhang, M. Oestreich, *Chem. Soc. Rev.* **2021**, 50, 2010.

[9] H. Miura, Y. Hachiya, H. Nishio, Y. Fukuta, T. Toyomasu, K. Kobayashi, Y. Masaki, T. Shishido, *ACS Catal.* **2021**, 11, 758.

[10] Related 1-substituted 4-hydroxy-1-allenylsilanes have been prepared through the copper-catalyzed S<sub>N</sub>2'-type silylation of 1-substituted 1-alkynylepoxides and through the copper- and nickel-catalyzed decarboxylative silylation of alkyne-substituted cyclic carbonates with Et<sub>3</sub>SiBpin: a) X.-H. Chang, Z.-L. Liu, Y.-C. Luo, C. Yang, X.-W. Liu, B.-C. Da, J.-J. Li, T. Ahmad, T.-P. Loh, Y.-H. Xu, *Chem. Commun.* **2017**, 53, 9344; b) K. Guo, W. Kleij, *Org. Lett.* **2020**, 22, 3942; c) K. Guo, Q. Zeng, A. Villar-Yanez, C. Bo, A. W. Kleij, *Org. Lett.* **2022**, 24, 637.
[11] Allenylmetals, including allenylborons, are known to react with carbonyl derivatives mainly through an $S_2C^2-$pathway. For an excellent review, see: J. A. Marshall, B. W. Gung, M. L. Grachan, *Modern Allene Chemistry*, Vol. 2 (Eds: N. Krause, A. S. K. Hashmi), Wiley-VCH, Weinheim, **2004**, pp. 493–592.

[12] For a discussion on hydrolysis of allenylboranes, see: M. M. Midland, *J. Org. Chem.* **1977**, *42*, 2650.

[13] $Et_3Bpin$ was readily prepared in 67% isolated yield by iridium-catalyzed borylation of $Et_3SiH$ with $B_2pin_2$ following the procedure reported: T. A. Boebel, J. F. Hartwig, *Organometallics* **2008**, *27*, 6013.

[14] In both cases, the conversion was almost complete (> 95%) yielding the corresponding enyne (~15%) and an unidentified product (~80%) which led to the starting epoxide by silica gel filtration. We therefore assumed that the $S_{N1}$ with epoxides *trans*-13e and 13f proceeds very slowly, affording epoxides borylated in the acetylenic position, which then undergo proto-deborylation on silica gel.

[15] The related *anti*-S$_{N1}$ from dialkylzincate complexes derived from ethynylepoxides has been reported: a) A. Denichoux, L. Debien, M. Cyclinsky, M. Kaci, F. Chemla, F. Ferreira, A. Pérez-Luna, *J. Org. Chem.* **2013**, *78*, 134; b) V. N. Bochatay, L. Debien, F. Chemla, F. Ferreira, O. Jackowski, A. Pérez-Luna, *C. R. Chim.* **2017**, *20*, 609.

[16] All attempts to purify products (aS*,S*)-14 by flash chromatography over silica gel (SiO$_2$) or neutral alumina (Al$_2$O$_3$) were unsuccessful, leading to enynes 17 as the main products.

[17] Product (*anti*,*syn*)-18ac was recrystallized from pentane/CH$_3$Cl$_2$ at room temperature. Deposition number 2310124 for (*anti*,*syn*)-18ac contains the supplementary crystallographic data for this paper. These data can be obtained free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

[18] The nucleophilic addition of allenylborane species to aldehydes is assumed to proceed through a chair-like transition state mode, see: a) R. Haruta, M. Ishiguro, N. Ikeda, H. Yamamoto, *J. Am. Chem. Soc.* **1982**, *104*, 7667; b) H. Yamamoto in *Comprehensive Organic Synthesis*, Vol. 2 (Ed: C. H. Heathcock), Pergamon, Oxford, U. K., **1991**, pp. 81–98; c) C.-H. Ding, X.-L. Hou, *Chem. Rev.* **2011**, *111*, 1914.

[19] For a discussion on Houk model, see: K. N. Houk, N. G. Rondan, Y.-D. Wu, J. T. Metz, M. N. Paddon-Raw, *Tetrahedron* **1984**, *40*, 2257. [Correction added on 30 April 2024, after first online publication: Reference 19 was updated in this version.]

Manuscript received: January 25, 2024

Revised manuscript received: February 7, 2024

Version of record online: February 29, 2024