Silylcupration of Allene with the Lower Order Cuprate (t-BuPh₂Si)CuCNLi and Reaction with α,β-Unsaturated Nitriles. An Unusual Diaddition Process Leading to Disilyl Functionalised Ketones

Abstract

The lower order cuprate (t-BuPh₂Si)CuCNLi, prepared by mixing one equivalent of t-butyldiphenylsilyllithium and one equivalent of copper(I) cyanide, reacts with allenes in the presence of α,β-unsaturated nitriles showing a regiochemistry that depends on the electronic nature of the nitrile. Electron-poor nitriles give monoadducts (allylsilane containing nitriles) whereas electron-rich nitriles lead to diadducts (allylsilane-vinylsilane containing ketones), the latter resulting from an unusual diaddition (1,2 and 1,4) process. An explanation is advanced and a reliable mechanism ­proposed.

allenes - silylcupration - allylsilanes - cuprates - regiochemistry

References

For selected examples of Si-Cu of allenes, see:

1a Fleming I. Pulido FJ. Chem. Commun.  1986,  1010 

1b Fleming I. Rowley M. Cuadrado P. González AM. Pulido FJ. Tetrahedron  1989,  45:  413 

1c Blanco FJ. Cuadrado P. González AM. Pulido FJ. Fleming I. Tetrahedron Lett.  1994,  35:  8881 

1d Barbero A. Cuadrado P. González AM. Pulido FJ. Fleming I. Sanchez A. J. Chem. Soc., Perkin Trans. 1  1995,  1525 

1e Barbero A. Castreño P. Pulido FJ. Org. Lett.  2003,  5:  4045 

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2a Barbero A. Cuadrado P. González AM. Pulido FJ. Fleming I. Sanchez A. J. Chem. Soc., Perkin Trans. 1  1995,  1525 

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For other authors references, see:

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9

General Procedure: Typically, 6 mmol of t-BuPh₂SiLi^3a,4 were added by syringe to a suspension of CuCN (6 mmol) in THF (12 mL) at 0 °C and the mixture was stirred for 30 min under argon. The resulting dark solution was cooled at -40 °C and a slight excess of allene (1,2-propadiene) was added from a balloon. The mixture was stirred for 45 min, then 6 mmol of BF₃·OEt₂ were added at once. After stirring 10 min, 3 mmol of the corresponding nitrile (Table [1] ) were added at -40 °C and the mixture stirred at this temperature for an additional hour. After gentle warming up to 0 °C, the mixture was hydrolysed with sat. NH₄Cl solution, extracted with Et₂O, dried with MgSO₄ and evaporated. The crude was chromatographed (EtOAc-hexanes) on silica gel (230-400 mesh), yielding 10-14 (Scheme [2] , Table [1] ). Compound 12: ¹H NMR (300 MHz, CDCl₃): δ = 7.69-7.31 (m, 20 H), 5.98 (s, 2 H), 4.57 (s, 1 H), 4.47 (s, 1 H), 3.03 (s, 2 H), 2.17 (d, J = 14.7 Hz, 1 H), 2.12 (d, J = 14.7 Hz, 1 H), 2.10 (m, 1 H), 1.97 (dd, J = 16.5, 4 Hz, 1 H), 1.80 (dd, J = 16.5, 9.5 Hz, 1 H), 1.22 (s, 9 H), 1.10 (s, 9 H), 0.71 (d, J = 6.8 Hz, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 207.5, 150.4, 139.2, 136.4, 134.5, 133.9, 133.7, 129.3, 129.1, 127.6, 127.4, 108.9, 50.3, 48.7, 36.4, 28.6, 27.9, 19.2, 18.7, 18.5. IR (film): 1695, 1625, 1100, 900 cm^-1. MS: m/z (%) = 628 (3), 239 (20), 199 (100), 135 (60). HR-FAB: 629.368.