Silanes with acyl halides

Allylic silanes react with aldehydes, in the presence of Lewis acids, to give an allyl-substituted alcohol. In the case of benzylic silanes, this addition reaction has been induced with Mg(C104)2 under photochemical conditions. The addition of chiral additives leads to the alcohol with good asymmetric induction. In a related reaction, allylic silanes react with acyl halides to produce the corresponding carbonyl derivative. The reaction of phenyl chloroformate, trimethylallylsilane, and AICI3, for example, gave phenyl but-3-enoate. ... 

Palladium-mediated addition of silyl stannane reagents to alkynyl ethers has been employed for the synthesis of aliphatic acyl silanes in very good yields via the intermediate a-alkoxy-/J-stannyl vinyl silanes (enol ethers of acyl silanes)82. In a second palladium-catalysed step, the vinyl stannane moiety could be coupled to suitable halides before hydrolysis to the acyl silanes with trifluoroacetic acid (Scheme 11). 

A much more general method for acyl silane synthesis involving silyl diazo intermediates is illustrated in Scheme 1688. The lithiated derivative of trimethylsilyl diazomethane reacts smoothly with alkyl halides in THF solution to give a-trimethylsilyl diazoalkanes in good yields. Oxidative cleavage of the diazo moiety is effected using 3-chloroperbenzoic acid in benzene solution, to give access to a wide variety of acyl silanes in yields of up to 71%. A phosphate buffer (pH 7.6) is used to prevent side reactions. Aromatic acyl silanes clearly cannot be prepared by this chemistry since an aromatic nucleophilic substitution reaction would be required. 

There is a great deal of variety possible in the nature of the group Z. The pioneering, systematic mechanistic studies on this type of reaction utilized trialkyltin groups [3]. The earliest, and still most widely used, synthetic applications also involve the use of allyltins. Other Z groups have included sulfides, sulfoxides, sul-fones, silanes, cobaloximes and halides. The reactions also occur with a wide variety of carbon-based radicals, from simple alkyl radicals to acyl radicals, and have been carried out in both inter- and intramolecular fashions [4]. 

The low ionic character of the aluminium-silicon bond has been cleverly utilized to develop a very mild, general and effective synthesis of acyl silanes, successful for aliphatic, aromatic, heteroaromatic, a-aUcoxy, a-amino and even a-chiral and a-cyclopropyl acyl sUanes. Acyl chlorides are treated with lithium tetrakis(trimethylsilyl)aluminium or lithium methyl tris(trimethylsilyl) aluminium in the presence of copper(I) cyanide as catalyst to give the acyl silanes in excellent yields after work-up. Later improvements include the use of 2-pyridinethiolesters in place of acyl halides, allowing preparation of acyl silanes in just a few minutes in very high yields indeed (Scheme 9) °, and the use of bis(dimethylphenylsilyl) copper lithium and a dimethylphenylsilyl zinc cuprate species as nucleophiles. 

Use of the silylated ether (39), which is easily prepared from methoxyallene, allows alkyl halides to be converted into E-a.jS-unsaturated aldehydes or acyl-silanes with a three-carbon chain extension (e.g. Scheme 49)/ Alternatively a,jS-unsaturated aldehydes can be prepared by a prototropic rearrangement of... 

So far, no crystallographic evidence for adducts of silanes with aldehydes, ketones, esters, or acyl halides has been reported [81]. Dimethylformamide [86-88] and tetramethylurea [89], however, are known to enter the silicon coordination sphere (e.g., in 13 and 14, respectively). In a similar manner amine-M-oxides (e.g., in 15) [90], phosphine oxides (e.g., in 16 and 17) [90,91], and phosphoric amides [92-94] form hypercoordinated Si complexes. Although dimethyl sulfoxide (DMSO) increases the silicon coordination number (as shown Si NMR spectroscopy) [49], crystallographic evidence for a siUcmi complex with DMSO Ugand(s) is still lacking [81]. 

Alkylations. Treatment of 1 with various primary alkyl halides provides the corresponding substituted dithianes (eqs 3-5 ). Removal of the dithiane under the Lewis acid conditions, illustrated in eqs 3-5, unmasks the acyl silane for subsequent transformations such as photolysis, radical reactions, and heterocyclic synthesis. Other conditions for removing the dithiane moiety of 2-substituted-2-f-butyldimethylsilyl-l,3-dithianes include anodic oxidation, ceric ammonium nitrate (CAN)/NaHC03 in CH3CN/H2O, iodomethane/CaC03 in THF/H20, 8 and l2/CaC03 in THF/H2O. The formyl sUane of 2-t-butyl-dimethylsilyl-l,3-dithiane has also been reported." ... 

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