Silanes reaction with carbonyl compounds

Analogous to its reaction with carbonyl compounds (see 6.3.4), benzyltrimethyl-silane undergoes a fluoride-induced nucleophilic substitution reaction on pyridine-1-oxides and quinoline-l-oxide to form 2-benzylpyridines (>70%) and 2-benzyl-quinoline (65%), respectively [57], Allyltrimethylsilane reacts with pyridine-l-oxide to produce 2-propenylpyridine (56%). 

Lithiated allyl silanes LiCH2CH =CHSiMe3 show a useful y-regio-preference in reactions with carbonyl compounds [13]. With the lithiated N,0 and S-analogues this regiochemistry is very variable (2,6,8], sometimes totally unexpected and not very useful from a preparative point of view. 

An interesting 3-carbon ring-expansion reaction utilizing the allyl silane (87) has been used to convert the cyclic ketone (88) into muscone (89) in high yield. In its reaction with carbonyl compounds, the trimethylsilylallyl anion acts as a j8-acyl anion equivalent, enabling the preparation of y-lactones (90). ... 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

In 1968 Peterson published his results about the reaction of a-silylated carbanions with carbonyl compounds to achieve /3-hydroxyalkyl-silanes. He found that instantaneous elimination affords unsaturated hydrocarbons ( Peterson olefmation )180 Thus, the lithiated trimethyl-benzyl-silane (274) plus benzophenone (135) give via lithium l,l,2,-triphenyl-2-TMS-ethanolate (275) 1,1,2-triphenyl-ethene (276)180 ... 

Peterson reaction (olefination). Reaction of a-silylated carbanions with carbonyl compounds, yielding (3-hydroxyl-alkyl silanes, which undergo instantaneous elimination to afford olefins. 

The reactions of ailylboranes, -silanes and -stannanes with carbonyl compounds and imines always take place with double-bond migration, and the structural stability of the reagent is a common problem that is encountered ( 2.7). The reactions of ailylboranes and -boronates occur without catalysis, while those of allylsilanes and -stannanes usually require the presence of a Lewis add [253], The mechanism of the reactions of allylboron derivatives is concerted, and the addition occurs via a six-membered cyclic transition state. A slightly distorted chair transition state model in which the oxygen of the carbonyl group is coordinated to the boron atom is usually invoked (Figure 6.42). Various steric and polar interactions dictate whether the Si or Re face of the prochiral aldehyde is attacked (models C j... 

Reagents of the type (1) can be prepared by the reaction of triphenylvinyl-silane with organolithium compounds. They react with carbonyl compounds to... 

Homopropargylic alcohols are available by reaction of carbonyl compounds with allenyl silanes.105 Full details of this chemistry include preparations of variously substituted allenysilanes together with methods for their reaction using TiCl4. The same compounds are available in optically active form by use of chiral allenylboronic esters.106 This method is rather similar to the chemistry outlined in Scheme 27 for the preparation of homoallylic alcohols. A third method for the preparation of homoallylic... 

Trimethylsilylstyrenes are formed by reaction of alka-nesulfonyl chlorides with TMSCHN2 in the presence of triethyl-amine (eq 13). TMSC(Li)N2 reacts with carbonyl compounds to give a-diazo- -hydroxy silanes which readily decompose to give O, /3-epoxy silanes (eq 14). However, benzophenone gives diphenylacetylene under similar reaction conditions (eq 15). ... 

Reactions of similar organoalkynyl/allenyl silanes containing various metals with carbonyl compounds to give propargylic products have been reported. " Conjugate addition of allenyl stan-nanes to Q , -unsaturated carbonyl compounds to give propargylic products can be effected in the presence of titanium(IV) chloride. ... 

The reactions of allylic silanes with carbonyl compounds also require the use of Lewis acid catalysts such as TiC and BF3... 

Peterson went on to describe reactions of several lithiated silanes with carbonyls compounds, all giving the desired alkenes in good yields, albeit with very little stereoselectivity. In 1975 however, Peterson and Hudrlik published their studies on the stereoselective elimination of the hydroxyalkylsilyls. The reduction of 5-trimethylsilyl-4-octanone 10 was carried out with DIBAL-H to give one diastereoisomer, 11. The authors found that elimination with sodium or potassium hydride gave /ra 5-4-octene as the major isomer 12, while elimination under acidic conditions resulted in predominantly c/5-4-octene 13. Mild conditions were employed, affording stereochemical purity of up to 95% with excellent yields. 

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