Vinylsilanes Friedel-Crafts reactions

Classically, vinylsilanes have served as nucleophiles in Friedel-Crafts acylation and related reactions with strong electrophiles.4,4a 89 These strategies have been employed in the synthesis of natural products.90,9011... 

Vinylsilanes are proper compounds which have won great appreciation as facile intermediates for many sorts of application. They are prepared either by addition reactions of acetylenes (vide supra) or by elimination reactions of saturated organosilanes. The main reactions, carried out with vinylsilanes, are addition reactions to obtain saturated organosilanes or electrophilic substitutions of the silyl group under Friedel-Crafts conditions where a rapid cleavage of the silyl moiety occurs (Scheme 8). 

Acyclic a,/3-unsaturated acid chlorides do not undergo this acylation. However, cyclic vinylsilanes also undergo Friedel-Crafts acylation with a,/S-unsaturated acid chlorides to give dienones of type a, which are cyciized by SnCl to cyclopentenones. A typical example is formulated for the reaction of 5-methyl-l-tri-methylsilylcyclopentene (3) with j3,iS-dimethylacryloyl chloride. The first step is conducted with AICI3 and gives 4, which after isolation is cyciized with SnCU to a mixture of 5 and 6. The cyclopentenone 6 is obtained in 55% overall yield after isomerization with RhCU. 

A significant advance in the use of Friedel-Crafts acylation of alkenes to prepare divinyl ketones was the employment of vinylsilanes to control the site of electrophilic substitution. Two groups have developed this approach to cyclopentenone annulation using slightly different strategies. In the method described by Magnus the reagent vinyltrimethylsilane (80) is used primarily as an ethylene equivalent (equation 44). The construction of bicyclic systems followed readily as Nazarov cyclization proceeded under the reaction conditions. Tin(lV) chloride was found to be the most effective promoter of the overall transformation. As expected the position of the double bond is thermodynamically controlled. 

Vinylsilanes (8, 491-492) aHylic alcohols (9, 340). Details are available for conversion of a ketone to a vinylsilane in which the C—Si bond has replaced the C==0 group (enesilylation). The reaction affords the less substituted vinylsilane in the case of unsymmetrical ketones. The paper includes details for use of vinylsilanes for cyclopentenone annelation by Friedel-Crafts acylation with acryloyl chlorides and subsequent cyclization of penladienyl cations (9,498-499).1... 

The GaCls-promoted reaction of alkynylsilanes with arenes, fhen treatment wifh MeLi followed by hydrolysis, yields -arylvinylsilanes (Scheme 10.206) [538]. When D2O is added in fhe hydrolysis step, the a-carbon of fhe vinylsilane product is deuterated. This indicates fhe presence of vinylgallium species in fhe reaction mixture. The arylation mechanism probably involves the Friedel-Crafts-type reaction of GaClj-coordinated alkynylsilanes at the -position. 

The stability of the -C -Si< bond has been known for a long time But on the other hand they are reactive compounds which undergo either - as precursors to vinylsilanes - various types of addition reactions or - as only silyl-protected acetylenes - an electrophilic substitution under Friedel-Crafts conditions in presence of catalytic amounts of Lewis acids The — SiR3 moiety has a highly useful protecting and/or activating function. 

Several examples of Friedel-Crafts acylation of vinylsilanes have appeared.High asymmetric induction is observed in the reaction of the vinylsilanes (93) with MeLi, yielding compounds (94) with silicon transposed from carbon to oxygen. 

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