Stannanes acyl halides

In the presence of Pd° catalysts, hexaalkyldistannanes will react with aryl, heteroaryl, vinyl, or acyl halides to give the corresponding aryl-, heteroaryl-, vinyl-, or acyl-stannanes,53 for example 54... 

Despite the lack of examples of Friedel-Crafts acylations catalyzed by Lewis acids, - reactions of stannanes with acyl halides catalyzed by palladium species have found considerable use for the preparation of ketones. Since alkyl groups are only transferred slowly from tin, more rapid transfer to the acyl chloride is observed for alkynyl, alkenyl and allyl, as well as aryl and benzyl, groups. This leads to a versatile synthesis of ketones.Acylations of alkenylstannanes are both regio- and stereo-specific. 

Use of di-M-butylstannyl dichloride along with an acyl or silyl halide leads to addition of allylstannanes to the aldehydes.106 Reaction is also promoted by butylstannyl trichloride.107 Both SnCl4 and SnCl2 also catalyze this kind of addition. Reactions of tetraallyl-stannane with aldehydes catalyzed by SnCl4 also appear to involve a halostannane intermediate. It can be demonstrated by NMR that there is a rapid redistribution of the allyl group.108... 

The added advantage of the C (1 )-stannylated glycals is their abUity to participate in palladium-catalyzed coupling reactions with organic halides, a process independently reported by Beau [75] and Friesen [81]. Vinyl stannane 237 can be benzylated, allylated or acylated provided that appropriate catalysts are used [75,77] and representative examples are given in Scheme 59. The C-arylation of... 

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). 

All types of electrophiles have been used with 2-lithio-l,3-dithiane derivatives, including alkyl halides, sulfonates, sulfates, allylic alcohols, arene-metal complexes, epoxides, aziridines, carbonyl compounds, imines, Michael-acceptors, carbon dioxide, acyl chlorides, esters and lactones, amides, nitriles, isocyanates, disulfides and chlorotrialkylsilanes or stannanes. The final deprotection of the dithioacetal moiety can be carried out by means of different types of reagents in order to regenerate the carbonyl group by heavy metal coordination, alkylation and oxidation184 or it can be reduced to a methylene group with Raney-nickel, sodium or LiAIII4. 

This alternative to the Friedel-Crafts reaction, extensively developed by Stille and coworkers, is particularly important, since the reaction conditions are essentially neutral, and so provides a method for acylation of compounds containing an acid-sensitive functionality which would preclude the use of the Friedel-Crafts reaction. Reaction temperatures are often below 100 C, and high (1000-fold) turnovers of the catalyst have been achieved. Solvents employed include chloroform, toluene, and, on occasions, HMPA. Some reactions have been carried out under an atmosphere of carbon monoxide to prevent excessive decarbonylation of the acyl palladium intermediate. Indeed, carbonylative coupling of alkenylstannanes with allyl halides in the presence of carbon monoxide ca. 3 atm or greater 1 atm =101 kPa) offers an alternative to the Friedel-Crafts acylation, ketones being formed by the reaction of the stannane with the acyl species formed by carbon monoxide insertion into the allyl palladium intermediate. ... 

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