Ynamines silylated

Sihcon is most important in the cycloaddition of alkynyl silyl sulphides to ynamines, the silyl shift giving the 4-silylcyclobut-2-enethione which, on S-methylation and desilylation, yields the cyclobutadiene (equation 10)18. 

On the other hand, this method is particulary suitable for ynamines where R = Et and higher alkyl groups, where the haloacetylene method is unpractical. This method could be extended even to the preparation of silylated ynamines (27)41 58). 

Silylated amines are formed as side-products. Silylated ynamines are thermally very stable and can be distilled at normal pressure without decomposition 41 As a corollary, their reactivity is reduced as compared to 2-alkyl ynamines 59). The same procedure was applied to the synthesis of a few triorganogermyl ynamines (28) 58). 

Metal exchange. Lithiated ynamines 44 are. smoothly silylated to the corresponding silyl ynamines (84) 42>59 136). Such ynamines formally replace the very sensitive non-substituted ynamines but the increase in stability entails a loss of reactivity toward electrophilic reagents because silicon acts as a weak electron acceptor. 

Silylated ynamines react only with very reactive acid chlorides, but the analogous stannyl ynamines 46 are much more versatile and have recently become available in fifty gram lots (85) 95 ... 

The addition takes place exclusively in a cis fashion when silylated ynamines are used 42, l37) to give 58a having maleic acid configuration. The isomers with fumaric acid configurations are formed in 40-50% conversions upon sensitized photoisomerization in benzene solutions, 37). Methyl propiolate behaves in a like manner 42). 

Diphenylketene . s0-151.155-156) 9-fluorenylidene ketene156) and cyanoketenes (112)l53) have been thoroughly studied. In all cases, the silylated, germylated and stannylated ynamines add across the carbonyl group of ketenes to furnish a whole series of new vinyl ynamines 67 and of ynamine ketones 68 (110) upon mild hydrolysis (110-113). 

Silylated amines are formed as side-products. Silylated ynamines are thermally very stable and can be distilled at normal pressure without decomposition As a corollary,... 

The yields are lower because of competing hydrolysis during the aqueous work-up. Lithioynamines 44 must be used with unreactive halides such as carbamoyl chlorides and imidoyl halides (93) The scope of this reaction seems to be limited only by the availability of the requisite acyl chlorides. It is less surprising, therefore, that a single paper reports the preparation of about sixty push-pull acetylenes In contrast, silylated ynamines, react only with very strong acylation agents, e.g. trichloroacetyl and nitrobenzoyl chloride as well as with phosgeniminium chloride. 

The silylated lithium ynamine 192 is generated by metaUation with BuLi of 5 -methyl Af-phenyl-trimethylsilylethanimidothioate (190), via a ketenimine intermediate (191), as shown in equation 75. The interconversion between 191 and 192 can be discerned by silylation taking place at both the N- and -positions to afford the Af-silylynamide 193 and the bis(/ -silyl)ketenimine 194. In equation 76 are shown three possible synthetic... 

Carbodiitnides (332 Scheme 55) form amidines on treatment with NaBH4 in alcohols, Grignard reagents, sodium phenylacetylide, carbon monoxide in the presence of s-butyl- or t-butyl-lithium and nitrones catalyzed by HBF4. In the reaction of stannylated or silylated ynamines with carbodiimides... 

Ynamines and silylketenes couple to give allenecarboxamides, the silyl substituent inducing H migration to give silyl cyclobutenones... 

In the reaction of alkenyl silyl sulfides with ynamines, the thioketenes 90 are generated as intermediates which undergo electrocyclization to give the four-membered ring compounds 91... 

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