Silyl enamines

It is interesting to note that condensation of the N,N-bis(silylated) enamine 538 with a variety of chalcones such as benzalacetophenone 735 proceeds, via 539 and subsequent cyclization and oxidation, to pyridines such as 540 [106, 108] whereas persilylated co-amino ketones such as the 2-substituted pyridine 541 cyclize, via 542, in 29% yield, to the pyrrole 543 [109] (Scheme 5.36). 

The thermal [1] or photochemical [5] isomerization of N-silylated allylamine in the presence of Fe(CO)5 provides the corresponding N-silylated enamines 7a and 7b. Z-enamine 7b does not react in any of the examined cycloadditions. The cyclopropanation of E-enamine 7a with methyl diazoacetate under copper(I) catalysis provides the donor-acceptor-substituted cyclopropane 9 [1], which can be converted in good yield into the interesting dipeptide 10 [6]. 

Synthetic building blocks with latent 1,4-functionality like the 5,6-dihydro-4N-1,2-oxazine 8 are accessible in a [4+2]-cycloadditon of N-silylated enamine 7a and in situ generated nitrosoalkenes [6],... 

A wide range of olefins can be cyclopropanated with acceptor-substituted carbene complexes. These include acyclic or cyclic alkenes, styrenes [1015], 1,3-dienes [1002], vinyl iodides [1347,1348], arenes [1349], fullerenes [1350], heteroare-nes, enol ethers or esters [1351-1354], ketene acetals, and A-alkoxycarbonyl-[1355,1356] or A-silyl enamines [1357], Electron-rich alkenes are usually cyclopropanated faster than electron-poor alkenes [626,1015],... 

Reaction of allylpyrrolidine with n-butyllithium in TMEDA leads, after silylation, to a silylated enamine further treatment with n-butyllithium results in the 1,3-migration of the trimethylsilyl group to the corresponding SMA derivative in poor yield after silylation. In the presence of magnesium bromide, 1-trimethylsilylallylpyrrolidine is formed selectively in moderate yield.125... 

Substituted bicyclo[ . 1.0]alkanes may also be obtained by condensation of secondary amines with 2-haloketones. A variety of nucleophilic reactions can be carried out on the intermediate cyclopropaniminium salt 116251 (Scheme 108). Competing alkene scission and cyclopropanation occurs on reaction of enamines with pentacarbonyl-chromium carbene complexes252 (Scheme 109). N-Silylated allylamines and their derived N-silylated enamines undergo rhodium or copper catalysed cyclopropanation by methyl diazoacetate253 (Scheme 110). 

Photochemical Fe(CO)5-induced rearrangement of silylated allyl amine 9 gave N-silylated enamine 1015, which on subsequent Cu-catalyzed cyclopropanation by methyl diazoacetate afforded cyclopropane derivative 11. The use of an optically active catalyst gave an asymmetric induction of 56% ee for the cis isomer and 20% ee for the trans isomer. Further acid-induced ring cleavage afforded the -formyl ester 12, whereas reduction and desilylation produced aminocyclopropane carboxylic acid 13 (equation 2). 

Treatment of the silylated enamine 24 with phenacyl bromide affords the salt 81, which is hydrolysed by water to the keto aldehyde 82. Condensation with aniline then yields 3-(trimethylsilylmethyl)-l,5-diphenylpyrrole 83 (equation 36)36. 

The hydrosilation of aromatic nitriles is promoted by Co2(CO)g to give )V,Ai-bis(silyl)benzylamines (equation 12). Aliphatic nitriles require higher temperatures. a, -Unsaturated nitriles with substituents in a-and/or /3-positions give iV,Ai-bis(silyl)enamines as major products. ... 

Silyl enamines have been prepared from cyanohydrins RR QOSiMcaiCN by reductive silylation with McaSiCl/Li. Photodesilylation by HCl gas in dry diethyl ether or by trimethylchlorosilane in methanol gave, after neutralization, the enamines, RR C=C(NH2)SiMc3 which were surprisingly stable. 

The catalytic hydrosilylation of 1-aza- and 1,4-diaza-1,3-dienes R N=CH-CH=CHR [R = Me,Ph R = McjCH, McjC, (Me2CH)2CH] and R N=CH-CH=NR were performed with rhodium(I) complexes. Under mild conditions N-silylated enamines, such as R N(SiR )CH=CHCH2R or R NCH=CHN(SiR )RS were formed as the main products. ... 

Allyl-SMP 1 is easily to deprotonate with the Lochmann-Schlosser base at -78 °C in TBME. Subsequent silylation with DMPSCl provides quantitatively the -silylated enamine 2, which can be metallated directly again in the same pot... 

Trimethylsilylazide attacks citraconic anhydride with regiospecificity and catalytically adds to aldehydes in the presence of ZnCl2-18-crown-6. Silylazides react with vinyl and other alkenylsilanes to give bis(silyl)enamines through the thermal rearrangement of intermediate triazolines. Other azides give silyl-aziridines, which also result when silyl bromoazides, obtained from alkenylsilanes, are reduced with LiAlHi. ... 

Further, enamines (38) or allylamines (39) are deprotonated to give ambident 1-aminoallyl anions which react with chlorotrimethylsilane with high regio-selectivity to yield the versatile silylated enamines (40) only, and which may be hydrolysed, or further deprotonated and subsequently alkylated to give enamines (41), which themselves may be isolated or hydrolysed without further purification (Scheme 54). Similarly, the silylated enamines (43) are derived from (42). Surprisingly, compounds (43) react with a variety of carbon electrophiles in the presence of TiCU to give products derived from substitution at the y-position (Scheme 55). A(,iV-Bis(silyl)enamines react with carbonyl compounds in the presence of fluoride ion to provide an interesting route to 2-aza-l,3-dienes, Enamine products also result from the rearran ment of thioimidates derived... 

Taking advantage of the usually high enantioselectivity of the addition of diphe-nylprolinol silyl enamines to nitroolefins the Enders group developed a large palette of domino sequences [7], including Michael/5 2 tandems in the presence of aliphatic aldehydes and the (o-iodonitroalkenes [5f]. Cyclopentanes were obtained, albeit in a low chemical yield and diastereoselectivity (20%, 70 30 dr) but with excellent enantioselectivity [irans 94% ee, ds 95% ee). The presence of benzoic acid co-catalyst increased both chemical yield and enantioselectivity... 

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