The extraordinary reactions of phenyldimethylsilyllithium with N,N-disubstituted amides

Abstract

Phenyldimethylsilyllithium reacts with N,N-dimethylamides in a variety of ways, depending upon the stoichiometry, the temperature and, most subtly, on the structure of the amide, with quite small-seeming changes in structure leading to profound changes in the nature of the products. When equimolar amounts of the silyllithium reagent and N,N-dimethylamides 6 are combined in THF at −78 °C, and the mixture quenched at −78 °C, the product is the corresponding acylsilane 8. If the same mixture is warmed to −20 °C before quenching, the product is a cis enediamine 11. The enediamines are easily isomerised from cis to trans, easily oxidised to dienediamines 15, and, with more difficulty, hydrolysed to α-aminoketones 13. If two equivalents of the silyllithium reagent are used, the product is an α-silylamine 20. The mechanism of formation of the enediamines appears to be by way of a Brook rearrangement of the tetrahedral intermediate 17 followed by loss of a silanoxide ion to give a carbene or carbene-like species. The ‘carbene’ combines with the Brook-rearranging nucleophile to give an intermediate 28, which loses another silanoxide ion to give the enediamine. The same carbene can be attacked by a second equivalent of the silyllithium reagent to give the α-silylamine 20. Other nucleophiles, like alkyllithiums, phenyllithium, and tributylstannyllithium also trap the carbene to give products 48–52. The intermediate anions in these reactions, when benzylic, can be further trapped with alkylating agents to give the products 33, 34 and 53–55. In special cases, the anion formed by attack on the carbene can be trapped by intramolecular reactions displacing internal leaving groups, as in the formation of the enamine 37 and the cyclopentane 41, or attacking a carbonyl group, as in the formation of the indanone 61, or attacking a double or triple bond, as in the formation of the cyclopentanes 71 and 75. In another special case, the carbene reacts with vinyllithium to give an allyllithium intermediate 56, which selectively attacks another molecule of carbene to give eventually the γ-aminoketone 58. Small changes in the structure of the amide lead to a variety of other pathways each of which is discussed in the text. Notably, each member of the homologous series of amides Ph(CH₂)_nCONMe₂ gives rise to a substantially different product: when n = 0, the reaction is normal, and the yield of the α-silylamine 20e is high; when n = 1, proton transfer in the intermediate anion 64 and displacement of the phenyl group leads to the silaindane 66; when n = 2, fragmentation of the intermediate anion 80, and capture of the carbene by benzyllithium leads to the 1,4-diphenylbut-2-ylamine 83; and when n = 3, proton transfer in the intermediate anion 67 and displacement of the phenyl group leads to the silacyclopentane 69.