Imines, reactions with silenes

In addition to undergoing cycloaddition reactions with alkenes and al-kynes, silenes readily undergo cycloaddition reactions with heteroatom multiple bonds such as C=0 and C=N, most commonly when the trapping reagent for the silene is either an aldehyde, ketone, or imine. In many... 

Some remarkable chemistry is observed when silenes react with heteroatom systems, in particular carbonyl compounds (]>C=0) and imines Q>C=N—R). The reaction with ketones was first described by Sommer (203), who postulated formation of an intermediate siloxetane which could not be observed and hence was considered to be unstable even at room temperature, decomposing spontaneously to a silanone (normally isolated as its trimer and other oligomers) and the observed alkene [Eq. (14)]. Many efforts have been made to demonstrate the existence of the siloxetane, but it is only very recently that claims have been advanced for the isolation of this species. In one case (86) an alternative formulation for the product obtained has been advanced (204). In a second case (121) involving reaction of a highly hindered silene with cyclopentadienones,... 

Thus, the alkyl-substituted silenes of the family (Me3Si)2Si=C(OSiMe3)R give with triphenylimine the unstable 514 which is converted completely, faster upon photolysis or more slowly in the dark, into the silaazetidine 515 (equation 174). For the adamantylsilene 150 the complete conversion from the acylpolysilane to 515 (R = 1-Ad) requires 5 days and proceeds in an overall yield of 94%248. The mesitylsilene 349 forms no [4 + 2] cycloadduct, and the only product of the reaction with triphenylimine detected after 24 h is the silaazetidine 515 (R = Mes)248. The imine component also influences the product distribution of the reaction. For example, no [4+2] cycloadducts are formed in the reaction of silenes (Me3Si)2Si=C(OSiMe3)R with A-fluorenylidineaniline and only silaazetidines have been detected248. 

Stable 1,2-siloxetanes can be obtained by reaction of transient silenes with acetone (Scheme 19) <19960M2554> or benzaldehyde (Scheme 20) <2001EJI481>. Other examples of the reaction of silenes with ketones or imines have also been reported <1995CB1241>. 

The reactions of 1,1-dimethyl-2,2-bis(trimethylsilyl)silene (54) with carbonyl compounds such as acetone and ethyl acetate, and particularly with benzophenone imine, are very much faster than its ene reactions with hydrocarbons such as isobutene244,246,266, pointing to the involvement of the nucleophilic lone pairs in the former case. 

Another class of compounds that can be included here are fluorene derivatives. For example, fluoreno[l,9- /e]-l,2-azasilines such as (63) are synthesized from the reaction of imines with silenes <930M3666> whilst oxasilin analogues such as (64) are formed from the reaction of silenes with fluorenone <870M1246>. 

The temperature at which a cycloaddition reaction of a neopentylsilene takes place (detected by the elimination of LiCl) has turned out to be dependent on the reaction partners added as substrate. This implies that an interaction between the substrate and A or B or the substrate and C occurs somewhere along the reaction pathway depicted above. For the system Cl3SiCH=CH2/LiBut/R2C=NR it was observed that the imine initiates and supports the salt elimination from the species A/B. Based on the knowledge that silenes are stabilized by external donors [1] we conclude that with carbon unsaturated compounds x-donor interactions instead of cr-donor complexes may be possible as well for the lithiated species (D) as for the silene itself (E). 

Dichloroneopentylsilene is formed in situ by reaction of trichlorovinylsilane with LirBu [1], The [2+2] cycloaddition to imines yields Si-dichloro functionalized 2-silaazetidines in a preparative scale [2], When aldimines are used as trapping agents for the silene, the resulting SiN-four membered ring compounds are isolated as syn/anti-isomers (syn/anti 2/1). 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

The addition of N-(trimethylsilyl)benzophenone imine to 1,1-dimethyl-2,2-bis(trimethylsilyl)silene (54)243 proceeds in a fashion quite analogous to the reaction of benzophenone, but is much slower. Both the 2 + 2 (56) and the 4 + 2 (55) cycloadducts have been isolated (equation 74). The former is formed at only about half of the rate of the latter but dominates in the equilibrium. As described in Section III.A.2.C, the small amount of 54 present in the equilibrium can be trapped by other added reagents so that 56 represents a convenient means of storing the silene 54, which cannot be stored neat since it rapidly dimerizes244,246. Another such store is provided by the silenate adduct of 54 with NMe3, which reacts with N-(trimethylsilyl)benzophenone imine to yield 55 and 56266. 

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