Imines silylimines

Silylimines fi-lactams.1 This lithium amide converts aldehydes, even enol-izable ones, into silylimines. Thus acetaldehyde can be converted into the imine (2), which cannot be isolated but which reacts with lithium ester enolates to form... 

A catalytic asymmetric in situ reduction of N-H imines has been achieved in a sequence in which trifluoroacetophenones, ArCOCF3, are first converted to silylimines [using LiN(SiMe3)2], and then on to give trifluoromethylated amine salts, Ar-C(CF3)-NH2.HC1, in good to excellent yield and ee.5s The intermediate N-H imines can be isolated via methanolysis of the N-Si bond, while the enantioselective reduction can be carried out using a chiral borane auxiliary. 

Optically active TV-sulfonylamino alcohols derived from D-camphor or norephedrine were found to be efficient chiral ligands for the enantioselective allylboration of iV-silylimines (Equation (170)) 646-648 B-Allyl(diisopinocampheyl)borane allylated iV-diisobutylaluminum imines with 87% ee (Equation (171)).649,650... 

Silylketene acetals react with imines under acidic conditions (Scheme 22). With N-alkylimines in the presence of a stoichiometric amount of TiCU, -lactams are formed in good yields. N-Aryl- " and N-trimethyl-silylimines also react under acidic conditions but yield only open-chain products. On the other hand, bis(trimethylsilyl)ketene acetals yield 3-lactams with both N-alkyl- and N-aryl-imines (Scheme 22). °... 

The same group has recently reported new findings on the reaction of -BuLi with W-(metallo)imines [23b]. They examined the addition of -BuLi to an AT-alu-minoimine, W-borylimine, and AT-silylimine in the presence of chiral nitrogen ligands including (-)-sparteine and proline-derived amino alcohols (Scheme 5). 

Zinc enolates of (V -bissilylglycine esters, according to van Koten and coworkers, condense with N-silylimines with high anti selectivity (Table 20 entries 18-20). °° These results parallel those previously described in Section 4.1.3.2.2.i using N-ary - and A/-alkyl-imines and are consistent with a C(Z,E)t transition state. Reactions of N-silylimines are slightly less anti selective, however, than those of N-aryl- and A/-alkyl-imines. Retroaldolization prior to cyclization has been cited as a possible cause for the erosion in stereoselectivity based on the slow rates of cyclization found in the reactions of entries 19 and 20. 

Cainelli, Martelli and coworkers have reported an interesting case of combined syn-anti and dia-stereofacial selectivity using chiral A/-silylimine (199), prepared in situ from (S)-O-TBDMS-lactic aldehyde (198). 2 As shown in Scheme 41, condensation of the lithium enolate of r-butyl butyrate with A -silylimine (199) affords essentially a single p-lactam (2(M)), contaminated with only 4% of the corresponding other trans diastereomeric 3-lactam. The authors propose that the high level of diastereofacial selectivity (14 1) is due to the formation of lithium chelate (201), which undergoes attack by the enolate from the least hindered ir-face of the imine. The authors do not discuss the unusual anti selectivity of this reaction. 

Addition to the C=N of an imine forms the magnesium amide, which is hydrolyzed to an amine. Isolated C=N double bonds of simple imines are relatively unreactive toward organomagnesium compounds, and may instead be depro-tonated at the a-position. Oximes and other N-substituted imines may react better, although an alternative sequence can produce aziridines. Imminium ions, because of their positive charge, are more reactive, and are useful in making tertiary amines. A-Silylimines yield primary amines after hydrolysis. Grignard reagents also add to one C=N of carbodiimides to produce amidines, and to nitrones to produce hydroxylamines. Some varied examples are shown in Scheme 6. 

Turning now to heteroatomic cases, the N—O, O—C, S—C, and N—C 1,2-anionic rearrangements have been described earlier in this chapter. Anionic rearrangements from doubly bonded nitrogen to carbon in imines, however, do not seem to have been studied. If carbanions can be generated from Al-silylimines, N=C rearrangement seems likely ... 

The yv-silylimines are monomeric compounds that are stable under anhydrous conditions. Since the metal-nitrogen bond is easily hydrolyzed, the silylimines can be considered a protected, stabilised form of the corresponding elusive imines of ammonia which are known to be very unstable, readily polymerising to triazines and other products. 

Sulfur ylides represent another family of nucleophiles structurally coupled with a leaving group. It is therefore to be expected that they will react with imines to give aziridines via an addition-substitution sequence. In fact, the sulfur ylid obtained by metallation c trimethylsulfonium iodide with n-butyl lithium reacts at -30 C in THF with both silylimines and p-methoxyphenyl imines to give the corresponding aziridines. Again a single isomer is obtained in the case of (S)-lactaldehyde imines (Scheme 22). 

Recently, we have studied the addition of organometallic compounds to aluminum-imines and silylimines which are easily prepared from nitriles and aldehydes ly treatment with diisobutyl aluminum hydride (DIBAH) and lithium hexamethyl disilyl... 

The corresponding aryl and alkyl silylimines undergo reaction in the same way but much fastCT requiring only one equivalent of the organometallic compound (Table S). With silylimines, the reaction is complete in less that one hour at -78 C. The yields of amines are somewhat Iowa- than those obtained with aluminum imines. In fact the trimethylsilylimines behave in part as silylating reagents toward organometallic compounds. 

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