Lithium amides complexed

Complexed lithium amides, 37 108-131 attachment of Lewis base molecules, 37 113-114... 

Fig. 34. Structural types for complexed lithium amides (RR NLixLi, L = a Lewis base (a) ladders with n = 2 and 3 (b) dimeric rings (c) monomers (CIPs and SSIPs). ...

Recent studies have begun to explore the consequences of complexed lithium amide solution structures for reaction mechanisms and rates. Double-labeling (6Li and 15N) NMR experiments, allied to colligative measurements, show that lithium diphenylamide in THF/hydrocarbon solutions exists as a cyclic oligomer at low THF concentrations, probably a bi- or trisolvated dimer [Fig. 48, structure (I) or (II), respectively]... 

The conjugate addition of Grignard reagents to 2-cyclohexenone was promoted by catalytic amounts (2-4 mol %) of alkylcopper(I) complexes of the lithium amide prepared from N- (R)-1 -phenylethyl]-2-[(/ )-l-phenylethyliminojcycloheptatrienamine, Li[CuR(CHIRAMT)]52,11. However, 3-substituted cyclohexanones were obtained in very low ee (4-14%). 

Irradiation of matrix-isolated imidazole-2-carboxylic acid gave the 2,3-dihydro-imidazol-2-ylidene-C02 complex (31) characterized by IR spectroscopy and calculated to lie 15.9 kcal mol above the starting material. A series of non-aromatic nucleophilic carbenes (32) were prepared by desulfurization of the corresponding thiones by molten potassium in boiling THF. The most hindered of the series (32 R = Bu) is stable indefinitely under exclusion of air and water and can be distilled without decomposition. The less hindered carbenes slowly dimerize to the corresponding alkenes. Stable aminoxy- and aminothiocarbenes (33 X = O, S) were prepared by deprotonation of iminium salts with lithium amide bases. The carbene carbon resonance appears at 260-297 ppm in the NMR spectrum and an X-ray structure determination of an aminooxycarbene indicated that electron donation from the nitrogen is more important than that from oxygen. These carbenes do not dimerize. 

ALkoxyalkyl-substituted arenechromium complexes have been lithiated enantioselec-tively in the benzylic position without control over planar chirahty and the tendency for lithium amides to lithiate at a benzyhc position rather than on the ring itself poses a problem with some substrates. In the lithiation of 433, an element of planar stereochemistry is introduced during a benzyhc lithiation (Scheme 176) . Exploiting... 

A high level of enantioselectivity in an acyclic system has been reported in the rearrangement of tricarbonylchromium(O) complexes of allyl benzyl ethers using chiral lithium amide base 73 (equation 38) . Upon treatment with 1.1 equivalents of lithium amide 73 and 1 equivalent of LiCl at —78 to —50°C, ether 74 afforded the rearrangement product R)-75 in 80% yield with 96% ee. The effect of substituents on the chemical yields and enantioselectivity of the [2,3]-Wittig rearrangement was also studied (see Table 3). 

An interesting result, contributed by Gibson (nee Thomas) and coworkers, should be added " The tricarbonylchromium complexes 258 are readily deprotonated by the chiral bis(lithium amide) 234 and alkylated to provide highly enantioenriched alkylation products 260 (equation 62). The decisive intermediate 259 presumably is formed by abstraction of the pro-S proton and is attacked from above. Related work has been published by Ewin and Simpkins . 

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