Amines allylic site selectivity

In 1991, Kessar and coworkers demonstrated that the kinetic barrier could be lowered by complexing the tertiary amine with BF3, snch that i-BuLi is able to deprotonate the ammoninm compound, which can be added to aldehydes and ketones as shown by the example in Scheme 4a. Note the selectivity of deprotonation over vinyl and allyl sites. A limitation of this methodology is that the ylide intermediate does not react well with alkyl hahde electrophiles. To get aronnd this, a seqnence that begins with the stannylation and decomplexation shown in Scheme 4b was developed. The stannane can be isolated in 94% yield (Scheme 4b) and snbseqnently snbjected to tin-lithium exchange to afford an unstabilized lithiomethylpiperidine that is a very good nucleophile. However, isolation of the stannane is not necessary and a procedure was devised in which the amine is activated with BF3, deprotonated, stannylated, decomplexed from BF3 with CsF, transmetalated back to lithium and alkylated, all in one pot (Scheme 4c). ... 

Site selectivity with allylic alcohols and amines. 836... 

Synthetic highlight Diastereoselective production of rac-menthol from its aromatic precursor is achieved by site-selective isopropylation and diastereoselective hydrogenation to the all-trans racemate. Enantioselective allylic amine-enamine-imine rearrangement of an acyclic diene-allylic amine, catalyzed by an Rh(I)-(—)-BINAP complex, affords (—)-menthol the process has been scaled-up to production of 1,000 tons/year. 

Scheme 2.28 [FePcJ -catalyzed regio- and site-selective allylic C(sp )-H bond amination [142]...

A C(ip )-H amination of trimethylsilyl-substituted cyclopropenes proceeds via a tandem regioselective ene reaction between cyclopropenes and azodicarboxylate to generate a hydrazodicarboxylate intermediate followed its site-selective allylic transposition (Scheme 85). ... 

Similar selectivities are encountered with substrates 228. In the case of Irons-22, the product with overall retention, trans-2, can be obtained predominantly by conducting the reaction in the presence of lithium chloride, which blocks the coordination sites on palladium in the intermediate 7t-allyl complex and thus suppresses isomerization of the starting material via syn migration of acetate from the metal to the rc-allyl ligand. A large excess of amine has been found to further enhance the diastereoselectivity of this process. 

These mesoporous mixed titania-silica oxides are hydrophilic materials and are excellent catalysts for epoxidations of olefins, allylic alcohols and a,jff-unsaturated ketones with alkyl hydroperoxides in non-aqueous media [37]. Their performance can be improved even further by adding organic or inorganic bases to neutralize acid sites present on the surface [38,39], The latter cause side-reactions, especially with acid sensitive epoxides. Amine addition was particularly effective and led to the development of a mesoporous Ti-Si mixed oxide containing surface-tethered tertiary amino groups as an active, selective, and recyclable catalyst for the epoxidation of allylic alcohols [38]. 

---