Enamines allylic rearrangements

Aromatic enamines were prepared by dehydroha logenation of /3-bromo-amines with strong base. While trans enamines were thus formed, one obtained mostly cis enamines from rearrangement of the corresponding allylic amines under similar reaction conditions (646). Vicinal endiamines were obtained from S-dichloroamines and lithium amides (647). 

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. 

Enantioselective Allylic Amine-Enamine-Imine Rearrangement... 

On thermolysis, appropriately substituted A-allyl-A-silyloxy enamines 19 undergo smooth [3,3]-sigmatropic rearrangements to the corresponding A-silyloxy imino ethers laP (equation 5). Two stereogenic centers are created but no reference to chiral induction is referred. High diastereoselectivity was observed and short reaction times favoured the syn A-silyloxy imino ether diastereomers. 

Alkylation of enamines may lead to the formation of N-alkylated product, which on heating is converted to C-alkyl compound (This rearrangement is common with allylic halide, alkyl halide or a-haloacetic ester. 

Opitz.G. (1961) Enamines. Vll. Course of the allyl- and propargyl-aUenyl rearrangements in the alkylation of enamines. Justus Liebigs Ann. Chem., 122—132. 

It can be seen from the examples displayed above that the Claisen rearrangement of allyl vinyl ethers with an amino substituent at C(n and C(2) proceeds much faster than that of allyl vinyl ether itself. Several models98- 00 have been proposed in order to interpret the substituent effect on the rate of Claisen rearrangement. Both the acceleration of the rearrangements of / -allyloxyenamine and 0-allylketene TV, 0-acetals and deceleration of the reaction of enamine 120 are in agreement with the prediction of the models. 

JV-Alkyl-JV-allyl enamines 128 (X = NR), like their carbon and oxygen counterparts 128 (X = CH2jO), can undergo [3,3]-sigmatropic rearrangement to form y,<5-unsaturated imines 129 (X = NR) (equation 27). This reaction has been defined as a 3-aza-Cope or the aliphatic amino-Claisen rearrangement8. 

The acceleration of the 3-aza-Cope rearrangement of A-allylenamines through formation of a cationic quaternary nitrogen center (enammonium salt) has obvious advantages. However, direct allylation of enamines in order to form the enammonium ions is unsatisfactory, and difficult. Moreover, the rearrangement-hydrolysis product is often contaminated by the C-allylated product when an unsymmetrical allyl halide is employed... 

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