CATALYTIC RHODIUM-MEDIATED TETRAENE CARBOCYCLIZATIONS

Certain rhodium complexes ([RhCl3-2 Ar3P], {[(1,5-COD)RhCl](2)-2 Ar3P} , (Ph(3)P)(3)RhCl, [(Ph(3)P)(2)Rh(NBD)]PF6) catalyze the cyclization-a mine trapping of the prototypical tetraene 8. The chemical yields are quite acceptable, up to 89% when one takes into account the unreacted tetraene recovered in reusable form. The best conversion is obtained i n 2,2,2-trifluoroethanol (TFE, 75 degrees C) using RhCl3 as the cataly st precursor. The stereochemistry of the products obtained via rhodium catalysis complements that obtained with palladium. The cyclization o f 8, promoted by the combination [RhCl3-2 Ph(3)P] in TFE (75 degrees C ), affords the morpholine-trapped cyclopentane (67%) bearing a cis rel ative stereochemistry between the side chains and predominantly the Z geometry within the newly formed side chain (i.e., a mixture of 9a (mi nor component) and 9b (major component)). In contrast, palladium catal ysis affords predominantly the trans-cyclopentane derivative 9c. The c yclization of 8, promoted by the combination [RhCl3-2 (p-Me(2)N-C6H4)( 3)P] in TFE (75 degrees C), affords the morpholine-trapped cyclopentan e 12 (63%), an isomer that may arise via the double-bond isomerization of 9a,b. Rhodium-catalyzed cyclization and trapping by N-benzylmethyl amine affords predominantly cis-13a (R(1) = CH(2)Ph, R(2) = Me, 81%), and the reaction with diethylamine gives a 1:1 cis-trans mixture of 13 b (R(1) = R(2) = Et, 70% based on recovered 8). Attempts to cyclize an d trap tetraene 8 with benzylamine, diisopropylamine, phthalimide, and N-methyl-p-toluenesulfonamide were unsuccessful under the conditions of [RhCl3-2 Ph(3)P] catalysis in TFE.