Hydroboration. 70. The Polycyclic Hydroboration of Acyclic and Cyclic Trienes with Borane in Tetrahydrofuran and Triethylamine-Borane. Reexamination of the Stereochemistry of Isomeric Perhydro-9b-boraphenalenes.

Article abstract of DOI:10.1021/jo00204a020

The hydroboration of 1,4,8-nonatriene (1), 5-methyl-1,4,8-nonantriene (2), 1,3,7-octatriene (3), and trans,trans,trans-1,5,9-cyclododecatriene (4) with borane in tetrahydrofuran (THF) and triethylamine-borane was studied.The hydroboration of the acyclic trienes with borane in THF at 0 deg C produced monomeric bicyclic boranes in low yields.Moreover, thermodynamically stable organoboranes with fused six-membered rings, such as 9-boradecalin (15) and 10-methyl-9-boradecalin (20), were not formed as the major monomeric cyclic products.However, thermal treatment of the hydroboration product from 1 produced 15 as the only monomeric bicyclic o oganoborane in high yield.The thermodynamically favored major product from 2 was 20 which amounted to 62percent of the monomeric boranes formed in 74percent yield.With 1 and 2, both borane in THF and triethylamine-borane gave comparable results after thermal treatment.However, in the hydroboration of 3, use of triethylamine-borane resulted in a considerably higher yield of perhydro-8-boraindan.The carbonylation-oxidation of 15 provided an 80:20 mixture of cis- and trans-9-decalols.The ratio of cis-10-methyl-9-decalol to its trans isomer obtained by the carbonylation-oxidation of 20 was at least 62:1.The carbonylation-oxidation of 3 produced only cis-8-hydrindanol.The hydroboration of the cyclic triene 4 followed by thermal treatment at 160 deg C for 6 h produced a mixture consisting largely of cis,trans-perhydro-9b-boraphenalene (6) and a constitutional isomer in 83percent yield.However, thermal treatment at 200 deg C for 6 h provided a 92:8 mixture of 6 and its all-cis isomer 5, uncontaminated with the constitutional isomer.The hydroisomerization of 5 or 6 at 220 deg C in the presence of triethylamine under 140 atm of hydrogen gave an equilibrium mixture of 88+/-1percent 5 and 12+/-1percent 6.The difference between thefree energies of 5 and 6 was estimated to be 1.9+/-0.1 kcal/mol.The stereochemistry of 5 and 6 established by 1H NMR examination of the pyridine adducts of 5 and 6 as well as of free 5 and 6.The original structural assignments by Rotermund and Koester must therefore be reversed.

Full text of DOI:10.1021/jo00204a020

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