Synthesis of 1H-1-benzazepines by thermolysis of 2a,7b-dihydro-3H-cyclobut[b]indoles

Abstract

Syntheses and thermolyses of 3-substituted 2a,7b-dihydro-3H-cyclobut[b]indoles and some 2a-methyl derivatives are described. X-Ray crystal-structure analyses were carried out on the 3-acetyl and 3-pivaloyl derivatives, which both crystallise in the monoclinic system; space group C2/c, Z= 8, a= 15.277(7), b= 9.035(5), c= 14.485(11)Å, β= 104.27(5)° for the former, and space group P2₁/c, Z= 4, a= 8.696(7), b= 15.227(12), c= 9.428(8)Å, β= 102.67(7)° for the latter. The crystal structures were solved by direct methods, and atomic parameters refined to R= 0.118 (for the acetyl derivative) and R= 0.108 (for the pivaloyl derivative). No abnormal results were found. Direct thermolyses of the dihydrocyclobut[b]indoles resulted in the formation of 1-substituted 1H-1-benzazepines, N-substituted 1-naphthylamines, and 1-substituted indoles, whose relative distributions depend upon the nature of the substrate and the reaction temperature. The presence of silver ion significantly lowered the temperature necessary for ring opening and gave the 1H-1-benzazepines in variable yields. The 3-benzoyl-2a-methyl derivative, when heated in the presence of silver ion, gave both the rearrangement product 1-methyl dihydro-3H-cyclobut[b]indole and its ring-opened derivative, in addition to the 2-methyl-1H-1-benzazepine. The thermal and photochemical behaviour of these 1 H-1-benzazepines is also described.