Nucleophilic Attack on an Electronically Unsaturated Triosmium Cluster Complex of 2-Methylbenzoxazole: An Unusual Oxidative Ring Opening

In the course of our studies of nucleophilic attack on electronically unsaturated benzoheterocycle triosmium clusters we have studied the reaction of the 2-methylbenzoxazole complex (μ-H)Os₃(CO)₉(μ₃-η²-2-CH₃–C₇H₃NO) (1) with hydride followed by protonation with acid. In sharp contrast to our previous studies with related benzoheterocycle triosmium clusters, where the nature of the heterocycle controls the regiochemistry of nucleophilic attack, we observe here an unusual ring opening of the heterocyclic ring, coupled with rearrangement of the carbocyclic ring to a 2-imino-ethyl-phenol complex (μ-H)Os₃(CO)₉(μ₃-η³-N=CHCH₃–C₆H₃(OH)) (2). Deuterium labeling experiments verify initial attack by hydride at the 2-position followed by protonation at oxygen. Reaction of 1 with two equivalents of hydride followed by two equivalents of acid results in reduction of the C=N bond in 2 and on standing in air, oxidation of the carbocyclic ring occurs to give the 2-ethyl-amino hydroquinolyl derivative (μ-H)Os₃(CO)₉(μ₃-η³-NHCH₂CH₃–C₆H₃(2-O)(5-OH)) (3). The solid-state structure of 3 is reported and a plausible mechanism, supported by deuterium labeling experiments, is presented, for the formation of 2 and 3.