Mechanistic Studies of the Copolymerization Reaction of Aziridines and Carbon Monoxide to Produce Poly-β-peptoids

The coupling of carbon monoxide and aziridines has been shown to be selective for comonomer-alternating enchainment in the presence of PhCH2C(O)Co(CO)4 to afford poly-β-peptoids. In this article,
we have investigated the mechanistic aspects of the reaction of CO and N-butylaziridine by means of in
situ infrared spectroscopy employing CH3C(O)Co(CO)3L (L = PPh3 (1) and P(o-tolyl)3 (2)) as precatalysts.
Precatalyst 1 exists in solution under catalytic conditions as an equilibrium mixture of 1 and CH3C(O)Co(CO)4, and affords both poly-β-butylalanoid and the corresponding lactam. By way of contrast, precatalyst
2 which possesses the sterically bulky and labile P(o-tolyl)3 ligand, affords only the acyl cobalt tetracarbonyl
species in solution during catalysis with concomitant selective production of the copolymer. Kinetic studies
conducted with precatalyst 2 showed the coupling reaction to have a first order dependence on catalyst,
a first order dependence on N-butylaziridine, and only a slight dependence on the concentration of CO
over the pressure range 17−69 bar. The working mechanistic model for the copolymerization reaction
involves first aziridine insertion into the cobalt-acyl bond, rate determining ring opening by the cobaltate
species, followed by the migratory CO insertion.