Ring opening proposed catalytic cycle

The observation that the metal carbene complex, (CO)5W = C(Ph)2 [22], catalyzed the polymerization of cyclic olefins to ring opened polymers containing the diphenylmethylene unit of the catalyst provided additional evidence that carbenes were involved in the catalytic cycle. The formation of the initiating metal carbenes in the classic systems that consist of transition metal halides and alkylating agents was proposed to involve metal alkylation followed by oc-hydrogen loss, Eq. (6). Methane and propene were detected in the early stages of these reactions [23]. 

As a consequence, a poly(P-alanine) can only be formed by a ring-opening polymerization, in which the chain elongation proceeds directly by reaction of P-lactam with an acyl-enzyme intermediate. Because of the low nucleophilicity of the lactam nitrogen, this process requires activation of P-lactam by a water molecule. On this basis we have proposed the catalytic cycle for a CALB-catalyzed polymerization of P-lactam depicted in Figure 14.9. The mechanism described is in accordance with experimental data and is assisted by in-depth computational calculation of the reactions involved. It contains two starting steps (I and II), five steps for the chain elongation (III—VII), and is completed by the release of the polymer (VIII). 

The aryl pyridyl ketone 66 cannot be formed by triazolo ring opening in basic medium. It has been proposed that the cationic intermediate formed in the catalytic cycle is in equilibrium with an azonium form, which with traces of water loses nitrogen, then a probable extrusion of Pd gives an alcohol, which is rapidly oxidized to ketone by air (Scheme 12). 

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