Mecchanism of the reaction of methoxycarbonylcarbene as revealed by CIDNP. V. Thermal reaction of methyl diazoacetate with benzyl ethers.

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When methyl diazoacetate is allowed to react thermally with dibenzyl, benzyl phenyl and benzyl ethyl ethers, insertion into the benzylic carbon–oxygen bonds takes place inter alia to give PhCH2CH(OR)CO2CH3(3, R=PhCH2, Ph, and CH3CH2, respectively). Strongly polarized signals due to the insertion products 3 were observed in the 1H and 13C NMR spectra of the reacting mixture. Application of Kaptein's rules to the CIDNP signals demonstrates the formation of 3 by the cage recombination of radical pairs ‾PhCH2··CH(OR)CO2CH3 (2). Product analyses by VPC have also been performed and escape products toluene and ROCH2CO2CH3 (4) inherent to the radical pairs were detected. In the reaction with benzyl ethyl ether, 4 (R=PhCH2) without noticeable CIDNP is produced about six times more than 4 (R=CH3CH2). No methyl α-benzyloxybutyrate is formed. The results indicate that the ethyl group is more easily cleaved than the benzyl via a non-radical path. Formation of an ylide intermediate (PhCH2)R\overset⊕O–\overset\ominusCHCO2CH3 (1, R=CH3CH2) followed by the Hoffmann type β-elimination of ethylene is the most reasonable explanation. Homolysis of the same ylide 1 is considered to give the radical pair 2 (R=CH3CH2). A similar process appears to lead solely to radical pairs 2 (R=PhCH2, Ph) in the case of ylides lacking β-hydrogen. The potentiality of this ylide mechanism in the reactions of ethers is discussed.
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