Ruthenacyclobutane Intermediates Derived from Phosphonium Alkylidene Complexes

Ruthenacyclobutane Intermediates Derived from Phosphonium Alkylidene Complexes 

The observation of the first substituted ruthenacyclobutanes occurred somewhat serendipitously. Upon the exposure of 22c to propene (30 equiv) at —40 C, we observed the presence of the ethylene-derived metallacycle 23 in 45% conversion via NMR after 3h [32]. However, much of the mass balance of the reaction remained to be accounted for, as 95% of 22c had been consumed. 

2D-COSY, ROESY, and HMQC analyses of the reaction mixture were used to determine the identities of 25a and 26a. Remarkably, we later found that, if the 

EXSY experiments were performed at -87 °C (186 K) to determine the rate of exchange between the cis and trans stereoisomers of metallacycles 26a-c and 27a-c (Table 8.2). These rate constants incorporate the rates of metallacycle cycloreversion, alkylidene rotation, and cycloaddition to interconvert the two species. Given that propene, 1-butene, and 1-hexene are all Type I olefins for cross-metathesis [42], it is not unexpected that these results correlate with each other. 

Most of the key chemical shifts of 30 were found to be similar to those of 23, with the exception that both the and the C of the methine carbon in 30 were signifi- 

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