Ethylenes ethylene-derived metallacycles

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. 

A particular subset of these ligated dialkyls are those in which the ligand is tethered to the cyclopentadienyl ligand. Thus, chromacyclopentane and cycloheptane derivatives stabilized by the (7/ 7/ -Me4C5CH2CH2NMe2) ligand have been prepared and their reactivity supports the intermediacy of such metallacycles in the catalytic trimerization of ethylene. A variety of donor-ligand-substituted cyclopen-tadienylchromium(III) complexes with amino and phosphino substituents has been prepared and screened for an ethylene polymerization activity. ... 

The cyclopentadienyl and neopentylidene derivatives (> -Cp)M(CHCMe3)Cl2 and (>/ -C5H4Me)M(CHMe3)Cl2 (M = Ta, Nb) also react with ethylene, prolylene and styrene to give the organic products of jS-hydrogen transfer of the metallacyclobutane intermediate " " . Propylene exclusively affords the a,jS-disubstituted metallacycle, and styrene the a,a -disubstituted one ... 

In 2008, Piers and coworkers [43] reported the preparation and spectroscopic characterization of ruthenacyclobutane and ruthenium alkylidene/olefin complexes associated with ring-closing metathesis (RCM) catalysis. Exploiting the reversibility associated with the RCM reaction, phosphonium alkylidene complex 28a was reacted with 1 equiv of ethylene at —78 °C, followed by the addition of an excess of RCM product 29 (2-3 equiv), to afford a 90% NMR yield of metallacycle 30 (Scheme 8.9). This use of reverse engineering was found to minimize the formation of the thermodynamically favored, unsubstituted ruthenacycle 23 to only 10% yield. Later studies found that, similar to the phenomena that had been observed with propene, the a-monosubstituted metallacycle derived from ethylene and the propagating alkylidene of 30 could also be observed if the reaction temperature was further lowered (e.g., —76 C) [41]. It was additionally reported that the use of the more bulky phosphonium alkylidene 28b afforded lower reaction yields relative to 28a (60-70% vs. 90%). 

This is to be compared with the products of codimerization of ethylene with 1-butene (R = Et vide supra). As for tantalum, the /3-substituted metallacycle preferentially forms over the a-derivative. 

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