Metallacycles ethylene-derived

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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