Complexes rhenacyclobutadienes

The crystal structure of a Fischer-type metallacyclobutadiene complex of rhenium 6 has also been reported <1993JA9986, 1990JOMC1>. The bond lengths between Re and the -carbons (2.18 and 2.13 A) are very similar, but the two C-C bond lengths are considerably different (C(l)-C(2) 1.36 A, C(2)-C(3) 1.45 A). In addition, the C(3)-0(4) bond distance (1.30 A) is much longer than C(5)-0(6) (1.19 A) but shorter than C(5)-0(7) (1.34 A). Therefore, the structure of rhenacyclobutadiene 6 is expected to be best represented by significant contributions from the resonance structures 6a and 6b and a smaller contribution from structure 6c. 

The reaction of the rhenium alkylidyne complex 277 with diisopropyl-acetylene and with diethylacetylene [Eq. (196)] demonstrates the sensitivity of metathesis reactions toward steric factors (57). With diisopropylace-tylene an alkylidyne complex is obtained whereas the reaction with diethylacetylene gives a metallacyclobutadiene. In the metathesis reactions the alkyne with the bulkiest groups cleaves most easily from intermediate metallacyclobutadiene complexes. The rhenacyclobutadienes with the smallest substituents thus become sinks and slow down the effective rate of metathesis. The alkylidyne alkylidene rhenium complex 278 is an active olefin metathesis catalyst (52). Reaction with hexene transforms the neo-pentylidene group into a propylidene group as shown in Eq. (197). 

The reaction of 1,2,3-triphenylcyclopropenylium tetraduoroborate (28) with sodium pentacar-bonylrhenium afforded the cycloprop-2-enyl-rhenium complex 39. Upon ultraviolet irradiation or heating, complex 39 split off one carbonyl ligand to give the rhenacyclobutadiene complex 40, which was in equilibrium with the )7 -cyclopropenyl-rhenium rr-complex 41. The structure of 40 was determined by X-ray crystallography. ... 

---