Rhenacyclobutadienes reaction

Rhenacyclobutadienes are more stable toward loss of an acetylene or further reaction with an internal acetylene than other metallacyclobutadienes268-269,278 519. However, whereas the low-temperature l3C NMR spectra (-50 °C) of the rhenacyclobutadienes are consistent with their X-ray structure, coalescence of the ring signals is observed at room temperature, indicating a rapid exchange of the ReC3-ring carbons. Since no loss of acetylene is observed, this was attributed to a rapid equilibrium of the rhenacyclobutadienes with their -cyclopropenyl counterparts (equations 249 and 250)268,269,319. 

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. ... 

---