Rhodium complexes cyclobutadiene

In contrast to cycloaddition reactions of phosphaalkenes, cycloaddition reactions between phosphaalkynes and other unsaturated systems are comparatively rare. Indeed, there are only a limited number of reports for monophos-phacyclobutadiene) complexes, which are obtained from the corresponding phosphaalkyne. Relatively recently, the reaction of phosphaalkynes with highly electron deficient alkynes was reported <19990M4838>. Treatment of a CF3C=CGF3-coordinated dimeric rhodium complex with phosphaalkynes in hexane at — 20°C followed by warming to room temperature afforded the red air- and moisture-stable phosphete complexes 60 in ca. 50% isolated yields. When phosphaalkynes are allowed to react with a kinetically stabilized cyclobutadiene, 2-Dewar-phosphinines, for example 93 (Equation 30), are obtained <1998S1305>. 

Biphenylenes undergo oxidative addition to various low-valent metals to form the corresponding dibenzometalloles [7]. The reaction with Cp RhfPMcj) involved C-H activation prior to C-C activation, as with the case of C-C activation of cyclopropane [7c]. On the other hand, the reaction with [Rh(Pi-Pr3)2] initially formed the r -arene complex, which led to the dibenzorhodacycle [7g]. Density functional theory (DFT) calculation suggested the C-C activation proceeds via the rhodium Tj -cyclobutadiene intermediate (Scheme 1.4). 

Sekiguchi s work concerning the use of silylated cyclotetrasUadienes and related ligands covered also several cobalt (17) and rhodium complexes [237,422,425]. Related work on a monosila cyclobutadiene complex was reported by Kira and coworkers [426]. 

Cycloalkylation 4, 832 suppl. 28 Cyclobutadiene rhodium triarylphosphine complexes... 

The formal 2 -I- 2-cycloaddition reaction of platinum(ll) acetilides (14) with TCNE and TCNQ initially produced 7,7,8,8-tetracyano-p-quinodimethane cyclobuta-1, 3-dienes that undergo retroelectrocyclization to produce feis-cycloadducts (15, 16). In all examples investigated, the triple bond adjacent to the platinum atom did not react with the TCNE or the TCNQ (Scheme 5). The 2-I-2-cycloaddition of substituted cyclobutadiene-AlCl3 complexes (18) with methyl phenylpropynoate (17) produced Dewar benzene intermediates (19) that readily converted to 1,2,3,4-tetramethyl- and 1,2,3,4-tetraethyl-fluorenes (20) (Scheme 6). The first rhodium-catalysed inter-molecular 2-1-2-cycloaddition of terminal alkynes with e-deflcient alkenes yielded substituted cyclobutenes in high yields (99%) and complete regioselectivity. An 8-quinolinolatorhodium/phosphine catalyst (21) was employed in this reaction. ... 

A rhodium-catalyzed cycloisomerization reaction of triyne 137 to 141 involves cleavage of the C=C triple bond (Scheme 7.49) [68]. The following reaction pathway is proposed initially, oxidative cyclization produces the rhodacycle 138, which then undergoes reductive elimination. The rhodium cyclobutadiene complex 139 is thus generated, and then undergoes oxidative addition to produce the rhodacycle 140. This isomerization from 138 to 140 would reduce the steric congestion of the heUcal structure. Subsequently, a cycloaddition reaction between the rhodacycle and the pendant alkyne moiety takes place to afford 141. 

---