Highly Chemo-, Regio-, and Enantioselective Rhodium-Catalyzed Cross-Cyclotrimerization of Two Different Alkynes with Alkenes

Rhodium-Catalyzed Asymmetric [2+2+2] Cycloaddition of 1,6-Enynes with Racemic Secondary Allylic Alcohols through Kinetic Resolution

It has been established that a cationic rhodium(I)/P-phos complex catalyzes the asymmetric [2+2+2] cycloaddition of 1,6-enynes with racemic secondary allylic alcohols to produce the corresponding chiral bicyclic cyclohexenes, possessing three stereogenic centers, as a single diastereomer with excellent ee values. Mechanistic experiments revealed that the present cycloaddition proceeds through the kinetic resolution of the racemic secondary allylic alcohols, in which one enantiomer preferentially reacts with the 1,6-enyne.

Reaction of [TpRh(C2 H4 )2 ] with Dimethyl Acetylenedicarboxylate: Identification of Intermediates of the [2+2+2] Alkyne and Alkyne-Ethylene Cyclo(co)trimerizations

The reaction between the bis(ethylene) complex [TpRh(C2 H4 )2 ], 1, (Tp=hydrotris(pyrazolyl)borate), and dimethyl acetylenedicarboxylate (DMAD) has been studied under different experimental conditions. A mixture of products was formed, in which TpRh(I) species were prevalent, whereas the presence of trapping agents, like water or acetonitrile, allowed for the stabilization and isolation of octahedral TpRh(III) compounds. An excess of DMAD gave rise to a small amount of the [2+2+2] cyclotrimerization product hexamethyl mellitate (6). Although no catalytic application of 1 was achieved, mechanistic insights shed light on the formation of stable rhodium species representing the resting state of the catalytic cycle of rhodium-mediated [2+2+2] cyclo(co)trimerization reactions. Metallacyclopentene intermediate species, generated from the activation of one alkyne and one ethylene molecule from 1, and metallacyclopentadiene species, formed by oxidative coupling of two alkynes to the rhodium centre, are crucial steps in the pathways leading to the final organometallic and organic products.