Rhenium-Catalyzed C-H and C-C Bond Activation

Link: http://www3.interscience.wiley.com/cgi-bin/abstract/114130502/ABSTRACT

From Prof. Yoshikazu Horino at University of Toyama, Japan

This work appeared recently in ACIEE Highlights as a minireview on a specific type of reaction in the field of C-H and C-C bond activations using Rhenium and Ruthenium catalysts. The work featured in this minireview is the work being done by other leaders in the field, such as Kuninobu and Takai.The first catalyst being discussed is [{ReBr(CO)3(thf)}2] which was found to work well in the reaction with aldimine 1 as shown in Scheme 1 above. The mechanism was proposed as the activation of the aryl C-H bond ortho to the aldimine group resulting in the C-Re bond formation, where the metal center is coordinated by the proximal N group of the aldimine. Thus, it is required that the metal center lost a CO in order for this coordination to occur. This CO-loss hypothesis was later confirmed by other experiments. Also, the nitrogen had to be present to coordinate to the metal and thus stabilized the intermediate.

Another point worth mentioning is the reverse of product composition with different phenyl acetylene derivative was used without significant loss of yield. (compare ratio of 5 and 6 in the scheme above).

The reaction with aldimne 1 with a ruthenium catalyst was found to take a different course of action. When [Ru3(CO)12] was used as a catalyst, no metal migratory insertion on the aldimine occurred following insertion of ruthenium to acetylene, and the reaction stopped at product 9 (Scheme 2).

The same rhenium catalyst as above was also used in a reaction to form indene derivatives from aromatic ketones with alpha,beta-unsaturated esters. In this case, p-anisidine was needed as a co-catalyst for the reaction to proceed via the intermediacy of the ketimine 11, of which the nitrogen of the ketimine then directed the activation of C-H bond by the metal center followed by cyclization with unsaturated ester (Scheme 3).

The same rhenium complex was also found to facilitate the vinylation of 1,3-diketones and beta-keto esters with acetylenes as illustrated in the scheme below.

Kuninobu and Takai also found that their rhenium complex in the presence of an isocyanide as a ligand can also catalyze a ring expansion reaction of cyclic 1,3-dicarbonyl compounds with acetylenes as demonstrated in Scheme 5 below. It is important that isocyanide is used as a ligand. In the absence of an isocyanide, only alpha-vinylated product was obtained.

As shown in the scheme above, the retro-aldol (path A) and the deMayo-type reaction (path B) were proposed as a possible reaction mechanism of this process. Although experimental data to differentiate between the two proposed mechanisms has not been discussed, mechanistic studies on a related ruthenium-catalyzed [2+2] cycloaddition reactions of olefins with acetylenes are consistent with the deMayo reaction (path B). While [ReBr(CO)5] and [MnBr(CO)5] also catalyze the synthesis of 35 in modest yield (45% and 59%, respectively), neither [Ru3(CO)12], [RuH2(CO)(PPh3)3], PtCl2, AuCl3, nor GaCl3 promote formation of 35 under identical reaction
conditions.

This paper gave a good reactivity profile of the rhenium catalyst and the recent technology on its effect on C-H and C-C activation reactions, in comparison with other metal catalyst. This is a good place to come back to consult with when this type of reaction/activation is desired.