Catalytic version of the Intramolecular Pauson-Khand Reaction

Metal-Assisted Cycloaddition Reactions in Organotransition Metal Chemistry

“…Only more recently turnover numbers of several hundred have been achieved with modified cobalt catalysts under CO pressure. [781][782][783] The reaction is very regioselective with respect to the acetylene, and the CO group is normally found next to the bulkier R group of the alkyne (R 1 in eq 125), i.e., with terminal acetylenes, the substituted terminal is exclusively found at the C-2 position of the cyclopentenone. 756,775 This infers that the olefin coordinates to cobalt from the less hindered side of complex 480.…”

The Pauson–Khand Reaction: the Catalytic Age Is Here!

“…[12] Subsequently, a catalytic conversion of enynes into cyclopentenones employing phosphites as coligands was reported by Jeong, Chung, and co-workers (Table 1, entry 3). [10] Remarkably, under 1 atm of CO the use of phosphites as coligands did not show any positive effect on the PKR conversions (they were either ineffective or detrimental); the advantages of this procedure were only apparent at pressures of 3 atm. Based on their own previous work on the stoichiometric version of the reaction, [13] Sugihara and Yamaguchi used hard Lewis bases to effect both the inter-and the intramolecular CPKR, employing 0.01-0.03 equivalents of octacarbonyldicobalt(0).…”

“…Despite the significant use of phosphane-and phosphitesubstituted alkyne complexes in the stoichiometric PKR, in which coordination leads to a decrease in the rate and overall efficiency of the reaction, [12,22] and despite the fact that triphenylphosphite additive in the [Co 2 (CO) 8 ]-mediated catalytic PKR leads to an improved reaction efficiency, [10] only recently was it shown that preformed complexes of phosphanes and phosphites can catalyze the PKR under atmospheric pressures of carbon monoxide. [32,33] The TONs observed in work by our own group with these stable complexes were comparable with those of other similar mild systems (Scheme 8).…”

“…[4] However, the employment of promoters such as tertiary amine N-oxides or DMSO in the CPKR produced only trace amounts of the desired products. [10] Presumably, the regeneration of Co 0 species within the catalytic process is not permitted under these oxidizing conditions.…”

“…[4,5] At that time, catalytic systems that could induce asymmetry were only speculative, presumably because in most of the cases they required very harsh conditions (high temperatures and very high pressures of carbon monoxide). [6,10,18,20] In 1996, however, Buchwald and Hicks reported the first example of an asymmetric PKR involving a catalytic amount of a chiral titanocene complex (Scheme 12). [52] Following their own work on the use of the highly active species [Cp 2 Ti(CO) 2 ] as a catalyst for the cyclization of enynes under mild conditions, they succeeded in inducing asymmetry by employing the enantiomerically pure analogue (S,S)-[(ebthi)Ti(CO) 2 ] (8)-generated in situ from (S,S)-[(ebthi)Ti(Me) 2 ] (Scheme 13, ebthi = ethylene-1,2-bis(h 5 -4,5,6,7-tetrahydro-1-indenyl)).…”