New promoters for the molybdenum hexacarbonyl- mediated Pauson–Khand reaction

Abstract

A systematic study of new additives for the stoichiometric molybdenum hexacarbonyl-mediated Pauson-Khand reaction resulted in the discovery of several active compounds such as tetrasubstituted thioureas, ester and amide derivatives of phosphoric acid, quaternary ammonium bromides and phosphine oxides. Tributylphosphine oxide (TBPO) was the most efficient additive providing PK products in moderate to good yields. Some experimental evidences were found which support a non-oxidative Mo(CO) 6 activation by TBPO.

Synthesis of Chiral Cyclopentenones

“…The classical version involved a stoichiometric amount of cobalt carbonyl, while the recent catalytic variants require substoichiometric amounts of transition metal carbonyls. To circumvent the high temperatures and long reaction times necessary to effect the classical PK cycloaddition, a range of promoters of the reaction have been applied, such as Nmethylmorpholine N-oxide (NMO), 111 trimethylamine N-oxide (TMANO), 112 phosphine oxides, 113 alkyl sulfides, 114 thioureas, 115−117 hard Lewis bases, 118 etc. The Pauson−Khand annulation is one of the most powerful tools for the preparation of cyclopentenones, structural units or key intermediates in the synthesis of complex molecules.…”

Synthesis of Cyclopentitols by Ring-Closing Approaches

“…[225][226][227] The classical version of PKR involved a stoichiometric quantity of cobalt carbonyl, while the recent catalytic variants require substoichiometric amounts of cobalt or other transition metals. In order to circumvent the high temperatures and long reaction times necessary to effect the PK cycloaddition, different promoters of the reaction have been widely applied, such as N-methylmorpholine N-oxide, 228 trimethylamine N-oxide, 229 phosphine oxides, 230 alkyl sulphides, 231 thioureas, 232,233 and hard Lewis bases, 234 and it has been found that the selectivity of the reaction depends both on the substrate structure and on the nature of the metal carbonyl/promoter. 235 The cyclization has been also performed as environmentally friendly green protocols, such as in dry-state adsorption conditions, 236 under microwave irradiation, 237,238 in ionic liquids, 239,240 in water, 241,242 and in supercritical ethylene.…”

“…More recently, it has been found that PdCl 2 coordinated to a thiourea ligand could also catalyze an intramolecular reaction. − The classical version of PKR involved a stoichiometric quantity of cobalt carbonyl, while the recent catalytic variants require substoichiometric amounts of cobalt or other transition metals. In order to circumvent the high temperatures and long reaction times necessary to effect the PK cycloaddition, different promoters of the reaction have been widely applied, such as N -methylmorpholine N -oxide, trimethylamine N -oxide, phosphine oxides, alkyl sulphides, thioureas, , and hard Lewis bases, and it has been found that the selectivity of the reaction depends both on the substrate structure and on the nature of the metal carbonyl/promoter…”

Phosphine-catalyzed regio- and stereo-selective hydroboration of ynamides to (Z)-β-borylenamides

“…Thus, in the presence of HBpin, a zwitterionic vinylphosphonium intermediate 4 forms a new B–C bond on the β carbon to produce 5 , which can eliminate PBu 3 to afford keteniminium 6 . 31 P NMR of the reaction mixture under standard conditions showed the disappearance of PBu 3 within 30 min 25 with a concomitant appearance of a new peak at (25.1 ppm) 26 that can be attributed to 5 or 6 (bottom), which can undergo 1,3-hydride shift to produce the desired product 2 . Hydride transfer occurs on the orbital of the sp hybridized central carbon of 6 that is on the same side of Bpin to exclusively afford cis products (Scheme 4).…”