Low-energy pathway for pauson-khand reactions: synthesis and reactivity of dicobalt hexacarbonyl complexes of chiral ynamines
A family of dicobalt hexacarbonyl complexes of 1-(dialkylamino)-2-(trimethysilyl)acetylenes (3a-f) derived both from achiral and chiral amines [a, morpholine; b, (S)-2-methoxymethylpyrrolidine; c, (2R,5R)-2, 5-bis(methoxymethyl)pyrrolidine; d, (+/-)-trans-2, 5-bis(benzyloxymethyl)pyrrolidine; e, (2R,5R)-2, 5-dimethylpyrrolidine; f, (S)-(alpha-methylbenzyl)benzylamine] has been prepared by a one-stage process from dichloroacetylene. The methanolysis at room temperature of these complexes (MeOH/K(2)CO(3)) induces the selective cleavage of the carbon-silicon bond, leading to the thermally unstable terminal ynamine complexes 12a-f. The Pauson-Khand reaction of 12a-f with strained olefins (norbornadiene and norbornene) takes place at unprecedentedly low temperatures (-35 degrees C) in the absence of chemical promoters. Diastereoselectivities of up to 94:6 are recorded in these reactions with the ynamine complexes derived from C(2) symmetrical chiral auxiliaries (12c,d). The high reactivity depicted by terminal ynamines in the Pauson-Khand reaction has been analyzed by theoretical semiempirical procedures [PM3(tm)] and with density functional theory (VWN/B88), and appears to reflect an easy CO loss from the ynamine-dicobalt hexacarbonyl complexes.
Balsells
J
Unitat de Recerca en Sintesi Asimetrica (URSA), Departament de Quimica Organica, Universitat de Barcelona, c/Marti i Franques, 1-11, 08028-Barcelona, Spain.
Vazquez
J
Moyano
A
Pericas
MA
Riera
A