A Study of [Co₂(alkyne)(binap)(CO)₄] Complexes (BINAP=(1,1′‐Binaphthalene)‐2,2′‐diylbis(diphenylphosphine))

Abstract

Understanding the interaction of chiral ligands, alkynes, and alkenes with cobaltcarbonyl sources is critical to learning more about the mechanism of the catalytic, asymmetric Pauson-Khand reaction. We have successfully characterized complexes of the type [Co2(alkyne)(binap)(CO)4] (BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine)) and shown that diastereomer interconversion occurs under Pauson-Khand reaction conditions when alkyne=HC[triple bond]CCO2Me. Attempts to isolate [Co2(alkyne)(binap)(CO)x] complexes with coordinated alkenes led to the formation of cobaltacyclopentadiene species.

A Study of (Binap)(enyne)tetracarbonyldicobalt(0) Complexes

“…A resonance corresponding to binap at δ −14.6 ppm was also observed. The 31 P NMR spectrum of 4 was consistent with the 31 P NMR data obtained for diastereoisomers 2 a and 2 b , the structures of which had been confirmed by X‐ray crystallography;10 this, together with the X‐ray characterisation obtained for complex 6 (see below), provided the necessary assurance that complex 4 was the desired binap cobalt derivative of enyne 3 . The two diastereoisomers of 3 , formed in a 2:1 ratio, could not be separated by chromatography or recrystallisation.…”

“…Given that alkyne coordination is the accepted first event in the PKR, we subsequently examined the reactivity of 1 towards alkynes. We found that complex 1 reacted with a range of alkynes to form mixtures of two diastereoisomeric complexes as exemplified by 2 a and 2 b (Scheme ; structures confirmed by X‐ray crystallography) 10. In the case of 2 , the two isomers were separable and 31 P NMR studies revealed that they interconverted without decomposition at 75 °C 10…”

“…One of the proposed pathways for the interconversion of the diastereoisomers of 2 is illustrated in Scheme 10. The barrier to alkyne rotation is lowered by locating it on just one cobalt atom, the binap‐free cobalt being selected for steric reasons.…”

Transition metals in organic synthesis: Highlights for the year 2005

“…Asymmetric cobalt-catalyzed intermolecular Pauson-Khand reactions were reported [997]. An asymmetric intermolecular Pauson-Khand reaction of cobalt-alkyne complexes having a chiral bridging ligand [998] and a rhodium-catalyzed asymmetric Pauson-Khand reaction in water [999] was described.…”

Regioselectivity, Stereoselectivity and Catalysis in Intermolecular Pauson-Khand Reactions: Teaching an Old Dog New Tricks

“…While our abovementioned basal anti-bridged BINAP cobalt-alkyne complex was obtained by mixing the cobalt-alkyne complex with (R)-BINAP, 65 Gibson first prepared a precatalyst from either Co 2 (CO) 8 or Co 4 (CO) 12 and (R)-BINAP and then added the alkyne 68 in order to get the chelated complex. Thus, the order of addition seems to play an important role.…”