Hydrophosphinations calcium

Scheme 18 Proposed mechanism for calcium-catalyzed alkene hydrophosphination (for simplicity, coordinated THF is not shown in 12 or other intermediates)...

Hydrophosphination of carbodiimides to yield phosphaguanidines was catalyzed by several related metal complexes. For example, after carbodiimide coordination by the calcium phosphide complex 12 (see Scheme 18 above), P-C bond formation involving a four-centered transition state and rearrangement was proposed to yield the N,N-bound intermediate 13. Addition of the P-H bond across the Ca-N bond, followed by dissociation of the product, then regenerates the phosphide intermediate (Scheme 23) [37]. Related chemistry was reported recently using the calcium phosphide complex Ca(PPh2)2(THF)4 [38]. 

Keywords Barium Calcium Catalysis Dehydrocoupling Dehydrogenation Group 2 Heavier Alkaline Earth Heterofunctionalization Hydroamination Hydroboration Hydrogenation Hydrophosphination Hydrosilylation Magnesium Strontium... 

Calcium-based catalysts have also shown promise in hydrophosphination reactions (Scheme 4.299) [62]. Treatment of phenylacetylene with diphenylphosphine in the presence of a p-diketiminate-ligated calcium complex afforded an excellent yield of the vinylphosphine. Furthermore, the reaction was highly selective for the formation of the F-stereoisomer. This chemistry provided a transition metal-free route to the synthesis of these valuable targets. 

SCHEME 4.301 Using calcium and ytterbium compounds as catalysts for hydrophosphination reactions [468]. 

Rather unusual hydrophosphination of alkenes was reported using calcium-catalyzed transformation [131]. The reaction was carried out in benzene at 25-75 °C for 13-36 h and resulted in 78-95% conversion. Anti-Markovnikov addition products were formed with high selectivity. Proposed reaction mechanism involves insertion of the C=C bond into the Ca-P bond followed by metathesis stage (Scheme 8.48). The metathesis stage was found as rate determining for the catalytic cycle. Thus, if an activated alkene is utilized in the reaction insertion a more rapid insertion stage compared to slow metathesis stage results in formation phosphine-terminated polymers [131]. 

Crimmin MR, Barrett AGM, Hill MS, Hitchcock PB, Procopiou PA (2007) Calcium-catalyzed intermolecular hydrophosphination. Organometallics 26 2953-2956... 

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