Palladium-catalyzed hydrophosphinylation

Scheme 5-22 Stoichiometric reactions relevant to the proposed mechanism for palladium-catalyzed hydrophosphinylation of alkynes...

Very interestingly, the regioselectivity of the palladium-catalyzed hydrophosphinylation can be switched by addition of a tiny quantity of diphenylphosphinic acid or other acidic compounds (Scheme 39) [35]. Thus, the linear product (19 8), which is the major product under normal conditions (i.e., without the phosphinic acid), becomes the minor regioisomer, and instead, the branched isomer (19a) is formed as the prevailing product. Furthermore, the addition of the phosphinic acid enhances the catalytic activity. Note that the quantity of the palladium catalyst is 5 mol% relative to the reactant, and that the addition of only 1 mol% (relative to the reactant) of the phosphinic acid results in a substantial change in the regioselectivity. New palladium species, which are much more active in the catalysis, appear to be generated upon addition of the phosphinic acid or other acidic additives. 

Other analogues of PALA were also synthesized. A chemical sequence using isoprenylphosphinate 121 as common precursor to three targeted structures 108-110 has been elaborated. Phosphinic acid precursor 120 was obtained by palladium-catalyzed hydrophosphinylation from isoprene and hypophosphorous acid... 

Deprele S, Montchamp J-L (2002) Palladium-catalyzed hydrophosphinylation of alkenes and alkynes. J Am Chem Soc 124 9386-9387... 

Scheme5-24 Eq. (1) Proposed mechanism for Eq. (2) Stoichiometric reactions relevant to the phosphinic acid-modified palladium-catalyzed proposed mechanism hydrophosphinylation of alkynes.

Han, L.-B., Choi, N., and Tanaka, M., Oxidaative addition of HP(0)Ph2 to platinum(O) and palladium(O) complexes and palladium-catalyzed regio- and stereoselective hydrophosphinylation of alkynes, Organometallics, 15, 3259,... 

Han L-B, Zhao C-Q, Onozawa S-y, Goto M, Tanaka M. Retention of configuration on the oxidative addition of P—H bond to platinum(O) complexes the first straightforward synthesis of enantiomerically pure P-chiral alkenylphosphinates via palladium-catalyzed stereospecific hydrophosphinylation of alkynes. J. Am. Chem. Soc. 2002 124 3842-3843. 

---