| Kagan's method for obtaining selectivity information from racemic catalysts was applied to the Pt-catalyzed asymmetric benzylation of secondary bis(phosphines) and also expanded to account for cases of substrate control and substrate dissociation. The catalysts Pt((+/-)-BenzP*)(Ph)(Cl) and Pt((R,R)-BenzP*)(Ph)(Cl) were synthesized and screened in a set of reactions with the substrate PhHP(CH2)3PHPh, a variety of benzyl halides (YnC6H5-n)CH2X (Y = alkyl, -OMe, -Br, -CF3, -F, -CN, -Ph; X = -Cl, -Br), and NaOSiMe 3 as a base. C6H5CH2Br was found to be the most selective alkylating agent for this reaction, and the results were compared to those obtained by Brian Anderson using Pt((R,R)-Me-DuPhos)(Ph)(Cl) as catalyst. Pt((R,R)-BenzP*)(Ph)(Cl) (dr = 5.5 +/- 0.5, er = 27 +/- 13 ) was predicted to be a more selective catalyst than Pt((R,R)-Me-DuPhos)(Ph)(Cl), but some substrate dissociation was observed (positive half-alkylation test) due to ligand steric effects, and substrate control with negative cooperativity was detected by comparison with the same reaction utilizing Pt((+/-)-BenzP*)(Ph)(Cl) (dr 4.4 +/- 0.4, predicted x = 0.92, er = 120). Lithium-mediated P-phenyl bond cleavage of Ph2PCH2C(CH3)2CH2PPh2 gave the gem-dimethyl secondary bis(phosphine) substrate PhHPCH2C(CH3)2CH2PHPh. Asymmetric alkylation of PhHPCH2C(CH3)2CH2PHPh using the catalysts Pt((+/-)-BenzP*)(Ph)(Cl) (dr = 2.0, predicted x = 0.80, er = 16) and Pt((R,R)-BenzP*)(Ph)(Cl) (dr = 2.0, er = 2) confirmed that the Thorpe-Ingold effect was successful in preventing substrate dissociation (negative half alkylation test). However, the added steric demand of the two methyl groups exacerbated the negative cooperativity observed previously. |
