Selective formation of a C3-symmetric P-stereogenic tris(phosphine) via platinum-catalyzed asymmetric alkylation of a tris(secondary phosphine)

Alkylation of MeC(CH2PHPh)31 with 2-cyanobenzyl bromide and NaOSiMe3 using the catalyst precursor Pt((R,R)-Me-DuPhos)(Ph)(Cl) gave a mixture of tris(phosphines) enriched in C3-MeC(CH2PPh(CH2(o-C6H4CN))33; Cu-catalyzed phosphination yielded C1-enriched 3. Oxidation of 3 by sulfur or H2O2 formed MeC(CH2P(E)Ph(CH2(o-C6H4CN))3 (E = S 4; E = O 5). Repeated recrystallization gave a low yield of (R,R,R)-4, which was crystallographically characterized. Tris(phosphine) 3 was formed with a diastereomeric ratio (dr = C3/C1) of 2.1(2) and enantiomeric ratios of 54(10) and 3.8(7) for C3-3 and C1-3 respectively, which showed that the selectivity of the triple alkylation was not the same at each site (substrate control).

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