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Influence of the size of the phosphine ligand on primary-secondary alkyl isomerization equilibria with (Me2NCS2)Pt(PR3)(alkyl) complexes

1993; Elsevier BV; Volume: 452; Issue: 1-2 Linguagem: Inglês

10.1016/0022-328x(93)83201-6

1872-8561

Daniel L. Reger, Yan Ding, David G. Garza, Lukasz Lebioda,

Synthetic Organic Chemistry Methods

The reactions of (Me2NCS2)Pt(PR3)Cl with Grignard or alkyl lithium reagents lead to the preparations of (Me2NCS2)Pt(PR3) (alkyl) (R  Me, Et, Ph, Cy, o-CH3C6H4, C6F5) complexes. The reactions are successful at or below room temperature for R  Me, Et, Ph, but reactions with the starting chloride containing phosphines with larger cone angles need to be heated in refluxing Et2O or THF. The alkylplatinum complexes are extremely stable, and can be heated in solution to 100°C without decomposition. At 120°C, the alkyl ligand undergoes a slow isomerization reaction that leads to a mixture of the linear and branched isomers. For example, heating either (Me2NCS2)Pt(PMe3)(η1-CH2CH2CH2CH3) or (Me2NCS2)Pt(PMe3)(η1-CH(CH3)CH2CH3), in xylene produces a 9/1 mixture, respectively, of the two isomers. Similar equilibrium mixtures are observed for the analogous PEt3 and PPh3 complexes and also for complexes of all three ligands in which the alkyl ligand is a propyl substituent. The bulky phosphines where R is Cy and o-CH3C6H4 shift the equilibria to favor the linear isomers, but some of the branched isomer is still observed at equilibrium. The solid state structure of (Me2NCS2)Pt(PCy3)(sec-butyl) has been determined crystallographically. Crystal data: triclinic, P1, a  9.288(3) Å, b  19.538(5) Å, c  8.854(3) Å, α  99.76(2)°, β  91.98(2)°, γ  103.02(2)°, V  1538 Å3, Z  2, RF  5.2% and RwF  6.4%. The steric influence of the bulky phosphine causes a substantial increase in the P-Pt-C(butyl) angle when compared to the structures of analogous complexes containing phosphine ligands with small cone angles.