Alkene Metatheses in Transition Metal Coordination Spheres: Effect of Ring Size and Substitution on the Efficiencies of Macrocyclizations That Join<i>trans</i>Positions of Square-Planar Platinum Complexes

Abstract

Reactions of KPPh2 and Br(CH2)nCHCH2 give the phosphines PPh2(CH2)nCHCH2 (n = a, 4; b, 6; c, 8; d, 9; 95−41%), which are combined with the platinum tetrahydrothiophene complex [Pt(μ-Cl)(C6F5)(S(CH2CH2−)2)]2 to give trans-(Cl)(C6F5)Pt(PPh2(CH2)nCHCH2)2 (3a−d, 71−54%). When treated with Grubbs' catalyst, ring-closing alkene metatheses occur to give 13- to 23-membered macrocycles with trans-spanning diphosphine ligands (96−85%, including some dimeric or oligomeric byproducts). The mixtures of CC isomers are hydrogenated (1 atm, 10% Pd/C) to give (6a−d), which are isolated in 72−50% yields. Comparable results are obtained with (1) the second-generation dihydroimidazolylidene Grubbs' catalyst and (2) a series of compounds derived from the dimethylated phosphine Ph2P(CH2)2C(CH3)2(CH2)3CHCH2, in turn prepared by sequential reactions of BrCH2CH2C(CH3)2CH2CH2Br with BrMgCH2CHCH2/Li2CuCl4 and KPPh2. The crystal structures of 6a−d are analyzed, but no special features that would promote intramolecular macrocyclizations are noted. A reaction of [Pt(μ-Cl)(C6F5)(S(CH2CH2−)2)]2 and the diphosphine Ph2P(CH2)14PPh2 leads to a multitude of products and little 6b (<15%).