Phenylphosphatrioxa-adamantanes: bulky, robust, electron-poor ligands that give very efficient rhodium(I) hydroformylation catalysts

The cage phosphines 1,3,5,7-tetramethyl-6-phenyl-2,4,8-trioxa-6-phosphaadamantane (1a) and 1,3,5,7-tetraethyl-6-phenyl-2,4,8,trioxa-6-phosphaadamantane (1b) have been made by the acid catalysed addition of PhPH2 to the appropriate beta-diketones; the acid used (HCl, H3PO4 or H2SO4) and its concentration affect the rate and selectivity of these condensation reactions. Phosphines 1a and 1b react with [PdCl2(NCPh)(2)] to form complexes trans-[PdCl2(1a)(2)] (2a) and trans-[PdCl2(1b)(2)] (2b) as mixtures of rac and meso diastereoisomers. The platinum(II) chemistry is more complicated and when 1a or 1b is added to [PtCl2(cod)], equilibrium mixtures of trans-[PtCl2L2] and [Pt2Cl4L2] (L = 1a or 1b) are formed in CH2Cl2 solution. Meso/rac mixtures of trans-[MCl(CO)(1a)(2)] M = Ir (6a) or Rh (7a) are formed upon treatment of MCl3.nH(2)O with an excess of 1a and the anionic cobalt complex [NHEt3][CoCl3(1a)] (9) was isolated from the product formed by CoCl2.6H(2)O and 1a. The nu(CO) values from the IR spectra of 6a and 7a suggest that 1a resembles a phosphonite in its bonding to Rh and Ir. Crystal structures of meso-2a, meso-2b, rac-6a and 9 are reported and in each case a small intracage C-P-C angle of ca. 94degrees is observed and this may partly explain the bonding characteristics of ligands 1a and 1b. The cone angles for 1a and 1b are similar and large (ca. 200). Rhodium complexes of ligands 1a and 1b are hydroformylation catalysts with similarly high activity to catalysts derived from phosphites. The catalysts derived from 1a and 1b gave unusually low linear selectivity in the hydroformylation of hexenes. This feature has been further exploited in quaternary-selective hydroformylations of unsaturated esters; catalysts derived from 1a give better yields and regioselectivities than any previously reported catalyst.