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

Abstract

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 PhPH(2) to the appropriate beta-diketones; the acid used (HCl, H(3)PO(4) or H(2)SO(4)) and its concentration affect the rate and selectivity of these condensation reactions. Phosphines 1a and 1b react with [PdCl(2)(NCPh)(2)] to form complexes trans-[PdCl(2)(1a)(2)](2a) and trans-[PdCl(2)(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 [PtCl(2)(cod)], equilibrium mixtures of trans-[PtCl(2)L(2)] and [Pt(2)Cl(4)L(2)](L = or ) are formed in CH(2)Cl(2) solution. Meso/rac mixtures of trans-[MCl(CO)(1a)(2)] M = Ir (6a) or Rh (7a) are formed upon treatment of MCl(3).nH(2)O with an excess of 1a and the anionic cobalt complex [NHEt(3)][CoCl(3)(1a)](9) was isolated from the product formed by CoCl(2).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. 94 degrees is observed; 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 degrees). 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.

Highly Regioselective Rhodium‐Catalysed Hydroformylation of Unsaturated Esters: The First Practical Method for Quaternary Selective Carbonylation

“…[25] In an initially unrelated project, we collaborated with the Pringle group to show the remarkably high activity of phenylphosphatrioxaadamantanes (1 a) in rhodium-catalysed hydroformylation of hex-1-ene. [26] This paper also notes that this easily prepared, air-stable phosphane seemed to hold some advantage in the hydroformylation of methyl atropate in two preliminary reactions under unoptimised conditions. This observation led us to thoroughly investigate the effect of ligands, solvents, temperature, pressure, and substrate in hydroformylation of unsaturated esters, and here we describe how these studies have led to a practical procedure for the synthesis of quaternary substituted aldehydes, and enabled high-yielding a-selective hydroformylation of 1,2-substituted unsaturated esters for the first time.…”

Mononuclear and Heterodinuclear Metal Complexes of Nonsymmetric Ditertiary Phosphanes Derived from R2PCH2OH

“…While a few crystallographic examples of sterically hindered phosphanes containing this cage have been reported in the literature [15][16][17][18][19]21,25] this structure represents Table 1. Selected spectroscopic data for compounds 1-10.…”

“…10 ppm with respect to the phenyl-modified phosphaadamantanes, AdP-Ph, previously reported. [19] Upon second condensation the 31 P resonance for the -PAd group is shifted upfield by ca. 10 ppm to -41.5 ppm (for 3a) and -42.0 ppm (for 3b).…”

“…Given the paucity of palladium complexes with adamantane-based phosphorus ligands documented in the literature [17,19,25] we prepared the palladium(II) complex 4 by bridge cleavage of the known [34] dimeric compound {Pd(κ 2 -C,N-C 16 H 16 N)Br} 2 with 2a in refluxing acetone [Equation (1)]. Compound 4 displayed a singlet at δ(P) 18.7 ppm in the 31 P{ 1 H} NMR spectrum (Table 1) and furthermore was characterised by single-crystal X-ray crystallography.…”

Development of a Ligand Knowledge Base, Part 1: Computational Descriptors for Phosphorus Donor Ligands

“…This map is useful in locating unusual trialkylphosphine ligands (such as the adamantane-derived ligand 61) as being similar to more familiar ligands (notably alkyl and aryl phosphites) in neighboring regions of ligand space. Most interestingly, the rhodium complex of ligand 61 is an active hydroformylation catalyst [37] -not a characteristic of orthodox trialkylphosphines, but commonplace for trialkylphosphites. In addition, the consequences of systematic ligand variations can be visualized (e.g.…”