The coordination chemistry and ethene hydromethoxycarbonylation catalysis with the diphosphine o-C 6 H 4 (CH 2 P t Bu 2 )(CH 2 PPh 2 ) (L 3 ) is reported and the results compared with the analogous chemistry of the symmetrical diphosphines o-C 6 H 4 (CH 2 P t Bu 2 ) 2 (L 1 ) and o-C 6 H 4 (CH 2 PPh 2 ) 2 (L 2 ). Palladium-catalyzed ethene hydromethoxycarbonylation studies under the commercial catalytic conditions are reported. The results obtained using L 1-3 as supporting ligands show that the catalysts derived from L 3 and L 1 have similar activity and selectivity for methyl propanoate (MeP). In addition, the Pd-L 3 catalyst has much greater longevity than the Pd- 6), and [PtCl(CH 3 )(L 3 )] (9). At equilibrium, complex 9 is a 90:1 mixture of geometric isomers 9a (with CH 3 trans to the t Bu 2 P) and 9b (with Cl trans to the t Bu 2 P). The fluxionality of complex 3, detected by 1 H NMR, is interpreted in terms of the conformation of the seven-membered chelate. The complexes [Pt(CH 3 )(PMe 3 )(L 3 )]Cl (10b) and [PtH(PPh 3 )(L 3 )]Cl (12b) are formed as essentially single isomers with CH 3 /H trans to the Ph 2 P group. The palladium complexes [PdCl 2 (L 3 )] ( 13), [PdCl(CH 3 )(L 3 )] (14a/14b), and [PdH(PCy 3 )-(L 3 )]BF 4 (15b) have been made by similar methods to their platinum analogues. The factors controlling the relative isomer stabilities are explored experimentally and computationally. The complexes [PtCl 2 (L 4 )] ( 16) and [PtCl(CH 3 )(L 4 )] (17a/17b) where L 4 = o-C 6 H 4 (CH 2 P n Bu 2 )(CH 2 PPh 2 ) are reported, and the geometric isomers of 17 are almost isoenergetic. The crystal structures of 3, 14a, 15b, and 16 have been determined by X-ray crystallography. DFT calculations on complexes of the type [Pt(X)(Y)(L 3 )] gave only small calculated differences in energy between the geometrical isomers (0-4 kcal mol -1 ), which are consistent with the experimental observations. It is suggested that repulsive intramolecular H 3 3 3 H interactions (between the Pt-CH 3 and PC(CH 3 ) 3 groups) determine which isomer predominates. The reasons for the favorable catalytic properties of the Pd-L 3 catalyst are probed by 13 CO reactions with the model complexes 9a/9b and 14a/14b, and the structures of the resulting acyl complexes are assigned on the basis of 13 C and 31 P NMR and IR spectroscopy. From these studies, it is suggested that the reason for the Pd-L 3 catalyst resembling the Pd-L 1 catalyst in terms of selectivity is that the crucial acyl intermediates are similar.

The following unsymmetrical diphosphines have been prepared: o-C6H4(CH2PtBu2)(PR2) where R = PtBu2 (L3a); PCg (L3b); PPh2 (L3c); P(o-C6H4CH3)2 (L3d); P(o-C6H4OCH3)2 (L3e) and o-C6H4(CH2PCg)(PCg) (L3f) where PCg is 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl. Hydromethoxycarbonylation of ethene under commercially relevant conditions has been investigated in the presence of Pd complexes of each of the ligands L3a–f and the results compared with those obtained with the commercially used o-C6H4(CH2PtBu2)2 (L1a). The Pd complexes of the bulkiest ligands L3a, L3b and L3f are highly active catalysts but the Pd complexes of L3c, L3d and L3e are completely inactive. The crystal structures of the complexes [PtCl2(L1a)] (1a) and [PtCl2(L3a)] (2a) have been determined and show that the crystallographic bite angles and cone angles are greater for L1a than L3a. Solution NMR studies show that the seven-membered chelate in 1a is more rigid than the six-membered chelate in 2a. Treatment of [PtCl(CH3)(cod)] with L3a–f gave [PtCl(CH3)(L3a–f)] as mixtures of 2 isomers 3a–f and 4a–f. The ratio of the products 4:3 ranges from 100:1 to 1:20, the precise proportion is apparently governed by a balance of two competing factors, steric bulk and the antisymbiotic effect. The palladium complexes [PdCl(CH3)(L3b)] (5b/6b) and [PdCl(CH3)(L3c)] (5c/6c) react with labelled 13CO to give the corresponding acyl species [PdCl(13COCH3)(L3b)] (7b/8b) and [PdCl(13COCH3)(L3c)] (7c/8c). Treatment of [PdCl(13COCH3)(L)] with MeOH gave CH3(13)COOMe rapidly when L = L3b but very slowly when L = L3c paralleling the contrasting catalytic activity of the Pd complexes of these two ligands.

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A switch in enantioselectivity is observed when iodine is used as a co-catalyst in the hydrogenation of unfunctionalised enamines. Mechanistic studies implicate a stepwise protonation-hydride reduction pathway.

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SummaryIn the hydrogenation of sluggish unactivated enamine substrates, Rh complexes of electron-deficient phosphines are demonstrated to be far more reactive catalysts than those derived from triphenylphosphine. These operate at low catalyst loadings (down to 0.01 mol %) and are able to reduce tetrasubstituted enamines. The use of the sustainable and environmentally benign solvent (R)-limonene for the reaction is also reported with the amine isolated by acid extraction.

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Activity. -This method uses low catalyst loadings and is suitable for the hydrogenation of tetrasubstituted enamines. -(TIN, S.; FANJUL, T.; CLARKE*, M. L.; Beilstein J. Org. Chem. 11 (2015) 622-627, http://dx.doi.org/10.3762/bjoc.11.70 ; Sch. Chem., Univ. St. Andrews, St. Andrews, Fife KY16 9ST, UK; Eng.) -M. Tismer