Intermolecular alkene and alkyne hydroacylation with beta-S-substituted aldehydes: mechanistic insight into the role of a hemilabile P-O-P ligand.

A straightforward to assemble catalytic system for the intermolecular hydroacylation reaction of beta-S-substituted aldehydes with activated and unactivated alkenes and alkynes is reported. These catalysts promote the hydroacylation reaction between beta-S-substituted aldehydes and challenging substrates, such as internal alkynes and 1-octene. The catalysts are based upon [Rh(cod)(DPEphos)][ClO(4)] (DPEphos=bis(2-diphenylphosphinophenyl)ether, cod=cyclooctadiene) and were designed to make use of the hemilabile capabilities of the DPEphos ligand to stabilise key acyl-hydrido intermediates against reductive decarbonylation, which results in catalyst death. Studies on the stoichiometric addition of aldehyde (either ortho-HCOCH(2)CH(2)SMe or ortho-HCOC(6)H(4)SMe) and methylacrylate to precursor acetone complexes [Rh(acetone)(2)(DPEphos)][X] [X=closo-CB(11)H(6)Cl(6) or [BAr(F) (4)] (Ar(F)=3,5-(CF(3))(2)C(6)H(3))] reveal the role of the hemilabile DPEphos ligand. The crystal structure of [Rh(acetone)(2)(DPEphos)][X] shows a cis-coordinated diphosphine ligand with the oxygen atom of the DPEphos distal from the rhodium. Addition of aldehyde forms the acyl hydride complexes [Rh(DPEphos)(COCH(2)CH(2)SMe)H][X] or [Rh(DPEphos)(COC(6)H(4)SMe)H][X], which have a trans-spanning DPEphos ligand and a coordinated ether group. Compared to analogous complexes prepared with dppe (dppe=1,2-bis(diphenylphosphino)ethane), these DPEphos complexes show significantly increased resistance towards reductive decarbonylation. The crystal structure of the reductive decarbonylation product [Rh(CO)(DPEphos)(EtSMe)][closo-CB(11)H(6)I(6)] is reported. Addition of alkene (methylacrylate) to the acyl-hydrido complexes forms the final complexes [Rh(DPEphos)(eta(1)-MeSC(2)H(4)-eta(1)-COC(2)H(4)CO(2)Me)][X] and [Rh(DPEphos)(eta(1)-MeSC(6)H(4)-eta(1)-COC(2)H(4)CO(2)Me)][X], which have been identified spectroscopically and by ESIMS/MS. Intermediate species in this transformation have been observed and tentatively characterised as the alkyl-acyl complexes [Rh(CH(2)CH(2)CO(2)Me)(COC(2)H(4)SMe)(DPEphos)][X] and [Rh(CH(2)CH(2)CO(2)Me)(COC(6)H(4)SMe)(DPEphos)][X]. In these complexes, the DPEphos ligand is now cis chelating. A model for the (unobserved) transient alkene complex that would result from addition of alkene to the acyl-hydrido complexes comes from formation of the MeCN adducts [Rh(DPEphos)(MeSC(2)H(4)CO)H(MeCN)][X] and [Rh(DPEphos)(MeSC(6)H(4)CO)H(MeCN)][X]. Changing the ligand from DPEphos to one with a CH(2) linkage, [Ph(2)P(C(6)H(4))](2)CH(2), gave only decomposition on addition of aldehyde to the acetone precursor, which demonstrated the importance of the hemiabile ether group in DPEphos. With [Ph(2)P(C(6)H(4))](2)S, the sulfur atom has the opposite effect and binds too strongly to the metal centre to allow access to productive acetone intermediates.

Intramolecular alkyl phosphine dehydrogenation in cationic rhodium complexes of tris(cyclopentylphosphine).

[Rh(nbd)(PCyp(3))(2)][BAr(F) (4)] (1) [nbd = norbornadiene, Ar(F) = C(6)H(3)(CF(3))(2), PCyp(3) = tris(cyclopentylphosphine)] spontaneously undergoes dehydrogenation of each PCyp(3) ligand in CH(2)Cl(2) solution to form an equilibrium mixture of cis-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 a) and trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 b), which have hybrid phosphine-alkene ligands. In this reaction nbd acts as a sequential acceptor of hydrogen to eventually give norbornane. Complex 2 b is distorted in the solid-state away from square planar. DFT calculations have been used to rationalise this distortion. Addition of H(2) to 2 a/b hydrogenates the phosphine-alkene ligand and forms the bisdihydrogen/dihydride complex [Rh(PCyp(3))(2)(H)(2)(eta(2)-H(2))(2)][BAr(F) (4)] (5) which has been identified spectroscopically. Addition of the hydrogen acceptor tert-butylethene (tbe) to 5 eventually regenerates 2 a/b, passing through an intermediate which has undergone dehydrogenation of only one PCyp(3) ligand, which can be trapped by addition of MeCN to form trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(PCyp(3))(NCMe)][BAr(F) (4)] (6). Dehydrogenation of a PCyp(3) ligand also occurs on addition of Na[BAr(F) (4)] to [RhCl(nbd)(PCyp(3))] in presence of arene (benzene, fluorobenzene) to give [Rh(eta(6)-C(6)H(5)X){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (7: X = F, 8: X = H). The related complex [Rh(nbd){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] 9 is also reported. Rapid ( approximately 5 minutes) acceptorless dehydrogenation occurs on treatment of [RhCl(dppe)(PCyp(3))] with Na[BAr(F) (4)] to give [Rh(dppe){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (10), which reacts with H(2) to afford the dihydride/dihydrogen complex [Rh(dppe)(PCyp(3))(H)(2)(eta(2)-H(2))][BAr(F) (4)] (11). Competition experiments using the new mixed alkyl phosphine ligand PCy(2)(Cyp) show that [RhCl(nbd){PCy(2)(Cyp)}] undergoes dehydrogenation exclusively at the cyclopentyl group to give [Rh(eta(6)-C(6)H(5)X){PCy(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (17: X = F, 18: X = H). The underlying reasons behind this preference have been probed using DFT calculations. All the complexes have been characterised by multinuclear NMR spectroscopy, and for 2 a/b, 4, 6, 7, 8, 9 and 17 also by single crystal X-ray diffraction.

[Ir(PPh3)2(H)2(ClCH2CH2Cl)][BArF4]: a well characterised transition metal dichloroethane complex.

Reaction of [Ir(PPh(3))(2)(COD)][BAr(F)(4)] with H(2) in dichloroethane solution results in [Ir(PPh(3))(2)(H)(2)(ClCH(2)CH(2)Cl)][BAr(F)(4)], which has been fully characterised by X-ray crystallography, NMR spectroscopy and ESI-MS. Its activity towards alkene hydrogenation has been compared with analogous CH(2)Cl(2) complexes.

The role of halogenated carborane monoanions in olefin hydrogenation catalysed by cationic iridium phosphine complexes.

Iridium hydridophosphine complexes of general formula [Ir(PR3)2H2(anion)](PR3= PPh3, PMe2Ph; anion =[1-closo-CB(11)H(6)Cl(6)]-, [1-closo-CB(11)H(6)I(6)]-, [BAr(F)4]-) have been prepared by hydrogenation of cyclooctadiene precursor complexes. Solid-state structures of selected examples of these complexes reveal intimate contacts between the carborane anion and cation, with the anion binding through two lower-hemisphere halogen ligands. In CD2Cl2 solution the very weakly coordinating anions [1-closo-CB(11)H(6)Cl(6)]- and [BAr(F)4]- are suggested to favour the formation of solvent complexes such as [Ir(PR3)2H2(solvent)n][anion], while the [1-closo-CB(11)H(6)I(6)]- anion forms a tightly bound complex with the cationic iridium fragment. Calculated DeltaG values for anion reorganisation in d8-toluene reflect this difference in interaction between the anions and cation. With the bulky anion [1-closo-CB(11)Me(5)I(6)]- different complexes are formed: Ir(PPh3)H2(1-closo-HCB(11)Me(5)I(6)) and [(PPh3)3Ir(H2)H2][1-closo-HCB(11)Me(5)I(6)] which have been characterised spectroscopically. Diffusion measurements in CD2Cl2 are also consistent with larger, solvent coordinated, complexes for the more weakly coordinating anions and a tighter interaction between anion and cation for [1-closo-CB(11)H(6)I(6)]-. All the complexes show some ion-paring in solution. Comparison with data previously reported for the [1-closo-CB(11)H(6)Br(6)]- anion shows that this anion--as expected--fits between [1-closo-CB(11)H(6)Cl(6)]- and [1-closo-CB(11)H(6)I(6)]- in terms of coordinating ability. Although not coordinating, the large [1-closo-CB(11)H(6)Cl(6)]- and [BAr(F)4)]- anions do provide some stabilisation towards the metal centre, as decomposition to the hydride bridged dimer [Ir2(PPh3)4H5]+ is retarded. This is in contrast to the [PF6]- salt where decomposition is immediate. As expected, complexes with the smaller phosphine PMe2Ph form tighter interactions with the carborane anions. These observations on the interaction between anion and cation in solution are reflected in benchmark hydrogenation studies that show a significant attenuation in rate of hydrogenation of cyclohexane on using the [1-closo-CB(11)H(6)I(6)]- anion or complexes with the PMe2Ph phosphine. We also comment on the reusability of the catalysts and their tolerance to water and oxygen impurities. Overall the catalyst with the [1-closo-CB(11)H(6)Br(6)]- anion shows the best combination of rate of hydrogenation, reusability and tolerance to impurities.