Backbone-functionalised ruthenium diphosphine complexes for catalytic upgrading of ethanol and methanol to iso-butanol.

Efficient catalysts for Guerbet-type ethanol/methanol upgrading to iso-butanol have been developed via Michael addition of a variety of amines to ruthenium-coordinated dppen (1,1-bis(diphenylphosphino)ethylene). All catalysts produce over 50% iso-butanol yield with >90% selectivity in 2 h with catalyst 1 showing the best activity (74% yield after this time). The selectivity and turnover number approach 100% and 1000 respectively using catalyst 6. The presence of uncoordinated functionalised donor groups in these complexes results in a more stable catalyst compared to unfunctionalised analogues.

Rhenium Complexes Bearing Tridentate and Bidentate Phosphinoamine Ligands in the Production of Biofuel Alcohols via the Guerbet Reaction.

We report a variety of rhenium complexes supported by bidentate and tridentate phosphinoamine ligands and their use in the formation of the advanced biofuel isobutanol from methanol and ethanol. Rhenium pincer complexes 1-3 are effective catalysts for this process, with 2 giving isobutanol in 35% yields, with 97% selectivity in the liquid fraction, over 16 h with catalyst loadings as low as 0.07 mol %. However, these catalysts show poorer overall selectivity, with the formation of a significant amount of carboxylate salt solid byproduct also being observed. Production of the active catalyst 1d has been followed by 31P NMR spectroscopy, and the importance of the presence of base and elevated temperatures to catalyst activation has been established. Complexes supported by diphosphine ligands are inactive for Guerbet chemistry; however, complexes supported by bidentate phosphinoamine ligands show greater selectivity for isobutanol formation over carboxylate salts. The novel complex 7 was able to produce isobutanol in 28% yield over 17 h. The importance of the N-H moiety to the catalytic performance has also been established, giving further weight to the hypothesis that these catalysts operate via a cooperative mechanism.

Manganese Diphosphine and Phosphinoamine Complexes Are Effective Catalysts for the Production of Biofuel Alcohols via the Guerbet Reaction.

We report a variety of manganese-based catalysts containing both chelating diphosphine (bis(diphenylphosphino)methane (dppm: 1, 2, and 7) or 1,2-bis(diphenylphosphino)ethane (dppe: 3)), and mixed-donor phosphinoamine (2-(diphenylphosphino)ethylamine (dppea: 4-6)) ligands for the upgrading of ethanol and methanol to the advanced biofuel isobutanol. These catalysts show moderate selectivity up to 74% along with turnover numbers greater than 100 over 90 h, with catalyst 2 supported by dppm demonstrating superior performance. The positive effect of substituting the ligand backbone was also displayed with a catalyst supported by C-phenyl-substituted dppm (8) having markedly improved performance compared to the parent dppm catalysts. Catalysts supported by the phosphinoamine ligand dppea are also active for the upgrading of ethanol to n-butanol. These results show that so-called PNP-pincer ligands are not a prerequisite for the use of manganese catalysts in Guerbet chemistry and that simple chelates can be used effectively.

Bidentates versus monodentates in asymmetric hydrogenation catalysis: synergic effects on rate and allosteric effects on enantioselectivity.

C 1-Symmetric phosphino/phosphonite ligands are prepared by the reactions of Ph 2P(CH 2) 2P(NMe 2) 2 with ( S)-1,1'-bi-2-naphthol (to give L A ) or ( S)-10,10'-bi-9-phenanthrol (to give L B ). Racemic 10,10'-bi-9-phenanthrol is synthesized in three steps from phenanthrene in 44% overall yield. The complexes [PdCl 2( L A,B )] ( 1a, b), [PtCl 2( L A,B )] ( 2a, b), [Rh(cod)( L A,B )]BF 4 ( 3a, b) and [Rh( L A,B ) 2]BF 4 ( 4a, b) are reported and the crystal structure of 1a has been determined. A (31)P NMR study shows that M, a 1:1 mixture of the monodentates, PMePh 2 and methyl monophosphonite L 1a (based on ( S)-1,1 '-bi-2-naphthol), reacts with 1 equiv of [Rh(cod) 2]BF 4 to give the heteroligand complex [Rh(cod)(PMePh 2)( L 1a )]BF 4 ( 5) and homoligand complexes [Rh(cod)(PMePh 2) 2]BF 4 ( 6) and [Rh(cod)( L 1a ) 2]BF 4 ( 7) in the ratio 2:1:1. The same mixture of 5- 7 is obtained upon mixing the isolated homoligand complexes 6 and 7 although the equilibrium is only established rapidly in the presence of an excess of PMePh 2. The predominant species 5 is a monodentate ligand complex analogue of the chelate 3a. When the mixture of 5- 7 is exposed to 5 atm H 2 for 1 h (the conditions used for catalyst preactivation in the asymmetric hydrogenation studies), the products are identified as the solvento species [Rh(PMePh 2)( L 1a )(S) 2]BF 4 ( 5'), [Rh(S) 2(PMePh 2) 2]BF 4 ( 6') and [Rh(S) 2( L 1a ) 2]BF 4 ( 7') and are formed in the same 2:1:1 ratio. The reaction of M with 0.5 equiv of [Rh(cod) 2]BF 4 gives exclusively the heteroligand complex cis-[Rh(PMePh 2) 2( L 1a ) 2]BF 4 ( 8), an analogue of 4a. The asymmetric hydrogenation of dehydroamino acid derivatives catalyzed by 3a, b is reported, and the enantioselectivities are compared with those obtained with (a) chelate catalysts derived from analogous diphosphonite ligands L 2a and L 2b , (b) catalysts based on methyl monophosphonites L 1a and L 1b , and (c) catalysts derived from mixture M. For the cinnamate and acrylate substrates studied, the catalysts derived from the phosphino/phosphonite bidentates L A,B generally give superior enantioselectivities to the analogous diphosphonites L 2a and L 2b ; these results are rationalized in terms of delta/lambda-chelate conformations and allosteric effects of the substrates. The rate of hydrogenation of acrylate substrate A with heterochelate 3a is significantly faster than with the homochelate analogues [Rh( L 2a )(cod)]BF 4 and [Rh(dppe)(cod)]BF 4. A synergic effect on the rate is also observed with the monodentate analogues: the rate of hydrogenation with the mixture containing predominantly heteroligand complex 5 is faster than with the monophosphine complex 6 or monophosphonite complex 7. Thus the hydrogenation catalysis carried out with M and [Rh(cod) 2]BF 4 is controlled by the dominant and most efficient heteroligand complex 5. In this study, the heterodiphos chelate 3a is shown to be more efficient and gives the opposite sense of optical induction to the heteromonophos analogue 5.

Cyclopropenylidene carbene ligands in palladium C-C coupling catalysis.

A palladium complex supported by a 2,3-diphenylcyclopropenylidene carbene ligand is a highly active and robust catalyst for Heck and Suzuki coupling reactions.

Chiral palladium bis(phosphite)PCP-pincer complexes via ligand C-H activation.

The synthesis of a range of chiral palladium bis(phosphite) pincer complexes has been achieved via C-H activation of the parent ligands and one of the complexes formed shows good activity in the catalytic allylation of aldehydes.

Catalytic conversion of methanol/ethanol to isobutanol--a highly selective route to an advanced biofuel.

Catalysts based on ruthenium diphosphine complexes convert methanol/ethanol mixtures to the advanced biofuel isobutanol, with extremely high selectivity (>99%) at good (>75%) conversion via a Guerbet-type mechanism.

Cyclopropenylidene carbene ligands in palladium catalysed coupling reactions: carbene ligand rotation and application to the Stille reaction.

Reaction of [Pd(PPh(3))(4)] with 1,1-dichloro-2,3-diarylcyclopropenes gives complexes of the type cis-[PdCl(2)(PPh(3))(C(3)(Ar)(2))] (Ar = Ph 5, Mes 6). Reaction of [Pd(dba)(2)] with 1,1-dichloro-2,3-diarylcyclopropenes in benzene gave the corresponding binuclear palladium complexes trans-[PdCl(2)(C(3)(Ar)(2))](2) (Ar = Ph 7, p-(OMe)C(6)H(4)8, p-(F)C(6)H(4)9). Alternatively, when the reactions were performed in acetonitrile, the complexes trans-[PdCl(2)(NCMe)(C(3)(Ar)(2))] (Ar = Ph 10, p-(OMe)C(6)H(4)11 and p-(F)C(6)H(4)) 12) were isolated. Addition of phosphine ligands to the binuclear palladium complex 7 or acetonitrile adducts 11 and 12 gave complexes of the type cis-[PdCl(2)(PR(3))(C(3)(Ar)(2))] (Ar = Ph, R = Cy 13, Ar = p-(OMe)C(6)H(4), R = Ph 14, Ar = p-(F)C(6)H(4), R = Ph 15). Crystal structures of complexes 6·3.25CHCl(3), 10, 11·H(2)O and 12-15 are reported. DFT calculations of complexes 10-12 indicate the barrier to rotation about the carbene-palladium bond is very low, suggesting limited double bond character in these species. Complexes 5-9 were tested for catalytic activity in C-C coupling (Mizoroki-Heck, Suzuki-Miyaura and, for the first time, Stille reactions) and C-N coupling (Buchwald-Hartwig amination) showing excellent conversion with moderate to high selectivity.

Chiral triaryl phosphite-based palladacycles and platinacycles: synthesis and application to asymmetric Lewis acid catalysis.

The optically pure monophosphites P(OAr)(BINOLate) (7, where Ar = 2,4-di-tert-butylphenyl) have been prepared by treatment of PCl2(OAr) with R- or S-BINOL. Treatment of [PdCl2(NCMe)2] with 7 gave [PdCl2(7)2] (9) or the binuclear orthometallated complex [Pd2Cl2(7-H)2] (8) depending on the reaction conditions. Bridge cleavage reactions of 8 gave [PdCl(7-H)(L)] with L trans to carbon when L = PPh3 or 7 and cis to carbon when L = N-heterocyclic carbene. Treatment of [PtCl2(NCtBu)2] with 7 gave [PtCl2(7)2] (18) which upon further reaction with PtCl2 furnished a mixture of binuclear [Pt2Cl2(7-H)2] (17) and cis-[PtCl(7-H)(7)] (19). The palladium complexes containing cyclometallated 7 were screened for catalysis of 1,4-conjugate addition of phenylboronic acid to cyclohexen-2-one and the allylation of benzaldehyde with allyltributyltin. Conversions were generally high in each case but enantioselectivities were low (15% e.e. at best). The X-ray crystal structures of 8, 17 and [PdCl(7-H)(NHC)] (10a, where NHC = 1,3-(dimesityl)imidazolidin-2-ylidene) have been determined.

Bulky 4-phosphacyclohexanones: diastereoselective complexations, orthometallations and unprecedented [3.1.1]metallabicycles.

The 4-phosphacyclohexanones, 2,2,6,6-tetramethyl-1-phenyl-4-phosphorinanone (La), 1,2,6-triphenyl-4-phosphorinanone ((Ph)Lb), 1-cyclohexyl-2,6-diphenyl-4-phosphorinanone ((Cy)Lb) and 1-tert-butyl-2,6-diphenyl-4-phosphorinanone ((Bu)Lb) have been made by modifications of literature methods. Phosphines (R)Lb are each formed as mixtures of meso- and rac-diastereoisomers. Isomerically pure rac-(Ph)Lb, rac-(Cy)Lb and meso-(Bu)Lb can be isolated by recrystallisation from MeCN. Heating mixtures of isomers of (R)Lb with TsOH leads to isomerisations to give predominantly the meso-(R)Lb. The complex trans-[PdCl2(La)2] (1) is readily made from [PdCl2(NCPh)2] but the analogous platinum complex 2 has not been detected and instead, cyclometallation at the 3-position (alpha to the ketone) in the phosphacycle occurs to give trans-[PtCl(La)(La-3H)] (3) (where La-3H = La deprotonated at the 3-position) featuring a [3.1.1]metallabicycle as confirmed by X-ray crystallography. The analogous palladabicycle 4 has been detected upon treatment of 1 with Et3N in refluxing toluene. The type of complex formed by (R)Lb depends on which diastereoisomer (meso or rac) is involved. rac-(Ph)Lb (a mixture of R,R- and S,S-enantiomers, labelled alpha and beta) forms trans-[MCl2(rac-(Ph)Lb)2], M = Pd (5) or Pt (6), as mixtures of diastereoisomers (alphaalpha/betabeta and alphabeta forms). The structure of alphaalpha-6 has been determined by X-ray crystallography. Ligand competition experiments monitored by 31P NMR showed that Pd(II) and Pt(II) have a significant preference to bind rac-(Ph)Lb over meso-(Ph)Lb. meso-(Bu)Lb reacts with [PtCl2(NCBu(t))2] under ambient conditions to give the binuclear complex [Pt2Cl2(meso-(Bu)Lb-2'H)2] (7) where orthometallation has occurred on one of the exocyclic phenyl substituents as confirmed by X-ray crystallography. rac-(Bu)Lb reacts with [PtCl2(NCBu(t))2] to give a mononuclear cyclometallated species assigned the structure trans-[PtCl(rac-(Bu)Lb-2'H)((Bu)Lb)] (8) on the basis of its 31P NMR spectrum. rac-(Cy)Lb reacts with [PtCl2(NCBu(t))2] in refluxing toluene to give trans-[PtCl2(rac-(Cy)Lb)2] (9) and the crystal structure of alphabeta-9 has been determined.

Special effects of ortho-isopropylphenyl groups. Diastereoisomerism in platinum(II) and palladium(II) complexes of helically chiral PAr3 ligands.

The coordination chemistry of the four phosphines, P{C6H3(o-CH3)(p-Z)}3 where Z = H (1a) or OMe (1b) and P{C6H3(o-CHMe2)(p-Z)}3 Z = H (1c) or OMe (1d) with platinum(II) and palladium(II) is reported. Mononuclear complexes trans-[PdCl2L2](L = 1a,b) and trans-[PtCl2L2](L = 1a-c) have been prepared and the crystal structures of trans-[PdCl2(1b)2] and trans-[PtCl2(1c)2] as their dichloromethane solvates have been determined. The structures show that in these complexes, the ligands adopt g+ g+ a conformations. Examination of the Cambridge Structural Database confirms this to be one of only two conformer types that tri-o-tolylphosphines adopt and the only viable conformer in 4 and 6 coordinate complexes. The binuclear complexes trans-[Pd2Cl4L2](L = 1c,d) are formed even when an excess of the bulky 1c,d is used in the synthesis and the crystal structure of trans-[Pd2Cl4(1c)2] as its chloroform solvate is reported. Reaction of [PtCl2(NCBu(t))2] with 1b-d in refluxing toluene gave the cycloplatinated species [Pt2Cl2(L - H)2] where L - H is phosphine 1b-d deprotonated at one of the ortho-methyl carbon atoms. Variable temperature 31P and 1H NMR spectroscopy reveals that all the complexes reported are fluxional. The processes are analysed in terms of restricted P-C and P-M rotations that give rise to diastereoisomeric rotamers because of the helically chiral orientations of the aryl substituents. For the complexes of the bulky ligands 1c,d, rotation about the P-C bond is slow on the NMR timescale even up to 75 degrees C. The crystal structure of the cyclometallated complex [Pt2Cl2(1d - H)2] has been determined.

Bulky triarylarsines are effective ligands for palladium catalysed Heck olefination.

The four arsines, As{C6H3(o-CH3)(p-Z)}3{Z=H (2a) or OMe (2b)} and As{C6H3(o-CHMe2)(p-Z)}3{Z=H (2c) or OMe (2d)} react with [PdCl2(NCPh)2] or [PtCl2(NCBu(t))2] to give trans-[MCl2L2] or trans-[M2Cl2(mu-Cl)2L2]. The crystal structures of trans-[PdCl2(2a)2] and [PtCl2(2d)2] have been determined, the latter as its dichloromethane solvate. The structures show that in these complexes, the ligands adopt gga type conformations as do all analogous tri-o-tolyl- and tri-o-isopropylphenylphosphines in square-planar and octahedral complexes. The variable-temperature NMR behaviour of the complexes shows that they are fluxional due to restricted As-C bond rotation. The rate of the fluxionality is more rapid than in the analogous phosphine complexes and this is associated with longer As-C and As-M bonds allowing more free movement. The catalytic activity of the palladium complexes of the arsines and their phosphine analogues for the reaction of 4-bromoacetophenone and n-butyl acrylate has been screened. The results show that the arsines are generally superior to the phosphines as ligands for this catalysis. Tri(o-isopropylphenyl)phosphine and tri(o-isopropylphenyl)arsine are superior to tri-o-tolylphosphine as ligands for this Heck reaction and a p-methoxy substituent improves the arsine catalyst but not the phosphine catalyst. The phosphine catalysts are superior to the arsine catalysts for the reaction of 4-chloroacetophenone and n-butyl acrylate. These observations are discussed in the context of ligand stereoelectronic effects, as measured by the Tolman electronic parameter, nuCO of the [NiL(CO)3]{L=AsAr3 or PAr3}.

The electron-poor phosphines P{C6H3(CF3)2-3,5}3 and P(C6F5)3 do not mimic phosphites as ligands for hydroformylation. A comparison of the coordination chemistry of P{C6H3(CF3)2-3,5}3 and P(C6F5)3 and the unexpectedly low hydroformylation activity of their rhodium complexes.

The fluoroaryl phosphines P{C6H3(CF3)2-3,5}3 (La) and P(C6F5)3 (Lb) form the complexes trans-[MCl2(La)2] and trans-[MCl2(Lb)2](M = Pd or Pt) which have been isolated and fully characterised. 31P NMR studies of competition experiments show that the stability of trans-[PdCl2L2] is in the order L = Lb< La