Ligand Bite Angle Effects in Metal-catalyzed C-C Bond Formation

Chalcogen−Chalcogen Bonds in Edge-Sharing Square-Planar d8 Complexes. Are They Possible?

A theoretical study of the formation of X-X bonds in complexes with the general formula [M(2)(mu-X)(2)L(4)] (M = group 10 and X = group 16 elements) having d(8) transition-metal atoms is presented. The existence of two energy minima for some complexes, with short and long X-X distances, is shown by density functional theory calculations, and the factors responsible for it are analyzed, including a strong influence of the nature of the metals and ligands on the relative stability of the two isomers. The influence of the bite angle of chelating terminal ligands and the nature of the donor atom on the relative stabilities of the two isomers are also discussed.

Chiral biphenyl diphosphines for asymmetric catalysis: stereoelectronic design and industrial perspectives

Two original chiral diphosphines, SYNPHOS and DIFLUORPHOS, have been synthesized on multigram scales. Their steric and electronic profiles have been established in comparison with the commonly used 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl and 6,6'-dimethoxy-2,2'-bis(diphenylphosphino)-1,1'-biphenyl ligands. A screening study of the four ligands in Ru(II)-catalyzed asymmetric hydrogenation of prochiral ketones and olefins has been performed. It revealed that the stereoelectronic features of the ligand and the substrate deeply influenced the enantioselectivities obtained in asymmetric hydrogenation, SYNPHOS and DIFLUORPHOS being fully complementary in terms of enantioselectivity for this reaction.

Supraphos: A Supramolecular Strategy To Prepare Bidentate Ligands

We report a new strategy for the preparation of chelating bidentate ligands, which involves just the mixing of two monodentate ligands functionalized with complementary binding sites. In the current example, the assembly process is based on selective metal-ligand interactions, using phosphite zinc(II) porphyrins 1-6 and the nitrogen donor ligands b-i. From only 16 monodentate ligands, a library of 60 palladium catalysts based on 48 bidentate ligand assemblies has been prepared. The relatively small catalyst library gave a large variety in the selectivity of the alkylation of rac-1,3-diphenyl-2-propenyl acetate. Importantly, small variations in the building blocks lead to large differences in the enantioselectivity imposed by the catalyst (up to 97% ee).

Tandem or Sequential Coupling−IMDA Cycloaddition Approach to Highly Fused Polycarbocycles

The Diels-Alder cycloadduct made from 3,5-dibromo-2-pyrone and 2-bromo-styrene was successfully transformed stereoselectively into a series of novel benzotetracycles via a tandem or sequential Pd-catalyzed coupling-intramolecular Diels-Alder (IMDA) cycloaddition. The resulting polycarbocycles can be readily converted into a bicyclo[3.3.1]nonane system upon ozonolysis of the internal double bond.

Rhodium-catalyzed 1,4-addition of arylboronic acids to alpha,beta-unsaturated carbonyl compounds: large accelerating effects of bases and ligands

The effects of ligands and bases in the rhodium(I)-catalyzed 1,4-addition of arylboronic acids to alpha,beta-unsaturated carbonyl compounds were reinvestigated to carry out the reaction under mild conditions. Rhodium(I) complexes possessing a 1,5-cyclooctadiene (cod) and a hydroxo ligand such as [RhOH(cod)](2) exhibited excellent catalyst activities compared to those of the corresponding rhodium-acac or -chloro complexes and their phosphine derivatives. The reaction was further accelerated in the presence of KOH, thus allowing the 1,4-addition even at 0 degrees C. A cationic rhodium(I)-(R)-binap complex, [Rh(R-binap)(nbd)]BF(4), catalyzed the reaction at 25-50 degrees C in the presence of Et(3)N with high enantioselectivities of up to 99% ee for alpha,beta-unsaturated ketones, 92% for aldehydes, 94% for esters, and 92% for amides.

Alkylation of phosphine boranes by phase-transfer catalysis

The alkylation of phosphine boranes with various electrophiles proceeds with good to excellent yields in a biphasic solution in the presence of tetrabutylammonium bromide as a phase-transfer catalyst. [reaction: see text]

Alcoholysis of acylpalladium(II) complexes relevant to the alternating copolymerization of ethene and carbon monoxide and the alkoxycarbonylation of alkenes: the importance of Cis-coordinating phosphines

The mechanism and kinetics of the solvolysis of complexes of the type [(L-L)Pd(C(O)CH(3))(S)](+)[CF(3)SO(3)](-) (L-L = diphosphine ligand, S = solvent, CO, or donor atom in the ligand backbone) was studied by NMR and UV-vis spectroscopy with the use of the ligands a-j: SPANphos (a), dtbpf (b), Xantphos (c), dippf (d), DPEphos (e), dtbpx (f), dppf (g), dppp (h), calix-6-diphosphite (j). Acetyl palladium complexes containing trans-coordinating ligands that resist cis coordination (SPANphos, dtbpf) showed no methanolysis. Trans complexes that can undergo isomerization to the cis analogue (Xantphos, dippf, DPEphos) showed methanolyis of the acyl group at a moderate rate. The reaction of [trans-(DPEphos)Pd(C(O)CH(3))](+)[CF(3)SO(3)](-) (2e) with methanol shows a large negative entropy of activation. Cis complexes underwent competing decarbonylation and methanolysis with the exception of 2j, [cis-(calix-diphosphite)Pd(C(O)CH(3))(CD(3)OD)](+)[CF(3)SO(3)](-). The calix-6-diphosphite complex showed a large positive entropy of activation. It is concluded that ester elimination from acylpalladium complexes with alcohols requires cis geometry of the acyl group and coordinating alcohol. The reductive elimination of methyl acetate is described as a migratory elimination or a 1,2-shift of the alkoxy group from palladium to the acyl carbon atom. Cis complexes with bulky ligands such as dtbpx undergo an extremely fast methanolysis. An increasing steric bulk of the ligand favors the formation of methyl propanoate relative to the insertion of ethene leading to formation of oligomers or polymers in the catalytic reaction of ethene, carbon monoxide, and methanol.

Ruthenium dihydrogen complexes with wide bite angle diphosphines

The wide bite angle diphosphines homoxantphos (10,11-dihydro-4,5,-bis(diphenylphosphino)dibenzo[b,f]oxepine), sixantphos (4,6-bis(diphenylphosphino)-10,10-dimethylphenoxasilin), and thixantphos (2,8-dimethyl-4,6-bis(diphenylphosphino)phenoxathiin) were used to prepare cis[MH(2)(diphosphine)(2)] complexes (1a-f) by reaction of [Ru(cod)(cot)] (cod = cyclo-octa-1,5-diene, cot = cyclo-octa-1,3,5-triene) with 2 equiv of the diphosphine under dihydrogen pressure. The electronic properties of the thixantphos ligand were varied. Complexes 1a-f can be protonated with HBF(4) or CF(3)COOH to yield hydrido(dihydrogen) complexes cis[MH(H(2))(diphosphine)(2)](+) (2a-f), which were characterized by VT (variable temperature) NMR and T(1) measurements. These complexes show fast hydrogen atom exchange between the eta(2)-H(2) and the terminal hydride at all temperatures studied. They are thermally unstable toward dihydrogen loss yielding the cationic monohydride complexes cis[MH(diphosphine)(2)](+) (3a-f). Coordination of the eta(2)-H(2) is dominated by sigma --> d donation, and hence, the H-H distance is hardly influenced by the electronic properties of the ligands.

Understanding the correlation of structure and selectivity in the chiral-phosphoramide-catalyzed enantioselective allylation reactions: solution and solid-state structural studies of bisphosphoramide.SnCl(4) complexes

Complexation of bisphosphoramides, linked by various length methylene tethers, with tin tetrachloride was studied by both solution NMR spectroscopy and single-crystal X-ray crystallography. The formation of cis-configured, octahedral 1/1 bisphosphoramide.SnCl(4) complexes was supported by both crystallographic and solution NMR studies. In addition, the formation of such complexes was shown to be highly dependent on the tether length and solution concentration. Single-crystal X-ray analysis of bisphosphoramide.SnCl(4) complexes also provided detailed information on the stereochemical environment relevant to the enantioselectivity of asymmetric allylation reactions.

Internal olefins to linear amines

The selective synthesis of linear amines from internal olefins or olefin mixtures was achieved through a catalytic one-pot reaction consisting of an initial olefin isomerization followed by hydroformylation and reductive amination. Key to the success is the use of specially designed phosphine ligands in the presence of rhodium catalysts. This reaction constitutes an economically attractive and environmentally favorable synthesis of linear aliphatic amines.

Palladium-catalyzed cross-coupling reaction of bis(pinacolato)diboron with 1-alkenyl halides or triflates: convenient synthesis of unsymmetrical 1,3-dienes via the borylation-coupling sequence

The synthesis of 1-alkenylboronic acid pinacol esters via a palladium-catalyzed cross-coupling reaction of bis(pinacolato)diboron (pin(2)B(2), pin = Me(4)C(2)O(2)) with 1-alkenyl halides or triflates was carried out in toluene at 50 degrees C in the presence of KOPh (1.5 equiv) and PdCl(2)(PPh(3))(2)-2Ph(3)P (3 mol %). The borylation of acyclic and cyclic 1-alkenyl bromides and triflates was achieved in high yields with complete retention of configuration of the double bonds. The method was applied to the one-pot synthesis of unsymmetrical 1,3-dienes via the borylation-coupling sequence.