Electron-rich Phosphine Research Articles

ChemInform Abstract: The Reactions of Stable Nucleophilic Carbenes with Main Group Compounds

Publisher Summary This chapter discusses various reactions of stable nucleophilic carbenes with main group compounds. Special interest is associated with carbenes that feature the attachment of donor groups to the carbenic carbon as they behave as nucleophiles and, in some instances, can be isolated. Stable nucleophilic carbenes that are capable of forming isolable complexes with a variety of main group species in oxidation states ranging from +1 to +6. The majority of the complexes that have been reported thus far possess 1:1 stoichiometry; however, there are several instances of 2:1 complexation and one example of a 3:1 complex. Although, in principle, it is possible to write double-bonded (carbene)CuEXn canonical forms to describe the interactions between carbenes and main group entities (EXn), in the stable nucleophilic carbene complexes the bonding is predominantly of the donor–acceptor type—namely (carbene)C→EX n . As such, the chemistry of these two-electron donor carbenes bears a strong-resemblance to that of electron-rich phosphines.

Efficient palladium-catalyzed N-arylation of indoles.

[equation--see text] The N-arylation of indoles, including a variety of substituted ones, has been carried out using bulky, electron-rich phosphines as the supporting ligand in combination with Pd(2)(dba)(3). Using this catalyst system, the efficient coupling of indole and a variety of substituted indoles with aryl iodides, bromides, chlorides, and triflates can be achieved.

Highly Active and Selective Catalysts for the Formation of α-Aryl Ketones

Bulky, electron-rich phosphine ligands with a biphenyl backbone, when combined with Pd(OAc)2, give highly active catalysts for the α-arylation of ketones. The ligand 2-methyl-2‘-dicyclohexylphosphinobiphenyl is particularly effective, and with 0.1−1.0 mol % Pd, a large variety of aryl halides and ketones react efficiently and with high selectivity. For two types of substrates, the ligands BINAP and Xantphos are more effective than the biphenyl-based ligands. It is also shown that K3PO4 can be used as the base in these reactions, and that base-sensitive functional groups are better tolerated if this is used instead of NaOtBu or NaHMDS. In some cases, α-aryl ketones can be produced without adding a ligand to the reaction. Although the substrate scope of the ligandless conditions is limited, some combinations react in high yield, and in one case, 100 000 turnovers were obtained. The results of experiments on the Pd-catalyzed arylation of diethyl malonate, cyclic 1,3-diketones, and nitroalkanes are also reported.

P–N bond formation as a route to highly electron rich phosphine ligands

Phosphines containing two N-bound pyrrolidine groups and one alkyl or aryl group are unusually electron rich σ-donor ligands when compared to either tris(N-pyrrolidinyl)phosphine or trialkyl- and triaryl-phosphines.

Reactions of catecholborane with iridium complexes: molecular structure of trans-IrHCl(CO)(Bcat)(PPh3)2

Reactions of certain iridium complexes with catecholborane (HBcat) gave boron-substituent redistribution products ([Bcat2]−, B2cat3) with concomitant formation of a complex mixture of iridium hydrides. While "IrCl(PPh3)(COE)n" generated in situ from [Ir(μ-Cl)(COE)2]2 (COE = cyclooctene) (2) plus 2 equivalents of PPh3 also promoted HBcat degradation, this catalyst system was extremely active and selective for alkene hydroborations. Oxidative addition of HBcat to the metal centre can be the sole reaction pathway, however, with iridium complexes containing electron-rich phosphines. The unsaturated boryl complex IrHCl(Bcat)(PPri3)2 was prepared and isolated in high yield from reaction of HBcat with 2 in the presence of four equivalents of PPri3. Ancillary carbonyl ligands also appear to facilitate oxidative addition of HBcat to an iridium centre. Addition of HBcat to trans-IrCl(CO)(PPh3)2 gives the first structurally characterized metal-carbonyl boryl complex trans-IrHCl(CO)(Bcat)(PPh3)2 (7). Colourless crystals of 7 are triclinic, [Formula: see text] (No.2) with two molecules per unit cell of dimensions a = 12.223(2) Å, b = 12.684(2) Å, c = 14.652(1) Å, α = 70.81(1)°, β = 72.47(1)°, and γ = 71.92(1)°.

Synthesis and structure of the tris(tetrathiafulvalenyl)phosphine, P(TTF)3

An efficient synthesis of P(TTF)3(TTF = tetrathiafulvalene), a unique electron-rich phosphine, is reported together with its complete characterisation by 31P NMR, cyclic voltammetry and an analysis of its single-crystal X-ray structure.

Synthesis and spectroscopic properties of the ‘mixed’ seven-co-ordinate complexes [MI2(CO)3L(L′)][M = Mo or W; L,L′= PPh3, PPh2(C6H11), PPh(C6H11)2, AsPh3, or SbPh3] and rearrangement of selected [MI2(CO)3L(L′)] to [HL][MI3(CO)3L′]. X-Ray crystal structure of [PPh2(C6H11)H][WI3(CO)3(SbPh3)]·Et2O

Reaction of the complexes [MI2(CO)3(NCMe)2](M = Mo or W) with 1 equivalent of L(L = PPh3, AsPh3, or SbPh3) in CH2Cl2 at room temperature followed by 1 equivalent of L′[L′= PPh3, PPh2(C6H11), PPh(C6H11)2, AsPh3, or SbPh3]in situ affords the ‘mixed’ seven-co-ordinate compounds [MI2(CO)3L(L′)]. Low-temperature (-70 °C, CD2Cl2)13C n.m.r. spectra of the triphenylantimony complexes [MI2(CO)3L(SbPh3)][L = PPh3, PPh2(C6H11), or PPh(C6H11)2] showed two carbonyl resonances. This indicated capped octahedral geometry with a carbonyl ligand in the unique capping position. Some of the neutral seven-co-ordinate complexes [MI2(CO)3L(L′)] containing phosphines disproportionated in CH2Cl2 to give the salts [HL][MI3(CO)3L′]. 31P N.m.r. spectroscopy and negative ion mass spectrometry confirmed that the more electron-rich phosphine is protonated in [HL][MI3(CO)3(PPh3)][L = PPh2(C6H11) or PPh(C6H11)2]. The structure of the complex [PPh2(C6H11)H][WI3(CO)3(SbPh3)]·Et2O was determined by X-ray analysis. The co-ordination geometry about the tungsten is capped octahedral with a carbonyl ligand in the unique capping position. The capped face contains two carbonyl groups and the triphenylantimony ligand, with the three iodide ligands occupying the uncapping face. The low-temperature (-70 °C, CD2Cl2) spectrum of the related complex [NBun4][WI3(CO)3(SbPh3)] prepared by treating [WI2(CO)3(NCMe)(SbPh3)]in situ with 1 equivalent of [NBun4]I in CH2Cl2showed two carbonyl resonances at δ= 209.68 and 235.50 p.p.m. with an intensity ratio of 2 : 1. The lower-field resonance can be ascribed to the unique capping carbonyl ligand and the higher-field resonance to the two equivalent carbonyl ligands on the octahedron. The low-temperature 13C n.m.r. spectrum of [NBun4][MoI3(CO)3(PPh3)] shows a single resonance at δ= 222.20 p.p.m. which suggests a different capped octahedral structure with the triphenylphosphine ligand capping the three carbonyl ligands.