Borane complexes metal-phosphine

Synthesis of MOP ligands from the BINAPO-borane complex by phosphine oxide/metal... 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

The rhenium complex 76 related to 74b was also prepared recently by Labinger and Bercaw using another synthetic strategy. 2 In this case, the pendant borane moieties were introduced by hydroboration of unsaturated phosphines in the coordination sphere of the metal. The cationic rhenium complex 75 featuring two diphenyl(vinyl)phosphines was readily converted into the corresponding bis(phosphine-borane) complex 76 (Scheme 45). The coordination mode of 76 was substantiated spectroscopically (5 nB = 87.7 ppm) and crystallographically. 

The chemistry of secondary phosphine oxides, R2P(H)0 and their phosphi-nous acid tautomers, R2POH, has continued to attract attention. The study of the phosphinous acid tautomers has been aided by the development of stereoselective procedures for direct conversion of secondary phosphine oxides to the phosphinous acid-boranes (83). Treatment of the secondary phosphine oxide with either a base-borane complex or boron trifluoride and sodium borohyd-ride provides the phosphinous acid-borane with predominant inversion of configuration at phosphorus. The phosphinous acid tautomers are usually trapped as ligands in metal complexes and further examples of this behaviour have been noted. Discrimination of enantiomeric forms of chiral phosphinous acids, Ph(R)OH, coordinated to a chiral rhodium complex, has been studied by NMR. °° Palladium complexes of di(t-butyl)phosphinous acid have found application as homogeneous catalysts.A lithium salt of the tellurophos-phinite Ph2PTeH has been prepared and structurally characterised. ... 

As has been the pattern in recent years, there has been considerable interest in the synthesis and characterisation of phosphide reagents derived from metals other than lithium, sodium, and potassium, and also in studies of the structure of metallophosphides in the solid state. A new route to P-chiral phosphine-boranes of high enantiopurity is afforded by treatment of the borane complexes of methyl(phenyl)phosphine with a copper(I) reagent, giving the copper-phosphido intermediate (83), which, on subsequent treatment with an iodoarene in the presence of a palladium(O) catalyst, gives the related chiral t-phosphine-borane (84), with retention of configuration at phosphorus. Organophosphido systems... 

Chiral phosphines are widely used as auxiliaries for various metal-catalyzed asymmetric reactions and can be prepared from stable phosphine-borane complexes. Secondary P-chiral phos-phine-boranes can be prepared by reductive lithiation of the corresponding tertiary phosphine-borane using LN (eq Likewise, P-chiral tertiary phosphine ligands can be produced by the reductive lithiation of phosphinite-boranes followed by alkylation, both proceeding with retention of configuration (eq 18). ... 

It is difficult to know exactly how to draw the bonding in metal complexes and there are often several different acceptable representations. There is no problem when the metal forms a a bond to atoms such as Cl or C as the simple line we normally use for covalent bonds means exactly what it says. The problems arise with ligands that form a bonds by donating both their electrons, and with Jt complexes. Everyone writes phosphine-borane complexes with two charges but we normally draw the same sort of bond between a phosphine and, say, Pd as a simple line with no charges. 

At low temperatures phosphine reacts with perchloric acid to produce crystals of phosphonium perchlorate, PH CIO4, which are explosive and very sensitive to heat, moisture and friction. Phosphine readily forms addition complexes in which a metal-phosphorus bond is present, for example, H3P AICI3, H3P TiCl4, (H3P) Cr(CO)3 and HjP Co(NO)(CO)2 (Chapter 8.11). Borane complexes are also formed (Chapter 9.1). 

Crystal structure analyses have indicated that, in some phosphine metal borane complexes, the borane cage is bonded to the metal atom through the H atoms. 

A quite new class of catalysts based on early main group metals (Ca, Sr, and K) was recently reported to promote general conversion of conjugated double bond (137). The catalytic reaction is initiated by the formation of a highly reactive metal hydride that adds either to an alkene or to a silane. The regiochemistry for the hydrosilylation of 1,1-diphenylethylene (DPE) catalyzed by calcium complex can be completely controlled by the polarity of the solvent. Amine borane and phosphine borane complexes were successfully used as effective catalysts for hydrosilylation of organic compounds with internal unsaturated bond (138) that cannot be selectively hydrosilylated in the presence of Pt catalysts. 

The first borane-metal carbonyl derivatives contain the rather unstable anions [Re(CO)5(BH3)]-, [Re(COs)(BH3)2]-, and [Mn(C05)BH3]-, and a somewhat more stable species is [Mn(CO)4(BH3)P(C6Hs)3]. The increased stability of the phosphine-substituted borane complex suggests a role for the phosphine primarily as a a donor, as also indicated by the NMR spectrum of [Mn(CO)4P(C6H5)3] (371). An interesting series of metal derivatives of... 

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