Phosphine borane adducts

Figure 20 The primary phosphine-borane adduct RPH2-BH3 (29) with the highly electron-withdrawing aromatic group R = p-CF3C6H4. (Adapted from ref. 49.)...

The proton n.m.r. spectra of these adducts have been intensively studied. The BH3 resonance in the spectrum of CHjPHj BH3 consists of a 1 1 1 1 quartet due of coupling between the boron nucleus ( B, 80% natural abundance /= 3/2) and the directly bonded protons (/bh" = 99 Hz). Each component of the quartet is further split into a doublet of triplets due to coupling with the phosphorus nucleus (/pH" = 16 Hz) and the two protons bonded to phosphorus, respectively. The PH -signal is, as is typical for phosphine-borane adducts, a doublet with/pH = 375 Hz. All n.m.r. data for the two types of adducts are given in Table 11. 

Nucleophilic Attack at Other Atoms. A highly efficient general synthesis of secondary- and tertiary- phosphine-borane adducts is afforded by treatment of the phosphine with sodium borohydride in the presence of acetic acid in THF. Any carbonyl groups present in the phosphine undergo concomitant... 

Nucleophilic Attack at Other Atoms.- Interest has continued in the synthesis of phosphine-borane adducts. Complexes of tetramethyldiphosphine with borane and various halogenoboranes have been used to generate a series of open-chain and cyclic phosphino-boranes. Adducts of 1,3,5-diazaphosphorinanes involving two... 

Nucleophilic Attack at Other Atoms. - The chemistry of phosphine-borane adducts continues to develop. The diphenylphosphine-borane adduct has been employed in the synthesis of linear systems having a skeleton consisting of alternating phosphorus and boron atoms, e.g., (73). Unusual salt-like systems, e.g., (74), have been isolated from the reactions of trialkylphosphines with the dimethylsulphide adduct of dibromoborane. 

Stereochemistry — The conformational analysis of various deuteriated ethyl-phosphine-borane adducts and dimethyl methylphosphonates were based on vibrational spectral data. The stereochemistry of diethylphosphonyl acetamide,the unusual vinyl compounds (60 X = L.E.P., O, and a number of l,3,2-dioxaphosphorinanes have been studied. It was found that the Raman-active ring vibration is related to the orientation of the phosphoryl group. Conformational data for dioxaphosphepane was compared with calculated parameters. A low-temperature study of several cyclopropyl-phosphonates (61) revealed rotational isomerization about the P-O bonds but not about the P-C bond. The variable temperature i.r. and Raman spectra of the silyl phosphates (62) also revealed rotational isomerism. ... 

Pt-catalyzed alkylations of bis(secondary) phosphines were diastereo- and enan-tioselective, yielding eiuiched mixtures of C2-symmetric rac and meso bis(tertiary) phosphines. Removal of the meso isomer by recrystallization of a phosphine— borane adduct enabled synthesis of the enantiomerically pure DiPAMP analog 39 by this approach (Scheme 64) [115]. 

Nucleophilic attack at other atoms. The chemistry of phosphine-borane adducts has continued to generate interest. Simple borane adducts of primary vinyl-, ethynyl- and allenyl-phosphines have been prepared and studied by a range of spectroscopic and theoretical techniques. The same group has also shown that attachment of the BH3 unit to a variety of primary phosphines results in a substantial increase in the intrinsic acidity of the system in the gas-phase. Group III halide adducts of the type Bu 2PH EX3 (E = B, Al, Ga or In X = Cl or Br) are accessible from the reactions of the secondary phosphine with the trihalides at room temperature. The solid state structure and reactivity of these adducts was also reported. Treatment of l,8-bis(diphenylphosphino)naphthalene with the borane-dimethylsulfide complex in ether solvents affords a simple monoborane adduct of the diphosphine irrespective of the molar ratio of the... 

Borohydride salts react with trivalent phosphorus compounds to give a variety of boranes, phosphines and phosphine-borane adducts (9.20-9.23). The reaction between sodium borohydride and phosphorofluoridic acid produces diborane in about 80% yield, and is a convenient method of preparation of the latter. 

The first method to be used for the preparation of (cT -cT ) cyclophosphino boranes was the thermal decomposition of secondary phosphine-borane adducts, formed by mixing diborane with a secondary phosphine. 

Phosphine boranes, adducts between phosphines and a BH3 unit, have been known for over 70 years but it was not until the 1990s that they became essential intermediates in the synthesis of P-stereogenic compounds, thanks to... 

Polyphosphinoboranes containing phosphorus and boron linked to each other in polymeric chains have remained elusive for a long time. Recently, catalytic dehydrogenation of the phosphine-borane adducts has been found to be effective to prepare the linear polymer. Thus, thermal treatment of PhPH2.BH3 in the presence of catalytic amounts of [(l,5-COD)Rh( j,-Cl)] affords the linear polymer poly(phenylphosphinoborane), [PhHPBH2]n (see Eq. 1.13) [28-30]. High polymers with of about 33,000 have been isolated by this procedure. 

Polyphosphinoboranes, [RPH-BH2]n, are a recent class of polymers that contain a P-B repeat unit in the polymer backbone [25, 26]. These polymers are prepared by a rhodium-catalyzed dehydropolymerization procedure. The monomers that have been used for these polymers are the phosphine-borane adducts, RPH2.BH3. Dehydropolymerization of these adducts occurs by the use of rhodium complexes such as [ Rh(p-Cl)(l,5-cod) 2] or even by simple rhodium salts such as RhCU or RhCl3.xH20. The polymerization is carried out in neat conditions (no solvent) (see Eq.5.13). 

The molecular weights of these polymers have been determined by various methods. For example, the molecular weights (M ) of [PhPHBH2]n and [iBuPHBH2]n as determined by static light scattering methods are 20,000 and 13,100, respectively. The phosphorus chemical shifts of the phosphine-borane adducts and the linear polymers are nearly similar. Thus, while the P-NMR signal of PhPHz.BHj is at -47 ppm, the linear polymer resonates at -48.9 ppm. 

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