Phosphine boranes amines

Recently, Manners and co-workers have found another type of reaction, the inorganic process corresponding to Rh-catalysed dehydrocoupling of borane adducts, which is homogeneous or heterogeneous catalysed depending on the substrate, in particular the nature of the Lewis base, phosphine or amine respectively [15]. 

The phosphine boranes undergo a wide range of reactions, some of which are not possible for the corresponding phosphine. Removal of the borane is easily accomplished by treatment with a large excess of amine. 

Many phosphine-borane complexes Y3P BZ3 have been characterized. They include compouuds where Y = alkoxy, aUcyl, amino, halide, and hydride groups, and Z = alkyl, halide, and hydride. The stabilities of these complexes vary widely depending on the Lewis acidity and basicity of the boron and phosphorus moieties, respectively. The relative stabilities of Lewis acid-base complexes with BH3 are R3P > R3N > R3AS > R3Sb, but with BF3 the order is R3N > R3P > R3AS > RsSb. The stabihties of the borou halide complexes of phosphines follow the same order as the amine complexes BI3 > BBrs > BCI3 > BF3. 

Imamoto reported that Pd-catalyzed coupling of phosphine-borane with aryl halides is useful for preparation of asymmetric phosphines. Phosphines can be easily isolated from phosphine-boranes by exchange reaction with amines such as pyrrolidine and DABCO [10]. Lipshutz found that aryl nonaflates ( Nf = nonafluorobutanesulfonate) and triflates are good substrates for coupling with BHs-stabilized diaryIphosphines. Selective coupling with nonaflate without... 

Lewis acid is a substance that is electron deficient (e g. BF3), and a Lewis base is a substance that can donate electrons (e.g. amines, phosphines, boranes, ethers, sulfides etc) to form bonds or complexes with Lewis acids. 

Johansson MJ, Schwartz L, Amedjkouh M, Kann N (2004) New chiral amine ligands in the desymmetrization of prochiral phosphine boranes. Tetrahedron Asymmetry 15 3531-3538... 

Bis(phosphine)dihydroboron cations can be obtained by reacting phosphine boranes with hydrogen phosphonium salts (9.73). The bis(amine)dihydroboron cation is produced by simple reaction between ammonia and diborane (9.74), but the phosphine analogue is not obtained in this way. The cations in Equation 9.73 are somewhat less stable to water than their nitrogen analogues, but are resistant to oxidation and hydrolysis. 

Phosphine boranes would not be so popular if the borane group could not be easily removed. Two reliable methods have been developed. The first one is the treatment of phosphine boranes with amines (D) or alcohols, which act as borane acceptors. Although phosphine boranes are in general more stable than amine boranes, the presence of a large excess of a basic amine frequently suffices to displace the equilibrium and deprotect completely the phosphine. Typical amines used for this transformation are diethylamine, TMEDA, pyrrolidines, morpholine and DABCO. The reactions are usually carried out at room temperature or with moderate heating in neat amine with the exception of DABCO, which as it is a solid is typically used in toluene. 

Trialkylphosphine boranes are electron rich and possess very inert P-B bonds. These compounds are incompletely deprotected by amines but are efficiently deboronated by method E, developed by McKinstry, Livinghouse and co-workers. Treatment of phosphine boranes with an excess of certain strong acids such as methanesulfonic, trifluoromethanesulfonic or more often tetrafluoroboric in dichloromethane at low temperature (— 5 °C to room temperature) cleaves the borane group producing phosphonium salts, which are extracted with an aqueous base (NaOH or NaHCOs) to afford tertiary phosphines in good yields. The mechanism of the decomplexation is not clear, but it is thought to involve nucleophilic substitutions at the borane moiety, eventually leading to decoordination. 

Although methylphosphinite boranes are usually used as electrophilic precursors to phosphine boranes (see Section 4.3.3.1), they can also be deprotected by amines, affording the corresponding optically pure methylphosphinites. The deprotection reaction is very enantioselective, with retention of configuration at the P centre. Only a few free phosphinites have been prepared with the Juge-Stephan method these are listed in Figure 4.1. 

Methylphosphinite boranes react smoothly with organolithium reagents to afford the corresponding tertiary phosphine boranes, as will be discussed in the next section. However, phosphinite boranes are not electrophilic enough to react with other weaker nucleophiles such as alcohols, amines or thiols. More reactive precursors, capable of producing a wide variety of phosphorus compounds, were needed. In phosphorus chemistry halophosphines, and chlorophosphines 27 in particular, are essential synthons (Scheme 4.12) as nucleophilic (after transformation into metal phosphides 28) and electrophilic building blocks. 

From a-lithiated carbanions 72, formylated (73), carboxylated (74), silylated (75) and aminated (76) P-stereogenic phosphine boranes have been prepared without loss of optical purity. Some of them can be further functionalised to obtain other interesting compounds. Furthermore, oxidative coupling with Cu(II) salts affords C2 diphosphine boranes of the DiPAMP family (77) and... 

The carbanion derived from (phosphine borane 73 in 85% yield. This compound, although rather unstable, when freshly prepared reacted smoothly with amines and subsequently sodium triacetoxyborohydride under microwave irradiation (pW) to afford the desired p-aminophosphine boranes 79 in 84-96% yields and in optically pure form. 

The desired phosphine boranes were prepared from phosphorus trichloride in a few steps using inexpensive materials. Although the diastereoselectivities were not very high, particularly in the phosphine of entry 5, the meso-68 compounds could be efficiently separated by conventional methods (chromatography or recrystallisation) affording chemically pure C2 compounds. Their optical purities could be easily increased by recrystallisation because the meso isomers were found to be more soluble than the C2 counterparts with the exception of the compound in entry 1. It has to be said that 68 could not be deboronated by the classic treatment with amines and the acidic method had to be used (see Chapter 1, Section 1.3.2). ... 

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. 

Imamoto and coworkers described phosphine-borane complexes as a coupling partner for the synthesis of aryl phosphines with aryl electrophiles using Pd(PPh3),j at room temperature (Scheme 20.68) [228,229], The borane moiety can be easily removed ljy excess use of diethyl amine or morpholine. Gaumont and coworkers demonstrated the palladium-catalyzed C—P cross-coupling in imidazolium-based ionic liquid and that the catalyst can be recycled up to six cycles [230]. 

Apart from this special behavior, a reliable protection of the phosphine group during synthesis and storage can be achieved by adduct formation with BH3. Several methods are described in the literature to remove the BHj group before the catalytic reaction by the reaction of amines or acids [162]. Moreover, under hydroformylation conditions, CO can successfully compete with the phosphine for BH3 and thus initiate the breakage of the P-B bond [163]. Phosphine hgands generated in this manner from phosphine borane adducts did not differ in the catalytic properties observed with phosphines directly submitted to the catalyst formation. 

As dectron-delident or neutral chain transfer agents, silanes, boranes, and alanes are known and extensively investigated in single-site-catalyzed olefin polymerization in order to obtain heteroatom-ftmctionalized chain ends. Electron-rich chain transfer agents, such as phosphines and amines, have also been smdied on organolantharride-mediated polymerization systems. ... 

The use of complexes of borane ligated to A -heterocyclic carbenes (NHC) as radical hydrogen atom donors has been reported recently Complexes such as 53 and 54 have been postulated to be superior hydrogen donors than amine- and phosphine-boranes due to n-conjugation, resulting in a weakened B-H bond. 

Tetrasubstituted phosphinous amides of the type R2NPPh2 have been successfully arylated at phosphorus by the action of bromobenzene, in a process catalyzed by NiBr2, to give the aminophosphonium bromides [R2NPPh3] Br [109]. Other representative members of this class form phosphane-borane complexes [62], are aminated at phosphorus by chloramine to yield bis(amino)phos-phonium salts [110] and have been claimed to be protonated at phosphorus by ethereal tetrafluoroboric acid, as determined by NMR analysis [111]. 

The reaction between a Lewis acid R3M and a Lewis base ER3 is of fundamental interest in main group chemistry. Synthetic and computational chemists have investigated the influence of both the Lewis acid and the base on the solid state structure and the thermodynamic stability of the corresponding adduct, that is usually expressed in terms of the dissociation enthalpy De. This led to a sophisticated understanding of the nature of dative bonding interactions. In particular, reactions of boranes, alanes and gallanes MR3 with amines and phosphines ER3, typically leading to adducts of the type R3M <— ER3, have been studied.10... 

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