Phosphide boranes

Table 2.7 shows that the extension to other substrates is challenging at least under the optimised conditions for rac-2. Only the closely related substrate of entry 5 gave an enantioselectivity comparable to the parent system. Interestingly, in the other eases no precipitate was observed during the equilibration of the two enantiomers of the phosphide borane, suggesting that the precipitation is a key event in the success of the reaction and inducing it (with other solvents, for example) may be a way to improve the results of Table 2.1... 

The generated phosphide boranes are extremely nucleophilic and reacted smoothly with activated alkyl halides (entries 3-7), epoxides (entries 1, 2 and 8) and molecules with activated multiple bonds such as ethyl acrylate (entry 9), benzyne (10) or even fullerene (entry 14). Some C2-symmetric diphosphine... 

Reaction of 31 with t-BuLi at very low temperature did not afford the expected tert-butylphosphine borane but prompted the metallation of 31 to form a solution of phosphide boranes 63. The steric bulk of the t-Bu carbanion probably accounts for this reactivity. Phosphide boranes are very nucleophilic, and can be protonated to form secondary phosphine boranes 64 or alkylated to produce tertiary phosphines, 65. The reported yields for these compounds are in the range of 21-75% with ee values up to 99%. Although no details of this reaction have been yet published, it is likely that this transformation will attract considerable attention because 64 are very versatile synthons for the preparation of many other P-stereogenic compounds. 

Scheme 4.29 Preparation of phosphide boranes and related derivatives from 31.

The optically pure secondary phosphine boranes are easily deprotonated, giving extremely nucleophilic phosphide borane anions, which preserve the absolute configuration at the P atom at low temperatures. This fact prompted Danjo, Imamoto and co-workers to couple the phosphide boranes derived from 9 with aryl cation equivalents. In particular, they found that fluoroarene tricarbonylchromium complexes such as 17 were excellent substrates for the SNAr process due to the high electron deficiency of the arene ring and significant electrophilicity of the ipso carbon (Scheme 5.9). 

The same method was applied to u-difluorobenzene tricarbonylchromium with the aim of obtaining or / o-substituted monophosphines and diphosphines (Scheme 5.10). The first nucleophilic attack provided the desired mono-substituted compound (20) in 97% yield, but no traces of diphosphine borane could be detected, even using excess of phosphide borane. Compound 20 possesses a second stereogenic element (a plane) and it was found to be present as a mixture of separable diastereomers in 60% de. When this compound was reacted with MeMgBr it was found that the large steric hindrance of the phosphine borane group inhibited the second nucleophilic attack and therefore the deprotection of the phosphine was required (Scheme 5.10). 

Several diphosphine boranes have been prepared by taking advantage of the availability of a few optically pure secondary phosphine boranes (Section 5.2.1.1). After deprotonation, the phosphide boranes were easily alkylated by Az5(benzylic) substrates (Scheme 5.22). 

The phosphide borane anions are also reactive towards haloarenes, giving substitution products via SNAr. This fact was used by Imamoto and co-workers to prepare two ligands known as QuinoxP (Scheme 5.23). 

These two ligands were easily prepared by reaction of the enantiomerically pure phosphide borane anions with 2,3-dichloroquinoxaline followed by deboronation with TMEDA, in 75-80% combined yield. Remarkably, none of the ligands were either oxidised or epimerised on standing in air for months. 

More recently, Livinghouse and co-workers studied the effect of Cu(I) on Pd-catalysed arylation of secondary phosphine boranes. Apart from a marked beneficial effect on the coupling elSciency, they discovered that Cu(I) is able to suppress the base-mediated racemisation of the phosphide borane intermediate, to generate a configurationally stable metallophosphide 47 (Scheme 6.22). Consequently, tertiary phosphine boranes 48 were obtained with retention of configuration, in good yields and excellent enantioselectivities. 

Aiming at preparing a phosphino-borane ILjPBR with PB doublebond character,43 Karsch reacted the lithium phosphide LiPMes2 with tribromoborane (Scheme 19).44 Instead of the desired compound 25, they isolated the PB 26a that results formally from the 1,2-addition of a C-H bond of a mesityl group across the PB double bond of 25. The 31P and... 

These nucleophilic reagents react with most common electrophiles such as organohalides, tosylates, aldehydes, ketones, epoxides, and activated alkenes. It should be noted that many workers have found much higher yields if the phosphides are protected as phosphine oxide or borane anions (see Section 3). Phosphide reagents also react with activated arenes to give mixed aryl phosphines (Protocol 2). Metal phosphides therefore provide an alternative, complementary tertiary phosphine synthesis to the electrophilic routes outlined in Section 2.1. 

So far we have discussed the synthesis of useful P-chiral phosphorus intermediates that can be converted into a variety of phosphines by reaction with a nucleophile (and deprotection). However, Imamoto and co-workers found that one-electron reducing agents react with resolved menthyl phosphine oxides and boranes to give metal phosphides that retain their configuration at phosphorus (Scheme 26).49... 

Other ligands have been synthesized more recently utilizing the same synthetic and theoretical philosophy.52,53 A chiral ditosylate or dimesylate is generally reacted with a metal phosphide to produce the diphosphine ligand. There have been problems with these displacement reactions with secondary ditosylates as has been discussed earlier. More recent procedures report success with phosphine oxide or borane anions. 

Ignition or explosive reaction with metals (e.g., aluminum, antimony powder, bismuth powder, brass, calcium powder, copper, germanium, iron, manganese, potassium, tin, vanadium powder). Reaction with some metals requires moist CI2 or heat. Ignites with diethyl zinc (on contact), polyisobutylene (at 130°), metal acetylides, metal carbides, metal hydrides (e.g., potassium hydride, sodium hydride, copper hydride), metal phosphides (e.g., copper(II) phosphide), methane + oxygen, hydrazine, hydroxylamine, calcium nitride, nonmetals (e.g., boron, active carbon, silicon, phosphoms), nonmetal hydrides (e.g., arsine, phosphine, silane), steel (above 200° or as low as 50° when impurities are present), sulfides (e.g., arsenic disulfide, boron trisulfide, mercuric sulfide), trialkyl boranes. 

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... 

The chemistry of boron-phosphorus compounds has been reviewed. Numerous boron-phosphorus derivatives have been reported, but relatively few boron-arsenic or boron-antimony compounds have been described. Boron-phosphorus compounds are similar in many ways to boron - nitrogen derivatives, but the tendency to share bonding electrons in covalent tetrahedral compounds is much more evident with phosphorus than with nitrogen. In fact, most boron-phosphorus chemistry involves tetrahedral boron. They are typically either phosphine-borane complexes, such as RgP-BRj, or phosphinoboranes (R2PBR 2) , cyclic or polymeric derivatives of the hypothetical H3P BH3. The chemistry of these compounds and that of boron phosphate and thiophosphate is described below. Boron phosphides are discussed in Section 2.6. 

Some gaseous or liquid compounds of nonmetals and hydrogen ignite on contact with air and bum with a luminous flame. These are the phosphines, silanes, and boranes—the hydrides of the elements phosphorus, silicon, and boron, respectively. Of these, the luminously burning phosphines have practical importance because they can be generated by moistening of solid phosphides with plain water according to the (simplified) equations ... 

Both phosphorus and boron have a strong tendency to achieve tetrahedral coordination, the former by donation and the latter by acceptance of lone-pair electrons. The simplest example is provided by phosphine-borane, H3B-PH3, in which an electron octet is achieved around both the P and B atoms (9.1). Other compounds which contain tetrahedrally coordinated boron and phosphorus atoms include boron phosphide BP (Chapter 4.7), and boron phosphate BPO4 (Chapter 5.3). 

The formation of achiral phosphonium salts (14) or phosphide anions (15) explains the easy racemisation of 13 in the presence of traces of acids or bases. It has been found that 13 can be stabilised by the presence of a mild base, capable of neutralising any trace of protons but unable to form 15. A different way to stabilise secondary phosphines is the quenching of the electron pair by the formation of phosphine boranes (see Section 1.3). 

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. 

It was found that 14 was over-oxidised to phosphinous add borane 15, probably due deprotonation of the formed secondary phosphine borane and immediate oxidation of the phosphide anion. Interestingly, the analysis of the optical purity of 16, prepared by methylation of 16, showed that the reactions were enantioselective. 

Even with an excess of tert-butylmethylphosphide, the desired compound 55 was not obtained and only 20 could be isolated, probably due to both steric and electronic reasons. Accordingly, the X-ray structure of 20 shows effective shielding of the ipso carbon of the remaining fluorine atom by the phosphine borane group. Deboronated 20 (Scheme 5.10) did not react with the phosphide either. Unexpectedly, when the reaction mixture containing 19 and the phosphide treated with acid at low temperature the main product turned out to be the p ra-substituted diphosphine borane 54. Compound 54 arises from nucleophilic attack at the meta position to the fluoro group therefore the... 

Sulfonimidophosphonates Phosphides s. Alkali phosphides Phosphinaminoboranes 16, 374 Phosphine, reactions with aldehydes, aliphatic 16, 696 —, reductions with — 17, 346 Phosphine boranes... 

Diborane(6) catalyzes the polymerization of imidazole-borane (see Section 2.2.4.3, p. 19), and it forms HBNH when subjected to an a.c. discharge plasma in the presence of NH3 [9, 10]. Boron phosphide, BP, is deposited as single crystal wafers when diborane(6) is thermally decomposed in the presence of PH3 in an H2 atmosphere [11]. Solvent-free diborane(6) reacts with CH3-U(r 5-C5H5)3 via the intermediate (r 3-CH3-BH3)U(r 5-C5H5)3 to form (r 3-BH4)U(r 5-C5H5)3 [12]. 

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