Phosphino boronate

Boron-capped thio- and phosphino-metallaborate compounds with bridgehead boron of types 151a and 151b (Figure 16 corresponding references are presented in Table 11) have been extensively studied in the past decade. 

Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are complementary to the metallic catalytic methods. For example lithium aluminium hydride, sodium borohydride and borane-tetrahydrofuran have been modified with enantiomerically pure ligands161. Among those catalysts, the chirally modified boron complexes have received increased interest. Several ligands, such as amino alcohols[7], phosphino alcohols18 91 and hydroxysulfoximines[10], com-plexed with the borane, have been found to be selective reducing agents. 

In addition to the results described, enantioselective access to 2-phosphino alcohols could be accomplished, too [71]. Starting from a borane-protected a-phosphino aldehyde hydrazone 91 as the key intermediate and available by two different approaches, the enantioselective synthesis of the desired 2-phosphino alcohols 93 could be accomplished. Thus, the electrophilic phosphinylation of aldehyde hydrazones 90 (via route I with the chlorodiphenylphosphine-borane adduct or via route II with chlorophosphines and subsequent phosphorus-boron bond formation) and the alkylation of phosphino acetaldehyde-SAMP hydrazones 92 (route III) was carried out (Scheme 1.1.26). 

Sesquiammino-boron Trichloride, 2BC13.3NH3, is also formed when dry ammonia gas is passed into liquid boron trichloride. The ammine does not fume on exposure to air, is less volatile than ammonium chloride, and may be sublimed without decomposition. Water decomposes it with formation of ammonium chloride, hydrochloric acid, and a borate. Besson 2 described a substance of composition 2BC13. 9NH3, which he obtained from boron phosphino-chloride and ammonia. This is not attacked by moist air and does not lose ammonia below 50° C., but water decomposes it immediately. 

Tricoordinated boron compounds (boranes) are coordinatively unsaturated and their chemistry is dominated by reactions in which complexes are formed. These complexes are either neutral molecules (borane complexes), anions (borates) or boron cations. Space limitations mean that little or no attention will be paid to complexes containing several boron atoms and to species of the type L-BH3, [BH,]- and [L2BH2]+ (L = neutral ligand), discussed in detail in several books and reviews. Similarly, little attention will be paid to the plethora of metal borates and the cyclic and polymeric amino- and phosphino-boranes. 

Pinacolborane is extensively used in the borylation of aryl halides 114 in the presence of a base (typically pyridine or Et3N or KOAc) and catalytic amount of PdCl2(DPPF) to furnish aryl boronates 115 (DPPF = l,l -bis(diphenyl-phosphino)ferrocene Equation 7) <1997JOC6458, 2000JOC164>. Pinacolborane is compatible with esters, ketones,... 

The p-RgPBsHg compounds discussed above apparently do not isomerize to the corresponding 2-substituted derivatives as the silicon and germanium derivatives do. This is not unexpected in view of the fact that the bridging phosphino-group is connected to the two basal borons by two 2-center two electron bonds as opposed to the one 3-center two electron bond which presumably binds the Group IV moieties previously discussed. 

Development from aminoboron chemistry led to phosphino- and arsino-boron compounds. Diborane and phosphine react at —110° to give B2Hg.2PHg, much less stable than the diammoniate. Trimethylamine displaces phosphine from it quantitatively at —40°, suggesting the ionic structure H3P+.BH3- ... 

Figure 1 The molecular structure of, and numbering system for, 5jS-fi-diphenyl phosphino-decaborane A). Boron atoms are represented by open circles, carbon atoms by shaded circles, hydrogen atoms by small shaded circles, and the phosphorus atom by a large open circle (Reproduced by permission from Inorg. Chem., 1973,12, 288)...

Fourteen new tricarbonylchromium complexes of benzodiazaboroles, phenyl-boranes and related compounds have been prepared and characterired. In all compounds n.m.r. data indicate that the Cr(CO)8 moiety is bonded to the benzo and phenyl rings. New types of co-ordination compounds of chromium, molybdenum, and tungsten with phosphino-aminoboranes of general formula (COsMPRSBRS, (CO)4M(PRia)aBR , and (CO)3M(PRS)B have been synthesized. Suitably substituted carbonyl compounds of the same metals reportedly react with l-phenyl-4,5-dihydroborepin (35) to yield stable complexes in which the boron atom apparently participates in metal-ligand bonding. "... 

FIGURE 26. Covalent poly(boronate)s were first identified as (a) the borono-hydra-zone analogs before (b-e) ester linked materials were identified, (d and e) Phosphino-integrated poly(boronate)s display reactivity and structural motifs similar to the monomeric precursors. 

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