Boron complex compounds, covalent

The Lewis acid character of many boron compounds and the ability of boron to participate in multicenter bonds lead to a vast and complex chemistry. Boron exclusively forms covalent bonds in an irmnense variety of compounds, often... 

The chemistry of boron-phosphorus compouuds 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 teudeucy to share boudiug electrons in covalent tetrahedral compounds is much more evident with phosphorus thau with uitrogeu. lu fact, most boron phosphorus chemistry iuvolves tetrahedral borou. They are typically either phosphiue-boraue complexes, such as R3P BR j, or phosphinoboranes (R2PBR2) , 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. 

In covalent azides, the pseudohalogen azide RN3 has an angular structure as in HN3. Triazidoborazine [H3N3B3(N3)3] and other boron azides, for example, salts of the tetraazidoborate ion B(N3 (4 have been considered as boron nitride precursors (see Boron-Nitrogen Compounds). The M(Ns)3 azido complexes of the other group 13 elements (Al, Ga, In, Tl) and their M(N3)4 tetraazido anions are all known. They and their derivatives are also used as precursors for the nitrides. 

Although Annual Reviews of N.M.R. Spectroscopy are primarily intended to cover specific years and, for the purpose of the written account, 1965 to the early months of 1968 are covered we are tabulating the available data on the simple ter- and tetra-covalent boron compounds and of the boron complexes with Lewis bases. Our purpose for doing this is twofold firstly to bring together much of this scattered information, and secondly to correct some mistakes, or draw attention to anomalous results that have appeared in separate accounts. 

It is known that boronic acids can bind with hydroxyl compounds, including polyols such as PVA, through the complex formation of a reversible covalent bonding [56, 57]. 

The catalytic cysteine-215 of PTP-1B is highly reactive toward oxidation and electrophiles and as such it is a tempting target for fragment discovery. Ockey and Gadek assembled a set of 19 reversible electrophiles, such as aldehydes, nitriles and boronic acids.1271 They then used electrospray ionization mass spectrometry to look for one-to-one complexes and three of the compounds were found to form covalent complexes. The dissociation constants ranged from 25 to 150 p,M and one of the compounds was also able to inhibit PTP-1B with an IC50 of 60 xM. 

In covalent organolithium compounds and covalent Grignard reagents neither the lithium nor the magnesium possesses a valence electron octet. This is energetically disadvantageous. In principle, the same mechanism can be used to stabilize these metals that monomeric boranes BH3 n Rb use to attain a valence electron octet at the boron atom (Section 3.3.3) the formation either of oligomers or, with suitable electron pair donors, of Lewis acid/Lewis base complexes. 

From the known, differential complexing between boronic acids and polyhydroxy compounds, it follows that carbohydrate mixtures may be separated by column-chromatographic methods that exploit the differences. Nucleoside and nucleotide boronates have been separated on columns of anion-exchange resins,90 and sugars and alditols have been shown to be differentially retained on such resins in the sulfonated phenylboronic acid form,64 but perhaps the best uses of column chromatography in this connection have incorporated the resolving powers of insoluble polymers to which boronic acid groups have been covalently bonded. Such insoluble forms of boronates have been synthesized either by substitution of polysaccharide derivatives, or by polymerization of suitable arylboronic acids. 

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