A boron analogues

We have already briefly mentioned the discovery of scandium in the chapter devoted to REEs (see p. 130). Although many of scandium s properties are similar to those of rare earths, D. I. Mendeleev predicted that the element would be a boron analogue in the third group of the periodic system. His prediction proved to be accurate enough. Scandium was discovered by the Swedish chemist L. Nilson on March 12, 1879, his article On Scandium, a New Rare Metal was published and on March 24 it was discussed at a session of the Paris Academy of Sciences. 

TUN-KYI, R. SCHWYZER. L-o-Carboranylalanine, a boron analogue of phenylalanine. Helv. Chim. Acta 59 2184-2187. 

A boron analogue of uracil has been reportedly prepared and its structure proposed (52) on the basis of i.r. and u.v. data. Several boron-substituted derivatives of uracil (53), the pyrimidine (54), and 2 -deoxyuridine (55) have been synthesized and characterized. Two approaches to the synthesis of boron-substituted pyrimidines and borazaroquinazolines have been discussed. ... 

This carboxyborane can undergo an amine exchange reaction with Hquid ammonia (eq. 7) to yield the boron analogue of glycine, the simplest alpha-amino acid (13). There has been a great deal of work on the pharmacological activity of these amino acid analogues (14). 

Laurence, P. R., and C. Thomson. 1982. The Boron Analogue of Glycine A Theoretical Investigation of Structure and Properties, J. Mol. Struct. (Theochem) 88, 37-43. Laurence, P. R., and C. Thomson. 1981. A Comparison of the Results of PCILO and Ab Initio SCF Calculations for the Molecules Glycine, Cysteine and N-Acetyl-Glycine, Theoret. Chim. Acta (Berl.) 58, 121-124. 

Terphenyl derivatives of boron were originally synthesized with the object of preparing compounds with B-B triple bonds. It was thought that these aryl ligands would be effective in the stabilization of triple bonds in a similar manner to their success in the stabilization of double bonds. For example, a B-B triple-bonded boron analogue of a substituted acetylene might be accessible by reduction of an ArBX2 or l,2-dimethoxydiborane(4) species as shown by Eq. (3). 

When selecting atom substitutions for new molecule design, chemists usually look only to the right of carbon in the periodic table. The contrarian looks to the left and finds boron—commonly viewed as a metal, but in fact quite nonmetallic in many respects. In his excellent review of boron analogues of biomolecules, Morin showed why working with boron is so attractive <94T12521>. Here are some of the unique potential applications for any new boron compound ... 

A theoretical (AMI molecular orbital) treatment of boron analogues of the katapinand series as well as macrotricyclic compounds related to 16 has also recently appeared. As with the ammonium-based host molecules the macrotricyclic hosts containing four boron atoms exhibited a greater degree of anion specificity as a consequence of the rigidity of their binding sites. In all cases, size selective complexation of halide anions was observed with accompanying decreases in B - B distances and partial sp -> sp rehybridization. ... 

A series of metal complexes containing trimethylamine boranecarboxylato ionligand [103904-11-6], (CH3)3N BH COO, have been prepared with Co (III), Co (II), Zn(II), Ca(II), Cr(III), and Fe(III) (39,40). This ligand, derived from the boron analogue of the amino acid glycine, behaves similady to organic carboxylato ligands. 

Borazine is isoelectronic with benzene, as B=N is with C=C, (Fig. 16.21). in physical properties, borazine is indeed a close analogue of benzene. The similarity of the physical properties of the alkyl-substituted derivatives of benzene and borazine is ever more remarkable. For example, the ratio of the absolute boiling points of the substituted borazines to those of similarly substituted benzene is constant. This similarity in physical properties led to a labeling of borazine as "inorganic benzene." This is a misnomer because tbe chemical properties of borazine and benzene are quite different Both compounds have aromatic rr clouds of electron density with potential for delocalization over all of the ring atoms. Due to the difference m electronegativity between boron and nitrogen, the cloud in borazine is "lumpy" because more electron... 

A boron-phosphorus analogue of borazine has been synthesized rather recently.73... 

Most of the work reported in this area is limited to carbocyclic systems. The recent developments with the boron analogues of the cyclobutenyl/homocyclopropenium and norbomenyl/norbornadienyl cations point to the potential importance of cyclopropyl homoconjugation and homoaromaticity in a much wider sphere of organic systems. This will likely be an area where there will be considerable further work. 

Bishomoaromatic stabilization energies [(35) -> (36)] were obtained by computational methods this study was part of a paper describing the experimental preparation of a zwitterionic analogue of such compounds (i.e. where X is boron).76... 

The presence of a transition metal is not necessarily required for hydrocarbon insertion. Alkyne incorporation has been reported for boracyclobutenes, as well as metallacyclobutene complexes of the transition elements. Boracyclobutene 51, a reactive intermediate prepared in situ (Section 2.12.9.2.1), inserts an additional equivalent of trimethylsilylacetylene into the B-C(sp2) bond to yield boracyclohexadiene 52 (Scheme 7). This isomerizes to the interesting bridged compound 53, an analogue of a nonclassical carbocation <1994AGE2306>. The related boracyclobutene 7 inserts the alkyne to yield a persistent boracyclohexadiene 54, but this product clearly arises from insertion into the boracyclobutene carbon-carbon bond rather than a boron-carbon bond <1994AGE1487>. 

The rapid development of carborane chemistry is mainly due to their practical applications. For instance, the potential utility of carborane polymers as gaskets, O-rings, and electrical connector inserts has been reported. Their functionality for solvent extraction of radionuchdes as well as the potential medicinal value of the isoelectronic and isostructural boron analogues of biologically important molecules has been the subject of many review articles. For example, a number of boron compounds have been found to possess anti-inflammatory and antiarthritic activity in animal model studies. Boron compounds have also been implicated in studies designed to probe the importance of the so-called anionic subsite of acetylcholine esterase and Ach receptors. But, by far the most interesting practical apphcations of carboranes are in areas of boron neutron capture therapy (BNCT) and supramolecular assembly. 

Our BNCT research has focused on radiolabeling BP A find on the creation of boron analogues of an unnatural cyclic a-amino acid, 1-aminocydobutanecarboxylic acid (ACBC), as a carrier molecule, Figure 3. This unnatural amino acid is known to be preferentially retained in intracerebral tumors. In fact, carbon-11 labeled ACBC, 3, is used for imaging brain tumors at the University of Tennessee Medical Center.6 Recently, we reported the syntheses of a m-carboranyl containing ACBC derivatives which were lipophilic in nature.7... 

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