Boron heterocycles

Allyl derivatives of B- and B,0-heterocycles in asymmetric synthesis 90MI48. 

Cyclic boronic esters in asymmetric synthesis 88ACR294 89T1859. Cyclic a-halo boronic esters in asymmetric synthesis 89CRV1535. B-Heterocycles as structure fragments of polymers 88UK1529. B,N-Heterocycles in the synthesis of boron nitride 90CRV73. B,P-Heterocycles 90AG(E)449. 

Molecular structures of B,S- and B,N,O-heterocycles 90MI4. Oxazaborolidine or cyclic diamidoborane in asymmetric synthesis 90YGK980. 

---

Synthesis of boron heterocycles for neutron capture therapy 94YGK1044. 

C. Boron Heterocycles 1. Chemistry of Individual Classes of B-Heterocycles... 

The 2 1 reaction of 9-BBN with a series of dicarboxylic acids, namely oxalic acid, malonic acid, 2,2-dimethylmalonic acid, and succinic acid, in dimeth-oxyethane gives in some cases dimeric and in other cases macrocyclic (acyloxy)diorganoboranes. This has been proved by IR spectroscopy (all C = O groups are bidentate), B-NMR 5 = 10 ppm) and X-ray crystallography [47]. With oxalic acid two structures are possible (IV and V), of which the first with a five-membered boron heterocycle instead of a four-membered one is the more probable formulation (Fig. 13). 

Aziridine esters are a- and -amino acid derivatives at the same time. A characteristic reaction of a-amino acids is their reaction with triethylboron to give boroxazolidines. We showed that aziridinecarboxylic acids exhibit the expected behavior in their reaction with triethylboron, viz., that they form stable boroxazolidines 34 (Scheme 19) [29]. These boron heterocycles can be reconverted into the free amino acids by treatment with 8-hydroxyquinoline. 

One example showing a serious discrepancy of the frontier electron method was reported by Dewar H8,ii9). This is 10,9-borazaphenanthrene, and the value of / -B) was reported to have been calculated by the Pople method, but the parameters usyd were not indicated. Fujimoto s calculation by the Pariser-Parr-Pople method 120>, in perfect disagreement with Dewar s, gives the most reactive position as 8, which parallels experiment. The ambiguity involved in the integral values adopted seems to be serious, so that the establishment of parametrization for boron heterocycles is desirable. 

The platinum-catalyzed diboration has been applied to some functionalized alkynes such as 1,3-diynes,44 1-borylalkynes,45 1,2-diborylethynes,46,47 and alkynylphosphonates.45 In particular, diboration of 1,2-diborylethyne gives tetraborylethene, which is a potential precursor for new boron heterocycles (Equation (2)).46,47... 

As rich and varied as the chemistry of boron-carbon heterocycles is, its versatility is extended considerably by the introduction of additional elements as ring heteroatoms (T able 6). At this writing, development in this area has involved primarily N- and S-containing rings, but examples of P- and O-containing boron heterocycles are also known. 

Tris(dimethylamino)borane similarly fails to 3deld a trivalent boron heterocycle, but gives the spiro-compound (CLXV). A monocyclic system of type (CLXVI) does arise from diethylaminodiphenylborane, and of type (CLXVII) from phenylboronic acid and its analogues 436). 

Fig. 28. (a) The arachno structure of benzvalene and nido structures of C5H5+ and CsMee . (b) The boron heterocycles Ph4C4BPh and H4C4B2F2. 

Another boron heterocycle with a formula apparently appropriate for a nido-pyramidal structure is the compound H4C4B2F2 (193) (Fig. 28). The fluorine substituents in this molecule, however, can 7r-bond to the boron atoms, thereby generating a quinonoid electronic structure. Elsewhere in boron cluster chemistry, the presence of halogen substituents appears to modify the skeletal electron requirements of the cluster. 

Other unsaturated boron heterocycles, such as borazines and borabenzenes, form transition metal complexes with the expected nido geometry, as exemplified by compounds (Et3N3B3Et3)Cr(CO)3 (123) and (CBH5BPh)Mn(CO)s (110) (Fig. 29). 

Boron Heterocycles as Platforms for Building New Bioactive Agents... 

This contribution first surveys some of the attractive properties of boron, briefly describing applications that have been developed mostly with non-aromatic boron-containing compounds. It then examines many of the stable, formally aromatic boron heterocycles that have been reported to dale, covering much of the pertinent literature through the end of 1999. With the sum of these two parts, I hope the reader will gain an appreciation of the untapped potential held by boron heterocycles, especially for constructing new bioactive agents. 

Our laboratory conducted the most extensive investigation of the 2,3,1-benzodiazaborines reported to date. We analyzed 25, l,2-dihydro-l-hydroxy-2,3,l-benzodiazaborine (26), and certain derivatives related to 26 by multisolvent H, C, "B, and NMR using isotopically-enriched ( C, N) compounds <97JA7817>. The X-ray crystal structures of 25 and 26 were obtained first, and that of the 2-methyl derivative 39 was determined soon thereafter <98AX(C)71>. The topography (internal geometry, intramolecular associations) of 39 was found to be most similar to 26, but some subtle 25-like characteristics were found. All three boron heterocycles were shown to exist in planar form in protic solution just like they do in the... 

Inverting the orientation of the C4-N3 imine unit of a 2,3,1-diheterabotine gives a boron heterocycle with a markedly different chemical reactivity. In effect, the weakly basic oxime- or hydrazone-type imine nitrogen in the 2,3,1-diheteraborine is replaced by a much more basic imidate- or amidine-type imine nitrogen in the 2,4,1-diheteraborine. Likely, the Lewis acid tendency of the boron is enhanced by the ready protonation of this basic N4, and the formation of a stable borate-based zwitterion becomes thermodynamically favored. 

A tricyclic boron heterocycle (48) related to these was synthesized recently along a different route starting torn 2-guanidinobenzinaidazole <98HAC399>. This time, we are fortunate enough to have an X-ray crystal structure to scrutinize, and can identify featiues consistent with an extensively delocalized positive charge counterbalancing the borate anion. 

WHAT DOES THE FUTURE HOLD FOR BORON HETEROCYCLES ... 

The X-ray crystal structures shown in this article were created with CambridgeSoft s ChemSD Pro 5.0 using coordinates obtained from the source, the CSD (Cambridge Structural Database) or the PDB (Protein Data Bank), or those determined by Paul D. Robinson at Southern Illinois University (SIU) in collaboration with the author. Most of the author s work on boron heterocycles first at SIU and more recently at SRI International (the nonprofit research institute formerly known as the Stanford Research Institute) was funded by NIH grant GM448I9. 

---