Boron heterocycles structures

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). 

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. 

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... 

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. 

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. 

Mass spectral data have frequently been used in the structural determination of boron heterocycles. One paper has been devoted to the mass spectra of some six-membered boron-nitrogen systems. It was concluded that the spectra could be interpreted analogously to their hydrocarbon counterparts. In all cases the molecular peak was the base peak of the spectrum (68T6755). Doubly charged molecular ions, a feature typical of aromatic compounds, are often encountered. It should be noted, however, that some certainly non-aromatic aminoboranes give such doubly charged ions as well. 

One chemical property quoted in support of aromatic character is kinetic stability towards hydrolytic break-down. Many boron heterocycles have been stated to be more stable than expected for organoboranes. On the other hand, several saturated cyclic boron compounds are stable as well, suggesting that the cyclic structure itself is favorable. 

M. P. Groziak, A. D. Ganguly, P. D. Robinson, Boron heterocycles bearing a peripheral resemblance to naturally occurring purines - design, syntheses, structures, and properties, /o r a/ of the American Chemical Society 1994, 116, 7597. 

A method for the B-arylation of boron heterocycle 89 has been developed and used to synthesize the BN for C=C analog 90 of the nonsteroidal anti-inflammatory drug (NSAID) felbinac (Scheme 28) (2013AG(I)9316). An X-ray crystal structure of the immediate precursor to the acid 90 was determined. The synthesis of the B-N for C=C analog 91 of 4-methoxy-ira s-stilbene has been accompHshed in a similar Rh-catalyzed dehydrogenative coupling reaction (20140L3340). An X-ray crystal structure of the N-benzylated derivative of product 91 was reported. 

A synthesis of 6-aryl-8H-dibenzo[d,/i][l,3,7,2]dioxazaborecin-8-ones, which are bridgehead bicyclo[4.4.0]boron heterocycles of general structure 101 (R = H, Cl = H, F, Et R = 2-OMe, 4-Br), has been developed and features a one-pot three-component reaction of a 2-aminobenzoic acid, a 2-hydroxybenzaldehyde, and an arylboronic acid (Scheme 33) (2012T3377). The reaction is conducted in CCI4 under microwave irradiation and gives the bridgehead bicyclo[4.4.0]boron heterocycles 101 in 95—98% yields. One of the compounds (101, R = R = R = H) was solved crystaUographicaUy. A similar three-component additive-free method... 

Stable boron heterocycles sometimes arise unexpectedly. The imine 113, produced from sahcylaldehyde and NH4OAC in the presence of dimethyl malonate, undergoes reduction with NaBH4 or NaBH3CN or NaBH(OAc)3 to give the cyclic boramide 114 (Scheme 36) (2014T8614). The X-ray crystal structure of this fidrly stable compound has been determined mechanistic studies revealed an oxazaborinane intermediate in the reaction pathway. 

A similar investigation looked at the condensation of dehydroacetic acid JV-aroylhydrazones with B(OH)3/Ac20 mixtures (known to produce B(OAc)3) to give the boron heterocycles 115a—g in 66—84% yield (Scheme 37) (2015T7245). An X-ray crystal structure determination of derivative 115a was obtained. 

The current year has seen the appearance of comprehensive data on known carbaboranes up to the end of 1976. - Other reviews have discussed Group IV carbaboranes, carbaborane crystal structures, wfo-monocarbaboranes, car-baborane nomenclature, metallocarbaboranes, and triple decker complexes with boron heterocycles as ligands. Theoretical studies have considered molecular orbitals utilized in bonding carbaboranes and their relationship to cage counting rules. ... 

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