The synthesis and characterization of four novel boron-containing bibenzimidazoles related to the DNA minor groove binder Hoechst 33258 are reported. Such compounds, particularly their 1°B-enriched forms, have potential as agents for boron neutron capture therapy which is used in the treatment of cancers.

The irradiation of various types of aromatic ethers with ultraviolet light has been shown to yield o- and p-substituted phenols, resulting from C-O bond fission followed by recombination of the two fragments. Also formed in these photolyses was the parent phenol, produced presumably by abstraction of hydrogen from the solvent by the aryloxy fragment. The ethers investigated were aryloxyacetic acids,1 allyl aryl ethers,2 benzyl phenyl ether, and diphenyl ether. ��� Mixture experiments have shown that for both the aryloxyacetic acids and allyl aryl ethers the rearrangement is intramolecular and most probably proceeds by formation of a radical-pair enclosed in a solvent cage.

The geminate self-reactions of the title methyl, ethyl and butyl ester radicals-(2a-c), formed by decomposition of the corresponding azo precursors (1a-c) in the presence of stable nitroxide radical scavengers, were found on the basis of product analysis to comprise combination and disproportionation in the ratios 56 : 44 (methyl), 58 : 42 (ethyl) and 47 : 53 (butyl). In the absence of radical scavengers, extensive oligomerization is observed. Hydrogenation and degradation were used in conjunction with g.l.c.-m.s . to deduce the identities of the dimeric, trimeric and tetrameric products, which were in most cases subsequently confirmed by isolation and n.m.r . analysis. Of particular interest is the highly regioselective disproportionation of radical (3) to give dimethyl 4-methylpent-1-ene-2,4-dicarboxylate (8), and the further reaction of (8) with (2a) to form branched oligomers (10) and (15).

A series of compounds have been synthesized in which the basic 2-phenylbibenzimidazole structure of Hoechst 33258 has been modified to include various combinations of bromo, iodo, methoxy, amino, alkylamino and nitro groups in the terminal phenyl ring. Both sequential and convergent synthetic routes have been followed using coupling reactions of both imino ethers and aldehydes to 1,2-diamines. All these compounds were characterized by a combination of f.a.b. mass spectrometry and 'H and 13C n.m.r. spectroscopy including inverse detection of long-range heteronuclear CH correlations (HMBC).

Eighteen 1-methyl-1-phenylethyl (cumyl) (1) and 1-phenylethyl (styryl) (2) carbocations have been prepared in known concentration in FSO3H/SbF5 solution from their corresponding carbinols and their 1H and 13C spectra have been measured. The correlations of both the methyl 1H shifts [δH(Me)] and methyl 13C shifts [δC(Me)] with the cationic carbon shifts [δ(C+)] have been determined. In the case of δC(Me) curves are obtained indicating non-linearity of chemical shifts with charge density. Correlations of δ(C+) with Brown's σ+ constants are only fair, the deviation from linearity being interpreted in terms of non-linear dependence of either δ(C+) or σ+, or both, on charge density at this level of charge. The use of an enhanced substituent constant (σ++) and/or the dual substituent parameter equation improves the correlation. Non-linearity in the plot of δ(C 1) against σ+ reported for 2-arylnorborn-5-en-2-yl(4) and 2-arylnorbornan-2-yl cations (5) has been shown to occur also in the case of δC(Me) of the cumyl cations (1). The implications of this finding with respect to the structures of (4) and (5) are discussed.