Phenanthridine bromination

Phenanthridine-6-carboxylic acids synthesis, 2, 415 Phenanthridines amination, 2, 236 bromination, 2, 320 hydrogenation, 2, 328 nitration, 2, 319 nomenclature, 2, 5 5-oxides... 

Phenanthridine (74) was converted by NBS into the 2-bromo derivative (40%) (55JA6379), but the bromine-sulfuric acid-silver sulfate reagent gave low yields of 1-, 4-, and 10-bromophenanthridines in the ratio (1 6.4 9.5), a reactivity order which contrasts with that found in nitration (1 > 10 > 4 > 2) (69AJC1105). Phosphoryl chloride converted phenanthridine 5-oxide into the 6-chloro derivative, but when that position was blocked by a phenyl substituent, the reductive chlorination process gave a 2-chloro compound (84MI2). 

Whereas treatment of phenanthridine with AT-bromosuccinimide gives only 2-bromo-phenanthridine, use of bromine and silver sulfate in 92% sulfuric acid gives, in low yields, 10-, 4- and 1-bromo-phenanthridines in the ratio 9.5 6.4 1 together with some dibromo products (Scheme 11) (69AJC1105). This order of reactivity contrasts with that observed for nitration of phenanthridine (Section 2.06.2.1.2). 

In a reinvestigation of earlier work (33LA(505)103) by Diels and Alder, Acheson et al. (60JCS1691) established that the stable isomer obtained by addition of two moles of dimethyl acetylenedicarboxylate to one of pyridine was the 4/7-quinolizine (96) and that this with bromine was oxidized to a quinolizinium salt (97 Scheme 65). 4iT-Quinolizines obtained from isoquinoline (62JCS748) and phenanthridine (63JCS3888) were similarly aromatized to afford benzo[a]quinolizinium (98) and dibenzo[a,c]quinolizinium ions (99) respectively. 

Reports of bromination of phenanthridine (11.14) are contradictory, claiming (1) a 40% yield of 2-bromophenanthridine (55JA6379) and (2) a positional reactivity order of 10 > 4 > 2, the isomer yields being 6, 4, and 0.7%, respectively, with —1% of dibromo products (69AJCI105). These reports both contrast with nitration, in which positional order is I > 10 > 8 > 3. 

In some cases an intramolecular arylation (Section A.l.i, f, equations 85 and 107) is problematic. Cyclic Af-(2-bromophenylethylenaminones instead of benzazepines with LiNEt2 yield indolines. Photolysis seems to be a good alternative method. A photolytic reaction of the same reagents leads to the desired benzazepines in high yield204-207 (equation 146). Carbazoles and phenanthridines can also be obtained by photocycliza-tion of brominated N-phenyl, N-benzyl derivatives of cyclic enaminones in yields similar to those in the base promoted cyclization (see equations 84, 106 and 107). 

Charge-transfer (C-T) bands have been located in the spectra of solutions of phenanthridine in 1,2-dimethoxyethane containing bromine solutions containing up to a 2 1 mole ratio of halogen to base were examined, but the structure of the species involved is not clear. Phenanthridine satisfies the conditions necessary for both n and tt- donation and n donation is apparently involved in the charge-transfer interaction with iodine. The equilibrium constant for this reaction has been determined spectrophotometrically, but the claimed correlation (for a series of A -heteroaromatic bases) between values (in 50% ethanol) and these C-T equilibrium constants appears to be an unsatisfactory one and in any case lacks theoretical justification, since it is doubtful whether dissociation constants provide, in general, an accurate measure of w-ionization potentials. In particular, the excellent correlation in the case of phenanthridine is probably fortuitous, since the authors report that w-halogen interactions are markedly sensitive to steric factors which are almost... 

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