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B. P. McIntyre et al. / Tetrahedron Letters 45 (2004) 7709–7711
Table 1. Preparative photoreactions of nitrobenzenes in 8.8M aq HBr
Reactant
Conditions
Products
% Yield
Nitrobenzene
10mL, 0.025M, 4h
20mL, 0.010M, 4h
20mL, 0.010M, 30h
20mL, 0.020M, 18h
2,4,6-Tribromoaniline
96
86
90
47
39
3-Nitrobenzoic acid
3-Nitrophenol
4-Nitrophenol
2,4,6-Tribromo-3-aminobenzoic acid
2,4,6-Tribromo-3-aminophenol
2,3,6-Tribromo-4-aminophenol
2,3,5,6-Tetrabromohydroquinone
Analysis by NMR of the products extracted into ether
revealed a 45:55 ratio of hydroquinone to 2-bromo-
hydroquinone. Since hydroquinone has no absorbance
above 340nm and concentrated aqueous HBr has very
slight absorption, light absorption by HBr is the logical
source of bromine. The extent of this is about
0.00003mol/L/h, which could account for 2% of the bro-
mination of the 4-nitrophenol products and 0.12% of the
nitrobenzene product. If the competition for light is con-
sidered in which reactants absorb at least 97% of the
light entering the reaction tube, direct photolysis of
HBr is negligible. The probable source of excess bromi-
nation in the case of 4-nitrophenol is air oxidation of a
hydroquinone to a benzoquinone followed by electro-
philic addition of HBr.
Conversions were about 10%, and azoxybenzene in eth-
anol was the actinometer.7 Starting material concentra-
tions were sufficient to absorb 98% of the broadband
light transmitted through the uranium glass filter into
a Pyrex test tube with an internal diameter of 1.4cm.
The quantum yields are reported in Table 2. The effects
of substituents on the quantum yields are similar to
those for the photoreaction of nitroaromatics with
hydrochloric acid:4 electron withdrawing substituents
have little effect on the efficiency, but electron donor
substituents reduce the efficiency.
The mechanism proposed4 for the HCl photoreaction
involves electron transfer from halide ion to a predomi-
nant n,p* excited state of the nitroaromatic as the pri-
mary event, and this is consistent with all that is
known for the photoreaction with HBr. Electron donor
substituents on nitrobenzenes lower the energy of the
p,p* state relative to the close-lying n,p* state, causing
the p,p* state, which is unreactive, to become the lower
energy state.4,8 That the range of the suppressive effect
of donors on the quantum yield is less than 10-fold in
the HBr case compared with 440-fold in the HCl case
(for 3-nitroanisole compared with nitrobenzene) adds
further support to the electron transfer hypothesis for
the HBr case. The free energies for Xꢀ ! X + eꢀ in
water are 293kJ/mol for chloride ion and 201kJ/mol
for bromide ion, suggesting that electron abstraction
from chloride ion should indeed be much more selective
than from bromide ion.
The report6 that nitrosobenzene reacts thermally with
hydrohalic acids to give halogenated reduction products
including 2,4-dihaloanilines furnishes a clue concerning
the reaction sequence. In a reinvestigation of this chem-
istry, we found that 4-bromonitrosobenzene in an acetic
acid solution has its blue-green color discharged imme-
diately on addition of concentrated hydrobromic acid,
and that 2,4,6-tribromoaniline is produced in 95% iso-
lated yield. This result is consistent with the reaction
sequence shown in Scheme 2. It is likely that the photo-
chemistry generates a bromonitrosobenzene derivative
that reacts rapidly and thermally to give the observed
products. This offers an explanation for the occurrence
of a hydroquinone from 4-nitrophenol as well as the
introduction in that case of bromine in the aminophenol
meta to the nitrogen function. The expected 3-bromo-4-
nitrosophenol has a tautomer, 2-bromobenzoquinone
monoxime, that can hydrolyze to give eventually the
hydroquinone derivative. The oxime can also undergo
electrophilic additions of HBr across the conjugated
oxime or imine to generate the observed aminophenol.
Table 2. Quantum yields of photohydrobromination of nitroaromatics
Compound
M in 8.8M aq HBr
Quantum yield
Nitrobenzene
0.020
0.020
0.010
0.020
0.16
0.15
3-Nitrobenzoic acida
3-Nitrophenol
4-Nitrophenol
Quantum yields of disappearance of nitroaromatics
were determined by monitoring the loss of UV absorb-
ance of diluted samples at the wavelength maximum
after first determining the absorbance at completion.
0.029
0.038
a Solution was 1:9 (v:v) HOAc: 8.8M aq HBr.
NHOH
NH2
NO
Br
NO2
dark
HBr
dark
HBr
Br
hν
Br
Br
HBr
Br
Br
Scheme 2.