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iodine to continue the catalytic cycle; H3PO2 is therefore the ultimate source of reducing
equivalents. However, an iodine source is necessary: we found in a control experiment less than
2% conversion of benzophenone into diphenylmethane when iodine was omitted from the
reaction medium. Hydrogen iodide has previously been shown to reduce aryl carbinols to the
corresponding aralkanes in acidic solution.8 Indeed, we have found that our postulated
intermediate benzhydrol is reduced much faster than benzophenone under our standard
conditions.9
Scheme 1.
Summary. An acetic acid solution of hypophosphorous acid containing a catalytic quantity of
iodine reduces diaryl ketones to the corresponding diarylmethylene compounds (ArCOAr%
ArCH2Ar%) in close to quantitative yield. This reaction is competitive in terms of yield,
convenience, and general synthetic utility with the traditional processes for this conversion, e.g.
the Wolff–Kishner10 and Clemmensen11 reductions, and with more recent processes such as
reduction by silanes.12,13 Arylalkyl ketones are reduced more slowly, and dialkyl ketones slowest
of all. The carbonꢀcarbon double bond of some a,b-unsaturated ketones such as benzal-
acetophenone (1) is reduced much faster than the carbonyl group by H3PO2ꢀI2; we will report
further on this reaction later because we are still exploring several unusual processes we have
observed with activated alkenes.
Aqueous hydrogen iodide, either alone, in refluxing acetic acid, or with added elemental
phosphorus, has previously been found to reduce quinones, hydroquinones, polycyclic aromatic
phenols, and a few aryl ketones to the corresponding arenes.14 The reactions in Table 1 occur
substantially more slowly when aqueous hydrogen iodide is substituted for iodine. This may
possibly be due to the higher water content of the medium. Finally, the present procedure has
an economic advantage, with hydriodic acid and hypophosphorous acid currently selling for ca
$60/mole and $10/mole, respectively.
Reduction of 4-bromobenzophenone (representative experiment): Iodine (1.00 g, 4.0 mmol) and
acetic acid (50 mL) were stirred together under N2 in a flask fitted with a condenser and a
dropping funnel. Hypophosphorous acid, 50% aq. (2 mL, 19.3 mmol) was added and the
mixture was heated to reflux. A solution of 4-bromobenzophenone (3.18 g, 12.0 mmol) in 15 mL
of acetic acid was added over a period of 1–2 h. The mixture was then stirred and refluxed for
an additional 24 h, cooled, diluted with water and extracted with hexane. The hexane was dried
of MgSO4. Analysis by GC-mass spectrometry showed that the hexane contained only a single
constituent, 4-bromodiphenylmethane. The hexane was concentrated and purified by flash
1
chromatography to afford 4-bromodiphenylmethane15 (2.70 g, 84%); H NMR (300 MHz,
CDCl3: l 3.95 (s, 2H), 7.06–7.43 (m, 9H); mass spectrum: 246, 248 (25, p+); 167 (100, p-Br).