The nonenolizable phenylglyoxal reacts with PhIO/
HBF4 (48% in water) to benzoic acid under CÀC
cleavage (entry 2, Table 2). Aryl aldehydes with a small
enol content such as 2-methoxy-2-phenylethanol (entry 3,
Table 2) do not give rise to the expected cleavage pro-
ducts: methyl benzoate is found as a side product in
only 3% yield, but the main product is benzaldehyde in
65% yield. Unbranched R-aryl acetaldehydes react to the
corresponding contracted aldehydes in good to excellent
yield (entries 4 and 5, Table 2). R-Aryl alkyl aldehydes
react to the corresponding ketones in good yields (entries
6À12, Table 2). The best yields are obtained with R,
R-diaryl acetaldehydes as substrates (entries 13À17,
Table 2), probably due to their low pKE values. According
to entries 6 and 7, Table 2, there is only a mild influence
of donor and acceptor substituents on the reaction
rates, the latter slowing down the reaction. In view
of the fact that some of the iodane reagents are quite
labile, some of the reaction times listed in Table 2 may
seem surprisingly long. Experimental evidence leads us
to assume that the lifetimes of the in situ prepared
complexes depend strongly on the reaction conditions,
especially if 1,4-dioxane is used as a solvent. We believe
that 1,4-dioxane coordinates to the hypervalent iodane
complex yielding longer lived species more akin in stabi-
lity to 3a, 6, or 7.
Scheme 2. Proposed Mechanism of the Oxidative Cleavage of
R-Aryl Aldehydes with Iodosylbenzene Reagents
The reaction mechanism we thus propose is outlined in
Scheme 2. We assume that intermediate 9 is first formed
in equilibrium with the corresponding intermediate 10.
Under nucleophilic attack, a five-membered cyclic inter-
mediate is formed. Precedence for the formation of the five-
membered cyclic intermediate 11 is found in the literature
for the decarboxylation of R-keto carboxylic acids14a,b and
for the oxidative cleavage of acetophenone.14c Seemingly,
Figure 1. React-IR analysis of the reaction of phenylacetalde-
hyde with PhIO/HBF4 (50À55% in diethyl ether) in dichloro-
methane. Carbonyl vibration of phenylacetaldehyde (A) (blue
spectrum): 1728 cmÀ1, formaldehyde (B), formic acid (C) and
benzaldehyde (D) (green spectrum): 1743 cmÀ1, 1720 cmÀ1 and
1705 cmÀ1
.
(6) Justik, M. W.; Koser, G. F. Tetrahedron Lett. 2004, 45, 6159.
(7) Saltzman, H.; Sharefkin, J. G. Org. Synth. 1963, 43, 60.
(8) (a) Miyamoto, K.; Tada, N.; Ochiai, M. J. Am. Chem. Soc. 2007,
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Chem. 2010, 75, 7416.
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J. M.; Frenz, B. A J. Org. Chem. 1976, 41, 3609.
(10) (a) Zhdankin, V. V.; Tykwinski, R.; Caple, R.; Berglund, B.;
Koz’min, A. S.; Zefirov, N. S. Tetrahedron Lett. 1988, 29, 3703.
(b) Zhdankin, V. V.; Mullikin, M.; Tykwinski, R.; Berglund, B.;
Koz’min, A. S. J. Org. Chem. 1989, 54, 2605. (c) Ochiai, M.; Varvoglis,
A.; Zhdankin, V. V.; Koser, G. F.; Wirth, T.; Tohma, H.; Kita, Y. Top.
Curr. Chem. 2003, 224, 60. (d) Ochiai, M.; Miyamoto, K.; Shiro, M.;
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(11) (a) Toullec, J. Adv. Phys. Org. Chem. 1982, 18, 1. (b) Kresge, A. J.
Chem. Tech 1986, 250.
Figure 2. Reference spectrum of benzaldehyde and the mixture
of benzaldehyde and formic acid in dichloromethane. Carbonyl
vibration of benzaldehyde (D) (blue spectrum) and formic acid
(C) (red spectrum): 1705 and 1724 cmÀ1 (the measuring accuracy
of the react-IRTM spectroscopy device is (4 wave numbers).
(12) Fritz, J. S.; Hammond, G. S. Quantitative Organic Analysis;
Wiley: New York, 1957.
(13) (a) Harcourt, M. P.; More O’Ferrall, R. A. J. Chem. Soc., Perkin
Trans. 2 1995, 1415. (b) Harcourt, M. P.; More O’Ferrall, R. A. J. Chem.
Soc., Chem. Commun. 1987, 822. (c) Toullec, J. In The Chemistry of
Enols; Rappoport, Z., Ed.; Wiley: Chichester, 1990. (d) Chiang, Y.; Kresge,
A. J.; Walsh, P. A. J. Am. Chem. Soc. 1986, 108, 6314. (e) Chiang, Y.;
Kresge, A. J.; Krogh, E. T. J. Am. Chem. Soc. 1988, 110, 2600.
(f) Capponi, N.; Gut, I.; Wirz, J. Angew. Chem., Int. Ed. Engl. 1986, 25, 344.
(14) (a) Moriarty, R. M.; Gupta, S. C.; Hu, H.; Berenschot, D. R.;
White, K. B. J. Am. Chem. Soc. 1981, 103, 686. (b) Moriarty, R. M.;
Vaid, R. K.; Duncan, M. P.; Ochiai, M.; Inenaga, M.; Nagao, Y.
Tetrahedron Lett. 1988, 29, 6913. (c) Moriarty, R. M.; Prakash, I.;
Penmasta, R. J. Chem. Soc., Chem. Commun. 1987, 202.
Carbonyl vibration of formaldehyde in the gas phase: 1743 cmÀ1 16
.
the anion BF4À does not play a role in the outlined mechan-
ism. We believe, however, that BF4À is of critical importance
for the stability of the in situ formed reagents and thus for
their overall reactivity toward the substrate aldehydes.
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Org. Lett., Vol. 14, No. 19, 2012