with a higher peak current. The nature of this peak is
reminiscent of an electron transfer followed by a chemical
reaction. In comparison, the reverse scan depicts a substan-
tially suppressed cathodic peak exhibiting a plateau around
0.2 V. More importantly, the forward scan of the second
cycle shows an anodically shifted descending peak around
0.40 V with a prepeak around 0.35 V. The rise in peak
current along with the retention of voltammetric pattern
following the addition of more amount of 1a indicates the
involvement of the electrochemical reaction, while the
position of new prepeak at 0.35 V in close proximity with
the anodic peak of the original redox couple reveals the
of R-diazoketone, 1a, containing pyridine as the nucleophilic
probe gives rise to a UV-vis absorption band around λ )
422 nm, indicating the formation of R-ketocarbene interme-
diate 6 and/or ketene 7 (Scheme 1). However, it is difficult
to discriminate between these two intermediates due to their
similar peak positions in the UV-vis absorption band arising
from pyridine-R-ketocarbene ylide3,4 and pyridine-ketene
ylide.21
We propose two nonclassical electron-transfer pathways
for the Wolff rearrangement (Scheme 1). The E(v), C, E(V)
pathway22 represented by steps iii, iv, and v, respectively,
leading to the formation of R-ketocarbene 6 (involving
structure 5, which is more or less similar to the Fischer
carbene intermediate) followed by the instantaneous genera-
tion of ketene 7. Alternatively, the E(v), C, C, E(V) pathway
represented by steps iii, iv, ix, and x, respectively, involves
the rearrangement of carbene cation radical,11 ultimately
leading to ketene 7.
regeneration19 of electrocatalytic species Agn . However, a
+
reverse scan of the second cycle is unable to show two
cathodic peaks corresponding to respective anodic peaks due
to the plateau-like nature of the cathodic response. Successive
cycles show a similar voltammetric pattern with decreased
peak current. The anodic peaks move in a more positive
direction, while a cathodic plateau shifts more negatively
with increasing scan rate. However, voltammograms beyond
ν . 0.1 Vs-1 are electrochemically silent. Qualitative
comparison between the nature of anodic (peak) and cathodic
(plateau) peaks indicates the occurrence of a kinetically faster
oxidation process. This analysis is in agreement with the shift
of OCP to more positive values following the onset of the
Wolff rearrangement (vide supra). Significantly, the zero
current curve-crossing phenomenon with thermodynamically
more facile second electron transfer step than the first is
absent, collectively suggesting the occurrence of a nonclas-
sical electron-transfer process.16,17
Another remarkable evidence for these mechanistic fea-
tures (Scheme 1) arises from CPC by the application of an
appropriate oxidation potential at room temperature (303 K),
which leads to the realization of Wolff rearranged carboxylic
acids from R-diazoketones, 1a-e, in excellent yield (Table
1). On the other hand, CPC of the R-diazoketone, 1a, using
Pt as anode (1.2 V vs Ag/AgCl) gives merely a mixture of
neutral reaction products.
In conclusion, we have shown that the addition of Ag(I)
oxide to the solution of R-diazoketone results in the in situ
generation of silver nanoclusters which function as electron
mediators in Wolff rearrangement. More importantly, this
work provides an efficient preparative electrochemical route
to realize Wolff-rearranged products in excellent yields at
room temperature. The technique can be advantageously
utilized in the homologation of naturally occurring R-amino
acids to â-amino acids, some of the key building blocks of
protenogenic â-peptides. We strongly believe that several
reactions are similarly catalyzed by metal nanoclusters,23 and
further studies in this direction are in progress.
The voltammograms of various R-diazoketones, 1b-f
(Table 1), evoke subtle yet informative features leading to
the understanding of stereoelectronic effects associated with
the nonclassical electron-trasfer process. For example, among
aromatic diazoketones, the anodic peak potential (Epa)
increases from the iodo, 1d (0.41 V) through 1c (0.47 V) to
1e (0.59 V), indicating the relative order of thermodynamic
difficulty to oxidize the respective diazoketones. Furthermore,
the magnitude of ∆Ep as well as the difference of the half-
0
+
wave potential (E1/2) of Agn /Agn redox couple in the
presence and absence of respective R-diazoketeones also
increase in a similar manner, leading to a decrease in the
rate of the nonclassical electron-transfer process (Supporting
Information, SI-3). These results are in good agreement with
the previous reports.20 Significantly, the CPC of 3-diazo-
camphor, 1f (vide infra), results in the exclusive formation
of the tricyclic ketone 11, arising from an intramolecular
insertion of R-ketocarbene intermediate into the C-H bond
(Scheme S-1, Supporting Information).5 This substantiates
the characteristic evidence of involvement of a nonclassical
E(v), C, E(V) pathway (vide infra).
Acknowledgment. J.S. acknowledges the University
Grant Commission (UGC), India, for the award of a senior
research fellowship.
Supporting Information Available: Detailed experi-
mental procedures, TEM images, UV-vis spectra, additional
CV, and characterization of various products. This material
OL052981W
Application of an anodic bias on Agn-coated platinum
electrode (0.5 V vs Ag/AgCl) in aqueous acetonitrile solution
(21) Chiang, Y.; Kresge, A. J.; Popik, V. V. J. Am. Chem. Soc. 1999,
121, 5930.
(22) (a) We strongly believe that relative stability of adsorbed conforma-
tion of R-ketocarbene over Agn will play a decisive role in selecting the
reaction pathway. Two possible pathways for the rearrangement from
intermediate 5 are decomplexation of Agn followed by the rearrangement
of free R-ketocarbene 6 (Scheme 1) and rearrangement of R-ketocarbene
followed by the decomplexatio of Agn (not shown in Scheme 1). (b)
Bogdanova, A.; Popik, V. V. J. Am. Chem. Soc. 2004, 126, 11293.
(23) Moreno-Manas, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638.
(19) Note: By increasing the amount of Ag2O coated on the working
electrode, we observed a rise in peak current with the retention of
voltammetric pattern indicating the regeneration of electrocatalytic species
+
Agn (Supporting Information, SI-2.2).
(20) Yukawa, Y.; Tsuno, Y.; Ibata, T. Bull. Chem. Soc. Jpn. 1967, 40,
2613.
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Org. Lett., Vol. 8, No. 6, 2006