Wolff Rearrangement
FULL PAPER
(
0.29 V) differ considerably, and consequently, Ag accepts
is generic to several reactions catalyzed by metal nanoclus-
n
[31,32]
an electron from DHB. This leads to the formation of bulk
ters;
further studies are essential to reveal this.
silver along with 1,4-benzoquinone, thus indirectly empha-
0
n
sizing the necessity of a backelectron donation by Ag to
+
regenerate the electrocatalyst species Agn , along with the
formation of ketene intermediate.
Experimental Section
For experimental details, please see the Supporting Information.
The in situ UV-visible spectrum (see Supporting Informa-
tion Part 7) recorded by applying an anodic bias (0.5 V vs
Ag/AgCl), using a Ag electrode, to a solution of a-diazoke-
n
tone 1a in an electrolyte solution of 0.1m tetrabutylammo-
niumtetrafluoroborate in dichloromethane containing pyri-
dine as the nucleophilic probe, exhibits the presence of a
strong absorption band around l=418 nm, indicating the in-
volvement of either a-ketocarbene intermediate 6 or ketene
Acknowledgements
S.G.S., N.K.C. and S.P.C. gratefully acknowledge the Council of Scientific
Industrial Research (CSIR), New Delhi (India), for financial support.
&
7
. However, it is difficult to pinpoint the species due to
[
1] B. M. Trost, I. Fleming, Comprehensive Organic Synthesis, Wiley,
New York, 1991.
close proximity of bands arising from a-ketocarbene–pyri-
dine ylide
With the help of information obtained from various elec-
trochemical experiments and the in-situ UV-visible analysis
[14–16]
[29]
and ketene–pyridine ylide.
[2] M. P. Doyle, M. A. Mckarvey, T. Ye, Modern Catalytic Methods for
Organic Synthesis with Diazo compounds, Wiley, New York, 1998.
[
3] S. G. Sudrik, T. Maddanimath, N. K Chaki, S. P. Chavan, S. P.
Chavan, H. R. Sonawane, K. Vijayamohanan, Org. Lett. 2003, 5,
(
see below), we propose two nonclassical electron-transfer
2
355–2358.
pathways involving different reaction intermediates (which
requires further confirmation) for the Wolff rearrangement
[4] W. Kirmse, Eur. J. Org. Chem. 2002, 2193–2256.
[
5] K. P. Zeller, A. Blocher, P. Haiss, Mini-Rev. Org. Chem. 2004, 1,
91–308.
6] M. S. Newman, P. F. Beal, J. Am. Chem. Soc. 1950, 72, 5163–5165.
2
(
Scheme 1). The CPC response of a-diazoketones 1a–e over
[
Ag further substantiates this procedure of electron swap-
n
[7] A. J. Fry, in The Chemistryof the Diazonium and Diazo Groups
(Ed.: S. Patai), Wiley Interscience, Chichester (UK), 1978, pp. 489–
498.
ping (see below). The E(›), C, E(fl)—electron transfer reac-
tions preceding (›) and following (fl) chemical reaction(s)
in opposite directions—pathway represented by steps iii, iv,
and v, respectively, leads to the formation of a-ketocarbene
intermediate 6, which spontaneously rearranges to ketene 7.
The E(›), C, C, E(fl) pathway involves the rearrangement
of CRC 4 culminating in direct realization of ketene 7.
The CPC response of a-diazoketones 1a–e (2.5mm) at an
appropriate potential (see Table 1 and Supporting Informa-
tion Part 8) ultimately gives Wolff rearranged carboxylic
acids 8a–e in excellent and reproducible yield. A divided-
[
8] V. E. Petrosyan, M. E. Niyazymbetov, Russ. Chem. Rev. 1989, 58,
44–653.
9] V. Parker, D. Bethell, J. Am. Chem. Soc. 1987, 109, 5066–5072.
6
[
[
10] It is pertinent to note that mass spectrometry detects the formation
of CRCs from a-diazoketones in the gas phase. a) The Chemistryof
the Diazonium and Diazo Groups, Part I and Part II (Ed.: S. Patai),
Wiley Interscience, Chichester (UK), 1978; b) J. S. Splitter, In Ap-
plication of Mass Spectrometry (Ed.: J. S. Splitter, F. Tureck), VCH,
Weinheim (Germany), 1994.
[11] a) Recently with the help of mass spectroscopic technique, Stotlz
and co-workers have detected the formation of charged, Fischer
type, copper and silver a-ketocarbene intermediates during metal-
mediated Wolff rearrangement of diazomalonates. R. R. Julian, J. A.
May, B. M. Stoltz, J. L. Beauchamp, J. Am. Chem. Soc. 2003, 125,
cell assembly made up of a Ag -coated Pt plate as anode
n
and a graphite plate as cathode were used in aqueous aceto-
nitrile. UV-visible and TEM analysis of the anodic material
revealed the invariant nature of the electrocatalyst. In con-
trast, CPC of a-diazoketone 1a using Pt as the anode gave
uncharacterizable, neutral products.
4
478–4486; b) it is pertinent to note that mass spectrometry detects
the formation of CRCs from a-diazoketones in the gas phase; c) see
reference [10a].
[
[
12] D. A. Van-Galen, M. P. Young, M. D. Hawley, R. N. Mcdonald, J.
Am. Chem. Soc. 1985, 107, 1465–1470.
13] C. R. Jones, J. Org. Chem. 1981, 46, 3870–3873.
[14] J. P. Toscano, M. S. Platz, J. Am. Chem. Soc. 1995, 117, 1712–1721.
[15] I. Likhotvorik, Z. Zhu, E. L. Tae, E. Tippmann, B. T. Hill, M. S.
Platz, J. Am. Chem. Soc. 2001, 123, 6061–6068.
In conclusion, we have revealed a unique nonclassical
process, which involves electron transfer preceding and fol-
lowing chemical reaction(s) in opposite directions, with spe-
cial emphasis on the mediating role of silver nanoclusters,
during the Wolff rearrangement of a-diazoketones. Apart
from providing an efficient preparative electrochemical
route for the Wolff rearrangement without loosing the elec-
trocatalyst, this method has potential as an elegant homo-
logation of the naturally occurring a-amino acids to b-amino
acids—the key building blocks of proteinogenic b-pep-
[
16] A. P. Scott, M. S. Platz, L. Radom, J. Am. Chem. Soc. 2001, 123,
069–6076.
6
[
17] S. G. Sudrik, S. P. Chavan, K. R. S. Chandrakumar, S. Pal, S. K.
Date, S. P. Chavan, H. R. Sonawane, J. Org. Chem. 2002, 67, 1574–
1
579.
[
18] The rearrangement occurs either in concerted or stepwise manner
depending on the conformational equilibrium of the a-diazoketones
and the mode of activation (photochemical verses thermal); for ex-
ample, Csizmadia and co-workers have shown photochemically in-
duced Wolff rearrangement of 3-diazo-butan-2-one involves oxiirene
intermediates: a) G. Imre, J. F. Csizmadia, O. P. Strausz, J. Am.
Chem. Soc. 1968, 90, 7360–7361; b) see reference [5].
19] Differential pulse voltammetric response of monodispersed gold
nanoparticles reveals the occurrence of quantized oxidative and re-
ductive charging of an electrical double layer. a) S. Chen, R. S.
[30]
tides.
We believe that a similar procedure of electron
swapping is involved in the generation of copper and rhodi-
[1,2]
um carbenoids,
some of the well-known intermediates in
cyclopropanation and CꢀH insertion reactions, respectively,
from a-diazocarbonyl compounds. In addition, we hope that
the occurrence of this nonclassical electron-transfer process
[
Chem. Eur. J. 2006, 12, 859 – 864
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
863