Table 1. Features of Silver Nanoclusters (Agn) Formed under
Various Conditions of Silver(I) Catalysis of the R-Diazoketonea
silver nanoclusters (Agn)
silver(I)
compd
UV-vis particle size yield of 3
coreagent λmax (nm)
(nm)
(%)
i
AgOCOPh Et3N
308
304
433
436
347
308, 440
0.5-15
91
86b
87
92
68
76
ii AgNO3
iii Ag2O
iv Ag2O
NH3
Et3N
Na2S2O3
1-10
250c
258d
1-5
v
Ag2O
vi AgOCOPh
1-14
a General Procedure. The silver catalyst (5 wt % R-diazoketone) and
either silver nanoclusters (Agn) prepared following the appropriate literature
procedure5,12-14,16 excluding the R-diazoketone at room temperature (for
cases i-iv) or the silver(I) compound (for the cases v and vi) is added in
small portions at the regular interval of 20 min to the aqueous 1,4-dioxane
solution of 2-hexanone-1-diazo-6-phenyl 1 (0.1 M) at 60 °C until the
complete decomposition of 1 (0.5 h to 1 h). A usual solvent extractive
workup followed by purification furnished the Wolff rearrangement product
3, which matched the reported physical and spectral characteristics.
b Chromatographically isolated yield of an amide of the carboxylic acid 3.
c TEM core size. d From SEM images.
Figure 1. UV-visible spectroscopic followup of the formation
of Newman’s silver reagent prepared by mixing triethylamine and
silver(I) benzoate for (A) 1 min, (B) 5 min, (C) 10 min, (15 min;
UV-visible spectra in aqueous 1,4-dioxan) of (E) triethylamine,
(F) silver(I) benzoate.
thiosulfate,14 and silver(I) benzoate15,16 (AgOCOPh) in
combination with Et3N are more frequently used. The use
of coreagents along with silver(I) compounds leads to a more
reproducible15 formation of the rearrangement product in
relatively higher yields at much lower temperatures. For
example, AgOCOPh15,16 in combination with Et3N catalyzes
the Wolff rearrangement of R-diazoketones at room tem-
perature. Although several suggestions such as reduction of
the activation energy, solvent action on the silver(I) com-
pounds leading to catalysis under homogeneous conditions
or as a base to abstract the most acidic R-hydrogen resulting
in the formation of carbanion have been made, the exact role
of these additives is not well understood.
In this communication, we reveal for the first time
experimental proof17-19 that electron donation by the addi-
tives to silver(I) compounds leads to the formation of silver
nanoclusters (Agn) during the reaction, which presumably
catalyzes the Wolff rearrangement of R-diazoketones. The
latter feature is further confirmed by the formation of
6-phenyl-hexanoic acid 3 from R-diazoketone 1 following
the portion wise addition of preformed silver nanoclusters20
(Table 1; entries i-iv). The evidence from the surface
plasmon absorption18 of the silver nanocluster formed during
the reaction along with transmission electron micrographs
(TEM) give unambiguous confirmation for the involvement
of nanoclusters. These results may prove to be germane to
the understanding of the catalysis of several organic reactions
by metal nanoclusters, especially the corelation of cluster
size with catalytic activity.
Transitory formation of an orange color during the
preparation of Newman’s silver reagent15,16 using AgOCOPh
and Et3N kindled our curiosity to know more about the nature
of silver catalyst. UV-visible spectroscopy (Figure 1) reveals
the presence of a charge-transfer absorption band between
λ ) 517 and 308 nm. With increasing time, the absorption
in the visible region diminishes in intensity along with a
concomitant rise in the peak intensity in the ultraviolet region.
Significantly, this latter absorption (around λ ) 308 nm)
corresponds to the surface plasmon characteristic of silver
nanoclusters (Agn).18 It is relevant to note that silver ions
are devoid of any UV-visible absorption, while metallic
silver absorbs in the vacuum ultraviolet region. The plasmon
absorption peak at λ ) 308 nm (Figure 1) shifts bathochro-
mically with time to give a relatively stable optical response
around λ ) 436 nm after about 12 h (Figure 2A and
Supporting Information). This phenomenon can be under-
stood in terms of the growth of particles of chemically
identical silver clusters (Agn). TEM and selected area electron
diffraction (SAED) at various regions of the sample specimen
taken at different intervals provide conclusive evidence in
this direction. For example, the TEM image obtained after
half an hour (Figure-3A) show the presence of nearly
monodispersed spherical particles having an average size of
1.4 nm (σ ( 0.048), while similar analysis performed after
12 h (Figure 3B) exhibits the formation of relatively larger
clusters (0.5-15 nm) with an average particle size of 4.6
nm (σ ( 4.2). Importantly, all the electron diffraction patterns
(inset of Figure 3) are identical to the structure of metallic
silver (FCC, a ) 4.104 Å). In addition, the silver surface
(14) Furniss, B. S.; Tatchell, A. R. Vogel’s Textbook of Practical Organic
Chemistry, 4th ed.; ELBS-Longmann: London, 1978; p 485.
(15) Newman, M. S.; Beal, P. F. J. Am. Chem. Soc. 1950, 72, 3.
(16) Lee, V.; Newman, M. S. Org. Synth. 1970, 50, 77.
(17) Aiken, J. D., III; Aiken, R. G. J. Mol. Catal. A.: Chem. 1999, 145,
1.
(18) Henglein, A. J. Phys. Chem. 1993, 97, 457.
(19) Mulvaney, P. In Semiconductors Nanoclusters-Physical, Chemical,
and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier: Amsterdam,
1987, pp 99-124.
(20) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J.
Chem. Soc., Chem. Commun. 1994, 801. We prepared silver nanoclusters
using silver(I) benzoate and sodium borohydride as the reducing agent.
Deliberate addition of this nanocluster to R-diazoketone 1 gives Wolff
rearranged carboxylic acid 3 in 80% chemical yield.
2356
Org. Lett., Vol. 5, No. 13, 2003