A silver-catalyzed Büchner reaction

Article abstract of DOI:10.1016/j.tetlet.2005.02.052

A silver scorpionate complex, derived from the highly fluorinated [HB(3,5-(CF₃)₂Pz)₃]^-, catalyzes the addition of ethyl diazoacetate to benzene rings, providing norcaradienes, which undergo electrocyclization to provide the corresponding cycloheptatriene. These reactions are surprisingly selective for addition to the aromatic moiety rather than C-H insertion.

Full text of DOI:10.1016/j.tetlet.2005.02.052

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 2453–2455
¨
A silver-catalyzed Buchner reaction
Carl J. Lovely,* R. Greg Browning, Vivek Badarinarayana and H. V. Rasika Dias*
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, TX 76019, USA
Received 16 November 2004; revised 8 February 2005; accepted 9 February 2005
Abstract—A silver scorpionate complex, derived from the highly fluorinated [HB(3,5-(CF3)2Pz)3]ꢀ, catalyzes the addition of ethyl
diazoacetate to benzene rings, providing norcaradienes, which undergo electrocyclization to provide the corresponding cyclohepta-
triene. These reactions are surprisingly selective for addition to the aromatic moiety rather than C–H insertion.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Some time ago, we initiated a program to investigate the
catalytic potential of several metal complexes derived
from highly fluorinated analogs (e.g., 1a and 2) of the
classical tris(pyrazolyl)borate (scorpionate) ligands.1
Along these lines, our labs have established that the cop-
per(I) complexes (e.g., 1a) are effective nitrene transfer
agents2 and carbene transfer agents,3 providing aziri-
dines and cyclopropanes, respectively, from alkenes,
and C–H insertion products with alkanes. More recently
we have examined the ability of the related silver(I) ad-
duct 2 to function as a nitrene and carbene transfer cata-
lyst (Fig. 1). Although we have not demonstrated that
nitrene transfer reactions are feasible with this complex,4
it effectively catalyzes carbene insertion reactions into
C–H and C–X (X = Cl, Br) bonds.3,5,6
In a continuation of these efforts we attempted to extend
the chemistry of this latter complex, by examining its
utility in cyclopropane formation by carbenoid addition
to alkenes. On treatment of a dichloromethane solution
of styrene with ethyl diazoacetate (4, EDA) in the pres-
ence of 5 mol % of 2, a reaction took place as evidenced
by the liberation of nitrogen. However, analysis of the
1
surprisingly complex H NMR spectrum of the crude
reaction mixture indicated that the anticipated simple
addition to the olefinic bond was not the major product
(Scheme 1, 3 + 4 ! cis/trans 5). There were several
unexpected signals, other than unreacted styrene, in
the vinylic region of the spectrum that initially could
not be accounted for. However on further analysis it be-
came clear that, in addition to the expected cyclopro-
pane, addition to the aromatic moiety had occurred
followed by a ring expansion, that is, a Buchner reaction
¨
had taken place (Scheme 1).7,8 It should be noted that in
addition to the ring expansion product, insertion of the
carbene into the C–Cl bond of dichloromethane had oc-
curred.5 To simplify matters, the reaction was conducted
CO2Et
CO2Et
CO2Et
4
N2
5
6
CO2Et
2, CH2Cl2
Figure 1. Scorpionate catalysts.
Cl
3
CO2Et
CO2Et
Cl
Keywords: Carbene; Scorpionates; Catalysis; Ring-expansion.
*
Corresponding authors. Tel.: +1 817 272 5446; fax: +1 817 272 3808
(C.J.L.); tel.: +1 817 272 3813; fax: +1 817 272 3808 (H.V.R.D.); e-mail
addresses: lovely@uta.edu; dias@uta.edu
7
8
Scheme 1.
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.02.052

2454
C. J. Lovely et al. / Tetrahedron Letters 46 (2005) 2453–2455
using benzene as a substrate, and much to our delight, a
single cyclohepatriene product (11a) was obtained in
74% yield (Table 1). A small quantity of the C–Cl inser-
tion product 8 was also obtained (ca. 25%), so to prevent
this competing reaction pathway, the reaction was
repeated in neat benzene, under these conditions the ring
expansion product was obtained in a comparable 75%
yield (based on EDA).
Given that these reaction conditions initiated the addi-
tion and rearrangement with benzene, the generality of
the reaction with substituted derivatives was investi-
gated. As can be seen in Table 1, both electron rich
and electron deficient systems participate in the rear-
rangement, but with varying efficiencies and selectivities
Table 1. Yields and isomer ratios of the silver catalyzed Buchner9
reaction benzene derivatives and EDA
¨
CO Et
2
CO Et
2
CO Et
2
4
N
2
`
(vis a vis isomer distribution).
2, CH2Cl2
R
11a-f
R
R
In general, the more electron rich systems react with bet-
ter efficiencies than the more electron poor systems,
although all substituents appear to lead to reduced
yields compared to the parent substrate. This latter
observation is in keeping with the electrophilic nature
of these metallocarbenoid systems. These observations
parallel the results obtained by Noels and co-workers
9a-f
10a-f
Substrate
Product
Yielda,b (%)
Isomer ratio (2:3:4)
CO2Et
with rhodium catalysts, but the yields are lower.10,13 Per-
9a
11a10,11
74
75c
N/A
N/A
´
ez and co-workers have reported that related complexes
1b and 1c will catalyze carbene transfer to aromatics.11
In their studies with alkylbenzenes it was found that
both addition (formation of 11) and benzylic C–H inser-
tion were observed to varying extents, depending on the
catalyst. With catalyst 2, no evidence of aryl C–H inser-
tion was noted and only in the reaction of 9b was a small
amount of benzylic C–H insertion (Fig. 2, 12), ca. 4–5%,
CO2Et
CH3
CH3
9b
11b11
64d
62c,e
1:1.8:2.4
CO2Et
OCH3
1
observed in the H NMR spectrum of the crude reac-
tion mixture. In a related study it has been demonstrated
that this complex is an effective catalyst for C–H inser-
OCH3
tion and so the selectivity for the Buchner pathway is
¨
9c
11c12
40
1:0:19
quite interesting.3 To probe this issue further, the reac-
tion of mesitylene under these conditions was investi-
gated. Interestingly, the major pathway was again the
CO2Et
Cl
Buchner reaction, although C–H insertion did occur to
¨
an appreciable extent ca. 28%. The selectivity for the
Buchner pathway is quite remarkable when this out-
¨
Cl
9d
11d12
49f
1:8.5:18
come is normalized for the number of reactive sites,
which suggest that addition to C@C occurs ca. 2–7·Õs
faster than C–H insertion. In addition to chemoselectiv-
ity issues, these addition–rearrangement reactions can
give rise to up to three regioisomeric products. In gen-
CO2Et
CO2Me
CO2Me
9e
11e12
14f
0:1:1
eral our results mirror those of Noels and co-workers
10,11
´
and Perez and co-workers,
with the 4-isomer pre-
CO Et
2
dominating, although the scorpionate systems appear
to give rise to a greater proportion of the 3-isomer
compared to the rhodium systems.
9f
11f11
35g
N/A
In conclusion, we have discovered a novel silver-cata-
lyzed carbene addition reaction that proceeds in good
to moderate yields. Silver-catalyzed homogeneous pro-
cesses of any type are rare,6 and to our knowledge, this
a The yields are the average of at least two runs and refer to isolated
material and, unless noted, were conducted in CH2Cl2.
b In the reactions conducted in CH2Cl2, there was 15-25% of the C–Cl
1
insertion product 8 observed in the H NMR spectrum of the crude
study represents the first use of silver(I) in Buchner
¨
reaction mixture.
c These reactions were conducted in the neat arene.
d 4% of the C–H insertion product was observed in the 1H NMR
spectrum of the crude reaction mixture.
OEt
OEt
e 5% of the C–H insertion product was observed in the 1H NMR
spectrum of the crude reaction mixture.
O
O
f The purified material was contaminated with ca. 5% of the C–Cl
insertion product.
g 28% of the C–H insertion product 13 was observed in the 1H NMR
spectrum of the crude reaction mixture.
12
13
Figure 2. C–H insertion products from 9b and 9f.

C. J. Lovely et al. / Tetrahedron Letters 46 (2005) 2453–2455
2455
Chem. Soc. 2001, 123, 11101; (c) Yang, M.; Webb, T. R.;
Livant, P. J. Org. Chem. 2001, 66, 4945; (d) Doyle, M. P.;
Ene, D. G.; Forbes, D. C.; Pillow, T. H. Chem. Commun.
1999, 1691; (e) Maguire, A. R.; Buckley, N. R.; OÕLeary,
P.; Ferguson, G. J. Chem. Soc., Perkin Trans. 1 1998,
4077; (f) Moody, C. J.; Miah, S.; Slawin, A. M. Z.;
Mansfield, D. J.; Richards, I. C. J. Chem. Soc., Perkin
Trans. 1 1998, 4067; (g) Manitto, P.; Monti, D.; Zanzola,
S.; Speranza, G. J. Org. Chem. 1997, 62, 6658.
chemistry. Typically, rhodium(II) compounds are the
most widely used catalysts for this important process.8
The yields of silver scorpionate mediated process appear
to be somewhat lower than with the rhodium-based sys-
tems, but this may be due to the slow decomposition of
2. The presence of a B–H moiety presumably leads to
reduction of the Ag(I) to Ag(0), and attendent loss of
activity.1 Current efforts are focused on identifying cata-
lysts that retain the reactivity of 2, but are more stable.
9. General procedure: Ethyl diazoacetate (114 mg,
1.00 mmol) in CH2Cl2 (5 mL) was added by syringe pump
over 4 h to a stirred solution of the arene 9 (5.00 mmol)
and 2 (40 mg, 0.05 mmol) in CH2Cl2 (1.5 mL) under
nitrogen in a flask shielded from light. The resulting
solution was stirred overnight at room temperature under
nitrogen, diluted with CH2Cl2 (20 mL), filtered through
Celite, and concentrated under reduced pressure. The
residue was purified by flash chromatography (SiO2, 4:1
hexanes/ether) to give the cycloheptatriene 11 (and C–Cl
insertion product 8) as a colorless oil.
Acknowledgements
This work was supported by the Robert A. Welch Foun-
dation (Y-1362 (C.J.L.) and Y-1289 (H.V.R.D.)) and
NSF (CHE-0314666 for H.V.R.D.). The NSF (CHE-
9601771) is thanked for partial funding of the purchase
of a 500 MHz NMR spectrometer.
10. Anciaux, A. J.; Demonceau, A.; Noels, A. F.; Hubert, A.
´
J.; Warin, R.; Teyssie, P. J. Org. Chem. 1981, 46,
873.
References and notes
11. Morilla, M. E.; Diaz-Requejo, M. M.; Belderrain, T. R.;
Nicasio, M. C.; Trofimenko, S.; Perez, P. J. Organomet-
allics 2004, 23, 293.
12. The spectroscopic data for these compounds is essentially
identical to that obtained for the corresponding Buchner
¨
1. Trofimenko, S. Scorpionates: The Coordination Chemistry
of Polypyrazolylborate Ligands; Imperial College: London,
1999; Pettinari, C.; Santini, C. In Comprehensive Coordi-
nation Chemistry II; Elsevier, 2004; Vol. 1, pp 159–210.
2. Dias, H. V. R.; Lu, H.-L.; Polach, S. A.; Goh, T.; Browning,
R. G.; Lovely, C. J. Organometallics 2002, 21, 1466.
3. Dias, H. V. R.; Browning, R. G.; Richey, S. A.; Lovely, C.
J. Organometallics 2004, 23, 1200.
4. (a) Cui, Y.; He, C. J. Am. Chem. Soc. 2003, 125, 16202; (b)
Cui, Y.; He, C. Angew. Chem., Int. Ed. 2004, 43, 4210.
5. Dias, H. V. R.; Browning, R. G.; Polach, S. A.; Diyabal-
anage, H. V.; Lovely, C. J. J. Am. Chem. Soc. 2003, 125,
9270–9271.
6. For other silver catalyzed processes see citations in Ref. 3.
Also see: (a) Sweis, R. F.; Schramm, M. P.; Kozmin, S. A.
J. Am. Chem. Soc. 2004, 126, 7442; (b) Driver, T. G.;
Woerpel, K. A. J. Am. Chem. Soc. 2004, 126, 9993; (c)
Bachmann, S.; Fielenbach, D.; Anker Jørgensen, K. Org.
Biomol. Chem. 2004, 4210.
products derived from methyl diazoacetate, Ref. 10.
Selected 1H NMR (500 MHz, CDCl3) data for: 11c (4-
isomer) d = 6.20 (ddd, J = 8.1, 6.7, 1.2 Hz, 1H), 6.07 (ddd,
J = 9.9, 1.7, 1.7 Hz, 1H), 5.86 (dd, J = 6.7, 2.0 Hz, 1H),
5.61 (dd, J = 9.9, 5.9 Hz, 1H), 5.26 (ddd, J = 10.0, 5.2,
0.7 Hz, 1H), 4.25 (q, J = 7.1 Hz, 2H), 3.66 (s, 3H), 2.69
(dd, J = 5.5, 5.5 Hz, 1H), 1.30 (t, J = 7.1 Hz, 3H). Com-
pound 11d (4-isomer) d = 6.81 (dd, J = 6.4, 1.0 Hz, 1H),
6.59 (ddd, J = 11.4, 5.7, 0.5 Hz, 1H), 5.53–5.46 (m, 2H),
5.20–5.14 (m, 1H), 4.25 (q, J = 7.2 Hz, 2H), 2.71 (ddd,
J = 6.0, 5.9, 1.3 Hz, 1H), 1.30 (t, J = 7.2 Hz, 2H); (3-
isomer) = 6.58 (ddd, J = 10.9, 5.7, 0.5 Hz, 1H), 6.47–6.45
(m, 1H), 6.42-6.38 (m, 1H), 6.32 (dd, J = 9.4, 5.7 Hz, 1H),
5.76 (dd, J = 8.2, 7.6 Hz, 1H), 4.24 (q, J = 7.4 Hz, 2H),
2.48 (ddd, J = 5.7, 5.0, 0.7 Hz, 1H), 1.27 (t, J = 7.4 Hz,
3H). 11e (1:1 mixture of 3- and 4-isomers) = 7.20–7.16 (m,
1H), 7.00–6.90 (m, 1H), 6.86–6.80 (m, 1H), 6.63 (dd,
J = 10.1, 6.5 Hz, 1H), 6.42–6.36 (m, 2H), 5.71–5.69 (m,
1H), 5.52 (ddd, J = 8.9, 5.5, 1.5 Hz, 1H), 5.34–5.31 (m,
1H), 4.82–4.78 (m, 1H), 4.27 (q, J = 7.1 Hz, 4H), 3.83 (s,
3H), 3.79 (s, 3H), 1.32–1.26 (m, 6H). For the correspond-
ing conjugated isomers of 11c see: Garst, M. E.; Roberts,
V. A. J. Org. Chem. 1982, 47, 2188.
7. The Buchner reaction is the thermal or photochemically
¨
induced carbene addition to an aromatic, forming a
norcaradiene derivativem followed by ring expansion to
provide a cycloheptatriene. Typically, under these condi-
tions mixtures of conjugated and non-conjugated isomers
are obtained. Buchner, E.; Curtius, T. Chem. Ber. 1885,
¨
18, 2371.
8. Transition metal catalyzed variants of the Buchner reac-
¨
tion are known, and tend to produce non-conjugated
isomers due to the generally milder conditions employed.
For some recent papers see: (a) Pirrung, M. C.; Liu, H.;
Morehead, A. T., Jr. J. Am. Chem. Soc. 2002, 124, 1014;
(b) Merlic, C. A.; Zechman, A. L.; Miller, M. M. J. Am.
13. This can be explained in part due to differences in the
reaction conditions employed. In NoelsÕ work, the aryl
system was used as both substrate and solvent, whereas in
this work the reactions were conducted in CH2Cl2 in most
cases.