Direct observation of a cationic gold(I)-bicyclo[3.2.0]hept-1(7)-ene complex generated in the cycloisomerization of a 7-phenyl-1,6-enyne

Article abstract of DOI:10.1002/anie.201301640

The reaction of enyne 1 with a 1:1 mixture of [LAuCl] and AgSbF₆ in CD₂Cl₂ at -20 °C gave the gold complex 2 in 97 % yield (NMR spectroscopy). Warming a solution of 2 at 25 °C led to 1,3-H migration (t_1/2≈16 min) to form the gold complex 3 with 96 % selectivity. ¹³C NMR analysis of 2 and 3 showed predominant metallacyclopropane character of the gold-bicyclo[3.2.0]heptene bond. Copyright

Full text of DOI:10.1002/anie.201301640

Angewandte
Chemie
DOI: 10.1002/anie.201301640
Cycloaddition
Direct Observation of a Cationic Gold(I)–Bicyclo[3.2.0]hept-1(7)-ene
Complex Generated in the Cycloisomerization of a 7-Phenyl-1,6-
enyne**
Rachel E. M. Brooner, Timothy J. Brown, and Ross A. Widenhoefer*
The cycloisomerization of enynes catalyzed by electrophilic
noble-metal complexes,[1] in particular Au and Pt,[2] has
attracted considerable attention owing to the formation of
skeletal rearrangement products, including vinylcyclopen-
tenes (B and C), bicyclo[4.1.0]heptenes (D), and bicyclo-
[3.2.0]hept-6-enes (E) from 1,6-enynes (A; Scheme 1).[3,4]
these cationic complexes, as no organometallic intermediate
has been observed spectroscopically in any of these trans-
formations.[6–10]
Platinum(II),[11] gold(I),[12,13] and rhodium(II)[14] com-
plexes catalyze the cycloisomerization of 7-aryl-1,6-enynes
(A, R = Ar) to form vinylcyclopentenes C and/or bicyclo-
[3.2.0]hept-6-enes E (Scheme 1). Directly implicated in these
and related[15,16] transformations is the strained bicyclo-
[3.2.0]hept-1(7)-ene species I, presumably generated through
6-endo-cyclization followed by 1,2-alkyl migration from
cyclopropyl carbene II and consumed either by ring opening
to form B and/or 1,3-hydrogen migration to form
E
(Scheme 1).[11–14] Possible contributors to I include the meta-
lated cyclobutyl carbenium ion Ia, the p-alkene complex Ib,
and the metallacyclopropane complex Ic. In this context, it
appeared likely that contribution of the latter forms would
engender stability to I not realized in the corresponding
cyclopropyl carbene intermediates II or III, such that I might
represent a local minimum on the reaction coordinate.
Indeed, here we report the selective generation, spectroscopic
characterization, and reactivity analysis of a gold–bicyclo-
[3.2.0]hept-1(7)-ene complex formed in the gold-catalyzed
cycloisomerization of a 7-phenyl-1,6-enyne.
Scheme 1. Ligand- and substrate-dependent pathways for enyne cyclo-
addition catalyzed by electrophilic noble-metal complexes.
Toward detection of a reactive bicyclo[3.2.0]hept-1(7)-ene
complex, we investigated the gold(I)-catalyzed conversion of
the 7-phenyl-1,6-enyne 1 into the 6-phenylbicyclo[3.2.0]hept-
6-ene 2 reported by Echavarren (Scheme 2).[12] In an initial
experiment, a 1:1:1 mixture of 1, [LAuCl] (L = P(tBu)2(o-
biphenyl)), and AgSbF6 in CD2Cl2 was mixed thoroughly at
ꢁ788C. 31P and 1H NMR analysis at ꢁ808C showed formation
of an approximately 9:1 mixture of p-alkene and p-alkyne
Fꢀrstner first posited that these outcomes were consistent
with the intermediacy of metal-stabilized nonclassical cyclo-
propylmethyl-, cyclobutyl-, and homoallylic carbocations/
=
carbenes accessed through attack of the C C moiety on
a metal-complexed C C bond (Scheme 1),[5] and mechanistic
ꢀ
thought in this area has evolved largely within this conceptual
framework. The involvement of nonclassical carbocations/
carbenes is supported by a wealth of indirect experimental
evidence, including trapping experiments, isotopic labeling
studies, and stereochemical analyses, and through numerous
computational studies.[1,2] Absent, however, is direct exper-
imental evidence regarding the structure and reactivity of
[*] R. E. M. Brooner, T. J. Brown, Prof. R. A. Widenhoefer
French Family Science Center, Duke University
Durham, NC 27708–0346 (USA)
E-mail: rwidenho@chem.duke.edu
[**] We thank the NSF (CHE-0555425) for support of this research.
REMB was supported in part through Charles Bradsher and Joe
Taylor Adams Fellowships (Duke University) and T.J.B. was
supported in part through a C. R. Hauser fellowship (Duke
University).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301640.
Scheme 2. Gold-mediated conversion of 1 into 4.
Angew. Chem. Int. Ed. 2013, 52, 1 – 4
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Angewandte
Communications
complexes 3a and 3b that together constituted around 90%
of the reaction mixture (Scheme 2).[17–19] When this solution
was warmed at ꢁ208C for 8 h, 3a and 3b were consumed to
form the gold–7-phenylbicyclo[3.2.0]hept-1(7)-ene complex 4
in 97% yield (1H NMR spectroscopy).[17] Complex 4 was
likewise the only phosphine-containing species observed by
31P NMR spectroscopy (d = 65.4) during the catalytic con-
version of 1 into 2 (Scheme 2).
without detectable formation of vinylcyclopentene
(Scheme 3).[24]
1
As was the case with 4, the JC6C7 coupling constant of 6
(1JC6C7 = 35 Hz) was diminished significantly relative to that of
free 2 (1JC6C7 = 63 Hz).[25] The strain of 2, although attenuated
relative to 5,[23] is apparently sufficient to significantly bias the
ꢁ
gold alkene bond of 6 toward the metallacyclopropane
structure. However, the lower strain of 2 relative to 5 was
evidenced by the about 60-fold lower binding affinity of 2 to
the [LAu]+ fragment relative to that of 5, determined by
Complex 4 was thermally unstable and was characterized
in solution at or below ꢁ208C by NMR spectroscopy and by
comparison with free 7-phenylbicyclo[3.2.0]hept-1(7)-ene 5
displacement of the weak s-donor ligand NCArF (NCArF =
(see below).[20] Coordination of gold to the C1 C7 bond of the
NC-3,5-C6H3(CF3)2)
from
[LAu(NCArF)]+
SbF6
ꢁ
ꢁ
bicyclo[3.2.0]heptene ligand was established by the large
upfield shift (Dd = ꢁ20) of the C1 carbon atom of 4 relative to
5 and by the presence of phosphorous coupling to both the C1
(d = 126.9; JCP = 13.8 Hz) and C7 carbon atom (d = 141.2;
(Scheme 4).[26] The higher binding affinity of 5 relative to 2
is significant, considering that the binding affinity of the
trisubstituted alkene 2-methyl-2-butene to [LAu]+ is approx-
imately 350 times greater than is that of the tetrasubstituted
alkene 2,3-dimethyl-2-butene.[18]
J
CP = 7.5 Hz) of the bicycloheptene ligand of 4 in the
13C NMR spectrum. 1H–1H NOESY analysis established
binding of the [LAu]+ fragment to the convex face of the
bicycloheptene ligand. Regarding the electronic structure of
4, the C7 (d = 141.2) and C7-phenyl resonances (d = 131.8,
129.1, 127.2) of 4 were shifted downfield only slightly relative
to those of free 5 (d = 137.59 (C7), 128.6, 127.7, 125.0 (Ph)) in
the 13C NMR spectrum;[21] this observation points to modest
accumulation of positive charge at the C7 carbon atom of 4
and hence nominal contribution of the cyclobutyl cation
structure 4a. Rather, the 1JC1C7 coupling constant of 4 (1JC1C7
=
Scheme 4. Equilibrium binding affinities of 2 and 5 to [LAu]+ relative
to NCArF (NCArF =NC-3,5-C6H3(CF3)2).
33 Hz) was diminished significantly relative to that of 5
(1JC1C7 = 61 Hz),[21] consistent with sp3 hybridization of the C1
and C7 carbon atoms of the bicycloheptene ligand of 4. This
observation points to significant d!p* back-bonding and the
predominant contribution of the metallacyclopropane struc-
ture 4c (Scheme 2).[22] The metallacyclopropane character of
4 stands in sharp contrast to the predominant s-bonding
character of extant gold–p-alkene complexes,[18] a discrepancy
attributed to the strain associated with free 5.[23]
In summary, the stoichiometric reaction of 1,6-enyne
1 with [LAuCl]/AgSbF6 at ꢁ208C led to selective (97%)
formation of the gold–bicyclo[3.2.0]hept-1(7)-ene complex 4,
which is the only phosphine-containing species observed in
the gold-catalyzed conversion of 1 into 2. Complex 4 under-
goes rapid 1,3-hydrogen migration at room temperature (t1/2
ꢂ 16 min) to form the gold–bicyclo[3.2.0]hept-1(7)-ene com-
plex 6 with more than 90% selectivity. 13C NMR analysis of
both 4 and 6 established the dominant metallacyclopropane
character of these complexes, which differs markedly from the
predominant s-donating character of known cationic, two-
coordinate gold–p-alkene complexes.[18]
Treatment of 4 with pyridine or nBu4N+ Brꢁ (1 equiv) at
ꢁ208C led to quantitative formation of free 5 and [LAu(py)]+
ꢁ
SbF6 or [LAuBr], respectively (Scheme 3); warming the
latter solution at 258C led to the slow (t1/2 ꢂ 9 h) decompo-
sition of 5 without detectable formation of 2 or vinylcyclo-
pentene. Conversely, warming a solution of 4 at 258C led to
rapid (t1/2 ꢂ 16 min) 1,3-hydrogen migration to form the
thermally sensitive gold–6-phenylbicyclo[3.2.0]hept-6-ene
complex 6 in 82% yield at 85% conversion (96% selectivity)
Received: February 26, 2013
Published online: && &&, &&&&
Keywords: alkene ligands · cycloaddition · gold ·
.
homogeneous catalysis · metallacycles
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1
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[24] The 1H NMR resonances corresponding to 6 became obscured at
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[25] Determined by analysis of the 13C-labeled isotopomers 6-6-13C1
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6
(ꢁ208C, 500 MHz, aliphatic
resonances only): d = 4.66 (d, J = 4.8 Hz, 1H), 3.99 (dd, J = 2.9,
7.9 Hz, 1H), 3.67 (s, 3H), 3.21 (ddd, J = 1.4, 3.0, 7.6 Hz, 1H), 3.15
(s, 3H), 2.69 (d, J = 14.0 Hz, 1H), 2.55 (d, J = 14.4 Hz, 1H), 2.29
(dd, J = 8.2, 14.4 Hz, 1H), 2.08 (dd, J = 7.8, 14.0 Hz, 1H), 1.30 (d,
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=
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Angewandte
Communications
Communications
Cycloaddition
R. E. M. Brooner, T. J. Brown,
R. A. Widenhoefer*
&&&& — &&&&
Direct Observation of a Cationic Gold(I)–
Bicyclo[3.2.0]hept-1(7)-ene Complex
Generated in the Cycloisomerization of
a 7-Phenyl-1,6-enyne
The reaction of enyne 1 with a 1:1 mixture
of [LAuCl] and AgSbF6 in CD2Cl2 at ꢁ208C
gave the gold complex 2 in 97% yield
(NMR spectroscopy). Warming a solution
of 2 at 258C led to 1,3-H migration (t1/
2ꢂ16 min) to form the gold complex 3
with 96% selectivity. 13C NMR analysis of
2 and 3 showed predominant metalla-
ꢁ
cyclopropane character of the gold
bicyclo[3.2.0]heptene bond.
4
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Angew. Chem. Int. Ed. 2013, 52, 1 – 4
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