The [4+2], not [2+2], mechanism occurs in the gold-catalyzed intramolecular oxygen transfer reaction of 2-alkynyl-1,5-diketones

Article abstract of DOI:10.1002/anie.201005514

[2+2] or [4+2]? Mechanistic studies on the gold-catalyzed intramolecular oxygen transfer of 2-alkynyl-1,5-diketones were performed using isotopic experiments and quantum chemical calculations. The results demonstrate that the transformation follows a [4+2

Full text of DOI:10.1002/anie.201005514

Communications
DOI: 10.1002/anie.201005514
Gold Catalysis
The [4+2], not [2+2], Mechanism Occurs in the Gold-Catalyzed
Intramolecular Oxygen Transfer Reaction of 2-Alkynyl-1,5-
Diketones**
Le-Ping Liu,* Deepika Malhotra, Robert S. Paton, K. N. Houk,* and Gerald B. Hammond*
The chemistry of the oxonium and aminium ions formed from
alkynylic aldehydes or imines by transition metals, Lewis
acids, Brønsted acids, or even electrophiles such as iodine
(Scheme 1), has attracted wide interest. These intermediates
undergo both intermolecular and intramolecular cycloaddi-
tions to carbon–carbon multiple bonds, to give myriad
products of synthetic importance.[1,2–10]
Recently, we developed a convenient approach to 2-
alkynyl-1,5-dicarbonyl derivatives, 1, from the Michael addi-
tion of activated allenes to electron-deficient olefins.[11] We
envisioned that if a furanium intermediate[12] (Scheme 2)
Scheme 2. Formation of furanium ions from propargylic ketones.
could be generated from the alkynylketone 1 containing a
quaternary propargylic carbon, then the usual path A[12]
would be blocked, leaving path B to engender novel trans-
formations of the oxonium intermediate.
We have now found that after only 5 min at room
temperature, using gold catalysis,[13,14] 2-alkynyl-1,5-diketone
1a furnished cyclopentenylketone 2a—an intramolecular
oxygen transferred product—in excellent yield (Scheme 3,
top).
Scheme 3. Gold-catalyzed intramolecular oxygen transfer of alkynylke-
tones.
Of special interest is the oxygen transfer from a carbonyl
group to a carbon–carbon triple bond through an oxonium
intermediate, also known as alkyne–carbonyl metathesis.[15]
This has also been showcased in Yamamoto and co-workersꢀ
recent papers on gold or TfOH-catalyzed intramolecular
oxygen transfer of w-alkynylketones to the corresponding
cyclic enones (Scheme 3, bottom).[16] A [2+2] pathway has
been invoked for this oxygen transfer.[15,16]
Scheme 1. Formation of oxonium or aminium ions from alkynylic
aldehydes or imines.
[*] Dr. L.-P. Liu, D. Malhotra, Prof. Dr. G. B. Hammond
Department of Chemistry, University of Louisville
2320 South Brook Street, Louisville, KY 40292 (USA)
Fax: (+1)502-852-3899
A [2+2] pathway might be invoked to rationalize the
outcome in our gold-catalyzed, intramolecular oxygen trans-
fer of 2-alkynyl-1,5-diketones to form the corresponding
cyclopentenylketones. However, the fact that this reaction
could be completed in minutes at room temperature, and with
higher yields than those of previously reported oxygen
transfers,[15,16] prompted us to propose an alternative [4+2]
mechanism (Scheme 4), and to investigate the scope of this
reaction. Herein, we describe the results from experimental
and theoretical investigations that establish this new mech-
anism.
E-mail: leping.liu@louisville.edu
gb.hammond@louisville.edu
Dr. R. S. Paton, Prof. Dr. K. N. Houk
Department of Chemistry and Biochemistry,
University of California, Los Angeles
607 Charles E. Young Drive East, Los Angeles, CA 90095 (USA)
E-mail: houk@chem.ucla.edu
[**] We are grateful to the National Science Foundation for financial
support (CHE-0809683, EPS-0447479) and the Royal Commission
for 1851 (R.S.P.). Professor K. N. Houk is grateful to the National
Science Foundation (CHE-0548209) for financial support. We also
thank Prof. Dr. Eugene Mueller for generously donating us the
H218O, and to Neeraj Kumar and Prof. Dr. Pawel M. Kozlowski for
their initial contributions to the computational work.
To elucidate which pathway—the well-accepted [2+2]
mechanism or our newly proposed [4+2] pathway[17]—was
responsible for the gold-catalyzed intramolecular oxygen
transfer of 2-alkynyl-1,5-diketones, we designed an isotopic
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005514.
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Figure 1. Synthesis of substrate 1b-18O and the 13C NMR spectrum.
[a] Determined by MS (ESI).
Scheme 4. Isotopic labeling experiment for mechanistic studies.
labeling experiment (Scheme 4). We speculated that if an 18
O
atom could be incorporated into one of the carbonyl groups of
the substrate, then the 13C NMR spectrum[18] of the reaction
product could be used to locate the 18O atom, and provide
clues as to the more favorable mechanistic pathway. Hence,
using alkynyldiketone 1b-18O (R = p-methoxyphenyl) as a
model substrate, if the reaction follows a [2+2] route then 18
O
would end up on the left carbonyl group in 2b-18O-a
(Scheme 4, top), whereas it would be incorporated on the
benzoyl group in 2b-18O-b if the reaction follows a [4+2]
pathway (Scheme 4, bottom).
Substrate 1b-18O was synthesized from the 18O exchange
of compound 1b with H218O under acidic condition
(Figure 1).[19] The 18O exchange happened only at the
methyl carbonyl group, as indicated by 13C NMR spectrosco-
py. A d = 0.05 ppm (5 Hz) upfield chemical shift[18] was found
on carbon 1, whereas no chemical shift change occurred on
carbon 2. With substrate 1b-18O in hand, we carried out the
gold-catalyzed oxygen transfer reaction and the product 2b-
18O was obtained in quantitative yield (Figure 2), without
any 18O loss as determined by its ESI mass spectrum. It was
found that only carbon 4 in the product 2b-18O exhibited the
d = 0.05 ppm (5 Hz) upfield chemical shift in its 13C NMR
spectrum (also see Supporting Information). The absence of
any detectable 18O incorporation at carbon 3 demonstrates
that the [2+2] pathway is disfavored, and instead it is a [4+2]
pathway that is the favored mechanism for the gold-catalyzed
intramolecular oxygen transfer of 2-alkynyl-1,5-diketones.
Since our experimental studies established the [4+2]
pathway to be the preferred mechanism, we turned to
quantum chemical calculations to seek further verification
of the preferred pathway and to understand the origins of the
selectivity.[20] We performed calculations at the “double-
hybrid” density functional level of theory with the B2PLYP
functional. This method replaces a fraction of the semi-local
correlation energy by a non-local correlation energy expres-
Figure 2. 18O isotopic experiment and the 13C NMR spectrum.
[a] Determined by MS (ESI). [b] NMR yield.
sion that employs the Kohn–Sham orbitals in second-order
perturbation theory and delivers improved energetics over
hybrid density functionals such as B3LYP. Results obtained at
this level were also compared with the standard B3LYP and
M06-2X functionals, and from single-point energy calcula-
tions at the SCS-MP2 level: we found that all methods were in
agreement over the preferred mechanism (for a full compar-
ison refer to the Supporting Information). DFT studies of AuI
catalysis have been shown to give qualitative and quantitative
insight into the catalytic cycle.[21] All calculations were
performed with Gaussian09[22] and use a LANL2DZ effective
core potential for Au and Pople double or triple-z basis sets
for all other elements.[23]
The competing [2+2] and [4+2] reaction coordinates were
computed for the substrate shown in Scheme 5. Calculations
were also performed for a larger substrate for which the aryl
groups of 1a are modeled by phenyl groups, giving very
similar results. The computed reaction coordinate for each
substrate is shown in full in the Supporting Information. In
accord with our experimental findings, the [4+2] pathway is
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Communications
dihedral angle (0.88) between the alkyne and attacking
carbonyl not present in TS-1 due to the constraints of the 7-
membered ring (48.98), and also in the intermolecular
reaction (75.88) due to sterics. The low computed barrier for
TS-4 is consistent with the rapid conversion seen experimen-
tally. TS-5 is concerted but highly asynchronous, with forming
À
À
bond lengths of 1.66 ꢁ (C O) and 2.67 ꢁ (C C), and lies only
4.4 kcalmolÀ1 above the starting complex. Overall, the large
energetic preference of TS-4 over TS-1, which was seen at all
levels of theory we tested, supports the idea that the [4+2]
pathway is dominant in the gold-catalyzed oxygen transfer of
2-alkynyl-1,5-diketones, which is in accordance with our 18O
isotopic experiments.
We then proceeded to investigate the scope of this gold-
catalyzed intramolecular oxygen transfer of 2-alkynyl-1,5-
diketones. Both aromatic and aliphatic-containing substrates
were screened, and the corresponding products 2b–2j were
obtained in excellent yields (Table 1).
Table 1: Gold-catalyzed intramolecular oxygen transfer of 2-alkynyl-1,3-
diketones.[a]
Scheme 5. B2PLYP/6-311+G(d,p)//B2PLYP/6-31G(d) computed reac-
tion profile. Relative energies in kcalmolÀ1
.
computed to be the more favorable. The rate-limiting step in
each pathway is the intramolecular nucleophilic addition to
the Au-coordinated alkyne—the barrier for this step is
computed to be 6.8 kcalmolÀ1 lower for the formation of
the five-membered ring oxonium intermediate C than for
seven-membered ring oxonium A. This energetic preference
is also observed in the stabilities of the oxonium ions
themselves, with C considerably more stable by 16.1 kcal
molÀ1. The subsequent transformations are all computed to be
feasible, with the barrier to [4+2] cyclization lying only
4.4 kcalmolÀ1 above the starting complex.
The key transition structures for the intramolecular
alkyne addition are shown in Figure 3, along with the [4+2]
cyclization transition state (TS). Why is 5-endo-dig TS-4 so
drastically preferred over 7-endo-dig TS-1? By way of
comparison, we computed the barrier for the intermolecular
addition of acetone to Au-coordinated butyne to be 19.3 kcal
molÀ1, so TS-4 appears to be remarkably stable rather than
any inherent instability of TS-1. Acceleration of the ring-
closing due to the presence of a quaternary center cannot fully
explain the low barrier of 8.8 kcalmolÀ1, since replacing this
carbon with methylene only raises the barrier by 0.7 kcal
molÀ1. However, TS-4 does benefit from an almost planar
Entry
R1, R2
Yield [%]/2[b]
1
2
3
4
5
6
7
8
9
10
C6H5, Me 1a
p-MeOC6H4, Me 1b
p-ClC6H4, Me 1c
n-C6H13, Me 1d
iPr, Me 1e
tBu, Me 1 f
Me, Me 1g
Bn, Me 1h
CypCH2, Me 1i
Ph, Et 1j
2a, 98
2b, 99
2c, 92
2d, 99
2e, 96
2 f, 95
2g, 91
2h, 94
2i, 99
2j, 98
[a] General reaction conditions: 2-alkynyl-1,5-diketone
CH2Cl2 2.0 mL. [b] Yields of isolated product.
1 0.3 mmol,
In conclusion, we have discovered a novel [4+2] pathway
for the gold-catalyzed intramolecular oxygen transfer of 2-
alkynyl-1,5-diketones to cyclopentenylketones. This mecha-
nism was confirmed by 18O isotopic labeling experiments and
quantum chemical calculations. Various substrates were
employed in the reaction and the corresponding products
were obtained in excellent yields under very mild conditions.
Further exploration and applications of this methodology and
calculations on the mechanism are underway in our groups.
Experimental Section
General procedure for gold-catalyzed oxygen transfer of 2-alkynyl-
1,5-diketones to the corresponding cyclopentenylketones: To
a
solution of 2-methyl-1-phenyl-2-(phenylethynyl)hexane-1,5-dione
(1a; 46 mg, 0.20 mmol) in dichloromethane (1.0 mL) was added
AuCl (2 mg, 0.010 mmol). The mixture was stirred for 5 min at room
temperature. Afterwards the solvent was removed under reduced
pressure and the residue was subjected to a flash column chromatog-
raphy (eluent: ethyl acetate/n-hexane 1:15) to give product 2a (45 mg,
Figure 3. B2PLYP/6-31G(d) transition structures for addition to the
alkyne and for [4+2] cyclization.
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~
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1
1446, 1283, 973 cmÀ1; H NMR (CDCl3, 500 MHz): d = 1.58 (3H, s),
1.70 (3H, s), 2.14–2.18 (1H, m), 2.48–2.53 (1H, m), 2.56–2.61 (1H, m),
2.76–2.78 (1H, m), 7.39–7.55 (6H, m), 7.48–7.84 ppm (4H, t, J =
7.0 Hz)); 13C NMR (CDCl3, 126 MHz): d = 17.2, 24.5, 36.0, 38.5,
65.5, 128.1, 128.4, 128.5, 129.3, 131.4, 132.4, 137.3, 139.4, 140.8, 148.7,
196.5, 204.5 ppm. Anal. calcd. for C21H20O2: C 82.86, H 6.62; found: C
82.68, H 6.76.
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Received: September 2, 2010
Published online: October 28, 2010
Keywords: cycloaddition · gold catalysis · oxygen transfer ·
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