Development of a povarov reaction/carbene generation sequence for alkenyldiazocarbonyl compounds

Article abstract of DOI:10.1002/anie.201205692

Rings aplenty: A HOTf-catalyzed (Tf=trifluoromethanesulfonyl) Povarov reaction of alkenyldiazo species has been developed and delivers diazo-containing cycloadducts stereoselectively (see scheme). The resulting cycloadducts provide access to six- and seven-membered azacycles through the generation of metal carbenes as well as the functionalization of diazo group. Copyright

Full text of DOI:10.1002/anie.201205692

Angewandte
Chemie
DOI: 10.1002/anie.201205692
Synthetic Methods
Development of a Povarov Reaction/Carbene Generation Sequence for
Alkenyldiazocarbonyl Compounds**
Appaso Mahadev Jadhav, Vinayak Vishnu Pagar, and Rai-Shung Liu*
Metal-catalyzed cycloadditions of alkenyldiazo reagents are
useful tools to access carbo- and heterocycles.[1] These diazo
compounds are chemically sensitive toward both Brønsted or
Lewis acids. Their reported cycloadditions rely heavily on the
formation of metal carbenes to initiate regio- and stereo-
selective [3+n] cycloadditions (n = 2–4) with suitable
dipolarophiles.[2–4] A noncarbene route was postulated for
a few copper-catalyzed cycloadditions of these diazo species,
but they resulted in complete diazo decomposition.[3a,4a,5]
Doyle and co-workers reported[4a] a [3+2] cycloaddition
of the alkenylrhodium carbene A with imines to give
dihydropyrroles (Scheme 1a). We proposed a cycloaddition
the tetrahydroquinoline derivatives 3 and 3’, as well as the
tetrahydro-1H-benzo[b]azepine species 4.
Access to these frameworks are valuable for the prepa-
ration of several bioactive molecules including 2-phenyl-2,3-
dihydroquinolone,[8a] L-689,560,[8b] torcetrapib,[8c] martinellic
acid,[8d] OPC-31260,[8e] OPC-51803,[8f] and tetraperalone A
(Figure 1).[8g] Their specific biological functions have been
well documented.[8]
Scheme 1. Cycloaddition routes for alkenyldiazo species. a) Metal
carbene routes (current pathways).[2–4] b) Cycloaddition route (this
work).
Figure 1. Accessible biologically active molecules.
route with analogous reagents to secure their versatile diazo
functionalities. Toward this objective, we developed an
unprecedented cycloaddition route (Scheme 1b) involving
an initial formal [4+2] cycloaddition (Povarov[6,7] reaction)
between the diazo species 1 and N-aryl imines to give isolable
diazo-containing cycloadducts (2) stereoselectively. The sub-
sequent formation of their metal carbenes allows efficient
production of six- and seven-membered azacycles including
Shown in Table 1 are our efforts to realize a Povarov[6,7]
reaction between the N-benzylideneaniline [I(a)] and ethyl
vinyldiazoacetate (1a) using various acid catalysts; the
duration of reaction pertains to the complete consumption
of 1a. We selected I(a) as the target because of its great
affinity toward metal coordination, possibly impeding diazo
decomposition. As shown in entry 1, treatment of a 1,2-
dichloroethane (DCE) solution of 1a (1.0 equiv) and I(a)
(1.1 equiv) with [Rh2(OAc)4] (2.5 mol%) at 288C (4 h) gave
a complex mixture of products. Under the same reaction
conditions, Cu(OTf)2 (5 mol%, entry 2) gave the cycloadduct
2a (18%) together with the E- and Z-configured unsaturated
esters 3a (48%) and 3a’ (27%), respectively, and they were
separable on a silica gel column. We sought to improve the
chemoselectivity by using various gold catalysts (entries 3–5).
[PPh3AuCl]/AgNTf2 gave the unsaturated esters 3a and 3a’ in
55 and 30% yields, respectively (entry 3). With [P(tBu)2(o-
[*] A. M. Jadhav, V. V. Pagar, Prof. Dr. R.-S. Liu
Department of Chemistry, National Tsing Hua University
Hsinchu (30013) (Taiwan)
E-mail: rsliu@mx.nthu.edu.tw
[**] We thank the National Science Council, Taiwan, for financial support
of this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205692.
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Table 1: Test of Povarov reactions on various acid catalysts.[a]
the E-configured ester [D1]-3a and Z-configured ester [D1]-
3a’, respectively, with full deuterium content at their olefin
positions. The treatment of 3a’ with [P(tBu)2(o-biphenyl)-
AuNTf2] in DCE (288C, 2 h) gave mixture of 3a and 3a’
(1.1:1). We postulate that the 1,2-hydride shift proceeds
stereoselectively for the Au and Rh carbenes B; the former
favors E-alkene selectivity[10] whereas the latter produces
Z alkenes preferably.[11]
Entry
Catalyst (mol%)
T [8C]
t [h]
Yield [%][b]
2a
3a
3a’
[c]
We tested the reactions of the diazo reagents 1a–c with
various imines [I(a)–(i)] to generalize the stereoselective
synthesis of the cycloadducts 2, together with the E- and Z-
esters 3 and 3’ (Table 2). We obtained the cycloadducts 2 and
E-configured esters 3 directly from the starting reagents using
1
2
3
4
5
6
7
8
[Rh2(OAc)4] (2.5)
Cu(OTf)2 (5)
28
28
28
28
60
28
28
28
4
2
2
2
4
2
2
0.2
–
18
–
60
–
82
–
–
–
48
55
32
84
–
27
30
–
–
–
[PPh3AuCl]/AgNTf2 (5)
[LAuCl]/AgNTf2 (5)
[LAuCl]/AgNTf2 (5)
[IPrAuCl]/AgNTf2 (5)
AgNTf2 (5)
–
–
–
–
HOTf (3)
95
Table 2: Stereoselective synthesis of nitrogen heterocycles.
[a] 1a (1.0 equiv), I(a) (1.1 equiv), [1a]=0.57m. [b] Product yields are
given for products isolated after purification using a silica gel column.
[c] Complicated mixture of products. L=P(tBu)2(o-biphenyl), Tf=tri-
fluoromethanesulfonyl.
Entry 1[a] (X)
I[b] (Y, R1)
Yield [%]
biphenyl)AuNTf2] (entry 4), we obtained the desired 2a as
the major product (60%) in addition to 3a (32%). With this
gold catalyst in hot DCE (608C, 4 h), we obtained 3a in 84%
yield. To our pleasure, the use of the less acidic [IPrAuNTf2]
(IPr= 1,3-bis(diisopropylphenyl)imidazol-2-ylidene) allowed
efficient production of 2a exclusively (entry 6). In a control
experiment, AgNTf2 alone gave a complex mixture of
products in DCE (288C). Particularly appealing is HOTf
(3 mol%), which gave 2a stereoselectively (d.r. > 20:1) with
a yield of up to 95% after a short reaction time (entry 8).[7] We
postulate that the tetrahydroquinoline framework in 2a is
slightly basic and protects the diazo group from HOTf
decomposition. The stereochemistry of 2a is supported by
the 1H NOE data. The structure of 3a was confirmed by X-ray
diffraction.[9]
The diazo-containing cycloadduct 2a is can be used in the
stereoselective syntheses of 3a and 3a’ via the carbene
intermediate B (Scheme 2). The treatment of 2a with [P-
(tBu)2(o-biphenyl)AuNTf2] in DCE (288C, 2 h) gave 3a in
97% yield, whereas [Rh2(OAc)4] (2.5 mol%) gave only 3a’ in
94% yield under the same reaction conditions. With deuter-
ated [D1]-2a (X = D), the same Au and Rh catalysts delivered
2[c]
3[d]
3’[e]
1
2
3
4
5
6
7
8
9
10
1a (Et)
1a
1a
1a
1a
1a
1a
1a
I(b) (OMe, Ph)
I(c) (CF3, Ph)
I(d) (Cl, Ph)
2b: 83 3b: 77 3b’: 68
2c: 89 3c: 79 3c’: 72
2d: 84 3d: 82 3d’: 67
I(e) (H, 4-MeOC6H4) 2e: 84 3e: 74 3e’: 73
I(f) (H, 4-ClC6H4)
I(g) (H, 2-furanyl)
I(h) (H, nPr)
2 f: 89
3 f: 75
3 f’: 72
2g: 86 3g: 76 3g’: 70
2h: 81 3h: 72 3h’: 60
I(i) (H, CO2Et)
2i: 84
2j: 89
3i: 74
3j: 86
3i’: 67
3j’: 73
1b (tBu) I(a) (H, Ph)
1c (Bn) I(a)
2k: 84 3k: 85 3k’: 74
[a] 1 (1.0 equiv). [b] Imine (1.1 equiv), [substrate]=0.57m. [c] For reac-
tion using HOTf. Yields are given for product isolated after purification
using a silica gel column. [d] For reaction using [LAuNTf2]. Yields are
given for product isolated after purification using a silica gel column.
[e] For reaction using H+/Rh. Yields are given for product isolated after
purification using a silica gel column. L=P(tBu)2(o-biphenyl).
HOTf (3 mol%) and [P(tBu)2(o-biphenyl)AuNTf2] (5 mol%)
respectively, under the optimum reaction conditions detailed
in entries 5 and 8 of Table 1. Production of the Z-configured
esters 3’ relies on an initial treatment of the reagents with
HOTf (3 mol%), and subsequent addition of [Rh2(OAc)4]
(2.5 mol%) to the same DCE solution. The three reactions
worked well for the imines I(b)–(d) bearing para-methoxy,
para-trifluoromethyl, and para-chloro substituents, respec-
tively, on their aniline moieties. The resulting products 2b–d,
3b–d, and 3b’–d’ were obtained with satisfactory yields
(entries 1–3). For I(e),(f) bearing 4-MeOC6H4 and 4-ClC6H4,
respectively, the corresponding products 2e,f, 3e,f, and 3e’,f’
were obtained with yields greater than 72% (entries 4 and 5).
These catalytic reactions can be used for the imines I(g)–(i)
bearing 2-furanyl, n-propyl, and ester groups, respectively,
thus giving the desired compounds 2g–i, 3g–i, and 3g’–i’ in
reasonable yields (entries 6–8). We tested the reactions on the
a-diazo esters 1b,c, thus producing the expected cycloadducts
2j,k by using HOTf as the catalyst. We obtained the E- (3j,k)
Scheme 2. Transformations into metal carbenes.
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and Z-configured (3j’,k’) esters using the Au and Rh catalysts,
respectively (entries 9 and 10).
To expand the scope of the diazo substrates, we examined
the HOTf-catalyzed cycloaddition of the b-substituted diazo-
ester 1d with I(a) in DCE (288C, 0.2 h), thus giving the
desired cycloadduct 2l (d.r. > 20:1) in 81% yield (Scheme 3).
tively (entries 3 and 4). The seven-membered azacycles 4d
and 4e were also produced in good yields (75–76%). For the
remaining diazoesters 1h–j (entries 5–7), we obtained two
diastereomeric products (d.r. = 4:1–4.4:1) for compounds 2q
(R2 = n-butyl), 2r, and 2s which were separable on a silica gel
column. Gold carbene generation in situ for these isomeric
mixtures gave the seven-membered azacycles 4 f–h in 67–
82% yields. The molecular structure of 4 f was confirmed by
X-ray diffraction.[9]
Equation (1) highlights the functionalization of the diazo
group of 2a and 2m. Dimethyldioxirane (DMDO)[12] oxida-
tion of the acetyl derivatives of these two azacycles in cold
THF (08C) gave the a-keto esters 5a and 5b in 66 and 68%,
yields respectively.
Scheme 3. Reactions of the 2-substituted alkenyldiazo species 1d.
TBS=tert-butyldimethylsilyl.
Interestingly, the treatment of the cycloadduct 2l with
[P(tBu)2(o-biphenyl)AuNTf2] in hot DCE (708C, 1 h) gave
the seven-membered azacycle 4a in 82% yield, albeit with
low diastereoselectivity (d.r. = 1.6:1). [Rh2(OAc)4] gave the
same azacycle with a better d.r. value (4.1:1), but a decreased
yield (62%).
Scheme 4 depicts a plausible path to rationalize the
stereochemical course of the resulting cycloadducts, a path
involving the initial reaction of HOTf with 1d.[7] According to
These Povarov reactions are amenable to the b-substi-
tuted diazoesters 1e–j using HOTf as a catalyst (3 mol%),
and the resulting cycloadducts 2m–s were produced with
a diastereoselectivity of greater than 4:1 (Table 3). We also
obtained the seven-membered azacycles 4b–h in a one-pot
operation involving treatment of the cycloadducts 2 in situ
with [P(tBu)2(o-biphenyl)AuNTf2] (5 mol%) in DCE (288C,
1 h). For the methyl-substituted diazo species 1e, cycloaddi-
tion with I(a) and I(l) gave the corresponding cycloadducts
2m and 2n stereoselectively (d.r. > 20:1), and the seven-
membered azacycles 4b and 4c were obtained in 67–76%
yields (entries 1 and 2). The stereochemistry of 2m was
determined by X-ray diffraction.[9] Excellent diastereoselec-
tivity was also obtained for the cycloadducts 2o and 2p, which
were derived from the diazo substrates 1 f and 1g, respec-
Scheme 4. A plausible model for stereochemistry.
a preferable stepwise mechanism,[6b] the formation of an
À
initial C C bond generates the chairlike transition-state C
which has the small hydrogen and OTBS substituents in axial
positions, thus generating the cycloadduct 2l with the
observed stereochemistry. For other less polarized alkenyl-
diazo species, including 1a–c and 1e–j, the cycloadditions
likely proceed in a concerted endo fashion.[6c–e]
Table 3: [4+2] cycloadditions and ring expansions.
Inspired by our success in the preceding Povarov reactions
catalyzed with triflic acid, we investigated accessing diazo-
containing oxacycles by an unprecedented oxa-Povarov
reaction via the postulated benzylidene(phenyl)oxonium D
(Table 4). Treatment of an equimolar mixture of ethyl vinyl-
diazoacetate (1a; 0.57m) and (diphenoxymethyl)benzene
(6a) with HOTf (3 mol%) in cold DCE (À308C, 10 min)
led to the rapid decomposition of 6a into phenol and
benzaldehyde. After some optimization, we found that the
presence of a-phenylethylamine (2 mol%) in this triflic acid
system delivered the desired diazo-containing oxacycle 7a as
a single diastereomer in 75% yield (entry 1). The role of this
primary amine is to decrease the acidity of the reaction
medium. The stereochemistry of 7a was confirmed by
1H NOE and NMR data. This oxa-Povarov reaction worked
well for the acetals 6b–e to give the desired oxacycles 7b–
Entry 1[a] (R2)
I[b]
Yield [%]
2[c]
4[d]
1
2
3
4
5
6
7
1e (Me)
1e
1 f (Cy)
1g (Bn)
1h (nBu)
1i (3-methylbutyl)
I(a) 2m: 82 (d.r.>20:1) 4b: 76
I(l)
I(a) 2o: 80 (d.r.>20:1)
I(a) 2p: 83 (d.r.>20:1)
I(a) 2q: 86 (d.r.=4:1)
I(a) 2r: 88 (d.r.=4.4:1)
2n: 72 (d.r.>20:1)
4c: 67
4d: 75
4e: 76
4 f: 73
4g: 67
4h: 82
1j (3-methyl-3-butenyl) I(a) 2s: 88 (d.r.=4.4:1)
[a] 1 (1.0 equiv). [b] Imine (1.1 equiv), [substrate]=0.57m. [c] For reac-
tion using HOTf. Yields are given for product isolated after purification
using a silica gel column. [d] For reaction using H+/Au. Yields are given
for product isolated after purification using a silica gel column.
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Table 4: Feasibility of an oxa-Povarov reaction.
[1] Selected reviews: a) M. P. Doyle, M. A. McKervy, T. Ye, Modern
Catalytic Methods for Organic Synthesis with Diazo Compounds,
Wiley, New York, 1998; b) A. Padwa, M. D. Weingarten, Chem.
Rev. 1996, 96, 223; c) H. M. L. Davies, J. R. Denton, Chem. Soc.
Rev. 2009, 38, 3061; d) M. P. Doyle, R. Duffy, M. Ratnikov, L.
Zhou, Chem. Rev. 2010, 110, 704; e) H. M. L. Davies, D. Morton,
Chem. Soc. Rev. 2011, 40, 1857; f) Z. Zhang, J. Wang, Tetrahe-
dron 2008, 64, 6577.
Entry
1[a] (X)
6[b] (Y, Z)
7: Yield [%][c]
[2] Selected examples for carbocyclic cycloadducts, see: a) L. Deng,
A. J. Giessert, O. O. Gerlitz, X. Dai, S. T. Diver, H. M. L. Davies,
J. Am. Chem. Soc. 2005, 127, 1342; b) H. M. L. Davies, Adv.
Cycloaddit. 1999, 5, 119; c) H. M. L. Davies, B. Xing, N. Kong,
D. G. Stafford, J. Am. Chem. Soc. 2001, 123, 7461; d) H. M. L.
Davies, T. J. Clark, H. D. Smith, J. Org. Chem. 1991, 56, 3819;
e) Y. Liu, K. Bakshi, P. Zavalij, M. P. Doyle, Org. Lett. 2010, 12,
4304; f) J. P. Olson, H. M. L. Davies, Org. Lett. 2008, 10, 573.
[3] For oxacyclic cycloadducts, see: a) X. Xu, W.-H. Hu, P. Y.
Zavalij, M. P. Doyle, Angew. Chem. 2011, 123, 11348; Angew.
Chem. Int. Ed. 2011, 50, 11152; b) M. P. Doyle, W. Hu, D. J.
Timmons, Org. Lett. 2001, 3, 3741.
1
2
3
4
5
6
7
8
1a (OEt)
1a
1a
1a
1a
1b (OtBu)
1k (Ph)
1l (nPr)
6a (H, H)
6b (H, OMe)
6c (H, Cl)
6d (Me, H)
6e (Cl, H)
6a
7a: 75
7b: 77
7c: 72
7d: 69
7e: 56
7 f: 74
7g: 76
7h: 68
6a
6a
[a] 1a (0.57m). [b] Acetal (1.1 equiv). [c] Yields are given for product
isolated after purification using a silica gel column.
[4] For azacyclic cycloadducts, see selected reviews: a) M. P. Doyle,
M. Yan, W. Hu, L. Gronenberg, J. Am. Chem. Soc. 2003, 125,
4692; b) J. Barluenga, G. Lonzi, L. Riesgo, L. A. Lꢀpez, M.
Tomas, J. Am. Chem. Soc. 2010, 132, 13200; c) M. Yan, N.
Jacobsen, W. Hu, L. S. Gronenberg, M. P. Doyle, J. T. Colyer, D.
Bykowski, Angew. Chem. 2004, 116, 6881; Angew. Chem. Int. Ed.
2004, 43, 6713; d) X. Wang, X. Xu, P. Zavalij, M. P. Doyle, J. Am.
Chem. Soc. 2011, 133, 16402; e) Y. Lian, H. M. L. Davies, J. Am.
Chem. Soc. 2010, 132, 440; f) X. Xu, M. O. Ratnikov, P. Y.
Zavalij, M. P. Doyle, Org. Lett. 2011, 13, 6122; g) V. V. Pagar,
A. M. Jadhav, R.-S. Liu, J. Am. Chem. Soc. 2011, 133, 20728;
h) R. P. Reddy, H. M. L. Davies, J. Am. Chem. Soc. 2007, 129,
10312.
e stereoselectively (entries 2–5). The generality of this
unprecedented cycloaddition is again manifested with its
applicability to various alkenyldiazo compounds (1b,k and l),
thus giving the cycloadducts 7 f–h in 68–76% yields
(entries 6–8). Using our methods discussed above, the com-
pound 7a was converted into the dicarbonyl species 8a, and
the Z- and E-configured esters 8b and 8c, respectively, with
61–91% yields (Scheme 5).
[5] Y. Qian, X. Xu, X. Wang, P. Zavalij, W. Hu, M. P. Doyle, Angew.
Chem. 2012, 124, 6002; Angew. Chem. Int. Ed. 2012, 51, 5900.
[6] Povarov reactions refer to the formal [4+2] cycloadditions of N-
aryl imines with enol ethers or enamines. See reviews: a) L. S.
Povarov, Russ. Chem. Rev. 1967, 36, 656; b) V. V. Kouznetsov,
Tetrahedron 2009, 65, 2721; c) D. Bello, R. Ramꢀn, R. Lavilla,
Curr. Org. Chem. 2010, 14, 332; d) M. A. McCarrick, Y. D. Wu,
K. N. Houk, J. Org. Chem. 1993, 58, 3330; e) A. Whiting, C. M.
Windsor, Tetrahedron 1998, 54, 6035.
Scheme 5. Chemical functionalization of diazo compound 7a.
[7] For Povarov reactions catalyzed by Brønsted acids, see selected
examples: a) H. Xu, S. J. Zuend, M. G. Woll, Y. Tao, E. N.
Jacobson, Science 2010, 327, 986; b) T. Akiyama, H. Morita, K.
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Chem. Soc. 2011, 133, 14804; e) H. Ishitani, S. Kobayashi,
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2010, 46, 327; g) L. He, M. Bekkaye, P. Retailleau, G. Masson,
Org. Lett. 2012, 14, 3158.
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Reported catalytic cycloadditions on alkenyldiazo
reagents are inevitably accompanied by a diazo decomposi-
tion. We have developed a HOTf-catalyzed Povarov reaction
for these diazo species to give diazo-containing cycloadducts
stereoselectively. The resulting cycloadducts provide access to
various six- and seven-membered azacycles, by either the
generation of metal carbenes or the functionalization of the
diazo group. Our preliminary results also reveal the feasibility
of oxa-Povarov reactions, which are unknown in the liter-
ature. Future work will be focused on the asymmetric versions
of these new reactions.
Received: July 17, 2012
Revised: September 5, 2012
Published online: October 18, 2012
Keywords: carbenes · cycloaddition · diazo compounds ·
.
heterocycles · synthetic methods
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Haigh, M. B. Roe, J. Med. Chem. 2008, 51, 8124; g) C. Li, X. Li,
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[9] CCDC 905645 (2m), CCDC 905646 (3a), and CCDC 905647
(4 f) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif. Full details are provided in the Supporting
Information.
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