Highly regio-, diastereo-, and enantioselective [2+2+2] cycloaddition of 1,6-enynes with electron-deficient ketones catalyzed by a cationic Rh I/H8-binap complex

Article abstract of DOI:10.1002/anie.200704758

(Chemical Presented) Two quaternary carbon centers are created in the title reaction to form fused dihydropyrans (see scheme). The same catalyst promotes the ortho functionalization of aryl ketones with 1,6-enynes with excellent regio- and enantioselectivity. Z=amide, C(CO₂Me)₂, O; E=CO₂Et, Ac; R¹=Me, aryl, CO₂Me; R ²=Me; R³=Me, CO₂Et.

Full text of DOI:10.1002/anie.200704758

Communications
DOI: 10.1002/anie.200704758
Asymmetric Catalysis
Highly Regio-, Diastereo-, and Enantioselective [2+2+2]
Cycloaddition of 1,6-Enynes with Electron-Deficient Ketones
Catalyzed by a Cationic RhI/H8-binap Complex**
Ken Tanaka,* Yousuke Otake, Hiromi Sagae, Keiichi Noguchi, and Masao Hirano
Transition-metal-catalyzed [2+2+2] cycloaddition reactions
are efficient methods for the construction of six-membered
carbocycles and heterocycles.[1] For the synthesis of six-
membered oxygen heterocycles, [2+2+2] cycloaddition reac-
tions involving carbonyl compounds are potentially attractive.
Catalytic [2+2+2] cycloaddition reactions of diynes with
carbonyl compounds[2] in the presence of various transition-
metal complexes have been reported, for example, with Ni,[3]
Ru,[4] and Rh.[5] However, these reactions frequently failed to
produce oxygen heterocycles, and instead dienones were
obtained as the major products as a result of electrocyclic ring
opening of the initially formed fused a-pyrans (Scheme 1). In
products (Scheme 2). Although a Ni0/imidazolylidene-cata-
lyzed reaction of a 1,7-enyne with benzaldehyde has been
reported, poor regioselectivity and b-hydride elimination
Scheme 2. Transition-metal-catalyzed [2+2+2] cycloaddition of1,6-
enynes with ketones.
prior to reductive elimination from the corresponding nick-
elacycle were observed.[3b,7] Our research group recently
reported the reaction of 1,6-diynes with carbonyl compounds
to give dienones and ortho-functionalized aryl ketones under
the catalysis of cationic complexes of RhI with modified
binap ligands (binap = 2,2’-bis(diphenylphosphanyl)-1,1’-
binaphthyl).[8,9] Herein, we describe the [2+2+2] cycloaddi-
tion of 1,6-enynes with electron-deficient ketones to give
fused dihydropyrans with two quaternary carbon centers
under the catalysis of a cationic RhI/H8-binap complex, as well
as the ortho functionalization of aryl ketones with 1,6-enynes
in the presence of the same catalyst. Both reactions proceed
with excellent regio-, diastereo-, and enantioselectivity.
We first investigated the reaction of the tosylamide-linked
1,6-enyne 1a, which contains a geminally disubstituted alkene
moiety, with ethyl pyruvate (2a) in the presence of cationic
RhI/binap-type bisphosphane complexes (Table 1). The reac-
tion proceeded at 808Cto give the desired fused dihydro-
pyran 3aa as a single regioisomer and a single diastereomer in
excellent yield and with high enantioselectivity in the
presence of the catalyst [Rh(cod)2]BF4/(R)-binap (10 mol%;
Table 1, entry 1). The use of the sterically demanding binap-
type bisphosphane ligands (R)-tol-binap and (R)-xyl-binap
led to a significant decrease in the yield of 3aa (Table 1,
entries 2 and 3). Although the use of (R)-segphos led to an
improvement in the enantioselectivity, the yield of 3aa
decreased to 58% (Table 1, entry 4). Both high yield and
high enantioselectivity were observed with the ligand (R)-H8-
binap (Table 1, entry 5).
Scheme 1. Transition-metal-catalyzed [2+2+2] cycloaddition of1,6-
diynes with carbonyl compounds.
contrast, a transition-metal-catalyzed [2+2+2] cycloaddition
of enynes 1 to ketones 2 would furnish fused dihydropyrans 3
or 4 with two quaternary carbon centers,[6] bicyclic com-
pounds that would not be converted into ring-opened
[*] Prof. Dr. K. Tanaka, Y. Otake, H. Sagae, Dr. M. Hirano
Department ofApplied Chemistry, Graduate School ofEngineering,
Tokyo University ofAgriculture and Technology, Koganei,
Tokyo 184-8588 (Japan)
Fax: (+81)42-388-7037
E-mail: tanaka-k@cc.tuat.ac.jp
Prof. Dr. K. Noguchi
Instrumentation Analysis Center, Tokyo University ofAgriculture
and Technology, Koganei, Tokyo 184-8588 (Japan)
[**] This research was partly supported by a Grant-in-Aid for Scientific
Research on Priority Areas (No. 19028015, Chemistry ofConcerto
Catalysis) from the Ministry of Education, Culture, Sports, Science
and Technology (Japan). We thank Yusuke Aida (TUAT) for his
experimental assistance and Takasago International Corporation for
the gift of modified binap ligands. H8-binap=2,2’-bis(diphenyl-
phosphanyl)-5,5’,6,6’,7,7’,8,8’-octahydro-1,1’-binaphthyl.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
(entry 6).[10] When the reaction of 1a with 2a was carried out
at 258Cwith 20 mol% of the Rh catalyst, almost complete
enantioselectivity was observed (Table 2, entry 2).
Table 1: Screening ofligands for the Rh-catalyzed [2 +2+2] cycloaddition
of 1a with ethyl pyruvate (2a).
We found that 1,6-enynes with tosylamide (1a; Table 2,
entries 1–5), malonate (1b, entries 7–9), and oxygen (1c and
1d, entries 10–13) linkages could be used in the cycloaddition.
With respect to the substituent R1 at the alkyne terminus, 1,6-
enynes substituted not only with methyl (1a; Table 2,
entries 1–5), but also with phenyl and 4-bromophenyl
groups (1c and 1b, entries 7–11), participated in the reaction.
In the case of the 4-bromophenyl-substituted 1,6-enyne 1b,
improved yield and enantioselectivity were observed when
the reaction was carried out at 258Cwith 20 mol% of the Rh
catalyst (Table 2, entry 9). The reaction of the methoxycar-
bonyl-substituted 1,6-enyne 1d with 2a furnished the corre-
sponding dihydropyran 3da in low yield as a result of the
rapid [2+2+2] homocycloaddition of 1d (Table 2, entry 12);
however, when the highly electron deficient ketone 2d was
used, the corresponding dihydropyran 3dd was obtained in
good yield (entry 13). Although the 1,6-enyne 1e with a
monosubstituted alkene moiety reacted with 2a, the alcohol
5ea was obtained as the major product along with the
expected dihydropyran 3ea (Table 2, entry 14). Furthermore,
the use of diethyl ketomalonate (2d) instead of 2a led to the
almost exclusive formation of the alcohol 5ed (Table 2,
entry 15).[11,12]
Entry
Ligand
Yield [%][a]
ee [%]
1
2
3
4
5
(R)-binap
>99
22
<10
58
91
92
–
96
95
(R)-tol-binap[b]
(R)-xyl-binap[c]
(R)-segphos[d]
(R)-H8-binap[e]
>99
[a] Yield ofthe isolated product. [b] 2,2 ’-Bis(di-p-tolylphosphanyl)-1,1’-
binaphthyl. [c] 2,2’-Bis(di(3,5-xylyl)phosphanyl)-1,1’-binaphthyl. [d] (4,4’-
Bi-1,3-benzodioxole)-5,5’-diylbis(diphenylphosphane). [e] 2,2’-Bis(diphe-
nylphosphanyl)-5,5’,6,6’,7,7’,8,8’-octahydro-1,1’-binaphthyl.
cyclooctadiene, Ts=p-toluenesulfonyl.
cod=1,5-
We then explored the scope of this process with respect to
the two substrates (Table 2). Not only ethyl pyruvate (2a;
Table 2, entry 1), but also 2,3-butanedione (2b; entry 3) and
diethyl ketomalonate (2d, entry 5), underwent the desired
reaction with 1a to furnish the expected dihydropyran in high
yield; however, the reaction of ethyl phenylglyoxylate (2c)
with 1a gave the expected dihydropyran 3ac in low yield
(Table 2, entry 4), and acetone (2e) failed to react with 1a
We applied this regio-, diastereo-, and enantioselective
[2+2+2] cycloaddition to the synthesis of the enantiomeri-
cally enriched spirocyclic com-
pound 3cf by using 1-methylisatin
(2 f) as the carbonyl substrate
(Scheme 3).[13] Importantly, neither
the regioisomer 4 nor the other
diastereomer was detected in the
crude product mixture for this reac-
tion or any of those described in
Table 2. The absolute configuration
of the dihydropyran product (À)-
3ab was determined by the anom-
Table 2: RhI+/(R)-H8-binap-catalyzed regio-, diastereo-, and enantioselective [2+2+2] cycloaddition of
1,6-enynes 1 with ketones 2.
Entry 1 (Z, R1, R2)
2 (R3, E)
3
Yield [%][a]
ee [%]
95
>99
98
97
97
–
98
alous
(Figure 1.)[14]
Scheme 4 depicts
dispersion
method
1
1a (NTs, Me, Me)
2a (Me, CO2Et)
2a (Me, CO2Et)
2b (Me, Ac)[d]
2c (Ph, CO2Et)
2d (CO2Et, CO2Et)
2e (Me, Me)[f]
(À)-3aa
(À)-3aa
>99
91
73
24
89
<1
49
33
2[b,c] 1a (NTs, Me, Me)
a
possible
3
4
5
6
7
8
1a (NTs, Me, Me)
1a (NTs, Me, Me)
1a (NTs, Me, Me)
1a (NTs, Me, Me)
(3R,6S)-(À)-3ab[e]
(+)-3ac
mechanism for the selective forma-
tion of the dihydropyran (3R,6S)-
3ab: The 1,6-enyne 1a reacts with
the rhodium center of the catalyst
to form the rhodacyclopentene A as
(À)-3ad
3ae
1b (C(CO2Me)2, Ar,[g] Me) 2a (Me, CO2Et)
1b (C(CO2Me)2, Ar,[g] Me) 2b (Me, Ac)[d]
(+)-3ba
(+)-3bb
(+)-3ba
92
9[b,c] 1b (C(CO2Me)2, Ar,[g] Me) 2a (Me, CO2Et)
82
>99
a
result of a steric interaction
between the Rh-CH2 moiety and
the equatorial P-Ph group of (R)-
H8-binap. Subsequently, 2,3-butane-
dione (2b) coordinates to A to form
complex B. The insertion of 2b
followed by the reductive elimina-
tion of rhodium then furnishes
(3R,6S)-3ab.
10
11
12
13
1c (O, Ph, Me)
1c (O, Ph, Me)
2a (Me, CO2Et)
2d (CO2Et, CO2Et)
2a (Me, CO2Et)
2d (CO2Et, CO2Et)
2a (Me, CO2Et)[d]
(+)-3ca
(+)-3cd
67
64
17
>99
96
98
93
1d (O, CO2Me, Me)
1d (O, CO2Me, Me)
(À)-3da
(À)-3dd
(À)-3ea
61
14[b,h] 1e (NTs, Me, H)
25 (30[i])
94
(>99[i,j], 81[i,k]
– (52[l])
)
15[b,h] 1e (NTs, Me, H)
2d (CO2Et, CO2Et)[d]
3ed
<1 (70[l])
[a] Yield ofthe isolated product. [b] Catalyst: 20 mol%. [c] The reaction was carried out at 25 8C in
CH2Cl2. [d] Compound 2: 2 equivalents. [e] The absolute configuration of (À)-3ab was determined by X-
ray crystallographic analysis (Figure 1). [f] Acetone (2e) was used as both substrate and solvent. [g] Ar=
4-BrC6H4. [h] Ligand: (R)-binap. [i] Data for 5ea (d.r. 2.8:1). [j] The ee value ofthe major diastereomer.
[k] The ee value ofthe minor diastereomer. [l] Data of r 5ed.
Next, we examined the reaction
of 1,6-enynes with electron-rich aryl
ketones (Table 3). In analogy with
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Table 3: Rh-catalyzed regio- and enantioselective ortho functionalization
ofaryl ketones 2 with 1,6-enynes 1.[a]
Scheme 3. Synthesis ofthe enantiomerically enriched spirocyclic com-
pound (+)-3cf.
Entry
1
1
2 (R2, R3)
Ligand
6
Yield [%][b] ee [%]
1e 2g (Ph, H) (R)-H8-
binap
1e 2g (Ph, H) (R)-seg-
phos
(+)-6eg 55
(+)-6eg 59
96
95
2
3
4
1e 2g (Ph, H) (R)-binap
1e 2h (Me, H) (R)-H8-
binap
(+)-6eg 26
97
98
(R)-(+)-6eh[c] 34
5
6
7
1 f 2i (Me,
OMe)
1 f 2j (Me,
CF3)
(R)-H8-
binap
(R)-H8-
binap
(+)-6fi
34
94
–
6fj
<1
Figure 1. ORTEP drawing of(3 R,6S)-(À)-3ab drawn at the 50%
probability level.
1a 2g (Ph, H) (R)-H8-
6ag <1
–
binap
[a] The enyne 1 was added over a period of5 min (see the Supporting
Information). [b] Yield of the isolated product. [c] The absolute config-
uration of( +)-6eh was determined by X-ray crystallographic analysis
(Figure 2).
functionalized aryl ketone (+)-6eh was determined by the
anomalous dispersion method (Figure 2).[14]
In conclusion,
a
cationic RhI/(R)-H8-binap complex
catalyzes the [2+2+2] cycloaddition of 1,6-enynes with
Scheme 4. Possible mechanism for the selective formation of the
dihydropyran (3R,6S)-3ab.
the reactions of 1,6-diynes with electron-rich aryl ketones,[8a]
when the 1,6-enyne 1e was added over 5 min to a solution of
benzophenone (2g) and the [Rh(cod)2]BF4/(R)-H8-binap
catalyst (5 mol%) in CH2Cl2 at 258C, the ortho-functional-
ized aryl ketone 6eg was isolated in 55% yield (Table 3,
entry 1).[15–17] Although the desired product was obtained in
comparable yield when the ligand (R)-segphos was used
(Table 3, entry 2), the use of (R)-binap led to a significantly
lower yield of 6eg (entry 3).[18] These reactions proceeded
with perfect regioselectivity and high enantioselectivity.
Acetophenone (2h) also participated in the reaction with
1e (Table 3, entry 4). Although 2i, in which the meta
substituent R3 is an electron-donating group, reacted with
1 f to provide the expected product 6 fi as a single regioisomer
(Table 3, entry 5), the presence of an electron-withdrawing
group at the meta position (in 2j) led to a complete shutdown
of the reaction (entry 6). Furthermore, the 1,6-enyne 1a with
a geminally disubstituted alkene moiety failed to react with
2g. Instead, 1a underwent [2+2+2] homocycloaddition
(Table 3, entry 7). The absolute configuration of the ortho-
Figure 2. ORTEP drawing of( R)-(+)-6eh drawn at the 50% probability
level.
electron-deficient ketones to give fused dihydropyrans con-
taining two quaternary carbon centers with excellent regio-,
diastereo-, and enantioselectivity. Electron-rich aryl ketones
react with 1,6-enynes in the presence of the same catalyst to
give ortho-functionalized aryl ketones with excellent regio-
and enantioselectivity. Further studies to expand the scope of
the two reactions and elucidate the reaction mechanisms are
in progress.
Experimental Section
Representative procedure: (R)-H8-binap (18.9 mg, 0.030 mmol) and
[Rh(cod)2]BF4 (12.2 mg, 0.030 mmol) were dissolved in CH2Cl2
(2.0 mL) in a Schlenk tube under an argon atmosphere, and the
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mixture was stirred at room temperature for 5 min. H2 was introduced
into the resulting solution, which was then stirred at room temper-
ature for 0.5 h, concentrated to dryness, and dissolved in (CH2Cl)2
(0.5 mL). A solution of 1c (55.9 mg, 0.300 mmol) and 2 f (96.7 mg,
0.600 mmol) in (CH2Cl)2 (1.5 mL) was added at room temperature,
and the resulting mixture was stirred at 808Cfor 16 h. The reaction
mixture was then concentrated, and the product was purified by
preparative TLC(hexane/ethyl acetate 2:1) to furnish ( +)-3cf
(88.8 mg, 0.256 mmol, 85%, 92% ee, single diastereomer) as a pale
yellow oil. [a]D251= + 52.8 degcm3 gÀ1 dmÀ1 (c = 2.04 ” 10À2 gcmÀ3 ace-
tone; 92% ee); H NMR (CDCl3, 300 MHz): d = 7.29 (d, J = 7.5 Hz,
1H), 7.20 (t, J = 7.5 Hz, 1H), 7.15–6.99 (m, 4H), 6.87–6.75 (m, 2H),
6.57 (d, J = 7.5 Hz, 1H), 4.59 (d, J = 10.5 Hz, 1H), 4.45 (d, J = 13.8 Hz,
1H), 4.09 (d, J = 13.8 Hz, 1H), 3.93 (d, J = 10.5 Hz, 1H), 3.91 (d, J =
8.1 Hz, 1H), 3.62 (d, J = 8.1 Hz, 1H), 3.00 (s, 3H), 1.58 ppm (s, 3H);
13C NMR (CDCl3, 75 MHz): d = 175.6, 144.3, 144.1, 135.3, 129.9,
128.7, 128.6, 128.0, 127.5, 127.2, 124.4, 122.9, 108.2, 79.1, 77.0, 69.7,
68.3, 41.4, 25.8, 23.0 ppm; IR (neat): n˜ = 2964, 1715, 1090, 1052,
705 cmÀ1; HRMS (FAB): m/z calcd for C22H21NO3: 380.1532; found:
380.1562 [M + H]+; HPLC(chiralcel OD-H, hexane/ iPrOH 90:10,
1.0 mLminÀ1): tR(major isomer): 8.5 min, tR(minor isomer): 12.9 min.
Proc. Natl. Acad. Sci. USA 2004, 101, 5363; c) I. Denissova, L.
Barriault, Tetrahedron 2003, 59, 10105.
[7] For the Au-catalyzed diastereoselective intermolecular addition
of carbonyl compounds to 1,6-enynes to give fused [5,5] ring
systems, see: M. Schelwies, A. L. Dempwolff, F. Rominger, G.
Helmchen, Angew. Chem. 2007, 119, 5694; Angew. Chem. Int.
Ed. 2007, 46, 5598.
[8] a) K. Tanaka, Y. Otake, A. Wada, K. Noguchi, M. Hirano, Org.
Lett. 2007, 9, 2203; a similar publication appeared soon after-
wards: b) K. Tsuchikama, Y. Yoshinami, T. Shibata, Synlett 2007,
1395.
[9] For selected reports by our research group of [2+2+2] cyclo-
addition reactions of alkynes under the catalysis of cationic RhI/
modified-binap complexes, see: a) K. Tanaka, K. Shirasaka, Org.
Lett. 2003, 5, 4697; b) K. Tanaka, G. Nishida, A. Wada, K.
Noguchi, Angew. Chem. 2004, 116, 6672; Angew. Chem. Int. Ed.
2004, 43, 6510; c) K. Tanaka, K. Toyoda, A. Wada, K. Shirasaka,
M. Hirano, Chem. Eur. J. 2005, 11, 1145; d) K. Tanaka, K.
Takeishi, K. Noguchi, J. Am. Chem. Soc. 2006, 128, 4586; e) K.
Tanaka, H. Sagae, K. Toyoda, K. Noguchi, M. Hirano, J. Am.
Chem. Soc. 2007, 129, 1522; f) G. Nishida, K. Noguchi, M.
Hirano, K. Tanaka, Angew. Chem. 2007, 119, 4025; Angew.
Chem. Int. Ed. 2007, 46, 3951, and references therein; for similar
reactions of nitriles, see: g) K. Tanaka, N. Suzuki, G. Nishida,
Eur. J. Org. Chem. 2006, 3917; h) A. Wada, K. Noguchi, M.
Hirano, K. Tanaka, Org. Lett. 2007, 9, 1295; for similar reactions
of isocyanates and isothiocyanates, see: i) K. Tanaka, A. Wada,
K. Noguchi, Org. Lett. 2005, 7, 4737; j) K. Tanaka, A. Wada, K.
Noguchi, Org. Lett. 2006, 8, 907; for similar reactions of alkenes,
see: k) K. Tanaka, G. Nishida, H. Sagae, M. Hirano, Synlett 2007,
1426.
Received: October 15, 2007
Published online: January 8, 2008
Keywords: asymmetric catalysis · cycloaddition · enynes ·
.
ketones · rhodium
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presence of an alkyne moiety and found that the carbonyl group
of an alkynyl ketoester reacted with a 1,6-enyne, although the
cycloadduct could not be isolated in pure form. For the [2+2+2]
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[11] When the ligand (R)-binap was used, the cycloaddition products
were obtained in higher yields than with (R)-H8-binap.
[12] Similar heterocycle formation during the transition-metal-cata-
lyzed cycloisomerization of enynes was reported; for Ru, see:
a) M. Mori, Y. Kozawa, M. Ishida, M. Kanamaru, K. Onozuka,
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Communications
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À
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