Cationic rhodium(I)/H8-binap complex catalyzed [2+2+2] cycloadditions of 1,6- And 1,7-diynes with carbonyl compounds

Article abstract of DOI:10.1002/ejoc.200900185

We have established that a cationic rhodium(I)/H₈-binap complex catalyzes [2+2+2] cycloadditions of a variety of 1,6and 1,7-diynes with both electron-deficient and electron-rich carbonyl compounds, leading to dienones in high yield under mild reaction conditions. In the reactions with acyl phosphonates, the reactivity of 1,6- and 1,7-diynes was highly dependent on their own structures. The addition of chelating diethyl oxalate effectively promoted the [2+2+2] cycloadditions involving acyl phosphonates, presumably due to the equilibrium formation of the desired 1:1 rhodium complex of the diyne and the acyl phosphonate by facile ligand exchange between the diyne and weakly coordinated diethyl oxalate. In the reactions involving bifunctional carbonyl compounds or unsymmetrical 1,6-diynes, high chemo- or regioselectivities were observed. Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejoc.200900185

FULL PAPER
DOI: 10.1002/ejoc.200900185
Cationic Rhodium(I)/H8-binap Complex Catalyzed [2+2+2] Cycloadditions of
1,6- and 1,7-Diynes with Carbonyl Compounds
Yousuke Otake,[a] Rie Tanaka,[a] and Ken Tanaka*[a]
Keywords: Carbonyl compounds / Cycloaddition / Enones / Diynes / Rhodium
We have established that a cationic rhodium(I)/H8-binap
complex catalyzes [2+2+2] cycloadditions of a variety of 1,6-
and 1,7-diynes with both electron-deficient and electron-rich
carbonyl compounds, leading to dienones in high yield under
mild reaction conditions. In the reactions with acyl phos-
phonates, the reactivity of 1,6- and 1,7-diynes was highly de-
pendent on their own structures. The addition of chelating
diethyl oxalate effectively promoted the [2+2+2] cycload-
ditions involving acyl phosphonates, presumably due to the
equilibrium formation of the desired 1:1 rhodium complex of
the diyne and the acyl phosphonate by facile ligand ex-
change between the diyne and weakly coordinated diethyl
oxalate. In the reactions involving bifunctional carbonyl com-
pounds or unsymmetrical 1,6-diynes, high chemo- or regio-
selectivities were observed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
rhodacyclopentadiene was reported in cationic rhodium(I)/
biphep complex catalyzed carbonyl Z-dienylations through
multicomponent reductive coupling of aldehydes and α-
keto esters mediated by hydrogen in the presence of a cata-
lytic amount of triphenylacetic acid, but this example is not
the straightforward [2+2+2] cycloaddition of alkynes with
carbonyl compounds.[10] Thus, a study concerning the
[2+2+2] cycloaddition of diynes with carbonyl compounds
by using a rhodium(I) complex as a catalyst has not been
reported. Furthermore, a catalyst that can be used in
[2+2+2] cycloaddition reactions of diynes with both elec-
tron-deficient and electron-rich carbonyl compounds has
not been developed.
In contrast, our research group reported cationic rhodi-
um(I)/biaryl bisphosphane complex catalyzed [2+2+2] cy-
cloadditions of diynes with both electron-deficient and elec-
tron-rich Csp–heteroatom multiple bonds under mild reac-
tion conditions,[11,12] which prompted our investigation into
cationic rhodium(I)/biaryl bisphosphane complex catalyzed
[2+2+2] cycloadditions of diynes with both electron-de-
Introduction
Transition-metal-catalyzed [2+2+2] cycloadditions of
diynes with Csp–heteroatom multiple bonds such as nitriles
and heterocumulenes are valuable methods to construct
various heterocycles in a highly atom economical manner.[1]
Although a number of transition-metal complexes can cata-
lyze or mediate these reactions, analogous [2+2+2] cycload-
ditions involving Csp2–heteroatom multiple bonds such as
aldehydes and ketones have been reported in a limited
number of examples.[2–11] The pioneering work for such a
catalytic [2+2+2] cycloaddition was reported in nickel(0)/
monophosphane complex catalyzed reactions of 1,6- and
1,7-diynes with aldehydes.[2] After this report, Cp*Ru(cod)-
Cl-catalyzed reactions involving electron-deficient ketones
(diethyl ketomalonate and indanetrione)[3,4] and nickel(0)/
imidazolylidene complex catalyzed reactions involving elec-
tron-rich aldehydes and ketones were reported.[5,6] Other
than these catalytic reactions, the reactions using stoichio-
metric transition-metal complexes were also reported in co-
balt-mediated reactions involving electron-rich aldehydes
and ketones[7] and zirconium-mediated reactions involving
diethyl ketomalonate.[8] For the rhodium-catalyzed reac-
tions, [2+2+2+1] cycloadditions of diynals with CO by
using a neutral rhodium(I) complex, [Rh(cod)Cl]2, as a cat-
alyst furnished intramolecular [2+2+2] cycloaddition prod-
ucts as byproducts.[9] Carbonyl insertion into a cationic
[a] Department of Applied Chemistry, Graduate School of Engi-
neering, Tokyo University of Agriculture and Technology,
Koganei, Tokyo 184-8588, Japan
Fax: +81-42-388-7037
E-mail: tanaka-k@cc.tuat.ac.jp
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Scheme 1. Cationic rhodium(I)/H8-binap complex catalyzed
[2+2+2] cycloadditions of diynes with carbonyl compounds.
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Y. Otake, R. Tanaka, K. Tanaka
FULL PAPER
ficient and electron-rich carbonyl compounds. In this paper,
we describe our study concerning [2+2+2] cycloadditions
of 1,6- and 1,7-diynes with various carbonyl compounds,
leading to dienones through thermal electrocyclic ring
opening of fused α-pyrans,[13] catalyzed by a cationic rhodi-
um(I)/H8-binap complex under mild reaction conditions
(Scheme 1).[14–16]
Results and Discussion
Cationic Rhodium(I)/H8-binap Complex Catalyzed [2+2+2]
Cycloadditions of 1,6- and 1,7-Diynes with Carbonyl
Compounds
Figure 1. Structures of the bisphosphane ligands.
We first investigated the reaction of malonate-linked 1,6-
diyne 1a with an electron-deficient carbonyl compound [di-
ethylketomalonate(2a)]inthepresenceofvarious[Rh(cod)2]-
BF4/bisphosphane complexes (5 mol-%) as shown in
Table 1. After screening the bisphosphane ligands (Fig-
ure 1; Table 1, Entries 1–6), we were pleased to find that the
use of H8-binap showed the highest catalytic activity and
selectivity, and dienone 3aa was obtained in excellent yield
(Table 1, Entry 3). Removal of the cod ligand by hydrogena-
tion and the use of the cationic rhodium(I) complex and
not the neutral one are essential for this transformation
(Table 1, Entries 7 and 8).
and 7)] reacted with 1,6-diyne 1a to give the corresponding
dienones or α-pyrans in excellent yields under mild condi-
tions.
Table 2. Scope of acyclic tri- and dicarbonyl compounds 2a–g in
the reactions with 1,6-diyne 1a.[a]
Table 1. Screening of rhodium catalysts for [2+2+2] cycloaddition
of 1,6-diyne 1a with diethyl ketomalonate (2a).[a]
[a] [Rh(cod)2]BF4 (0.015 mmol), H8-binap (0.015 mmol), 1a
(0.30 mmol), 2 (0.33–0.60 mmol), and CH2Cl2 (2.0 mL) were used.
[b] Isolated yield. Products were isolated as a mixture of E/Z iso-
mers. [c] At 50 °C in (CH2Cl)2. [d] Products were isolated as a mix-
ture of dienone 3 and α-pyran 4.
[a] Rh complex (0.0075 mmol of Rh), bisphosphane (0.075 mmol),
1a (0.15 mmol), 2a (0.165 mmol), and (CH2Cl)2 (1.5 mL) were
used. The active catalyst was generated in situ by hydrogenation
(H2: 1 atm, r.t., 0.5 h). [b] Determined by 1H NMR spectroscopy.
[c] Without hydrogenation.
The successful [2+2+2] cycloadditions of 1,6-diyne 1a
with electron-deficient acyclic carbonyl compounds
prompted our investigation into the reactions of 1,6-diyne
1a with electron-rich acyclic carbonyl compounds as shown
in Table 3. Pleasingly, a variety of aromatic (2h–l; Table 3,
Thus, we explored the scope of this process with respect Entries 1–5) and aliphatic aldehydes (2m–o; Table 3, En-
to electron-deficient acyclic carbonyl compounds as shown tries 6–8) could participate in this reaction to give the corre-
in Table 2. Not only 1,2,3-tricarbonyl compounds [diethyl sponding dienones in excellent yields at room temperature.
ketomalonate (2a; Table 2, Entry 1)] but also a variety of Not only aldehydes but also an electron-rich ketone [ace-
1,2-dicarbonyl compounds [α-keto esters (2b–e; Table 2, En- tone (2p)] could react with 1a at 50 °C, although a large
tries 2–5) and 1,2-diketones (2f and 2g; Table 2, Entries 6 excess of 2p was required (Table 3, Entry 9).[17]
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Cationic Rhodium(I)/H8-binap Complex Catalyzed [2+2+2] Cycloadditions
Table 3. Scope of acyclic monocarbonyl compounds 2h–p in the
reactions with 1,6-diyne 1a.[a]
Table 4. Scope of cyclic carbonyl compounds 2q–v in the reactions
with 1,6-diyne 1a or 1b.[a]
[a] [Rh(cod)2]BF4 (0.015 mmol), H8-binap (0.015 mmol), 1a
(0.30 mmol), 2 (0.33 mmol), and CH2Cl2 (2.0 mL) were used. In
Entry 9, 2p was used as the solvent. [b] Isolated yield. Products
were isolated as a mixture of E/Z isomers. [c] Ligand: tol-binap. At
50 °C. [d] Product was isolated as a mixture of dienone 3ap and α-
pyran 4ap.
The reactions of 1,6-diynes with cyclic carbonyl com-
pounds 2q–v were also examined as shown in Table 4. Five-
membered 1,2-dicarbonyl compounds 2q–s reacted with
1,6-diyne 1a or 1b at room temperature to give the corre-
sponding dienones in excellent yields (Table 4, Entries 1–
3).[18] However, six-membered 1,2-dicarbonyl compound 2t
failed to react with 1a even at elevated temperatures (80 °C),
and the starting materials were recovered (Table 4, Entry 4).
Five-membered monocarbonyl compound [cyclopentanone
(2u)] failed to react with 1a due to the rapid homo-[2+2+2]
cycloaddition of 1a (Table 4, Entry 5), but six-membered
monocarbonyl compound [cyclohexanone (2v)] was able to
react with 1a at 50 °C to give a mixture of dienone 3av and
α-pyran 4av in good yield (Table 4, Entry 6).
As shown in Tables 2–4, 1,2-dicarbonyl compounds show
high reactivity, whereas monocarbonyl compounds show
low reactivity in the present [2+2+2] cycloaddition. There-
fore, the reactions of 1,6-diyne 1a with 1,3- and 1,4-dicar-
bonyl compounds 2w–y (1.1 equiv.) were also examined as
shown in Scheme 2. 1,3-Dicarbonyl compounds 2w and 2x
reacted with 1a at room temperature to give the corre-
sponding dienones 3aw and 3ax, respectively, in good
yields, whereas 1,4-dicarbonyl compound 2y failed to react
with 1a due to the rapid homo-[2+2+2] cycloaddition of 1a.
Next, the scope of diyne substrates was investigated as
shown in Table 5. With respect to 1,6-diynes, not only ma-
lonate-linked internal 1,6-diyne 1a (Table 5, Entries 1 and
2) but also tosylamide-, ether-, and 1,3-dimethoxypropane-
linked internal 1,6-diynes 1b–e could be employed for this
reaction (Table 5, Entries 3–7). Furthermore, internal 1,7-
[a] Reactions were conducted by using [Rh(cod)2]BF4
(0.015 mmol), H8-binap (0.015 mmol),
1
(0.30 mmol),
2
(0.33 mmol), and CH2Cl2 (1.5 mL) at r.t. for 3 h. In Entries 5 and
6, ketones 2u and 2v were used as the solvent. [b] Isolated yield.
Products were isolated as a mixture of E/Z isomers. [c] (CH2Cl)2
(3.0 mL) was used. [d] At 80 °C. [e] Diyne 1a remained unchanged.
[f] Homo-[2+2+2] cycloaddition of 1a proceeded. [g] At 50 °C.
[h] Product was isolated as a mixture of dienone 3av and α-pyran
4av.
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Y. Otake, R. Tanaka, K. Tanaka
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diyne 1a, which was recovered unchanged (Scheme 3). Sur-
prisingly, 5a smoothly reacted with tosylamide-linked 1,6-
diyne 1b at room temperature to give desired dienone 6ba
in good yield by using 10 mol-% of the [Rh(cod)2]BF4/H8-
binap complex (Scheme 3).
Scheme 2. [2+2+2] Cycloadditions of 1,6-diyne 1a with 1,3- and
1,4-dicarbonyl compounds 2w–y.
Scheme 3. Reactions of 1,6-diynes 1a and 1b with acetyl phos-
phonate 5a.
diyne 1f could also participate in this reaction (Table 5, En-
try 8). Although terminal 1,6-diyne 1g failed to react with
keto ester 2e due to the rapid homo-[2+2+2] cycloaddition
of 1g (Table 5, Entry 9), terminal 1,7-diyne 1h, which shows
low reactivity toward the homo-[2+2+2] cycloaddition,
could react with 2e to give the corresponding dienyl alde-
hyde 3he in moderate yield (Table 5, Entry 10).
Reaction conditions for the [2+2+2] cycloaddition of 1b
with 5a were then optimized as shown in Table 6. Like the
cycloadditions with tricarbonyl compound 2a, H8-binap
was the best ligand for this reaction (Table 6, Entries 1–4).
The use of 2 equiv. of 5a further improved the yield of 6ba,
and the reaction could be completed within 1 h (Table 6,
Entry 5). Unfortunately, decreasing the catalyst loading to
5 mol-% dramatically lowered the yield of 6ba (Table 6, En-
try 6).
Table 5. Scope of 1,6- and 1,7-diynes 1a–h in the reactions with 2a
or 2e.[a]
Table 6. Optimization of reaction conditions for the rhodium-cata-
lyzed [2+2+2] cycloaddition of 1,6-diyne 1b with acetyl phos-
phonate 5a.[a]
[a] [Rh(cod)2]BF4 (0.015 mmol), H8-binap (0.015 mmol),
1
(0.30 mmol), 2 (0.33 mmol), and CH2Cl2 (1.5 mL) were used.
[b] Isolated yield. Products were isolated as a mixture of E/Z iso-
mers. [c] At 50 °C in (CH2Cl)2. [d] Homo-[2+2+2] cycloaddition of
1g proceeded.
[a] [Rh(cod)2]BF4
0.010 mmol), 1b (0.10 mmol), 5a (0.11–0.20 mmol), and CH2Cl2
(1.5 mL) were used. [b] Isolated yield.
(0.0050–0.010 mmol),
ligand
(0.0050–
The scope of this process was then examined with respect
to both the acyl phosphonates and the diynes as shown in
Table 7. Not only acetyl phosphonate (5a; Table 7, Entry 1)
but also benzoyl phosphonate (5b; Table 7, Entry 2) could
participate in this reaction. In contrast, the reactivity of di-
Cationic Rhodium(I)/H8-binap Complex Catalyzed [2+2+2]
Cycloadditions of 1,6- and 1,7-Diynes with Acyl
Phosphonates
We have recently found that alkynylphosphonates are ynes was highly dependent on their own structures. Tosyl-
suitable substrates for cationic rhodium(I)/H8-binap com- amide- and ether-linked internal 1,6-diynes 1b and 1d could
plex catalyzed [2+2+2] cycloadditions.[19] Therefore, we an- react with 5b (Table 7, Entries 2 and 3), whereas malonate-
ticipated that acyl phosphonates, which are useful building and dimethoxypropane-linked internal 1,6-diynes 1a and 1e
blocks for the synthesis of functionalized phosphorus com- and internal 1,7-diyne 1f failed to react with 5b, and they
pounds through alkylation[20] or olefination,[21] would show remained unchanged (Table 7, Entries 4–6). In the case of
high reactivity in cationic rhodium(I)/H8-binap complex terminal diynes, although 1,6-diyne 1g failed to react with
catalyzed [2+2+2] cycloadditions.[22] Unfortunately, acetyl 5b due to the rapid homo-[2+2+2] cycloaddition of 1g
phosphonates 5a failed to react with malonate-linked 1,6- (Table 7, Entry 7), 1,7-diyne 1h could react with acyl phos-
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Cationic Rhodium(I)/H8-binap Complex Catalyzed [2+2+2] Cycloadditions
phonates 5a and 5b to give the corresponding dienyl alde- Table 8. [2+2+2] Cycloadditions of 1,6-diyne 1a with benzoyl phos-
phonate 5b in the presence of ethyl phenylglyoxylate (2e).[a]
hydes 6ha and 6hb, respectively, in good yields (Table 7, En-
tries 8 and 9).
Table 7. Scope of 1,6- and 1,7-diynes 1 in the reactions with 5a or
5b.[a]
[a] [Rh(cod)2]BF4 (0.020 mmol), H8-binap (0.020 mmol), 1a
(0.20 mmol), 5b (0–0.22 mmol), 2e (0–0.40 mmol), and CH2Cl2
(1.5 mL) were used. [b] Isolated yield. E/Z isomers were isolated
separately. [c] Isolated yield. Products were isolated as a mixture of
[a] [Rh(cod)2]BF4 (0.020 mmol), H8-binap (0.020 mmol),
1
E/Z isomers. [d] Diyne 1a remained unchanged.
(0.20 mmol), 5 (0.40 mmol), and CH2Cl2 (1.5 mL) were used.
[b] Isolated yield. [c] Product was isolated as a mixture of E/Z iso-
mers. [d] The product could not be isolated in a pure form. [e] Di-
yne 1 remained unchanged. [f] Homo-[2+2+2] cycloaddition of 1g
proceeded.
We assumed that bidentate coordination of α-keto ester
2e to rhodium might promote the formation of the desired
1:1 rhodium complex of 1,6-diyne 1a and benzoyl phos-
phonate 5b, which will be discussed in the section of the
reaction mechanism. Therefore, the reaction of 1a and 5b
in the presence of chelating diethyl oxalate (7), which can-
not react with 1a, was examined as shown in Table 9.[23] We
were pleased to find that desired product 6ab was obtained
in excellent yield in the presence of 1.1 equiv. of 7 (Table 9,
As α-keto ester 2e is a highly reactive coupling partner
for the cationic rhodium(I)/H8-binap catalyzed [2+2+2] cy-
cloaddition with 1,6-diyne 1a (Table 2, Entry 5), it would
be interesting to investigate a competitive [2+2+2] cycload-
dition of 1a with acyl phosphonate 5b and α-keto ester 2e.
We anticipated that 1a would selectively react with 2e, lead-
ing to ester-substituted dienone 3ae. Contrary to our expec-
tation, 1a reacted with both 5b and 2e (Scheme 4).
Table 9. [2+2+2] cycloadditions of 1,6- and 1,7-diynes 1 with acyl
phosphonates 5 in the presence of diethyl oxalate (7).[a]
Scheme 4. Competitive [2+2+2] cycloaddition of 1,6-diyne 1a with
carbonyl compounds 5b and 2e.
As α-keto ester 2e plays an important role in the present
catalysis, the amount of 2e was examined as shown in
Table 8. Decreasing the amount of 2e decreased the yield of
dienone 3ae and increased the yield of dienone 6ab (Table 8,
Entries 3–5).
[a] [Rh(cod)2]BF4 (0.020 mmol), H8-binap (0.020 mmol),
1
(0.20 mmol), 5 (0.22 mmol), 7 (0.040–0.22 mmol), and CH2Cl2
(1.5 mL) were used. [b] Isolated yield. [c] The product could not be
isolated in a pure form. [d] Product was isolated as a mixture of E/
Z isomers. [e] Diyne 1f remained unchanged.
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Y. Otake, R. Tanaka, K. Tanaka
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Entry 1). The amount of 7 could be reduced to a catalytic group of unsymmetrical 1,2-diketone 8a was more reactive
amount (Table 9, Entry 2). Under these optimized reaction than its acetyl group to give dienone 9aa as a major product
conditions, the reactions of both benzoyl and acetyl phos- (Table 10, Entry 1). As the cationic rhodium(I)/H8-binap
phonates 5b and 5a with malonate-, ether-, and dimeth- complex is a highly effective catalyst for [2+2+2] cycload-
oxypropane-linked 1,6-diynes 1a, 1d, and 1e, respectively, ditions of 1,6-diynes with alkenes[24] and alkynes,[18,25]
proceeded in high yields (Table 9, Entries 2–7). However, chemoselective cycloadditions involving alkenyl and alkynyl
1,7-diyne 1f failed to react with 5b even in the presence of carbonyl compounds were also examined. In the case of
7 (Table 9, Entry 8).
acrolein (8b), the carbonyl group of 8b exclusively reacted
with 1a (Table 10, Entry 2).[26] Like acrolein (8b), the car-
bonyl groups of alkynyl aldehyde 8c and alkynyl keto ester
8e exclusively reacted with 1a (Table 10, Entries 3 and 5),
but the alkyne moiety exclusively reacted with 1a in the case
of alkynyl ketone 8d to furnish hexasubstituted benzene
10ad in excellent yield (Table 10, Entry 4).[27] Furthermore,
the carbonyl group of 2-alkynylbenzaldehyde 8f exclusively
reacted with 1a (Table 10, Entry 6).
Cationic Rhodium(I)/H8-binap Complex Catalyzed Chemo-
or Regioselective [2+2+2] Cycloadditions of 1,6-Diynes with
Carbonyl Compounds
Chemoselective [2+2+2] cycloadditions of 1,6-diyne 1a
with bifunctional carbonyl compounds 8 were examined, as
shown in Table 10. As both dimethyl and diphenyl-substi-
tuted symmetrical 1,2-diketones 2f and 2g reacted with 1,6-
diyne 1a to give the corresponding dienones 3af and 3ag,
respectively, in high yields (Table 2, Entries 6 and 7), a
chemoselective cycloaddition between acetyl and benzoyl
groups is of interest. The study revealed that the benzoyl
Next, regioselective [2+2+2] cycloadditions of unsym-
metrical 1,6-diynes 1i–k with carbonyl compounds were
also examined, as shown in Table 11. The reaction of un-
Table 11. Regioselective [2+2+2] cycloadditions of unsymmetrical
1,6-diynes 1i–k with carbonyl compounds 2a, 5a, and 5b.
Table 10. Chemoselective [2+2+2] cycloadditions of bifunctional
carbonyl compounds 8a–f with 1,6-diyne 1a [Z = C(CO2Me)2].[a]
[a] Isolated yield. [b] Reactions were conducted with the use of
[Rh(cod)2]BF4
(0.015 mmol),
H8-binap
(0.015 mmol),
1
(0.30 mmol), and 2a (0.33 mmol) at 80 °C in (CH2Cl)2 (1.5 mL)
(Entry 1) or at r.t. (Entry 3) in CH2Cl2 (1.5 mL) for 3 h. [c] Reac-
tions were conducted with the use of [Rh(cod)2]BF4 (0.020 mmol),
H8-binap (0.020 mmol), 1 (0.20 mmol), 5 (0.40 mmol), and CH2Cl2
(1.5 mL) at r.t. for 1 h. [d] Homo-[2+2+2] cycloaddition of 1k pro-
ceeded.
[a] Reactions were conducted by using [Rh(cod)2]BF4
(0.015 mmol), H8-binap (0.015 mmol), 1a (0.30 mmol), 8 (0.33–
1.50 mmol), and CH2Cl2 (1.5 mL) at r.t. for 3 h. [b] Isolated yield.
Products were isolated as a mixture of E/Z isomers. [c] For 8 h.
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Cationic Rhodium(I)/H8-binap Complex Catalyzed [2+2+2] Cycloadditions
symmetrical 1,6-diyne 1i, possessing a methyl group and a ynes proceeded in high yields in the reactions of diynes with
methoxycarbonyl group at each alkyne terminus, with di- electron-rich ketones [acetone (2p) and cyclohexanone (2v)].
ethyl ketomalonate (2a) furnished the corresponding dien- Therefore, the formation of rhodacyclopentadiene interme-
ones 3ia and 11ia with moderate regioselectivity (Table 11, diate A is more likely. Indeed, the reactions of alkynyl
Entry 1). On the contrary, the reaction of 1,6-diyne 1j with ketone 13, which would generate bicyclic oxarhodacyclop-
benzoyl phosphonate 5b furnished the corresponding dien- entene intermediate D, with monoynes 14 did not furnish
one 6jb with complete regioselectivity (Table 11, Entry 2). the corresponding cycloadducts 15, and 12 was recovered
Although 2a reacted with unsymmetrical 1,6-diyne 1k, pos- unchanged even at the elevated temperature (Scheme 6).[29]
sessing a methyl group at an alkyne terminus, in very low
yield due to the rapid homo-[2+2+2] cycloaddition of 1k
(Table 11, Entry 3), acetyl phosphonate 5a smoothly re-
acted with 1k to give the corresponding dienone 6ka in
good yield with complete regioselectivity (Table 11, En-
try 4).
Mechanistic Consideration Regarding Cationic Rhodium(I)/
H8-binap Complex Catalyzed [2+2+2] Cycloadditions of
1,6- and 1,7-Diynes with Carbonyl Compounds
Scheme 6. Reactions of alkynyl ketone 13 and monoynes 14.
A possible mechanism for the regioselective [2+2+2] cy-
cloaddition of unsymmetrical 1,6-diyne 1i with tricarbonyl
compound 2a is shown in Scheme 7. The reaction of 1i and
2a with rhodium generates an equilibrium mixture of inter-
Scheme 5 depicts a possible mechanism for the cationic
rhodium(I)/H8-binap complex catalyzed [2+2+2] cycload-
dition of diyne 1 with carbonyl compound 2. Diyne 1 reacts
with rhodium to generate rhodacyclopentadiene intermedi-
mediates E and F through the bidentate coordination of
ate A. Insertion of carbonyl compound 2 into intermediate
2a and 1i. Oxidative coupling of two alkyne moieties of 1i
A generates intermediate B.[28] Reductive elimination of
rhodium followed by electrocyclic ring opening furnishes
dienone 3. Alternatively, diyne 1 and carbonyl compound 2
react with rhodium to generate oxarhodacyclopentene in-
termediate C. Insertion of another alkyne moiety of 1 into
intermediate C also can generate the same intermediate B.
furnishes rhodacyclopentadiene intermediate G. Insertion
of the carbonyl group of 2a would furnish intermediate H
and/or intermediate J. However, the regioselective forma-
tion of intermediate H might be predominant due to the
steric bulk of the methoxycarbonyl group of intermediate
G and the stabilization of intermediate H through the coor-
dination of the carbonyl oxygen atom to the cationic rho-
dium. Reductive elimination of rhodium from intermediate
H followed by electrocyclic ring opening furnishes dienone
11ia. Alternatively, oxidative coupling of the highly reactive
electron-deficient alkyne moiety[25] of 1i and the carbonyl
group of 2a would furnish oxarhodacyclopentene interme-
diate I. Insertion of another alkyne moiety of 1i furnishes
intermediate J. Reductive elimination of rhodium followed
by electrocyclic ring opening furnishes regioisomeric dien-
one 3ia. The reaction of 1i and 2a might proceed preferably
through oxarhodacyclopentene I, which results in the for-
mation of dienone 3ia as a major product. According to
this consideration and the low yields of homo-[2+2+2] cy-
cloaddition byproducts of diynes, oxarhodacyclopentene C
(Scheme 5) would be a major intermediate in the reactions
of diynes with tri- and dicarbonyl compounds.
A possible explanation for the significant acceleration of
the [2+2+2] cycloadditions with acyl phosphonates 5b by
Scheme 5. Two possible pathways for the cationic rhodium(I)/H8-
employing tosylamide-linked 1,6-diyne 1b as a coupling
binap complex catalyzed [2+2+2] cycloaddition of diyne 1 with car-
partner or adding diethyl oxalate (7) to malonate-linked
1,6-diyne 1a is shown in Scheme 8. The reaction of tosylam-
bonyl compound 2.
In the reactions of diynes with aldehydes, homo-[2+2+2] ide-linked 1,6-diyne 1b and acyl phosphonate 5b with rho-
cycloaddition of diynes proceeded in very low yields. Oxa- dium generates an equilibrium mixture of intermediates K
rhodacyclopentene intermediate C could not furnish the and L through the bidentate coordination of both 1b and
homo-[2+2+2] cycloaddition products of diynes, which sug- 5b to rhodium. Intermediate L subsequently undergoes oxi-
gests that oxarhodacyclopentene C might be the major in- dative coupling, leading to oxarhodacyclopentene interme-
termediate. In contrast, homo-[2+2+2] cycloaddition of di- diate M. Insertion of another alkyne moiety of 1b followed
Eur. J. Org. Chem. 2009, 2737–2747
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2743

Y. Otake, R. Tanaka, K. Tanaka
FULL PAPER
Scheme 7. Possible mechanism for the regioselective [2+2+2] cycloaddition of unsymmetrical 1,6-diyne 1i with 1,2,3-tricarbonyl compound 2a.
by reductive elimination of rhodium and electrocyclic ring
opening furnishes dienone 6bb. The absence of homo-
[2+2+2] cycloaddition product of 1b through the rhodacy-
clopentadiene intermediate would support this mechanism.
In the reaction of malonate-linked 1,6-diyne 1a, acyl phos-
phonate 5b, and rhodium, two molecules of 5b coordinate
to rhodium in a bidentate fashion to generate intermediate
K as a result of the higher coordination ability of 5b than
that of 1a, which results in no conversion of 1a. In contrast,
an equilibrium mixture of intermediates K, N, and O would
be generated in the presence of diethyl oxalate (7). Subse-
quent ligand exchange between 1a and weakly coordinated
7 generates the desired 1:1 rhodium complex P of 1a and
5b, which furnishes dienone 6ab through oxarhodacyclop-
entene intermediate Q.[30] Successful [2+2+2] cycload-
ditions of dimethoxypropane-linked 1,6-diyne 1e with acyl
phosphonates 5 in the presence of diethyl oxalate (7) could
reasonably be explained by the same mechanism. Ligand
exchange between 1,7-diyne 1f bearing no heteroatom in
the tether and 7 would be difficult to proceed, which results
in no conversion of 1f.
The exclusive formation of dienone 6jb from unsymmet-
rical 1,6-diyne 1j and benzoyl phosphonate 5b might be ex-
plained by the mechanism shown in Scheme 9. Oxidative
coupling of the highly reactive terminal alkyne moiety of 1j
and the carbonyl group of 5b furnishes oxarhodacyclopen-
tene intermediate R, which provides dienone 6jb.
Scheme 9. Possible mechanism for the regioselective [2+2+2] cyclo-
addition of unsymmetrical 1,6-diyne 1j with benzoyl phosphonate
5b.
Scheme 8. Possible mechanism for rhodium-catalyzed [2+2+2] cy-
Like the formation of 6jb, the exclusive formation of di-
cloadditions of tosylamide-linked 1,6-diyne 1b with benzoyl phos-
enone 6ka from unsymmetrical 1,6-diyne 1k and acetyl
phosphonate 5a might be explained by oxidative coupling
phonate 5b and malonate-linked 1,6-diyne 1a with 5b in the pres-
ence of diethyl oxalate (7).
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Eur. J. Org. Chem. 2009, 2737–2747

Cationic Rhodium(I)/H8-binap Complex Catalyzed [2+2+2] Cycloadditions
of the highly reactive terminal alkyne moiety[25] of 1k and
Experimental Section
the carbonyl group of 5a, leading to oxarhodacyclopentene
General Methods: 1H NMR spectra were recorded at 300 MHz
(JEOL AL 300). 13C NMR spectra were obtained with complete
proton decoupling at 75 MHz (JEOL AL 300). HRMS data were
obtained with a Bruker micrOTOF Focus II and a JEOL JMS-700
instrument. Infrared spectra were obtained with a JASCO FT/IR-
4100. All reactions were carried out under an atmosphere of argon
or nitrogen in oven-dried glassware with magnetic stirring.
intermediate S (Scheme 10).[31]
Starting Materials: Diynes 1a,[32] 1b,[11c] 1c,[25g] 1d,[33] 1e,[32] 1g,[34]
1i,[35] 1j,[35] and 1k[3] and carbonyl compounds 2e,[36] 8c,[37] 8e,[38]
and 8f[18] were prepared according to literature procedures. Anhy-
drous CH2Cl2 (No. 27099-7) and anhydrous (CH2Cl)2 (No. 28450-
5) were obtained from Aldrich and used as received. The H8-binap
and segphos ligands were obtained from Takasago International
Corporation. Rhodium complexes were obtained from Umicore.
All other reagents were obtained from commercial sources and
used as received.
Scheme 10. Possible mechanism for the regioselective [2+2+2] cy-
cloaddition of unsymmetrical 1,6-diyne 1k with acetyl phosphonate
5a.
Finally, complete intermolecular [2+2+2] cycloadditions
of two terminal monoynes with activated carbonyl com-
pounds were investigated at room temperature in the pres-
ence of the [Rh(cod)2]BF4/H8-binap complex (5 mol-%).
Methyl propargyl ether (16a, Figure 2) failed to react with
diethyl ketomalonate (2a) as a result of the rapid homo-
cyclotrimerization, leading to trisubstituted benzenes 17
(Figure 2). Under the same reaction conditions, no conver-
sion of 1-dodecyne (16b, Figure 2) was observed. Similarly,
the homo-cyclotrimerization of 16a and no conversion of
16b were observed in the reactions with benzoyl phos-
phonate 5b. Thus, the use of chelating diynes is necessary
to promote the desired [2+2+2] cycloaddition of two alkyne
units with carbonyl compounds.
Typical Procedure for the [2+2+2] Cycloaddition of Diynes with
Carbonyl Compounds: The H8-binap ligand (9.5 mg, 0.015 mmol)
and [Rh(cod)2]BF4 (6.1 mg, 0.015 mmol) were dissolved in CH2Cl2
(2.0 mL), and the mixture was stirred at room temperature for
5 min. H2 was introduced into the resulting solution in a Schlenk
tube. After stirring at room temperature for 1 h, the resulting solu-
tion was concentrated to dryness and dissolved in CH2Cl2
(0.5 mL). To this solution was added dropwise a solution of 1a
(70.9 mg, 0.300 mmol) and 2e (58.8 mg, 0.330 mmol) in CH2Cl2
(1.5 mL) at room temperature. The mixture was stirred at room
temperature for 3 h. The resulting solution was concentrated and
purified by preparative TLC (hexane/ethyl acetate, 3:1), which fur-
nished 3ae (121.4 mg, 0.293 mmol, 98% yield; Table 2, Entry 5) as
a pale-yellow oil. E/Z, 29:71. IR (neat): ν = 2954, 1738, 1660, 1268,
˜
732 cm–1. Data for the Z isomer: 1H NMR (CDCl3): δ = 7.42–7.30
(m, 3H), 7.29–7.23 (m, 2H), 4.09 (q, J = 7.2 Hz, 2H), 3.78 (s, 6H),
3.47 (t, J = 1.5 Hz, 2H), 3.38 (t, J = 1.5 Hz, 2H), 2.33 (s, 3H), 1.87
1
(s, 3H), 1.14 (t, J = 7.2 Hz, 3H) ppm. Data for the E isomer: H
NMR (CDCl3): δ = 7.29–7.23 (m, 3 H), 7.16–7.10 (m, 2 H), 4.22
(q, J = 7.2 Hz, 2 H), 3.65 (s, 6 H), 3.16 (t, J = 1.5 Hz, 2 H), 3.09
(t, J = 1.5 Hz, 2 H), 2.23 (s, 3 H), 2.19 (s, 3 H), 1.25 (t, J = 7.2 Hz,
3 H) ppm. 13C NMR (CDCl3): δ = 195.6, 171.5, 171.2, 167.8, 166.8,
152.7, 150.6, 141.9, 140.5, 136.2, 135.8, 135.2, 133.4, 132.2, 132.1,
129.0, 128.29, 128.25, 128.0, 127.7, 127.6, 60.9, 57.0, 56.6, 53.0,
45.7, 45.1, 41.1, 28.7, 28.6, 20.2, 19.8, 14.1, 13.9 ppm. HRMS (EI):
calcd. for C23H26O7 414.1679 [M]+; found 414.1733.
Figure 2. Terminal monoynes 16 and homo-cyclotrimerization
product 17.
Conclusions
In conclusion, we have established that a cationic rhodi-
um(I)/H8-binap complex catalyzes [2+2+2] cycloadditions
of a variety of 1,6- and 1,7-diynes with both electron-de-
ficient and electron-rich carbonyl compounds, leading to di-
enones in high yields under mild reaction conditions. The
reactions of diynes with carbonyl compounds might pro-
ceed via an oxarhodacyclopentene intermediate and/or a
rhodacyclopentadiene intermediate depending on the struc-
Typical Procedure for the [2+2+2] Cycloaddition of Diynes with Acyl
Phosphonates in the Presence of Diethyl Oxalate: The H8-binap li-
gand (12.6 mg, 0.020 mmol) and [Rh(cod)2]BF4 (8.1 mg,
0.020 mmol) were dissolved in CH2Cl2 (2.0 mL), and the mixture
was stirred at room temperature for 5 min. H2 was introduced into
the resulting solution in a Schlenk tube. After stirring at room tem-
perature for 1 h, the resulting solution was concentrated to dryness
tures of the carbonyl compounds. In the reactions with acyl and dissolved in CH2Cl2 (0.5 mL). To this solution was added
dropwise a solution of 1a (47.3 mg, 0.200 mmol), 7 (5.8 mg,
0.040 mmol), and 5b (53.3 mg, 0.220 mmol) in CH2Cl2 (1.0 mL) at
room temperature. The mixture was stirred at room temperature
for 1 h. The resulting solution was concentrated and purified by
preparative TLC (hexane/ethyl acetate/triethylamine, 5:3:1), which
furnished (E)-6ab (46.7 mg, 0.0976 mmol, 49% yield) as a pale-
yellow oil and (Z)-6ab (44.6 mg, 0.0932 mmol, 47% yield; Table 9,
phosphonates, the reactivity of 1,6- and 1,7-diynes was
highly dependent on their own structures. The addition of
chelating diethyl oxalate effectively promoted the [2+2+2]
cycloadditions presumably as a result of the equilibrium
formation of the desired 1:1 rhodium complex of the diyne
and the acyl phosphonate by facile ligand exchange be-
tween the diyne and weakly coordinated diethyl oxalate. In
the reactions involving bifunctional carbonyl compounds or
unsymmetrical 1,6-diynes, high chemo- or regioselectivities
were observed.
Entry 2) as a pale-yellow oil. Data for (E)-6ab: IR (neat): ν = 2984,
˜
1
2955, 2909, 1738, 1685, 1238, 1024, 703 cm–1. H NMR (CDCl3):
δ = 7.25–7.15 (m, 3 H), 7.07–6.97 (m, 2 H), 4.12–3.91 (m, 4 H),
3.60 (s, 6 H), 3.10–3.02 (m, 2 H), 2.95 (s, 2 H), 2.35 (d, J = 3.3 Hz,
Eur. J. Org. Chem. 2009, 2737–2747
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2745

Y. Otake, R. Tanaka, K. Tanaka
FULL PAPER
3 H), 2.18 (s, 3 H), 1.20 (t, J = 7.2 Hz, 6 H) ppm. 13C NMR
(CDCl3): δ = 195.4, 171.0, 152.0, 151.7, 151.6, 151.4, 137.4, 137.3,
133.8, 129.8, 128.8, 128.7, 127.8, 127.5, 127.3, 61.9, 61.8, 56.8, 53.0,
44.9, 40.8, 29.0, 20.2, 20.1, 16.2, 16.1 ppm. 31P NMR (121 MHz,
CDCl3): δ = 15.6 ppm. HRMS (ESI): calcd. for C24H31NaO8P
[6]
[7]
Nickel-catalyzed [4+2+2] cycloadditions of 1,6-diynes with cy-
clobutanones were reported; see: M. Murakami, S. Ashida, T.
Matsuda, J. Am. Chem. Soc. 2006, 128, 2166–2167.
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501.1654 [M + Na]+; found 501.1648. Data for (Z)-6ab: IR (neat):
[8]
T. Takahashi, Y. Li, T. Ito, F. Xu, K. Nakajima, Y. Liu, J. Am.
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1
ν = 2984, 2951, 2905, 1737, 1659, 1254, 1022, 702 cm–1. H NMR
˜
(CDCl3): δ = 7.40–7.28 (m, 3 H), 7.22–7.16 (m, 2 H), 3.96–3.70 (m,
4 H), 3.74 (s, 6 H), 3.70–3.30 (m, 4 H), 2.33 (s, 3 H), 1.79 (d, J =
2.4 Hz, 3 H), 1.10 (t, J = 7.2 Hz, 6 H) ppm. 13C NMR (CDCl3): δ =
195.8 152.4, 152.2, 150.1, 150.0, 136.8, 136.6, 133.9, 130.5, 129.11,
129.05, 128.4, 128.0, 127.5, 127.4, 61.9, 61.8, 57.3, 53.0, 45.6, 41.0,
28.9, 21.1, 20.9, 16.1, 16.0 ppm. 31P NMR (121 MHz, CDCl3): δ =
14.7 ppm. HRMS (ESI): calcd. for C24H31NaO8P 501.1654 [M +
Na]+; found 501.1642.
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[12]
[13]
Acknowledgments
This work was supported partly by a Grant-in-Aid for Scientific
Research from MEXT, Japan (No. . 20675002 and 19028015). We
are grateful to Takasago International Corporation for the gift of
H8-binap and segphos and to Umicore for generous supply of the
rhodium complexes.
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A part of this work was previously communicated; see: a) K.
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2746
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Eur. J. Org. Chem. 2009, 2737–2747

Cationic Rhodium(I)/H8-binap Complex Catalyzed [2+2+2] Cycloadditions
Chim. Acta 1975, 58, 748–760. For hetero-Diels–Alder reac-
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[27] The cyano group exclusively reacted with 1a in the case of ace-
tyl and benzoyl cyanides; see: ref.[11c]
[23] We deeply appreciate one of the referees of our previous paper
[28] For examples of carbonyl insertion into a Rh–C bond, see: a)
C. Krug, J. F. Hartwig, J. Am. Chem. Soc. 2002, 124, 1674–
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[29] 5-Alkynals spontaneously cyclized to generate intramolecular
hydroacylation products (cylopentanones) in the presence of
the cationic rhodium(I)/biaryl bisphosphane catalyst at room
temperature; see: K. Takeishi, K. Sugishima, K. Sasaki, K.
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[30] Equilibrium coordination of the ester carbonyl oxygen atom
versus the alkyne moiety of a malonate-linked 1,6-diyne is pro-
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[31] A similar regioselectivity to this was observed in the ruthe-
nium-catalyzed [2+2+2] cycloadditions of 1,6-diynes, pos-
sessing a methyl group at the alkyne terminus, with diethyl ke-
tomalonate; see: ref.[3]
(ref.[14b]) for this valuable suggestion.
[24] For examples of the cationic rhodium(I)/biaryl bisphosphane
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Received: February 22, 2009
Published Online: April 8, 2009
Eur. J. Org. Chem. 2009, 2737–2747
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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