Rhodium-catalyzed novel carbonylative carbotricyclization of enediynes [8]

Full text of DOI:10.1021/ja993977g

J. Am. Chem. Soc. 2000, 122, 2385-2386
2385
As a part of our continuing studies on the metal-catalyzed,
silicon-initiated carbometalations, we looked at the reaction of
dedec-11-ene-1,6-diynes. To our surprise, the reaction of the
enediynes with PhMe2SiH in the presence of a rhodium catalyst
under ambient CO afforded fused 5-7-5 ring structures incor-
porating CO. We describe here a novel silicon-initiated carbon-
ylative carbotricyclization of enediynes promoted by rhodium
catalysts, which provides a rapid and useful method for the
construction of fused 5-7-5 ring systems.
Rhodium-Catalyzed Novel Carbonylative
Carbotricyclization of Enediynes
Iwao Ojima* and Seung-Yub Lee
Department of Chemistry
State UniVersity of New York at Stony Brook
Stony Brook, New York 11794-3400
The reaction of enediyne 7a (0.5 mmol) with PhMe2SiH (1.0
mmol) catalyzed by Rh(acac)(CO)2 (1 mol %) in toluene (2.5
mL) under CO at 70 °C for 1h gave cyclopenta[e]azulene 8a and
bis(cyclopentylidene) compound 9a as major products and alde-
hyde 10a as minor product in 70% overall yield (8a:9a:10a )
36:43:21; isolated by column chromatography on silica gel) (Table
1, entry 1) (eq 3).
ReceiVed NoVember 12, 1999
Transition metal-catalyzed carbocyclization of compounds
containing organic π-systems has played an important role in
organic syntheses.1 Both the intermolecular and intramolecular
carbocyclization is a powerful method for construction of cyclized
compounds that are useful for further synthetic transformation.
Among various metals, nickel, ruthenium, cobalt, rhodium, and
palladium catalysts have been commonly used for carbocycli-
zations.2-4
In the course of our studies on silicon-initiated carbometalation
reactions, silyformylation,5a silylcyclocarbonylation (SiCCa),6,7
and silycarbocyclization (SiCaC)7 have been discovered. We
reported a stereospecific cascade SiCaC reaction8 of dodec-6-
ene-1,11-diynes (eq 1). In this reaction, although cyclization of
the vinyl-[Rh] moiety of intermediate 2 to the vinylsilane moiety
to form the third ring was conceptually possible, it did not take
place. Simple reductive elimination preferentially occurred to give
the corresponding biscyclopentyl 3.
Since the one-step formation of 8a was synthetically very
attractive, we investigated the reaction variables to optimize the
product selectivity for 8a. Results are summarized in Table 1.
The reactions under higher CO pressure (10 atm) suppress the
formation of 9a (entry 2). Lower reaction temperatures, smaller
amounts of PhMe2SiH, and higher dilution also disfavor the
formation of 9a. Addition of P(OPh)3 completely suppresses the
formation of 10a (entry 3), while the use of PPh3 as the additive
substantially favors the formation of 10a and reduces the yield
of 8a (entry 4). The use of THF as solvent gives better product
selectivity for 8a. Under dilute reaction conditions, the product
selectivity for 8a is dramatically increased (entries 6-8, 10, and
11). Reaction with 0.5 equiv of PhMe2SiH is sufficient for high
overall yield (entries 7, 10, and 11). This clearly indicates that
the hydrosilane is recycled in the catalytic system. However, the
reaction with 0.1 equiv of the hydrosilane does not give complete
conversion in 72 h, and 8a was isolated in 33% yield (entry 8).
In the absence of PhMe2SiH, no reaction takes place after 72 h,
recovering enediyne 7a (entry 9). Thus, PhMe2SiH is proven
necessary for this reaction to occur. The best result is achieved
by using 0.5 equiv of PhMe2SiH at 0.015 M concentration, giving
8a as the single product in 92% isolated yield (entry 7). As for
the catalyst, Rh(acac)(CO)2, Rh4(CO)12, and [Rh(CO)2Cl]2 show
similar efficacy. For hydrosilanes in this process, the reactions
with Ph2MeSiH and (EtO)3SiH give 8a exclusively although the
reactions are slower than that with PhMe2SiH. Trialkylsilanes
behave differently, that is, the reaction with Et3SiH gives a 9:1
mixture of 8a and 9a-TES in low conversion, while that with
In the case of trialkynes, silylcarbotricyclization (SiCaT) takes
place to give fused tricyclic compounds 6a and 6b in good to
excellent yields (eq 2).9 Reactions catalyzed by rhodium clusters
showed high selectivity to 6a.
(1) For recent reviews on metal-catalyzed carbocyclization, see: Grotjahn,
D. B. In ComprehensiVe Organometallic Chemistry II; Hegedus, L. S., Ed.;
Pergamon/Elsevier Science: Kidlington, 1995; Vol. 12; pp 703, 741; Lautens,
M.; Klute, W.; Tam, W. Chem. ReV., 1996, 96, 49; Ojima, I.; Tzamarioudaki,
M.; Li, Z.; Donovan, R. J. Chem. ReV. 1996, 96, 635.
(2) (a) Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1984, 23, 539. (b)
Cruciani, P.; Aubert, C.; Malacria, M. J. Org. Chem. 1995, 60, 2664 and
references therein.
(3) (a) Grigg, R.; Scott, R.; Stevenson, P. Tetrahedron Lett. 1982, 23, 2691.
(b) Grigg, R.; Scott, R.; Stevenson, P. J. Chem. Soc., Perkin Trans I 1988,
1357.
(4) Negishi, E.; Harring, L. S.; Owczarczyk, Z.; Mohamud, M. M.; Ay,
M. Tetrahedron Lett. 1992, 33, 3253.
(6) (a) Ojima, I.; Donovan, R. J.; Eguchi, M.; Shay, W. R.; Ingallina, P.;
Korda, A.; Zeng, Q. Tetrahedron 1993, 49, 5431. (b) Ojima, I.; Vidal, E.;
Tzamarioudaki, M.; Matsuda, I. J. Am. Chem. Soc. 1995, 117, 6797. (c) Ojima,
I.; Li, Z.; Donovan, R. J.; Ingallina, P. Inorg. Chim. Acta 1998, 270, 279. (d)
Matsuda, I.; Fukuta, Y.; Tsuchihashi, T.; Nagashima, H.; Itoh, K. Organo-
metallics 1997, 16, 4327.
(7) (a) Ojima, I.; Donovan, R. J.; Shay, W. R. J. Am. Chem. Soc. 1992,
114, 4, 6580. (b) Ojima, I.; Tzamarioudaki, M.; Tsai, C.-Y. J. Am. Chem.
Soc. 1994, 116, 3643. (c) Ojima, I.; Fracchiolla, D. A.; Donovan, R. J.; Banerji,
P. J. Org. Chem. 1994, 59, 7594. (d) Ojima, I.; Kass, D. F.; Zhu, J.
Organometallics 1996, 15, 5191. (e) Ojima, I.; Zhu, J.; Vidal, E. S.; Kass, D.
F. J. Am. Chem. Soc. 1998, 120, 6690.
(8) Ojima, I.; McCullagh, J. V.; Shay, W. R. J. Organomet. Chem. 1996,
521, 421.
(9) Ojima, I.; Vu, A. T.; McCullagh, J.; Kinoshita, A. J. Am. Chem. Soc.,
1999, 121, 3230.
(5) (a) Ojima, I.; Clos, N.; Donovan, R. J.; Ingallina, P. Organometallics
1990, 9, 3127. (b) R. Tanke; R. H. Crabtree J. Am. Chem. Soc., 1990, 112,
7984-7989.
10.1021/ja993977g CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/23/2000

2386 J. Am. Chem. Soc., Vol. 122, No. 10, 2000
Communications to the Editor
Table 1. Silicon-Initiated Carbonylative Carbotricyclization of 7a a
Scheme 1
product ratio
(h) (%)b 8a 9a 10a
R3SiH
entry cat. (equiv.)
temp CO solvent time yield
(°C) (atm) (M)
1
2
A
A
A
A
A
A
PhMe2SiH
(2.0)
PhMe2SiH
(2.0)
PhMe2SiH
(1.0)
PhMe2SiH
(1.0)
PhMe2SiH
(1.0)
PhMe2SiH
(1.0)
70
60
50
50
22
22
1
toluene
(0.2)
1
70
55
62
52
65
36 43 21
10 toluene 10
55
9
36
0
(0.2)
toluene 10
(0.2)
toluene 10
(0.2)
THF
(0.2)
THF
(0.15)
3c
4d
5
1
1
1
1
65 35
10 32 58
70 15 15
12
24
6
82 100
92 100
33 100
0
0
7
A
PhMe2SiH 22
(0.5)
1
THF
(0.015)
30
0
0
8
9
A
A
B
C
A
A
A
A
PhMe2SiH
(0.1)
PhMe2SiH
(0.0)
PhMe2SiH
(0.5)
PhMe2SiH
(0.5)
PhMe2SiH
(0.5)
(EtO)3SiH
(0.5)
Et3SiH
(0.5)
tBuMe2SiH 22
(0.5)
22
22
22
22
22
22
22
1
1
1
1
1
1
1
1
THF
(0.015)
THF
(0.015)
THF
(0.015)
THF
(0.015)
THF
(0.015)
THF
(0.015)
THF
96
96
17
23
96
96
96
96
0
0
8
0
0
0
0
0
0
0
0
0
0
0
0
0
10
11
12
13
14
15
78
92
80 100
42 100
34 100
reaction of 7g gives 8g that contains two fused bispyrrolidine
rings in 85% isolated yield (entry 6), while the reaction of 7h
that contains dipropargyl ether as well as propargyl allyl ether
moieties is sluggish to give 8h in 50% yield after 48 h (entry 7).
A possible mechanism for this novel carbonylative carbotri-
cyclization is proposed in Scheme 1. The reaction begins with
the insertion of the terminal alkyne moiety into the Si-[Rh] bond
of the hydrosilane-Rh oxidative adduct. Carbocyclization then
occurs to give (Z)-dienyl[Rh](H) intermediate a. Because of the
steric hindrance between the vinylsilane and the vinyl-[Rh]
moieties, a isomerizes to c via b through the “Ojima-Crabtree
mechansim”.5 Subsequent carbocyclization to d, followed by
reductive elimination, should give 9. The CO insertion to d gives
acyl-[Rh](H) intermediate e. Reductive elimination of e should
yield aldehyde 10. Carbocyclization of e gives tricyclic intermedi-
ate f that has the silicon and the [Rh] moieties in syn positions.
Subsequent â-silyl elimination7e,9 should afford the fused 5-7-5
tricyclic product 8 and regenerate the active catalyst species, R3-
Si-[Rh](H). The proposed mechanism can nicely accommodate
the observed three products under non-optimized conditions,
dilution effects,10 and the fact that only substoichiometric hy-
drosilane is needed for this novel process.
12
0
90 10
(0.015)
THF
(0.015)
0
0
a Reaction was run with endyne (0.25-0.5 mmol scale), hydrosilane
and Rh-catalyst (1 mol %) in a solvent under CO. Catalyst A )
Rh(acac)(CO)2, B ) Rh4(CO)12, C ) [Rh(CO)2Cl]2. b Isolated yields.
c P(OPh)3 (3 equiv to catalyst) was added as ligand. d PPh3 (3 equiv to
catalyst) was added as ligand.
Table 2. Silicon-Initiated Carbonylative Carbotricyclization of 7a
Further studies on the scope and limitation of this novel
carbonylative carbotricyclization process as well as its applications
to the syntheses of biologically active compounds are actively
underway in these laboratories.
Acknowledgment. This research was supported by grants from the
National Science Foundation and the National Institutes of Health
(NIGMS). Generous support from Mitsubishi Chemical Corp. is gratefully
acknowledged. The author (S.Y.L) thanks Dr. An T. Vu at Stony Brook
for valuable discussion and support.
a Reaction was run with an endyne (0.25 mmol), PhMe2SiH (0.13
mmol) and Rh(acac)(CO)2 (1 mol %) in THF (17 mL) under CO (1
atm). b Isolated yield.
Supporting Information Available: Experimental procedures, spec-
tral data for all new compounds 7a-g, 8a-h, 9a, and 10a (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
JA993977G
t-BuMe2SiH does not give any conversion after 96 h. Accordingly,
PhMe2SiH is the best hydrosilane for this process so far examined.
Next, other than ester groups that were included in 8a, we
looked at the heteroatom and functional group tolerance of this
novel carbotricyclizaton process. Thus, various substrates contain-
ing ether, ester, hydroxyl, and sulfonamide groups were employed
for the reaction. Results are summarized in Table 2. As Table 2
shows, these functional groups and heteroatoms are well tolerated
in this reaction to give the corresponding expected fused 5-7-5
tricyclic compounds 8 in good to excellent isolated yields. The
(10) The observed remarkable dilution effects on the product selectivity
appears to be ascribed to the relative concentration of CO in solution to those
of catalyst and hydrosilane, i.e., the relative concentration of CO is higher in
dilute solution if the amounts of the catalyst and hydrosilane are kept constant.
As Scheme 1 illustrates, the reductive elimination of intermediate d can be
suppressed by higher CO concentration, directing the reaction to the formation
of intermediate e. Also, the reductive elimination of d and e is believed to be
significantly accelerated by another molecule of hydrosilane.5a,7 Thus, the low
concentration of hydrosilane should slow the reductive elimination in these
two steps, leading to the selective formation of intermediate f and then the
product 8.