2346
Organometallics 1988, 7, 2346-2360
Mechanistic Studies on the Reaction of Chromium Carbene Complexes with Acetylenes: Furan Formation and the Dependence of the Product Distribution on the Stereochemistry of Reaction Intermediates J. Stuart McCallum,” Fen-Ann Kunng, Scott R. Gilbertson, and William D. Wulff**’b Searle Chemistry Laboratory, Department of Chemistry, The University of Chicago, Chicago, Illinois 60637 Received April 13, 1988
The reaction of [(2-furyl)methoxymethylene]pentacarbonylchromiumwith acetylenes produces two products in which a new aromatic nucleus is constructed: a furan and a phenol. The mechanism for the formation of these two products is studied for the reaction of methyl 4-pentynoate and with other acetylenes. Furan and phenol products have been previously observed for the reaction of carbene complexes with acetylenes, but the mechanism for phenol formation is far from understood and the mechanism for furan formation has not been previously studied. Trapping experiments with alcohols fail to distinguishbetween two possible mechanisms for phenol formation but do provide the first evidence that links the stereochemistry of reaction intermediates with the product distribution from the reaction of carbene complexes with acetylenes. It is proposed (at least for the reactions of (2-fury1)carbenecomplexes of chromium) that the stereoseledivityin the formation of reaction intermediates,and thus the product distribution,is determined by an irreversible electrocyclic ring opening of a metallacyclobutene intermediate. This proposal is used as a model to correlate the distributionbetween furan and phenol products and the nature of the substituents on the acetylene. Two reasonable mechanisms are proposed for the formation of the furan nucleus. A distinction between these two mechanisms is made on the basis of 13C-,170-, and 180-labelingexperiments. The reaction of Fischer carbene complexes with acetylenes is multifaceted since under appropriate conditions up to nine structurally different types of organic products can be obtained.2 These include 4-alko~yphenols,~ phen o l ~f, ~~ r a n s , ~ ~ ? ~ vinylketenes,3c~”,~ cyclohexadienones,2c~3~7c 1,3-dienes,8and p y r o n e ~ . ~ The ” ~ ~first product ever isolated1° from this type of reaction was a 4-alkoxyphenol of the type 2 which (1)(a) Dow Chemical predoctoral fellow, 1982-1984. (b) Eli Lilly Young Scholar, 1986-1987. (2)For reviews of the chemistry of Fischer carbene complexes see: (a) Dotz, K. H.; Fischer, H.; Hofmann, P.; Kreissel, F. R.; Schubert, U.; Weiss, K. Transition Metal Carbene Complexes; Verlag Chemie: Deerfield Beach, FL, 1984. (b) Doh, K. H. Angew. Chem.,Int. Ed. Engl. 1984,23,587. (c) Wulff, W. D.; Tang, P. C.; Chan, K. S.; McCailum, J. S.; Yang, D. C.; Gilbertson, S. R. Tetrahedron 1985,41,5813.(d) Wulff, W. D. In Aduances in Metal-Organic Chemistry; Liebeskind, L. S., Ed.; JAI: Greenwich, CT, 1987;Vol. 1. (3) (a) Wulff, W. D.; Kaesler, R. W.; Peterson, G. A,; Tang, P. C. J . Am. Chem. SOC. 1985,107,1060. (b) Semmelhack, M. F.; Park, J. Organometallics 1986,5,2550. (c) Xu, Y. C.; Wulff, W. D. J. Org. Chem. 1987,52,3263.(d) Wulff, W. D.; Xu, Y. C. Tetrahedron Lett. 1988,415. (4)Dotz, K. H. J. Organomet. Chem. 1977,140,177. (b) Dotz, K. H.; Dietz, R.; Neugebauer, D., Chem. Ber. 1979,112,1486.(c) Semmelhack, M. F.; Bozell, J. J.; Keller, L.; Sato, T.; Spiess, E. J.; Wulff, W.; Zask, A. Tetrahedron 1985,41,5803. (d) Wulff, W. D.; Kaesler, R. W. Organometallics 1985,4,1461. (e) Wulff, W. D.; Gilbertson, S. R.; Springer, J. P. J . Am. Chem. SOC.1986,107,5823. (5)(a) Dotz, K. H.; Pruskil, I. Chem. Ber. 1978,111,2059. (b) Dotz, K. H.; Neugebauer, D. Angew. Chem., Znt. Ed. Engl. 1978,17,851.(c) Dotz, K. H.; Dietz, R. E.; Kappenstein, C.; Neugebauer, D.; Schubert, U. Chem. Ber. 1979,112,3682.(d) Foley, H. C.; Strubinger, L. M.; Targos, T. S.; Geoffroy, G. L. J. Am. Chem. SOC.1983,105,3064. (e) Pruskil, I.; Schubert, U.; Ackermann, K.; Dotz, K. H. Chem. Ber. 1983,116, 2337. (f) Yamashita, A. Tetrahedron Lett. 1986,27,5915.(g) Combie, R.C.; Rutledge, P. S.; Tercel, M.; Woodgate, P. D. J . Organomet. Chem. 1986, 315,171.(h) Dotz, K. H.; Strum, W. J. Organomet. Chem. 1986,310,C22. (i) Chan, K. S.; Peterson, G. A.; Brandvold, T. A.; Faron, K. L.; Challener, C. A.; Hyldahl, C.; Wulff, W. D. J . Organomet. Chem. 1987,334, 9. (6)(a) Dotz, K. H.; Dietz, R. E. J . Organomet. Chem. 1978,157,C55. (b) Yamashita, A.; Scahill, T. A.; Toy, A. Tetrahedron Lett. 1985,2969. (c) Dotz, K. H.; Strum, W. J . Organomet. Chem. 1985,285, 205. (d) Yamashita, A.;Toy, A. Tetrahedron Lett. 1986,27, 3471. (7)(a) Dotz, K. H. Angew. Chem., Znt. Ed. Engl. 1979,18,954. (b) Dotz, K. H.; Fuegen-Koster, B. Chem. Ber. 1980,113,1449. (c) Tang, P. C.; Wulff, W. D. J. Am. Chem. SOC.1984,106, 1132. For examples of metal ketene complexes not derived from carbene complexes see ref 23. (8)Macomber. D. W. Ormnometallics 1984.3. 1589. (9)Semmelhack, M. F.; T-mura, R.; Schnatter, W.; Springer, J. J. Am. Chem. SOC.1984.106. 5363. (10) Dotz, K. H. Ahgew. Chem., Int. Ed. Engl. 1975,14, 644.
is normally the predominant product of the reaction, and the conditions necessary for the optimization of this product are beginning to be defined!‘ The 4-alkoxyphenols are produced in an annulation process that gives rise to a new benzene ring that is the result of the assembly of pieces indicated in structure 4 in the coordination sphere of the metal. This benzannulation reaction has received considerable attention in the last few years which is largely a result of the growing recognition of its utility in synthetic organic chemistry.2 Although proposals have been made for and some data have been collected in regard to the mechanism for the formation of 4-alkoxyphenols and for some of the other structural types from these r e a c t i o n ~ , ~no , ~mechanistic ~J~ study has been directed to the mechanism of furan formation from the reaction of a carbene complex and an acetylene. Furans are usually minor products from the reactions of chromium carbene complexes and acetylenes and have been reported only on rare occasion^.^^^^ One of the best characterized furan products from these reactions is the ferrocenyl-substituted furan 6 for which the structure was confirmed by an X-ray diffraction We have recently found that a general furan synthesis is possible from the reaction of cobalt carbene complexes and acetylenes and have suggested a mechanism for their format i ~ n We . ~ herein ~ report on studies directed toward the elucidation of the mechanisms for the formation of both furan and for 4-alkoxyphenols from the reaction of chromium carbene complexes and acetylenes, and the results may be applicable to the reactions of complexes of other metals as well. PhCiCPh
-
5
0 I
6
45%
(11)(a) Fischer, H.; Mulheimer, J.; Markl, R.; Dotz, K. H. Chem. Ber. 1982,115,1355.(b) Casey, C. P. In Reactiue Intermediates; Jones, M., Jr., Moss, R. A., Eds.; Wiley: New York, 1981;Vol. 2.
0276-1333/8~/2307-2346$01.50/0 0 1988 American Chemical Society
Organometallics, Vol. 7, No. 11, 1988 2347
Reaction of Chromium Carbene Complexes with Acetylenes Scheme I
3 1
0
4
Scheme IV
Scheme I1
C02Me
9
7
Me02C 14
11
sphondin 10
CQMe
I
Scheme I11 n Me:=>
Me0
OH 8
OMe
C
12
2) air, HOTS.benzene 8OOC. 4AoYeYes
OMe
1 1 22%
52%
OMe
8
(COIMe
%co2Me
L
MeOH 70'
15
OMe 9
lCO15Cr =c
C 0 2 Me
OMe
F
THF, 85'
ti
0
C Me0
C02Me
OMe 8
12
50%
23%
one dlaStereOmel
r
1
13
In the course of a total synthesis of sphondin (10) utilizing the general approach outlined in Scheme 11, we were led to investigate the benzannulation of the (2-fury1)carbene complex 7 and methyl 4-pentyn~ate.'~The expected regiochemistry for this reaction'J3 is that indicated in the benzopyran 9, and thus lactonization and dehydrogenation are all that would remain to complete the synthesis of sphondin. The reaction of the carbene complex 7 with methyl 4-pentynoate (8) in fact gave a 251.0 mixture of the benzofuran 9 and the 5-furylfuran 11 in 73% yieldlZa (Scheme 111). However, all attempts to further optimize this reaction for intermediate 9 in the synthesis of sphondin failed. It was observed during these studies directed to the synthesis of sphondin that the reaction of the furyl complex 7 and methyl 4-pentynoate in methanol produced the benzofuran 9 in approximately the same yield as in THF. (12) (a) Wulff, W. D.; McCallum, J. S.;Kunng, F. A. J. Am. Chem. SOC.,in press. (b) For other discussions concerningthe reactions of furyl carbene complexes with acetylenes see ref 14 and Dotz, K. H.; Dietz, R. Chem. Ber. 1978, 111, 2517. (13) (a) Wulff, W. D.; Tang, P. C.; McCallum,J. S.J. Am. Chem. SOC. 1981, 103, 7677. (b) Dotz, K. H.; Muhlemeier, J.; Schubert, U.; Orama, 0. J. Organomet. Chem. 1983,247, 187.
However, none of the furan product 11 could be observed. In its stead was isolated the enol ether 12 in 23% yield as one diastereomer. The formation of the benzannulated product 9 is not affected by methanol whereas it appears that an intermediate on the way to furan 11 is intercepted by methanol to produce the enol ether 12. In order to establish that the enol ether 12 was not a secondary product of this reaction arising from the furan 11, we performed a spiked experiment. When the reaction of the carbene complex 7 and acetylene 8 was carried out under the same conditions, except that 0.2 equiv of furan 11 was added to the initial reaction mixture, the reaction then provided the benzofuran 9 and the enol ether 12 in the same ratio and also provided an 86% recovery of the spiked furan 11. With a single exception? the reaction of complex 7 with methyl 4-pentynoate is the only known example of a reaction of a carbene complex and an alkyne that produces a mixture of furan and 4-alkoxyphenol products without contamination by any of the other possible structural types and thus appears to be an ideal case for investigating the mechanism for the formation of furans and 4-alkoxyphenols. On the basis of one of the proposed mechanisms for 4-alkoxyphenol f o r m a t i ~ n , ~a ~likely ~~J~ intermediate that would be expected to give rise to the enol ether 12 is the vinylketene complex 13. The stereochemistry of the vinylketene complex 13 is left unspecified, since initially the stereochemistry of the enol ether 12 was not determined. The regiochemistry of acetylene incorporation in benzofuran 9 is in accord with that previously established for this reaction'J3 and has been chemically confirmed in this case rather simply since attempts to isolate 9 on silica gel gives a 1.0:2.4 mixture of the phenol 9 and its corresponding lactone 9a.lZaThe assignment of the structure of the furan 11, however, is not as straightforward. The crystal structure of the previously reported furan 6 reveals that the aryl and methoxyl groups on the carbene carbon separate and become the C-2 and C-5 substituents on the
McCallum et al.
2348 Organometallics, Vol. 7, No. 11, 1988 Scheme VI
Scheme V. Structural Correlation between Furan 11 and Enol Ether 12
11
1% HCI
-
&OM'
Hpd-C
MeOHlH,O
93%
-
JoMe
1%HCl
EtOAc
0
99%
12
MeOHIH20 0
100% 17
16-2
,OMe
furan ring.4b However, the regiochemistry of incorporation of an unsymmetrical acetylene into a furan nucleus has not been previously e ~ t a b l i s h e d . Therefore, ~~ both of the structural possibilities, i.e., compounds 11 and 14, must be considered as possibilites for the product from this reaction since they could not be easily distinguished by their spectral properties. On the other hand, the enol ether 12 can be clearly distinguished from its regioisomeric possibility 15, simply on the basis of proton-proton couplings in the NMR spectra. The structure of the furan product from this reaction was established as 11 on the basis of a structural correlation of 11 and enol ether 12 by chemical conversion of both to the diester 17 as indicated in Scheme V. Treatment of furan 11 with 1%aqueous HC1 in methanol and water in the presence of air resulted in hydrolysis and oxidation to give the unsaturated keto ester 16-2. The stereochemistry about the double bond in this structue was established by NOE experiments that are described in the Experimental Section. Hydrogenation of 16-Z produced the saturated keto ester 17 which was found to be identical with material produced from acid hydrolysis of the enol ether 12. The stereochemistry about the double bond in enol ether 12 can be assigned as the 2 configuration on the basis of the absence of an NOE enhancement of the vinyl hydrogen upon irradiation at any of the three methoxy absorptions. A confirmation of the Z stereochemistry for this reaction product was established in the following experiments. The purified enol ether 12 that was obtained from the reaction mixture was irradiated in a benzene solution through a Pyrex filter at 2537 A for 1h. The resulting solution was determined to contain a 1:l mixture of the E and 2 isomers of 12 from the proton NMR spectrum. The new set of
photolysis
L
I 254 nm
benzene, 4 h I:< mixture
12-2
12-E
10 5e' '
NOE
absorptions that were produced upon photolysis could be assigned as those of the E configuration of 12 by the observation of a 10.5% enhancement of the new vinyl proton Hd,upon irradiation at the new vinylmethoxy absorption Hh,. The lH NMR spectra of 12-2 and of the mixture of 12-2 and 12-E and of the NOE experiments can be found in the supplementary material. The initial reaction product must therefore have the 2 configuration about the enol ether double bond. Care must be taken during the isolation of 12-2 in order to avoid isomerization which was observed to occur if UV lamps were used for detection during preparative radial TLC on a Chromatotron. Before continuing with the results of the present study and with the discussion of the implication of the stereochemistry of the enol ether 12, the evidence for the intermediacy and the stereochemistry of vinylketene intermediates in the reactions of metal carbene complexes with alkynes that
20
El-C-C-El
0 cco,,cd,
-'
SnPh3
22
93 %overall
2l.E
has been published in the literature will first be reviewed. The earliest and perhaps still the strongest piece of evidence for the intermediacy of vinylketene complexes in the reaction of carbene complexes with acetylenes is the isolation of v i n y l k e t e n e ~ .Dotz ~ ~ ~ reported7b ~ that the vinylketene 19-E could be isolated in 52% yield, and an X-ray diffraction analysis revealed that the double bond had the correct stereochemistry necessary for the benzannulation reaction to occur. All of the vinylketene complexes that have been isolated from the reactions of chromium complexes have been with silyl-substituted acetylenes. It was argued that 19-E was isolable as a consequence of the steric interaction between the two silyl groups that would arise when they are held in the cis relationship that exists in an q4-vinylketenecomplex of the type 13 (Scheme 111). However, vinylketenes can also be isolated from the reactions of carbene complexes and (trimethylsi1yl)a~etylene~~~' which suggests that the known strong stabilizing effect that silyl substituents have on ketenes is coming into play as well. The fact that vinylketenes can be isolated certainly indicates that they are formed in the reactions of carbene complexes with acetylenes; however, it does not necessarily mean that they are intermediates on the way to the formation of the benzannulated 4-alkoxyphenol products. This point is illustrated by the reaction of cobalt complex 20. The reaction of complex 20 with diethylacetylene produced the first example of an v4-complexedvinylketene from the reaction of a carbene complex and an acetylene.4e However, the reactions of cobalt carbene complexes do not give benzannulated products! The vinylketene complex 21-E is converted to the furan 22 when warmed even though it has the correct stereochemistry about the double bond to produce the benzannulated product. I t cannot be discerned from these data whether vinylketene complexes are in fact intermediates on the way to the benzannulated products or whether vinylketene complexes are formed on a side pathway that can, in instances where the vinylketene is particularly stabilized by silyl substituents, lead to the isolation of vinylketenes as alternate products. The formation of the enol ether 12 (Scheme 111) does not represent the first time that vinylketene intermediates have been intercepted by alcohols. Y a m a ~ h i t a , Dotz? ~J~ and our group4dhave previously reported the isolation of alcohol-trapping products of vinylketene intermediates from the reaction of carbene complexes with acetylenes. Examples from the work of Dotz and Yamashita are presented in Schemes VI1 and VIII. Dotz has reported four examples, all intramolecular and related to the reaction of the p-tolyl complex 23 and 4-pentyn-1-01.~This reaction gave the normal benzannulated product 24 in 44% (14)(a) Yamashita, A.; Scahill, T. A. Tetrahedron Lett. 1982, 3765. (b) Yamashita, A. J. Chem. SOC.1985,107, 5823. (15) Huisgan, R.; Mayr, H. J. Chem. SOC., Chem. Commun. 1976, 55.
Reaction of Chromium Carbene Complexes with Acetylenes
Organometallics, Vol. 7, No. 11, 1988 2349
Scheme VI1
Scheme IX 0 1
r
I
PH1
M = YCO),
26.E
- MOoxH +
M e O Y C O z E l
1 5 equiv EtOH
27-E
0
0 - 4 OMe
OEI
(CO)&r =C THF 6 P C
-
OH I
COzEI
36%
27.2
0 II
OEf OMe
7
28
\
34
21%
+
44 %
37 O Me0 %-
COzEI COZEt 27.2
20 %
yield along with a single diastereomer of the enol ether 25, which was assigned as having the E configuration on the basis of the anisotropic effects of the aryl group on the shift of the vinyl hydrogen that could be anticipated for each isomer. This result by itself suggests that only one isomer of the vinylketene complex 26 is involved which either can lead to the trapping product 25-E or undergo cyclization to the benzene ring to generate the benzannulated product 24. This result was invoked as evidence, albeit circumstantial, that vinylketene complexes are on the pathway to the benzannulated products. A consideration of the results published by Yamashita that are presented in Scheme VI11 only serves to confuse the issue. It was reported that the reaction of the 2-fury1 complex 7 with ethyl propiolate gave both the E and 2 isomers of the ketene product 27 and none of the benzannulated product when the reaction was carried out in THF in the presence of 1.5 equiv of ethan01.l~~ The assignment of the stereochemistry of the two isomers of 27 was made on the basis of the relative proton shifts for the vinyl hydrogen of each isomer. In the absence of trap,14b the benzannulated phenol 28 was isolated in 21 % yield; however, the ketene trapping product was still obtained, but this time only as one isomer! The origin of the second ethoxyl group in 27-2 was not determined, but it is certain that it must be derived from ethyl propiolate, but what is less certain is that it is derived from free ethanol. This was the first time that both isomers of enol ether products were isolated, suggesting that both isomers of a vinylketene complexed intermediate were present in solution, assuming 27-E and 27-2 are configurationally stable under the reaction and isolation conditions. The question that arises is, if both isomers of the complexed vinylketenes are present in the reactions of the fury1 complexes, are they also present in the reactions of phenyl complexes? If this is not the case 89 is suggested by the results in Scheme VII, then the questions are: what are the factors that control the stereochemistry of the formation of vinylketene com; plexes and how are they affected by the nature of the aryl
substitutent on the carbene carbon? The issue of the stereochemistry of reaction intermediates has not been addressed in the mechanisms that have been proposed for the benzannulation reaction. The mechanism that has been proposed by D o t ~ ~involves ~Jl~ the v4-vinylketene complexed intermediate 35, and this mechanism is presented in Scheme IX along with a mechanism that has been proposed by Caseyllb which involves the chromacyclohexadiene 36 as the key intermediate rather than a vinylketene complex. At the present time no evidence has been presented that can rule out either mechanism although the circumstantial evidence at this point favors the ketene mechanism. Both mechanisms have the chromacyclobutene and vinylcarbene complex intermediates 33 and 34 in common. Both of these mechanistic accounts have ignored the issue of the stereochemistry of any of the reaction intermediates and have simply assumed that the chromacyclobuteneintermediate 33 (Scheme IX) always undergoes ring opening to give the E isomer of the vinylcarbene intermediate 34 since it is this configuration that is necessary for cyclization to the chromacyclohexadiene intermediate 36 in Casey's mechanism and also the necessary stereochemistry for the electrocyclic ring closure of the vinylketene complex 35 in Dotz's mechanism. In light of the results shown in Scheme VIII, Y a m a ~ h i t a has l ~ ~suggested that the vinylcarbene intermediate 34 (Scheme IX) can exist as E and 2 isomers and that they are in an equilibrium that is determined by the nature of the aryl substituent of the starting carbene complex 30. We shall consider two issues with respect to the stereochemistry of intermediates from the reaction of furylcarbene complexes with acetylenes. First, are the stereoisomers of the vinylcarbene intermediates 34 and/or the vinylketene complexes 35 in equilibrium with respect to product formation? Second, what are the factors that control the formation and/or the population of these stereoisomers? When stereochemical considerations are applied to the general mechanism in Scheme IX for the specific reaction of the 2-furylcarbene complex 7 and methyl 4-pentynoate (8), the resulting mechanism is presented in Scheme X. One clear conclusion to be drawn from the reactions in
2350 Organometallics, Vol. 7, No. 11, 1988
McCallum et al. Scheme X
t 8
7
-
co
36
I I
37
OH
MeQd
12-2
-W
f 12-E
THF and methanol (Scheme 111) is that the furan and phenol products are on different pathways and that intermediates in furan formation can be trapped with methanol and those involved in phenol formation cannot be trapped. The fact that the furan product 11 is replaced by the (2)-enol ether 12 in methanol suggests that the (Zhvinylketene complex 35a is an intermediate in furan formation. As mentioned above there are two proposed mechanisms to account for phenol formation and one of them involves the E isomer of the vinylketene complex 35. From the results of the reaction of complex 7 and acetylene 8 in THF and methanol (Scheme 111) it is also clear that if the phenol 9 is formed from the E isomer of the ketene complex 35, then the E and 2 isomers of the ketene complex 35 cannot be in rapid equilibrium with respect to product formation since the yield of the phenol 9 does not change when the solvent is changed from THF to methanol and at the same time the furan 11 is completely trapped. If phenol formation occurs via Casey's mechanism, then in order that cyclization to 36 occur the vinylcarbene complex intermediate 34 must have the E configuration of the vinylketene complex 35a- 2 that is intercepted on the way to the furan product. That nonequilibrating stereoisomeric intermediates are involved in these reactions is further supported by the reactions shown in Scheme XI. The partition between phenol and furan products was 2.3:l.O from the reaction of complex 7 with methyl 4-pentynoate (8), but with diethylacetylene the partition ratio increases to 23:l.O. Furthermore, at this different partition ratio there is no crossover in the trapping experiment with methanol; the furan product is intercepted to give exclusively the (2)-enol ether 43 in the same yield as the furan, and the formation of the phenol product 41 is unaffected. The assignment of the stereochemistry of the enol ether was assigned by NOE experiments similar to those described for enol ether 12 and can be found in the Experimental Section. Irra-
Scheme XI OH
7
b(
82%
42
4
Yo
OH OMe
0 EIC-CEI MeOH 70'C
* OMe
7
4I
82 Y e
43.2
3
Ofn
one diastereomer
diation of a benzene solution of 43-2 produced a 1:1.3 mixture of Z and E isomers. Only the E isomer showed an enhancement (4.3%) at the 3-fury1 proton upon irradiation of the methine proton. All of the results from the reaction of the furylcarbene complexes that have been discussed are consistent with either of the two mechanisms for the formation of the benzannulated product that were presented in Scheme IX. In Dotz's m e ~ h a n i s m ' ~ Jthe l ~ vinylketene complex 35-E is proposed to be an intermediate in the formation of the benzannulated product, and in Casey's mechanismllb the chromacyclohexadiene 36 is an intermediate in the formation of the benzannulated product. The results of the reactions of the fury1 complexes described here at first glance may suggest that the vinylketene complex 35-Eis not on the pathway to the benzannulated product 9 since the yield of the phenol was unaffected when the reaction was run in methanol. However, is it reasonable that cyclization of the ketene complex 35-E is faster than its reaction with methanol when methanol is the solvent? This is, in fact, the case with free vinylketenes as is illustrated with the electrocyclic ring opening of the cyclobutenone 44 in methanol where the cyclicized product 46 is produced with 8:l selectivity over the methanol-trapping
Reaction of Chromium Carbene Complexes with Acetylenes Scheme
Organometallics, Vol. 7, No. 11, 1988 2351
XI1
Scheme
XI11 0
OH OH ICOiSCr
THF, 800 C, 14 Hr 49.2
OMe 5n 22%
48 %
Me0 42
51.2
O
4
49.2
41
7
49.E
25 %
product 47 (eq 3).21 Also the cobalt ketene complex 21-E is stable to methanol and can only be converted to an 0,y-unsaturated ester with methoxide. It would thus be specious to suggest that the fact that the ketene complex 35-E is not trapped with methanol is evidence that it is not present in solution.
44
45
46
8 1
47
Dotz has reported& that an (E)-vinylketene complex on the way to phenol formation could be intercepted with an alcohol in an intramolecular trapping experiment (Scheme VII). This suggests that, if in the present case the vinylketene complex 35-E is an intermediate in the formation of the phenol 9 and cyclization is faster than trapping with methanol solvent, perhaps providing an intramolecular trap may increase the likelihood of interepting the ketene complex before cyclization occurs. The reaction of the 2-fury1 complex 7 with 4-pentyn-1-01, however, gave only the 2 isomer of the enol ether 49 (Scheme XII). Thus the intramolecular trapping experiment in this case provides no evidence for the presence of the (E)-vinylketene complexed intermediate. The reaction of complex 7 with 4-pentyn-1-01in acetonitrile does give some of the E isomer of the enol ether and therefore evidence for the intermediacy of the (E)-vinylketene complex. However, it is not clear whether the (E)-vinylketene complex that was intramolecularly trapped in the acetonitrile reaction to give the lactone 49-E is actually an intermediate in the THF reaction and/or an intermediate in phenol formation. It is well-known16"that more polar solvents, such as acetonitrile, will accelerate carbon monoxide insertion reactions, and therefore it is possible that the effect of acetonitrile is just to cause carbon monoxide insertion in the vinylcarbene complex intermediate 34-E to give the vinylketene complex 35-E rather than cyclization to the chromacyclohexadiene 36 (Scheme M) which could be the normal course of events in THF. It is also possible that the acetonitrile simply displaces the vinylketene ligand from the metal and that the free (E)-vinylketene can be partially trapped. The intermolecular reaction of the 2-fury1 complex 7 with acetylene 8 in acetonitrile also is different than the same reaction in THF. The lactonized version of the phenol 9 is obtained in 12% yield along with the dimer 51 in 42% yield as only the 2 diastereomer. The furan product 11 could not be detected in this reaction. In this case the formation of the furan is thwarted by the trapping (16) (a) For leading references see: Principles and Applications of Organotransition Metal Chemistry; Collman, J. P.; Hegedus, L. S., Norton, J. R., Finke, R. G., Eds.; University Science Books: Mill Valley, CA, 1987. (b) Ibid., p 786.
-It
of either the (2)-vinylketene complex 35 or the free (2)vinylketene by the phenol functionality of the benzannulated product 9. Why this particular trapping reaction is much faster in acetonitrile than in THF is not known, but one possibility is that the acetonitrile displaces the vinylketene ligand from the metal generating a free vinylketene that is incapable of going on to the furan product and waits for the much less reactive trapping agent (compared to methanol). The fact that we failed to obtain any solid evidence for the presence of the E isomers of vinylketene complexes by using either inter- or intramolecular trapping experiments, and the chemical shifts that we observed for the various enol ethers that we had isolated and characterized led us to suspect that the stereochemistry of the enol ether 25 from the reaction of the p-tolyl complex 23 and 4pentyn-1-01 had been incorrectly assigned&(Scheme VII). We found that this was in fact the case. This reaction was repeated and a colorless oil was isolated in 25% yield which had the same spectral properties that had been previously reported for the enol ether 25-E.6C An NOE experiment
-
/photolysis
P
c
b
v
O
o
(4)
254 nm benzene,l h
zs-z
13 % NOE
O%NOE
from reaction of 23
25-E
,
m,xtUre
on this material revealed that there was no enhancement of the vinyl hydrogen signal upon irradiation of the enol ether methoxy. When the isolated enol ether was photochemically isomerized, however, the newly formed isomer was found to give a 13% enhancement for the same experiment. Therefore, the initially formed enol ether 25 from this reaction is the 2 isomer and not the E isomer as reported. Thus there is no evidence for the intermediacy of an (Ebvinylketene complex in the intramolecular trapping experiments with 4-pentyn-1-01either with the reaction of the 2-fury1 complex 7 or with the p-tolyl complex 23. All of the alcohol-trapping experiments to date are suggestive of the presence of vinylketene complexes in solution; however, they do not rule out the mechanism for phenol formation that involves the chromacyclohexadiene intermediate 36 in Scheme IX, and the unambiguous delineation of the mechanism for phenol formation will have to await more cleverly designed experiments. A summary of pertinent proton NMR data for the enol ether products is presented in Table I. In addition to the NOE enhancements observed for the E isomers that greatly facilitate their characterization, it can also be observed that there is a general trend in these enol ethers for the vinyl hydrogen to be shifted downfield when it is syn to the aryl group. These chemical shift trends for aryl vinyl ethers are well established in the 1iterat~re.l~" The opposite trend is observed for the methme proton Hb which is deshielded in the E isomer relative to the Z isomer. The results of the trapping experiments are consistent with nonequilibrating stereoisomeric intermediates in this
2352 Organometallics, Vol. 7, No. 11, 1988
McCallum et al.
Table I. 'HNMR Data for Enol Ether Productsa
Scheme XV. Possible Constructions of the Furan Product .Fu,
,tOMe
H-CEC-R
Z-FU.~,~\~~O-M~
..-
F
c-c.
R
H'
7
Em1 Ether
2-Fu
11
*c@o C f O M e
2-F~.~,~~~foMe c
6
12
4
4
.
3
iCH212C02Me
550 d
501 d
Me
536383
480418
10
576355
502383
8
-
c-C>
R
H'
d
Z-FU.~
4.c ? O M e
2-Fu.C,o~;fO-Me \\
H' {CH2l2CO2Me @OM'
I/
C
c-c.
ti-CaC-R
51
N
I
H-CsC-R El
/I
I
crico,,
R
5 5 4 403
Scheme XVI
OMe
OAc
CDC13. Enhancement of Ha in the E isomer upon irradiation a t the vinyl methoxy of the E isomer. cReference 14a. d N o t rea
ported. eReference 6c. fThis work.
g
Not assigned.
(
t HZ0
1) HC(0n-Bu),. HCI c
t
~
0
1
56
MeOH
5
~
4
h
+
'd
2) LiAlH.
Scheme XIV Me
"
+
''0 - 15 5 % "0 - 23 0 Yo - 61 5
J r ( C 0 l 3
7
+
"0 - 30 8 % 170- 16 8 % "0 - 50 4 % '
a was prompted by the fact that only the Z isomer of the ketene complex 35a goes to the furan. The proximity of the methoxy group to the ketene carbon is suggestive that the first step may be nucleophilic attack by the methoxy oxygen on the ketene. Mechanism b includes an initial q4 to q2 transformation of the vinylketene complex 35a-Z to the chromacyclopentenoneintermediate 40a. This process creates an additional open coordination site which in Scheme XIV is shown occupied by a solvent molecule. Alternatively, this site could be filled by internal chelation of the furan oxygen. Migration of the methoxyl group in 40a to the metal center leads to the cyclic carbene complex 52 which then reductively eliminates the methoxy and acyl substituents to generate the new carbene complex 53. Attack at the carbene carbon by the ester carbonyl oxygen with subsequent fragmentation and elimination of the chromium and its ligands gives rise to the furan 11. Apart from mechanistic possibilities, furan formation can be considered just in terms of the possible constructions of the pieces involved. As outlined in Scheme XV there are three possible constructions for the furan product if the following two assumptions are made: (1)the two carbons of the acetylene remain bonded and (2) the carbon and oxygen of the carbon monoxide remain bonded. Of the three possible constructions indicated in Scheme XV the mode in which furan formation actually occurs could not be identified by a single labeling experiment. However, the combined results of the two labeling experiments indicated in Scheme XV would allow for a distinction between the three possible constructions of the furan nucleus. The oxygen-labeled carbene complex 7+was prepared with the oxygen isotopic abundances as indicated in Scheme XVI. Oxygen labeled methanol was prepared by the method of Knowles" from water which was 23% 1 7 0 and 61.5% l80.The labeled carbene complex was then prepared by an alcoholysis of the acetoxycarbene complex
I' Meo+F
A
""'. "
CO2Me 52
53
54
reaction which give rise to different product types. The stereodifferentiating step according to the proposed mechanism for this reaction (Scheme IX) would be at the metallacyclobutene intermediate 33a (Scheme X) which can undergo electrocyclic ring opening to give either the (2)-or (E)-vinylcarbene complex intermediate 34. It has recently been pointed out16iJ6bthat another possible mechanistic pathway for phenol formation involves the carbon monoxide insertion in 33 to give the metallacyclopentene intermediate 40. This intermediate could be the origin of stereodifferentiation of subsequent intermediate depending on whether the chromium moves up or down in the q2 to q4 conversion that takes it to a vinylketene complexed intermediate. However, before this issue of whether stereodifferentiation occurs at intermediate 33 or 40, the question of the mechanism by which the Z isomer of the vinylketene complex 35a-Z goes to the furan 11 will first be addressed. A t this time there have been no published studies directed to the mechanism by which furan formation occurs, although a mechanism has been previously proposed.4e Two reasonable mechanistic possibilities for the conversion of the vinylketene complex 35a-Z to the furan 11 are illustrated in Scheme XIV. A consideration of mechanism
(17)Abbot, S. S.; Jones, S. R.; Weinman, S. A.; Bockhoff, F. M.; McLafferty, F. W.; Knowles, J. R. J. Am. Chem. SOC.1979, 101, 4323.
Organometallics, Vol. 7, No. 11, 1988 2353
Reaction of Chromium Carbene Complexes with Acetylenes Scheme XVII
7
Table 11. Effect of Solvent on the Reaction of Complex 7 with Methyl 4-Pentynoate
7
*
24 -fold 13C
enhancement
7
11'
'O
56 according to the procedure originally described by Connors.18 Carbene complex 7f was found to be enriched at the methoxy oxygen with 50.4% l80and 18.8% 170. Upon reaction of the labeled complex 7' with acetylene 8 it was determined that the labeled oxygen was exclusively incorporated into the methyl ether of 1 IT on the basis of the 170NMR chemical shift of 24.7 ppm (H20standard). If the 170label had been incorporated into the furan ring then a much greater downfield shift in the 170NMR would have been expected.lg For example, in the 170natural abundance NMR of 2-methoxyfuran (H20standard) the methoxy oxygen has an absorption at 6 38.2 and the furan oxygen at 6 217.8. The conclusion drawn from the 170NMR experiment that the oxygen label in carbene complex 7t upon reaction with acetylene 8 is incorporated exclusively at the methoxy oxygen of furan 11 can be corroborated by the l80chemical shift perturbationm of the 13C NMR spectrum of llf. The pair of doubled peaks were found at 60.093,60.065 ppm and 155.846, 155.833 ppm, indicating that the labeled oxygen is attached to an aryl carbon and to the methoxy carbon. These labeling studies with 170and l80clearly rule out mechanism a outlined in Scheme XIV and construction B that is indicated in Scheme XV. The oxygen of the methoxy group in the carbene complex 7 remains with the methyl group, and thus only constructs A and C in Scheme XV are possible. The remaining question is whether the carbene carbon becomes the carbon at the 2-position in the furan product as depicted by construct A or whether it becomes the carbon at the 5-position as depicted by construct C. A labeling experiment that will identify the origins of the carbons at the 2- and 5-positions in the furan product 11 is indicated in Scheme XVII and involves labeling the carbons of the carbon monoxide ligands in the carbene complex 7. This was done as indicated in Scheme XVII following a procedure that has been reported by Casey for the labeling of similar complexes.21 The unlabeled complex 7 was exposed to 1 atm of 13C-labeled carbon monoxide in a THF solution at 80 "C. After 2 days the starting material is reisolated and found to contain 26% 13C. The reaction of the labeled complex 7* with acetylene 8 produced the carbon-labeled furan 11* plus the carbon-labeled benzofuran product 9*. Identification of the position of the l3C label in furan 11* would have been difficult without the l80chemical shift perturbations that were measured for the 180-labeledfuran 1 It. The l80perturbations from the carbon spectrum indicate that the carbon in the 2position of furan 11 has a resonance of 155.84 ppm. It is precisely this absorption that is enriched 25-fold in the (18) (a) Connors, J. A.; Jones, E. M. J. Chem. SOC. A 1971,3368. (b) Connors, J. A,; Jones,E. M. J. Chem. SOC.A 1971,1974. (19) Diehl, P.; Fluck, E.; Kosfeld, R. NMR: Basic Princ. B o g . 1981, 17, 1. (20) Webb, G . A. Annu. Rep. NMR Spectrosc. 1983, 15, 188. (21) Casey, C. P.; Cesa, M. C. Organometallics 1982, 1, 87.
yield of 9,a,b % 45 30 52 33 50
solv hexane benzene THF CH3CN CH30H
yield of 11," % 15 30 22
