J. Org. Chem. 1985,50, 3526-3535
3526
2-methyltetrahydrofuran and placed in a 4-mm quartz tube. The solution was then degassed, sealed, and kept under liquid nitrogen.
Acknowledgment. The authors wish to thank Drs. V. P. Senthilnathan and B. B. Wright for technical assistance with the Curie Law measurements. Registry No. l a , 62617-73-6; l b , 91424-03-2; IC,91424-04-3; Id, 91424-05-4; le, 91424-06-5; If, 91424-07-6; 6, 1729-99-3; 7, 19310-98-6; 8b, 91424-13-4; SC, 91424-08-7; 8d, 91424-19-0; 9b,
91424-14-5; 9 ~91424-09-8; , 9d, 91424-20-3; lob, 91424-15-6; lob (tosylhydrazone), 91424-16-7; lOc, 91424-10-1; 1Oc (tosylhydrazone), 91424-11-2; 10d, 91424-21-4; 10d (tosylhydrazone), 91424-22-5; l l b , 91424-17-8; Ilc, 91424-12-3; l l d , 91424-23-6; 12, 91424-25-8; 13, 91424-26-9; 14, 91424-27-0; 15, 91424-28-1; 15 (tosylhydrazone), 91424-29-2; 16,91424-30-5; 17,91424-31-6; 18, 91424-32-7; 18 (tosylhydrazone), 91424-33-8; 19, 91424-34-9; 5(iodomercuri0)-8-methyl-1-naphthoic acid, 91424-18-9; 5bromo-8-methyl-1-naphthoylchloride, 91424-24-7; cuprous phenymercaptide, 1192-40-1.
On the Conjugative Isomerizations of P,y-Unsaturated Esters. Stereochemical Generalizations and Predictions for 1,3-Prototropic Shifts under Basic Conditions Simon G. Alcock, Jack E. Baldwin,* Rolf Bohlmann, Laurence M. Harwood, and Jeffrey I. Seeman Dyson Perrins Laboratory, University of Oxford, Oxford OX1 3QY, U.K.
Received December 4 , 1984 An investigation of the base-catalyzed conjugative isomerization of a series of @,y-unsaturatedesters to their corresponding a,@-unsaturatedesters was performed. It was found that, with sodium hydride in THF, methyl 3-butenoate isomerized initially to a 5:l ratio of (2)-to @)-methyl 2-butenoates; the Z E ratio is time dependent, and after several days, the thermodynamic ratio 1:23 = Z E was obtained. The isomerization appears to be catalytic in NaH, as it proceeds with less than 1molar equiv of base, no hydrogen evolution is observed, and the reaction rate is approximately f i s t order in NaH and zero order in ester. Under the same conditions (2)-methyl 3-hexenoate isomerized stereoselectively to @)-methyl 2-hexenoate while (E)-methyl 3-hexenoate isomerized to a 2:l mixture of (2)and (E)-methyl 2-hexenoates. These product ratios are far from the isomeric compositions obtained under equilibrating conditions. To investigate further the stereochemical outcome of these isomerizations, three isomeric &y-unsaturated methyl esters were studied (a) methyl 3-ethyl-3-butenoate isomerized exclusively to (,!?)-methyl 3-methyl-2-pentenoate; (b) @)-methyl 3-methyl-3-pentenoate isomerzied exclusively to (2)-methyl 3-methyl2-pentenoate; (c) @+methyl 3-methyl-3-pentenoate isomerized exclusively to (E)-methyl 3-methyl-2-pentenoate. In the latter three cases, dimerization was not observed presumably due to steric effects. Related results were observed for a smaller series of @,y-unsaturatedamide isomerizations. Examination of the literature on olefin isomerizations reveals a general trend that the current results exemplify. Thus, in the absence of severe steric factors or cation-anion complexation, deprotonation a t allylic positions kinetically preferentially forms the anion possessing a cisoid crotyl subunit (if available) regardless of initial substrate conformation. The stereochemical 2 and 2 -E geometry conversions in kinetic 1,2-transpositions of olefins. consequences of this results in E This generalization can also be applied to the stereochemical results of ketone, ester, and hydrazone enolate formation, base-catalyzed exchange in polysubstituted aromatics and heteroaromatics, and other reactions involving the formation of allylic or benzylic anions.
-
CY,@and P,y-unsaturated esters play important roles in organic chemistry. The reactivity of these groups to both nucleophiles and electrophiles under a variety of reaction conditions has made them ideal precursors in many organic chemical syntheses, apd numerous natural products possess these structural subunits. While it is well-known that a,@-and P,y-unsaturated esters can readily isomerize to each other under a variety of condition^,'-^ there are significant gaps in our full understanding of the mechanistic basis of these reactions and also in our ability to control the course of the isomerizations. Most of the studies published to and cer-
(1) (a) PineTH.;Stalick, W. M. ”Base Catalyzed Reactions of Hydrocarbons and Related Compounds”; Academic Press: New York; 1977. (b) Pines, H. ‘Chemistry of Catalytic Hydrocarbon Conversions“; Academic Press: London; 1981. (c) Yanovksaya, L. A.; Shakhidayatov, K. Russ. Chem. Rev. (Engl. Transl.) 1970, 39, 859. (2) For leading references, see: (a) Rhoads, S. J.; Chattopadhyay, J. K.; Wadi, E. E. J. Org.Chem. 1970, 35, 3352. (b) Hine, J.; Kanagasabapathy, V. M.; Ng,P. J. Org. Chem. 1982, 47, 2745. (c) Hine, J.; Flachskam, N. w. J. Am. Chem. SOC.1973, 95, 1179. (3) Sakai, M.; Nishikawa, S.; Koike, H.; Sakakibara, Y.; Uchino, N. Bull. Chem. SOC.Jpn. 1978,51,2970. Sakai, M. Bull. Chem. SOC.Jpn. 1977,50, 1232. For thermal isomerization, see: McGreer, D. E.; Chiu, N. W. K.Can. J. Chem. 1968, 46, 2225. For photochemical isomerizations, see: Skinner, I. A,; Weedon, A. C. Tetrahedron Lett. 1983, 4299. Lewis, F. D.; Oxman, J. D. J. Am. Chem. SOC.1981, 103, 7345. 0022-3263/85/1950-3526$01.50/0
Scheme I
8’
R3
R‘ R’CH
CHR1R2 \
’
X C 0 2 R
R 2R4 0:‘
Scheme I1
tainly the bulk of the early investigations,l deal with the thermodynamic equilibrium of these unsaturated esters (4) Rathke, M. W.; Sullivan, D.
Tetrahedron Lett. 1972, 4249.
0 1985 American Chemical Society
J. Org. Chem., Vol. 50, No. 19, 1985 3527
Conjugative Isomerizations of P,y-Unsaturated Esters Scheme I11 R2
1
3
2
COtMe
Scheme V
4
5
6
(Scheme I). Since the pioneering studies of Rathke4 and Schlessinger,5 the regiochemistry and (less frequently) stereochemistry of deprotonation followed by kinetic protonation! alkylation,' and acylation8of a,P-unsaturated esters to the P,y-counterparts have been widely studied (Scheme 11). To date, very little is known regarding kinetic conjugative isomerizations of &-punsaturated esters to a,P-unsaturated esters (Scheme 111). In this paper, we will present our recent experimental results and mechanistic arguments with the aim of filling some of this void. The stereochemical consequences of this conjugative isomerization will then be discussed and related isomerization of P,y-unsaturated amides will be considered. Evidence for the reaction process under study being a catalytic, surface phenomenon will be presented also. Finally, we will propose a unified generalization for the stereochemistry of the isomerization of substituted olefins under kinetic, anionic conditions.
"I
.t
11
f i g . la
b
I
' hl.1 Figure 1. Comparative rates of formation of 4, 5, and 6. Experimentally obtained equilibrium ratios for this system ( K = [5]/[6]) are as follows K(200-500 "C; gas phase) = 4.5, K(195 "C; liquid phase) = 7.0; K(117"C; NaOMe, HMPA) = 6.3, K(77 "C; NBS, CCl,). = JO.% Extrapolation of these values to 20 "C leads to an equilibrium distribution of K = 10-30.
Results Kinetic deconjugation of a,p-unsaturated esters to their P,y-unsaturated counterparts involves the formation of a molar equivalent of an enolate anion which is subsequently quenched to the corresponding P,y-unsaturated e ~ t e r . ~ - ~ the 3-methyleneester 1 with a range of bases (Scheme IV). The inverse, kinetic conjugative process also formally inWhile most reagents led to mixtures of E and 2 a,P-unvolves an anionic intermediate. For both of these countsaturated products 2 and 3, respectively, sodium hydride erpart isomerizations to be experimentally attainable they was unique in promoting stereoselective rearrangements must pass through different reaction surfaces. Only then to the E a,P-unsaturated ester 2.1° It must be stressed can directional differentiation be obtained. that while the E product would be expected to predomiOur survey of the available literature suggested one nate in any thermodynamically controlled conjugation,' reaction system for consideration as a candidate for a its sole formation is indicative of the absence of rapid kinetic conjugative process. Cardillog had previously reequilibration among 1-3. The most obvious feature of the ported the stereochemical consequences of conjugation of sodium hydride system (Scheme 111) which distinguishes it from the other bases used for conjugation or deconjugation (Scheme 11) is its heterogeneity. We anticipated (5) Herrmann, J. L.; Kieczykowski, G. R.; Schlessinger, R. H. Tetrahedron Lett. 1973, 2433. that with this reagent, there was reason t o propose the (6) Harris, F. L.; Weiler, L. Tetrahedron Lett. 1984, 1333. formation of an anionic intermediate different from that (7) Koyama, H.; Kogure, K.; Mori, K.; Matsui, M. Agric. Biol. Chem. existing in the cases of deconjugation. 1972,36,793. Zimmerman, M. P. Synth. Commun. 1977,7,189. Kende, A. S.; Constantinides, D.; Lee, S. J.; Liebeskind, L. Tetrahedron Lett. In order to test the validity of our assumptions, it was 1975,405. Ortiz de Montellano, P. R.; Hsu, C. K. Tetrahedron Lett. 1976, decided to investigate the effects of sodium hydride on the 1974, 4215. Katzenellenbgen, J. A.; Crumrine, A. L. J.Am. Chem. SOC. conversion of methyl 3-butenoate to ( E ) - or (Z)-methyl 96,5662. For reactivity of y,b-unsaturated @-oxo esters via their dianions, see: van den Goorgergh, J. A. M.; van der Gen, A. Recl. Trau. Chim. 2-butenoates (Scheme V). Pays-Bas 1983, 102, 393 and references cited therein. Evidence both theoretical and experimental, exists which (8) Kajikawa, A.; Morisaki, M.; Ikekawa, N. Tetrahedron Lett. 1975, indicates that the cisoid crotyl anion system is more stable 4135. While acylation frequently occurs at the y-position, see the work of Hudlicky for a-reactivity in the Reformatsky reaction: Short, R. P.; than the transoid arrangement.'l This has been proposed
Revol, J.-M.; Ranu, B. C.; Hudlicky, T. J. Org. Chem. 1983, 48, 4453. (9) Cardillo, G.; Contento, M.; Sandri, S.; Panunzio, M. J.Chem. Soc., Perkin Trans 1 , 1979, 1729.
(10) Mialhe, Y.; Vessiere, R. Bull. Chim. SOC.Fr. 1968, 4181.
3528 J. Org. Chem., Vol. 50, No. 19, 1985
Alcock et al.
Scheme VI
T
J-
c
CO *Me
Scheme VIIa /'f Br
Me02C
8
i,ii,iii
*
10
( C02Me
9
( C02Me
T' ?a
C0$3"
12b
12c
+lo
11
to be due to hyperconjugative involvement of the 2 substituent to form a six-electron cyclic delocalized system, although this electronic effect may in some cases be overwhelmed by secondary steric factors.12 The 3-butenoate system was chosen so that, in initial studies, these additional steric effects would be absent. The reaction of methyl 3-butenoate 4 with sodium hydride could be followed conveniently by capillary GC analysis of aliquots of the reaction mixture. In the early stages of reaction the 2-isomer 6 was formed a t roughly 5 times the rate of the E isomer (Figure la). After this time the E isomer continued to increase at the expense of the 2 isomer to give a final product mixture ratio of 23:l (E)-5-(2)-6 after several days (Figure Ib). The time dependence of the product ratios indicated that the system was not operating under purely kinetic conditions and that significant conclusions regarding the stereochemistry of the kinetic rearrangement could be drawn only from the early stages of reaction. During the course of reaction, substantial material loss occurred which was found to be due to saponification of the esters and dimer 7a,b formation, the latter process being more important. C02Me
7-TOZH iv
ZZzMe 7b
Several pertinent features of the reaction should be stressed. It was found that the E:Z ratio of products a t a given conversion was largely independent of the quantity of sodium hydride used (0.3-10.0 equiv), but the reaction rate was found to be close to first order in concentration in NaH and zero order in ester 4 concentration. No hydrogen evolution could be observed during the course of the reaction, and direct analysis of the clear supernatant obtained after allowing the mixture to settle gave the same GC profiles as those obtained following aqueous quenching of the total mixture. The reaction also demonstrated an induction period that was critically dependent upon the means of preparation of the sodium hydride and varied from 15 min to 6 h. We interprete these observations as implying a surface reaction with sodium hydride playing a catalytic role. Such a mechanistic dichotomy might explain the different stereochemical outcome of reactions involving sodium hydride and the other homogeneous conjugative or deconjugative processes. The observation of an initial, more rapid formation of the 2-conjugated ester in this case led us to address the question of stereochemical transfer using the (2)-and (11) (a) Houk, K. N.; Strozier, R. W.; Rondan, N. G.; Fraser, R. R.; Chauqui-Offermanns, N. J. Am. Chem. SOC.1980,102,1426. (b) Hoffman, R.; Olofson, R. A. J . Am. Chem. SOC.1966,88,943. (c) Bartmess, J. E.;Hehre, W. J.; McIver, R. T.; Overman, L. E. J . Am. Chem. SOC. 1977, 99, 1976. (d) David, V.; Hartmann, J.; Schloser, M. Helu. Chim. Acta 1974,57, 1567. (e) Hartmann, J.; Schlosser, M. J . Am. Chem. SOC. 1976, 98, 4674. (12) Dill, J. D.; Hehre, W. J.; Pople, J. A.; Schleyer, P. v. R. Tetrahedron 1977, 33, 2497 and references cited therein.
i
'C02Me
75
16
a Reagents: ( i ) NiBr, n - B u L i , T H F , -78 " C ; ( i i ) L i C H , C O , - t - B u , -78 --t 20 "C; ( i i i ) H,O+; ( i v ) T F A , 1 h, 20 'C; ( v ) M e I , K , C O , , M e , C = O , 18 h, 20 "C; ( v i ) NaH,
Et,O, 20 "C.
(E)-3-hexenoateesters 8 and 9 (Scheme VI). Esters 8 and 9 and other substrates to be discussed subsequently are important extensions of this work in that the 0,y-double bond possesses stereochemical features not present in the compounds studied by Cardillog(e.g., 1) or in 4. As shown in Scheme VI, the starting ester possessing 2 double bond configuration gave totally E-conjugated product 10 but the E 0,y-unsaturated ester gave a ratio of E:Z products of 7.0:3.8. The low stereoselectivity in the isomerization of E starting material, giving a rearranged product mixture in which the major constituent had retained initial double bond stereochemistry, has to be interpreted in the light of the much slower rate of isomerization of the 2 isomer (20 h) compared with the E isomer (3 h) and with the subsequent increase in 2 E isomerization of the products. Indeed it should be noted that neither isomerization gives a product mixture that represents the equilibrium ratio of the (E)-and (2)-2-hexenoatesystem (K[lO]/[ll] > 20 at 59 "C based on methyl pentenoate equilibrations2b). Additionally, the preponderance of dimerization with these substrates makes any rigorous stereochemical interpretation difficult. In order to investigate the stereochemical course of such double bond transpositions and obtain unambiguous results it was thus necessary to design and examine substrates whose primary reaction products were less susceptible to further reaction, either isomerization or dimerization. To this end we chose to study the three isomeric P,y-unsaturated esters 12a, 13, and 14 (Scheme VII) all of which on conjugation would lead to the same pair of a,P-unsaturated esters 15 and 16. Ester 12a was prepared via Rathke condensation13of tert-butyl a-lithioacetate with 2-bromo-1-butene followed by hydrolysis of the tert-butyl ester 12b to the acid 1212 and methylation with Me1/K2CO3in acetone (Scheme VII). The literature methods were followed for the preparation of substrate esters 13 and 1414aand the mixture of conju-
-
(13) Millard, A. A.; Rathke, M. W. J . Am. Chem. SOC.1977,99,4833. (14) (a) Cornforth, D. A.; Opara, A. E.;Read, G. J . Chem. SOC.C 1969,
2799. (b) Zimmerman, H. E.; Robbins, J. D.; McKelvey, R. D.; Samuel, C. J.; Sousa, L. R. J. A m . Chem. SOC.1974, 96, 4630.
J. Org. Chem., Vol. 50, No. 19, 1985 3529
Conjugative Isomerizations of @,-punsaturatedEsters
Table I. Summary of Kinetic Conjugative Experiments for Highest Yield of Monomeric Conjugated Products reacn time for highest conversion mixture composition, % equiv to conjugated starting E Z ratio EIZ B.r-unsaturated substrate of NaH Droducts. min material Droduct Droduct
-
~
~~
26
51
0.50
7
0.57
4
CO2Me
1.80
1224
49
4
1.90
225
21
7
a
10.00
960
706
a
B
\/-_7/Co2Me 9 &02Me
12 a
5.0OC
1200
38
9
30
0.30
5.0OC
1080
28
41
5
8.20
24
0.25
13
COzMe
14
-CONMe2 /
-
0.89C
10
15d
6
6.60
160
55
40
5.5
1070
43
4
17
-CONMe2
a
21
CONMe2
6
0.66
23
Only one conjugated isomer detected. Isolated yield. THF used as solvent. starting material from Z-conjugated product by GC.
gated esters 15 and 1614b(15:16 = 2.3:l). It was hoped that the substitution pattern of the products would lower their propensity for Michael addition and therefore resultant dimerization. As a series, the three P,y-unsaturated esters possess a substitution pattern about the double bond which might permit an estimation of the effect of increasing steric factors opposing any electronic effects in the rearrangement. The reactions were performed by using a large excess of sodium hydride in light in the previous observations that the amount of sodium hydride affected the rate of isomerization but not the isomer ratio. It was felt that shorter reaction times might further reduce the amount of dimerization occurring. The configurations of esters 13-16 were confirmed by the appropriate NOE difference experiments. The thermodynamic ratio of 15:16 at room temperature was determined (2.3:l) by the action of methanolic sodium methoxide on the esters. The ester 12a, possessing the exo-methylene unit, rearranged to give exclusively the E-product 15. Gratifyingly, no dimerization was observed during the course of the reaction. Treatment of 13 with sodium hydride gave only the thermodynamically less stable 2-isomer 16 (after 24 h). This is entirely in accord with the results using methyl (E)-3-hexenoate 8, as 13 can be considered to be a “buttressed” E P,y-unsaturated ester. With longer reaction periods some 2 E isomerization of the product occurred, but no dimerization was observed. Crucially, the 2-isomer 14 yielded exclusively the E-product 15. The three reactions in Scheme VI1 together form a complementary set of stereospecific conjugative isomerizations with regard to transfer of double-bond stereochemistry. The rate of isomerization is strikingly substituent dependent, the order of reactivity being 12a >> 14 > 13. We briefly examined the scope of these isomerizations by considering the related properties of P,y-unsaturated amides. As shown (Scheme VIII), N,N-dimethyl 3-butenamide (17) isomerizes under the NaH/THF conditions
-
Yields estimated by NMR due to inability to separate Scheme VI11
CONMe
17
19
18
r-
20
CONMez
CONMe2
22
21 /
23
CONMe,
22
25
24
to a 4:l mixture of 18-19. This kinetic stereoselectivity in favor of the 2 isomer is directly analogous to the results found for methyl 3-butenoate (Scheme V). Dimer formation became dominant at longer reaction times with 17, with NMR analysis suggesting the structure 20 for this dimer. Scheme VIII also summarizes the results for the more highly substituted. P,y-unsaturated amides which we have examined. The (2)-hexenamide 21 isomerized relatively rapidly to the E a,P-unsaturated amide 22 in modest yield, while the (E)-hexenamide 23 reacted sluggishly to yield mostly dimer 24 and a low percentage of a,p-unsaturated amides 22 and 25 with the 2 isomer in a 2:l predominance. We tentatively conclude that, as in the examples of Schemes VI and VII, the reaction stereochemistry follows the E 2 and 2 E course observed in the ester series. The low yields of conjugated amides and the high dimerization yields discouraged us from further studying these substrates from both a mechanistic and synthetic point of view. Table I summarizes the chromatographic yields and product composition at the time of highest observed concentration of the conjugated products resulting from sodium hydride treatment of the
-
-
3530 J. Org. Chem., Vol. 50, No. 19, 1985
@,y-unsaturatedsubstrates 4, 8, 9, 12-14, 17, 21, and 23.
Summary of Results The results with regard to the ester and amide conjugative isomerizations can be summarized as follows: 1. The NaH/ether reaction condition results in the isomerization of a variety of P,y-unsaturated esters to their a,@-unsaturatedanalogues. The stereochemical profile clearly indicates that the initial isomerization is kinetically controlled, but as the reaction time increases secondary processes can compete and, at long reaction times, a thermodynamic product ratio can be obtained. Increasing substitution decreases the relative rates of both secondary isomerization and side reactions. The isomerization products may be obtained without the need for an external quenching reagent, and while it is clear that the anionic intermediates are involved, it is also evident that the reaction intermediates do not resemble those obtained in the complementary a,@-unsaturatedester to @,y-unsaturated ester isomerization. 2. For the methyl 3-butenoate system, the initial product is the (2)-olefin, but for the analogous ethyl 3ethyl-3-butenoate, the E product is obtained, a stereochemical result seemingly opposite to but actually identical with the unsubstituted methyl 3-butenoate case. 3. Stereoselective isomerizations are observed for substrates in which there is stereochemical labeling of the @,y-doublebond. Thus, E @,y-unsaturatedesters isomerize primarily to the 2 a,@-unsaturatedesters, whereas 2 @,yunsaturated esters isomerize primarily to the E a,@-unsaturated esters. The stereoselective esters. The stereoselective @,y-to a,@-unsaturatedester isomerizations observed in this work demonstrate that the intermediates involved in these two reactions are distinct both from each other, and also from the intermediates of the deconjugative process. 4. The NaH/THF conditions involve a catalytic isomerization process, as the isomerizations can frequently be carried out with
