1550 COOK,HERSCHER, SCHULTZ, AND BURKE
Bicyclic Enamines.
V.
The Journal of Organic Chemistry
Cumulated Cyclopropylenamines1v2
A. GILBERTCOOK,SUSAN B. HERSCHER, DIANAJ. SCHULTZ, AND JONATHAN A. BURKE Department of Chemistry, Valparaiso University, Valparaiso, Indiana 46S8S Received September 16, 1969 Cyclopropylcarbinylaminals were studied. Certain of these aminals possess double bonds in such a steric position that they aid in the formation of cumulated cyclopropylenamines. These cumulated cyclopropylenamines show uv maxima of high intensity in the 300-nm region. The origins of these observed phenomena are discussed. The basicities of various cyclopropyl methyl ketones are reported and compared with those of other methyl ketones. The basicity of the tricyclic 2-acetyltricyclene is much stronger than that of other reported cyclopropyl ketones, with a ~ K B H of I - -4.06.
One of the earliest reports of the formation of enamines3 describes the formation of what is probably a cumulated dienamine.4 This synthesis is carried out by treating an a,p-unsaturated aldehyde such as acrolein with 2 mol of a secondary amine such as piperidine
0 -H,O
3
0
II
CH,=CHCH
+
3-
2 HN
(=SCH,CH=CHN3 1
.L
4
-
1,)
cH2=C=CHN3
I
2
to form amino enamine 1 (a vinylogous aminal). Vacuum distillation of 1 produced cumulated dienamine 2. This type of compound has been virtually ignored since this early report.5 The cyclopropyl group has many chemical and physical properties which are analogous to those observed in alkene groups.e This would lead one to suspect that tertiary cyclopropylamines behave like enamines in such typical reactions as alkylation and acylation. However, this similarity in chemical behavior between cyclopropylamines and enamines has not been o b ~ e r v e d . ~ Although cyclopropylamines themselves do not show any of the typical enamine properties, the properties of cumulated cyclopropylenamines is a significant area for investigation. The problem of constructing such a system for investigation then arises. The reaction of secondary amines with ketones or aldehydes is the most widely used method of synthesizing enamines,a so the reaction of secondary amines with cyclopropyl ketones or aldehydes should be a fruitful approach. The treatment of methyl cyclopropyl ketone (3) with pyrrolidine in the presence of an acid catalyst produced, beside ring-fission products,s a small amount of enamine 4. Identification of 4 was made through its ir spectrum and spectral and elemental analysis of its perchlorate salt 5.9 The ir spectrum of the enamine (1) For the previous article in the series, see A. G . Cook and W. M. Kosman, Tetrahedron Lett., 5847 [l9881. (2) Support of this work by a grant from the Petroleum Research Fund of the American Chemical Society and by a Valparaiso University Grant is gratefully acknowledged. (3) For a comprehensive review of enamines, see “Enamines: Synthesis, Structure, and Reactions,” A. G . Cook, Ed.. Marcel Dekker, Inc., New York, N. Y., 1989. (4) C. Mannich, K. Handke, and K. Roth, Chem. Ber., 69, 2112 (1938). ( 5 ) L. A. Paquette and M. Rosen, J . Amer. Chem. Soo., 89, 4102 (1987). (8) M. Yu Lukina, U s p . Khim, 81, 419 (1982). (7) R . A . Fouty, Ph.D. Dissertation, University of Pennsylvania, 1982. (8) A . G. Cook and K. E. Ungrodt, unpublished data. (9) J. V. Paukstelis, Ph.D. Dissertation, University of Illinois, 1984. He made this ternary iminium salt b y the direct reaction of ketone 8 with pyrrolidium perchlorate.
I>=6CH3 6
product indicated that a trace of cumulated cyclopropyl enamine 6 might also be present, but it was very unstable and was not positively identified. The reaction of cyclopropanecarboxaldehyde (7) with piperidine results in the formation of aminal 8.’O I n a similar manner aminal 9 is produced when 7 is
n=l
10
r 7
11
treated with pyrrolidine. These aminals are thermally stable, and hence they are distillable and not readily converted into “cumulated” enamines, as was vinylogous aminal 1, which was studied by Mannich and coworkers. Aminals are commonly produced by the reaction of a secondary amine with an aldehyde,a~ll-la (10) K. C. Brannock, A. Bell, R. D. Burpitt, and C. A. Kelly, J . Org. Chem., 29, 801 (1984). (11) C . Mannioh and H. Davidsen, Chem. Ber., 69, 2108 (1938). (12) R. Dulou, E. Elkik, and A. Veillard, Bull. SOC.Chim. Fr., 987 (1980). (13) A. Dornow and W. Sohacht, Chem. Ber., 82,484 (1949).
CUMULATED CYCLOPROPYLENAMINES 1551
VoZ. 36, No. 5, May 19YO but upon distillation 1 mol of the amine is usually eliminated and an enamine is formed. It has been shown that a,minals and enamines are in equilibrium in some cases.14 Aminal 9 is reduced to amine 10 by treatment with lithium aluminum hydride, a reaction parallel to the borohydride reduction of aminalll to the monoamine reported by Szmuszkovicz. Stable aminals are formed from aldehydes with no a hydrogens such as benzaldehyde, l6 from cyclopropanones, 15*17,'8 and from cyclopropylcarboxaldehydes such as 7. Apparently, cyclopropanones form stable aminals rather than enamines because of the excessive ring strain that would be introduced by the formation of an enamine, but the question remains as to the reluctance of aminals formed from cyclopropylcarboxaldehydes to yield enamines under the normal conditions. There are three steps which together determine the overall ra,te of enamine f o r m a t i ~ n ,and ~ , ~the ~ last of
13
these steps (step C) has direct bearing on the question at hand. Step C depends upon the ease of losing a proton from the @-carbonatom of the ternary iminium ion. The base-catalyzed exchange of deuterium for the methine hydrogen in isopropyl methyl ketone (13) is much more rapid than that for the methine hydrogen in cyclopropyl methyl ketone (3).20121 I n fact, the basecatalyzed exchange of deuterium for cyclopropyl methine hydrogens is essentially nonexistentz2in spite of the s character of the exocyclic bonding orbitals.23 The endocyclic carbon-carbon bonds in cyclopropane are sp4.12 h y b r i d i ~ e d ~and ~ - ~ can ~ have pseudoconjugation with an adjacent r-electron system when the plane of the cyclopropane ring is parallel to the axis of the p 0rbita1.~~-~6 The exocyclic carbon-carbon bonds (14) P. Ferruti, D. Pocar, and G. Bianchetti, Uazz. Chim. Ital., 97, 109 (1967). (15) J. Samuszkovica, E. Cerda, M. F. Grostic, and J. F. Zieserl, Jr., Tetrahedron Lett., 3969 (1967). (16) R. A. Henry and W. M. Dehn, J . Amer. Chem. Sac., 71, 227 (1949). (17) W. J. M. Van Tilborg, S. E. Schaafsma, H. Steinberg, and T. J. DeBoer, Rec. Trau. Chim. Pays-Bas, 86, 417 (1967). (18) N.J. Turro, Accounts Chem. Res., '2, 25 (1969). (19) G. Stork, A. Brizaolara, H. Landesman, J. Samuszkovica, and R. Terrell, J . Amer. Chen. Sac., 86, 207 (1963). (20) C. Rappe and W. H. Sachs, Tetrahedron, 34, 6287 (1968). (21) W. T. Van Wijnen, H. Steinberg, and T. J. DeBoer, Rec. Trau. Chim. Pays-Bas, 87,844 (1968). (22) H.W. Amburn, K . C. Kauffman, and H. Shechter, J . Amer. Chem. Sac., 91, 530 (196R). (23) D. J. Cram, "Fundamentals of Carbanion Chemistry," Academic Press, New York, N. Y., 1965,pp 23 and 49-52. (24) C. A. Coulson and W. E. Moffitt, Phil. Mag., 40, 1 (1949). (25) A. D. Walah, Trans. Faraday Sac., 46, 179 (1949). (26) L. L. Ingrahm, "Steric Effects in Organic Chemistry," M. S. Newman, Ed., John Wiley & Sons, Inc., New York, N. Y., 1956,p 479. (27) W. A. Bernett, J . Chem. Educ., 44,17 (1967). (28) N. C. Deno, H. G. Richey, Jr., J. S. Lui, D. N. Lincoln, and J. 0. Turner, J . Amer. Chem. Sac., 87,4533 (1965). (29) H.C. Brown and J. D . Cleveland, ibid., 88, 2051 (1966).
are ~ phybridized. ~ * ~Any~ change in the endocyclic carbon-carbon bonds toward spa hybridization causes increased strain in the ring. Thus the orbital geometry of the carbanionlike transition states in this basecatalyzed hydrogen exchange makes meaningful delocalization of charge impossible; i.e., because of the cyclopropane ring the unshared electron pair is in an orbital with a smaller amount of p character than would be desirable for maximum overlap with the carbonyl p orbital. The formation of an enamine from an hemiaminal or an aminal is an analogous situation, since the transition state in step C in going from the ternary iminium ion to the enamine would be very similar to the carbanion-like transition states in the base-catalyzed hydrogen exchange, and for similar reasons the enamine does not readily form. It has been shown from heats of hydrogenation studies that a double bond exocyclic to a cyclopropane ring is very highly strained,a6as would be expected from the theoretical model, since this would mean the use of a p orbital in the cyclopropyl carbon and spxwould have x
