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Exploratory theoretical study of the [2 + 2] cycloaddition between ketene and formaldehyde
THEO CHEM ELSEVIER
Journal of Molecular Structure (Theochem) 313 (1994) 361-364
Short communication
Exploratory theoretical study of the [2 + 2] cycloaddition between ketene and formaldehyde J e a n - M a r c P o n s a, A g n 6 s P o m m i e r a, M i c h e l R a j z m a n n a'*, D a n i e l L i o t a r d b aURA C N R S 1411, Centre de St. J~rome, boite 561, F-13397 Marseille Cedex 20, France bLaboratoire de Physico-chimie Thkorique, URA C N R S 503, Univ. Bordeaux I, 351, cours de la Liberation, F-33405 Talence Cedex, France Received 27 June 1994; accepted 15 July 1994
Abstract Two reaction paths, leading from formaldehyde 1 and ketene 2 to 2-oxetanone 3, are examined. M e c h a n i s m A, involving the initial forma"tlon o f the C2-O3 b o n d , a stepwise m e c h a n i s m with biradical character, is found to be m o r e favoured than mechanism B, a concerted, but highly-asynchronous mechanism with zwitterionic character involving the initial f o r m a t i o n o f the C4 C5 bond.
1. Introduction The [2 + 2] cycloaddition between ketenes and carbonyl compounds leading to ¢3-1actones (2-oxetanones) is a synthetically-significant reaction [1]. In connection with our experimental knowledge in this area [2], we have investigated the cycloaddition * Corresponding author. 1This preliminary communication will be followed by an ab initio study on the parent system. Nevertheless, AM1, which enables calculations on more complex systems we are interested in, proved to be a reliable method for this type of calculations. Indeed, we were able, using AM 1/CI, to reproduce qualitatively ab initio results (MC-SCF/6-31G*) reported recently by Palmer et al. on the Paterno-Biichi reaction. Both reaction paths were characterized as diradical ones and geometries of calculated transition states and intermediates of both paths were very similar to the ab initio ones [7]. 2 A value of CI = 6 proved to be sufficient for calculations on reaction path B but, because of molecular orbital crossing occuring in reaction path A, in that case, calculations were performed with CI = 8.
mechanism from a theoretical viewpoint. Indeed, although the cycloadditions between ketenes and olefins [3] and, more recently, between ketenes and imines [4], have been widely studied, the cycloaddition between ketenes and aldehydes has received very little attention. However, in the only theoretical study we are aware of [5], ketene was found to behave as an electron donor when opposed to formaldehyde; a behaviour which contrasts that in the analogous reaction of ketene with olefines or imines.
2. Methodology In this communication we report results, obtained by use of the semiempirical AM 1 method [6], 1 on the nature of the reaction paths and the saddle points leading to the formation of 2-oxetanone 3 (/%lactone) from formaldehyde I and ketene 2. Calculations were performed with configuration interaction (CI) 2 and reaction paths determined by
0166-1280/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0166-1280(94)03925-9
J.-M. Pons et al./J. Mol. Struct. (Theochem) 313 (1994) 361 364
362
0 1
-
(a) 01
3o 1
0
.-,.
U2
., "H t ~ 01
2 \
Co)
3
/ 4
Scheme 1. T w o reaction p a t h s t o w a r d s ~-lactone 3; m e c h a n i s m A versus m e c h a n i s m B.
IRC (intrinsic reaction coordinate). Both the initial formation of the C2-O3 bond (mechanism A) and the initial formation of the C4-C5 bond (mechanism B) have been examined (Scheme 1).
3. Results and discussion
In mechanism A, the formation of/3-1actone 3 occurs through a reaction path (Scheme 2) involving three transition states and two intermediates; the highest transition state TSa2 being 31.6kcalmo1-1 higher than the reagents ( 1 + 2 ) (Table 1). The attack of the formaldehyde oxygen atom 03 on the central carbon atom C2 of ketene occurs in an anti fashion and the highest transition state TSa2 (Scheme 2) corresponds to the rotation leading from anti intermediate Ial to the syn intermediate Ia2. Both intermediates, Ial and Ia2, and transition state TSa2 possess a significant biradical character, illustrated by a strong contribution of
monoexcited configurations 2. This biradical character can also be deduced from the values of net atomic charges of carbon atoms C4 and C5; indeed for these three saddle points 6(C4) and 6(C5) are equal to -0.22 ± 0.02. Such a character appears as soon as the C2-O3 distance reaches 1.4 A and disappears with the formation of the C4 C5 bond. This character was found to be responsible for a discontinuity we found when trying to perform these calculations at the R H F level. Finally, 9lactone 3 is formed via a conrotatory ring closure as can be seen from Scheme 2. Unlike mechanism A, the reaction path of mechanism B displays a single transition state TSb (Scheme 3) (Table 2). Despite a careful search, no anti transition state or intermediates could be found. The activation energy of the reaction is, in this case, 48.7 kcal mol 1 and the values of net atomic charges of atom 03 and C2 are 6(03) = -0.57 and 6(C2) = +0.44. This clear separation of charges suggests that the mechanism
Table 1 Salient properties o f m e c h a n i s m A critical p o i n t s
l + 2 TSal lal TSa2
Ia2 TSa3 3
d(c.~ o3/
d(c4 c5)
01C2C4
C503C2C4
H6C503C2
H8C4C203
AHf a
AH~b
3.625 1.571 1.387 t .403 1.392 1.393 1.399
4.853 3.543 3.609 3.202 2.839 2.433 1.550
179.8 146.9 129.9 128.6 127.3 132.4 142.5
159.5 180.0 180.0 -90.0 -24.0 - 19.6 0.0
-99.1 0.0 0.0 -0.5 -5.2 - 59.6 117.5
8.5 180.0 180.0 - 178.2 170.8 132.3 115.8
-48.5 c -20.7 -27.6 - 16.9 -20.7 - 18.1 -56.6 c
27.8 31.6 30.4
H e a t s o f f o r m a t i o n in kcal m o l 1. b E n e r g y of a c t i v a t i o n [AH~ = ETS -- E(1+2)] in k c a l m o l 1. c These values are c a l c u l a t e d w i t h o u t the key w o r d O P E N (2,2) which is n o t suitable to calculate energies of reactants and products.
363
J.-M. Pons et al./J. Mol. Struct. (Theochem) 313 (1994) 361-364
E
(kcal.mo1-1) TSa I
-20 /
-40 -
TSa2 . . . . . . . . . "-.....
/ 1+2,/"
.-'"
TSa3
. ...........
la 2
Ial ~
"',
-60 r
Scheme 2. Formation of ¢3-1actone 3 through reaction path A. Structure of calculated transition states TSa I, TSa2 and TSa3 and intermediates Ial and Ia 2.
J
E
(kcal-mol-1) 200-20 -40 -
1+2// "'.
3
-60 Scheme 3. Formation of 3-1actone 3 through reaction path B. Structure of calculated transition state TSb.
is a closed-shell one; such a hypothesis is confirmed by the contribution from the fundamental configuration which was calculated to be 99%. Finally, results obtained with configuration interaction 2 were very similar to those obtained when working at the R H F level.
mechanism, by about 17 kcal mol i. Studies are currently underway to examine the effects of substituents on the mechanism of the reaction, particularly, the introduction of a silyl group on the ketene and the addition of a Lewis acid to the reactants.
4. Conclusion
Acknowledgements
This work shows that mechanism A, a non concerted mechanism with significant biradical character, is more favourable than mechanism B, a closed-shell concerted but highly asynchronous
We are grateful to Prof. Philip Kocienski for helpful advice and to the C I M M and the Centre de Calcul de St. J~rome for providing computational facilities.
Table 2 Salient properties of TSb
References
TSb
d(c: o3/
d(c4 cs)
O1C2C4
C204C5C3
AHf a AH~ b
2.187
1.745
171.6
-30.2
0.04
Heat of formation in kcalmol 1. h Energy of activation in kcalmol -l .
48.69
[1] A. Pommier and J.-M. Pons, Synthesis, (1993) 441. [2] J.-M. Pons and P. Kocienski, Tetrahedron Lett., 30 (1989) 1833. J.-M. Pons, A. Pommier, J. Lerpiniere and P. Kocienski, J. Chem. Soc., Perkin Trans. 1, (1993) 1549.
364
J.-M. Pons et al./J. Mol. Struct. (Theochem) 313 (1994) 361 364
[3] L.A. Burke, J. Org. Chem., 50 (1985) 3149. X. Wang and K.N. Houk, J. Am. Chem. Soc., 112 (1990) 1754.
F. Bernardi, A. Bottoni, M.A. Robb and A. Venturini, J. Am. Chem. Soc.~ 112 (1990) 2106. E. Valenti, M.A. Pericas and A. Moyano~ J. Org. Chem., 55 (1990) 3582. [4] D.C. Fang and X.Y. Fu, Int. J. Quant. Chem., 43 (1992) 669. J.A. Sordo, J. Gonzalez and T.L. Sordo, J. Am. Chem. Soc., 114 (1992) 6249. F.P. Cossio, J.M. Ugalde, X. Lopez, B. Lecea and C. Palomo, J. Am. Chem. Soc., 115 (1993) 995.
R. Lopez, T.L. Sordo, J.A. Sordo and J. Gonzalez. J. Org. Chem., 58 (1993) 7036. F.P. Cossio, A. Arrieta, B. Lecea and J.M. Ugalde, J. Am. Chem. Soc., 116 (1994) 2085. [5] S. Yamabe, T. Minato and Y. Osamura, J. Chem. Sot., Chem. Commun., (1993) 450. [6] M.J.S. Dewar, E.G. Zoebish, E F . Healy and J.J.P. Stewart, J. Am. Chem. Soc., 107 (1985) 3902. All calculations were performed with the .xMp.xc4s code, Semichem, Shawnes, KS 66216, USA. [7] I.J. Palmer, I.N. Ragazos, F. Bernardi, M. Olivucci and M.A. Robb, J. Am. Chem. Soc., 116 (1994) 2121.
Journal of Molecular Structure (Theochem) 313 (1994) 361-364
Short communication
Exploratory theoretical study of the [2 + 2] cycloaddition between ketene and formaldehyde J e a n - M a r c P o n s a, A g n 6 s P o m m i e r a, M i c h e l R a j z m a n n a'*, D a n i e l L i o t a r d b aURA C N R S 1411, Centre de St. J~rome, boite 561, F-13397 Marseille Cedex 20, France bLaboratoire de Physico-chimie Thkorique, URA C N R S 503, Univ. Bordeaux I, 351, cours de la Liberation, F-33405 Talence Cedex, France Received 27 June 1994; accepted 15 July 1994
Abstract Two reaction paths, leading from formaldehyde 1 and ketene 2 to 2-oxetanone 3, are examined. M e c h a n i s m A, involving the initial forma"tlon o f the C2-O3 b o n d , a stepwise m e c h a n i s m with biradical character, is found to be m o r e favoured than mechanism B, a concerted, but highly-asynchronous mechanism with zwitterionic character involving the initial f o r m a t i o n o f the C4 C5 bond.
1. Introduction The [2 + 2] cycloaddition between ketenes and carbonyl compounds leading to ¢3-1actones (2-oxetanones) is a synthetically-significant reaction [1]. In connection with our experimental knowledge in this area [2], we have investigated the cycloaddition * Corresponding author. 1This preliminary communication will be followed by an ab initio study on the parent system. Nevertheless, AM1, which enables calculations on more complex systems we are interested in, proved to be a reliable method for this type of calculations. Indeed, we were able, using AM 1/CI, to reproduce qualitatively ab initio results (MC-SCF/6-31G*) reported recently by Palmer et al. on the Paterno-Biichi reaction. Both reaction paths were characterized as diradical ones and geometries of calculated transition states and intermediates of both paths were very similar to the ab initio ones [7]. 2 A value of CI = 6 proved to be sufficient for calculations on reaction path B but, because of molecular orbital crossing occuring in reaction path A, in that case, calculations were performed with CI = 8.
mechanism from a theoretical viewpoint. Indeed, although the cycloadditions between ketenes and olefins [3] and, more recently, between ketenes and imines [4], have been widely studied, the cycloaddition between ketenes and aldehydes has received very little attention. However, in the only theoretical study we are aware of [5], ketene was found to behave as an electron donor when opposed to formaldehyde; a behaviour which contrasts that in the analogous reaction of ketene with olefines or imines.
2. Methodology In this communication we report results, obtained by use of the semiempirical AM 1 method [6], 1 on the nature of the reaction paths and the saddle points leading to the formation of 2-oxetanone 3 (/%lactone) from formaldehyde I and ketene 2. Calculations were performed with configuration interaction (CI) 2 and reaction paths determined by
0166-1280/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0166-1280(94)03925-9
J.-M. Pons et al./J. Mol. Struct. (Theochem) 313 (1994) 361 364
362
0 1
-
(a) 01
3o 1
0
.-,.
U2
., "H t ~ 01
2 \
Co)
3
/ 4
Scheme 1. T w o reaction p a t h s t o w a r d s ~-lactone 3; m e c h a n i s m A versus m e c h a n i s m B.
IRC (intrinsic reaction coordinate). Both the initial formation of the C2-O3 bond (mechanism A) and the initial formation of the C4-C5 bond (mechanism B) have been examined (Scheme 1).
3. Results and discussion
In mechanism A, the formation of/3-1actone 3 occurs through a reaction path (Scheme 2) involving three transition states and two intermediates; the highest transition state TSa2 being 31.6kcalmo1-1 higher than the reagents ( 1 + 2 ) (Table 1). The attack of the formaldehyde oxygen atom 03 on the central carbon atom C2 of ketene occurs in an anti fashion and the highest transition state TSa2 (Scheme 2) corresponds to the rotation leading from anti intermediate Ial to the syn intermediate Ia2. Both intermediates, Ial and Ia2, and transition state TSa2 possess a significant biradical character, illustrated by a strong contribution of
monoexcited configurations 2. This biradical character can also be deduced from the values of net atomic charges of carbon atoms C4 and C5; indeed for these three saddle points 6(C4) and 6(C5) are equal to -0.22 ± 0.02. Such a character appears as soon as the C2-O3 distance reaches 1.4 A and disappears with the formation of the C4 C5 bond. This character was found to be responsible for a discontinuity we found when trying to perform these calculations at the R H F level. Finally, 9lactone 3 is formed via a conrotatory ring closure as can be seen from Scheme 2. Unlike mechanism A, the reaction path of mechanism B displays a single transition state TSb (Scheme 3) (Table 2). Despite a careful search, no anti transition state or intermediates could be found. The activation energy of the reaction is, in this case, 48.7 kcal mol 1 and the values of net atomic charges of atom 03 and C2 are 6(03) = -0.57 and 6(C2) = +0.44. This clear separation of charges suggests that the mechanism
Table 1 Salient properties o f m e c h a n i s m A critical p o i n t s
l + 2 TSal lal TSa2
Ia2 TSa3 3
d(c.~ o3/
d(c4 c5)
01C2C4
C503C2C4
H6C503C2
H8C4C203
AHf a
AH~b
3.625 1.571 1.387 t .403 1.392 1.393 1.399
4.853 3.543 3.609 3.202 2.839 2.433 1.550
179.8 146.9 129.9 128.6 127.3 132.4 142.5
159.5 180.0 180.0 -90.0 -24.0 - 19.6 0.0
-99.1 0.0 0.0 -0.5 -5.2 - 59.6 117.5
8.5 180.0 180.0 - 178.2 170.8 132.3 115.8
-48.5 c -20.7 -27.6 - 16.9 -20.7 - 18.1 -56.6 c
27.8 31.6 30.4
H e a t s o f f o r m a t i o n in kcal m o l 1. b E n e r g y of a c t i v a t i o n [AH~ = ETS -- E(1+2)] in k c a l m o l 1. c These values are c a l c u l a t e d w i t h o u t the key w o r d O P E N (2,2) which is n o t suitable to calculate energies of reactants and products.
363
J.-M. Pons et al./J. Mol. Struct. (Theochem) 313 (1994) 361-364
E
(kcal.mo1-1) TSa I
-20 /
-40 -
TSa2 . . . . . . . . . "-.....
/ 1+2,/"
.-'"
TSa3
. ...........
la 2
Ial ~
"',
-60 r
Scheme 2. Formation of ¢3-1actone 3 through reaction path A. Structure of calculated transition states TSa I, TSa2 and TSa3 and intermediates Ial and Ia 2.
J
E
(kcal-mol-1) 200-20 -40 -
1+2// "'.
3
-60 Scheme 3. Formation of 3-1actone 3 through reaction path B. Structure of calculated transition state TSb.
is a closed-shell one; such a hypothesis is confirmed by the contribution from the fundamental configuration which was calculated to be 99%. Finally, results obtained with configuration interaction 2 were very similar to those obtained when working at the R H F level.
mechanism, by about 17 kcal mol i. Studies are currently underway to examine the effects of substituents on the mechanism of the reaction, particularly, the introduction of a silyl group on the ketene and the addition of a Lewis acid to the reactants.
4. Conclusion
Acknowledgements
This work shows that mechanism A, a non concerted mechanism with significant biradical character, is more favourable than mechanism B, a closed-shell concerted but highly asynchronous
We are grateful to Prof. Philip Kocienski for helpful advice and to the C I M M and the Centre de Calcul de St. J~rome for providing computational facilities.
Table 2 Salient properties of TSb
References
TSb
d(c: o3/
d(c4 cs)
O1C2C4
C204C5C3
AHf a AH~ b
2.187
1.745
171.6
-30.2
0.04
Heat of formation in kcalmol 1. h Energy of activation in kcalmol -l .
48.69
[1] A. Pommier and J.-M. Pons, Synthesis, (1993) 441. [2] J.-M. Pons and P. Kocienski, Tetrahedron Lett., 30 (1989) 1833. J.-M. Pons, A. Pommier, J. Lerpiniere and P. Kocienski, J. Chem. Soc., Perkin Trans. 1, (1993) 1549.
364
J.-M. Pons et al./J. Mol. Struct. (Theochem) 313 (1994) 361 364
[3] L.A. Burke, J. Org. Chem., 50 (1985) 3149. X. Wang and K.N. Houk, J. Am. Chem. Soc., 112 (1990) 1754.
F. Bernardi, A. Bottoni, M.A. Robb and A. Venturini, J. Am. Chem. Soc.~ 112 (1990) 2106. E. Valenti, M.A. Pericas and A. Moyano~ J. Org. Chem., 55 (1990) 3582. [4] D.C. Fang and X.Y. Fu, Int. J. Quant. Chem., 43 (1992) 669. J.A. Sordo, J. Gonzalez and T.L. Sordo, J. Am. Chem. Soc., 114 (1992) 6249. F.P. Cossio, J.M. Ugalde, X. Lopez, B. Lecea and C. Palomo, J. Am. Chem. Soc., 115 (1993) 995.
R. Lopez, T.L. Sordo, J.A. Sordo and J. Gonzalez. J. Org. Chem., 58 (1993) 7036. F.P. Cossio, A. Arrieta, B. Lecea and J.M. Ugalde, J. Am. Chem. Soc., 116 (1994) 2085. [5] S. Yamabe, T. Minato and Y. Osamura, J. Chem. Sot., Chem. Commun., (1993) 450. [6] M.J.S. Dewar, E.G. Zoebish, E F . Healy and J.J.P. Stewart, J. Am. Chem. Soc., 107 (1985) 3902. All calculations were performed with the .xMp.xc4s code, Semichem, Shawnes, KS 66216, USA. [7] I.J. Palmer, I.N. Ragazos, F. Bernardi, M. Olivucci and M.A. Robb, J. Am. Chem. Soc., 116 (1994) 2121.