- Email: [email protected]
Pressure effect on reactions of C7H8 hydrocarbons
Physica 139 & 140B (1986) 729-731 North-Holland, Amsterdam
PRESSURE EFFECT ON REACTIONS OF
C7H s
HYDROCARBONS
G6rard JENNER and Mihalis PAPADOPOULOS Laboratoire de Pi(zochimie Organique, Universit( Louis Pasteur, 1, rue Blaise Pascal, 67008 Strasbourg, France
The reactions of three CTIt 8 valence isomers (quadricyclane, norbornadiene and cycloheptatriene) with activated carbonyl bonds were considered. Ouadricyclane and norbornadiene react with mesoxalates to give the same cycloadducts (oxetanes). While this result is normally expected with quadricyclane, it is surprising for norbornadiene. The determination of the volume profile for the reaction indicates a transition state occurring "earlier" on the reaction coordinate. A mechanism involving a common diradical intermediate is proposed. Cycloheptatriene reacts with mesoxalates only under high pressure giving an ene product. There was no evidence of a norcaradiene structure.
2. Addition of the C = 0 bond to quadricyclane and norbornadiene
I. Introduction
There are three interesting reactive hydrocarbons corresponding to the formula C7H s. The most strained is quadricyclane 1 which transforms when heated into bicyclo [2.2.1 .] heptadiene (norbornadiene) 2. The latter gives cycloheptatriene 3 at higher temperatures [1] (fig. 1). Depending on their structure the reactivity of the valence isomers differs and this will be shown by the pressure effect on the addition reactions of activated carbonyl bonds.
1
g
3
~--"~ 4
5
hp : 9 0 0 MPa - 80"C
D : o=c~ E, K.
Fig. 1.
C-- OH
Quadricyclane and norbornadiene react according to [It 2 + or2 + o"2] and [Tr2 + 7r2 + 7r2] processes, respectively. The mechanism of these reactions was shown, by a pressure study, to be fairly concerted when the participating bond of the partner is homonuclear [2]. The reaction products are the related cyclobutanes and homoDiels--Alder adducts, respectively. Heteronuclear bonds may not necessarly give concerted reactions. We have considered the carbonyl bond activated by ester groups. Submitted to 100°C, at ambient pressure, ,1 combines with dimethyl mesoxalate (E = E ' = C O 2 C H 3 ) or phenyl glyoxalate ( E = H , E' =CO2C6H5) to give oxetanes 4 [3]. In the same conditions, 2 reacts only with difficulty. Under high pressure, 2 adds readily mesoxalates and glyoxalates to lead also to oxetanes 4. While the result is normally expected with quadricyclane 1, it is rather surprising for norbornadiene 2, where a [4 + 2] homoadduct should form. The stereo-selectivity of oxetane 4 would suggest an apparent [2+2] cycloaddition. To account for this result, the volume profile of the reaction (2_+ dimethyl mesoxalate) was determined in the usual way (pressure effect on the kinetics) [4]. The volume of activation was found to be only -23.5 mL/mol and compared to the reaction volume (-31.8 mL/ mol). These values, from a geometric point of
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
730
G. Jenner and M. Papadopoulos
E" Fig. 2. view, indicate that the transition state is more distant from the final state than what is expected for a strictly concerted process. In other words, the transition state for the norbornadiene reaction occurs "earlier" on the reaction coordinate. Therefore, the concertedness of the process may be questioned. If concertedness is weakened, what could be the intermediate? 1 is known to give charge transfer complexes (CTFC) with electron deficient compounds [5]. The appearance of the conjugative interaction of the two cyclopropane rings in 1_seems to require a moderate (charge transfer) or a strong (cycloaddition) perturbation. This interaction distorts the initial structure to an intermediate structure with the possible creation of two distinct radical cations [6]. The activated carbonyl bond of mesoxalates or glyoxalates gives, in the presence of quadricyclane, a CTC and, during the reaction, 1 passes to a highly distorted molecule, which might not have symmetry elements. This intermediate would be the same for the reaction of activated ~ O bonds with either 1 or 2 and could be a possible polarized biradical such as shown in fig. 2. In that case, the reaction 2 + mesoxalate would be a stepwise process, like the corresponding quadricyclane reaction. As a matter of fact, although the normal oxetane is produced, the probable formation of a CTC and the absence of a reaction order strictly equal to 2 suggest rather a non-concerted process.
/ Reactions of CTH~ hydrocarbons
can be easily evidenced through the reaction products. For example, 3 combines with homonuclear bonds to form, in addition to other products, norcaradiene adducts [71. Another reaction occurring with _3 consists in the ene reaction, since 3 has allylic hydrogens which are easily abstracted by enophiles. Since mesoxalates have been proved to possess strong enophilic properties [8], we investigated their reaction with 3. However, the reaction does not occur at normal pressure in the temperature range 80-140°C. Submitted to high pressure (900MPa) at 80°C, dialkyl mesoxalates react with _3 producing two adducts in a 95:5 ratio and a total yield of 72% after 24 h. The major product was shown to be the ene adduct 5. The other product could not be separated from 5_. We thought that it would be the isomer possessing the norcaradiene structure; however, this could not be demonstrated. Thus, with heteronuclear bonds, 3 does not react in the usual way (norcaradiene [4 + 2] cycloaddition). The pressure effect on the kinetics of this reaction to determine the volume profile could not be investigated, since no reaction occurs at lower pressures.
4. Conclusion The addition of the carbonyl bond to CTH 8 hydrocarbons is greatly facilitated by pressure. While reactions involving quadricyclane may be possible at normal pressure, they occurred with extreme difficulty with norbornadiene and not at all with cycloheptatriene 3-. In some cases, kinetic studies under pressure enable the determination of volume profiles, hence contributing to the elucidation of the reaction mechanism.
References 3. Addition of the C--~O bond to cycioheptatriene Cycloheptatriene 3 is generally less reactive than 1 or 2. The triene possesses interesting properties since it is subjected to a cyclotautomerization process 3 ~ norcaradiene which
[1] C. Bleasdale and D.W. Jones. J. Chem. Soc. Chem. Comm. (1984) 1200. [2J G. Jenner and M. Papadopoulos, Tetrahedron Lett. 23 (1982) 4333; M. Papadopoulos and G. Jenner, Nouv. Chim. 7 (1983) 463. [3] M. Papadopoulos, R. Jost and G. Jenner, J. Chem. Soc., Chem. Comm. (1983) 221.
G. Jenner and M. Papadopoulos / Reactions of C7Hs hydrocarbons [4] G. Jenner, Bull. Soc. Chim. France II (1984) 275. [5] I. Tabushi and K. Yamamura, Tetrahedron 31 (1975) 1827. [6] H. Roth, J. Amer. Chem. Soc. 103 (1981) 7210. [7] M.J. Goldstein and A.H. Gevirtz, Tetrahedron Lett.
731
(1965) 4413; M. Balci and B. Atasoy, Tetrahedron Lett. 25 (1984) 4033. [8] O. Achmatowicz and M. Pietraszkiewicz, Tetrahedron Lett. 22 (1981) 4323.
PRESSURE EFFECT ON REACTIONS OF
C7H s
HYDROCARBONS
G6rard JENNER and Mihalis PAPADOPOULOS Laboratoire de Pi(zochimie Organique, Universit( Louis Pasteur, 1, rue Blaise Pascal, 67008 Strasbourg, France
The reactions of three CTIt 8 valence isomers (quadricyclane, norbornadiene and cycloheptatriene) with activated carbonyl bonds were considered. Ouadricyclane and norbornadiene react with mesoxalates to give the same cycloadducts (oxetanes). While this result is normally expected with quadricyclane, it is surprising for norbornadiene. The determination of the volume profile for the reaction indicates a transition state occurring "earlier" on the reaction coordinate. A mechanism involving a common diradical intermediate is proposed. Cycloheptatriene reacts with mesoxalates only under high pressure giving an ene product. There was no evidence of a norcaradiene structure.
2. Addition of the C = 0 bond to quadricyclane and norbornadiene
I. Introduction
There are three interesting reactive hydrocarbons corresponding to the formula C7H s. The most strained is quadricyclane 1 which transforms when heated into bicyclo [2.2.1 .] heptadiene (norbornadiene) 2. The latter gives cycloheptatriene 3 at higher temperatures [1] (fig. 1). Depending on their structure the reactivity of the valence isomers differs and this will be shown by the pressure effect on the addition reactions of activated carbonyl bonds.
1
g
3
~--"~ 4
5
hp : 9 0 0 MPa - 80"C
D : o=c~ E, K.
Fig. 1.
C-- OH
Quadricyclane and norbornadiene react according to [It 2 + or2 + o"2] and [Tr2 + 7r2 + 7r2] processes, respectively. The mechanism of these reactions was shown, by a pressure study, to be fairly concerted when the participating bond of the partner is homonuclear [2]. The reaction products are the related cyclobutanes and homoDiels--Alder adducts, respectively. Heteronuclear bonds may not necessarly give concerted reactions. We have considered the carbonyl bond activated by ester groups. Submitted to 100°C, at ambient pressure, ,1 combines with dimethyl mesoxalate (E = E ' = C O 2 C H 3 ) or phenyl glyoxalate ( E = H , E' =CO2C6H5) to give oxetanes 4 [3]. In the same conditions, 2 reacts only with difficulty. Under high pressure, 2 adds readily mesoxalates and glyoxalates to lead also to oxetanes 4. While the result is normally expected with quadricyclane 1, it is rather surprising for norbornadiene 2, where a [4 + 2] homoadduct should form. The stereo-selectivity of oxetane 4 would suggest an apparent [2+2] cycloaddition. To account for this result, the volume profile of the reaction (2_+ dimethyl mesoxalate) was determined in the usual way (pressure effect on the kinetics) [4]. The volume of activation was found to be only -23.5 mL/mol and compared to the reaction volume (-31.8 mL/ mol). These values, from a geometric point of
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
730
G. Jenner and M. Papadopoulos
E" Fig. 2. view, indicate that the transition state is more distant from the final state than what is expected for a strictly concerted process. In other words, the transition state for the norbornadiene reaction occurs "earlier" on the reaction coordinate. Therefore, the concertedness of the process may be questioned. If concertedness is weakened, what could be the intermediate? 1 is known to give charge transfer complexes (CTFC) with electron deficient compounds [5]. The appearance of the conjugative interaction of the two cyclopropane rings in 1_seems to require a moderate (charge transfer) or a strong (cycloaddition) perturbation. This interaction distorts the initial structure to an intermediate structure with the possible creation of two distinct radical cations [6]. The activated carbonyl bond of mesoxalates or glyoxalates gives, in the presence of quadricyclane, a CTC and, during the reaction, 1 passes to a highly distorted molecule, which might not have symmetry elements. This intermediate would be the same for the reaction of activated ~ O bonds with either 1 or 2 and could be a possible polarized biradical such as shown in fig. 2. In that case, the reaction 2 + mesoxalate would be a stepwise process, like the corresponding quadricyclane reaction. As a matter of fact, although the normal oxetane is produced, the probable formation of a CTC and the absence of a reaction order strictly equal to 2 suggest rather a non-concerted process.
/ Reactions of CTH~ hydrocarbons
can be easily evidenced through the reaction products. For example, 3 combines with homonuclear bonds to form, in addition to other products, norcaradiene adducts [71. Another reaction occurring with _3 consists in the ene reaction, since 3 has allylic hydrogens which are easily abstracted by enophiles. Since mesoxalates have been proved to possess strong enophilic properties [8], we investigated their reaction with 3. However, the reaction does not occur at normal pressure in the temperature range 80-140°C. Submitted to high pressure (900MPa) at 80°C, dialkyl mesoxalates react with _3 producing two adducts in a 95:5 ratio and a total yield of 72% after 24 h. The major product was shown to be the ene adduct 5. The other product could not be separated from 5_. We thought that it would be the isomer possessing the norcaradiene structure; however, this could not be demonstrated. Thus, with heteronuclear bonds, 3 does not react in the usual way (norcaradiene [4 + 2] cycloaddition). The pressure effect on the kinetics of this reaction to determine the volume profile could not be investigated, since no reaction occurs at lower pressures.
4. Conclusion The addition of the carbonyl bond to CTH 8 hydrocarbons is greatly facilitated by pressure. While reactions involving quadricyclane may be possible at normal pressure, they occurred with extreme difficulty with norbornadiene and not at all with cycloheptatriene 3-. In some cases, kinetic studies under pressure enable the determination of volume profiles, hence contributing to the elucidation of the reaction mechanism.
References 3. Addition of the C--~O bond to cycioheptatriene Cycloheptatriene 3 is generally less reactive than 1 or 2. The triene possesses interesting properties since it is subjected to a cyclotautomerization process 3 ~ norcaradiene which
[1] C. Bleasdale and D.W. Jones. J. Chem. Soc. Chem. Comm. (1984) 1200. [2J G. Jenner and M. Papadopoulos, Tetrahedron Lett. 23 (1982) 4333; M. Papadopoulos and G. Jenner, Nouv. Chim. 7 (1983) 463. [3] M. Papadopoulos, R. Jost and G. Jenner, J. Chem. Soc., Chem. Comm. (1983) 221.
G. Jenner and M. Papadopoulos / Reactions of C7Hs hydrocarbons [4] G. Jenner, Bull. Soc. Chim. France II (1984) 275. [5] I. Tabushi and K. Yamamura, Tetrahedron 31 (1975) 1827. [6] H. Roth, J. Amer. Chem. Soc. 103 (1981) 7210. [7] M.J. Goldstein and A.H. Gevirtz, Tetrahedron Lett.
731
(1965) 4413; M. Balci and B. Atasoy, Tetrahedron Lett. 25 (1984) 4033. [8] O. Achmatowicz and M. Pietraszkiewicz, Tetrahedron Lett. 22 (1981) 4323.