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Mechanism of the concerted ring expansion of singlet cyclopropyl nitrene
30 September 1994
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 228 ( 1994) 268-272
Mechanism of the concerted ring expansion of singlet cyclopropyl nitrene * Huabin Sun a, Chengbu Liu b,*,Liming Zhao ‘, Lu Deng d ’ Institute of Military Medicine, Jinan Command, Jinan 250014, People’s Republic of China b Institute of Theoretical Chemistry, Shandong University, Jinan 250100, Peoples Republic of China c Shandong Agricultural Managerial Official College, Jinan 250100, People’s Republic of China ‘Department ofPhysics, Georgia Southern University, Statesboro, GA 30460, USA Received 11 April 1994; in final form 10 July 1994
Abstract The isomerization by concerted ring expanding of singlet cyclopropyl nitrene has been studied using the self-consistent field (SCF) method with the 3-21G basis set. The transition state has been obtained. The disrotation-conrotation mechanism has been proposed and discussed with the Woodward-Hoffmann approach. The barrier height for the ring expansion is 38.89 kJ/ mol and the released energy is 232.21 kJ/mol. The relative structure data are given.
1. Introduction The concerted ring expansion of the biradical is an important kind of reaction in organic chemistry. Some specific characteristics of these reactions have been noted [ 1,2 1. For example, in studying the concerted ring expansion of cyclic carbenes with the Woodward-Hoffmann approach, Deng [ 31 has proposed the disrotation-conrotation or conrotation-disrotation mechanisms, and successfully explained the reactions. Nitrene (NH ) is an analogue of carbene, both are biradicals. Because of the higher stability of aromatic nitrene, nitrene chemistry deals mainly with aromatic nitrene. The stability of nonaromatic nitrene is rather low and the knowledge about this kind of compounds is scanty at present both theoretically and experimentally [ 5,6]. In addition, experimental
studies showed that the reaction of concerted ring expansion of phenyl nitrene takes place easily and cyclic imine is produced [ 7,8 1. Thus the isomerization reactions by concerted ring expansion of cycle hydrocarbyl nitrenes are probably efficient methods of synthesizing heteronitrogen compounds. It is thus evident that the studies of the mechanisms of the isomerization reactions of cycle hydrocarbyl nitrenes by concerted ring expansion have theoretical and practical significance. But up to now we have not found any report in the literature on a theoretical study of these reactions, and the mechanisms of these reactions are not yet clear. In this Letter we study the mechanism of the concerted ring expansion of singlet cyclopropyl nitrene using an ab initio method and explain the mechanism with the Woodward-Hoffmann approach.
* Supported by the National Natural Science Foundation of China. * Corresponding author. 0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO9-2614(94)00921-X
H. Sun et al. /Chemical Physics Letters 228 (1994) 268-272
2. Calculation method
The Hartree-Fock (HF) method is used to calculate the singlet reactant, transition state and product. The geometries are optimized by using the gradient technique at the 3-2 1G level. The force constant matrix has only one negative eigenvalue at the transition state. The convergence criteria for geometry optimizations are that the maximum force of the atomic inner coordinates (bond length and bond angle) in the molecule systems are smaller than 1.2~ 10m2 eV a< ’ and 1.2 x 10 -’ eV radian- ‘, respectively. The calculated deviations of bond length, bond angle and self-consistent field are within 0.000 1 nm, 0.1’ and lo-‘, respectively. All calculations are performed with the Gaussian-86 program.
(a)
269
3. Results and discussions 3.1. Geometries of the reactant, transition state and product The optimized geometries of the reactant, transition state (TS) and product with 3-2 1G are given in Fig. 1. Let us discuss the geometries in Fig. 1 in the light of chemical bonding. In molecular orbital theory a chemical bond is considered to be formed from the interactions of the relative atomic orbitals. In the discussions of these interactions we consider only the atomic orbitals with larger coefficients in the molecular orbitals corresponding to the bonds. Cyclopropyl nitrene. In the molecule there is a conjugate bond rr: formed from the hyperconjugate in-
(W
Fig. 1. The optimized geometries of the reactant, transition state and product with 3-2 1G. Bond lengths in nm, bond angles in deg. (a) Cyclopropyl nitrene; (b) transition state; (c) I-hetero nitrogen cycle butene. Dihedral angles (deg): HICIz3= - 101.6 (a), - 115.5 (b), -113.7 (c). H2C,23=102.7 (a), 90.9 (b), 113.7 (c). H&&,=-102.7 (a), 110.9 (b), 114.5 (c). H&,p=101.6 (a), -110.6 (b), - 114.5 (c). HSCSzI= 102.7 (a), 244.4 (b), 180.0 (c). NC JL2= 103.0 (a), NCS2,=50.1 (b), 0.1 (c).
270
H. Sun et al. / Chemical Physics Letters 228 (1994) 268-272
teraction of the lone-electron pair p orbital at N with the conjugate bond pseudo x: [ 91 on the cyclopropyl ring. The p lone electron pair dislocates strongly to the ring. Thus the bond lengths of Cr-C2 and C3-N in the molecule are rather short. In the geometry optimization no particular symmetry restriction is imposed on the molecule. But the calculations show that the molecule has the symmetry C,. Transitionstate. The process forming the transition state is rather complicated. The calculations show that to reach the saddle point, the two terminal groups, CIHz and C3N, disrotate while the angle C,C& expands. Because of the rotations of the two groups the bond Ci-Cs partly breaks. The hybrid fashions of the atomic orbitals of Cr and C3 change from sp3 to sp*. At this time each of the atoms Ci and Cs has an unbonded p orbital with one electron. These two p orbitals interact with the vacant p orbital at N forming the x$ bond. I-Hetero nitrogen cycle butene. After having reached the saddle point, the terminal group C3N continues to rotate forward and the terminal group ClH2 reverses its direction of rotation, and the reao tion proceeds in a conrotation fashion this time. Because of the rotations of the two groups the bond CiCs breaks down. The electron on the p orbital of the atom CJ transfers to the vacant p orbital of the atom N while the hybrid fashion of the atomic orbitals of N changes from sp to sp*, and that of C, from sp* to sp3. Then the one-electron sp* orbital at N interacts with the one-electron sp3 orbital at C,, forming the o bond C-N, and the lone-electron pair orbital at N with the vacant p orbital at C3, forming a tt bond. Thus the bond between Ca and N is a double bond. This can be confirmed by the fact that the bond length of CS-N is 0.1277 nm (generally speaking, the length of the double bond C=N is 0.130 nm). 3.2. Activebarrierand releasedenergy The total and the relative energies of all singlet species are given in Table 1. The active barrier and the released energy are given in Table 2. The detinitions of the active barrier and the released energy are E,+ =E(TS)-E(R) and
, E, =E(TS)-E(P)
,
Table 1 Total energies (au) and relative energies (kJ/mol) of all singlet species (at the 3-2 1G level) Total energy (au)
Relative energy W/mol)
- 169.835503 - 169.820690 - 169.923958
0.0 38.89 -232.21
reactant TS product
Table 2 Active barriers and released energies (kJ/mol) E:
J%
AE
38.89
271.10
-232.21
AE=E(P)
-E(R),
respectively, where E(P), E(R) and E(TS) are the energies of product, reactant and transition state, respectively. The data in Tables 1 and 2 show that the isomerization of singlet cyclopropyl nitrene is an exothermic reaction. The active barrier E: is rather low, it is only 38.89 kJ/mol. This means that the isomerization of singlet cyclopropyl nitrene to 1-hetero nitrogen cycle butene takes place easily, the latter is thermodynamically more stable than the former. Cyclopropyl nitrene is more reactive than cyclopropyl carbene as concerns the isomerization reaction. 3.3. Charge distributionand orbitalinteraction The Mulliken populations of singlet reactant, transition state and product are given in Fig. 2. The charge populations in the valence orbitals of the skeleton atoms are given in Table 3. Fig. 2 shows that from the reactant to the transition state the negative charge of N increases from 0.302 to 0.427, the negative charge of Cr decreases from 0.221 to 0.186, and the positive charge of H2 increases from 0.176 to 0.198. This is an indication that the valence electrons transfer from the terminal group ClH2 to the vacant p orbital of N. Because of the orbital interactions the bond C-N is formed whose order is 0.132 while the bond Cl-C3 is weakened and its order decreases from 0.483 to 0.182. This is an indication that there is a x: bond in the transi-
IX Sun et al. / ChemicalPhysicsLetters 228 (I 994) 268-272
271
n4
(br
9
+o.rob Fig. 2. Mulliken populations of reactant, transition state and product. (a) Cyclopropyl nitrene; (b) transition state; (c) I-hetero nitrogen cycle butene.
state. This can also be seen from the data in Table 3. From the reactant to the transition state the 2p, charge of N changes from 0.248 to 1.026, and the charges on the three 2p orbitals of N are almost equal in the transition state. This means that the hybrid fashion of the orbitals of N changes from sp to sp2. After having reached the saddle point, the bond CrN continues to be strengthened until the o bond C,N is formed. Meanwhile the II bond between Cs and N is formed. This can be seen from the change of the order of the bond Cs-N; the bond order changes from 0.672 to 0.9 13. This indicates that the bond between C3 and N in 1-hetero nitrogen cycle butene is a typical double bond. Because of forming the double bond between Cs and N, the negative charge at N decreases from 0.427 to 0.361. This means that the II electrons tion
located between C3 and N are from the p lone-electron pair of N. 3.4. Analysis of the isomerization reaction The only negative eigenvalue of the force constant matrix at the transition state is -0.584. The corresponding reaction vector is V(2a+2c)=0.084R,-,,-0.285R,,-0.153
,
where the bond lengths and the bond angles are in bohr and radian respectively. From the vector it can
H. Sun et al. / ChemicalPhysicsLetters228 (1994) 268-272
272
Reactant
TS
Product
Cl 2Px 2P, 2P,
0.834 1.212 0.910
0.787 0.193 0.965
0.772 1.176 0.908
2Px 2P, 2P,
0.833 1.211 0.910
0.944 1.205 0.859
0.993 0.944 0.960
the p orbital at N with lone-electron pair interacts with the vacant p orbital at C3, forming the rt bond. The p lone-electron pair participates in the bond forming, four electrons, therefore, are involved after the transition state. It is of the Mobius type. According to the Woodward-Hoffmann approach the reaction can only proceed in a conrotation fashion after the transition state. Thus the concerted ring expansion of singlet cyclopropyl nitrene is via disrotation-conrotation processes.
C3 2P.x 2P, 2P,
0.794 1.060 1.143
0.856 0.975 0.832
0.993 0.994 0.784
4. Conclusions
N 2px 2P, 2P,
1.726 1.503 0.248
1.263 1.207 1.026
1.273 0.939 1.298
Table 3 Charge populations in the valence orbitals of the skeleton atoms (au) (singlet species)
c2
be seen that the main change from the reactant to the transition state is the decrease of the dihedral angle NC3C2CI, and the secondary change is the decrease of the angle NC3C2 and the dihedrals H2C1C2C3and HSC3C2C1.From the vector it can be confirmed that the transition state is at the saddle point in the reaction pathway from singlet cyclopropyl nitrene to lhetero nitrogen cycle butene. Based on the calculated results the reaction, from cyclopropyl nitrene to the transition state and then to 1-hetero nitrogen cycle butene, proceeds in a disrotation-conrotation fashion. This can be explained with the Woodward-Hoffmann approach. Before reaching the saddle point, the two terminal groups, C1H2 and C3N, d&rotate. When the bond Cl-C3 is partly broken the hybrid fashions of AOs of Ci and C3 change from sp3 to sp*. At this time each atom of Cr and C3 has a p orbital with one electron, The two p orbitals interact with the vacant p orbital of N, forming a rr: bond. Only two electrons are involved in this process. It is of the Htickel type. According to the Woodward-Hoffmann approach, the reaction can only proceed in a disrotation fashion before the transition state. After the transition state the terminal group C3N continues to rotate forward about the axis C2-C3 and the terminal group CIHz reverses its direction of rotation about the axis Ci-C2. Due to these rotations the bond Cl-C3 is broken while the p electron at C3 migrates to the vacant p orbital at N, and
The isomerization of cyclopropyl nitrene to l-hetero nitrogen cycle butene is via disrotation-conrotation. Two electrons are involved before the transition state and four electrons after that. They are of the Hiickel and the Mobius type respectively. These mechanisms can be extended to the concerted ring expansion of generalized cycle hydrocarbyl nitrene. In these reactions the atom N contributes zero and two electrons respectively before and after the transition state. Therefore the concerted ring expansions of cycle hydrocarbyl nitrenes with 4n + 2 electrons, for example O-N:, are via disrotation-conrotation, and that of cycle hydrocarbyl nitrenes with 4n electron, for example 0-N: and +z, are via conrotation-disrotation. ’
References [ 1] D.J. Pasto, M. Haley and D.M. Chipman, J. Am. Chem. Sot. 100 (1978) 5272. [2] A. Rouk, W.J. Bouma and L. Radom, J. Am. Chem. Sot. 107 (1985) 3780. [3] B.Z. Wang and C.H. Deng, Chem. Phys. Letters 142 (1987) [4] EZ. Wang and C.H. Deng, Tetrahedron 44 (1988) 7355. [ 51 P.A. Smith, in: Azides and nitrene, eds. E.F. Striven (Academic Press, Orlando, 1984) ch. 3. [6] P.V. Sudhakar and K. Lammertsma, J. Am. Chem. Sot. 113 (1991) 5219. [ 7 ] T. Fueno, V.B. Koutecky and J. Koutecky, J. Am. Chem. Sot. 105 (1983) 5547. [8]Y.Z. Li, J.P. Kirby, M.W. George, M. Poliakoff and G.B. Schuster, J. Am. Chem. Sot. 110 (1988) 8092. 191 G.X. Xu, L.M. Li and R.Z. Wu, Nature 3 (1988) 474.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 228 ( 1994) 268-272
Mechanism of the concerted ring expansion of singlet cyclopropyl nitrene * Huabin Sun a, Chengbu Liu b,*,Liming Zhao ‘, Lu Deng d ’ Institute of Military Medicine, Jinan Command, Jinan 250014, People’s Republic of China b Institute of Theoretical Chemistry, Shandong University, Jinan 250100, Peoples Republic of China c Shandong Agricultural Managerial Official College, Jinan 250100, People’s Republic of China ‘Department ofPhysics, Georgia Southern University, Statesboro, GA 30460, USA Received 11 April 1994; in final form 10 July 1994
Abstract The isomerization by concerted ring expanding of singlet cyclopropyl nitrene has been studied using the self-consistent field (SCF) method with the 3-21G basis set. The transition state has been obtained. The disrotation-conrotation mechanism has been proposed and discussed with the Woodward-Hoffmann approach. The barrier height for the ring expansion is 38.89 kJ/ mol and the released energy is 232.21 kJ/mol. The relative structure data are given.
1. Introduction The concerted ring expansion of the biradical is an important kind of reaction in organic chemistry. Some specific characteristics of these reactions have been noted [ 1,2 1. For example, in studying the concerted ring expansion of cyclic carbenes with the Woodward-Hoffmann approach, Deng [ 31 has proposed the disrotation-conrotation or conrotation-disrotation mechanisms, and successfully explained the reactions. Nitrene (NH ) is an analogue of carbene, both are biradicals. Because of the higher stability of aromatic nitrene, nitrene chemistry deals mainly with aromatic nitrene. The stability of nonaromatic nitrene is rather low and the knowledge about this kind of compounds is scanty at present both theoretically and experimentally [ 5,6]. In addition, experimental
studies showed that the reaction of concerted ring expansion of phenyl nitrene takes place easily and cyclic imine is produced [ 7,8 1. Thus the isomerization reactions by concerted ring expansion of cycle hydrocarbyl nitrenes are probably efficient methods of synthesizing heteronitrogen compounds. It is thus evident that the studies of the mechanisms of the isomerization reactions of cycle hydrocarbyl nitrenes by concerted ring expansion have theoretical and practical significance. But up to now we have not found any report in the literature on a theoretical study of these reactions, and the mechanisms of these reactions are not yet clear. In this Letter we study the mechanism of the concerted ring expansion of singlet cyclopropyl nitrene using an ab initio method and explain the mechanism with the Woodward-Hoffmann approach.
* Supported by the National Natural Science Foundation of China. * Corresponding author. 0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO9-2614(94)00921-X
H. Sun et al. /Chemical Physics Letters 228 (1994) 268-272
2. Calculation method
The Hartree-Fock (HF) method is used to calculate the singlet reactant, transition state and product. The geometries are optimized by using the gradient technique at the 3-2 1G level. The force constant matrix has only one negative eigenvalue at the transition state. The convergence criteria for geometry optimizations are that the maximum force of the atomic inner coordinates (bond length and bond angle) in the molecule systems are smaller than 1.2~ 10m2 eV a< ’ and 1.2 x 10 -’ eV radian- ‘, respectively. The calculated deviations of bond length, bond angle and self-consistent field are within 0.000 1 nm, 0.1’ and lo-‘, respectively. All calculations are performed with the Gaussian-86 program.
(a)
269
3. Results and discussions 3.1. Geometries of the reactant, transition state and product The optimized geometries of the reactant, transition state (TS) and product with 3-2 1G are given in Fig. 1. Let us discuss the geometries in Fig. 1 in the light of chemical bonding. In molecular orbital theory a chemical bond is considered to be formed from the interactions of the relative atomic orbitals. In the discussions of these interactions we consider only the atomic orbitals with larger coefficients in the molecular orbitals corresponding to the bonds. Cyclopropyl nitrene. In the molecule there is a conjugate bond rr: formed from the hyperconjugate in-
(W
Fig. 1. The optimized geometries of the reactant, transition state and product with 3-2 1G. Bond lengths in nm, bond angles in deg. (a) Cyclopropyl nitrene; (b) transition state; (c) I-hetero nitrogen cycle butene. Dihedral angles (deg): HICIz3= - 101.6 (a), - 115.5 (b), -113.7 (c). H2C,23=102.7 (a), 90.9 (b), 113.7 (c). H&&,=-102.7 (a), 110.9 (b), 114.5 (c). H&,p=101.6 (a), -110.6 (b), - 114.5 (c). HSCSzI= 102.7 (a), 244.4 (b), 180.0 (c). NC JL2= 103.0 (a), NCS2,=50.1 (b), 0.1 (c).
270
H. Sun et al. / Chemical Physics Letters 228 (1994) 268-272
teraction of the lone-electron pair p orbital at N with the conjugate bond pseudo x: [ 91 on the cyclopropyl ring. The p lone electron pair dislocates strongly to the ring. Thus the bond lengths of Cr-C2 and C3-N in the molecule are rather short. In the geometry optimization no particular symmetry restriction is imposed on the molecule. But the calculations show that the molecule has the symmetry C,. Transitionstate. The process forming the transition state is rather complicated. The calculations show that to reach the saddle point, the two terminal groups, CIHz and C3N, disrotate while the angle C,C& expands. Because of the rotations of the two groups the bond Ci-Cs partly breaks. The hybrid fashions of the atomic orbitals of Cr and C3 change from sp3 to sp*. At this time each of the atoms Ci and Cs has an unbonded p orbital with one electron. These two p orbitals interact with the vacant p orbital at N forming the x$ bond. I-Hetero nitrogen cycle butene. After having reached the saddle point, the terminal group C3N continues to rotate forward and the terminal group ClH2 reverses its direction of rotation, and the reao tion proceeds in a conrotation fashion this time. Because of the rotations of the two groups the bond CiCs breaks down. The electron on the p orbital of the atom CJ transfers to the vacant p orbital of the atom N while the hybrid fashion of the atomic orbitals of N changes from sp to sp*, and that of C, from sp* to sp3. Then the one-electron sp* orbital at N interacts with the one-electron sp3 orbital at C,, forming the o bond C-N, and the lone-electron pair orbital at N with the vacant p orbital at C3, forming a tt bond. Thus the bond between Ca and N is a double bond. This can be confirmed by the fact that the bond length of CS-N is 0.1277 nm (generally speaking, the length of the double bond C=N is 0.130 nm). 3.2. Activebarrierand releasedenergy The total and the relative energies of all singlet species are given in Table 1. The active barrier and the released energy are given in Table 2. The detinitions of the active barrier and the released energy are E,+ =E(TS)-E(R) and
, E, =E(TS)-E(P)
,
Table 1 Total energies (au) and relative energies (kJ/mol) of all singlet species (at the 3-2 1G level) Total energy (au)
Relative energy W/mol)
- 169.835503 - 169.820690 - 169.923958
0.0 38.89 -232.21
reactant TS product
Table 2 Active barriers and released energies (kJ/mol) E:
J%
AE
38.89
271.10
-232.21
AE=E(P)
-E(R),
respectively, where E(P), E(R) and E(TS) are the energies of product, reactant and transition state, respectively. The data in Tables 1 and 2 show that the isomerization of singlet cyclopropyl nitrene is an exothermic reaction. The active barrier E: is rather low, it is only 38.89 kJ/mol. This means that the isomerization of singlet cyclopropyl nitrene to 1-hetero nitrogen cycle butene takes place easily, the latter is thermodynamically more stable than the former. Cyclopropyl nitrene is more reactive than cyclopropyl carbene as concerns the isomerization reaction. 3.3. Charge distributionand orbitalinteraction The Mulliken populations of singlet reactant, transition state and product are given in Fig. 2. The charge populations in the valence orbitals of the skeleton atoms are given in Table 3. Fig. 2 shows that from the reactant to the transition state the negative charge of N increases from 0.302 to 0.427, the negative charge of Cr decreases from 0.221 to 0.186, and the positive charge of H2 increases from 0.176 to 0.198. This is an indication that the valence electrons transfer from the terminal group ClH2 to the vacant p orbital of N. Because of the orbital interactions the bond C-N is formed whose order is 0.132 while the bond Cl-C3 is weakened and its order decreases from 0.483 to 0.182. This is an indication that there is a x: bond in the transi-
IX Sun et al. / ChemicalPhysicsLetters 228 (I 994) 268-272
271
n4
(br
9
+o.rob Fig. 2. Mulliken populations of reactant, transition state and product. (a) Cyclopropyl nitrene; (b) transition state; (c) I-hetero nitrogen cycle butene.
state. This can also be seen from the data in Table 3. From the reactant to the transition state the 2p, charge of N changes from 0.248 to 1.026, and the charges on the three 2p orbitals of N are almost equal in the transition state. This means that the hybrid fashion of the orbitals of N changes from sp to sp2. After having reached the saddle point, the bond CrN continues to be strengthened until the o bond C,N is formed. Meanwhile the II bond between Cs and N is formed. This can be seen from the change of the order of the bond Cs-N; the bond order changes from 0.672 to 0.9 13. This indicates that the bond between C3 and N in 1-hetero nitrogen cycle butene is a typical double bond. Because of forming the double bond between Cs and N, the negative charge at N decreases from 0.427 to 0.361. This means that the II electrons tion
located between C3 and N are from the p lone-electron pair of N. 3.4. Analysis of the isomerization reaction The only negative eigenvalue of the force constant matrix at the transition state is -0.584. The corresponding reaction vector is V(2a+2c)=0.084R,-,,-0.285R,,-0.153
,
where the bond lengths and the bond angles are in bohr and radian respectively. From the vector it can
H. Sun et al. / ChemicalPhysicsLetters228 (1994) 268-272
272
Reactant
TS
Product
Cl 2Px 2P, 2P,
0.834 1.212 0.910
0.787 0.193 0.965
0.772 1.176 0.908
2Px 2P, 2P,
0.833 1.211 0.910
0.944 1.205 0.859
0.993 0.944 0.960
the p orbital at N with lone-electron pair interacts with the vacant p orbital at C3, forming the rt bond. The p lone-electron pair participates in the bond forming, four electrons, therefore, are involved after the transition state. It is of the Mobius type. According to the Woodward-Hoffmann approach the reaction can only proceed in a conrotation fashion after the transition state. Thus the concerted ring expansion of singlet cyclopropyl nitrene is via disrotation-conrotation processes.
C3 2P.x 2P, 2P,
0.794 1.060 1.143
0.856 0.975 0.832
0.993 0.994 0.784
4. Conclusions
N 2px 2P, 2P,
1.726 1.503 0.248
1.263 1.207 1.026
1.273 0.939 1.298
Table 3 Charge populations in the valence orbitals of the skeleton atoms (au) (singlet species)
c2
be seen that the main change from the reactant to the transition state is the decrease of the dihedral angle NC3C2CI, and the secondary change is the decrease of the angle NC3C2 and the dihedrals H2C1C2C3and HSC3C2C1.From the vector it can be confirmed that the transition state is at the saddle point in the reaction pathway from singlet cyclopropyl nitrene to lhetero nitrogen cycle butene. Based on the calculated results the reaction, from cyclopropyl nitrene to the transition state and then to 1-hetero nitrogen cycle butene, proceeds in a disrotation-conrotation fashion. This can be explained with the Woodward-Hoffmann approach. Before reaching the saddle point, the two terminal groups, C1H2 and C3N, d&rotate. When the bond Cl-C3 is partly broken the hybrid fashions of AOs of Ci and C3 change from sp3 to sp*. At this time each atom of Cr and C3 has a p orbital with one electron, The two p orbitals interact with the vacant p orbital of N, forming a rr: bond. Only two electrons are involved in this process. It is of the Htickel type. According to the Woodward-Hoffmann approach, the reaction can only proceed in a disrotation fashion before the transition state. After the transition state the terminal group C3N continues to rotate forward about the axis C2-C3 and the terminal group CIHz reverses its direction of rotation about the axis Ci-C2. Due to these rotations the bond Cl-C3 is broken while the p electron at C3 migrates to the vacant p orbital at N, and
The isomerization of cyclopropyl nitrene to l-hetero nitrogen cycle butene is via disrotation-conrotation. Two electrons are involved before the transition state and four electrons after that. They are of the Hiickel and the Mobius type respectively. These mechanisms can be extended to the concerted ring expansion of generalized cycle hydrocarbyl nitrene. In these reactions the atom N contributes zero and two electrons respectively before and after the transition state. Therefore the concerted ring expansions of cycle hydrocarbyl nitrenes with 4n + 2 electrons, for example O-N:, are via disrotation-conrotation, and that of cycle hydrocarbyl nitrenes with 4n electron, for example 0-N: and +z, are via conrotation-disrotation. ’
References [ 1] D.J. Pasto, M. Haley and D.M. Chipman, J. Am. Chem. Sot. 100 (1978) 5272. [2] A. Rouk, W.J. Bouma and L. Radom, J. Am. Chem. Sot. 107 (1985) 3780. [3] B.Z. Wang and C.H. Deng, Chem. Phys. Letters 142 (1987) [4] EZ. Wang and C.H. Deng, Tetrahedron 44 (1988) 7355. [ 51 P.A. Smith, in: Azides and nitrene, eds. E.F. Striven (Academic Press, Orlando, 1984) ch. 3. [6] P.V. Sudhakar and K. Lammertsma, J. Am. Chem. Sot. 113 (1991) 5219. [ 7 ] T. Fueno, V.B. Koutecky and J. Koutecky, J. Am. Chem. Sot. 105 (1983) 5547. [8]Y.Z. Li, J.P. Kirby, M.W. George, M. Poliakoff and G.B. Schuster, J. Am. Chem. Sot. 110 (1988) 8092. 191 G.X. Xu, L.M. Li and R.Z. Wu, Nature 3 (1988) 474.