- Email: [email protected]
Ab initio studies on the mechanism of cycloaddition reactions of isocyanic acid and methylenimine
THEO CHEM Journal of Molecular Structure (Theochem) 365 (1996) 219-223
Ab initio studies on the mechanism of cycloaddition reactions of isocyanic acid and methylenimine De-Cai Fang, Xiao-Yuan Fu* Chemistry Department, Beijing Normal Universiiy, Beijing 100875. People’s Republic of China
Received 20 September 1995; accepted 9 November 1995
Abstract The cycloaddition reactions of isocyanic acid and methylenimine leading to four- or six-membered ring products were studied theoretically. If the reaction is equimolar, a two-step mechanism via a cis intermediate is confirmed. The second step is a rate-controlling one with an energy barrier of 28.3 kcal mol-’ In addition, the cis intermediate (INT) can further react with another methylenimine or isocyanate molecule to form a six-membered ring adduct. For the system INT + H$=NH, the energy barrier is only 2.76 kcal mol-‘, while for the system INT + HN=C=O, it is 5.02 kcal mol-’ . This is consistent with the experimental fact that when one of the reactants is in excess, a six-membered ring adduct
is formed exclusively.
Keywords: Ab initio calculation; Cycloaddition; Isocyanic acid; Methylenimine
1. Introduction Imines undergo a [2 + 21 cycloaddition reaction with isocyanates to give 1 : 1 adducts [l-7]. If an excess of the imine or isocyanate is used, a sixmembered ring product will form exclusively [1,5-71. A stepwise dipolar mechanism was proposed by Ulrich in 1969 [7] (see Scheme 1) and, to our knowledge, no theoretical studies have been reported on this reaction mechanism. In this paper, we have used isocyanic acid and methylenimine as model compounds to study the reaction mechanism theoretically.
of the reactants, product and transition states (TSs) were optimized with RHF/6-31G and RHF/631G* methods, while for the mole ratio 1 : 2 or 2: 1 reactions, only the RHF/6-31G method was used to save CPU time. Vibrational analysis, at the same level as the optimized geometries, was carried out to check whether the geometries obtained here are really of the TS or not. All the calculations were done with the GAUSSIAN 92 program [8] on a DEC workstation computer.
3. Results and discussion 3.1. Reaction with mole ratio 1: 1
2. Methods of calculations For the equimolar * Corresponding author.
(1 : 1) reaction,
the geometries
We have located the stationary points on the potential energy surface of the titled reaction with the ab initio energy gradient method with 6-31G and 6-31G* basis sets. Two transition states (TSl
0166-1280/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0166.1280(95)04444-2
D.-C. Fan, X.-Y. Fu/Journal of Molecular Structure (Theochem)
220
365 (1996) 219-223
R’CH=Nl? + R’N=C=O
2:l
P2
I:2
I:1
P3
PI Scheme 1
and TS2) and one intermediate (INT) have been found on this reaction path. The TSs located at the HF/6-31G and HF/6-31G* levels have only one imaginary vibration frequency each, corresponding to the motion along the reaction coordinate. The geometries of the stationary points are given in Table 1, which shows that first the TS 1 and INT have Cs symmetry and a cis structure (see Fig. l(a)), with the Ni-C2 bond is being formed, while TS2 has a twisted structure (see Fig. l(b)),
with the dihedral angle LC4N1C3N4 = -18.2” (6-31G) or -18.7” (6-31G*). Consequently, a cis intermediate will be formed due to the nucleophilic attack of the nitrogen associated with the methylenimine on the carbon of the isocyanic acid. This process can also be rationalized by the frontier orbital interaction (see Fig. 2). Due to the interaction between the LUMO of isocyanic acid and the HOMO of methylenimine, 0.25 e (for TSl) and 0.31 e (for INT) have been transferred from
Table 1 The geometrical parameters for the optimized stationary points of the 1: 1 reaction INT
TSl 6-31G Cz-NI N3--Cz C4-N1 C4-N3 08-c2
WY, C4N
C2
O&2N, C4NCZN3
HsWCZW WzN,W
1.908 1.245 1.257 2.833 1.200 101.32 122.54
104.40
6-31G* 1.745 1.255 1.250 2.784
6-31G 1.565 1.273 1.261 2.692
1.186
1.238
104.03 125.59 106.82
108.97 124.16 109.72
0.0
0.0
0.0
180.0 180.0
180.0 180.0
180.0 180.0
Bond lengths in Angstroms, angles in degrees.
TS2 6-31G* 1.626 1.264 1.252 2.727 1.199 106.85 125.56 108.46 0.0
180.0 180.0
P
6-31G
6-31G*
1.473 1.331 1.323 2.133 1.222 100.56 106.54 122.98 -18.21 128.29 182.18
1.517 1.317 1.308 2.129 1.190 100.80 104.36 121.16 -18.69 113.19 181.89
6-31G 1.385 1.385 1.470 1.470 1.209 91.08 92.22 134.45 0.0 180.0 180.0
6-31G* 1.377 1.377 1.450 1.450 1.185 90.86 92.02 134.57 0.0 180.0 180.0
D.-C. Fan, X.-Y. FulJournal of Molecular Structure (Theochem)
if’&
Hf’\c,H7
221
365 (1996) 219-223
Ii,
“1
4\
Fig. 1. The numbering system for TS 1 and TS2, along with their imaginary vibrational modes. Reaction process
Fig. 3. The energy changes along the 1: 1 reaction path (the 6-31G results are in parentheses).
INT to product) is the rate-controlling step. The barrier of the rate-controlling step is 28.30 (6-31G) or 28.24 kcal mol-* (6-31G*). 3.2. Reaction with mole ratio 2 : 1
Fig. 2. The frontier orbital interaction between the HOMO of methylenimine and the LUMO of isocyanic acid.
methylenimine to isocyanic acid (at the HF/6-31G* level). The intermediate has some zwitterionic character. The INT can form a four-membered ring product via TS2, similar to the closed-ring reaction of cis-butadiene [9]. The relative energies for the stationary points are given in Fig. 3, which shows that the barrier of the first step (from R to INT) is 9.82 (6-31G) or 15.5 kcal mol-’ (6-3 lG*) and the second step (from
The INT can further react with a methylenimine molecule to form a six-membered ring product when one of the reactants, methylenimine, is in excess. We have located the transition state TS3 for this process at the HF/6-31G level (the geometric parameters are listed in Table 2). Furthermore, we have located a loose intermolecular complex (COM), in which there is a weak interaction between INT and methylenimine. The calculated binding energy at the HF/6-31G level is 9.85 kcal mol-’ . In COM, there is 0.013 e of charge transferred from methylenimine to INT, which would stabilize the complex.
Table 2 The geometries of TS3 and COM at the HF/6-31G level (see Fig. 4 for the numbering system)
N2--CI C3-N2 N4--C3 C5-N4 N6-C5 N6-G 09-G
TS3
COM
3.147 1.298 1.475 1.306 2.026 1.267 1.247
3.500 1.284 1.521 1.269 2.657 1.267 1.243
C3N2CI
N&3& GN4C3
N&N4 09C3N4
Bond lengths in Angstroms, angles in degrees.
TS3
COM
98.90 112.07 124.00 110.40 112.81
100.31 110.73 124.81 110.88 110.94
WWZCI CSNWZ N&N&3 09C3N4C5
TS3
COM
47.69 20.17 -61.41 -171.19
50.17 64.30 -83.12 -174.08
222
D.-C. Fan, X.-Y. FulJournal of Molecular Structure (Theochem)
36.5 (1996) 219-223
Table 3 The geometries of TS4 at the HF/6-31G level (see Fig. 5 for the numbering system) Bond length (A) Bond angle (deg)
Dihedral angle (deg)
N,-C, C3-Nz
N,C3N2C, &N&N2 N&NJ3 07C,NZC3 O&N&
N4-C3
CS-N4 Ns-C5 N6-Cl 07-G
Fig. 4. The numbering system for TS3 and COM.
In spite of the fact that the potential energy surface was scanned extensively, only COM and TS3 were found. Therefore, this process takes place in a concerted but asynchronous manner. The calculated activation barrier (from COM to P2) at the HF/6-31G level is 2.76 kcal mol-‘, implying that the reaction proceeds very easily. Our calculated results are consistent with the fact that if an excess of imine is used, a six-membered ring is formed exclusively [l-7]. 3.3, Reaction with mole ratio I : 2 In this section, we will investigate the case of excess isocyanic acid. We have located the transition state TS4. Vibrational analysis shows that TS4 has only one imaginary frequency and its vibrational mode corresponds to the motion along the reaction coordinate (see Fig. 5). The geometric parameters are listed in Table 3, which indicates that in TS4 the bond lengths Cl-N2 and N6-Cs are 2.157 and 2.475 A respectively. The structure of TS4 is a twisted six-membered ring. We have tried to explore the potential energy surface further, but
OS-C3
2.157 1.301 1.490 1.273 2.475 1.245 1.176 1.234
C3NZC, N&N2 C5N4C3 N&N4 07C1NZ O&N4
127.03 109.94 122.92 99.82 98.55 113.85
47.69 20.17 -61.41 139.33 -161.90
only TS4 has been found. The activation barrier of this reaction is calculated to be 5.02 kcal mol-‘, which is very low. The product also has a sixmembered ring, but its atomic constitution differs from the one discussed above. 4. Conclusions The results can be summarized as follows. (1) Ab initio studies indicate that the cycloaddition of methylenimine with an isocyanic acid, leading to a four-membered ring product, takes place in a two-step mechanism via a dipolar cis intermediate. The second step is rate determining. (2) The cis intermediate can react further with another methylenimine or isocyanic acid molecule to form a six-membered ring adduct. Both processes proceed very easily, but the former is more favourable. The substituent effect of these reactions is to be studied later.
Acknowledgements This project was supported by the National Natural Science Foundation of China and the Foundation of the Chinese Education Commission.
References
Fig. 5. The numbering system for TS4
[l] R. Richter, Chem. Ber., 102 (1969) 938. [2] U. Kraatz, Tetrahedron L&t., (1973) 1219. [3] B.A. Arbuzov and N.N. Zobova, Synthesis, (1974) 461. [4] G. Entenmann, Tetrahedron Lett., (1974) 3279.
D.-C. Fan, X.-Y. FulJournal of Molecular Siructure (Theochem)
[S] J. Biideker and K. Courault, Tetrahedron, 34 (1978) 101. [6] R. Richter and H. Ulrich, in A. Hassner (Ed.), Small Ring Heterocycles, Part 2, Wiley, New York, 1983, p. 528. [7] H. Ulrich, Act. Chem. Res., (1969), 187. [8] M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, J.B. Foresman, B.G. Johnson, H.B. Schlegel, M.A. Robb, ES. Replogle, R. Comperts, J.L. Adnres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.J. Defrees, J. Baker, J.J.P. Stewart and J.A. Pople, GAUSSIAN 92, Carnegie-Mellon Quantum Chemistry Publ. Unit. Pittsburgh PA, 1992. [9] (a) W. Kirmse, N.G. Rondan and K.N. Houk, J. Am. Chem. Sot., 106 (1984) 7989.
365 (1996) 219-223
223
(b) N.G. Rondan and K.N. Houk, J. Am. Chem. Sot., 107 (1985) 2099. (c) K.N. Houk, D.C. Spellmayer, C.W. Jefford, C.G. Rimbault, Y. Wang and R.D. Miller, J. Org. Chem., 53 (1988) 2125. (d) DC. Spellmeyer and K.N. Houk, J. Am. Chem. Sot., 110 (1988) 3412. (e) A.B. Buda, Y. Wang and K.N. Houk, J. Org. Chem., 54 (1989) 2264. (f) E.A. Kallel, Y. Wang, D.C. Spellmeyer and K.N. Houk, J. Am. Chem. Sot., 112 (1990) 6759.
Ab initio studies on the mechanism of cycloaddition reactions of isocyanic acid and methylenimine De-Cai Fang, Xiao-Yuan Fu* Chemistry Department, Beijing Normal Universiiy, Beijing 100875. People’s Republic of China
Received 20 September 1995; accepted 9 November 1995
Abstract The cycloaddition reactions of isocyanic acid and methylenimine leading to four- or six-membered ring products were studied theoretically. If the reaction is equimolar, a two-step mechanism via a cis intermediate is confirmed. The second step is a rate-controlling one with an energy barrier of 28.3 kcal mol-’ In addition, the cis intermediate (INT) can further react with another methylenimine or isocyanate molecule to form a six-membered ring adduct. For the system INT + H$=NH, the energy barrier is only 2.76 kcal mol-‘, while for the system INT + HN=C=O, it is 5.02 kcal mol-’ . This is consistent with the experimental fact that when one of the reactants is in excess, a six-membered ring adduct
is formed exclusively.
Keywords: Ab initio calculation; Cycloaddition; Isocyanic acid; Methylenimine
1. Introduction Imines undergo a [2 + 21 cycloaddition reaction with isocyanates to give 1 : 1 adducts [l-7]. If an excess of the imine or isocyanate is used, a sixmembered ring product will form exclusively [1,5-71. A stepwise dipolar mechanism was proposed by Ulrich in 1969 [7] (see Scheme 1) and, to our knowledge, no theoretical studies have been reported on this reaction mechanism. In this paper, we have used isocyanic acid and methylenimine as model compounds to study the reaction mechanism theoretically.
of the reactants, product and transition states (TSs) were optimized with RHF/6-31G and RHF/631G* methods, while for the mole ratio 1 : 2 or 2: 1 reactions, only the RHF/6-31G method was used to save CPU time. Vibrational analysis, at the same level as the optimized geometries, was carried out to check whether the geometries obtained here are really of the TS or not. All the calculations were done with the GAUSSIAN 92 program [8] on a DEC workstation computer.
3. Results and discussion 3.1. Reaction with mole ratio 1: 1
2. Methods of calculations For the equimolar * Corresponding author.
(1 : 1) reaction,
the geometries
We have located the stationary points on the potential energy surface of the titled reaction with the ab initio energy gradient method with 6-31G and 6-31G* basis sets. Two transition states (TSl
0166-1280/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0166.1280(95)04444-2
D.-C. Fan, X.-Y. Fu/Journal of Molecular Structure (Theochem)
220
365 (1996) 219-223
R’CH=Nl? + R’N=C=O
2:l
P2
I:2
I:1
P3
PI Scheme 1
and TS2) and one intermediate (INT) have been found on this reaction path. The TSs located at the HF/6-31G and HF/6-31G* levels have only one imaginary vibration frequency each, corresponding to the motion along the reaction coordinate. The geometries of the stationary points are given in Table 1, which shows that first the TS 1 and INT have Cs symmetry and a cis structure (see Fig. l(a)), with the Ni-C2 bond is being formed, while TS2 has a twisted structure (see Fig. l(b)),
with the dihedral angle LC4N1C3N4 = -18.2” (6-31G) or -18.7” (6-31G*). Consequently, a cis intermediate will be formed due to the nucleophilic attack of the nitrogen associated with the methylenimine on the carbon of the isocyanic acid. This process can also be rationalized by the frontier orbital interaction (see Fig. 2). Due to the interaction between the LUMO of isocyanic acid and the HOMO of methylenimine, 0.25 e (for TSl) and 0.31 e (for INT) have been transferred from
Table 1 The geometrical parameters for the optimized stationary points of the 1: 1 reaction INT
TSl 6-31G Cz-NI N3--Cz C4-N1 C4-N3 08-c2
WY, C4N
C2
O&2N, C4NCZN3
HsWCZW WzN,W
1.908 1.245 1.257 2.833 1.200 101.32 122.54
104.40
6-31G* 1.745 1.255 1.250 2.784
6-31G 1.565 1.273 1.261 2.692
1.186
1.238
104.03 125.59 106.82
108.97 124.16 109.72
0.0
0.0
0.0
180.0 180.0
180.0 180.0
180.0 180.0
Bond lengths in Angstroms, angles in degrees.
TS2 6-31G* 1.626 1.264 1.252 2.727 1.199 106.85 125.56 108.46 0.0
180.0 180.0
P
6-31G
6-31G*
1.473 1.331 1.323 2.133 1.222 100.56 106.54 122.98 -18.21 128.29 182.18
1.517 1.317 1.308 2.129 1.190 100.80 104.36 121.16 -18.69 113.19 181.89
6-31G 1.385 1.385 1.470 1.470 1.209 91.08 92.22 134.45 0.0 180.0 180.0
6-31G* 1.377 1.377 1.450 1.450 1.185 90.86 92.02 134.57 0.0 180.0 180.0
D.-C. Fan, X.-Y. FulJournal of Molecular Structure (Theochem)
if’&
Hf’\c,H7
221
365 (1996) 219-223
Ii,
“1
4\
Fig. 1. The numbering system for TS 1 and TS2, along with their imaginary vibrational modes. Reaction process
Fig. 3. The energy changes along the 1: 1 reaction path (the 6-31G results are in parentheses).
INT to product) is the rate-controlling step. The barrier of the rate-controlling step is 28.30 (6-31G) or 28.24 kcal mol-* (6-31G*). 3.2. Reaction with mole ratio 2 : 1
Fig. 2. The frontier orbital interaction between the HOMO of methylenimine and the LUMO of isocyanic acid.
methylenimine to isocyanic acid (at the HF/6-31G* level). The intermediate has some zwitterionic character. The INT can form a four-membered ring product via TS2, similar to the closed-ring reaction of cis-butadiene [9]. The relative energies for the stationary points are given in Fig. 3, which shows that the barrier of the first step (from R to INT) is 9.82 (6-31G) or 15.5 kcal mol-’ (6-3 lG*) and the second step (from
The INT can further react with a methylenimine molecule to form a six-membered ring product when one of the reactants, methylenimine, is in excess. We have located the transition state TS3 for this process at the HF/6-31G level (the geometric parameters are listed in Table 2). Furthermore, we have located a loose intermolecular complex (COM), in which there is a weak interaction between INT and methylenimine. The calculated binding energy at the HF/6-31G level is 9.85 kcal mol-’ . In COM, there is 0.013 e of charge transferred from methylenimine to INT, which would stabilize the complex.
Table 2 The geometries of TS3 and COM at the HF/6-31G level (see Fig. 4 for the numbering system)
N2--CI C3-N2 N4--C3 C5-N4 N6-C5 N6-G 09-G
TS3
COM
3.147 1.298 1.475 1.306 2.026 1.267 1.247
3.500 1.284 1.521 1.269 2.657 1.267 1.243
C3N2CI
N&3& GN4C3
N&N4 09C3N4
Bond lengths in Angstroms, angles in degrees.
TS3
COM
98.90 112.07 124.00 110.40 112.81
100.31 110.73 124.81 110.88 110.94
WWZCI CSNWZ N&N&3 09C3N4C5
TS3
COM
47.69 20.17 -61.41 -171.19
50.17 64.30 -83.12 -174.08
222
D.-C. Fan, X.-Y. FulJournal of Molecular Structure (Theochem)
36.5 (1996) 219-223
Table 3 The geometries of TS4 at the HF/6-31G level (see Fig. 5 for the numbering system) Bond length (A) Bond angle (deg)
Dihedral angle (deg)
N,-C, C3-Nz
N,C3N2C, &N&N2 N&NJ3 07C,NZC3 O&N&
N4-C3
CS-N4 Ns-C5 N6-Cl 07-G
Fig. 4. The numbering system for TS3 and COM.
In spite of the fact that the potential energy surface was scanned extensively, only COM and TS3 were found. Therefore, this process takes place in a concerted but asynchronous manner. The calculated activation barrier (from COM to P2) at the HF/6-31G level is 2.76 kcal mol-‘, implying that the reaction proceeds very easily. Our calculated results are consistent with the fact that if an excess of imine is used, a six-membered ring is formed exclusively [l-7]. 3.3, Reaction with mole ratio I : 2 In this section, we will investigate the case of excess isocyanic acid. We have located the transition state TS4. Vibrational analysis shows that TS4 has only one imaginary frequency and its vibrational mode corresponds to the motion along the reaction coordinate (see Fig. 5). The geometric parameters are listed in Table 3, which indicates that in TS4 the bond lengths Cl-N2 and N6-Cs are 2.157 and 2.475 A respectively. The structure of TS4 is a twisted six-membered ring. We have tried to explore the potential energy surface further, but
OS-C3
2.157 1.301 1.490 1.273 2.475 1.245 1.176 1.234
C3NZC, N&N2 C5N4C3 N&N4 07C1NZ O&N4
127.03 109.94 122.92 99.82 98.55 113.85
47.69 20.17 -61.41 139.33 -161.90
only TS4 has been found. The activation barrier of this reaction is calculated to be 5.02 kcal mol-‘, which is very low. The product also has a sixmembered ring, but its atomic constitution differs from the one discussed above. 4. Conclusions The results can be summarized as follows. (1) Ab initio studies indicate that the cycloaddition of methylenimine with an isocyanic acid, leading to a four-membered ring product, takes place in a two-step mechanism via a dipolar cis intermediate. The second step is rate determining. (2) The cis intermediate can react further with another methylenimine or isocyanic acid molecule to form a six-membered ring adduct. Both processes proceed very easily, but the former is more favourable. The substituent effect of these reactions is to be studied later.
Acknowledgements This project was supported by the National Natural Science Foundation of China and the Foundation of the Chinese Education Commission.
References
Fig. 5. The numbering system for TS4
[l] R. Richter, Chem. Ber., 102 (1969) 938. [2] U. Kraatz, Tetrahedron L&t., (1973) 1219. [3] B.A. Arbuzov and N.N. Zobova, Synthesis, (1974) 461. [4] G. Entenmann, Tetrahedron Lett., (1974) 3279.
D.-C. Fan, X.-Y. FulJournal of Molecular Siructure (Theochem)
[S] J. Biideker and K. Courault, Tetrahedron, 34 (1978) 101. [6] R. Richter and H. Ulrich, in A. Hassner (Ed.), Small Ring Heterocycles, Part 2, Wiley, New York, 1983, p. 528. [7] H. Ulrich, Act. Chem. Res., (1969), 187. [8] M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, J.B. Foresman, B.G. Johnson, H.B. Schlegel, M.A. Robb, ES. Replogle, R. Comperts, J.L. Adnres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.J. Defrees, J. Baker, J.J.P. Stewart and J.A. Pople, GAUSSIAN 92, Carnegie-Mellon Quantum Chemistry Publ. Unit. Pittsburgh PA, 1992. [9] (a) W. Kirmse, N.G. Rondan and K.N. Houk, J. Am. Chem. Sot., 106 (1984) 7989.
365 (1996) 219-223
223
(b) N.G. Rondan and K.N. Houk, J. Am. Chem. Sot., 107 (1985) 2099. (c) K.N. Houk, D.C. Spellmayer, C.W. Jefford, C.G. Rimbault, Y. Wang and R.D. Miller, J. Org. Chem., 53 (1988) 2125. (d) DC. Spellmeyer and K.N. Houk, J. Am. Chem. Sot., 110 (1988) 3412. (e) A.B. Buda, Y. Wang and K.N. Houk, J. Org. Chem., 54 (1989) 2264. (f) E.A. Kallel, Y. Wang, D.C. Spellmeyer and K.N. Houk, J. Am. Chem. Sot., 112 (1990) 6759.