reaction in acrolein, methylacrylate, methylmethacrylate, acryloylchloride, cinnamaldehyde and cinnamoylchloride. All calculations were performed at the HF ...
Reactivity of a,b-unsaturated carbonyl compounds towards nucleophilic addition reaction: a local hard–soft acid–base approach Paritosh Mondal,{ Kalyan Kr. Hazarika and Ramesh Ch. Deka*
Paper
Department of Chemical Sciences, Tezpur University, Napaam, Tezpur-784 028, Assam, India Received 11th February 2003, Accepted 5th March 2003 First published as an Advance Article on the web 18th March 2003
The Fukui function fk1 and local softness sk1 are assigned as reactivity parameters for nucleophilic addition reaction in acrolein, methylacrylate, methylmethacrylate, acryloylchloride, cinnamaldehyde and cinnamoylchloride. All calculations were performed at the HF level of theory using 6-31G, 6-31G** and TZV basis sets. The condensed local softness calculated using a Lo¨wdin population is compared with the local softness calculated from a Mulliken population. The most probable sites for nucleophilic attack on the a,bunsaturated carbonyl compounds are determined from local reactivity descriptors: they are quite reliable to predict the reactivity relative to atomic charges.
Introduction
Theoretical aspects
The chemical reactivity of a molecule is often interpreted in terms of charge distribution of its atoms. Atomic charges are used to indicate the preferred direction for a reagent to approach a substrate. Although the chemists have an intuitive feeling for the reactivity based on charge distribution, assigning precise reactivity from atomic charge failed in several cases.1,2 In recent years, reactivity descriptors3–7 such as hardness, softness, Fukui function etc. have emerged as powerful tools in predicting the reactive sites of molecules. These reactivity descriptors are derived from density functional theory. Global hardness and global softness8–12 represent the reactivity of a molecule as a whole. On the other hand, the Fukui function13 defines the reactivity of an atom in a molecule and it is a local property. Fukui function and local softness, which is closely related, are suited to describe the relative reactivity of different substrates. Pearson’s hard–soft acid–base (HSAB)12,14–16 principle suggested that hard–hard and soft–soft interactions are favorable over hard–soft interactions.17–19 Again it has been found that soft–soft interactions are preferred in the site of the maximum Fukui function, i.e. frontier control,20–22 but on the other hand, hard–hard interactions are preferred in the site of the minimum Fukui function,23,24 i.e. charge control. In this paper we have presented the reactivity parameters, the local softness sk1 and sk2 and Fukui functions fk1 and fk2 of a,b-unsaturated carbonyl compounds, namely acrolein (H2CLCHCHO), methylacrylate (H2CLCHCOOCH3), methylmethacrylate (H2CLC(CH3)COOCH3), cinnamaldehyde (C6H5CHLCHCHO), cinnamoylchloride (C6H5CHLCHCOCl) and acryloylchloride (H2CLCHCOCl), and the most reactive sites of nucleophilic attack were derived. In these compounds the carbonyl carbon is slightly positive, due to mesomeric and resonance effects. The b-carbon, which is conjugated with the carbonyl carbon develops partial positive charge. Experimentally it has been found that a nucleophile attacks at the b-carbon atom of a,b-unsaturated carbonyl compounds.25 Here we have investigated the reactive sites of these carbonyl compounds towards a nucleophile using a local hard–soft acid– base approach.
Within the framework of density functional theory (DFT)3 the global hardness (g)9–12 of an N electron system is defined as: ! 1 L2 E 1 L g~ ~ (1) 2 LN 2 2 LN vð~rÞ
{ Permanent address: Department of Chemistry, Darrang College, Tezpur–784 001, Assam.
24
vð~ rÞ
r
where E, N, m and v(r) are the energy, number of electrons, chemical potential and external potential, respectively. Again, the inverse of global hardness with a factor of ½ is defined as global softness (S), 1 LN S~ ~ (2) 2g L vð~rÞ The local property, Fukui function4 f(r) is defined as the r derivative of electron density, r(r), with respect to the number r of electrons at a constant external potential, v(r). Lrð~ rÞ f ð~ rÞ~ (3) LN vð~rÞ r Similarly local softness, s(r), is defined as the derivative of r electron density, r(r), with respect to the chemical potential, m, at constant external potential, Lrð~ rÞ Lrð~ rÞ LN sð~ rÞ~ ~ : ~f ð~ rÞ:S (4) L vð~rÞ LN vð~rÞ L vð~rÞ
Applying the finite difference approximation, three types of condensed Fukui functions23 are found from electronic population analyses.26 fkz ð~ rÞ~qk ðNz1Þ{qk ðN Þ, governing the nucleophilic attack on the system fk{ ð~ rÞ~qk ðN Þ{qk ðN{1Þ, governing the electrophilic attack on the system fk0 ð~ rÞ~½qk ðNz1Þ{qk ðN{1Þ=2, governing the
(5)
(6)
(7)
radical attack on the system where, qk(N), qk(N 1 1) and qk(N 2 1) are atomic charges for atom k in the N, (N 1 1) and (N 2 1)-electron systems. The Fukui functions fk1 and fk2 and local softness sk1 and
PhysChemComm, 2003, 6(6), 24–27 This journal is # The Royal Society of Chemistry 2003
DOI: 10.1039/b301675g
sk2 are calculated from SCF calculation using 6-31G, 6-31G** and TZV basis sets27,28 and all these calculations are performed with the program GAMESS.29
Results and discussion
preferable protonation sites in aniline and substituted anilines in the gas phase. Thus, the Fukui function and the local softness are found to be superior to atomic charges in determining the reactive sites in molecules. Fukui function
The a,b-unsaturated carbonyl compounds contain two reactive sites, the carbonyl group site and the carbon–carbon double bond site. Hence, two types of nucleophilic addition reaction can be assigned as the direct addition reaction and conjugated addition reaction. However, to what extent a given nucleophile undergoes the direct addition and conjugated addition is dependent on the steric and electronic factors. If the carbonyl carbon is more sterically congested and weakly electrophilic, the conjugated addition will occur more readily than the direct addition, on the other hand if the b-carbon is more sterically congested the direct addition will take place predominantly. Thus the nucleophilic reagents add to the conjugated system in such a way so as to form the most stable intermediate anion.30 The tendency of the a,b-unsaturated carbonyl compounds to undergo nucleophilic addition is not simply due to the electron withdrawing ability of the carbonyl group, but to the existence of a conjugated system that permits the formation of the resonance stabilized anion. Theoretically atomic charges, Fukui functions and local softness values can be calculated for each atom in the molecule and they provide an insight in directing the incoming nucleophiles for the attack on the carbon atoms of the a,b-unsaturated carbonyl compounds. Atomic charges The atomic charges derived from Mulliken population analysis (MPA) and Lo¨wdin population analysis (LPA) for Ca, Cb and Ccarbonyl atoms of a,b-unsaturated carbonyl compounds are given in Table 1. It is seen from Table 1 that both MPA and LPA derived charges have negative values for Ca and Cb atoms. Since the charges of the Ccarbonyl atoms are positive, a nucleophile should attack at the carbonyl atoms. However, experimentally it has been found that in most of the cases the nucleophilic addition takes place at the b-carbon atom. Hence it is difficult to predict the reactive sites of these compounds from atomic charge values. In our recent studies,31,33 Fukui function and local softness appear to be better reactivity descriptors for studying the acidity and basicity of zeolites. Roy et al.32 used the reactivity descriptors to study the most
The Fukui function values for the Ca, Cb and Ccarbonyl atoms are presented in Table 2. Both MPA and LPA derived Fukui functions for the Cb and Ccarbonyl atoms are positive. The negative Fukui function is observed for the Ca atom in acrolein and acryloyl chloride. The negative values of Fukui functions indicate a very low probability for nucleophilic attack to take place at those sites. Recently, Hirao and co-workers,31,32 suggested that the Fukui function of an atom should be always positive. They found that the Fukui function calculated using Hirshfeld population analysis become positive although MPA derived Fukui function for some atoms become negative in the same calculation. In our present study, the LPA derived Fukui functions are found to be positive for all the atoms. While comparing the Fukui function values calculated using 6-31G, 6-31G** and TZV basis sets, it appears that the b-carbon of each of the molecule has maximum values. This indicates that the incoming nucleophile will preferably attack the b-carbon atom which is in agreement with the experimental results.25 Moreover, the Fukui function values for b-carbon with less bulky groups at a and b positions are more than those of the molecules where more bulky groups are present at a and b positions. For all the molecules the reactivity of the atoms for nucleophilic attack decreases in the order: Cb w Ccarbonyl w Ca while considering the MPA derived Fukui functions. The LPA derived Fukui functions also predict the same trend bearing a disorder in case of methylacrylate. Local softness The local softness values of all the molecules calculated with different basis sets such as 6-31G, 6-31G** and TZV are given in Table 3. Like Fukui functions, local softness values predict the similar trend of reactivity. The reactivity of the atoms for nucleophilic attack decreases in the order: Cb w Ccarbonyl w Ca bearing slight disorder for LPA derived softness values. From the softness values a reactivity order for molecules could be derived. From Table 2 and Table 3 it is seen that Fukui function and local softness values are more at the b-carbon atoms in the molecules with less bulky groups. Hence, b-carbon
Table 1 The MPA and LPA derived charges of Ca, Cb and Ccarbonyl atoms of a,b-unsaturated carbonyl compounds Mulliken charge
Lo¨wdin charge
Compound
Basis Set
Ca
Cb
Ccarbonyl
Ca
Cb
Ccarbonyl
Acrolein Methylacrylate Methylmethacrylate Acryloyl chloride Cinnamaldehyde Cinnamoyl chloride
6-31G
20.260 20.198 20.031 20.194 20.315 20.241
20.289 20.304 20.337 20.279 20.081 20.072
0.305 0.759 0.771 0.246 0.309 0.241
20.214 20.185 20.089 20.200 20.235 20.229
20.138 20.145 20.171 20.117 20.009 20.012
0.184 0.363 0.366 20.205 0.187 20.213
Acrolein Methylacrylate Methylmethacrylate Acryloyl chloride Cinnamaldehyde Cinnamoyl chloride
6-31G**
20.235 20.209 20.090 20.196 20.301 20.259
20.215 20.229 20.259 20.209 20.033 20.029
0.374 0.787 0.810 0.392 0.381 0.395
20.183 20.156 20.054 20.166 20.206 20.194
20.138 20.146 20.174 20.124 20.015 0.001
0.153 0.278 0.279 0.163 0.151 0.165
Acrolein Methylacrylate Methylmethacrylate Acryloyl chloride Cinnamaldehyde Cinnamoyl chloride
TZV
20.326 20.342 20.161 20.278 20.321 20.260
20.220 20.198 20.244 20.209 20.146 20.136
0.328 0.645 0.651 0.358 0.345 0.349
20.251 20.211 20.158 20.253 20.256 20.266
20.129 20.133 20.156 20.107 20.018 20.002
0.213 0.368 0.378 0.109 0.219 0.119
PhysChemComm, 2003, 6(6), 24–27
25
Table 2 The MPA and LPA derived Fukui function at Ca, Cb and Ccarbonyl atoms of a,b-unsaturated carbonyl compounds MPA
LPA
Compound
Basis set
Ca
Cb
Ccarbonyl
Ca
Cb
Ccarbonyl
Acrolein Methylacrylate Methylmethacrylate Acryloyl chloride Cinnamaldehyde Cinnamoyl chloride
6-31G
20.045 0.043 0.001 20.033 0.018 0.017
0.180 0.162 0.169 0.177 0.133 0.139
0.125 0.109 0.129 0.082 0.056 0.033
0.025 0.138 0.101 0.027 0.067 0.058
0.294 0.284 0.269 0.286 0.204 0.214
0.191 0.102 0.123 0.139 0.093 0.068
Acrolein Methylacrylate Methylmethacrylate Acryloyl chloride Cinnamaldehyde Cinnamoyl chloride
6-31G**
20.021 0.066 0.039 20.007 0.038 0.039
0.178 0.163 0.170 0.175 0.126 0.133
0.118 0.114 0.129 0.098 0.053 0.046
0.068 0.171 0.137 0.074 0.094 0.090
0.295 0.292 0.275 0.289 0.189 0.197
0.181 0.098 0.110 0.136 0.092 0.070
Acrolein Methylacrylate Methylmethacrylate Acryloyl chloride Cinnamaldehyde Cinnamoyl chloride
TZV
20.010 0.092 20.009 20.004 0.052 0.037
0.241 0.247 0.276 0.243 0.153 0.163
0.163 0.089 0.101 0.101 0.082 0.050
0.049 0.159 0.109 0.053 0.080 0.075
0.333 0.333 0.318 0.323 0.217 0.223
0.203 0.099 0.123 0.139 0.108 0.071
Table 3 The MPA and LPA derived local softness of Ca, Cb and Ccarbonyl atoms of a,b-unsaturated carbonyl compounds MPA
LPA
Compound
Basis set
Ca
Cb
Ccarbonyl
Ca
Cb
Ccarbonyl
Acrolein Methylacrylate Methylmethacrylate Acryloyl chloride Cinnamaldehyde Cinnamoyl chloride
6-31G
20.121 0.106 0.002 20.078 0.056 0.054
0.482 0.403 0.421 0.421 0.407 0.439
0.335 0.272 0.318 0.195 0.171 0.104
0.068 0.342 0.249 0.063 0.203 0.182
0.786 0.706 0.667 0.679 0.626 0.673
0.509 0.254 0.305 0.332 0.285 0.215
Acrolein Methylacrylate Methylmethacrylate Acryloyl chloride Cinnamaldehyde Cinnamoyl chloride
6-31G**
20.050 0.156 0.093 20.017 0.112 0.120
0.469 0.389 0.405 0.433 0.369 0.409
0.302 0.273 0.306 0.241 0.154 0.141
0.177 0.407 0.327 0.182 0.274 0.277
0.772 0.695 0.653 0.714 0.550 0.608
0.486 0.234 0.263 0.337 0.268 0.216
Acrolein Methylacrylate Methylmethacrylate Acryloyl chloride Cinnamaldehyde Cinnamoyl chloride
TZV
20.026 0.234 20.022 20.011 0.163 0.120
0.668 0.631 0.702 0.643 0.481 0.519
0.453 0.229 0.258 0.266 0.258 0.162
0.137 0.406 0.277 0.140 0.253 0.239
0.923 0.852 0.809 0.856 0.681 0.709
0.564 0.253 0.3139 0.367 0.340 0.227
atoms of a,b-unsaturated carbonyl compounds are more reactive and prone for nucleophilic attacks when less bulky groups are present.
Acknowledgements
Conclusions
References
Local reactivity descriptors are shown to be very powerful in predicting the reactivity of a,b-unsaturated compounds. From the Fukui functions and local softness values it is concluded that the b-carbon atoms of the a,b-unsaturated carbonyl compounds are more reactive towards a nucleophile. The reactivities of the three types of carbon atoms are found to decrease in the order: Cb w Ccarbonyl w Ca in all of the molecules. The local softness values are dependent on the substituents present at the b-carbon atoms. The local softness values of b-carbon atoms with less bulky groups are greater than those with more bulky groups. These results provide an insight in comparing the intermolecular reactivities of various a,b-unsaturated carbonyl compounds. 26
PhysChemComm, 2003, 6(6), 24–27
RCD is grateful to the Department of Science and Technology (DST), New Delhi, India for financial support.
1 L. Salem, Electrons in Chemical Reaction: First Principles, J. Wiley, New York, 1982. 2 W. Langenaeker, K. Demel and P. Geerling, J. Mol. Struct. (THEOCHEM), 1992, 259, 317. 3 R. G. Parr and W. Yang, Density Functional Theory of Atoms and Molecules, Oxford University Press, New York, 1989. 4 R. G. Parr and W. Yang, J. Am. Chem. Soc., 1984, 106, 4049. 5 W. Yang and R. G. Parr, Proc. Natl. Acad. Sci. USA, 1985, 82, 6723. 6 S. K. Ghosh and M. Berowitz, J. Chem. Phys., 1985, 83, 2976. 7 R. K. Roy, S. Krishnamurty, P. Geerlings and S. Pal, J. Phys. Chem., 1998, 102, 3746. 8 M. Berkowitz and R. G. Parr, J. Phys. Chem., 1988, 88, 2554. 9 R. G. Pearson, J. Am. Chem. Soc., 1963, 83, 3533. 10 R. G. Pearson, Chemical Hardness: Applications from Molecules to Solid, Wiley-VCH Verlag GMBH, Weinheim, 1997.
11 R. G. Pearson, Science, 1966, 151, 172. 12 Chemical Hardness, Structure and Bonding, ed. K. D. Sen and D. M. P. Mingos, Springer-Verlag, Berlin, 1993. 13 C. Lee, W. Yang and R. G. Parr, J. Mol. Struct. (THEOCHEM), 1988, 163, 305. 14 R. G. Pearson, Coord. Chem. Rev., 1990, 100, 403. 15 Hard and Soft Acids and Bases, ed. R. G. Pearson, Dowden, Hutchinson and Ross, Stroudsburg, PA, 1973. 16 P. K. Chattaraj, J. Phys. Chem. A, 2001, 105, 511. 17 R. G. Pearson, J. Chem. Educ., 1987, 64, 561. 18 R. G. Parr and P. K. Chattaraj, J. Am. Chem. Soc., 1991, 113, 1854. 19 P. K. Chattaraj, G. H. Liu and R. G. Parr, Chem. Phys. Lett., 1995, 237, 171. 20 K. Fukui, Science, 1982, 217, 747. 21 K. Fukui, T. Yonezawa and H. Shingu, J. Chem. Phys., 1952, 20, 722. 22 K. Fukui, T. Yonezawa, C. Nagata and H. Shingu, J. Chem. Phys., 1954, 22, 1433. 23 W. Yang and W. J. Mortier, J. Am. Chem. Soc., 1986, 108, 5708. 24 Chemical Reactivity and Reaction Paths, ed. G. Klopman, Wiley, New York, 1974; G. Klopman, J. Am. Chem. Soc., 1968, 90, 223.
25 S. Patai and Z. Rappoport, in The Chemistry of Alkenes, ed. S. Patai, Interscience Publishers, London, 1964, p. 469. 26 R. S. Mulliken, J. Chem. Phys., 1955, 23, 1833. 27 R. Ditchfield, W. J. Hehre and J. A. Pople, J. Chem. Phys., 1971, 54, 724. 28 P. C. Hariharan and J. A. Pople, Mol. Phys., 1974, 27, 209. 29 GAMESS, General Atomic and Molecular Electronic Structure System, Department of Chemistry, North Dakota State University and Ames Laboratory, Iowa State University: M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S. Gordon, J. H. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S. J. Su, T. L. Windus, M. Dupuis and J. A. Montgomery, J. Comput. Chem., 1993, 14, 1347. 30 R. T. Morrison and R. N. Boyd, in Organic Chemistry, PrenticeHall, Inc., Englewood Cliffs, NJ, 6th edn., 1997. 31 R. C. Deka, R. K. Roy and K. Hirao, Chem. Phys. Lett., 2000, 332, 576. 32 R. K. Roy, K. Hirao and S. Pal, J. Chem. Phys., 2001, 133, 1372. 33 R. C. Deka, R. Vetrivel and S. Pal, J. Phys. Chem. A, 1999, 103, 5978.
PhysChemComm, 2003, 6(6), 24–27
27
