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5890

J. Org. Chem. 2001, 66, 5890-5896

Density Functional Theory Investigation of the Reactions of Isodihalomethanes (CH2X-X Where X ) Cl, Br, or I) with Ethylene: Substituent Effects on the Carbenoid Behavior of the CH2X-X Species David Lee Phillips*,† and Wei-Hai Fang*,‡ Departments of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S. A. R., P. R. China, and Beijing Normal University, Beijing 100875, P. R. China [email protected] Received June 11, 2001

We investigated the chemical reactions of isodihalomethane (CH2X-X) and CH2X radical species (where X ) Cl, Br, or I) with ethylene and the isomerization reactions of CH2X-X using density functional theory calculations. The CH2X-X species readily reacts with ethylene to give the cyclopropane product and an X2 product via a one-step reaction with barrier heights of ∼2.9 kcal/ mol for CH2I-I, 6.8 kcal/mol for CH2Br-Br, and 8.9 kcal/mol for CH2Cl-Cl. The CH2X reactions with ethylene proceed via a two-step reaction mechanism to give a cyclopropane product and X atom product with much larger barriers to reaction. This suggests that photocyclopropanation reactions using ultraviolet excitation of dihalomethanes most likely occurs via the isodihalomethane species and not the CH2X species. The isomerization reactions of CH2X-X had barrier heights of ∼14.4 kcal/mol for CH2I-I, 11.8 kcal/mol for CH2Br-Br, and 9.1 kcal/mol for CH2Cl-Cl. We compare our results for the CH2X-X carbenoids to results from previous calculations of the SimmonsSmith-type carbenoids (XCH2ZnX) and Li-type carbenoids (LiCH2X) and discuss their differences and similarities as methylene transfer agents. Introduction 1

The discovery by Simmons and Smith that a reagent prepared from a Zn/Cu couple and CH2I2 could efficiently convert a variety of alkenes into corresponding cyclopropanes has led to great progress in the synthesis of different cyclopropane derivatives. The Simmons-Smith reaction has become one of the most important synthetic methods in organic chemistry.1,2 A density functional theory calculation predicted the reaction between chloromethylzinc chloride (ClCH2ZnCl) and ethylene (which represents a model system for the Simmons-Smith reaction) does not occur easily as a result of a relatively high barrier of ∼25 kcal/mol on the reaction pathway.3 The Simmons-Smith reaction only occurs efficiently at high temperatures,1 and a number of experimental studies2 have been reported to modify the SimmonsSmith reaction and its reactivity. Ultraviolet photolysis of CH2I2 in the presence of olefins in room-temperature solutions4-7 can be used to make cyclopropanated products from olefins. Since these reactions occur with high stereospecificity and without much C-H insertion reaction, the methylene transfer agent (or carbenoid) is not a free carbene. †

The University of Hong Kong. Beijing Normal University. (1) Simmons, H. E.; Smith, R. D. J. Am. Chem. Soc. 1959, 81, 42564264. (2) Charette, A. B.; Marcoux, J.-F. Synlett 1995, 1197-1207 and references therein. (3) Bernardi, F.; Bottoni, A.; Miscione, G. P. J. Am. Chem. Soc. 1997, 119, 9, 12300-12305. (4) Blomstrom, D. C.; Herbig, K.; Simmons, H. E. J. Org. Chem. 1965, 30, 959-964. (5) Pienta, N. J.; Kropp, P. J. J. Am. Chem. Soc. 1978, 100, 655657. (6) Kropp, P. J.; Pienta, N. J.; Sawyer, J. A.; Polniaszek, R. P. Tetrahedron 1981, 37, 3229-3236. (7) Kropp, P. J. Acc. Chem. Res. 1984, 17, 131-137. ‡

Excitation of condensed phase CH2I2 by ultraviolet light,8-11 direct photoionization,12 and radiolysis13,14 produces photoproducts that have signature transient absorption bands at ∼385 nm with large intensity and ∼570 nm with moderate intensity. Several possible photoproducts such as trapped electrons, the CH2I radical, the CH2I2+ radical cation, and the isomer of CH2I2 have been proposed to be responsible for the ∼385 and/or ∼570 nm transient absorption bands.8-15 The solution phase ultraviolet photolysis of diiodomethane has also been examined by femtosecond transient absorption experiments.16-18 Although these femtosecond experiments exhibited similar behavior, three different interpretations of the results were given because of differences in the assignments of which photoproduct species was followed in a particular experiment.16-18 The ambiguity over which possible photoproduct species is actually responsible for the characteristic ∼385 nm intense transient absorption band led us to use transient resonance Raman spectros(8) Simons, J. P.; Tatham, P. E. R. J. Chem. Soc. A 1966, 854-859. (9) Mohan, H.; Rao, K. N.; Iyer, R. M. Radiat. Phys. Chem. 1984, 23, 505-508. (10) Maier, G.; Reisenauer, H. P. Angew. Chem., Int. Ed. Engl. 1986, 25, 819-822. (11) Maier, G.; Reisennauer, H. P.; Hu, J.; Schaad, L. J.; Hess, B. A., Jr. J. Am. Chem. Soc. 1990, 112, 5117-5122. (12) Andrews, L.; Prochaska, F. T.; Ault, B. S. J. Am. Chem. Soc. 1979, 101, 9-15. (13) Mohan, H.; Iyer, R. M. Radiat. Eff. 1978, 39, 97-101. (14) Mohan, H.; Moorthy, P. N. J. Chem. Soc., Perkin Trans. 2 1990, 277-282. (15) Brown, G. P.; Simons, J. P. Trans. Faraday Soc. 1969, 65, 3245-3257. (16) Schwartz, B. J.; King, J. C.; Zhang, J. Z.; Harris, C. B. Chem. Phys. Lett. 1993, 203, 503-508. (17) Saitow, K.; Naitoh, Y.; Tominaga, K.; Yoshihara, K. Chem. Phys. Lett. 1996, 262, 621-626. (18) Tarnovsky, A. N.; Alvarez, J.-L.; Yartsev, A. P.; Sundstro¨m, V.; Åkesson, E. Chem. Phys. Lett. 1999, 312, 121-130.

10.1021/jo010582i CCC: $20.00 © 2001 American Chemical Society Published on Web 08/01/2001

Reactions of Isodihalomethanes

copy experiments to clearly identify the photoproduct as isodiiodomethane (CH2I-I).19 We recently examined the chemical reactivity of CH2II, CH2I radical, and CH2I+ cation with ethylene using density functional theory calculations.20 We found that the CH2I-I species easily reacts with ethylene to produce cyclopropane and iodine molecule products via a one-step reaction with a barrier height ∼2.9 kcal/mol. However, the CH2I radical and CH2I+ cation have much more difficult reactions with ethylene to form cyclopropane product via a two-step reaction mechanism that forms relatively stable iodopropyl radical or iodopropyl cation intermediates and contains much larger barriers to reaction than that found for the CH2I-I species. These computational results and experimental results for the ultraviolet photolysis of CH2I2 in the solution phase indicates that the CH2I-I species is the methylene transfer agent for cyclopropanation reactions of olefins using ultraviolet excitation of CH2I2.20 Ultraviolet excitation of CH2I2 in the presence of olefins may provide a more efficient means of cyclopropanation than the Simmons-Smith reaction.20 We have observed a number of isopolyhalomethane species following ultraviolet photoexcitation (generally leading to excitation of n f σ* transitions localized on the C-X bonds) of polyhalomethanes in room-temperature solutions.21-25 It seems likely that formation of the isomer species via recombination of the two photofragments within the solvation shell in condensed phase environments is a general phenomena for many polyhalomethane molecules. We expect that there exists a similar reactivity of the isopolyhalomethane species with olefins. In this paper, we report density functional theory calculation results that investigate the chemical reactivity of several isopolyhalomethanes (CH2Cl-Cl, CH2BrBr, and CH2I-I) with ethylene to produce cyclopropane and halogen molecule products in order to elucidate the chemical reactivity of the CH2X-X species with olefins. We have also examined the corresponding reactions of the CH2Cl, CH2Br, and CH2I radicals with ethylene for comparison purposes and the isomerization reactions between the CH2X2 and CH2X-X species. We find that as the identity of the halogen atoms goes from I to Br to Cl, the barrier to reaction between both the CH2X-X and CH2X radical species with ethylene to form cyclopropane product increases, while the barrier for the isomerization reaction between the CH2X2 and CH2X-X species decreases. We discuss these trends in the cyclopropanation and isomerization reactions and probable implications for using isopolyhalomethanes and other carbenoids as methylene transfer agents.

Computational Details Density functional theory (DFT) was employed to examine the potential energy surfaces for addition reactions of ethylene (19) Zheng, X.; Phillips, D. L. J. Phys. Chem. A 2000, 104, 68806886. (20) Phillips, D. L.; Fang, W.-H.; Zheng, X. J. Am. Chem. Soc. 2001, 123, 4197-4203. (21) Zheng, X.; Phillips, D. L. Chem. Phys. Lett. 2000, 324, 175182. (22) Zheng, X.; Phillips, D. L. J. Chem. Phys. 2000, 113, 3194-3203. (23) Zheng, X.; Kwok, W. M.; Phillips, D. L. J. Phys. Chem. A 2000, 104, 10464-10470. (24) Zheng, X.; Fang, W.-H.; Phillips, D. L. J. Chem. Phys. 2000, 113, 10934-10946. (25) Kwok, W. M.; Ma, C.; Parker, A. W.; Phillips, D.; Towrie, M.; Matousek, P.; Phillips, D. L. J. Chem. Phys. 2000, 113, 7471-7478.

J. Org. Chem., Vol. 66, No. 17, 2001 5891 with CH2X-X molecules and CH2X radicals (where X ) Cl, Br, or I). B3LYP or UB3LYP levels of theory with C1 symmetry was used to fully optimize the stationary structures on the potential energy surfaces.26-30 The complete active space SCF (CASSCF) method31-35 was used only for examination of the CH2I-I species dissociation to CH2I + I or CH2 + I2 because these processes involve formation of different radicals. The CASSCF computations used 10 active electrons in eight orbitals as described in ref 20, and the CASSCF calculations are denoted by CAS(10,8) hereafter in the paper. Analytical frequency computations were done so as to confirm the optimized structure was at a minimum or at a first-order saddle point. Analytical frequency calculations were also used to find the zero-point energy correction for calculations of the relative energies. IRC (intrinsic reaction coordinate) calculations were done to confirm the transition states found connected the related reactants and products.36 A 6-311G** basis set was used for an initial search and to obtain the zero-point energies. The cc-pVTZ (for reactions containing Cl or Br atoms)37 and Sadlej-PVTZ (for reactions containing iodine) basis sets were used to optimize all the stationary structures found and to assess the dependence of the structure and energy on the basis set used for the calculations. The Sadlej-PVTZ basis set was contracted as (6s4p//3s2p),38 (10s6p4d//5s3p2d),39 and (19s15p12d4f//11s9p6d2f)40 for the H, C, and I atoms, respectively, and this basis set was comprised 290 basis functions contracted from 996 Gaussian functions. All of the calculations reported here made use of the Gaussian 98 program software suite.41 Relativistic effects may noticeably affect the computed energy of systems that contain heavy atoms (such as Br and I). However, we are interested in the relative energies (such as barrier heights to reactions and reaction energies) in our chemical reaction calculations. Thus, the energy errors coming from relativistic effects will partially cancel out in the calculated relative energies, and we expect that relativistic effects will not significantly perturb the reaction processes examined here. We note the caveat that relativistic effects due to spinorbit coupling may be noticeable and could modestly affect the results, but we have not considered spin-orbit relativistic effects because they are beyond the scope of our present study. (26) Becke, A. D. J. Chem. Phys. 1993, 98, 1372-1377. (27) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502-16513. (28) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 12001211. (29) Becke, A. D. Phys. Rev. A 1988, 38, 3098-3100. (30) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 58, 785-789. (31) Roos, B. O.; Taylor, P. R. Chem. Phys. 1980, 48, 157-173. (32) Siegbahn, P. E. M.; Almof, J.; Heiberg, A.; Roos, B. O. J. Chem. Phys. 1981, 74, 2384-2396. (33) Bernardi, F.; Bottini, A.; McDougall, J. J. W.; Robb, M. A.; Schlegel, H. B. Faraday Symp. Chem. Soc. 1984, 19, 137-147. (34) Frisch, M. J.; Ragazos, I. N.; Robb, M. A.; Schlegel, H. B. Chem. Phys. Lett. 1992, 189, 524-528. (35) Yamamoto, N.; Vreven, T.; Robb, M. A.; Frisch, M. J.; Schlegel, H. B Chem. Phys. Lett. 1996, 250, 373-378. (36) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154; J. Phys. Chem. 1990, 94, 5523-5527. (37) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007-1023. (38) Sadlej, A. J. Collect. Czech. Chem. Commun. 1988, 53, 19952016. (39) Sadlej, A. J. Theor. Chem. Acta 1992, 79, 123-140. (40) Sadlej, A. J. Theor. Chem. Acta 1992, 81, 339-354. (41) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr., Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; HeadGordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.

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Phillips and Fang

Figure 1. Schematic diagram showing the optimized geometry for selected reactants, transition state(s), intermediates, and reaction products for the reactions of CH2X-X and CH2X radical with ethylene to form cyclopropane product and the isomerization reaction of CH2X-X to form the parent CH2X2 molecule (where X ) Cl, Br, or I). The numbers present selected key structural parameters from the DFT computations using the cc-pVTZ basis set for Cl and Br and Sadlej-PVTZ basis set for I (bond lengths in Å and bond angles in deg) for the species shown (CH2X radicals, CH2X-X, TS1, TS2, IM, TS3, CH2X2, and TS4).

Results Reaction of CH2X-X (Where X ) Cl, Br, or I) with Ethylene. Figure 1 displays the optimized geometry found for the CH2X-X (where X ) Cl, Br, or I) species and their transition state (TS1) for reaction with ethylene to produce cyclopropane and halogen molecule products. Figure 1 shows values obtained from B3LYP computations using the cc-pVTZ basis set for systems containing Cl or Br atoms and the Sadlej-PVTZ basis set for systems containing I atoms. All three CH2X-X molecules approach ethylene (C2H4) asymmetrically and reacting with one of the CH2 groups from above the molecular plane. A very weakly bound complex may be formed as the two molecules move toward one another, but this has little effect on the chemical reaction. A transition state (TS1) was observed on the reaction path to the products cyclopropane (C3H6) and I2, and the TS1 structure is shown in Figure 1 along with selected optimized geometry structural parameters. The CdC and C-X bonds increase noticeably for TS1 compared to those of their

reactants (+0.025 and +0.103 Å for X ) I, +0.024 and +0.105 Å for X ) Br, and +0.034 and +0.095 Å for X ) Cl, respectively). The changes for the systems containing Br and I atoms are very similar to one another, while the Cl-containing system experiences more lengthening of the CdC bond and less lengthening of the C-Cl bond. The largest change in the TS1 structures compared to the reactant molecules takes place for the C-X-X angle, which becomes substantially larger in TS1 (by ca. +20.2° for X ) I, +20.7° for X ) Br, and +19.3° for X ) Cl). The observed structural changes in the TS1 geometry and the geometry of the reactants indicate the formation of a partial C-C bond between ethylene and CH2X-X accompanied by a weakening of the intramolecular CdC and C-X bonds. There is a small reduction in the X-X and C-X bond lengths, a small increase in the C-C and C-H bond lengths, and almost no change in the bond angles as the basis set size increases. Vibrational analysis of the TS1 structures found imaginary frequencies that ranged from 246.8i cm-1 for

Reactions of Isodihalomethanes

X ) I to 315i cm-1 for X ) Cl. The eigenvector corresponding to the negative eigenvalue of the force constant matrices shows that the internal coordinate reaction vectors are made up mostly of changes in the C1-C2 bond lengths and X2-X1-C1 bond angles (where X ) I, Br, or Cl). The reaction vector for the CH2I-I reaction with ethylene is 0.44 RC1-C3 + 0.55 RC1-C2 - 0.19 AI2-I1-C1. A similar reaction vector was found for X ) Br, while the reaction vector for the CH2Cl-Cl was found to be 0.38 RC1-C2 - 0.16 ACl2-Cl1-C1 with very little interaction between the C1 and C3 carbon atoms. From only the TS1 structure and imaginary modes, we cannot determine if TS1 connects the cyclopropane product with the ethylene and CH2X-X reactants. Thus, further IRC computations were done at the UB3LYP/6-311G** level to confirm the TS1 transition states connect the reactants of ethylene + CH2X-X to the products of cyclopropane (C3H6) + X2 (where X ) I, Br, or Cl). Our DFT results indicate that reaction of CH2X-X with ethylene to form cyclopropane and X2 proceeds essentially via a one-step process (i.e., an elementary reaction) and Figure 2a displays the potential energy profile for the cyclopropanation reactions. Examination of the TS1 for the reaction of CH2X-X with ethylene indicates that the barrier to reaction is in the entrance channel. The CH2I-I reaction with ethylene proceeds relatively easily with a barrier height of ∼2.9 kcal/mol, while the CH2Br-Br and CH2Cl-Cl reactions with ethylene occur less easily with barrier heights of 6.8 and 8.9 kcal/mol, respectively. This is likely due mainly to the increasing difficulty in breaking the C-X bond as the halogen atoms change from I to Br or Cl. In comparison with the barrier of about 25 kcal/mol for the Simmons-Smith cyclopropanation reaction, the reaction between the CH2X-X and ethylene proceeds substantially more easily. This predicts that ultraviolet excitation of dihalomethane molecules in the presence of olefins could provide a more facile pathway for organic synthesis of cyclopropane and its derivatives. We previously preformed B3LYP/6-311G** calculations to investigate the reaction of singlet carbene and ethylene to produce cyclopropane as a benchmark system. The B3LYP/6-311G** computed potential energy surface is downhill from the reactants (CH2 and CH2dCH2) to the cyclopropane product, and this agrees well with conclusions from previous theoretical work that found the energy along the reaction pathway decreases monotonically without a reaction barrier.42,43 Thus, we expect that the B3LYP computed barrier for the similar reaction of CH2I-I and CH2dCH2 to give cyclopropane and I2 is reasonable. Reaction of Chloromethyl, Bromomethyl, and Iodomethyl Radicals with Ethylene. The CH2X radicals also react with ethylene by approaching one of the CH2 groups of ethylene from above the molecular plane similar to the reactions of the CH2X-X species with ethylene. In contrast to the CH2X-X reactions with ethylene, which occur via a one-step elementary reaction mechanism (see preceding section), the CH2X reactions with ethylene to form cyclopropane product involves a two-step reaction mechanism with formation of a halopropyl radical intermediate. The first reaction step has (42) Zurawski, B.; Kutzelnigg, W. J. Am. Chem. Soc. 1978, 100, 2654-2659. (43) Sakai, S. Int. J. Quantum Chem. 1998, 70, 291-302.

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Figure 2. Schematic diagram showing the computed relative energies (in kcal/mol) for the reactants, transition state(s), intermediate, and reaction products for the reactions of CH2X-X (a) and CH2X with ethylene (b) to form cyclopropane product and the CH2X-X isomerization reaction (c) to form the parent CH2X2 molecule (where X ) Cl, Br, or I). See text for more details.

a transition state (denoted as TS2 in Figure 1) that has partial formation of a C-C bond between the CH2X radical and ethylene molecule. The C1-C2 bond lengths are 2.30 Å for X ) Cl and 2.286 Å for X ) Br from B3LYP/ cc-pVTZ computations and 2.328 Å for X ) I from B3LYP/ Sadlej-PVTZcalculations. The C3-C2-C1 bond angles are ca. 107-109° in TS2 (significantly larger than the ca. 97100° found for TS1), and this suggests that cyclopropane is not formed from TS2 but a halopropyl radical intermediate may be formed. IRC computations indicate that TS2 connects the CH2X radical and ethylene molecule reactants to an intermediate halopropyl radical (denoted by IM in Figures 1 and 2). The heavy atoms of the intermediate (IM) are in the same plane with a C3-C2C1-X1 dihedral angle of 180° and the six hydrogen atoms

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positioned above and below the heavy atom plane. The computed intermediate structure has Cs symmetry even though the initial guess geometry and computations were done using a C1 symmetry constraint. This is also the case for the transition state (TS2) structure. The computed structural parameters for TS2 and IM change very little upon increasing the basis set size from 6-311G** to cc-pVTZ (for X ) Br or Cl) or Sadlej-PVTZ (for X ) I). There are two possible reaction paths for the halopropyl radical intermediate (IM) to form cyclopropane and halogen atom product. The first possibility is a sequential path where the C-X bond breaking takes place first and then is followed by cyclopropanation. The second possibility is a concerted path where the C-X bond cleavage is simultaneously accompanied by the cyclopropanation. No transition state was found for the C-X bond cleavage in the sequential pathway where the C-X bond breaking takes place first and then is followed by cyclopropanation. We found that the second reaction pathway takes place. A transition state (labeled by TS3 in Figures 1 and 2) was found on the path from the intermediate (IM) to cyclopropane and halogen atom products, which is confirmed by IRC calculations at the UB3LYP/6-311G** level. Further evidence for this comes from the transition state (TS3) imaginary frequencies of 525i cm-1 for X ) I, 605i cm-1 for X ) Br, and 576i cm-1 for X ) Cl and their corresponding reaction vectors of 0.52 RI1-C1 - 0.57 AC3-C2-C1, 0.42 RBr1-C1 - 0.53 AC3-C2-C1 and 0.48 RCl1-C1 - 0.46 AC3-C2-C1, respectively. Figure 2b presents a simple schematic diagram of the potential energy surface for the reaction of the CH2X radicals with ethylene to form cyclopropane and halogen atom products. The values shown in Figure 2b are from UB3LYP/cc-pVTZ computations for X ) Br, Cl and UB3LYP/Sadlej-PVTZ calculations for X ) I with the UB3LYP/6-311G** zero-point energy correction included. Inspection of Figure 2b shows that as the halogen atom is changed from I to Br to Cl the barriers for the first and second reaction steps increase noticeably and the intermediate energy also increases. The reaction barriers for the first step are smaller than for the second step, so the cyclopropanation reaction step from the intermediate (IM) is the rate-determining step for the reaction of CH2X + ethylene to cyclopropane + X. The barriers for the ratedetermining step are much larger than those found for cyclopropanation via the CH2X-X species in the preceding section and this indicates that the photocyclopropanation reactions using ultraviolet excitation of dihalomethanes most likely occurs via the isodihalomethane species and not the CH2X species. Isomerization Reactions of CH2X-X (Where X ) Cl, Br, or I). Since the CH2X-X species are energetic molecules, the isomerization or dissociation reactions may conceivably compete with the reactions of CH2X-X with ethylene. The isomerization reactions of CH2I-I to form the parent CH2I2 molecule have been investigated by several groups.20,44,45 The barrier was first estimated to be ∼32.3 kcal/mol (134.6 kJ/mol) from an MP2/6-31G(d) calculation.44 Subsequent calculations using a B3LYP/ Sadlej-PVTZ level of theory estimated the isomerization barrier to be about 14.4 kcal/mol for CH2I-I to form CH2I2.20 The difference in the MP2/6-31G(d) and B3LYP/ (44) Glukhovtsev, M. N.; Bach, R. D. Chem. Phys. Lett. 1997, 269, 145-150. (45) Orel, A. E.; Kuhn, O. Chem. Phys. Lett. 1999, 304, 285-292.

Phillips and Fang

Sadlej-PVTZ computed values for the barrier heights for the CH2I-I isomerization reaction is mostly a method effect. The MP2 calculations tend to overestimate barrier heights by about 5-10 kcal/mol, and the DFT computations at the B3LYP level of theory tend to underestimate the barrier height by a few kcal/mol.46,47 Figure 2c displays the potential energy profile for the CH2X-X isomerization reactions. It is interesting that the barrier for the isomerization reaction decreases as the halogen atoms change from I to Br to Cl. This is somewhat counter intuitive since dissociation of the X-X bond by itself to give CH2X and X fragments would need additional energy as the halogen atoms changes from I to Br to Cl (the dissociation energies were estimated by the CAS(10,8)/6-311G** calculation). The transition state (TS4) geometry computed for the isomerization reactions is shown in Figure 1. Inspection of the transition state structure TS4 indicates that there is some simultaneous formation of a C-X bond and weakening of the X-X bond. This cooperative process probably leads to the isomerization reaction being more easily achieved as the halogen changes from I to Br to Cl. Discussion The B3LYP method tends to moderately underestimate reaction barrier heights. For example, B3LYP and CSSD(T) methods were used to investigate the intramolecular proton transfer in the ground state and the two lowest triplet states of malonaldehyde and found barrier heights of 3.0 kcal/mol in S0, 4.3 kcal/mol in T1, and 8.1 kcal/mol in T2 at the B3LYP level of theory and 4.3 kcal/mol in S0, 6.6 kcal/mol in T1, and 12.2 kcal/mol in T2 at the CSSD(T) level of theory.46 Similarly, Schaefer and coworkers47 recently examined the dissociation reaction of CH3CHO(T1) to CH3(S0) + HCO(S0) using B3LYP and RCCSD methods with a TZ2PF basis set and found values of 5468 and 6701 cm-1, respectively, for the vibrationless barrier height to dissociation (without the zero-point energy correction). The B3LYP computations usually underestimate the barrier height by a few kcal/ mol with respect to the CSSD(T) values,46,47 and we expect that our calculated barrier heights for the CH2X-X and CH2X reactions with ethylene are probably accurate to within a few kcal/mol. To estimate the relativistic effects on the relative energy for the reaction of CH2I-I with ethylene, additional B3LYP calculations were done with a Lan12DZ pseudopotential48-50 and the corresponding basis set augmented by one f function (d ) 0.3) on each I atom. The potential barrier, as an energy difference between TS1 (-140.63628 au) and CH2I-I (-62.01772 au) + CH2CH2 (-78.62318 au), is 4.0 kcal/mol. This is close to that of 2.9 kcal/mol from the B3LYP/cc-pVTZ calculations and indicates relativistic effects are small. As noted in the computational section, spin-orbit effects were not considered since they are beyond the scope of the present study. Using the B3LYP/6-311G** calculated entropies for CH2Br-Br (74.6 cal/mol‚K), CH2CH2 (52.3 cal/mol‚K), (46) Barone, V.; Adamo, C. J. Chem. Phys. 1996, 105, 11007-11019. (47) King, R. A.; Allen, W. D.; Schaefer, H. F., III J. Chem. Phys. 2000, 112, 5585-5592. (48) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270-283. (49) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284-298. (50) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.

Reactions of Isodihalomethanes

TS1 (73.4 cal/mol‚K), and TS4 (93.2 cal/mol‚K), the entropy change from the reactant (CH2Br-Br + CH2CH2) to TS1 is -33.7 cal/mol‚K, while the isomerization from CH2Br-Br to TS4 has an entropy change of about -1.2 cal/mol‚K. With the use of classical transition state theory, the entropic effects reduces the rate coefficient between CH2Br-Br and CH2CH2 by a factor of about 1.0D-8, while the rate coefficient for the isomerization is reduced by a factor of about 0.5. For the CH2Cl-Cl reactions, it is evident that the cyclopropanation reaction does not compete with the isomerization reaction. The cyclopropanation reaction is still dominant for the CH2I-I reactions. However, it is not clear to what extent the cyclopropanation reaction competes with the isomerization reaction, since both have similar barriers to reaction and similar rates. The moderate entropic effects appear to be important in the CH2Br-Br reaction, and this is consistent with the idea that competition between the cyclopropanation reaction and the isomerization reaction results in CH2Br-Br photocyclopropanation being less effective than the CH2I-I photocyclopropanation as observed experimentally.4,7,52,53 CH2X-X molecules can be formed in noticeable amounts in both low-temperature solids and/or room-temperature liquids.10,11,19-25 Recent nanosecond and picosecond timeresolved resonance Raman experiments19,23,25,51 have shown that both CH2I-I and CH2Br-Br are easily generated following ultraviolet excitation of diiodomethane and dibromomethane, respectively, in room-temperature solutions using both nonpolar (cyclohexane) and polar (acetonitrile) solvents. However, CH2Cl-Cl has so far only been observed using far ultraviolet (185 or 193 nm) excitation in low-temperature solids, and its stability is much less than that found for the CH2I-I and CH2BrBr species.10,11 Our present DFT results (see Figure 2) are consistent with these experimental observations in that decay of the CH2X-X species by isomerization back to the parent dihalomethane molecule is easiest for the CH2Cl-Cl species (barrier height of 9.1 kcal/mol) compared to the CH2Br-Br and CH2I-I species (with barrier heights of 11.8 and 14.4 kcal/mol, respectively). Inspection of the TS4 transition state optimized structure for the isomerization reaction in Figure 1 shows that there is simultaneous formation of a C-X bond as the X-X bond weakens. C-X bonds are generally stronger than the corresponding X-X bonds, and as the halogen atoms are changed from I to Br to Cl, the C-X bond energies increase in energy more than the X-X bond energies. This and the simultaneous C-X bond formation and X-X bond cleavage in the TS4 transition state most likely accounts for why the isomerization reaction barrier height decreases from 14.4 kcal/mol for CH2I-I to 11.8 kcal/mol for CH2Br-Br and to 9.1 kcal/mol for CH2Cl-Cl. Ultraviolet photoexcitation of both diiodomethane and dibromomethane in the presence of olefins has been found to give cyclopropanated products, with diiodomethane being more effective than dibromomethane.4,7,52,53 To our knowledge, there are no reports that ultraviolet photoly(51) Kwok, W. M.; Ma, C.; Parker, A. W.; Phillips, D.; Towrie, M.; Matousek, P.; Zheng, X.; Phillips, D. L. J. Chem. Phys. 2001, 114, 7536-7543. (52) Neuman, R. C., Jr.; Wolcott, R. G. Tetrahedron Lett. 1966, 6267-6272. (53) Marolewski, T.; Yang, N. C. J. Chem. Soc., Chem. Commun. 1967, 1225-1226.

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sis of dichloromethane by itself in the presence of olefins in room-temperature solutions produces noticeable amounts of cyclopropanated products. These experimental observations are consistent with our DFT computational results. For example, the barriers for the cyclopropanation reaction are larger for CH2Br-Br (6.8 kcal/mol) than CH2I-I (2.9 kcal/mol), while the corresponding isomerization reactions still occur with noticeably higher barriers to reaction (11.8 and 14.4 kcal/mol, respectively). These results indicate that cyclopropanation reactions should occur more readily for CH2I-I than CH2Br-Br, in agreement with experimental observations.4,7,52,53 However, the barriers for cyclopropanation and isomerization reactions for the CH2Cl-Cl species are almost the same (8.9 and 9.1 kcal/mol, respectively). Thus, the isomerization reaction will compete more effectively with the CH2Cl-Cl cyclopropanation reactions compared to those for the CH2Br-Br and CH2I-I. This is one reason for the very low stability of the CH2Cl-Cl species compared to either CH2Br-Br or CH2I-I. This and the stronger C-Cl bond that must initially be broken in the parent CH2Cl2 molecule to form CH2Cl-Cl indicate that CH2Cl-Cl will not be a very good carbenoid species consistent with its lack of successful use in photocyclopropanation reactions. The similarity of the CH2X-X molecule and CH2X radical attack toward the CH2 group of ethylene suggests that the CH2X-X carbenoid is not much more sterically demanding toward addition to olefins than the CH2X radical or the CH2X+ cation (which has been proposed to be the carbenoid species)7 or even the methylene species. This is consistent with the known reactivity of the carbenoid produced by ultraviolet photolysis of diiodomethane, which displays little sensitivity toward steric effects similar to methylene but does not allow appreciable C-H insertion reaction as would be expected for methylene. This indicates the carbenoid is not a free carbene.4-7 Inspection of the transition states for reaction of CH2X-X and CH2X with ethylene (TS1 and TS2 respectively in Figure 1) suggests that one of the properties for the CH2X-X species to potentially be good methylene transfer agents is that the C-X-X structure forces the C3-C2-C1 bond angle to move noticeably more toward the cyclopropane structure so that formation of the second C-C bond becomes likely. It is interesting to compare our results for the CH2X-X carbenoids to the carbenoids associated with SimmonsSmith-type reagents and Li-type reagents.1,3,52-56 A recent DFT study examined the reaction of ethylene with ClCH2ZnCl and found two reaction pathways, with one leading to cyclopropanation via a barrier of 24.7 kcal/mol and the other leading to insertion via a barrier of 36.0 kcal/mol.3 The active agent was found to have a colinear Cl-Zn-C geometry (∠ClZnC ) 180°,) which reacts with ethylene to give the cyclopropane product via a one-step reaction mechanism similar to the CH2X-X species. The ClCH2ZnCl carbenoid forms a transition state with ethylene where the CdC ethylene bond is only slightly longer (∼1.351 Å) than in free ethylene and the two new C-C bonds are forming asymmetrically with bond lengths of (54) Dargel, T. K.; Koch, W. J. Chem. Soc., Perkin Trans. 2 1996, 877-881. (55) Hermann, H.; Lohrenz, J. C. W.; Ku¨hn, A.; Boche, G. Tetrahedron 2000, 56, 4109-4115. (56) Nakamura, E.; Hirai, A.; Nakamura, M. J. Am. Chem. Soc. 1998, 120, 5844-5845.

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2.392 and 2.590 Å, respectively. This behavior is similar to the CH2X-X carbenoids (see TS1 in Figure 1) in that there is a weakening of the CdC ethylene bond accompanied by an asymmetric C-C bond formation, with one C-C bond noticeably stronger than the other. The CH2X-X carbenoid exhibits stronger C-C bond formation (from 2.28 Å for I to 2.045 Å for Cl) for the stronger C-C bond and weaker bond formation for the weaker C-C bond than in the ClCH2ZnCl carbenoid reaction. This appears to be due to the larger C1-C2-C3 angle in TS1 (∼96.8° for I, 99.7° for Cl) for the CH2X-X carbenoid species compared to the smaller angle in the transition state (82.5°) associated with the ClCH2ZnCl carbenoid reaction, which has a three-centered addition geometry.3 A DFT study was also reported by another group for the classical Simmons-Smith reagent ICH2ZnI reaction with ethylene.54 This study54 found that the C-Zn-I atoms were linear, with a 180° angle for the ICH2ZnI carbenoid, similar to the ClCH2ZnCl carbenoid.3 The computed optimized geometry for the ICH2ZnI carbenoid54 showed reasonable agreement with an X-ray crystal structure for the related bis(iodomethyl)zinc intermediate complexed with a glycol ether.57 The ICH2ZnI carbenoid approaches ethylene asymmetrically and reacts via a concerted onestep reaction mechanism similar to the ClCH2ZnCl carbenoid. The transition state structure shows the two C-C bond distances between the carbenoid and ethylene are ca. 2.383-2.395 and ca. 2.656 to 2.658 Å, respectively. The I-C-Zn angle decreases to smaller angles ∼67.9° to 70.4° while the C-I bond lengthens ∼0.486 to 0.587 Å to ∼2.772 to 2.781 Å. This transition state indicates that there is concerted formation of the ZnI2 leaving group and the cyclopropane product. The ClCH2ZnCl and ICH2ZnI carbenoid reactions with ethylene have transitions state structures similar to one another for the two C-C bond distances between the carbenoid and the angles between the three carbon atoms. However, the CH2X-X carbenoid species exhibit significantly different values for these structural parameters in TS1 with smaller C1-C2 and longer C1-C3 bond distances and a larger C1-C2-C3 bond angle. Comparison of the transition state geometry for CH2X-X to those for ClCH2ZnCl and ICH2ZnI carbenoid reactions shows that the CH2X-X carbon atom has a much more open structure where the X-X leaving group is only attached by one bond to the carbon atom. However, the ClCH2ZnCl and ICH2ZnI carbenoids have the ZnX2 leaving group attached by two bonds to their carbon atoms, giving a more hindered bulky structure. This suggests that the steric effects of the carbenoid species play an important role in their chemical reactivity. For example, the photocyclopropanation reaction (via the CH2I-I carbenoid) exhibits increasing rates of reaction upon going from mono- to tetrasubstituted alkenes, whereas the Simmons-Smith reaction carbenoid (using the diiodomethane-Zn complex) generally displays a reduced rate of reaction toward highly substituted alkenes.1-7 While Zn activates the parent CH2I2 molecule to become a carbenoid in the Simmons-Smith reaction1, it also gives this carbenoid a more bulky ZnI2 leaving group that causes it to become less reactive toward highly substituted alkenes compared to the more open structure CH2I-I carbenoid. (57) Denmark, S. E.; Edwards, J. P.; Wilson, S. R. J. Am. Chem. Soc. 1991, 113, 723-725.

Phillips and Fang

A very recent ab initio study examined the carbenoid behavior of LiCH2X and XZnCH2X (where X ) F, Cl, Br, or I) in reactions with ethylene to give cyclopropane product.55 The lithium carbenoids have a barrier to reaction that is relatively low (varying from 7.4 kcal/mol for F to 6.1 kcal/mol for I) and does not change much with the identity of the halogen atom, while the XZnCH2X carbenoids have higher barriers to reaction (varying from 31.9 kcal/mol for F to 17.0 kcal/mol for I) and change more noticeably with the identity of the halogen atoms.55 The lithium carbenoid gains from some weakening of the C-X bond by the C-Li bond, and in its cyclopropanation reaction transition state the Li-X is strongly decomplexed with the C-X bond cleavage compensated by the Li-X bond formation, which results in very similar reaction activation barriers.55 The behavior of the transition state for the lithium carbenoids is similar to that for the isomerization reaction of the CH2X-X carbenoids (see TS4 in Figure 1) where the formation of the C-X bond compensates the cleavage of the X-X bond. We note that there is a closer match between the Li-X and C-X bond energies in the Li carbenoids compared to the C-X and X-X bond energies in CH2X-X as the identity of the halogen is varied. This may account for the larger amount of change in the reaction activation barrier of the CH2X-X isomerization reaction compared to the lithium carbenoid cyclopropanation reaction as the type of halogen atom is varied. We note that the CH2X-X species are ground electronic state species like the XZnCH2X and LiCH2X carbenoids. The CH2I-I carbenoid species appears to be significantly more reactive toward ethylene than the XZnCH2X and LiCH2X carbenoids, and this suggests that further improvements in organometallic catalysts for cyclopropanation reactions are possible. The X-X leaving group attached to the CH2 moiety by a single bond in the CH2X-X carbenoid gives it a more open structure that allows it to easily react effectively with more highly substituted alkenes. However, the ZnX2 and LiX leaving groups are attached to their respective XZnCH2X and LiCH2X carbenoids by two bonds, and this appears to be more sterically demanding and results in lower reaction rates with more highly substituted alkenes. This suggests that an organometallic catalyst where the leaving group is attached to the CH2 moiety by a single bond (to give a more open structure and better access to the CH2 group) could allow more effective reaction with highly substituted alkenes. We are continuing both experimental and theoretical investigations of the chemistry and photochemistry of the novel and intriguing isopolyhalomethane species. Results of these studies will be reported in due course.

Acknowledgment. This work was supported by grants from the Committee on Research and Conference Grants (CRCG), the Research Grants Council (RGC) of Hong Kong, the Hung Hing Ying Physical Sciences Research Fund, and the Large Items of Equipment Allocation 1993-94 from the University of Hong Kong to DLP and the National Science Foundation of China (Grant No. 29673007) to WHF. Supporting Information Available: Selected output from the density functional theory computations for the reactions of CH2X-X and CH2X radicals (where X ) Cl, Br, or I) with ethylene and the isomerization reactions of the CH2X-X species. This material is available free of charge via the Internet at http://pubs.acs.org. JO010582I