Ab Initio and DFT Study of Homolytic Substitution Reactions of Acyl

Synopsis. Ab initio and DFT calculations predict that homolytic substitution reactions of acetyl radicals at the heteroatom in dimethylsilane, dimethy...
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Volume 28, Number 12, June 22, 2009

American Chemical Society

Articles Ab Initio and DFT Study of Homolytic Substitution Reactions of Acyl Radicals at Silicon, Germanium, and Tin Sonia M. Horvat* and Carl H. Schiesser School of Chemistry, The UniVersity of Melbourne, Victoria, Australia, 3010, and Bio21 Molecular Science and Biotechnology Institute, The UniVersity of Melbourne, Victoria, Australia, 3010 ReceiVed NoVember 2, 2008

Ab initio calculations using the 6-311G(d,p), cc-pVDZ, aug-cc-pVDZ, and (valence) double-ζ pseudopotential (DZP) basis sets, with (MP2, ROMP2, QCISD, CCSD(T)) and without (HF) the inclusion of electron correlation, and density functional (BHandHLYP) calculations predict that homolytic substitution reactions of acetyl radicals at the silicon atoms in dimethylsilane can proceed via both backside and frontside attack mechanisms. At the highest level of theory (CCSD(T)/aug-cc-pVDZ//MP2/aug-ccpVDZ), energy barriers (∆Eq) of 110.4 and 107.5 kJ mol-1 are calculated for the backside and frontside reactions, respectively. Similar results are obtained for reactions involving germanium and tin with energy barriers (∆Eq) of 97.6-191.7 and 100.8-171.3 kJ mol-1 for the backside and frontside mechanisms, respectively. These data suggest that both homolytic substitution mechanisms are feasible for homolytic reactions of acetyl radicals at silicon, germanium, and tin. BHandHLYP calculations provide geometries and energy barriers for backside and frontside transition states in good agreement with those obtained by traditional ab initio techniques. Introduction The concept and utilization of homolytic substitution (SH2) reactions has advanced in leaps and bounds since the discovery of free radicals by Gomberg at the beginning of the 20th century and as such are now regularly used in organic synthesis.1,2 Work in our laboratories has been focused on the design, application, and understanding of free radical homolytic substitution chemistry with the aim of developing novel synthetic methodology.3,4 To that end, we have published several ab initio studies with * Corresponding author. E-mail: [email protected]. (1) For leading reviews, see: (a) Walton, J. C. Acc. Chem. Res. 1998, 31, 99. (b) Schiesser, C. H.; Wild, L. M. Tetrahedron 1996, 52, 13265. (c) Saveant, J.-M. Tetrahedron 1994, 50, 10117. (d) Beckwith, A. L. J. Chem. Soc. ReV. 1993, 22, 143. (e) Rossi, R. A.; Palacios, S. M. Tetrahedron 1993, 49, 4485.

the aim of increasing our understanding of factors that affect and control the mechanism of homolytic substitution at several main group higher heteroatoms.4-8 It is generally agreed that (2) For some recent reports, see: (a) Carter, C. A. G.; Greidanus, G.; Chen, J.-X.; Stryker, J. M. J. Am. Chem. Soc. 2001, 123, 8872. (b) Stolzenberg, A. M.; Cao, Y. J. Am. Chem. Soc. 2001, 123, 9078. (c) Tichy, S. E.; Thoen, K. K.; Price, J. M.; Ferra, J. J., Jr.; Petucci, C. J.; Kenttaemaa, H. I. J. Org. Chem. 2001, 66, 2726. (d) Al-Maharik, N.; Engman, L.; Malmstro¨m, J.; Schiesser, C. H. J. Org. Chem. 2001, 66, 6286. (e) Kang, S.-K.; Seo, H.-W.; Ha, Y.-H. Synthesis 2001, 1321. (f) Bourgeois, M.-J.; Vialemaringe, M.; Campahnole, M.; Mountaudon, E. Can. J. Chem. 2001, 79, 257. (g) Carland, M. W.; Martin, L. R.; Schiesser, C. H. Tetrahedron Lett. 2001, 42, 4737. (h) Miller, J. B.; Salvador, J. R. J. Org. Chem. 2002, 67, 435. (i) Lightfoot, P.; Pareschi, P.; Walton, J. C. J. Chem. Soc., Perkin Trans. 2 2002, 918. (j) Benati, L.; Leardini, R.; Minozzi, M.; Nanni, D.; Spagnolo, P.; Strazzari, S.; Zanardi, G. Org. Lett. 2002, 4, 3079. (k) Hartung, J.; Gottwald, T.; Spehar, K. Synthesis 2002, 1469.

10.1021/om801016x CCC: $40.75  2009 American Chemical Society Publication on Web 05/26/2009

3312 Organometallics, Vol. 28, No. 12, 2009

homolytic substitution by an attacking radical (R) involves the approach of the radical at the heteroatom (Y) along a trajectory opposite the leaving group (Z). This backside mechanism can proceed either via a transition state 1 in which the attacking and leaving radicals adopt a collinear (or nearly so) arrangement, resulting in Walden inversion, or with the involvement of a hypervalent intermediate 2, which may or may not undergo pseudorotation prior to dissociation.1,5 In addition to the pathways for homolytic substitution described above, a mechanism involving frontside attack via transition state 3 has also been recently investigated. Calculations performed on the mechanism of the radical Brook rearrangement suggest that the frontside mechanism is involved in the 1,2migration of the group IV elements,4d as is the homolytic 1,2translocation reactions between the group IV elements.6 In addition, both of the above-mentioned mechanisms are predicted to be feasible for homolytic substitution involving methyl and acetyl radicals at disilane, digermane, distannane, silygermane, silylstannane, and germylstannane.8,9

It is well known that acyl radicals undergo homolytic substitution at heteroatoms.10 Interestingly, while there are several examples of homolytic substitution by acyl radicals at the halogens and chalcogens, we are aware of only one example of a group IV element involved in this sort of chemistry. Ryu and co-workers reported that acyl radical 5 undergoes ring closure at silicon with expulsion of a stannyl radical to afford 1,1-diphenylsilaacyclopentan-2-one (4) (Scheme 1).11 As part of an ongoing interest in homolytic substitution involving the main group higher heteroatoms, we now report (3) (a) Schiesser, C. H.; Smart, B. A. Tetrahedron 1995, 51, 6051. (b) Schiesser, C. H.; Smart, B. A.; Tran, T.-A. Tetrahedron 1995, 51, 10651. (c) Schiesser, C. H.; Smart, B. A. J. Comput. Chem. 1995, 16, 1055. (d) Fong, M. C.; Schiesser, C. H. Tetrahedron Lett. 1995, 36, 7329. (e) Schiesser, C. H.; Skidmore, M. A. Chem. Commun. 1996, 1419. (f) Lucas, M. A.; Schiesser, C. H. J. Org. Chem. 1996, 61, 5754. (g) Fong, M. C.; Schiesser, C. H. J. Org. Chem. 1997, 62, 3103. (h) Laws, M. J.; Schiesser, C. H. Tetrahedron Lett. 1997, 38, 8429. (i) Lucas, M. A.; Schiesser, C. H. J. Org. Chem. 1998, 63, 3032. (j) Schiesser, C. H.; Skidmore, M. A. J. Organomet. Chem. 1998, 552, 145. (k) Schiesser, C. H.; Wild, L. M. J. Org. Chem. 1999, 64, 1131. (l) Engman, L.; Laws, M. J.; Malmstro¨m, J.; Schiesser, C. H.; Zugaro, L. M. J. Org. Chem. 1999, 64, 6764. (m) Lucas, M. A.; Nguyen, O. T. K.; Schiesser, C. H.; Zheng, S.-L. Tetraherdon 2000, 56, 3995. (n) Kim, S.; Horvat, S. M.; Schiesser, C. H. Aust. J. Chem. 2002, 55, 753. (o) Horvat, S. M.; Kim, S.; Schiesser, C. H. Chem. Commun. 2003, 1182. (4) (a) Schiesser, C. H.; Smart, B. A.; Tran, T.-A. Tetrahedron 1995, 51, 3327. (b) Schiesser, C. H.; Wild, L. M. J. Org. Chem. 1998, 63, 670. (c) Schiesser, C. H.; Styles, M. L.; Wild, L. M. J. Chem. Soc., Perkin Trans. 1996, 2, 2257. (d) Schiesser, C. H.; Styles, M. L. Chem. Soc., Perkin Trans. 2 1997, 2335. (5) (a) Schiesser, C. H.; Wild, L. M. Aust. J. Chem. 1995, 48, 175 See also. (b) Howell, J. M.; Olsen, J. F. J. Am. Chem. Soc. 1976, 98, 7119. (c) Crammer, C. J. J. Am. Chem. Soc. 1990, 112, 7965. (d) Crammer, C. J. J. Am. Chem. Soc. 1991, 113, 2439. (6) Horvat, S. M.; Schiesser, C. H. J. Chem. Soc., Perkin Trans. 2 2001, 939. (7) Horvat, S. M.; Schiesser, C. H.; Wild, L. M. Organometallics 2000, 19, 1239. (8) Matsubara, H.; Horvat, S. M.; Schiesser, C. H. Org. Biomol. Chem. 2003, 1, 1199. (9) Matsubara, H.; Schiesser, C. H. Org. Biomol. Chem. 2003, 1, 4335. (10) (a) Ryu, I.; Okuda, T.; Nagahara, K.; Kambe, N.; Komatsu, M.; Sonoda, N. J. Org. Chem. 1997, 62, 7550. For leading reviews, see: (b) Chatgilialoglu, C.; Crich, D.; Komatsu, M.; Ryu, I. Chem. ReV. 1999, 99, 1991. (c) Ryu, I.; Sonoda, N. Angew. Chem., Int. Ed. Engl. 1996, 35, 1050. (11) Studer, A.; Amrein, S.; Matsubara, H.; Schiesser, C. H.; Doi, T.; Kawamura, T.; Fukuyama, T.; Ryu, I. Chem. Commun. 2003, 1190.

HorVat and Schiesser Scheme 1

the results of computational investigations into the homolytic substitution mechanism for reactions involving acetyl radicals with dimethylsilane, dimethylgermane, and dimethylstannane.

Computational Methods Ab initio and density functional theory calculations were carried using the Gaussian 03 program.12 Geometry optimizations were performed with standard gradient techniques at HF, MP2, and BHandHLYP levels of theory, using restricted and unrestricted methods for closed- and open-shell systems, respectively.13 Basis sets available in Gaussian 03 were used, as well as the (valence) double-ζ pseudopotential basis sets of Hay and Wadt14 supplemented with a single set of d-type polarization functions for the heteroatoms in this study (exponents d(ζ)Sn ) 0.200),15 together with the double-ζ all-electron basis sets of Dunning and Hay16 with an additional set of polarization functions (exponents d(ζ)C ) 0.75, d(ζ)O ) 0.85, and p(ζ)H ) 1.00), for C, O, and H. We refer to this basis set as DZP throughout this work. In previous work, results generated using DZP proved to be very similar to those obtained using 6-311G(d,p) for reactions involving silicon and chlorine.4c,d,7 All ground and transition states were verified by vibrational frequency analysis. Further single-point ROMP2, QCISD, and CCSD(T) calculations were performed on some of the MP2 and BHandHLYP optimized structures. When correlated methods were used, calculations were carried out by using the frozen core approximation. Values of 〈s2〉 never exceeded 0.88 before annihilation of quartet contamination and were mostly 0.8 at correlated levels of theory. Where appropriate, zero-point vibrational energy (ZPE) corrections have been applied. Natural bond orbital (NBO) analyses were carried out with NBO 5.017 linked through the Gaussian 03 program. (12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Borone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Peterson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G., Chen, W.; Wong, M. W.; Gonzalez, C. J.; Pople, A. Gaussian 03, ReVision B. 05; Gaussian Inc.: Pittsburgh, PA, 2003. (13) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, P. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (14) (a) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (15) Smart, B. A., Ph.D. Thesis, The University of Melbourne, 1994. (16) Dunning, T. H.; Hay, P. J. Modern Theoretical Chemistry; Plenum: New York, 1976; Chapter 1, pp 1-28.

Homolytic Substitution Reactions of Acyl Radicals

Organometallics, Vol. 28, No. 12, 2009 3313 Scheme 2

Table 1. Important Calculated Geometric Featuresa of the Transition States 6 (X ) Si) and 7 (X ) Si) Involved in the Reaction of Acetyl Radical with Dimethylsilane (Scheme 2, X ) Si) 6

7

method

r1

r2

θ

method

r1

r2

θ

UHF/6-311G(d,p) MP2/6-311G(d,p) MP2/cc-pVDZ MP2/aug-cc-pVDZ BHandHLYP/6-311G(d,p) BHandHLYP/cc-pVDZ BHandHLYP/aug-cc-pVDZ

2.145 Å 2.052 Å 2.075 Å 2.070 Å 2.051 Å 2.062 Å 2.062 Å

2.190 Å 2.182 Å 2.208 Å 2.195 Å 2.223 Å 2.236 Å 2.222 Å

168.7° 170.6° 172.2° 170.6° 170.1° 171.0° 169.8°

UHF/6-311G(d,p) MP2/6-311G(d,p) MP2/cc-pVDZ MP2/aug-cc-pVDZ BHandHLYP/6-311G(d,p) BHandHLYP/cc-pVDZ BHandHLYP/aug-cc-pVDZ

2.125 Å 2.039 Å 2.030 Å 2.030 Å 2.041 Å 2.054 Å 2.057 Å

2.207 Å 2.253 Å 2.266 Å 2.258 Å 2.171 Å 2.174 Å 2.169 Å

75.1° 72.7° 73.8° 72.5° 75.0° 75.8° 74.8°

Table 2. Calculated Energy Barriersa for the Forward (∆E1q) and Reverse (∆E2q) Homolytic Substitution Reactions of Acetyl Radical with Dimethylsilane and Imaginary Frequencies (ν)b of Transition States 6 (X ) Si) and 7 (X ) Si) 6

7

method

∆E1

∆E1 +ZPE

∆E2

∆E2q+ZPE

UHF/6-311G(d,p) MP2/6-311G(d,p) ROMP2/6-311G(d,p)//MP2/6-311G(d,p) QCISD/6-311G(d,p)//MP2/6-311G(d,p) CCSD(T)/6-311G(d,p)//MP2/6-311G(d,p) MP2/cc-pVDZ ROMP2/ cc-pVDZ//MP2/cc-pVDZ QCISD/cc-pVDZ//MP2/cc-pVDZ CCSD(T)/cc-pVDZ//MP2/cc-pVDZ MP2/aug-cc-pVDZ ROMP2/aug-cc-pVDZ//MP2/aug-cc-pVDZ QCISD/aug-cc-pVDZ//MP2/aug-cc-pVDZ CCSD(T)/aug-cc-pVDZ///MP2/aug-cc-pVDZ BHandHLYP/6-311G(d,p) QCISD/6-311G(d,p)//BHandHLYP/6-311G(d,p) CCSD(T)/6-311G(d,p)//BHandHLYP/6-311G(d,p) BHandHLYP/cc-pVDZ BHandHLYP/aug-cc-pVDZ

209.2 153.4 163.3 137.8 128.6 149.2 159.5 133.2 124.4 135.3 145.3 122.2 110.4 139.2 138.1 129.4 134.9 134.5

208.4 152.4

147.2 69.6 76.1 57.3 48.4 65.0 71.7 52.7 44.6 59.4 66.4 47.9 38.2 60.2 57.9

158.0 79.6

2848i 1220i

75.6

1176i

68.0

1179i

70.9

766i

56.1 60.0

67.1 69.5

742i 763i

a

q

q

148.5

133.7

139.0 134.9 134.1

q

ν

∆E1q

∆E1 +ZPE

∆E2q

∆E2q+ZPE

ν

198.5 142.2 149.4 137.8 128.9 139.3 147.0 134.4 125.8 120.9 127.9 118.4 107.5 140.6 143.6 132.5 145.0 143.0

199.7 143.1

136.5 58.4 62.2 57.2 48.7 55.1 59.2 53.9 46.0 45.0 49.0 45.1 35.3 61.7 63.5

149.3 70.4

803i 609i

66.9

607i

55.9

583i

73.3

445i

66.2 68.5

79.0 80.3

384i 382i

q

139.8

121.6

141.4 146.9 145.0

Energies in kJ mol-1. b Frequencies in cm-1.

Optimized geometries and energies for all transition structures in this study (Gaussian Archive entries) are available as Supporting Information.

Results and Discussion Homolytic Substitution Reaction of Acetyl Radical with Dimethylsilane (SiH2Me2). Extensive searching of the C4H11SiO potential energy surface at all of the levels of theory employed for optimization in this study located structures 618 and 718 (X)Si), as the lowest energy transition states for the reaction of the acetyl radical at the silicon atom in dimethylsilane (17) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0; Theoretical Chemical Institute; University of Wisconsin: Madison, WI, 2001. (18) Intrinsic reaction coordinate (IRC) calculations were carried out to verify that the transition states connect to the given reactants and products.

(Scheme 2). The important geometrical features of the transition states 6 and 7 (X ) Si) are summarized in Table 1, while calculated energy barriers (∆E1q and ∆E2q, Scheme 2) together with the corresponding imaginary frequencies are listed in Table 2. Full computational details are available as Supporting Information. Inspection of Figure 1 and Table 1 reveals that transition state 6 (X ) Si) is predicted to adopt a near collinear arrangement (θ ) 169-172°) of the attacking acetyl radical and the leaving methyl radical at all levels of theory employed, while structure 7 (X ) Si) involved in the analogous frontside chemistry is predicted to involve an attack angle (θ) of approximately 74°, at all levels; this angle is similar to those predicted for the frontside transition states involved in other homolytic substitution reactions involving silicon, germanium, and tin.7-9 The transition state (C-Si) separations (r1) in 6 (X ) Si) and 7 (X

3314 Organometallics, Vol. 28, No. 12, 2009

Figure 1. BHandHLYP/6-311G(d,p) optimized structure of transition states 6 (X ) Si) and 7 (X ) Si) for the backside and frontside substitution reactions of acetyl radicals at the silicon atom in dimethylsilane.

) Si) are predicted at all levels of theory to lie in the range 2.051-2.145 and 2.030-2.125 Å, respectively, while the (Si-C) distances (r2) in 6 (X ) Si) and 7 (X ) Si) are calculated to be 2.190-2.236 and 2.169-2.266 Å, respectively. These distances are in the expected ranges when compared with our previous calculations of homolytic substitution reactions involving silicon.4c,7-9 Not unexpectedly,7-9 the data provided by this computational study suggest that the energy requirements for both homolytic pathways are similar. In addition, these reactions are predicted to be significantly endothermic at all levels of theory, with the reverse energy barriers being some 60-90 kJ mol-1 lower than the forward barriers at all levels of theory employed. This is not surprising given the expected stability of the acetyl radical over the methyl radical.19 Inspection of Table 2 reveals that the energy barrier (∆E1q) for the forward reaction (Scheme 2) associated with 6 (X ) Si) and 7 (X ) Si) is calculated to be 209.2 and 198.5 kJ mol-1, respectively, at the UHF/6-311G(d,p) level of theory. As expected, inclusion of electron correlation (MP2/6-311G(d,p)) serves to lower the predicted energy barrier to 153.4 and 142.2 kJ mol-1 for the reaction involving 6 (X ) Si) and 7 (X ) Si), respectively, while further improvements in both the basis set quality and levels of correlation provide values of ∆E1q for the reaction involving 6 (X ) Si) that range from 149.2 kJ mol-1 (MP2/cc-pVDZ) to 137.8 kJ mol-1 (QCISD/6-311G(d,p)//MP2/ 6-311G(d,p)). In comparison, reactions involving 7 (X ) Si) are calculated to have values of ∆E1q in the range 139.3 to 137.8 kJ mol-1 at the same levels of theory. At the highest level of theory used (CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ), energy barriers (∆E1‡) of 110.4 and 107.5 kJ mol-1 are predicted for the reaction involving 6 (X ) Si) and 7 (X ) Si), respectively. BHandHLYP/6-311G(d,p) calculation provide energy barriers (∆E1q) of 139.2 and 140.6 kJ mol-1 for the reaction involving 6 (X ) Si) and 7 (X ) Si), respectively, while values of 134.5 to 143.0 kJ mol-1 are obtained at the BHandHLYP/aug-cc-pVDZ level of theory. Clearly both backside and frontside mechanisms are predicted to be feasible for homolytic substitution by an acetyl radical at dimethylsilane at all levels of theory employed. For example, at the MP2/6311G(d,p) level of theory the frontside attack mechanism is favored by 11.2 kJ mol-1, while at the BHandHLYP level of theory transition state 7 is favored by 1.4 kJ mol-1. It has been generally thought that acyl radicals are nucleophilic due to their high affinity toward CdC double bonds that have electron-withdrawing groups attached to the carbons.10b (19) Morihovitis, T.; Schiesser, C. H.; Skidmore, M. A. J. Chem. Soc., Perkin Trans. 2 1999, 2041.

HorVat and Schiesser

Figure 2. Orbital interaction diagram for the key bond-forming step involved in the homolytic substitution of acetyl radical at the heteroatom in dimethylsilane, germane, and stannane (transition states 6 and 7).

However, recent computational studies suggest that the acetyl and related radicals are ambiphilic in nature.20-24 Previous studies established that the BHandHLYP/6-311G(d,p) method is reliable for the study of acyl and related radical chemistry;21 accordingly we chose to perform all our natural bond orbital (NBO) analyses using this method. NBO analyses were performed for transition states 6 (X ) Si) and 7 (X ) Si) and reveal interesting SOMO f σ*Si-C and σSi-C f SOMO interactions and are represented pictorially in Figure 2. The former interaction, calculated to be worth 227 and 170 kJ mol-1 for the reactions involving backside and frontside attack mechanisms, respectively, are evident in the R spin-set, whereas the σSi-C f SOMO interactions, evident in the β spinset are calculated to contribute 4073 and 1139 kJ mol-1 for reactions involving 6 (X ) Si) and 7 (X ) Si). Clearly these calculations suggest that the nucleophilic character of the Si-C bond dominates significantly in this chemistry, with the σSi-C f SOMO interaction contributing 95% and 87% of the total orbital interaction for 6 (X ) Si) and 7 (X ) Si), respectively. Visualization of the Kohn-Sham orbitals generated at the BHandHLYP/6-311G(d,p) level of theory depicts the overlap of the two reacting units in transition states 6 (X ) Si) and 7 (X ) Si) (Figure 3). Clearly, these calculations indicate that the acetyl radical is behaving as an electrophilic radical in its reaction with dimethylsilane, an observation not unexpected, as it has been shown that the Si-C bond has a donor ability somewhat similar to that of an oxygen lone pair.25 Homolytic Substitution of Acetyl Radical with Dimethylgermane (GeH2Me2) and Dimethylstannane (SnH2Me2). Calculations for higher heteroatoms such as tin are complicated by the lack of availability of reliable all-electron basis sets, as well as the requirement for relativistic correction. Previous benchmarking studies, however, have shown that ab initio calculations for homolytic substitution chemistry involving main group higher heteroatoms using the double-ζ pseudopotential basis set of Hay and Wadt,14 supplemented with appropriate d-functions,15 are (20) Matsubara, H.; Falzon, C. T.; Ryu, I.; Schiesser, C. H. Org. Biomol. Chem. 2006, 4, 1920. (21) Schiesser, C. H.; Ritsner, I.; Wille, U. Chem. Commun. 2006, 4, 1067. (22) Kyne, S.; Schiesser, C. H.; Matsubara, H. Org. Biomol. Chem. 2007, 5, 3938. (23) Kyne, S.; Schiesser, C. H.; Matsubara, H. J. Org. Chem. 2008, 73, 427. (24) Krenske, E. H.; Schiesser, C. H. Org. Biomol. Chem. 2008, 6, 854. (25) Green, A. J.; Giordano, J.; White, J. M. Aust. J. Chem. 2000, 53, 285.

Homolytic Substitution Reactions of Acyl Radicals

Organometallics, Vol. 28, No. 12, 2009 3315

Figure 3. BHandHLYP/6-311G(d,p) generated Kohn-Sham orbitals for transition states 6 and 7 (X ) Si).

Figure 4. BHandHLYP/6-311G(d,p) (Ge) and BHandHLYP/DZP optimized structure of transition states 6 (X ) Ge, Sn) and 7 (X ) Ge, Sn) for the backside and frontside substitution reactions of acetyl radicals with dimethylgermane and dimethylstannane (Scheme 2, X ) Ge, Sn).

very similar to those obtained using the 6-311G(d,p) basis set (see above).4c,d,7With this in mind, we are comfortable in comparing data generated using the DZP basis set for calculations involving tin with those generated using 6-311G(d,p) for the remaining atoms in this study. Extensive searching of the C4H11XO (X ) Ge, Sn) potential energy surfaces located structures 6 (X ) Ge, Sn) and 7 (X ) Ge, Sn) as transition states for the homolytic substitution reactions (Scheme 2) at all levels of theory employed in this study. The important geometrical features of the transition states 6 (X ) Ge, Sn) and 7 (X ) Ge, Sn) are summarized in Figure 4 and Table 3, while the calculated energy barriers (∆E1q and ∆E2q, Scheme 2) together with the corresponding imaginary

frequencies are listed in Table 4. Full computational details are available as Supporting Information. Not surprisingly, transition states bear a striking resemblance to those calculated for the analogous reactions with dimethylsilane 6 (X ) Si) and 7 (X ) Si). Backside attack structures 6 (X ) Ge, Sn) are predicted to adopt a collinear arrangement of attacking and leaving radicals, while the frontside structures 7 (X ) Ge, Sn) of C1 symmetry are calculated to have angles (θ) of about 75°(Ge) and 70°(Sn) between attacking and leaving species. As can be seen in Figure 4 and Table 3, the C-X separations (r1) in 6 are calculated to lie in the range 2.140-2.269 and 2.335-2.450 Å for reactions involving dimethylgermane and dimethylstannane, respectively, while the X-C distances (r2) in 6 are predicted to be in the range 2.313 Å (X ) Ge) and 2.472 Å (X ) Sn). Similar trends are also observed for 7, where the transition state distances are calculated to lie in the narrow ranges 2.109-2.259 Å (Ge) and 2.316-2.545 Å (Sn). Inspection of Table 4 reveals that some interesting trends in energy are clearly evident. As can be seen in the table, all of the energy barriers (∆E2q) for the reverse reactions are always lower than those (∆E1q) for the forward process at all levels of theory; these reactions are predicted to be endothermic at all levels of theory. Calculated energy barriers (∆Eq) are also strongly affected by the inclusion of electron correlation. This computational study also suggests that the energy requirements for both homolytic pathways are similar. The calculated energy barriers (∆E1q) for the forward reaction (Scheme 2) associated with 6 (X ) Ge) range from 97.6 (CCSD(T)/aug-cc-pVDZ// MP2/aug-cc-pVDZ) to 191.7 kJ mol-1 (UHF/6-311G(d,p)), while similar barriers for the reaction involving 7 (X ) Ge) are calculated to range from 98.5 (CCSD(T)/aug-cc-pVDZ// MP2/aug-cc-pVDZ) to 185.4 kJ mol-1 (UHF/6-311G(d,p)). Similar trends are predicted for reactions involving tin, with barriers of 100.8 and 97.8 kJ mol-1 predicted at the CCSD(T)/ DZP//MP2/DZP level of theory for the backside and frontside attack mechanisms, respectively. However, the frontside mechanism is predicted to be favored slightly for reactions involving 7 (X ) Sn) at all levels of theory; CCSD(T)/DZP//MP2/DZP

3316 Organometallics, Vol. 28, No. 12, 2009

HorVat and Schiesser

Table 3. Calculated Important Geometric Featuresa of the Transition States 6 (X ) Ge, Sn) and 7 (X ) Ge, Sn) Involved in the Reaction of Acetyl Radical with Dimethylgermane and Dimethylstannane (Scheme 2, X ) Ge, Sn) 6

7

X

method

r1

r2

θ

r1

r2

θ

Ge

UHF/6-311G(d,p) MP2/6-311G(d,p) MP2/cc-pVDZ MP2/aug-cc-pVDZ BHandHLYP/6-311G(d,p) BHandHLYP/cc-pVDZ BHandHLYP/aug-cc-pVDZ UHF/DZP MP2/DZP BHandHLYP/DZP

2.269 Å 2.159 Å 2.152 Å 2.141 Å 2.160 Å 2.161 Å 2.163 Å 2.450 Å 2.354 Å 2.335 Å

2.307 Å 2.287 Å 2.290 Å 2.271 Å 2.308 Å 2.313 Å 2.302 Å 2.272 Å 2.426 Å 2.472 Å

172.8° 175.4° 175.9° 174.6° 173.7° 174.2° 173.1° 177.2° 177.6° 178.7°

2.259 Å 2.114 Å 2.109 Å 2.136 Å 2.132 Å 2.132 Å 2.136 Å 2.441 Å 2.313 Å 2.316 Å

2.336 Å 2.230 Å 2.302 Å 2.266 Å 2.280 Å 2.228 Å 2.266 Å 2.545 Å 2.504 Å 2.518 Å

74.1° 73.1° 73.8° 75.9° 76.0° 76.2° 75.9° 71.0° 71.1° 68.5°

Sn

Table 4. Calculated Energy Barriersa for the Forward (∆E1q) and Reverse (∆E2q) Homolytic Substitution Reactions of Acetyl Radical with Dimethylgermane and Dimethylstannane and Imaginary Frequencies (ν)b of Transition States 6 and 7 (X ) Ge, Sn) 6 X

method

Ge UHF/6-311G(d,p) MP2/6-311G(d,p) ROMP2/6-311G(d,p)//MP2/6-311G(d,p) QCISD/6-311G(d,p)//MP2/6-311G(d,p) CCSD(T)/6-311G(d,p)//MP2/6-311G(d,p) MP2/cc-pVDZ ROMP2/ cc-pVDZ//MP2/cc-pVDZ QCISD/cc-pVDZ//MP2/cc-pVDZ CCSD(T)/cc-pVDZ//MP2/cc-pVDZ MP2/aug-cc-pVDZ ROMP2/aug-cc-pVDZ//MP2/aug-cc-pVDZ QCISD/aug-cc-pVDZ//MP2/aug-cc-pVDZ CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ BHandHLYP/6-311G(d,p) QCISD/6-311G(d,p)//BHandHLYP/6-311G(d,p) CCSD(T)/6-311G(d,p)//BHandHLYP/6-311G(d,p) BHandHLYP/cc-pVDZ BHandHLYP/aug-cc-pVDZ Sn UHF/DZP MP2/DZP ROMP2/DZP//MP2/DZP QCISD/DZP//MP2/DZP CCSD(T)/DZP//MP2/DZP BHandHLYP/DZP QCISD/DZP//BHandHLYP/DZP CCSD(T)/DZP//BHandHLYP/DZP a

7

∆E1

∆E1 +ZPE

∆E2

∆E2q+ZPE

191.7 140.0 151.3 127.6 117.5 135.7 147.0 123.4 113.9 119.6 130.2 109.2 97.6 129.6 127.5 117.5 127.2 127.2 171.3 130.4 143.3 111.1 100.8 110.9 111.6 103.0

190.2 139.1

136.4 63.6 71.5

146.8 73.9

q

q

135.3

q

44.2 59.3 67.0 50.2 41.4

118.3

129.4 127.1 127.7 168.7 129.0

110.1

70.3 61.3

59.3 43.3 33.3 59.4 53.9 56.3 59.8 113.0 51.8 61.4 35.3 25.6 37.1 32.8 27.9

69.9 67.1 69.2 121.8 61.1

46.9

ν

∆E1q

1815i 185.4 1008i 134.1 140.9 130.6 120.7 951i 132.3 139.7 128.9 119.3 934i 111.4 118.1 110.4 98.5 627i 135.9 130.7 120.0 615i 134.0 630i 132.4 2966i 147.4 1173i 114.0 123.0 106.2 97.8 617i 107.7 105.9 97.6

∆E1 +ZPE

∆E2q

∆E2q+ZPE

ν

185.2 134.2

130.1 57.7 61.2 56.8 47.4 55.9 59.7 55.6 46.8 43.4 47.1 44.5 33.9 65.8 57.1

141.8 68.9

629i 531i

67.3 -

525i

54.5

501i

76.6

393i

63.1 65.0 89.1 35.5 41.1 30.3 22.6 33.9 27.1 22.5

73.9 75.1 46.2

390i 384i 560i 511i

44.5

354i

q

132.3 111.5

136.0 134.0 132.6 146.7 114.1

107.6

Energies in kJ mol-1. b Frequencies in cm-1.

calculations predict that the frontside process involving tin is favored by 3.0 kJ mol-1. NBO analyses were performed at the BHandHLYP/6311G(d,p) level of theory on transition states 6 (X ) Ge) and 7 (X ) Ge) and at the BHandHLYP/DZP level of theory on 6 (X ) Sn) and 7 (X ) Sn). Inspection of the NBO data reveals interactions between the acetyl radical (SOMO) and the C-X (X ) Ge, Sn) σ-bonds (Figure 2). Visualization of the Kohn-Sham orbitals (available as Supporting Information Figure S1) depicts the overlap of the two reacting units. For the reaction involving 6 (X ) Ge) and 7 (X ) Ge) the SOMO f σ*Ge-C interaction, calculated to be worth 226 and 230 kJ mol-1, respectively, is apparent in the R spin-set, with a contribution of 1914 and 1084 kJ mol-1 in the β spin-set from the σGe-C f SOMO interaction (Figure S1). This σGe-C f SOMO interaction is calculated to be worth 89% and 83% of the total orbital interaction for 6 and 7, respectively. In comparison, NBO analysis for attack of the acetyl radical at the tin atom in dimethylstannane reveals the SOMO f σ*Sn-C interaction to be worth 125 and 134 kJ mol-1 for transition states 6 (X ) Sn) and 7 (X ) Sn), respectively, evident in the R spinset. The σSn-C f SOMO interaction, evident in the β spin-set, is calculated to contribute 4896 and 4241 kJ mol-1 for 6 (X )

Sn) and 7 (X ) Sn), respectively, which is worth 98% and 97% of the total orbital interaction. Once again these calculations clearly indicate that the acetyl radical is acting as an electrophilic radical in its reactions with dimethylgermane and dimethylstannane. Although electronically different, homolytic substitution reactions (SH2) are, in terms of structural transformation, very similar to the corresponding nucleophilic substitution reactions (SN2). As such, it is interesting to compare our results with those of Bento and Bickelhaupt,26-28 who found that nucleophilic substitution reactions involving a frontside attack mechanism at the silicon, germanium, or tin atom have consistently higher activation energies than the analogous backside attack reaction, but that this preference becomes less pronounced in the progression down the group. This has been attrributed to steric repulsion between the nucleophile and leaving group.27 Similar steric interactions account for the trends observed in this study; indeed, in the case of SH2, the chemistry involving (26) Bento, A. P.; Bickelhaupt, F. M. J. Org. Chem. 2007, 72, 2201. (27) Bento, A. P.; Bickelhaupt, F. M. Chem. Asian J. 2008, 3, 1783. (28) Van Bochove, M. A.; Bickelhaupt, F. M. Eur. J. Org. Chem. 2008, 649.

Homolytic Substitution Reactions of Acyl Radicals

Organometallics, Vol. 28, No. 12, 2009 3317 Scheme 3

Table 5. Imported BHandHLYP/6-311G(d,p) (DZP) Calculated Geometric Featuresa of the Transition States 8 (X ) Si, Ge, Sn; R ) Et, i-Pr, t-Bu) and 9 (X ) Si, Ge, Sn; R ) Et, i-Pr, t-Bu) 8 R

method

r1

r2

θ

r1

r2

θ

Si

Et i-Pr t-Bu Et i-Pr t-Bu Et i-Pr t-Bu

BHandHLYP/6-311G(d,p) BHandHLYP/6-311G(d,p) BHandHLYP/6-311G(d,p) BHandHLYP/6-311G(d,p) BHandHLYP/6-311G(d,p) BHandHLYP/6-311G(d,p) BHandHLYP/DZP BHandHLYP/DZP BHandHLYP/DZP

2.073 2.095 2.111 2.184 2.208 2.226 2.362 2.386 2.405

2.183 2.187 2.195 2.270 2.276 2.285 2.433 2.431 2.442

170.0 167.8 166.1 173.7 170.9 169.4 178.0 175.6 174.3

2.042 2.085 2.067 2.175 2.200 2.231 2.369 2.368 2.440

2.153 2.184 2.138 2.225 2.277 2.235 2.439 2.507 2.435

79.7 75.9 85.5 78.5 75.3 83.2 73.9 73.1 78.6

Ge Sn

a

9

X

Distances in Å and angle in deg.

Table 6. BHandHLYP/6-311G(d,p) Calculated Energy Barriersa for the Forward (∆E1q) and Reverse (∆E2q) Homolytic Substitution Reactions of Acetyl Radical with Ethylmethylsilane, Isopropylmethylsilane, tert-Butylmethylsilane, Ethylmethylgermane, Isopropylmethylgermane, tert-Butylmethylgermane, Ethylmethylstannnane, Isopropylmethylstannane, and tert-Butylmethylstannane and Imaginary Frequencies (ν)b of Transition States 8 and 9 (X ) Si,Ge, Sn; R ) Et, i-Pr, t-Bu) 8

9

X

R

∆E1q

∆E1q+ZPE

∆E2q

∆E2q+ZPE

ν

∆E1q

∆E1q+ZPE

∆E2q

∆E2q+ZPE

ν

Si Si Si Ge Ge Ge Sn Sn Sn

Et i-Pr t-Bu Et i-Pr t-Bu Et i-Pr t-Bu

122.8 114.2 106.8 115.0 106.7 99.3 93.8 82.7 75.1

122.7 115.2 108.8 115.0 106.8 100.5 93.2 83.1 75.6

67.0 79.7 89.3 67.1 79.1 87.9 45.4 56.5 66.0

75.6 88.1 96.5 75.6 86.5 94.2 53.5 64.0 71.1

849i 912i 949i 675i 715i 745i 711i 770i 808i

128.5 122.9 122.6 125.7 117.7 119.1 92.1 82.0 78.7

129.6 126.2 125.4 126.4 119.7 121.5 92.5 83.9 81.9

72.6 88.4 105.1 77.8 90.1 107.6 43.5 55.9 69.6

82.4 99.1 113.1 87.0 99.4 115.3 52.7 64.7 77.4

446i 422i 429i 396i 380i 377i 375i 380i 358i

a

Energies in kJ mol-1. b Frequencies in cm-1.

tin is predicted to slightly favor the frontside mechanism. Furthermore, gas-phase nucleophilic substitution reactions have, in general, a single-well reaction profile;25,27 however, the introduction of bulky substituents at silicon can bring back reaction barriers.25,27 This single-well reaction profile is something that is not observed in the current chemistry involving silicon (Scheme 2, X ) Si), presumably because of the significantly less polar nature of SH2 reactions when compared with their nucleophilic counterparts. Effect of Alkyl Substitution: Homolytic Substitution Reaction of Acetyl Radical with XH2MeR (X ) Si, Ge, Sn, R ) Et, i-Pr, t-Bu). As described above, all of the reactions described so far have been endothermic; in other words, acetyl radicals prefer to be the leaving group rather than the attacking species. In order to explore the factors that affect this chemistry further, we briefly examined the effects of alkyl substitution on the reactions in question. Accordingly, the reactions of acetyl radicals with alkylmethylsilane, -germane, and -stannane (H3CXH2R), in which ethyl, isopropyl, and tert-butyl radicals were chosen as the

leaving groups, were modeled. Because of the significant increases in computational size and given that the BHandHLYP method preformed well in the above-mentioned work, the reaction profiles described in Scheme 3 were examined using this method. Thus, extensive searching of the CH3COXH2CH3R (X ) Si, Ge, Sn, R ) Et, i-Pr, t-Bu) potential energy surfaces at the BHandHLYP/6-311G(d,p) (Si, Ge) or BHandHLYP/DZP (Sn) levels of theory located transition states 8 (X ) Si, Ge, Sn, R ) Et, i-Pr, t-Bu) and 9 (X ) Si, Ge, Sn, R ) Et, i-Pr, t-Bu) as transition states in the homolytic substitution reactions of interest (Scheme 3). The important geometrical features of these structures are summarized in Table 5, and the calculated energy barriers are listed in Table 6. Full structural details are available as Supporting Information. Not unexpectedly, transition states 8 and 9 are calculated to have similar structures to 6 and 7. Backside attack structures are predicted to adopt near collinear arrangement of attacking and leaving radicals, while the frontside transition

3318 Organometallics, Vol. 28, No. 12, 2009

HorVat and Schiesser

states 9 are predicted to have C1 symmetry. Attacking angles (θ) are calculated to be about 73-85°, which are slightly larger than those in the transition states 7. As can be seen in Table 5, C-X distances (r1) in 8 and 9 are calculated to be longer than those in 6 and 7, while X-C separations (r2) in 8 and 9 are predicted to be shorter than those in 6 and 7, indicating that transition states 8 and 9 are calculated to be “earlier” than transition states 6 and 7. Inspection of Table 6 reveals that all of the reactions in question are still predicted to be endothermic, despite alkyl substitution on the leaving radical. However, calculated energy barriers (∆E1q) for the forward reaction decrease in the order methyl > ethyl > isopropyl > tert-butyl groups, while those (∆E2q) for the reverse process slightly increase in the same order of leaving radical. Importantly, the computational data indicate that both frontside and backside attack mechanisms are predicted to be feasible. The backside mechanism involving transition states 8 are favored by 6-16 kJ mol-1 for attack at silicon and by 10-20 kJ mol-1 for attack at germanium. Interestingly, frontside attack (9) is favored by 1.1 kJ mol-1 for attack at tin with expulsion of ethyl and isopropyl radicals, while the backside attack transition state (8) is favored when the tert-butyl radical is the leaving group.

employed. These high energy levels suggest these reactions are unlikely to be synthetically useful. However, this study has shed some light on the factors that control these types of reactions. In no system in this study is the difference in activation energy calculated for the backside and frontside attack mechanisms significantly large enough to rule out either mechanism; as such, both mechanism are predicted to be feasible. In addition calculations have predicted that the acetyl radical is acting predominately as an electrophilic radical in this chemistry, with NBO calculations suggesting that the σX-C f SOMO interaction are worth more than 83% of the total transition state orbital interaction for all reactions in this study. As expected, increasing the stability of the leaving radical is associated with commensurate reductions in the energy barriers with these reactions.

Conclusion

Supporting Information Available: Figure S1 and Gaussian Archive entries for the optimized transition structures (6-9). This material is available free of charge via the Internet at http://pubs.acs.org.

Ab initio and DFT calculations predict that homolytic substitution reactions of acetyl radicals at the heteroatom in dimethylsilane, dimethylgermane, and dimethylstannane, with the expulsion of methyl radical, are associated with high energy levels and are endothermic at all levels of theory

Acknowledgment. This work would not have been possible without the generous support of the Australian Research Council through the Centers of Excellence Program. We also gratefully acknowledge the support of the Victorian Institute for Chemical Sciences High Performance Computing Facility and the Australian Partnership for Advanced Computing.

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