Organometallics 2010, 29, 6739–6743 DOI: 10.1021/om100868b
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Ring-Opening Mechanism of Lithium Bromosilacyclopropylidenoids to Silaallenes Akin Azizoglu*,† and Cem B. Yildiz‡ †
Department of Chemistry, University of Balikesir, TR-10145, Balikesir, Turkey, and ‡ Department of Chemistry, Aksaray University, TR-68100, Aksaray, Turkey Received September 7, 2010
Density functional theory and ab initio quantum mechanical calculations elucidated the ringopening reactions of 1-bromo-1-lithiosilirane (3) and 2-bromo-2-lithiosilirane (4) to 2-silallene (6) and 1-silaallene (7), respectively. The ring-opening of 3 to 6 can proceed in a stepwise fashion with the intermediacy of a free silacyclopropylidene (9). Here, a high-energy barrier needs to be overcome in order to open the silacyclopropylidene ring and to generate 6. On the contrary, the ring-opening of 4 to 7 can occur in a concerted fashion. The chemistry of silaallenes has been attracting more and more interest in the past few decades because of their unique structures and the broad differences in their properties compared to the corresponding carbon compounds.1 The first stable silaallene was synthesized by West in 1993. It was stabilized by an extremely large steric hindrance around the SidCdC moiety and characterized by X-ray crystallography, revealing that it is slightly bent (173.5°), in contrast to the carbon analogue allene, which is linear.2 Another X-ray determination indicating a bending angle of 174.2° at the central sp carbon atom has been reported for a silyl-substituted 1-silaallene by Pietschnig.3 Unlike 1-silaallenes, 2-silaallenes have never been isolated but only postulated as transient species in some chemical reactions.4 Cyclopropylidenes, the carbenes or carbenoids of cyclopropanes, have been recognized as highly reactive carbon species and are frequently used as useful intermediates in organic synthesis.5 The reaction of 1,1-dibromocyclopropanes with alkyllithiums leads in many cases to fair yields of allenes, apparently derived through lithium-bromine exchange and
then loss of lithium bromide to give cyclopropylidenes.6 This reaction is called the Doering-Moore-Skattebøl reaction.7,8 Theoretical calculations from the group of Apeloig suggested that the ring-opening of disilacyclopropylidenes provides a possible route to 1,3-disilaallenes, R2SidCdSiR2, still unknown experimentally.9 More recently, we have investigated the ring-opening reactions of substituted lithium bromocyclopropylidenoids (1) to allenes (2) computationally and considered two pathways: the ring-opening reaction may proceed either in a concerted fashion or stepwise with the intermediacy of a free cyclopropylidene. In both cases, the loss of bromide ion determines the kinetics of the reaction. Moreover, cyclopropylidenes bearing an electron-donating group (þM) are extremely unstable and readily lead to the allene via a concerted pathway (Scheme 1). In contrast, bromocyclopropylidenoids with electron-withdrawing groups are particularly stable species.10 Herein we would like to present our results of theoretical calculations on 1-bromo-1-lithiosilirane (3) and 2-bromo-2lithiosilirane (4) and their corresponding free divalent ones. We also report their ring-opening mechanisms to form silaallenes.
*To whom correspondence should be addressed. E-mail: azizoglu@ balikesir.edu.tr. (1) For reviews, see: (a) Escudie, J.; Ranaivonjatovo, H.; Rigon, L. Chem. Rev. 2000, 100, 3639–3696. (b) Eichler, B.; West, R. Adv. Organomet. Chem. 2001, 46, 1–46. (c) Karni, M.; Apeloig, Y.; Kapp, J.; Schleyer, P. von, R. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John Wiley & Sons: Chichester, 2001; Vol. 3, pp 1-163. (d) Escudie, J.; Ranaivonjatovo, H. Organometallics 2007, 26, 1542– 1559. (2) (a) Miracle, G. E.; Ball, J. L.; Powell, D. R.; West, R. J. Am. Chem. Soc. 1993, 115, 11598–11599. (b) Trommer, M.; Miracle, G. E.; Eichler, B. E.; Powell, D. R.; West, R. Organometallics 1997, 16, 5737–5747. (3) Spirk, S.; Belaj, F.; Albering, J. H.; Pietschning, R. Organometallics 2010, 29, 2981–2986. (4) (a) Bertrand, G.; Manuel, G.; Mazerolles, P. Tetrahedron 1978, 34, 1951–1956. (b) Urbanova, M.; Volnina, E. A.; Gusel'nikov, L. E.; Bastl, Z.; Pola, J. J. Organomet. Chem. 1996, 509, 73–76. (c) Goetze, B.; Herrschaft, B.; Auner, N. Chem.;Eur. J. 1997, 3, 948–957. (5) (a) De Meijere, A.; Faber, D.; Heinecke, U.; Walsh, R.; Muller, T.; Apeloig, Y. Eur. J. Org. Chem. 2001, 663–680. (b) Mieusset, J. -L.; Brinker, U. H. J. Org. Chem. 2005, 70, 10572–10575. (c) Satoh, T. Chem. Soc. Rev. 2007, 36, 1561–1572. (d) Averina, E. B.; Sedenkova, K. N.; Borisov, I. S.; Grishin, Y. K.; Kuznetzova, T. S.; Zefirov, N. S. Tetrahedron 2009, 65, 5693–5701. (e) Kilbas, B.; Azizoglu, A.; Balci, M. J. Org. Chem. 2009, 74, 7075–7083.
(6) (a) Backes, J.; Brinker, U. H. In Houben-Weyl (Methoden der Organischen Chemie); Regitz, M., Ed.; Thieme: Stuttgart, Germany, 1989; Vol. E19b, pp 391-510. (b) Siegel, H. Top. Curr. Chem. 1982, 106, 55–78. (7) (a) Doering, W. v. E.; LaFlamme, P. M. Tetrahedron 1958, 2, 75– 79. (b) Moore, W. R.; Ward, H. R. J. Org. Chem. 1960, 25, 2073–2073. (c) Moore, W. R.; Ward, H. R.; Merritt, R. F. J. Am. Chem. Soc. 1961, 83, 2019– 2020. (d) Skattebøl, L. Tetrahedron Lett. 1961, 5, 167–172. (8) Some recent papers for the Doering-Moore-Skattebøl reaction: € (a) Algı, F.; Ozen, R.; Balcı, M. Tetrahedron Lett. 2002, 43, 3129–3131. € (b) Azizoglu, A.; Ozen, R.; H€okelek, T.; Balci, M. J. Org. Chem. 2004, 69, 1202–1206. (c) Christl, M.; Brauen, M.; Fischer, H.; Groetsch, S.; M€uller, G.; Leusser, D.; Deurlein, S.; Stalke, D.; Arnone, M.; Engels, B. Eur. J. Org. Chem. 2006, 5045–5058. (d) Azizoglu, A.; Demirkol, O.; Kilic, T.; Yildiz, Y. K. Tetrahedron 2007, 63, 2409–2413. (e) Eccles, W.; Jasinski, M.; Kaszynski, P.; Zienkiewicz, K.; Stulgies, B.; Jankowiak, A. J. Org. Chem. 2008, 73, 5732–5744. (f) Christl, M.; Fischer, H.; Arnone, M.; Engels, B. Chem.;Eur. J. 2009, 15, 11266–11272. (g) Mahlokozera, T.; Goods, J. B.; Childs, A. M.; Thamattoor, D. M. Org. Lett. 2009, 11, 5095–5097. (9) Sigal, N.; Apeloig, Y. J. Organomet. Chem. 2001, 636, 148–156. (10) Azizoglu, A.; Balci, M.; Mieusset, J. -L.; Brinker, U. H. J. Org. Chem. 2008, 73, 8182–8188.
r 2010 American Chemical Society
Published on Web 11/23/2010
pubs.acs.org/Organometallics
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Azizoglu and Yildiz Table 1. Calculated Bond Lengths and Bond Elongations in 3 and 4 As Compared to the Bond Lengths of H3Si-Br (2.229 A˚), H3Si-Li (2.473 A˚), Li-Br (2.167 A˚), H3C-Br (1.965 A˚), and H3C-Li (1.980 A˚), Respectively, at the B3LYP/6-31G(d) Level of Theory Si(1)-Br
3
d [A˚] 2.587
Si(1)-Li
elong. [%]
d [A˚]
16.1
2.452
C(2)-Br
4
Figure 1. Optimized structures and energies (E, in au) of 1-bromo-1-lithiosilirane (3) and 2-bromo-2-lithiosilirane (4). Bond lengths (A˚) and bond angles (deg) are at the B3LYP/6-31G(d) level. Scheme 1
(11) Frisch, M. J.; et al. et al. Gaussian 03, revision C.2; Gaussian, Inc.: Wallingford, CT, 2004. Full reference given in the Supporting Information. (12) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: Oxford, U.K., 1989. (13) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161–4162. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363–368. (c) IRC plots are available in the Supporting Information. (14) (a) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523– 5527. (b) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1991, 95, 5853–5860. (15) GaussView, Version 3.09; Dennington, R., II.; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, W. L.; Gilliland, R. Semichem, Inc.: Shawnee Mission, KS, 2003. (16) (a) Cramer, C. J.; Worthington, S. E. J. Phys. Chem. 1995, 99, 1462–1465. (b) Kakkar, R.; Padhi, B. S. Int. J. Quantum Chem. 1996, 58, 389–398. (17) (a) Barthelat, J.-C.; Trinquier, G.; Bertrand, G. J. Am. Chem. Soc. 1979, 101, 3785–3789. (b) Su, M.-D.; Amos, R. D.; Handy, N. C. J. Am. Chem. Soc. 1990, 112, 1499–1504.
elong. [%]
elong. [%]
-0.85
2.316
6.88
C(2)-Li
Li-Br
d [A˚]
elong. [%]
d [A˚]
elong. [%]
d [A˚]
elong. [%]
2.174
10.6
1.946
-1.72
2.350
8.44
Table 2. Calculated Wiberg Bond Orders for 3, 4, 5, TS1, TS2, and TS3 at the B3LYP/6-31G(d) Level of Theory
3 TS1 5 TS2
4 TS3
Initially, all structures were optimized using the Gaussian 03 suite of programs11 at the B3LYP/6-31G(d) level.12 The results of calculations at B3LYP/6-31G(d) were repeated with full geometry optimization at higher levels;: B3LYP/ 6-31þG(d,p), MPW1PW91/6-31þG(d,p), and MP2/6-31 þG(d,p). The stationary points were characterized as minima or transition structures by vibrational frequency calculations, and all energies reported here are corrected with unscaled zero-point vibrational energies. The intrinsic reaction coordinates (IRC)13 were also followed to verify the energy profiles connecting each transition state to the correct local minima, by using the second-order Gonzalez-Schlegel method.14 The computed structures were visualized by using the GaussView program. 15 Carbenes were considered as singlets because this represents the ground state of cyclopropylidenes.16 The singlet is also the ground-state for silacyclopropylidenes.17 1,1-Dibromosilirane and 2,2-dibromosilirane may react with methyllithium to give 1-bromo-1-lithiosilirane (3) and 2-bromo-2-lithiosilirane (4), respectively, via the lithiumbromine exchange reaction. Hence, we began by optimizing possible conformations of 3 and 4 at the B3LYP/6-31G(d) level, which led to their most stable conformers, shown in
Li-Br d [A˚]
Si(1)-Br
Si(1)-Li
Li-Br
0.580 0.075 0.026 0.014
0.124 0.125 0.125 0.132
0.121 0.255 0.282 0.279
C(2)-Br
C(2)-Li
Li-Br
0.838 0.426
0.101 0.096
0.092 0.137
Figure 1. From their calculated energy values, 1-bromo1-lithiosilirane (3) is much more stable, by 48.7 kcal/mol, than 2-bromo-2-lithiosilirane (4). The “classical” tetrahedral structures of 3 and 4 were not located on their potential energy surfaces. The structural data show that Li as well as Br are positioned on one side of the silirane ring plane and the C-Br bond is bridged by lithium, and all ligands of the carbene center are located in a cone. Hence, the geometric structures of 3 and 4 correspond to distorted-tetrahedron coordination. The CSiC bond angle and the BrLiSiC dihedral angle in 3 are 47.1° and 49.5°, respectively, while the SiCC bond angle and the BrLiCSi dihedral angle in 4 are 67.5° and 110.3°, respectively. Their remarkable structural data are also summarized in Table 1. A positive value of [%] indicates an elongation, whereas a negative value of [%] means shortening with regard to the reference bond. Concerning the Li-Br distances in 3 and 4, the elongations, as compared to the bond length of Li-Br (2.167 A˚), are in the range of 6.7% and 8.3%, respectively. Like the carbon analogue,8 the calculated C(2)-Li bond length on the carbene center of 4 is 1.946 A˚. Moreover, Si(1)-Li bond length in 3 is calculated as 2.452 A˚. This corresponds to minimal changes in the structures of 3 and 4 if compared with the C-Li bond length (1.980 A˚) in H3C-Li and the Si-Li bond length (2.473 A˚) in H3Si-Li, respectively. However, C(2)-Br and Si(1)-Br bond elongations are 10.6% (minimum for 4) and 16.1% (maximum for 3) as compared to the bond in H3C-Br (1.965 A˚) and that in H3Si-Br (2.229 A˚), respectively. This means that the most strongly elongated bond in the studied molecules is Si(1)-Br. Alternatively, Wiberg bond orders (WBO)18 of several bonds of 3 and 4 calculated by using NBO analyses are tabulated in Table 2 to quantify their chemical bond formation and disruption. A large WBO describes a strong covalent bonding interaction between two relevant atoms. The results show that the C(2)-Br bond of 4 has the strongest (18) Wiberg, K. B. Tetrahedron 1968, 24, 1083–1096.
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Figure 2. Calculated reaction path at the B3LYP/6-31G(d) level for the stepwise ring-opening of 1-bromo-1-lithiosilirane (3) to the complex 2-silaallene (6) with LiBr. Bond distances and bond angles are given in angstroms and degrees, respectively. WBO values are indicated in parentheses.
covalent interactions within the studied carbenoids due to its highest bond order value, 0.838. On the contrary, the Si(1)-Br bond of 3 has a WBO of 0.580, which describes a weak covalent bonding interaction. Furthermore, Si(1)-Li and C(2)-Li bonds have an ionic rather than covalent character because their calculated WBOs are quite small (0.124 and 0.101, respectively). In this work on the reactivity of 1-bromo-1-lithiosilirane (3) and 2-bromo-2-lithiosilirane (4), only two extreme cases will be considered: the results obtained from the calculations on lithium bromosilacyclopropylidenoids can give insight into the maximal effect of the lingering lithium and bromide ions upon the silylene reactivity, whereas computations of the free silacyclopropylidenes can help to describe the chemistry of the silylene. For the ring-opening reactions of lithium bromosilacyclopropylidenoids to silaallenes, two pathways can be considered: the reaction may either proceed stepwise with the intermediacy of a free silacyclopropylidene or in a concerted fashion. The energy profile for the stepwise ring-opening of 1-bromo-1-lithiosilirane (3) is shown in Figure 2. The elimination of LiBr from 3 is the first step of stepwise ringopening, and it starts readily with the transition state TS1 (3f5), where the Si-Br bond breaking occurs. Then, silylenoid 3 rearranges to the silicon-lithium complex 5, where positively charged Li is coordinated to the lone pair of the silylene. In 5, the Br atom interacts only with the Li atom and not with other atoms. The Si-Li distance is 2.686 A˚ at B3LYP/6-31G(d) and 0.457 A˚ longer than that in H3SiLi, while Li-Br distance is only 0.014 A˚ longer than in the LiBr
molecule, indicating that there exists a strong interaction between Li and Br atoms, which makes the interaction between Li and Si atoms weaker. The natural atomic charges on the silylenic Si(1) for 3, TS1, and 5 at the B3LYP/631G(d) level are þ0.443, þ0.791, and þ0.815, respectively. The results indicate that the release of the bromide ion from 3 causes the increase of electrophilic character of silicon. The NBO analysis also confirms this depiction of the release of lithium bromide. Silylenoid 3 is presented as a compound consisting of two units, a SiC2H4Br moiety with a formal charge of -0.870 and a lithium ion. In this description of structure 3, the Si-Br bond is strongly populated with an occupancy of 1.991 electrons. The interactions between the two units are principally because of electron donation from the lone pair on the silicon atom assisted by electron donation from the bromine. On the contrary, an association of a SiC2H4 moiety with LiBr describes the σ-complex structure 5, where the silacyclopropylidene unit is slightly positively charged (þ0.065 e) since electron donation to lithium still occurs as revealed by a secondorder perturbation analysis of the Fock matrix. The calculated energy barriers for the isomerization of silylenoid 3 to 5 at various levels (B3LYP/6-31G(d), B3LYP/ 6-31þG(d,p), MPW1PW91/6-31þG(d,p), and MP2/6-31þG(d,p)) are predicted to be 11.6, 12.6, 17.0, and 14.8 kcal/mol, whereas that for the back-reaction of 5 are found to 1, 0.7, 0.7, and 0.8 kcal/mol, respectively. The low backward values reveal that 5 undergoes rapid rearrangement even at low temperatures, with 3 being the dominant molecule. In the second step, the three-membered ring collapses to the
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Table 3. Calculated Energy Barriers (kcal/mol) for the Concerted and Stepwise Isomerizations of 3 and 4 at the B3LYP/6-31G(d), B3LYP/6-31þG(d,p) (in Parentheses) and MPW1PW91/6-31þG(d,p) (Underlined), MP2/6-31þG(d,p) (in Parentheses and Underlined) Levels of Theory stepwise a
3 4 a
concerted a
TS1
5
TS2
TS3b
11.6/(12.6)/17.0/(14.8) -c
10.6/(11.9)/16.3/(14.0) -c
48.5/(49.6)/60.7/(57.1) -c
-c 1.2/(1.7)/5.5/(5.2)
Energies in kcal/mol relative to 3. b Energies in kcal/mol relative to 4. c No structures on potential energy surface.
Figure 3. Calculated reaction path at the B3LYP/6-31G(d) level for the concerted ring-opening of 2-bromo-2-lithiosilirane (4) to the complex 1-silaallene (7) with LiBr. Bond distances and bond angles are given in angstroms and degrees, respectively. WBO values are indicated in parentheses.
complex 2-silaallene with LiBr by overcoming a very high activation energy barrier (TS2 (5f6)), 37.9 kcal/mol at B3LYP/6-31G(d). The overall barrier is also very high, 48.5 kcal/mol. A comparison of ring-opening of 3 to 6 with that of the all-carbon analogue, cyclopropylidene, to allene is of interest. The ring-opening of 3 to 6 follows a simple disrotatory motion of the methylene groups. The ringopening of cyclopropylidene to allene also starts with a disrotatory motion of the methylene groups, but additional geometry changes are required to reach the final linear geometry of allene.10,19 The overall energy barrier for stepwise ring-opening of unsubstituted cyclopropylidenoid to allene is low, 8.6 kcal/mol, but in this case the reaction is highly exothermic, by 32.6 kcal/mol (B3LYP/6-31G(d)).10 The WBO values of TS1 and TS2 computed by NBO analysis for stepwise ring-opening of 3 are also given in Figure 2 (in parentheses). From the result of TS1, it can be deduced that Si(1)-Br bond breaking occurs and the silicon-carbon bonds in the cyclopropane moiety get a little stronger. Moreover, the WBO values of Si(1)-C(2) and Si(1)-C(3) for TS2 are equal to 1.332, which indicates the formation of π-bonds. On the contrary, the WBO value of (19) Bettinger, H. F.; Schreiner, P. R.; Schleyer, P. v. R.; Schaefer, H. F. J. Phys. Chem. 1996, 100, 16147–16154.
C(2)-C(3) is 0.376, which represents the breaking of the σ-bond between C(2) and C(3). The stepwise ring-opening of 2-bromo-2-lithiosilirane (4) has been also investigated, but the attempts to locate a minimum for the structures related with stepwise ring-opening at the levels of B3LYP/6-31G(d), B3LYP/6-31þG(d,p), MPW1PW91/ 6-31þG(d,p), and MP2/6-31þG(d,p) lead directly to the corresponding allene. To indicate that no suitable geometry and energy results were obtained, dashes (-) were placed in Table 3. For 4, the formation of 1-silaallene can be computed only as a concerted process (TS3 (4f7)), where the isomerization of a carbenoid to a silaallene may occur readily without the intermediacy of a free carbene (Figure 3). The Si(1)-C(3) of TS3 has a low bond order value of 0.596, which indicates a weakening of the covalent bonding interactions between Si(1) and C(3) atoms. On the contrary, the Si(1)-C(2) and C(2)-C(3) bond orders for TS3 are higher, 1.063 and 1.408, respectively, which represents the formation of a π-bond between the interconnected atoms. Moreover, the calculated reaction barrier for the concerted ring-opening of 4 to 1-silaallene (7) is found to be very low, 1.2 kcal/ mol, but in this case the reaction is moderately exothermic, by 44.3 kcal/mol at the B3LYP/6-31G(d) level. A suitable transition structure of concerted ring-opening of 1-bromo-1-lithiosilirane (3) could not be found by a fuller
Article
Figure 4. IRC plot of 8 at the B3LYP/6-31G(d) level. Bond distances and bond angles of 7 are given in angstroms and degrees, respectively. WBO values are indicated in parentheses.
exploration of the potential energy surface. Instead, transition structure TS2 was located and characterized from the computations at the B3LYP/6-31G(d), B3LYP/6-31þG(d,p), MPW1PW91/6-31þG(d,p), and MP2/6-31þG(d,p) levels. IRC calculations linking transition structure TS3 to the complex 2-silaallene (6) with LiBr also could not be obtained (see Supporting Information). Like the results of Apeloig,20 we have also found that 1-silaallene (7) is energetically more stable than 2-silaallene (6) by 14.8 kcal/mol including zero-point energy at the MP2/6-31þG(d,p) level of theory. This may be the result of the low ability of silicon to participate in multiple bonds. Alternatively, the ring-openings to 1- and 2-silaallenes may occur starting from free carbene (8) and free silacyclopropylidene or 1-siliranylidene (9), respectively (Figures 4 and 5). Although the optimized geometry of 8 at the HF/ 3-21G level was reported long before,21 Sigal and Apeloig showed that the disilicon analogue of 8 is not a minimum on the potential energy surface at the B3LYP/6-31G(d), HF/ 6-31G(d), and MP2/6-31G(d) levels.9 Likewise, our calculations at all the theoretical levels examined herein reveal that the cyclopropylidene structure 8 is not a minimum on the potential energy surface. Instead, all attempts to locate this structure lead directly to 1-silaallene (7). Accordingly, no transition structures for the isomerization of 8 to 7 can be investigated by IRC calculations (Figure 4 and see Supporting Information). This finding means that no energy barrier for the conversion of 8 to 7 actually exists. The chemistry of silacyclopropylidene (9) has been the subject of various experimental and theoretical investigations.22,23 Maier and co-workers reported the matrix isolation of 9 from the reaction of silicon atoms with ethylene at 10 K for the first time. It was identified by the comparison of its experimental IR spectra and that obtained by DFT calculations at the BLYP/6-31G(d,p) level. For instance, the observed and computed symmetrical stretching frequencies of C(2)-C(3) in 9 were 1011.9 and 1029.9 cm-1, respectively.22 They are 1065.3, 1046.5, 1071.6, and 1083.9 cm-1 at the B3LYP/6-31G(d), B3LYP/6-31þG(d,p), MPW1PW91/ 6-31þG(d,p), and MP2/6-31þG(d,p) levels, respectively. Moreover, singlet and triplet states of silacyclopropylidene (20) Sigal, N.; Apeloig, Y. Organometallics 2002, 21, 5486–5493. (21) Gordon, M. S.; Koob, R. D. J. Am. Chem. Soc. 1981, 103, 2939– 2944. (22) Maier, G.; Peter, H.; Reisenauer, P.; Egenolf, H. Eur. J. Org. Chem. 1998, 1313–1317. (23) (a) Skancke, P. N.; Hrovat, D. A.; Borden, W. T. J. Phys. Chem. A 1999, 103, 4043–4048. (b) Becerra, R.; Cannady, J. P.; Dormer, G.; Walsh, R. J. Phys. Chem. A 2008, 112, 8665–8677.
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Figure 5. Calculated reaction path (at B3LYP/6-31G(d)) for the ring-opening of 9. Energies are given in kcal/mol. Bond distances and bond angles are given in angstroms and degrees, respectively. WBO values are indicated in parentheses.
(9) were computed at the B3LYP/6-31G(d) level, and the singlet state is determined to be of lower energy than the triplet one by 42.9 kcal/mol. Due to the high singlet-triplet energy separations, we consider only singlet 9, for which the ring-opening barrier leading to 2-silaallene (6) is also found to be 43.7, 42.7, 49.0, and 47.3 kcal/mol at the B3LYP/ 6-31G(d), B3LYP/6-31þG(d,p), MPW1PW91/6-31þG(d,p), and MP2/6-31þG(d,p) levels, respectively. Here, a highenergy barrier needs to be overcome in order to open the silacyclopropylidene ring and to generate 6. The ring-opening reaction of 9 is also highly endothermic by 26.1 kcal/mol at the B3LYP/6-31G(d) level. On the contrary, previous DFT calculations depicted that the activation energy barrier for the ring-opening of carbon analogue of 9 to allene is very low, 6.1 and 4.8 kcal/mol, and the reaction is moderately exothermic, by 67.9 and 69.3 kcal/mol at the B3LYP/ 6-31G(d) and B3LYP/TZP levels, respectively.10,19 In summary, the ring-opening reaction of 1-bromo-1lithiosilirane (3) to the complex 2-silaallene (6) can occur in a stepwise fashion with the intermediacy of a free silacyclopropylidene, whereas the ring-opening of 2-bromo-2-lithiosilirane (4) to the complex 1-silaallene (7) can proceed in a concerted fashion. Moreover, the ring-opening reaction of 3 is endothermic by 11.0 kcal/mol, while that of 4 is highly exothermic, by 44.3 kcal/mol. Thus, the ring-opening reaction for 4 is thermodynamically more favorable by 55.3 kcal/mol than the ring-opening of 3. We also obtained no structure as a minimum for cyclopropylidene 8, which is electronically unstable and readily leads to 1-silaallene. In contrast, silacyclopropylidene (9) is calculated to be stable, and a high-energy barrier needs to be overcome in order to generate 2-silaallene. Hence, the structural and ring-opening differences between silacyclopropylidene and cyclopropylidene demonstrate that carbon chemistry is a poor guide for predicting the properties of low-coordination silicon compounds.
Acknowledgment. We thank the Scientific and Technical Research Council of Turkey (TUBITAK) for generous financial support. We are also grateful to the reviewers for their helpful comments. Supporting Information Available: Tables listing energies, Cartesian coordinates, zero-point energies, and imaginary vibrational frequencies of transition states for all the calculated species. This material is available free of charge via the Internet at http://pubs.acs.org.
