Reactions of alkenes and alkynes with an acyclic silylene and heavier tetrylenes under ambient conditions

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INTRODUCTION
Silacycles have been studied because of their importance in organic syntheses and their ability to effect stereospecific, stereoselective, regioselective, and chemoselective carboncarbon bond formations under mild conditions. [1][2][3] Silacycles can be obtained by the addition of silylenes to unsaturated hydrocarbons. The groups of Gaspar and Seyferth reported first examples of stable silacyclopropenes by the addition of silylenes to alkynes in 1976. 4,5 The reaction of silylenes with alkenes afford silacyclopropanes (also called siliranes) 6 and were first synthesized by Seyferth in 1972. 7 Both the silacyclopropenes and silacyclopropanes are formed in a concerted stereospecific manner via [2+1] cycloadditions. 8 Additions of 1,3-dienes to silylenes have been found to depend on the steric bulk of the silylene subsituents. The reaction of Si(Mes)Tbt (Mes = 2,4,6trimethylphenyl;Tbt = 2,4,methyl}phenyl) with isoprene proceeds via a [2+1] cycloaddition to yield a vinylsilirane that rearranges to a silacyclopent-3-ene. 8b, 9 However, for a more sterically crowded dialkylsilylene 2,2,5,5tetrakis(trimethylsilyl)silacyclopentan-1,1-diyl, a direct [4+1] cycloaddition occurred to yield silacyclopent-3-ene. 10 In our previous investigations on the addition of the acyclic silylenes Si(SAr Me 6 )2 11 and Si(SAr iPr 4 )2 12 to the alkenes ethylene and norbornadiene, we obtained the silirane compounds (Ar iPr 4 S)2Si(CH2)2 and (Ar Me 6 S)2Si(C7H8). The first species features unique reversible binding to ethylene in toluene solutions with a Gibbs free energy of association (ΔGassn) of −24.9 ± 2.5 kJ mol −1 , 13 a value low enough to be greatly affected by entropic effects.
The compounds 1-3 were characterized by 1 H, 13 C{ 1 H}, and 29 Si NMR spectroscopy and by X-ray structural analysis. Compound 1 crystallizes in the centrosymmetric, orthorhombic space group Pccn. The C(31)−C(38) group is found to exist at half occupancy in two orientations of which only one is shown in Figure 1.
Compound 2 also crystallizes in the centrosymmetric, orthorhombic space group Pccn.
The molecular structure exhibits a mirror plane of symmetry perpendicular to the silacyclopropene ring (Si1-C31-C31A). Compound 3, which crystallizes in the triclinic space group P1, has no required symmetry element.
We were also interested to see if the germylene Ge(SAr Me 6 )2 and the stannylene Sn(SAr Me 6 )2 reacted with ethylene in a similar manner as with Si(SAr iPr 4 )2. However, no reaction was observed for the Ge and Sn analogues under similar reaction conditions. In order to understand why the reactivity of the heavier tetrylenes differs from those of their Si analogues, a series of DFT calculations was carried out for model systems E(SPh)2 (E = Ge, Sn). This simplified structure was used in our earlier study of electronic structure and reactivity of the silylene Si(SPh)2 with ethylene, reproducing the experimentally deduced energies in good precision. 13 The energies of the investigated reactions are summarized in Figure 4 along with those corresponding to the formation of the silirane (PhS)2SiCH2CH2. 13  It should be noted that the instantaneous interaction energy between E(SPh)2 and ethylene is negative for all elements E, whereas the overall reaction energy is negative only for E = Si. The differences in the calculated energies are due to the distortion energy, i.e. the energy required to distort the interactive fragments to the geometry they have in the product, which becomes more positive for the heavier tetrels germanium and tin. Thus, it is only in the case of the silirane (PhS)2SiCH2CH2 when the distortion energy is small enough that it does not completely negate the favorable bonding interactions between the tetrylene and ethylene. As a whole, the computational results are in agreement with the notion that, while going down the group 14, the lone pair at the tetrel center becomes more inert and the E−C bond energies decrease. The energy gain from the formation of two Ge−C and Sn−C bonds is simply not enough to overcome the energy required to break a C═C double bond and form the pseudo-tetrahedral bonding arrangement around the tetrel center.
After investigating the energetics of the reaction of ethylene with the heavier tetrylenes, the reaction of silylene Si(SPh)2 with the simplest alkyne, acetylene, was modelled for comparison ( Figure 5). The results show that the cycloaddition proceeds via single transition state with an activation barrier of 54 mol 1 (Gibbs energy with the zero level set at reactants); the relative energy of the product is 75 kJ mol 1 . Thus, the reaction is expected to be facile, as it is, and not reversible because the barrier from the product back to the reactants is 129 kJ mol 1 .

CONCLUSION
In conclusion, three-and five-membered silicon heterocycles can be easily accessed at ambient conditions by the reaction of the acyclic silylene Si(SAr iPr 4 )2 with the alkynes phenylacetylene and diphenylacetylene, and the diene 2,3-dimethyl-1,3-butadiene, yielding silacyclopropenes and a silacyclopentene, respectively. For the reactions of the germylenes and stannylenes E(SAr Me 6 )2 (E = Ge, Sn) with ethylene, the results of computational studies suggest that the E−C bonding interactions in the products (Ar Me 6 S)2ECH2CH2 are not able to overcome the energy required to distort the geometries of the fragments in order for the reaction to take place. Even the silirane product (Ar Me 6 S)2SiCH2CH2 is already fairly loosely bound, as evident from the reversible reaction observed experimentally and predicted computationally for (PhS)2SiCH2CH2.

COMPUTATIONAL DETAILS
The geometries of the model tetrylenes E(SPh)2 (E = Si, Ge, Sn), ethylene, and the corresponding products (SPh)2ECH2CH2 were fully optimized. Transition states connecting the two minima were searched with relaxed potential energy surface scans and optimizing the candidate structures (the ones with the highest energy) with a transition state search algorithm. Frequency calculations were then carried out to ensure that the stationary points found lie at true potential energy minima or show a single imaginary frequency corresponding to a vibration along the reaction coordinate (transition states). The interactions between the E(SPh)2 and ethylene fragments in the optimized products (SPh)2ECH2CH2 were analyzed by the Ziegler-Rauk-Morokuma energy decomposition scheme (EDA). 21 All geometry optimizations, potential energy surface scans, and frequency calculations were carried out with the Gaussian 09 program suite 22 using density functional theory (DFT) in conjunction with the PBE0 exchange correlation functional 23 and the def-TZVP basis sets. 24 A corresponding large core effective core potential (ECP) basis set was used for the Sn atom. 25 The EDA calculations were performed with the ADF 2013.01 suite of programs 26 using the PBE0 functional and the default TZ2P STO-type basis sets. 27 Scalar relativistic effects were taken into account with the zeroth order regular approximation (ZORA) as implemented in ADF. 28

ASSOCIATED CONTENT
Supporting Information. Spectral data and crystallographic details and CIFs for compounds 1(•1.5PhMe), 2, and 3(•1.5PhMe). Optimized geometries of the studied compounds and full details of the energy decomposition analyses. This material is available free of charge via the Internet at http://pubs.acs.org.