Facile access to silyl-functionalized N-heterocyclic olefins with HSiCl 3

N-heterocyclic olefins (NHOs), IPrCH2 (1) and SIPrCH2 (2) (IPrCH2 = {N(2,6-iPr2C6H3)CH}2CCH2 and SIPrCH2 = {N(2,6-iPr2C6H3)CH2}2CCH2), react with HSiCl3 and afford IPrCH(SiHCl2) (3) and SIPrCH(SiHCl2) (4), respectively. Compounds 3 and 4 have been isolated in almost quantitative yield. Interestingly, treatment of the silylene IPr·SiCl2 with 1 also affords 3, where silylene insertion into a C–H bond is observed. Computational analysis shows a high energy barrier for silylene insertion, therefore a protonation–deprotonation mechanism is more likely.

2][3][4] Among group 14 elements, carbenes and silylenes are the most studied reactive species. 53][4][5][6] Carbenes were believed to be non-isolable highly reactive intermediates.This situation changed when Bertrand et al. reported a phosphino stabilized carbene 7 in 1989.In 1991, Arduengo and co-workers reported the first thermally stable carbene as a N-heterocyclic carbene (NHC) (Scheme 1A). 8However, it took almost three years to isolate a silicon analogue of a NHC.][4] The electron richness and structure of the NHCs provide a unique class of s-donor ligands.NHCs have found widespread applications not only as ligands in transition-metal catalysis and organometallic chemistry, but also as organocatalysts in their own right. 10,11Furthermore, NHCs have been found to be very efficient neutral ligands in stabilizing highly reactive main group species. 3,4,12Among dihalosilylenes, some trapping reactions of condensed SiCl 2 with acetylene and benzene were already carried out by Timms in 1968. 13In 2009, the first monomeric dihalosilylenes were reported, which were stabilized by a NHC (Scheme 1C). 14,15Besides the classical method for preparing compounds with low-valent main group elements using alkali metals, a novel route was also disclosed to prepare NHC-stabilized dichlorosilylene (Scheme 1C; X = Cl). 14Reductive dehydrochlorination of HSiCl 3 with a NHC affords a NHC stabilized dichlorosilylene. 14Silylene C (X = Cl) has been found to be a strong s-donor ligand for transition metals. 16,17Silylene C (X = Cl) behaves as a Lewis base and some Lewis acid-base adducts of C with boranes have also been isolated. 18,19-heterocyclic olefin (NHO) (Schemes 1D and 2) and its Lewis acid-base adducts were reported by Kuhn et al. in 1994. 20 Scheme 2 illustrates various mesomeric forms (a, b, and c) of a NHO.Therefore, NHO (D) can be considered as a lighter congener of C. Interest in this seemingly unusual molecule aroused very recently.Rivard and co-workers have used NHOs (Scheme 2) with sterically demanding substituents and have isolated interesting compounds with low-valent Ge and Sn.21a Recently, NHOs have been shown to stabilize interesting boron compounds.21c,d Surprisingly, the chemistry of NHOs with silicon has not been explored so far.
Here, we report on the facile formation of silyl-functionalized NHOs 3 and 4 (Scheme 3) by the reaction of NHOs 1 and 2 with HSiCl 3 .Compound 3 can also be prepared by the reaction of NHC stabilized dichlorosilylene IPrÁSiCl 2 with 1 (Scheme 4).
Compounds 3 and 4 have been prepared in almost quantitative yield by the reaction of a NHO (1 or 2) with HSiCl 3 (Scheme 3).The insoluble side products 5 and 6 can be readily isolated by filtration and can be recycled to form NHOs 1 and 2. Compounds 3 and 4 crystallize as colourless crystals and are soluble in common organic solvents.Formation of compounds 3 and 4 can be readily observed from their NMR spectra. 22The 1 H NMR spectrum of each of the compounds 3 and 4 shows a doublet for the SiH proton accompanied by silicon satellite signals (J Si-H = 291.13Hz (3) and J Si-H = 295.67Hz ( 4)).Imidazoline ring NCH protons in 3 are magnetically non-equivalent, and each appears as a doublet.Similarly, NCH 2 protons of 4 exhibit a multiplet.Olefinic CH proton each in compounds 3 and 4 appears as a doublet due to coupling with the SiH proton. 13C NMR spectra of the compounds 3 and 4 show resonances consistent with their 1 H NMR spectral data. 223][4] The EI-mass spectrum of 3 exhibits the molecular ion peak at 500 (m/z).
Base induced disproportionation of HSiCl 3 to generate dichlorosilylene is well known. 14,23Use of NHO as a Lewis base has been shown by Kuhn, Rivard and others. 20,21We decided to use NHO as a base to generate silylene with HSiCl 3 . 14In analogy with the reaction of NHCs and HSiCl 3 , 14 dehydrochlorination of HSiCl 3 with a NHO base and subsequent silylene insertion into a C-H bond of NHO to afford 3 or 4 were assumed.However, computational analysis shows a high energy barrier for SiCl 2 insertion (Fig. 1). 22Therefore, formation of adducts I and II and subsequent deprotonation with the second molecule of the NHO base to afford 3 and 4 seem more plausible (Scheme 3).
We also carried out the reaction of IPrÁSiCl 2 (IPr = {(N(2,6-iPr 2 C 6 H 3 )CH} 2 C:)) with 1. Formation of compound 3 with the liberation of free IPr was readily observed from the 1 H and 29 Si NMR spectral studies (Scheme 4).However, in this case, high solubility of 3 and IPr in common organic solvents impedes their separation.In general, silylenes insert into the O-H, Si-H, S-H, C-Cl, and metal-hydrogen bonds. 24Insertion of a thermally stable silylene into a C-H bond is rather rare. 25Irrespective of the mechanism involved, reaction of IPrÁSiCl 2 with 1 to afford 3 is a clear silylene insertion into a C-H bond (Scheme 4).In this case, formation of an intermediate (III) and subsequent 1,2hydrogen migration to give 3 are plausible.However, the role of IPr as a proton transfer agent via protonation-deprotonation cannot be ruled out.
Suitable single crystals of 3 for X-ray diffraction studies were grown from a saturated benzene solution at room temperature  by the slow diffusion of n-hexane.Compound 3 crystallizes in the triclinic space group P% 1.The molecular structure of 3 is shown in Fig. 2. Solid state structure of compound 3 reveals the formation of a silyl-functionalized NHO with a SiHCl 2 group.Silyl-functionalized-NHO 3 features a distorted tetrahedral geometry at the four-fold coordinated silicon atom.The average Si-Cl bond length of 2.08 Å is consistent with those measured for the compounds with four-coordinate silicon. 26n this communication, we have presented direct access to silyl-functionalized NHOs 3 and 4 in almost quantitative yield by the reaction of 1 and 2 with HSiCl 3 .The only side products 5 and 6 are insoluble and can be easily separated.Computational analysis shows a high energy barrier for silylene insertion, therefore a protonation-deprotonation mechanism is more likely.Reaction of IPrÁSiCl 2 with 1 to yield 3 shows formal silylene insertion into a C-H bond.Easy separation of the products and the use of commercially available HSiCl 3 instead of silylene IPrÁSiCl 2 and facile access to silyl-functionalized NHOs 3 and 4 by the reaction of 1 and 2 with HSiCl 3 are more appealing.Further studies on the chemistry of 3 and 4 are currently in progress and the results will be published in due course.

Scheme 2 Scheme 3 Scheme 4
Scheme 2 Different mesomeric forms of a NHO.

Fig. 1
Fig.1Calculated energy profiles at BP86/def2-TZVP for the silylene insertion into a C-H bond.Values are the electronic energy (corrected with the ZPVE) given in kcal mol À1 .22