Selective Functionalization of a Bis-silylene

Functionalization of N-heterocyclic carbenes (NHCs) has an important influence on their stability, Lewis donor, and acceptor properties. In this study, we report on the selective functionalization of a four-membered N-heterocyclic bis-silylene (2,6-Ar2C6H3NSi:)2 (1) (Ar = 2,4,6-iPr3C6H2) with mono-oxygen sources N2O and Me3NO. Treatment of 1 with N2O results in the selective formation of mono-silylene (2,6-Ar2C6H3NSi(OH)2)(2,6-Ar2C6H3NSi:) (2) as a major product, along with a small amount of further oxidized product (2,6-Ar2C6H3NSi(OH)2)2 (3). Compound 2 is the first four-membered mono-silylene with a di-coordinate silicon atom.

Organosilicon compounds are valuable precursors in organic and organometallic synthesis, and materials science. 1 The availability of new stable compounds with low-valent silicon atoms (e.g., multiple bonded silicon compounds, silylenes and related species), which are otherwise only studied as short-lived reactive species, facilitates the preparation of new compounds which are difficult to isolate by traditional methods. 2,3The first stable silene and disilene were reported in 1981 by Brook et al. and West et al.,respectively. 4 The West group later succeeded in isolating an N-heterocyclic silylene (NHSi), 5 a compound containing a di-valent di-coordinate silicon atom with a lone pair of electrons.][8][9][10][11][12][13][14][15][16] Some of the interesting silicon compounds that are made available using silylene precursors include dimeric silaisonitrile, 9 silaoxirane, 10 silacyclopropene, 11 silaaziridine, 12 silacarbonyl compounds with a formal SiQO bond, 13 and silaheterocycles. 8,14Also, functionalization of a NHC 15 has been shown with IPrÁSiCl 2 (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). 16 few silicon compounds containing two silylene moieties in a single molecule have been prepared 14 using Lewis-base stabilization and are known as bis-silylenes.][8][9][10][11][12][13][14] To the best of our knowledge, there is no report on the reaction of a bis-silylene, in which only one silylene moiety is oxidized to a Si(IV) center, thereby maintaining the other silylene center.This is due to the high reactivity of these compounds.
Reactions of mono-and bis-silylenes with N 2 O to form Si(IV) compounds have been reported. 7,8Activation of small molecules by compounds containing low-valent main group elements that mimic transition metals has attracted attention during the last few years. 17Reactions of N 2 O and compounds with low-valent main group elements are quite varied.Severin and co-workers have recently reported the formation of the NHCÁN 2 O adduct by the reaction of NHC with N 2 O, which at higher temperature resulted in the formation of a ketone. 18ery recently, we have isolated the first dimeric silaisonitrile 9 (2,6-Ar 2 C 6 H 3 NSi:) 2 (1) (Ar = 2,4,6-iPr 3 C 6 H 2 ) (Scheme 1), which is stable at room temperature.Compound 1 features a fourmembered (Si 2 N 2 ) ring with two di-valent di-coordinate silicon atoms, each bearing a lone pair of electrons.Thus, 1 is the first base-free stable bis-silylene.Theoretical studies suggest that 1 is a four p-electron antiaromate. 9Therefore, partial oxidation of 1 to a mono-silylene should be favoured due to the formation of a two p-electron aromate. 14Herein we report on such a species.Selective functionalization of a bis-silylene 1 with N 2 O (Scheme 1) gives the mono-silylene (2,6-Ar 2 C 6 H 3 NSi(OH) 2 )-(2,6-Ar 2 C 6 H 3 NSi:) (2).
Treatment of a toluene solution of 1 with N 2 O yielded 2 as a major product (62%), whereas a further oxidized product, bissilanediol 3, was isolated as a minor product (8%).Reaction of 1 with Me 3 NO afforded only compound 3.Moreover, 2 reacts further with Me 3 NO to form 3. Compounds 2 and 3 were obtained as colorless crystals, soluble in common organic solvents and stable under an inert atmosphere.
The mechanism for the formation of 2 and 3 is at present unknown.Alkyne analogues of germanium and tin react with N 2 O and yield respective metal(II) hydroxides, 19,20 in which the hydrogen abstraction mechanism by radical type intermediates was suggested.Stable four-membered (E 2 N 2 ) (E = Si, Ge, or Sn) cyclic biradical species are known. 21,22Their unusual stability is explained by the presence of heteroatoms in the ring.To ascertain the origin of hydrogen in products 2 and 3, reaction of 1 with N 2 O in dry toluene-d 8 was carried out.Surprisingly, formation of compound 2 was again observed (as evident from similar NMR and IR spectral data), which was isolated in 56% yield. 23No signal was observed in the deuterium NMR spectrum of 2 for OD (D = 2 H) groups, prepared in toluene-d 8 .This excludes the involvement of radical type intermediates and hence hydrogen abstraction from the solvent molecules.Therefore, in the present study, we assume the initial formation of a silaketone (2 0 ) as an intermediate in the reaction of 1 with N 2 O, (Scheme 1) which upon reaction with residual water 23 would afford compound 2. Similarly, further oxidation of 2 with N 2 O would yield 3 0 , which upon reaction with residual water would give 3.
Silylenes and unsaturated silicon compounds are unstable in protic solvents and react immediately to form Si(IV) compounds.In view of this reactivity it seems unlikely to be able to isolate a silicon compound containing a silylene moiety together with an O-H functional group within the same molecule.The stability of silylene 2, which bears two hydroxyl groups on the Si(IV) silicon, may be explained by steric protection of the silylene moiety by the bulky terphenyl groups.Further reactivity studies of 2 with various substrates will be performed in the future to explore its properties.
Invariom refinements were performed for 2 and 3, taking into account non-spherical electron density in the aspherical scattering factors. 24These refinements lead to improvements in the figures of merit, the physical significance of the anisotropic displacement parameters, and highlight un-modeled features in the residual electron density.The improvement of the structural model can be illustrated by comparing the residual electron density map from IAM (independent atom model) and invariom refinement. 24This becomes most obvious for the phenyl ring as shown in Fig. 3.    5), O1-Si1-N2 122.17 (11), N1-Si1-N2 88.53( 9), N1-Si2-N2 86.45 (9).

Fig. 3
Fig.3Part of the molecular structure of 2 with anisotropic displacement parameters depicted at 10% probability.(a) Shows the un-modelled bonding residual electron density remaining from IAM refinement, whereas(b) illustrates the reduction of residual density in the same region after invariom refinement.24Residual electron density features are shown at a level of 0.14 eÅ À3 .