A Base‐Stabilized Sila‐β‐Lactone and a Donor/Acceptor‐Stabilized Silanoic Acid

Abstract

The carbonyl group I is ubiquitous and without doubt one of the most important functional groups in organic chemistry; it characterizes many compounds such as ketones, aldehydes, carboxylic acids, and many other related compounds. In marked contrast, the synthesis of the heavier silicon analogue II is still elusive owing to the weak and strongly polarized Si-O p bond, which accounts for a very strong tendency to give thermodynamically stable polysiloxanes.[1]M. Driess and co-workers reported that the silanone function can be efficiently stabilized using a donor/acceptor system III.[2-3] A base-stabilized silanon complex with a transition metal (type III) has also recently been reported.[4] M. Driess and co-workers also demonstrated that the stabilization of such species can be achieved by coordination of electron-donating ligands on the silicon center (type IV), which allowed the isolation of the first examples of a basesupported silaurea [5] and a silanoic silyl ester V.[6] We have also recently reported the synthesis of the first donorstabilized silacyclopropan-1-one of type IV.[7] Although these techniques allowed the synthesis of other silacarbonyl derivatives, such as silanoic acid anhydride VI [8] and pyridinium salts of silanoic acid VII, [9] only a few types of silacarbonyl compounds are available to date.[10] Here we present the synthesis of the first base-stabilized sila-b-lactone VIII and its unique reactivity with alcohols leading to the formation of a silanoic acid IX, more precisely, silicic acid monoethyl ester, which is a silicon analogue of monoethyl carbonate.It was known that silylenes react with dioxygen (O 2 ) to generate the corresponding transient silaester.[11] Similarly, the base-stabilized silacycloprop-1-ylidene 1 [12] readily reacts with one equivalent of dioxygen at 5 8C to give selectively the corresponding base-supported sila-b-lactone 2 (Scheme 1).The reaction proceeds regio-and diastereoselectively, which was indicated by the 31 P NMR spectrum showing only two signals for the b-lactone 2 in the same proportions as those for 1 (51.3 and 47.7 ppm; diastereomer ratio: 92:8). Derivative 2 is stable under inert conditions and was isolated as colorless crystals in 51 % yield. In the 29 Si NMR spectrum the signal corresponding to the silanone function appears as a doublet (d = À52.7 ppm, 2 J PSi = 11.2 Hz) at slightly lower field compared to 1 (d = À87.5 ppm), but in the range of base-stabilized silanones. [5,7] The two carbon atoms of the lactone ring were observed as two doublets at relatively low field (d = 80.1 ppm, 1 J CP = 85.8 Hz and d = 47.4 ppm, 2 J CP = 13.6 Hz). The 1 H NMR spectrum shows a doublet for the proton of the lactone ring with a large coupling constant (d = 4.51 ppm, 3 J PH = 20.4 Hz).The structure of 2 was unambiguously confirmed by X-ray diffraction analysis (Figure 1).[13] The structure reveals an essentially planar b-lactone four-membered ring (AE8 = 359.88) with an oxygen atom (O2) inserted between the silicon (Si1) and carbon (C1) atoms attached to the phospho...

NHCs in Main Group Chemistry

“…A major breakthrough in low-valent silicon chemistry was achieved in 2009, when the groups of Roesky and Filippou simultaneously reported the preparation of NHC-supported SiX 2 complexes (Scheme 93). Roesky et al described two different methods for the synthesis of (IDipp)SiCl 2 (602). 543 On the one hand, treating trichlorosilane with 2 equiv of IDipp provides 602 under carbene-induced dehydrochlorination in 78% yield.…”

“…The isolation of the NHC-supported silicon(II) dihalides heralded the start of a very fruitful era in organosilicon chemistry. Above all, Roesky's dichlorosilylene (IDipp)SiCl 2 (602) paved the way for the synthesis of a plethora of novel silicon compounds. Adduct 602 displays a nucleophilic and redox-active lone pair, thus acting as a source of molecular SiCl 2 .…”

“…In addition, compound 611 represents the dioxa analogue of NHCsupported trisilacyclopentene 609. The observed sharp signals in the 29 Si NMR spectra, varying from δ = −99.5 to δ = −142.5 The Rivard group used Roesky's dichlorosilylene (IDipp)-SiCl 2 (602) and the dichlorosilylene−borane adduct (IDipp)-SiCl 2 (BH 3 ) (614) as precursors for the synthesis of various "push−pull" stabilized silylene derivatives (Scheme 95). 633 Reaction of the hydride source lithium aluminum hydride with the dichlorosilylene−BH 3 adduct 614 provides the donor− acceptor-stabilized parent silylene complex 615 via chloride replacement.…”

“…29 The strong nucleophilic character of (IDipp)SiCl 2 (602) enabled the reaction with coordinatively labile tungsten complexes, providing the perhalogenated "push−pull" stabilized germasilene and stannasilene complexes 619. (IDipp)SiCl 2 (602) was further used to gain access to some multiply bonded Si(IV) complexes. Accordingly, the NHCstabilized dichlorosilaimines 533a−c and 535 were synthesized by treating 602 with the carbodiimide DippNCNDipp, metaterphenyl azides, or Ph 3 SiN 3 as previously mentioned (vide supra).…”

“…Indeed, the research groups of Roesky, Rivard, and Ghadwal described a series of Lewis base-supported Si(II) dichloride transition metal complexes (Figure 76). Reaction of (IDipp)SiCl 2 (602) with the metal carbonyls VCp(CO) 4 , Fe 2 (CO) 9 , and CoCp(CO) 2 gives the corresponding donor−acceptor-stabilized complexes 621−623 including the first structurally characterized vanadium−silylene complex. 637 SC-XRD analysis uncovered that the tetracoordinate Si(II) nuclei adopt distorted tetrahedral geometries in the solid state.…”