Oxygen-silicon coordination

An effective route to O Si pentacoordinate silicon chelates consists of the reaction of chloromethyl(dimethyl)chlorosilane (ClCH2SiMe2Cl, 8) with a variety of N- or O-trimethylsilylamides, 5 lactams, ureas, 5 

This reaction was applied to 0-TMS-hydrazides 1, and led to some interesting regioselective chemistry described below. 

The transsilylation reaction described above was studied with 8 and the silylated hydrazides l.33,34 The E,Z stereoisomers of la35 reacted in different ways, shown in Eqs. (6) and (7). Initially both isomers form unstable tetracoordinate silane intermediates (9a-E, 9a-Z), which can be observed at low temperatures in CD2C12 solutions by 29Si NMR spectroscopy. The intermediates 9a-E and 9a-Z could be observed for about 20 min at — 70 °C, before reacting further to form five-membered and six-membered chelate products 10a and 11a, respectively. At ambient temperature the reaction proceeds immediately without observation of intermediates. 

In the rearrangement reaction leading to 10a and 11a the nitrogen atom which is nearest and most accessible to the chloromethyl group displaces the chlorine atom in an internal SN2 type substitution, followed by migration of the displaced chloride to silicon. This rearrangement leads to the two different products, with five- and six-membered chelate cycles, 10a and 11a. While five-membered chelate cycles as in 10a are common in hypercoordinate silicon chemistry, the formation of six-membered chelates (11a) is unique.33,34 

Other silylacylimidates 1 presumably reacted in a similar manner. However, since in all but la only the Z-isomer was detectable by NMR spectroscopy, the only products obtained from all other 1 (lc-e,g,i) were the six-membered chelates llc-e,g,i.34,36,37 

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The framework of the structure consists of silicon tetrahedra (four oxygen ions coordinated about a silicon ion at the comers of an approximately regular tetrahedron) and aluminium octahedra and (or) tetrahedra, with Si-0 = 1.59 A, Al-0 = 1.89 A in octahedra, 1.75 A in tetrahedra, as observed in other aluminosilicate crystals. 

Fluoride ion first becomes coordinated with the silicon atom of tin reagent 33. Further coordination with the carbonyl oxygen atom of substrate 34 leads to an especially activated hypervalent silicon species 35.21 from which a stannyl anion is eliminated. I Ialogen-metal exchange between the tin anion and inyhc iodide 36 produces vinylic anion 37, which attacks in an intramolecular way the carbonyl group that has been activated through silicon coordination. Aqueous workup leads to alcohol 38. 

Similar amide-switching phenomena were reported in other compounds (142, 143). X-ray crystallography for 143 showed that only one amide-oxygen was coordinated to silicon (Si—O distance 1.992 A)173. However, in solution the two amide-moieties were NMR-equivalent and could not be resolved down to —65 °C in toluene-ds solution173. 

It was recognized at an early stage in the development of silicon-transition-metal chemistry that silicon-oxygen compounds often appeared, either as by-products from preparations or as decomposition products on heating or even on storage. At first, adventitious hydrolysis or oxidation was blamed, but it soon became clear that attack on silicon by oxygen of coordinated carbonyl groups was responsible. Since metal carbonyls are known to form adducts with Lewis acids such as compound (XXXIII) (286),... 

Fig. 2. (lower part of the figure) Oxygen-silicon longitudinal interatomic force constants in quartz, as a function of the distance between the pair of atoms. The full line is a guide for the eyes, and represents a decay proportional to the cube of the distance. The arrow indicates values that are larger than the range allowed by the coordinates. 

The possible reaction mechanism for the formation of 31 is shown in Scheme 15. Insertion of alkyne 14 into silazirconacyclopropane 3 gives silazirconacyclopentene 22. Then, insertion of carbon monoxide into the carbon-zirconium bond in silazirconacyclopentene 22 gives silazirconacyclohexenone 34, whose carbonyl oxygen would coordinate to zirconium metal. Then the zirconium carbon bond migrates onto silicon to afford oxazirconacyclohexene 36 via 35 [26]. Deuterolysis of 36 would afford 31-D2, which has two deuteriums. 

Nucleophilic addition of the trifluoromethyl group to aldehydes, ketones and other carbonyl compounds leads primarily to the corresponding trimethylsilyl ether this must subsequently be hydrolyzed to the alcohol. Because typical reaction conditions are very mild, the method is widely applicable, even for sensitive substrates. In contrast with most other methods, fluoride-induced perfluoroalkylation via silicon compounds also works for enolizable carbonyl compounds. With c(, -un-saturated substrates 1,2-addition directly to the carbonyl group is strongly preferred [64b]. If the oxygen is coordinated to a bulky Lewis acid, for example aluminum tri(2,6-bis(tert-butyl)phenoxide (ATPH), the 1,4-addition products are obtained selectively [73f] (Scheme 2.128). 

The highly covalent nature of the Si—O bond in silicas, Si02, with tetrahedral silicon coordinated by oxygen,... 

MALDl-TOF was used to deduce the three-dimensional structure of complex silsesquioxane polymers. These polymers have silicon coordinated with three bridging oxygen atoms in the form of [RSiOs/J and can form a wide variety of complex three-dimensional structures. Specifically, the branched-linear silsesquioxane may react with itself to form intramolecular closed loops. Four distinct levels of molecular structure were observed from the MALDl-TOF spectrum. 

The structure of monosilicic acid is assumed to involve silicon coordinated with four oxygen atoms as in amorphous vitreous silica and in crystalline quartz. Although there are rare minerals such as the stishovite form of SiOj (21) or thau-masite (22), in which silicon is coordinated with six o.xygen atoms, silicon in most oxides and silicates is surrounded by only four oxygen atoms. If the monomer had the structure HjSKOH), one would expect it to be a strong acid like the analogous HaSiF, but in fact it is a very weak acid. 

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