Silicon mechanistic studies

The concepts and basic approach used in studies of electrical fluctuations in corrosion processes proved to be very successful as well in mechanistic studies of electrode reactions taking place at materials covered by passivating films. A typical example is the electrochemical dissolution of silicon. From an analysis of the noise characteristics of this process, it has been possible to identify many features as well as the conductivity of the nanostructures of porous silicon being formed on the original silicon surface. 

Going back to mechanistic studies it is not clear if the reactions of nucleophiles with hexacoordinated silicon compounds are pure nucleophilic substitutions or if they take a more complex route. However there is another challenge to find whether the silicon atom can accept being in heptacoordination. Such a possible situation has been observed with a tricapped tetrahedron structure of a silane which has been proved to be isosteric with the corresponding germane of which the X-ray structure determination has been carried out. 

Several compounds containing Tt-bonds show reactions with 1 which most likely proceed via [2+1] or [4+1] cycloaddition processes, but no detailed mechanistic studies have been performed so far. Not unexpectedly, the electron-rich species 1 preferentially reacts with electron-poor substrates, and ring-strained or dipolar intermediates rearrange or react further to more stable products in a sometimes rather complicated and surprising fashion. In a few cases even the pentamethylcyclopentadienyl substituents at silicon are involved in the reaction pathways. 

Aldol reactions of silyl enolates are promoted by a catalytic amount of transition metals through transmetallation generating transition metal enolates. In 1995, Shibasaki and Sodeoka reported an enantioselective aldol reaction of enol silyl ethers to aldehydes using a Pd-BINAP complex in wet DMF. Later, this finding was extended to a catalytic enantioselective Mannich-type reaction to a-imino esters by Sodeoka s group [Eq. (13.21)]. Detailed mechanistic studies revealed that the binuclear p-hydroxo complex 34 is the active catalyst, and the reaction proceeds through a palladium enolate. The transmetallation step would be facilitated by the hydroxo ligand transfer onto the silicon atom of enol silyl ethers ... 

The most complete mechanistic studies have been made with trisubstituted silanes, which initially undergo an exclusive silicon-hydrogen bond cleavage. Retention of configuration occurs in the ozonolysis of 1-decahydronaphthylcyclohexylmethylsilane 68 to the corresponding hydroxide ... 

This review attempts to summarize the published literature and information in patents from the 1980s to the present. The effect of promoters will be discussed first because it was the understanding of their function which led to a quasi-second revolution in the area of the Direct Process. The second area discussed will be the effect of silicon morphology on selectivity and yield. The third area of this review will focus on mechanistic studies aimed at understanding the fundamental chemical and physical effects in the Direct Process. The fourth area of review will be the use of substrates other than MeCl in the Direct Process. Finally, a discussion will be presented on the recovery and use of by-products from MCS, previously considered waste products. 

The hydrolysis introduces all of the oxygen into the silicon oxide matrix, i.e. all of the oxygen comes from the water molecules. The underlying details of the chemistry are extremely complex12-14 and the detailed description of mechanistic studies is beyond the scope of the topic of this chapter, except for mechanistic studies which involve organic molecules and polymers — these are given in the various sections below. Here, some general aspects are summarized. 

The mechanism A very detailed mechanistic study of this phosphoramide-catalyzed asymmetric aldol reaction was conducted by the Denmark group (see also Section 6.2.1.2) [59, 60], Mechanistically, the chiral phosphoramide base seems to coordinate temporarily with the silicon atom of the trichlorosilyl enolates, in contrast with previously used chiral Lewis acids, e.g. oxazaborolidines, which interact with the aldehyde. It has been suggested that the hexacoordinate silicate species of type I is involved in stereoselection (Scheme 6.15). Thus, this cationic, diphosphoramide silyl enolate complex reacts through a chair-like transition structure. 

Modification with silicon compounds mechanistic studies... 

The reactions of atomic silicon cations with a large variety of neutral molecules have been reported and summarized in reviews focussed on the kinetic, energetic and the mechanistic study of these reactions by selected-ion flow tube (SIFT)10 and guided ion beam techniques11. 

Although reports of mechanistic studies are scarce for these reactions, Ouellette and Marks did propose a 1,3-dipolar attack by ozone (10). The corresponding intermediate was predicted to be a hydrotrioxide of silicon. 

Tietze et al. emphasized the usefulness of chiral trimethylsilyl ethers of readily accessible amino alcohol derivatives in allylation of aldehydes and ketones [43]. As a consequence, careful design of the norpseudoephedrine derivatives and proper choice of silicon Lewis acids have led to the convergent preparation of enantiomerically enriched secondary and tertiary homoallylic alcohols in high yields (Sch. 12) [43a], It should be noted that the configuration of the newly formed stereogenic center of the secondary homoallylic alcohols is the opposite of that in the allylation of ketones [43c], They also described in detail mechanistic studies of the above allylation reaction by use of and NMR. 

During the course of further mechanistic studies of this reaction, the reaction rate was found to be dramatically accelerated by incorporation of the silicon atom within a four-membered ring (Sch. 64) [106a], The reaction of trimethylsilyl ketene acetal 89a with 90 requires heating at 150 °C, and less than 25 % 91a is formed after 24 h. In marked contrast, the silacyclobutane counterpart 89b reacts completely and clearly with 90 within 24 h at 27 °C to furnish 91b quantitatively. This implies that reaction proceeds by way of a pentavalent organosilicon species in which a boat-like transition state is preferred. 

There was however a possible case of thermal migration of a silicon on a simple olefin in the largely overlooked 1985 report by Conlin [6] that a-methylenesilacyclobutane 1 cleanly isomerized to a mixture of silacyclopentenes 3 and 4 (Eq. 8). Although no mechanistic studies were performed, Conlin boldly proposed that this reaction proceeded through the intermediacy of carbene 2 which, if correct, would make this the sole example of a thermal isomerization of an untwisted olefin to a carbene. 

The first mechanistic studies of silanol polycondensation on the monomer level were performed in the 1950s (73—75). The condensation of dimethylsiloxanediol in dioxane exhibits second-order kinetics with respect to diol and first-order kinetics with respect to acid. The proposed mechanism involves the protonation of the silanol group and subsequent nucleophilic substitution at the silicone (eqs. 10 and 11). 

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