Reactivity pentacoordinated

The accelerated rate for alcoholysis with le, which was observed for the 10 % Pd/C catalytic system, was also seen with the Mn(CO)sBr catalyst. Reactions of le with primary, secondary or tertiary alcohols resulted in moderate yields of the corresponding silyl ketals after 2 h (Table 8 and 9). When mono-alkoxy silane from 3-hydroxy butyrate (lg) was treated with homoallyl alcohol in the presence of Mn(CO)sBr as the catalyst under the standard conditions, 76 % of the silyl ketal was obtained. These silyl ethers possess neighboring carbonyl groups that can participate in the reaction by forming a more reactive pentacoordinated silicon center upon addition of the silane to the metal center.. 

The Lewis acidic character of reactive pentacoordinate silicon compounds has been unequivocally confirmed by Corriu, Sakurai and Hosomi [90]. Allylsilicates prepared from allylsilanes and catechol can undergo allylation reaction with aldehydes in the absence of Lewis acid promoter (Sch. 51). 

The thermal reactions of silacyclopropanes cis- and trans-187a with benzaldehyde give a stereoisomeric mixture of an oxasilacyclopentane product and significant quantities of by-products whereas fhe t-BuOK-catalyzed reactions proceed efficiently under mild conditions wifh inversion of silacyclopropane configuration (Scheme 10.250) [680]. The latter reactions might involve initial formation of a more reactive pentacoordinate sihconate intermediate. The base-catalyzed system is not applicable to insertion of enohzable aldehydes. 

However, with aryl(trialkyl)silanes, sometimes for electronic reasons, the reaction does not take place. In order to reduce the electron density on the silicon atom, alkyl groups were replaced by fluorine. The fluorine effect could be explained as follows i) the van der Waals radius of fluorine is roughly comparable to that of hydrogen and hence the fluorine-substituted silyl group is not so bulky than the trimethylsilyl one ii) electronegativity of fluorine favours the formation of the reactive pentacoordinated silicate, enhances the Lewis acidity of the silicate and... 

Laine et al. [34] have described a process where SiOj is directly reacted with ethylene glycol and an alkali to produce reactive pentacoordinate silicates, which can be used to produce silicate materials. Laine has made stable precursor polymers (>670 K), some of which are liquid crystalline, by using catechol. Agaskar [35] has prepared organolithic macromolecular materials, which are hybrids containing silicate and organic molecules (functionalized spherosilicates) and can be used as precursors for microporous ceramic (Si-C-0) materials. 

Transition metal-free hydrosilylation of carbonyl compounds can be realized with the use of Brpnsted or Lewis acids as well as Lewis bases. Alkali or ammonium fiuorides (CsF, KF, TBAF, and TSAF) are highly effective catalysts for the reduction of aldehydes, ketones, esters, and carboxylic acids with H2SiPh2 or PMHS. Lithium methoxide promotes reduction of esters and ketones with trimethoxysilane. A generally accepted mechanism of Lewis base-catalyzed hydrosilylation of carbonyl compovmds involves the coordination of the nucleophile to the silicon atom to give a more reactive pentacoordinate species that is attacked by the carbonyl compound giving hexacoordinate silicon intermediates (or transition states), in which the hydride transfer takes place (Scheme 30) (235). 

Consideration of the stabilities of compounds of metals with various coordination numbers also accounts for the behavior of the reactive pentacoordinate [Cr(CO)5] . The very stable hexacoordinate Cr(CO)6 can only be reduced to this anion by very powerful reducing agents such as the blue solutions of alkali or alkaline earth metals in liquid ammonia (41). The [Cr(CO)s] anion is very reactive under conditions where hexacoordinate compounds of the type Cr(CO)5L may be formed (41 y 72, 73, 97). However, the [Cr(CO)s] anion has not yet been observed to react with two moles of halide to form heptacoordinate derivatives of the type R2Cr(CO)5. 

The frans-[Co(en)2(DMSO)Cl] ion aquates rapidly at 25°C in 0.1 Af HCIO4 with aq = 8.7 X 10 s The release of chloride is not a competitive reaction. Some 70.5% of the frans-aquo-chloro complex is initially produced. Subsequent to aquation of [Co(en)2(OH2)Cl] the isomers interconvert (fcisom = 9.90 x 10 s ) to give 79% cw-isomer at equilibrium. These results agree with those obtained independently for cis-and fra s-[Co(en)2(OH2)Cl]S206-H20, where ki m = 10.4 x 10 s to give 79% cis isomer. It is concluded that all the aquations are dissociative involving a common reactive pentacoordinate intermediate. 

Tri- and pentacoordinate phosphoms compounds often react by electron-pair mechanisms as demonstrated by the nucleophilic reactivity of the lone pair electrons in trivalent compounds, and the electrophilicity of the phosphoms atom in the pentavalent compounds. Some compounds also react by free-radical mechanisms. The theoretical and synthetic aspects of the chemistry of phosphoms compounds have been described (6—9). 

The role of the ligands is both to stabilize the Pd(0) state and to tune the reactivity of the palladium. The outline mechanism above does not specify many detailed aspects of the reaction that are important to understanding the effect of ligands, added salts, and solvents. Moreover, it does not address the stereochemistry, either in terms of the Pd center (tetracoordinate pentacoordinate , cisl, transl) or of the reacting carbon groups (inversion , retention ). Some of these issues are addressed by a more detailed mechanism.190... 

The cyanide exchange on [M(CN)4]2 with M = Pt, Pd, and Ni is a rare case in which mechanistic comparisons between 3d, 4d, and 5d transition-metal complexes. Surprisingly, the behavior of these metal square-planar centers leads to mechanistic diversity involving pentacoordinated species or transition states as well as protonated complexes. The reactivities of these species are strongly pH-dependent, covering 15 orders of magnitude in reaction rates.85... 

Today it is widely accepted that fivefold coordinated silicon plays a key role in the reaction mechanisms of the nucleophilic substitution having a trigonal bipyramidal transition state species which ressemble these transition states can be isolated in some special cases. The structural features fit well to kinetic data and possibly explain the significantly higher reactivity (proved by experimental data) of Si-pentacoordinated compounds compared to their tetracoordinated analoga. 

The reactivity of penta- and hexacoordinated silicon compounds has been described to be very different from the reactivity of the corresponding tetracoordinated derivatives [ 1], An increase in reactivity towards nucleophiles has been observed in the case of neutral and anionic pentacoordinated silicon compounds as exemplified by the following Schemes [2],... 

Alternatively, some conclusions can be derived from the relative reactivities of car-banions. For example, DePuy and colleagues13 made use of a clever method involving reactions of silanes with hydroxide ion to deduce acidities of such weak acids as alkanes and ethylene. The silane reacts with hydroxide ion to form a pentacoordinate anion that ejects a carbanion held as a complex with the hydroxysilane rapid proton transfer gives the stable silanoxide ion and the carbon acid (equation 5). 

The pentacoordinate oxazaphosphetidines 53 (Tip = tri(isopropyl)phenyl) are related to intermediates in the aza-Wittig reaction. Thermolysis of 53 shows that the compound displays two types of reactivity as an azaphosphetidine to give 51 and 52 and as an oxaphosphetane to yield 54 and 55 <00TL5237>. 

The direct protonation of isobutane, via a pentacoordinated carbonium ion, is not likely under typical alkylation conditions. This reaction would give either a tertiary butyl cation (trimethylcarbenium ion) and hydrogen, or a secondary propyl cation (dimethylcarbenium ion) and methane (37-39). With zeolites, this reaction starts to be significant only at temperatures higher than 473 K. At lower temperatures, the reaction has to be initiated by an alkene (40). In general, all hydrocarbon transformations at low temperatures start with the adsorption of the much more reactive alkenes, and alkanes enter the reaction cycles exclusively through hydride transfer (see Section II.D). 

In 1992, R.M. Laine (University of Michigan, Ann Arbor) announced the development of a process that transforms sand and other forms of silica into reactive silicates that can be used to synthesize unusual silicon-based chemicals, polymers, glasses, and ceramics. The Lame procedure produces pentacoordinate silicates directly from low-cost raw materials—silicon dioxide,ethylene glycol, and an alkali base. The mixture is approximately a 60 1 ratio of silica gel, fused silica (or sand) to metal hydroxide and ethylene... 

---