Synthetic Routes and Formation Mechanisms

The synthetic route used determines not only the mesostructure but also the macroscopic structure of the materials produced, and this is due in large part to differences in the formation mechanism in each route. The synthetic routes for mesoporous silica divide into three main types, characterised by the surfactant concentration in the initial solution. 

---

Ketene dimerization is the principal synthetic route to 4-methylene-2-oxetanones. This reaction proceeds very satisfactorily for ketene and methylketene, but disubstituted ketenes dimerize only to cyclobutane-1,3-diones. The cycloaddition reaction of r-butylcyanoketene to ketene and to methylketene gives a-cyanoalkylidene-/3-lactones in about 40% yield in addition to the cyclobutane-1,3-dione dimer of f-butylcyanoketene. A mechanism has been proposed for the formation of both types of dimers from a common zwitterionic intermediate (equation 111), with the relative amount of each product determined by the configurational equilibrium of the intermediate (80JOC4483, 75JOC3417). 

By formation of seven- from five-membered rings The mechanism of thermal isomerization of the cyclobutene compound in equation (74) to thiepin (50) is directly analogous to that found in the synthesis of dihydrothiepins (221) and (222 equation 65). Compound (50) was the first monocyclic thiepin to have been identified (74T2093). This synthetic route has also been applied to thiepin (148) (74T2431). 

Other methanofullerenes (not of the Bingel-type) were also found to be unstable after several reduction processes (68-70, Figure 23), [9-11] and under CPE they led to the isolation of [60]fullerene. A more recent study was conducted in THF to avoid the well-known reactivity of CH2CI2 towards the polyanions of C60 [72] and to explore the mechanisms involved during adduct removal [9b,10], Surprisingly, an electrochemically induced intermolecular adduct transfer was observed for the spiromethanofullerenes studied, and the regio-isomer distribution found in THF differed significantly from that obtained when the compounds are prepared by a direct synthetic route [10], The proposed mechanism for the formation of... 

Azahomoadamantane derivatives are known as potential biologically active compounds and several synthetic routes to this skeleton have been reported.113 The reaction of 5-methyl-4-azahomoadamant-4-ene (211) with dichlorocarbene resulted in the formation of 4-formyl-5-methylene-4-azahomoadamantane (214) in 66% yield.114 The isolation of this material is consistent with a mechanism involving carbene addi-... 

The thermal automerization and rearrangement reactions of PAHs have been widely investigated during the past two decades (for examples see refs. [31 e, g, 62-64]). The main objective was to understand the processes of formation of aromatic hydrocarbons in fuel rich flames and the mechanisms of transformation of the PAHs that have been observed at these elevated temperatures. In most cases, thermally initiated rearrangement reactions in the carbon skeletons of PAHs require high enthalpies of activation resulting in low product selectivities and poor overall yields. Because the expected products are often more effectively prepared by conventional routes, this approach has been used as a synthetic tool only in a few cases, e.g. the synthesis of azulenes [65] and the rearrangement of bifluorenylidenes to benzenoid hydrocarbons [38]. 

---