Condensation reactions unsatured species

The main synthetic route to high nuclearity metal carbonyl clusters involves a condensation process (/) a reaction induced by coordinatively unsaturated species or (2) a reaction between coordinatively saturated species in different oxidation states. As an example of (/), Os2(CO)22 can be condensed to form a series of higher coordinated species (89). 

N-Metalated azomethine ylides generated from a-(alkylideneamino) esters can exist as tautomeric forms of the chelated ester enolate (Scheme 11.8). On the basis of the reliable stereochemical and regiochemical selectivities described below, it is clear that the N-metalated tautomeric contributor of these azomethine ylides is important. Simple extension of the above irreversible lithiation method to a-(alkylideneamino) esters is not very effective, and cycloadditions of the resulting lithiated ylides to a,(3-unsaturated carbonyl compounds are not always clean reactions. When the a-(alkylideneamino) esters bear a less bulky methyl ester moiety, or when a,(3-unsaturated carbonyl compounds are sterically less hindered, these species suffer from nucleophihc attack by the organometalhcs, or the metalated cycloadducts undergo further condensation reactions (81-85). 

On the basis of mechanism, condensation processes can be divided into two broad categories (a) reactions induced by coordinatively unsaturated species and (b) reactions between coordinatively saturated species in different oxidation state (redox condensation). 

The condensation reactions of carbonyl metallates with neutral species, which are either coordinatively unsaturated or which will readily generate coordinatively unsaturated fragments, have also yielded a variety of mixed-metal clusters. The square-based pyramidal dianion [Fe5C(CO)i4] , for example, has been shown to react with a number of such species to yield octahedral FcjMC cluster compounds (Scheme 9) 269, 381). In some cases. 

For addition reactions with unsaturated organic compounds, all the reaction products contained the —SiF2SiF2— moiety, which led to the conclusion that the most reactive species in these co-condensation reactions must be the -SiF2SiF2 diradical (65, 66). The reaction mechanism for alkyne system is illustrated in Scheme 1. 

During the adsorption and desorption of thiophene on HZSM5 zeolites, reversible surface reactions of thiophene to unsaturated thiol-like species were found even at room temperature. Condensation reactions of thiophene to benzothiophene were observed over HZSM5 only but not over the NaZSMS samples [3]. Cleavage of the C-S bond and formation of SH surface groups were observed during thiophene adsorption on HY zeolites [4]. 

A complete analysis of the products reported in Fig. 1 requires some more comments on cyclopentadiene and benzene. Both are typical secondary products, and are mainly the result of successive addition and condensation reactions of alkenes and unsaturated radicals. Once a significant amount of ethylene and propylene is formed, vinyl and allyl radicals are present in the reacting system and form butadiene, via butenyl radicals. Successive addition reactions of vinyl and allyl-like radicals on alkenes and dialkenes sequentially explain the formation of cyclopentadiene and benzene. These reactions are discussed in-depth in the literature and will be also analysed in the coming paragraphs (Dente et al., 1979). It seems worthwhile mentioning that these successive reactions and interactions of small unsaturated radicals and species constitute the critical sub-mechanism for the correct evaluation of ethylene selectivity. In fact, once the primary decomposition of the hydrocarbon feed has largely completed, the primary products and mainly small alkenes can be... 

Figure 9 drastically simplifies the major reaction paths of alkyl-naphthalene components. Via H-abstraction and successive decomposition reactions, they can easily form, either naphthalenes with unsaturated side chains (vinyl, allyl or alkenyl side chains) or RSR and smaller decomposition products. The preferential radical attack on the alkyl side chain is in the benzyl position due to the weak hydrogen bond. This makes it easy to justify either the formation of RSR or the successive / -decomposition reaction to form vinylnaphthalene. The net result of the successive recombination and condensation reactions of these aromatic species is the formation of PAH of increasing molecular weight with a progressively lower hydrogen to carbon ratio. 

P-Diketone substrates have been valuable in the aza-annulation reaction with a,(J-unsaturated carboxylic acid derivatives, and both acyclic and cyclic (i-diketone species have been investigated. The simplest acyclic P-diketone, 150, underwent condensation reaction with BnNH2 to generate the... 

The general feature of syntheses of carbonyl clusters in most cases would appear to involve a condensation reaction of labile intermediates which have a lower nuclearity than that of the final product. One approach is via the production of coordinatively unsaturated species under conditions in which they can stabilize themselves only by condensation reactions. Coordinatively unsaturated species have been produced in a variety of ways, involving simple metal salts or carbonyl compounds as starting materials. 

The MgyAlOx activity declines in the acetone oligomerization reaction due to a blockage of both basic and acid active sites by a carbonaceous residue formed by secondary aldol condensation reactions. The key intermediate species for coke formation are highly unsaturated linear trimers that are formed by aldol condensation of mesityl oxide with acetone and remain strongly bound to the catalyst surface. The catalyst surface acid-base properties determine the preferential formation of a given trimeric intermediate, which in turn defines the chemical nature of the carbon deposit. Aromatic hydrocarbons are the main component of coke formed on acidic Al-rich MgyAlOx samples whereas heavy a,P-unsaturated ketones preferentially form on basic Mg-rich catalysts. 

Besides the already mentioned acidic aluminum chloride catalysts, alternative Friedd-Crafts catalysts such as supported acidic tin catalysts have also been developed. The tin-based catalysts were prepared by a method which closely resembled the already mentioned two-step grafting method devised for the aluminum chloride catalyst. Here, SnCU was anchored on silica materials modified with tetraalkylammonium chloride moieties obtained for example, from reaction with [3-(trimethoxysrlyl)propyl]octadecyldimethylammonium chloride, thereafter, reaction of the Lewis acid with the chloride moieties leads to formation of pentacoordinated anionic tin species forming catalytically active complexes (i.e. [R4N][SnCl5] species), associated with the surface. The supported tin catalysts were employed for condensation reactions of olefins with aldehydes forming unsaturated alcohols (Prins condensation. Scheme 5.6-2) [76]. 

Unsaturated polyesters (UP) are by far the most widely used resins in the composite industry [49]. UP resins are used in the highest volume due to their relatively low cost and good combination of thermo-mechanical properties and environmental durability. Typical properties of a cast UP resin are reported in Table 8.2. A UP resin is usually obtained from unsaturated polyesters dissolved in a reactive monomer. Unsaturated polyesters are linear polymers obtained by a condensation reaction between three chemical species saturated aromatic acids, unsaturated acids (maleic anhydride, fumaric acid), and glycols... 

Dissociative recombination of the ethyl ion can then lead to a species as saturated as C2H4. Since condensation reactions are generally not as important as carbon insertion reactions, the unsaturated nature of the hydrocarbon chemistry remains a salient prediction of ion-molecule chemistry, which... 

Condensation reactions induced by the presence of coordinately unsaturated species,... 

Condensation Reactions Induced by Coordinately Unsaturated Species... 

Frequently both processes may be carried out simultaneously in the same trial. In other cases, however, the precursors in a low oxidation state are prepared first and then the condensation reaction is induced by thermal, photochemical, or chemical generation of unsaturate species. 

---