Wittig-type reactivity

In addition to block copolymer synthesis by subsequent polymer growth along one polymer chain, Eq. (21), and the reaction sequence of Eqs. (19, 22—24), preformed polymer blocks can be linked via reactive end groups. Polynorbomene with one titanacyclobutane end group was reacted in a Wittig-type reaction with... 

Wittig-type olefination. These complexes convert esters to vinyl ethers and ketones to olefins in 78-100% yield. They are somewhat less reactive than the corresponding Ti complexes.1 In the olefination of aldehydes, ketones, and thioketones, the (Z)-isomer is... 

The electronic spectra of the triphenylphosphonium, triphenylarsonium and triphenyl-stibonium tetraphenylcyclopentadienylides confirmed the impression derived from their stabilities, basicities and reactivities in Wittig-type reactions, that the polarity of the ylidic bonding increased in the order P, As, Sb and concomitantly, the double-bond character decreased. Thus the longest-wavelength absorption peaks were at 288 nm (P), 291 nm (As), 349 nm (Sb) this was attributed to the less efficient overlap between the 2p-orbitals of the ylidic carbon atom and the d-orbitals of antimony, because of the greater size and diffuseness of the d-orbitals on going down the Periodic TableNone of these compounds was solvatochromic . 

In an attempt to increase the Wittig-type reactivity of amide derivatives the reaction of phosphonium ylides, e.g. (91), with thioimides, e.g. (92) and (93), has been investigated.49 Although the thio-Wittig reaction takes place, S-alkylation and oxidation-reduction occur in competition and reduce the usefulness of this reaction in synthesis. Further studies of the reactions of a-perfluoroacylalkylphosphonium salt (94),50.51 generated by... 

Actually, terminal metal carbene and alkylidene complexes are ubiquitous throughout the transition elements. The nomenclatural distinction between "carbene" and "alkylidene" represents a fundamental difference in reactivity. Metal carbene complexes usually behave as electrophiles, with typical reactions including cycloadditions to un-saturabed bonds (e.g. cyclopropanation of olefins). On the other hand, metal alkylidene complexes are nucleophilic, undergoing Wittig-type alkylations and olefin metathesis. 

The reaction is useful in the synthesis of acycHc imines [122-124] and heterocumulenes [112-117] and in the intramolecular formation of carbon-nitrogen double bonds in heterocycHc synthesis [112-117]. On the other hand aza-Wittig type reactions of iminophosphoranes with carbon dioxides, carbon disulphides, isocyanates, isothiocyanates and ketenes render access to functionalized heterocumulenes as highly reactive intermediates able to undergo a plethora of heterocycUzation reactions [112-117]. 

Instead of the usual strong and expensive bases, alkali carbonates or hydrogeno-carbonates can be used in Wittig-type reactions. For sonochemical applications, no obvious correlation of the reactivity with their lattice energy appears. The reaction applied to the synthesis of cinnamate esters is reasonably rapid even at room temperature, but sonication does not bring about any change in the isomeric ratio. 

The first example of a [l,2]-Wittig-type rearrangement was reported in 1924 by Schorigin, and was discovered during studies on the reactivity of benzyl ethers towards alkali metals. Schorigin found that heating a mixture of benzyl ethyl ether (4) with metallic sodium to 130-210 °C generated a small amount (13%) of phenyl ethyl carbinol (5). 

In Wittig-type reactions with aldehydes and ketones, arsonium ylides have been shown to give either epoxides or alkenes or mixtures thereof (Scheme 3.88) [140]. However, the semi-stabihzed yHde 457 can be directed onto either pathway by tuning the basicity of the solvent [141]. In pure TH F, the epoxide 459 was formed, whereas in THF/HMPA mixtures the conjugated diene 461 was obtained. This complete switch was observed for a variety of aldehydes and ketones. Rationalization lies in the assumption of zwitterionic intermediates 458 and 460, which react via different conformations. Thus, the anti-conformer 458 is reactive in unipolar solvents, presumably via aggregate formation to give the epoxide, whereas in the presence of HMPA the monomeric species 460 is formed, which undergoes syn-elimination. 

The Wittig-type olefination of carbonyl compounds is one of the characteristic reactions of carbene complexes. High-valent carbene complexes of early transition metals show ylide-like reactivity to vards carbonyl compounds. In 1976, Schrock first demonstrated that niobium and tantalum neopentylidene complexes 1 and 2, the typical nucleophilic Schrock-type carbene complexes, olefinate various carbonyl compounds including carboxylic acid derivatives [4]. 

Generally, arsonium ylides [62] are more reactive but less accessible than phos-phonium ylides. Recently, the chiral arsonium reagent 30 has appeared, and has been applied in asymmetric Wittig-type carbonyl olefinations. This first chiral arsonium reagent also bears 8-phenylmenthyl as a chiral auxiliary on its carboalkoxy portion [63], and gave moderate chemical yields and diastereoselectivities in the conversion of 4-substituted cyclohexanone derivatives to axially chiral non-racemic alkylidene cyclohexanes under the same reaction conditions as used for the related reactions with phosphorus reagents (Scheme 7.15). On the other hand, the corre-... 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

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>. 

In conclusion, phase transfer catalyzed Williamson etherification and Wittig vinylation provided convenient methods for the synthesis of polyaromatics with terminal or pendant styrene-type vinyl groups. Both these polyaromatics appear to be a very promising class of thermally reactive oligomers which can be used to tailor the physical properties of the thermally obtained networks. Research is in progress in order to further elucidate the thermal polymerization mechanism and to exploit the thermodynamic reversibility of this curing reaction. 

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