Vinylic derivatives, formation

The oxygen analogue of the Fischer cyclization requires the formation of 0-vinyl derivatives of iV-arylhydroxylarnines. These are readily converted to indoles but are less readily accessible than the arylhydrazones used for the Fischer cyclization. 

Much of the early work into the rhodium(II)-catalysed formation of oxazoles from diazocarbonyl compounds was pioneered by the group of Helquist. They first reported, in 1986, the rhodium(II) acetate catalysed reaction of dimethyl diazomalonate with nitriles.<86TL5559, 93T5445, 960S(74)229> A range of nitriles was screened, including aromatic, alkyl and vinyl derivatives with unsaturated nitriles, cyclopropanation was found to be a competing reaction (Table 3). 

Similar acetylene addition reactions take place with bis-cydopentadienylnickel carbonyl dimer (93). Changing from carbonyl to cyanide ligands seems to allow the formation of a true vinyl derivative. Thus, potassium pentacyanocobaltate, which may react as a dimer with a cobalt-cobalt bond (20), reacts with acetylene to give the adduct XV (31). The product was thought to be the trans isomer, but the data were not conclusive. 

It is apparent that a vicinal, trans donor subtituent favors the formation of an allylic product, whereas an acceptor group strongly biases the generation of a vinylic derivative. The regioselectivity is a result of the electronic transmission by the polar group in the same manner as discussed above [70]. 

The first mechanism is favored at present (see Sections IV.A-C) in many cases the formation of pyrroles clearly involves O-vinyl derivatives. 

In fact, most of this 30 kJmol1 stabilization is understandable from prior thermochemical findings. In particular, we recall that there is an essentially constant difference between the enthalpy of formation of corresponding phenyl and vinyl derivatives, <5(g Ph, Vi Z)=A fH°(g, PhZ) - Afi/°(g, Viz) - 30 kJ mol-1, regardless of the group Z. 

Photolysis of 2-bromo-4,4-dimethyl-2-cyclohexenone only affords reduction, even in a nucleophilic medium343,344. Apparently, this substrate is structurally not suitable to form a vinyl cation. Formation of vinyl radical-derived products is also the main process for all vinylic halides, if their irradiation is performed in an apolar medium. Such photochemical reductive dehalogenation and especially dechlorination reactions have been extensively studied in the past, not in the least because of their importance as abiotic transformation of persistent polychlorinated environmental pollutants. Examples are the cyclodiene insecticides aldrin and dieldrin, which contain a vicinal dichloroethene chromophore. In recent... 

Several ruthenium complexes are able to promote the classical Markovnikov addition of O nucleophiles to alkynes via Lewis-acid-type activation of triple bonds. Starting from terminal alkynes, the anti-Markovnikov addition to form vinyl derivatives of type 1 (Scheme 1) is less common and requires selected catalysts. This regioselectivity corresponding to the addition of the nucleophile at the less substituted carbon of the C=C triple bond is expected to result from the formation of a ruthenium vinylidene intermediate featuring a highly reactive electrophilic Ca atom. 

Fused dihydrooxazolium compounds react readily by nucleophilic displacement of the oxygen from the azine ring. Thus, 2-carboxy-2,3-dihydrooxazolo[2,3-a]pyridinium bromide reacts with oxygen or nitrogen nucleophiles to form 2-oxo- or 2-imino-l(2ff)-pyridinelactic acids (204) (52JA4906). Similarly, the pyridinium salt (205) reacts by substitution in potassium hydroxide solution, whereas potassium t-butoxide causes proton abstraction and formation of the JV-vinyl derivative (206) (79JA3607). 

Among the processes used for the formation of polyolefins, the longest-known but least selective one is free radical polymerization. A free radical species X produced e.g. by thermolysis of benzoyl peroxide or by photolysis of azabisisobutyronitrile (AIBN) - can react with the double bond of a vinyl derivative H2C=CHR to form a new radical of the type XCH2-CHR which can then add another H2C=CHR unit repetition of this process leads to polyolefin formation (Figure 2, top). This process works best for vinyl derivatives with unsaturated side groups, which provide resonance stabilization for an adjacent radical centre, e.g. with vinyl and acrylic esters, vinyl cyanides and vinyl chloride and with styrene and 1,3-dienes. It is extensively used in the emulsion polymerization of vinylic and acrylic derivatives and in the light-induced formation of photoresists for the nanofabrication of semiconductor chips and integrated electronic circuits. 

Formally similar reaction sequences occur in anionic polymerization. Here, a H2C=CHR double bond reacts with a strongly nucleophilic anion X to form a new carbon-centred anion XCH2-CHR. Continuation of this process leads to the formation of polymer chains, especially again for those vinyl derivatives... 

As reported (08DOC(423)66), the use of LiOH instead of KOH suppressed formation of N-vinyl derivatives 8a,b. 

Palladium catalysts are best known for oxidizing alkenes to ketones or vinyl derivatives. However, formation of a, unsaturated carbonyl compounds by UV irradiation of oxygenated solutions of alkoies in the presence of catalytic amounts of palladium s ts has been observed by Muzait - This reaction is believed to proceed through a ir-allylpalladium trifluoroacetate complex, e.g. (77). 

Isomerization may also modify the structure of the unsaturated product of deamination. In particular, vinyl derivatives afford more stable aromatic compounds, as reported in the case of the deamination of bi.s-dimethylaminomethyl cyclohexanone 236 (Fig. 86), which produces the dimethyl phenol 237 through a series of tautomeric equilibria. The formation of methyl-hydroxy thiophene 238 is explained similarly. ... 

Other Vinyl Derivatives. PVP is a nonionic surfactant used in 3% to 5% concentrations to increase viscosity of solutions. Although it exhibits surface-active properties similar to the cellulose ethers, PVP appears to have less abiUty to lower the interfacial tension at a water-oil interface. Nevertheless, in contrast to the cellulose ethers, PVP appears capable of forming hydrophilic coatings in the form of adsorbed layers. Because conjunctival mucin is believed to interact with the ocular surfece to form an adsorbing surface for aqueous tears, the formation by artificial means of a hydrophilic layer that would mimic conjimctival mucin (mucomimetic) appears to be clinically desirable. Both mucin- and aqueous-deficient dry eyes would benefit, because the wetting ability of the corneal surfece would be enhanced. 

This approach has been extended to protected 8-iodoadenosine, e.g. formation of 8-vinyl derivative 23. 

Despite these modifications, Ireland s original method of both vinyl phosphate formation and reduction remains the most popular. In studies on the cationic rearrangements of [4.3.2]propellanes, Smith et converted a tricyclic ketone, via the diethyl vinylphosphate derivative, into the corresponding alkene (Scheme 26). In a similar manner, Kamata et al prepared A - and A -steroids (Scheme 27) with excellent control of regiochemistry. As shown in this example, esters are susceptible to cleavage under the standard reduction conditions (Li, NHa, Bu OH, -35 C), while acetals survive. 

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