Vinyl fragmentation

Aliphatic Claisen Rearrangements Fluorine Substitution on the Vinyl Fragment... 

Tributylsilyl)trifluoroethene has been used as a precursor to the vinylic fragment.1516 This variation of the original methodology10 avoids the need for butyllithium, since proto-desilylation to give allyl difluorovinyl ether intermediates 34 is achieved with tetrabutylam-monium fluoride. The 2-fluoroalk-4-enoie acid derivatives 35 are produced in a one-pot procedure (Table 11). Various mechanistic possibilities have been discussed in order to account for the stereochemical outcome.16 The need for two equivalents of allylic alcohol limits the utility of this method. 

A second general approach to allyl /(-lluorovinyl ether intermediates involves the reaction of a /J-fluoro alcoholate with an allylic halide. Claisen rearrangement then produces a-fluoro carbonyl compounds. Although the tremendous rate acceleration caused by a-fluoro substitution in the vinyl fragment is not present here, the rearrangements take place under synthetically useful conditions. 

Acyl-chrysene 270 is formed on thermolysis of methoxyisochromene 109 in 50% yield (86UP3). By analogy with behavior of the adducts between 2-benzopyrylium salts and acetoacetic ester 235 (cf. Section III,C,4,d), one cannot exclude the electrocyclic ring-opening of the heterocyclic fragment in 109. The orf/io-quinonoid compound 274 thus formed is a diene, and the vinyl fragment in the unreacted adduct 109 plays the role of an external dienophile. 

Kinetic resolution can be accomplished by addition of allyl boronates to aldehyde groups adjacent to the tricarbonyliron fragment [59]. For the synthesis of ikaruga-mycin, Roush and Wada developed an impressive asymmetric crotylboration of a prochiral meso complex using a chiral diisopropyl tartrate-derived crotylborane (Scheme 1.25) [60]. In the course of this synthesis, the stereo-directing effect of the tricarbonyliron fragment has been exploited twice to introduce stereospedfically a crotyl and a vinyl fragment. 

Infrared absorption data for the compounds considered have been reported more often. Absorption of such vinyl ethers as 150a at 5.9-6.9 /xm corresponds to the vinylic fragment.28 33,86 Additional bands in the spectrum of 151 at 10.7 /am (Ref. 33) and of 152 at 10.18 /am (Ref. 86) show the presence of trans isomers. Alkene absorption of vinylthio ethers was observed29 83 84 between 6.1 and 6.2 /am, the upper limit corresponding83 84 to absorption by a terminal olefinic bond in such compounds as 154a or 159. 

The presence of the dimers malvidin-3-glucoside-ethyl-catechin (m/z 809, [M]" and malvidin-3-(6-p-coumaroyl)-glucoside-ethyl-catechin (m/z 955, [M] ) has been confirmed in wine and wine fractions with the help of ESI-MS. Other pigments (see below), in which the anthocyanin is joined to a flavanol via a vinyl fragment, can also be produced in the presence of acetaldehyde. 

In contrast to the behavior of 3, reductive carbon-carbon coupling between styryl and vinyl fragments of the saturated complex 5 is not observed, suggesting that the unsaturated ligand environment around the metallic center is a determinant factor for the coupling. 

The thermal decomposition of PA 6.6 is quite complex, and the products decomposed are affected by the temperature [57]. It was first reported that PA 6.6 eliminates cyclopentanone as the main decomposition product, but also some hydrocarbons, nitriles, and vinyl fragments [56], Wiloth [64] later suggested that the main mechanisms of thermal decomposition of PA 6.6 were based on the tendency of the adipic acid fragment to undergo cyclization. With the development of modern test methods, the thermal decomposition of PA 6.6 has been extensively investigated by mass spectrometry, IR spectrometry, evolved-gas analysis, and so forth. PA 6.6 fragments, hexamethylenediamine, ammonium, CO2, cyclopentanone, and many other products were detected [65-70]. 

The formation of 5.5-membered palladacycles becomes preferred again when a biphenyl or a phenyl vinyl fragment (that is two -bonds instead of two r/> -bonds) are involved in the cycle. At variance with palladium(O), which prefers to coordinate the arene as rf, the higher preference of palladium(ll) for -coordination gives rise also to five-membered palladacycles. 

The Pd-catalyzed reaction of 2-propynyl-l,3-dicarbonyls 62 with vinyl triflates in CH3CN at 60 °C under 2 atm of CO in the presence of Pd(PPh3)4 and K2CO3 provided acyl furans 63 containing a vinyl fragment (Scheme 9.30) [108]. 

Palladium complexes have also been used to generate vinyl ethers through transetherification reactions [103], In addition to simple substrates, this chemistry has been extended to steroidal substrates (Scheme 2.75) [104]. The catalyst was a phenanthroline complex of palladium acetate, and ethyl vinyl ether served as the source of the vinyl fragment. The reaction conditions were mild, but the dmation of the reaction was long (4 d). 

Treating chlorophosphines with nucleophilic vinyl fragments is one of the oldest ways to generate vinylphosphines. While some of the early woik in this area used organolead... 

Under similar conditions, migration of the vinyl fragment does not occur in 2-(l-hydroxy-2-carbamoyl-2-cyanovinyl)-l-methylpyrroles. The most probable cause is a strong intramolecular hydrogen bond between the hydroxyl proton and carbonyl group (x-ray diffraction (XRD) data), considerably lowering the acidity of the hydroxyl (-3.3 un. pX ) [568]. 

Thioacetals, products of N-Vmylpyrrole-2-carbaldehydes thiylation at the carbonyl group, contain reactive vinyl fragment that expands their synthetic potential and possibility of application in synthesis of porphyrins and related pigments. 

The expected oxidation of the pyrrole cycle, C=C and N=N double bonds, is not observed. The reason is a deep electron density transfer from the N-vinyl fragment to the azo group that decreases susceptibility of the vinyl group and pyrrole ring toward oxidizers. The fact that the stability of the molecule is ensured by the arylazo substituent is confirmed by deep oxidation of 2,3-dimethyl-N-vinylpyrrole in these conditions (NMR spectra of the products show no signals of the vinyl group and pyrrole ring). 

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