Leaving groups vinylic

As a consequence of the selenoxide instability, the oxidation into selenones is only possible if the syn-elimination reaction becomes difficult, as in the case of strained structures. Under these conditions, the selenone is deprotonated and the a-selenonylalkyllithiums formed can react as other selenium-stabilized car-banions. It must be added that the seleninyl [PhSe(O)-] and selenonyl [PhSe(02)-j substituents behave as very good leaving groups. Vinylic selenoxides and selenones can undergo an intramolecular displacement of the selenium moiety after nucleophilic addition to the double bond under basic conditions. Cyclopropanes and oxetanes have been synthesized in this way [1,2]. 

C-C bond formation via transition-metal-catalysed cross coupling of -activated enols has been reviewed (97 references) while less reactive than systems involving halide or triflate leaving groups, vinyl phosphates have significant advantages in terms of cost, stability, and low toxicity... 

Similar to Figure 1, the palmitoyl (C-16) group has also been enzymatically grafted onto HEC. The reaction entails the incubation of HEC, vinyl palmitate, and a suitable enzyme in N,N-dimethylacetamide (DMAc) at 50°C. Several enzymes were attempted satisfactory results were obtained with Pseudomonas fluorescens lipase (Amano P-30), Pseudomonas capecia lipase (Lipase PS), or Alcalase immobilized alkaline protease from Novozymes A/S as a catalyst. Note that in this case a polar aprotic solvent (DMAc) is used, and one of the reactants, vinyl palmitate, has a fricile leaving group (vinyl alcohol which leaves as acetaldehyde). 

The photocyclization of iV-vinylanilines is an e.xarnple of a general class of photocyclizations[l]. If the vinyl substituent has a potential leaving group or the reaction is carried out so that oxidation occurs, the cyclization intermediate can aromatize to an indole. 

Alkylation reactions are subject to the same constraints that affect all Sn2 reactions (Section 11.3). Thus, the leaving group X in the alkylating agent R—X can be chloride, bromide, iodide, or tosylate. The alkyl group R should be primary or methyl, and preferably should be allylic or benzylic. Secondary halides react poorly, and tertiary halides don t react at all because a competing E2 elimination of HX occurs instead. Vinylic and aryl halides are also unreactive because backside approach is sterically prevented. 

Esters of inorganic acids, including those given above and others, can be hydrolyzed to alcohols. The reactions are most successful when the ester is that of a strong acid, but it can be done for esters of weaker acids by the use of hydroxide ion (a more powerful nucleophile) or acidic conditions (which make the leaving group come off more easily). When vinylic substrates are hydrolyzed, the products are aldehydes or ketones. 

This mechanism is the same as the simple electrophilic one shown on page 970 except that the charges are reversed (lUPAC An- -Ae or A +Ah). When the alkene contains a good leaving group (as defined for nucleophilic substitution), substitution is a side reaction (this is nucleophilic substitution at a vinylic substrate, see p. 428). 

It is evident from the foregoing sections that simple alkylvinyl halides do not react via an Sn 1 mechanism, if at all, even under extreme solvolytic conditions (146,149). More reactive leaving groups, such as arylsulfonates, were clearly needed to investigate the possible solvolytic behavior of simple alkylvinyl systems, but the preparation of vinyl sulfonates until recently was unknown. Peterson and Indelicato (154) were the first to report the preparation of vinyl arylsulfonates via reaction of the appropriate disulfonate with potassium t-butoxide in refluxing f-butanol. They prepared and investigated the solvolysis of 1-cyclohexenyl tosylate 169 and c/s-2-buten-2-yl tosylate 170 and the corresponding p-bromobenzenesulfonates (brosylates). Reaction... 

An irreversible procedure for the lipase-catalyzed acylation using vinyl esters as acylating agent has been developed, where a leaving group of vinyl alcohol tautomerizes to acetaldehyde. In these cases, the reaction with the vinyl esters proceeds much faster to produce the desired compounds in higher yields, in comparison with the alkyl esters. 

The 6 -phosphines dppe, dppp, and dppb were also effective for this coupling. Entry 3 is an example of use of a vinyl triflate. Entries 4 and 5 illustrate the use of perfluorobu-tanesulfonate (nonaflate) as an alternative leaving group to triflate. The organozinc... 

The most comprehensive examination of the Rokita kinetic procedure from a synthetic standpoint was carried out by Barrero and coworkers.6 They examined the effects of various leaving groups, solvents, nucleophiles, and their equivalents on subsequent [4 + 2] cycloadditions. Avast excess of the intended nucleophile (50-100 equiv) must be employed, because the fluoride triggered (3-elimination proves nearly instantaneous at room temperature resulting in a high concentration of a species that is prone to undergo dimerization and other undesired side reactions that are irreversible at these low temperatures (Fig. 4.7). Use of fewer equivalents of the intended nucleophile led to a rapid drop off in yield. For example, 5-10 equivalents of ethoxy vinyl ether (EVE) affords only a 5-10% yield of the desired benzopyran... 

The triflate ion is such a good leaving group that even vinylic triflatres undergo SnI reactions and yield vinylic cations. 

Simple acetylenic allylic alcohols can be cyclized easily by tarf-butoxide, and since this base initiates prototropic shifts the vinylic link may.be E or Z such cyclizations proceed via exomethylene furans similar to 15.65 If necessary, UV irradiation can effect both the isomerization of an to a Z-alkene and the cyclization, leaving only the final adjustments to be made chemically.66 In a variation, methoxide plays the role of leaving group allowing very good yields to be attained (Scheme 10).67... 

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