Vinyl position

Dihydropyrans 88-90 are deprotonated at the vinylic position adjacent to oxygen by t-BuLi and the resulting anions add readily to alkyl halides, aldehydes, and ketones. Subsequent acid hydrolysis provides the products expected from reaction of an oj-functionalized pentanoyl anion 88 acts as HOCH2(CH2)3CO , 89 as 0CH(CH2)3C0 , and 90 as MeCO(CH2)3CO (77TL4187 81T3997). 

Two examples of this method for the synthesis of olefins deuteriated exclusively at the vinyl position by the simple use of deuteriated solvents are shown in equations 64198 and 65199. Due to the alkaline reaction conditions no subsequent isomerization of the initially produced alkene is observed. 

ADMET reaction. The 13C NMR spectrum also allows the scientist to distinguish between cis and trans internal sp2 carbons as well as the allylic carbons, which are adjacent to the internal vinyl position. Using quantitative 13C NMR analysis, the integration of the peak intensities between die allylic carbon resonances and diose of the internal vinyl carbons gives die percentage of trans/cis stereochemistry diat is present for the polymer.22 Empirically, the ratio of trans to cis linkages in ADMET polymers has typically been found to be 80 20. Elemental analysis results of polymers produced via ADMET demonstrate excellent agreement between experimental and theoretical values. 

Bromination and chlorination of alkanes and cycloalkanes can also take place by an electrophilic mechanism if the reaction is catalyzed by AgSbFs. " Direct chlorination at a vinylic position by an electrophilic mechanism has been achieved... 

There are two vinylic positions, highlighted in grey above. The vinylic position on the right has more alkyl gronps it is more substitnted. When Br ends up on the more substituted carbon, we call it a Markovnikov addition ... 

The vinylic carbon atoms, highhghted above, are both sp hybridized, and therefore trigonal planar. As a result, aU four groups (connected to the vinylic positions) are... 

Although this type of reaction is symmetry forbidden in an unadsorbed molecule, theoretical calculations showed that in a molecule adsorbed on transition metals, such a shift is allowed [3-5], Later, other theoretical calculations suggested another type of 1,3-hydrogen shift, one in which the allylic cxo-hydrogen is abstracted by the surface fi-om an adsorbed alkene (either 1,2-diadsorbed or n-complexed) and the resulting 7i-allyl species moves over the abstracted hydrogen in such a way that it adds to the former vinylic position and causes, in effect, a stepwise intramolecular 1,3-hydrogen shift (bottom shift) [6],... 

To distinguish between these two hydrogen shift mechanisms, ( S)-(-)-exo-2,4-dideuteroapopinene was constructed as a probe molecule. A top shift of the allylic endo-H will not affect the deuterium content of the molecule and no change should occur in the hydrogen content at any position, but a bottom shift of the allylic exo-D will decrease deuterium in the vinylic position (C2) and increase deuterium in the allylic position (C4). 

Nitrodesilylation (Eq. 2.17)36 and nitrodestanylation (Eq. 2.18)37 are efficient methods for the preparation of some kinds of nitroalkanes from readily available alkylsilanes or allylstan-nanes. Similar nitration also takes place at the vinylic positions (see Eq. 2.36 in Section 2.1.4). 

The stabilized anion 134 also attacks the terminal vinylic position of 131 exclusively to give cyclopropylideneethyl derivatives 135, which are synthetically useful building blocks [69], The use of chiral phosphanes as ligands leads to optically active methylenecyclopropane derivatives. (Scheme 47)... 

Reaction of organic halides with alkenes catalyzed by palladium compounds (Heck-type reaction) is known to be a useful method for carbon-carbon bond formation at unsubstituted vinyl positions. The first report on the application of microwave methodology to this type of reaction was published by Hallberg et al. in 1996 [86], Recently, the palladium catalyzed Heck coupling reaction induced by microwave irradiation was reported under solventless liquid-liquid phase-transfer catalytic conditions in the presence of potassium carbonate and a small amount of [Pd(PPh3)2Cl2]-TBAB as a catalyst [87]. The arylation of alkenes with aryl iodides proceeded smoothly to afford exclusively trans product in high yields (86-93%) (Eq. 61). 

Axelrad, G., Laosooksathit, S., and Engel, R., Halide exchange at vinylic position catalyzed by copper halide complexes of trivalent phosphorus, Synthetic Commun., 11, 405, 1981. 

PN and Parent Exo PX) have unsubstituted norbornenyl rings and differ from each other only in the stereochemistry of their ring fusion. These compounds have been elaborated by the incorporation of either a phenyl (tf>) or a carbomethoxy (C) group in the bridgehead (B) or vinyl (V) positions. Thus, the notation VN represents PN with a phenyl substituent at a vinyl position, while CBX represents PX with a carbomethoxy substituent at the bridgehead position. The isomerization and polymerization chemistry of these nine compounds (PN, PX, 4>BN, (jiVN, (jiVX, CBN, CBX, CVN, CVX) are the main concerns of our work. 

Reductive coupling of 1,1-dimethylallene and 5-nitro-2-furancarboxaldehyde under a deuterium atmosphere provides the product of ferf-prenylation incorporating deuterium at the interior vinylic position (80% H). This result is consistent with a mechanism involving allene-aldehyde oxidative coupling. However, alternate pathways involving allene hydrometallation to furnish allyliridium species cannot be excluded on the basis of these data (Scheme 10). 

An example of the vinylogous reactivity is the reaction of 52 with cyclopentadiene (Tab. 14.9) [77]. Rhodium(II) acetate-catalyzed decomposition of 52 in dichloro-methane, yields a 2 1 mixture of the bicyclic system 53 derived from the [3-1-4] cycloaddition, and the bicyclo[2.2.1]heptene 54 resulting from electrophihc attack at the vinylic position followed by ring closure. When Rh2(TFA)4 is used as the catalyst, bicy-clo[2.2.1]heptene 54 becomes the dominant product, while the reactivity of the vinyl terminus is suppressed using a hydrocarbon solvent as observed in the Rh2(OOct)4-cat-alyzed reaction in pentane, which affords a 50 1 ratio of products favoring the [3-1-4] cycloadduct 53. 

The point of attachment of is from the vinylic position to the hydroxamate carbon. 

A serious objection to the hydrogen separation mechanism is that it postulates that the hydrogen in vinylic position to the double bond is involved it seems more reasonable to expect that the hydrogen in allylic position would be more readily separated. Furthermore, the mechanism gives no consideration to the role of the catalyst other than to state that it is the means whereby the bonds of the reacting molecules are activated. The postulation of the intermediate formation of carbonium ion or esters seems to be more plausible. 

Geminal selectivity is also observed in cases where the bulky substituent is in a vinylic position . Examples with compounds 60-65 are shown in Table 10. 

On the basis of similar arguments, previously proposed, transition state TSi is expected to have lower energy than TS2 (Scheme 18) and thus account for the geminal selectivity of alkenes with the large L group in the vinyl position. 

Fluorine atoms have been introduced mainly at the vinylic positions 5, 6, 8, and 9, the allylic positons 7,10,and 13, and the terminal positions 2,19,and20. Onlyafew significant examples of these numerous works are discussed here ... 

Bromination and chlorination of alkanes and cycloalkanes can also take place by an electrophilic mechanism if the reaction is catalyzed by AgSbF. "2 Direct chlorination at a vinylic position by an electrophilic mechanism has been achieved with benzenescleninyl chloride PhSe(0)Cl and AIC13 or AIBr3. n However, while some substituted alkenes give high yields of chloro substitution products, others (such as styrene) undergo addition of Cl2 to the double bond (5-26).113 Electrophilic fluorination has already been mentioned (p. 690). 

Formation of 1,2-dioxetanes is usually characteristic of alkenes activated by heteroatom in the vinylic position. Most dioxetanes are unstable and decompose with concomitant chemiluminescence. The products are carbonyl compounds resulting from oxidative cleavage of the double bond. 

Substituent effects are in accord with the proposed mechanism. The Birch rule202 states that electron-releasing substituents (alkyl groups) deactivate the ring and direct reduction so that the major product has the maximum number of groups attached to the residual double bonds, that is, to vinyl positions. In other words,... 

Enol lactones with a halogen at the vinylic position have been synthesized as potential mechanism-based inactivators of serine hydrolyases <81JA5459). 5-Hexynoic acids (181) can be cyclized with mercury(II) ion catalysis to y-methylenebutyrolactones (182) (Scheme 41). Cyclization of the 6-bromo and 6-chloro analogues leads stereospecifically to the (Z)-haloenol lactones (trans addition) but is quite slow. Cyclization of unsubstituted or 6-methyl or 6-trimethylsilyl substituted 5-hexynoic acids is more rapid but alkene isomerization occurs during the reaction. Direct halolactonization of the 5-hexynoic acids with bromine or iodine in a two-phase system with phase transfer catalysis was successful in the preparation of various 5-halomethylene- or 5-haloethylidene-2-phenylbutyrolactones and 6-bromo-and iodo-methylenevalerolactones (Scheme 42). 

Lithiation of the exocyclic vinyl position of methylenecyclopropane is achieved by halogen exchange reaction of /-BuLi with (bromomethylene)cyclopropane in ether. The vinyl... 

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