Double bond formation nucleophilic trapping

If the substrate contains a nucleophile in addition to a vinylcyclopropane moiety, the halonium ion formed during the addition of bromine or iodine to the double bond, can be trapped which results in the formation of cyclization products. Examples of compounds that react in this way are 1-phosphoryl-substituted 2-vinylcyclopropanes, 2-vinylcyclopropyl-sub-stituted alkanoic acids, and 2-vinylcyclopropyl-substituted alkanols, which afford oxaphosphabicycloalkanes, bi- and polycyclic lactones and polycyclic ethers, respectively. However, 2-vinylcyclopropyl-substituted alkylamines behave differently, Very good yields have been obtained in some cases, e.g. formation of 4 from 3. ... 

Lin and coworkers described the formation of 5,9-methanobenzoannulenes by [(CpRu(PPh3)2(CH3CN)] -catalyzed allenylidene-ene reactions of orf/to-propenyl and orf/to-butenylphenyl propargyl alcohols. The processes probably involve the initial formation of aromatic vinylidenes as intermediates and these undergo nucleophilic attack by the pendant olefinic double bonds and final trapping with... 

Scheme 6 depicts a typical penicillin sulfoxide rearrangement (69JA1401). The mechanism probably involves an initial thermal formation of a sulfenic acid which is trapped by the acetic anhydride as the mixed sulfenic-acetic anhydride. Nucleophilic attack by the double bond on the sulfur leads to an episulfonium ion which, depending on the site of acetate attack, can afford either the penam (19) or the cepham (20). Product ratios are dependent on reaction conditions. For example, in another related study acetic anhydride gave predominantly the penam product, while chloroacetic anhydride gave the cepham product (7lJCS(O3540). The rearrangement can also be effected by acid in this case the principal products are the cepham (21) and the cephem (22 Scheme 7). Since these early studies a wide variety of reagents have been found to catalyze the conversion of a penicillin sulfoxide to the cepham/cephem ring system (e.g. 77JOC2887).

The formation of strained three- and four-membered rings by electrophile-initiated cyclization requires that the reaction be conducted under conditions which minimize the possibility of simple addition of the activating reagent across the double bond or reversal of the cyclization product to intermediates that can be trapped by external nucleophiles. 

Nucleophilic addition of the azide ion to fluorinated double bonds leads to the formation of a carbanion intermediate which, in the absence of protic solvents, can be trapped by electrophiles. For example, the fluorinated azide ester 2 is obtained from tetrafluoroethene (1) in high yield. ... 

Fig. 4.8, a monolayer of a barbituric acid derivative forms a hydrogen bonding network with aqueous triaminopyridine. Interestingly, cleavage of carbon-carbon double bonds was detected in this system. Upon the formation of the hydrogen bonding network, a few water molecules were trapped in a hydrophobic environment at the air-water interface. This enzyme-Uke behavior was explained by the enhanced nucleophilicity of the trapped water in this environment and the geometry associated with the reaction. 

Carbon-Carbon Bond Formation. The CAN-mediated oxidative generation of carbon-centered radicals has been extensively investigated. The radicals add to a C=C double bond resulting in the formation of a new carbon-carbon bond. The adduct radical can be further oxidized by another CAN molecule to give the carbocation, which is then trapped by a suitable nucleophile to give the final product. Active methylene compounds such as 1,3-dicarbonyls are among the typical substrates. For example, the CAN-mediated oxidative addition of dimedone to 1-phenylcyclohexene affords the corresponding 2,3-dihydrofuran... 

Reaction of e J<9-5-hydroxymethyl-2-norbornene with dichlorocarbene generated under phase transfer conditions leads to 3-chloro-5-oxatricyclo-[5.2.1.0 ]-dec-2-ene as the major product (see Eq. 2.21). Formation of this product probably involves initial addition of dichlorocarbene to the carbon-carbon double bond to yield a 1,1-dichlorocyclopropane which ionizes and ring-opens to form a chloro-substituted allylic carbonium ion. This cation is then trapped by the intramolecular nucleophilic alcohol [40]. 

With the C—C double bond fully substituted at the end proximal to the alkyne, the enyne moiety tends to cyclize in a 6-exo-dig manner, forming a relatively stable tertiary carbocation-containing intermediate (i.e., 20, Scheme 4.6), which can be trapped by nucleophiles in a cascade process. This is exemplified in elegant work by Sethofer et al. [10], where the cascade leads to the formation of three fused rings with high enantiomeric access when a chiral gold complex is used. 

Also the radical species are easily formed on treatment of l-ethyl-2,3-dicyano-1,4-diazinium [188, 189] and 1-ethyl-1,2,4-triazinium salts [190] with nucleophiles, as evidenced by dimerization of pyrazinyl radicals into the corresponding dimeric structure (Scheme 63). It is worth noting that the synthetic potential of the intermediate radicals can be used as trapped with compounds bearing C-C double or triple bonds, for instance by reacting with allyl carboranes. The latter reaction is accompanied by the hydrolysis of one cyano group and results in the formation of the corresponding 2,5-diazabicyclo[2,2,2]octenes (Scheme 63) [189]. 

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