Norbornenes electrophilic

The first documented example of the living ROMP of a cycloolefin was the polymerization of norbornene using titanacyclobutane complexes such as (207) 510-512 Subsequent studies described the synthesis of di- and tri-block copolymers of norbornenes and dicyclopentadiene.513 However, functionalized monomers are generally incompatible with the highly electrophilic d° metal center. 

All these protocols allow us to form a new carbocycle in a bimolecular process. The cyclization involving two different molecules besides CO has been realized, involving carbapalladation of norbornene, migratory insertion of CO, and subsequent intramolecular Heck-like attack at thiophene residue. Thallium acetate is required as electrophilic co-catalyst (Scheme 18). ... 

In anti addition to a cyclic substrate, the initial attack by the electrophile is also from the less-hindered face. However, many (though not all) electrophilic additions to norbornene and similar strained bicycloalkenes are syn additions.103 In these cases attack is always from... 

Stereoselectivity found for electrophilic additions to norbornene and l-methoxy-2-cyclohexene is believed to originate from secondary orbital interactions rather than from orbital distorsion at the reaction centre144. Rather different results have been obtained from an ab initio MO study of the norbornene hydroboration145. 

Electrophilic metal carbene complexes such as (CO)5W=C(Ph)OMe generally exhibit poor activity as catalysts for metathesis polymerisation, and higher temperatures are required to bring about the polymerisation of high-strained cycloolefins such as norbornene or cyclobutene [84,85], However, their activity can be enhanced by the addition of a Lewis acid such as TiCL into the polymerisation system [86]. Electrophilic complexes such as (CO)5W=CPh2 also generally exhibit poor activity but they are more active than those mentioned above and enable the polymerisation of various cycloolefins [87,88],... 

In fact, this ambiguity between polarization and secondary orbital interactions is akin to a similar ambiguity in orbital distortion explanations of stereoselectivity. Liotta suggested that the direction of attack on alkenes lacking a plane of symmetry was determined by orbital distortion due to a — ir mixing64. Fukui has proposed a similar explantion for the direction of attack of electrophiles on norbornene and related bicyclic systems65. It is believed that most stereoselectivity phenomena have been explained in this way by Liotta and Burgess66. ... 

An unusal /3-chlorodisulfide product 55a was formed in the reaction between norbornene and triphenylmethylsul-fenyl chloride (Scheme 20) <1993TL4289>. The structure was verified by X-ray crystallography. A similar result was obtained with bicyclo[2.2.2]octene. These products were probably formed by electrophilic attack of sulfur atom on the thiirane intermediate 54a and 54b. 

Thus, until the last decade, three families of catalysts have been reported to catalyze the addition, or vinyl-type homopolymerization of norbornene resulting in poly(2,3-bicyclo[2.2.1]hept-2-ene). These three catalyst types are the classical TiC -based Ziegler systems (type 1), the zirconocene/aluminoxane systems (type 2) and certain electrophilic palladium(II) complexes (type 3). 

Electrophilic sulfenes, R2CMSO2, are recognized for their ability to react with nucleophiles and for their 2ir participation in [2 + 2] and [4 + 2] cycloadditions (Chapter 5). Less well recognized is the demonstrated 4ir participation of vinyl sulfenes in Diels-Alder reactions with a select set of dienophiles. Thermolysis of thiete 1,1-dioxides including the parent thiete 1,1-dioxide (22) in the presence of norbornenes provided the Diels-Alder products derived from 4w participation of the vinyl sulfenes [Eq. (18)].40... 

If BuLi r-BuOK and a large excess of cyclopentene are allowed to interact under conditions similar to those used in the case of norbornene (Exp. 7), low to moderate yields of derivatization products are obtained after subsequent reaction with electrophilic reagents. Obviously, cyclopentene is metallated less easily than the strained olefin, and a considerable part of the base is consumed by reaction with THF. If this solvent is omitted, the conversion is very incomplete because of the slight solubility of the base couple in apolar solvents. The system BuLi r-BuOK TMEDA (equimolar amounts) has a good solubility in alkanes and is stable at temperatures up to about 0 °C [22] at higher temperatures TMEDA is attacked. The procedure below shows that cyclopentene can be metallated with satisfactory results using this ternary system. 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents, and the reactivity of the peroxy acids is increased by EWG substituents." These structure-reactivity relationships demonstrate that the peroxy acid acts as an electrophile in the reaction. Low reactivity is exhibited by double bonds that are conjugated with strongly EWG substituents, and very reactive peroxy acids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds. " Strain increases the reactivity of alkenes toward epoxidation. Norbornene is about twice as reactive as cyclopentene toward peroxyacetic acid." trani-Cyclooctene is 90 times more reactive than cyclohexene." Shea and Kim found a good correlation between relief of strain, as determined by MM calculations, and the epoxidation rate. ° There is also a correlation with ionization potentials of the alkenes. Alkenes with aryl substituents are less reactive than unconjugated alkenes because of ground state stabilization and this is consistent with a lack of carbocation character in the TS. 

Anhydrous Fe CH catalyzes the stereospecific epoxidation of norbornene, the demethylation of N. N-dimethylaniline, and the oxidative cleavage of PhCMe(OH)CMe(OH)Ph (and other a-diols) by hydrogen peroxide (Table 11 and Scheme 4). For each class of substrate, the products parallel those that result from their enzymatic oxidation by cytochrome P-450. The close congruence of the products indicates that the reactive oxygen in the Fe CH/HOOH model system and in the active form of cytochrome P-450 is essentially the same, with strong electrophilic oxene character (stabilized singlet atomic oxygen). 

The stereochemistry of electrophilic addition to norbornenes and norbomadienes is very interesting" . As these reactions form on intermediate carbocations, there arises, just as for the solvolysis of 2-substituted norbornanes and norbornenes, the question of the nature of these ions — classical or nonclassical. The electrophilic addi-... 

The main peculiarities of the addition of electrophilic reagents to norbornene are intensive skeletal rearrangements, formation of cis-adducts, exo-addition of both an electrophile and a nucleophile. The unusual process of cis-exo-addition was explained by nonclassical carbocations or rapid equilibrium of classical ions steric... 

Electrophiles (acid halides) have been reported to induce oxygen transfer from [Pd(Ph P)202] to cyclohexene and norbornene, affording... 

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