Styrene electrophilic addition

When an unsymmetrically substituted vinyl monomer such as propylene or styrene is polymerized, the radical addition steps can take place at either end of the double bond to yield either a primary radical intermediate (RCH2-) or a secondary radical (R2CH-). Just as in electrophilic addition reactions, however, we find that only the more highly substituted, secondary radical is formed. 

Numerous p-values for various electrophilic additions to styrene itself are available (Schmid and Garratt, 1977). Strictly speaking, the reaction constants measure only the sensitivity of the reaction to substituent effects they depend at the same time on the solvent, on the position of the transition state on the reaction coordinate (charge magnitude) and on the way in which substituent effects are transmitted (charge location). In particular, the observed trend of p-values for the chlorination ( — 3.22 Yates and Leung, 1980), bromination (—5.7 Ruasse et al, 1978) and sulfenylation ( — 2.41 ... 

Iodine was found to be an efficient catalyst for the aziridination of alkenes (Scheme 6) utilizing chloramine-T (A-chloro-A-sodio-p-toluenesulfonamide) as the nitrogen source. For example, when 2 equiv. of styrene (45a) were added to chloramine-T in the presence of a catalytic amount of iodine (10mol%) in a 1 1 solvent mixture of acetonitrile and neutral buffer, the corresponding aziridine (46) was obtained in 91% yield. The reaction proved to work with other acyclic and cyclic alkenes, such as oct-l-ene and cyclohexene. The aziridination of para-substituted styrene derivatives (45b-e) demonstrated that, as expected for an electrophilic addition, electron-rich alkenes reacted faster than electron-poor alkenes. However, with 1 equiv. of I2, mainly iodohydrin (47) was formed. A catalytic cycle has been proposed to account for the fact that only a catalytic amount of iodine is required (Scheme 1) ... 

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). 

The silver(I) fluoride/bromine and silver(I) fluoride/iodine systems can be exploited for generation in situ and electrophilic addition of "bromine monofluoride and iodine monofluoride to cyclohexene, mcthylenecyclohcxane, styrene, indene, and acenaphthylene.55... 

One of the most characteristic types of ground-state reaction for alkenes is electrophilic addition, often involving a proton acid as addend or catalyst. In the excited state similar reactions can occur, with water, alcohols or carboxylic acids as commonly encountered addends. However, there is a variety of photochemical mechanisms according to the conditions or substrate used. In a few instances it is proposed that the electronically excited state is attacked directly by a proton from aqueous acid, for example when styrenes are converted to l-arylethanols (2.47 the rate constant for such attack is estimated to be eleven to fourteen orders of magnitude greater than that for attack on the ground state, and the orientation of addition is that expected on the basis of relativecarbonium ion stabilities (Markowni-kov addition). 

Electrophilic additions of the halogens to alkynes have not been much studied. In acetic acid a given olefin reacts with bromine 103 to 105 times more rapidly than the corresponding acetylene. However, the relative rates are enormously solvent-dependent and decrease with solvent polarity. Thus bromina-tion of styrene is 2590 times faster than bromination of phenylacetylene in acetic acid, but only 0.67 times as fast in HzO. Solvation of the transition state must be... 

Partial ergot alkaloid substrates of 5 and related, conformationally fixed styrenes 6-8 have been found to undergo electrophilic additions (epoxidation, HOBr addition and... 

Halosilyl)styrenes undergo dimerization and trimerization on treatment with TfOH via electrophilic addition of the corresponding benzyl cation generated by protonation of the parent molecule386. 

Detailed investigation of C-14 KIE in simple electrophillic addition reactions has been presented by Kanska and Fry610-612 regarding the mechanism of addition of 2,4-dini-trobenzensulphenyl chloride to substituted styrenes labelled with C-14. Additions of 2,4-dinitrobenzenesulphenyl chlorides to / -substituted a- and -labelled styrenes have been carried out in acetic acid at 30.1 °C (equation 267) ... 

Electrophilic addition of sulphenyl halides to alkenes occurs, by all the evidence, via cyclic thiiranium ions (Mueller, 1969) and a comparison of the rates of addition to the double and triple bond would be quite interesting. Unfortunately, direct kinetic data for strictly comparable and typical cases are not available. Phenylacetylene has been reported (Kharasch and Yannios, 1964) to react 102 times slower than styrene (in acetic acid at 25°) with 2,4-dinitrobenzenesulphenyl chloride. On the other hand, Thaler (1969), by means of competitive experiments carried out in dilute paraffin solutions at — 70°, estimated that methane-sulphenyl chloride adds to mono- (and di-)alkylacetylenes 50-100 times more slowly than to the corresponding alkenes (cis) (but only ca. twice slower than to trans dialkylethylenes). The paucity of information does not allow generalizations and further work in this area seems desirable also with respect to the much larger rate differences observed in those bromine additions to triple and double bonds which also occur via bridged species. 

Electrophilic addition reactions are some of the most important processes (21) involving a cationic transition state or intermediate. The acid-catalysed hydration of arylalkenes such as styrene [32] is typical of such processes... 

Arylalkene bromination is a typical electrophilic addition to form an a-carbocation, but markedly non-linear structure-reactivity relationships were observed for brominations of styrene [32], trans-stilbene [37] and a-methylstilbene [38] (Ruasse and Dubois, 1972, 1974 Ruasse et al., 1978 Ruasse and Argile, 1983). Some of these curvatures could not be interpreted directly by the Y-T equation but some were related to a mechanistic changeover based on a multipathway scheme (Ruasse, 1990). Three pathways leading to the and carbocations and to the bromonium ion in the bromination of trans-stilbene (Ruasse and Dubois, 1972, 1974) are shown in Scheme 10. 

The mode of attack of stable carbenium ions on to alkenyl monomers has been proved in some cases to proceed via direct electrophilic addition. However, for certain monomers, e.g., styrene and a-methylstyrene, the mechanism of this reaction has not yet been established unambiguously. We believe that studies on model compounds such as substituted styrenes incapable of polymerising because of steric impediment could provide an answer to this important question. 

Reaction of DCT with styrene in polar protic solvents (water, acetic acid, and methanol) proceeds with the electrophilic addition of the halogen and the solvent moieties according to the Markovnikov rule. In carbon tetrachloride in the presence of an initiator (AIBN) addition of DCT and TBT to styrene proceeds to form anti-Markovnikov adducts and a small amount of 1,2-dihalo derivatives (69ZC325) (Scheme 72). 

The most common methods of C—B—X ring construction employ hydroboration/amination, aminolysis of 4-bromoalkylborinate esters, electrophilic additions of boron halides to o-amino-styrenes and -biphenyls, insertion of hydrazines into 1,2,5-thiadiborolenes, and aldol-type condensations. Specific examples of these processes are presented in the following sections 6.28.3.1 and... 

The formal electrophilic addition of BrF" to a double bond proceeds stereospecifically in an anti-sense as evidenced by the formation in high yields of trans-1 -bromo-2-fluorocycloalkanes from cis-cycloalkenes or of cis-1-bromo-2-fluorocyclododecane from trans-cyclododecene, respectively.5 The addition is regioselective with the observed regiochemistry being in accordance with the Markovnikov rule. For example, the bromofluorinations of a-substituted styrenes give the 1-bromo-2-fluoro-2-phenylalkanes with virtually complete regioselectivity only traces (<1%) of the regioisomeric adducts were detectable by 19F NMR spectroscopy... 

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