Reactivities of substituted styrene

Most SARs for 102 reactions are based on reactions in organic solvents (Wilkinson and Brummer, 1981). Foote and Denny (1971) measured reactivities of substituted styrenes toward 102 in MeOH and correlated the rate constants with the Hammett equation with p = -0.92. Winterle and Mill (1982) measured the oxidation of several of these same styrenes in water and found the same correlation of relative reactivities, but they found absolute reaction constants two to three times larger than in MeOH. 

Figure 7 Relative reactivities of substituted styrenes toward TohCHCl/ZnCh/ Et20 (CH2CI7, —70° C). (From Ref. 107.) kK 1 values in parentheses.

The low yields, which are observed among styrenyl adducts, reflect a combination of the poor reactivity of the styrene at the low temperature of the reaction. For example, the combination of t-butyl Grignard with the 2,4-bis-OBoc-benzyl alcohol 15 affords the corresponding benzopyran 50 in only 50% yield even when carried out in the presence of 5-10 equivalents of the styrene (method H, Fig. 4.27).27 Yields for substituted benzopyran styrene adducts are still lower (method G, Fig. 4.27). For example, addition of methyl lithium to 2,4-bis-OBoc-benzylaldehyde 5 followed by the addition of the dienophile and magnesium bromide affords benzopyran 51 in a paltry 27% yield. Method F is entirely ineffective in these cases, because the methyl Grignard reagent competes with the enol ether and with styrene 1,4-addition of methyl supercedes cycloaddition. 

More recently, a catalyst-free aqueous version of this strategy was proposed with simple acyclic 1,3-dicarbonyls, formaldehyde, and styrene or anilines derivatives (Scheme 40) [131], In the first case (Scheme 40), the very reactive 2-methylene-1,3-dicarbonyl intermediate reacts smoothly at 80°C with a variety of substituted styrenes to give the corresponding dihydropyrans in moderate to good yields. Remarkably, when styrenes were replaced by A-ethylaniline, a novel five-component reaction involving twofold excess of both formaldehyde and 1,3-dicarbonyl selectively occurred (Scheme 41). The result is the formation of complex fused pyranoquinolines following a Friedel-Craft alkylation - dehydration sequence to furnish the quinoline nucleus, which suffers the Hetero-Diels-Alder cyclization in synthetically useful yields. 

Bevington has continued his studies of the initiation reaction and of the reactivities of monomers towards reference radicals (69—71). A study of the polymerization of substituted styrenes was recorded (72). In methyl methacrylate polymerization by ammonium trichloroacetate in the presence of copper derivatives, the complexities of the initiation and termination reactions were elegantly unravelled by Bamford and Robinson using two differently labelled trichloroacetates (73). Apparently cyclic processes involving alternate oxidation and reduction of copper may arise. 

Soon after syndiospecific styrene polymerization, attention was directed to the homopolymerization of substituted styrenes as well as to their co-polymerization with styrene.956,957,964,1027-1029 Mono-Cp-based Ti systems are capable of homopolymerizing methyl-substituted styrenes and />-chlorostyrene, as well as co-polymerizing them with styrene. The general trend that emerged is that electron-withdrawing Cl substituents decrease the reactivity relative to styrene, whereas electron-releasing Me groups increase it. In both cases, syndiotactic co-polymers were obtained. 

The rate constant ky of the addition reaction of substituted styrenes to the living polystyrene anion has been measured in tetrahydrofuran at 25°C.373 When log ky was plotted against 6 C/3 of the substituted styrene, a linear correlation was obtained, showing that the lower the 7r-electron density on the /3-carbon, the higher the reactivity of the substituted styrene toward the polystyrene anion.363... 

Similarly, dichlorocarbene adds to substituted 2-arylpropenes with a p of -0.62, which is indicative of an electrophilic species. The magnitude of p is similar to that-observed for TT-complex formation of substituted styrenes by silver ion (-0.77), but significantly less than that for hydration (-4) or bromination (-4.3) of substituted styrenes. The relative reactivity of many carbenes and carbenoids with alkenes has been examined and the data accumulated and critically discussed in a comprehensive review. ... 

The substantial decrease in the absolute value of the polar reactivity constant from p = —1.4 for cycloaddition of substituted styrenes with dimethylke-tene (DMK) [39] to p = —0.73 for their cycloaddition to the much more sterically hindered (DPK) [40] = 2.55 [60, Table 4.1]) is consistent with a... 

Tabled, Monomer reactivity ratios (rU for copolymerization of substituted styrenes (Mi) with 1,1-diphenylethylene using -butyllithium as initiator [125, 129, 130] ...

We suggest that the explanation for this apparent anomaly is that the transition state for attack by rm-butoxyl radicals is more advanced than that for attack by less sterically hindered radicals and that as a result, the product radical stability plays a more significant role than it usually does in detemiining radical reactivity. A recent paper by the CSIRO group provides some support for this. Thus, while the rerr-butoxyl radical was found to behave as an electrophilic species towards a series of substituted styrenes, p/2m-substituents generally resulted in higher reactivities than those expected on the basis of the results for meta-substituents, suggesting a resonance contribution to the rate of addition. 

Thermoplastic resins produced from pure monomers such as styrene, alkyl-substituted styrenes, and isobutylene are produced commercially. An advantage of these resins is the fact that they are typically lighter in color than Gardner 1 (water-white) without being hydrogenated. Among the earliest resins in this category were those made from styrene and sold as Piccolastic. Styrene and alkyl-substituted styrenes such as a-methylstyrene are very reactive toward Friedel-Crafts polymerization catalysts. 

The traditional means of assessment of the sensitivity of radical reactions to polar factors and establishing the electrophilicity or nucleophilieity of radicals is by way of a Hammett op correlation. Thus, the reactions of radicals with substituted styrene derivatives have been examined to demonstrate that simple alkyl radicals have nucleophilic character38,39 while haloalkyl radicals40 and oxygcn-ccntcrcd radicals " have electrophilic character (Tabic 1.4). It is anticipated that electron-withdrawing substituents (e.g. Cl, F, C02R, CN) will enhance overall reactivity towards nucleophilic radicals and reduce reactivity towards electrophilic radicals. Electron-donating substituents (alkyl) will have the opposite effect. 

Waters61 have measured relative rates of p-toluenesulfonyl radical addition to substituted styrenes, deducing from the value of p + = — 0.50 in the Hammett plot that the sulfonyl radical has an electrophilic character (equation 21). Further indications that sulfonyl radicals are strongly electrophilic have been obtained by Takahara and coworkers62, who measured relative reactivities for the addition reactions of benzenesulfonyl radicals to various vinyl monomers and plotted rate constants versus Hammett s Alfrey-Price s e values these relative rates are spread over a wide range, for example, acrylonitrile (0.006), methyl methacrylate (0.08), styrene (1.00) and a-methylstyrene (3.21). The relative rates for the addition reaction of p-methylstyrene to styrene towards methane- and p-substituted benzenesulfonyl radicals are almost the same in accord with their type structure discussed earlier in this chapter. 

When one compares the brutto polymerization rate constants, a measure of the reactivity of monomers during cationic homopolymerizations is obtained. It was found for p-substituted styrenes that lg kBr increased parallel to the reactivity, which the monomers show versus a constant acceptor 93). The reactivity graduation of the cationic chain ends is apparently overcomed by the structural influence on the monomers during the entire process of the cationic polymerization. The quantitative treatment of the substituent influences with the assistance of the LFE principle leads to the following Hammett-type equations for the brutto polymerization rate constants ... 

The methyl substitution at a-position leads to an increase of the reactivity of styrene during polymerization as well as EDA-complex formation. However, the methyl substitution in p-position achieves an opposite effect. The strengthened complex formation connected with a further increase of the HOMO is faced with a drastically decreased polymerization rate. This can be explained by the well known steric effect of group hindrance around the p-C-atom under attack 72), as well as the polarity switch in the vinyl double bond. The p-C-atom in the p-methyl styrene possesses a... 

Styrenes may act as 2n and 4n components of the Diels-Alder reaction depending on the substitution site and the electronic effects of the substituent. Electron-donating groups at the a-carbon of the olefinic double bond enhance the dienic reactivity of styrenes [30]. 

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

The reactivity of the prototype o-QM as heterodiene in Diels-Alder cycloaddition reactions with several substituted alkenes such as methyl vinyl ether (MVE), styrene,... 

Some of the vinyl monomers polymerized by transition metal benzyl compounds are listed in Table IX. In this table R represents the rate of polymerization in moles per liter per second M sec-1), [M]0 the initial monomer concentration in moles per liter (M) and [C]0 the initial concentration of catalyst in the same units. The ratio i2/[M]0[C]0 gives a measure of the reactivity of the system which is approximately independent of the concentration of catalyst and monomer. It will be observed that the substitution in the benzyl group is able to affect the polymerization rate significantly, but the groups that increase the polymerization rate toward ethylene have the opposite effect where styrene is concerned. It would also appear that titanium complexes are more active than zirconium. The results with styrene and p-bromostyrene suggests that substituents in the monomer, which increase the electronegative character of the double bond, reduces the polymerization rate. The order of reactivity of various olefinically unsaturated compounds is approximately as follows ... 

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