Styrenes, ring-substituted

Analogous results were obtained for enol ether bromination. The reaction of ring-substituted a-methoxy-styrenes (ref. 12) and ethoxyvinylethers (ref. 10), for example, leads to solvent-incorporated products in which only methanol attacks the carbon atom bearing the ether substituent. A nice application of these high regio-and chemoselectivities is found in the synthesis of optically active 2-alkylalkanoic acids (ref. 13). The key step of this asymmetric synthesis is the regioselective and chemoselective bromination of the enol ether 4 in which the chiral inductor is tartaric acid, one of the alcohol functions of which acts as an internal nucleophile (eqn. 2). 

Pd(OAc)2 works well with strained double bonds as well as with styrene and its ring-substituted derivatives. Basic substituents cannot be tolerated, however, as the failures with 4-(dimethylamino)styrene, 4-vinylpyridine and 1 -vinylimidazole show. In contrast to Rh2(OAc)4, Pd(OAe)2 causes preferential cyclopropanation of the terminal or less hindered double bond in intermolecular competition experiments. These facts are in agreement with a mechanism in which olefin coordination to the metal is a determining factor but the reluctance or complete failure of Pd(II)-diene complexes to react with diazoesters sheds some doubt on the hypothesis of Pd-olefin-carbene complexes (see Sect. 11). 

The addition of two orf/io-methyl groups to ring-substituted cumyl carbocations results in greater steric crowding at the tetrahedral nucleophile adducts than at the corresponding a-methyl styrenes. This results in a decrease in ks for reaction of the carbocation with a nucleophilic solvent relative to kp for deprotonation of the carbocation. 

Fig. 6 Reactivity-structure relationship for the bromination of ring-substituted styrenes (data from Ruasse et ai, 1978). Competition between bromonium and carbocation intermediates.

M of ring substituted styrene or propenylbenzene in 30 ml tetrahydrofuran in V2L flask. Flush with N2 and add 33 ml 1 M borane in tetrahydrofuran (see procedure below for preparation). Stir one hour, add 3 ml water and 50 ml 3N NaOH, and then 215 ml 0.31 M fresh chloramine solution (prepared by treating dilute aqueous NH4OH with Na hypochlorite at 0° see BER 40,4586 (1907)). Keep at room temperature one hour, acidify with HCI, extract with ether, basify with NaOH and extract with ether and dry, evaporate in vacuum (or just basify and extract with ether and dry, evaporate in vacuum) to get the amine. 

The intramolecular coupling of enolethers with enolethers, styrenes, alkyl-substituted olefins, allylsilanes, and vinylsilanes was systematically studied by Moeller [69]. Many of these coupling reactions turned out to be compatible with the smooth formation of quaternary carbon atoms (Eq. 11) [70], which were formed diastere-oselectively and led to fused bicyclic ring skeletons having a ds-stereochemistry [71]. The cyclization is compatible with acid-sensitive functional groups as the allylic alkoxy group. Moeller has demonstrated in some cases that these reactions can be run without loss of selectivity and yield in a simple beaker with either a carbon rod or reticulated carbon as anode without potential control and a 6-V lantern battery as power supply [71]. 

The first CPO-catalyzed epoxidation was reported by McCarthy and White in 1983 [137]. Since this discovery, several research groups have intensively studied the substrate selectivity of this enzyme for the enantioselective epoxidation. As shown in Table 8, styrene and its ring-substituted derivatives are epoxidized by CPO in moderate enantiomeric excesses (entries 1 -8). In contrast, for the cis- -methylstyrene (entry 9) and tetrahydronaphthalene (entry 11) high enantio-selectivities were observed, while the trans-j3-methylstyrene was not converted by CPO (entry 10). 

Monomers which can add to their own radicals are capable of copolymerizing with SO2 to give products of variable composition. These include styrene and ring-substituted styrenes (but not a-methylstyrene), vinyl acetate, vinyl bromide, vinyl chloride, and vinyl floride, acrylamide (but not N-substituted acrylamides) and allyl esters. Methyl methacrylate, acrylic acid, acrylates, and acrylonitrile do not copolymerize and in fact can be homopolymer-ized in SO2 as solvent. Dienes such as butadiene and 2-chloro-butadiene do copolymerize, and we will be concerned with the latter cortpound in this discussion. 

Finally, addition reactions of the isolable phosphasilylcarbenes (13) to such electron-poor substrates as methyl acrylate, C4F9CH=CH2, and styrene afford cyclopropanes. The additions of 13a to (E)- or (Z)-p-deuteriostyrene are stereospecific, and the competitive additions of 13b to ring-substituted styrenes exhibit nucleophilic selectivity, consistent with singlet, nucleophilic carbene addition (Fig. 7.8). ... 

From the stereochemical evidence we might expect that the effect of substituents on the rate of bromination of substituted styrenes in polar solvents would not be cumulative.38 And, indeed, 23, 24, and 25, when brominated under the conditions of Table 7.7, have the relative rates shown. Furthermore, the logs of the rates of bromination of ring-substituted styrenes show a linear correlation... 

Dicarbonyl compounds (ethyl acetoacetate and diethyl malonate) have been added to indene in a cerium(rV) ammonium nitrate-mediated addition.119 Similar additions have been observed for ring-substituted styrenes.120... 

Kinetic studies of the addition of 2,4-dinitrobenzenesulphenyl chloride to cyclohexene in the presence of LiC104 have been interpreted in terms of an ion-pair mechanism. A similar conclusion has been arrived at for addition of (SCN)2 to cyclohexene and ring-substituted styrenes, RC6H4CH=CH2 (R = H, 4-Me, 4-C1, 3-C1)255. 

The mechanism of the isospecific polymerisation of styrene and its ring-substituted analogues in the presence of heterogeneous Ziegler-Natta catalysts is the same as for x-olefins. As regards the mode of monomer insertion, it has been reported as primary (1,2) insertion [37,38]. 

Reactivities of ring-substituted styrenes depend on the type and position of the substituents [18,19,27,40], Table 4.1 lists the relative reactivities of different vinylaromatic monomers in polymerisation with the heterogeneous TiCU AlEt3 catalyst [41],... 

Monocyclopentadienyl titanium derivatives are the most active precursors for catalysts possessing high syndiospecific polymerisation activity for styrene and ring-substituted styrenes. The polymerisation activity of biscyclopentadie-nyl titanium compounds activated with methylaluminoxane is lower than that of other soluble titanium-based catalysts [73]. 

Similarly, the same catalysts that promote the syndiospecific polymerisation of styrene also polymerise ethylene and a-olefins [106,107], ring-substituted styrenes [6] and conjugated dienes [44,74,108-110], These monomers can also be copolymerised with each other [111-114], Substituted styrenes, which yield syndiotactic polymers by polymerisation run with syndiospecific catalysts, form copolymers with styrene the polymerisation rate increases with increasing nucleophilicity of the comonomer. The random copolymers formed are co-syndiotactic [6,111,112]. 

Styrene and ring-substituted styrenes also form copolymers with carbon monoxide, similarly to ethylene and a-olefins. These styrene/carbon monoxide copolymers are of alternating, highly regioregular head-to-tail structure and are characterised by different stereoregularity (syndiotactic, isotactic) [115-117]. 

However, when (5,5)-3,3 -(2,3-butanediol)-2,2 -bipyridine or (/ )-3,3 -(l, 2-propanediol) -2,2 -bipyridine was used as the ligand [125], copolymers were obtained that had a higher content of isotactic triads. An effective control towards the isospecificity of copolymerisation (<98 % isotacticity in the copolymer) is fulfilled for the copolymerisation of ring-substituted styrene such as p-t-butylstyrene and carbon monoxide with catalysts containing cationic methylpalladium species [117] ... 

Explain why the polymerisability of ring-substituted styrenes increases with increasing electron density at the double bond undergoing polymerisation. 

The values of k2 for ring-substituted phenylacetylenes are well correlated by the Hammett relationship, using a+ constants, and p is very large, — 5-2. The p value compares well with those obtained in the acid-catalysed hydration of arylacetylene derivatives (see Table 1) and is entirely consistent with the formation of a vinyl cation 45 in which the positive charge is at the carbon next to the phenyl ring and is largely shared by it. Interestingly, the p value (by use of a+) based on k2 rate coefficients for the bromination of styrene in acetic acid is — 4-5. 

Finally (13), the relative rate constants for the reaction of ozone with selected ring-substituted styrenes in CC14 solution indicate that the second-order rate constants obey Hammett s equation log k = log k0 + per. The negative value of p (—0.91 0.03) confirms that for these olefins the ozone attack is electrophilic in nature. 

Gas Chromatographic Analyses. Were performed with a 5750 F M gas chromatograph, using a flame detector. Six-foot columns of silicone rubber UCW98 were used for styrene, the ring-substituted styrenes,... 

Fig. 19 The reactivity-structure relationship for the bromination of ring-substituted styrenes [32] in MeOH at 25°C competition between the bromonium ion k, . and carbocation kc pathways. For interpretation of symbols, see Rg. 1. The graph is drawn from the data of Ruasse et al. (W8). Copyright 1978 American Chemical Society.

---