A-substituted styrenes

Phenanthrene-l,4-diones have been prepared [52] by cycloaddition of a-substituted styrenes with an excess of 1,4-benzoquinone (Equation 2.18). Initial cycloadducts are oxidized by 1,4-benzoquinone. 

In contrast, product mixtures were obtained [53] when a-substituted styrenes were reacted with dimethylacetylenedicarboxylate (Equation 2.19). The products were formed via aromatization of the primary cycloadducts or by ene addition of a second molecule of DM AD. 

Each of the derivatives may be regarded as a substituted styrene, and classical styrene syntheses have been employed. Radical polymerization of the phenolic monomers (salicylate esters, 2-hydroxybenzophenones and hydroxyphenylbenzotriazoles) proceeds normally with AIBN as initiator, at least when oxygen is carefully excluded. It is expected that polymeric ultraviolet stabilizers, perhaps in combination with conventional stabilizer will make an important contribution to photostabilization technology. 

Alternating copolymers of N-allyl maleimide with a substituted styrene are promising negative working x-rav resists. These materials have sensitivities of 10-50 mJ/cm, providing a contrast of 1.5 or better, and appear capable of at least 0.5 m resolution. Further improvements in resist performance should be possible by optimization of polymer Mw and development conditions. 

Wullf and Hohn recently described several new stereochemical results (93). They reported the synthesis of a copolymer between a substituted styrene (M ) and methyl methaciylate (M2) having, at least in part, regular. . . M,M M2M MiM2. . . sequences. Polymerization involves the use of a chiral template to which the styrene monomer is loosely bound. After elimination of the template, the polymer shows notable optical activity that must be ascribed to the presence of a chiral stmcture similar to that shown in 53 (here and in other formulas methylene groups are omitted when unnecessaiy for stereochemical information). This constitutes the first stereoregular macromolecular compound having a three monomer unit periodicity. 

In the presence of BiCU, the hydroarylation of styrenes Ar,C(R)=CH2 with electron-rich arenes ArH afforded Markovnikov adducts Ar C(R)(Ar)Me selectively in good to high yields as a result of the C-H activation of ArH. Under arene-free conditions, the intermolecular hydroarylation of a-substituted styrenes and subsequent intramolecular hydroarylation produced the cyclic dimers of a-substituted styrenes in good yields.122,123... 

In addition, the same authors showed that ,/ -unsaturated aldehydes 132 could act both as CO and alkene source and give the PK products 134 upon reaction with different alkynes (133) (Scheme 41). As part of their study, they performed a cross reaction of cinnamaldehyde and a substituted styrene with phenylacetylene, isolating two PK products coming from both alkenes. This is a proof for a decarbonylative-[2 + 2+1] reaction pathway [147]. 

The addition of phenylselenenyl chloride to a-substituted styrenes in methanol is stereoselective and gives the product of anti addition of the phenylseleno and methoxy groups to the double bond. The (E)-alkcnc gives the (/ , S )-products while (Z)-alkenes give (S, S )-products28. 

Simple a-substituted styrenes are reduced in the presence of RuCl2(DuPhos)(DMF) . The reactivity of the ruthenium catalyst is enhanced by the addition of potassium te/t-butoxide, which may facilitate generation of a ruthenium hydride. The products are obtained under low hydrogen pressures and selectivities obtained are up to 89% ee (eq 8). Neutral Rh-DuPhos complexes catalyze the hydrogenation of a,3-unsaturated acids such as tiglic acid (eq 9). The product is obtained in quantitative yield and good enantioselectivity. ... 

Poly(styrenes) and poly(a-substituted styrenes) can bear various substituents on the aromatic ring. These substituents may affect other properties of the polymer, but their thermal stability is not very different from that of unsubstituted poly(styrene). Some of the idealized structures of the polymers in this group are shown below ... 

Similar to the poly(olefin-a/f-carbon monoxide), polymers alternating styrene or a substituted styrene unit and CO also are reported in literature [7], The idealized structure of these types of polymers is shown below ... 

Laus et al. also investigated a chain extension of pSt-TEMPO with a substituted styrene, phthalimide methylstyrene (PIMS) [146], with the objective of incorporating the PIMS, then deprotecting it to produce the amino functional polymers, according to Scheme 12. 

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... 

Although Markovnikov alkenylboranes have been known since 1975, "their Pd-catalyzed cross-couphng had not been reported until an improved procedure for the preparation of Markovnikov alkenylboranes in pure form was recently reported. " In these reactions, however, the primary alkyl groups compete with the 2-alkenyl groups and produce alkylbenzenes, but Pr, Bu, and Bu virtually do not. On the other hand, when the alkenylboranes are treated first with 1 equiv of trimethylamine A-oxide, which presumably gives alkenylalkoxyborane, the yields of the desired a-substituted styrenes increase dramatically (Scheme 24). 

Total yield of coupling products. The mono-a-substituted styrene was isolated in 12% yield. Fc = ferrocenyl. 

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