Styrenic double bonds

Styrene readily copolymerizes with many other monomers spontaneously. The styrene double bond is electronegative on account of the donating effect of the phenyl ring. Monomers that have electron-withdrawiag substituents, eg, acrylonitrile and maleic anhydride, tend to copolymerize most readily with styrene because their electropositive double bonds are attached to the electronegative styrene double bond. Spontaneous copolymerization experiments of many different monomer pair combiaations iadicate that the mechanism of initiation changes with the relative electronegativity difference between the monomer pairs (185). 

The styrene double bond in 9(ll)-dehydroestradiol 3-methyI ether (1) or its 8-dehydro counterpart is reduced by potassium or lithium in ammonia without affecting the aromatic ring estradiol 3-methyl ether (2) is formed from both compounds. Reduction of the corresponding 17-ketones occurs with partial or complete reduction of the carbonyl group. Lithium... 

In all of these cases, paUadium-catalyzed hydrosilylation proceeds via hydropalla-dation followed by reductive elimination of alkyl- and silyl group from the palladium. In the reaction of o-aUylstyrene (24) with trichlorosilane, which gives hydrosilylation products on the styrene double bond 25 and cycUzed product 26, the hy-dropalladation process is supported by the absence of side products which would result from the intermediate of the silylpaUadation process (Scheme 3-11) [37]. 

In a more recent contribution, O Shea and coworkers described a related process leading to substituted indoles 2-544 and 2-545 by an intermolecular addition of alkyllithium to a styrene double bond and reaction of the formed intermediate 2-543 with an appropriate electrophile (Scheme 2.123) [285]. Using DMF, C-2 unsub-... 

The triethylsilane/trifluoroacetic acid reagent system reduces alkenes to alkanes in poor to excellent yields depending largely on the ability of the alkene to form carbocations upon protonation. Under these conditions the more substituted olefins are reduced in better yields and styrene double bonds are reduced in high yields.127,202,207,221-228 The reduction of 1,2-dimethylcyclohexene with this reagent gives a mixture of cis- and trans- 1,2-dimethylcyclohexane.229 The formation of the trifluoroacetate esters is a side reaction.205,230... 

Intramolecular protonation on the more hindered face of a steroid from a neighbouring hydroxyl group best explains a reversal of diastereoselectivity in the Birch reduction of styrene double bonds.266 The kinetics and product distribution of lithium metal reduction of benzaldehyde to benzyl alcohol in THF have been studied electron transfer from Li to PhCHO occurs in a slow step, but absorption of the PhCHO onto the metal surface is also crucial in determining the overall kinetics. The proposed mechanism successfully accounts for the formation of minor products, benzoin and... 

The determination of the evolution of concentrations of different species and functional groups enables one to discern different paths present in the reaction mechanism. For example, Fig. 5.13 shows that as the molar ratio of styrene to polyester C=C double bonds (MR) increases from 1/1 to 4/1, the curves tend to shift downward. For MR = 4/1 there is a very low styrene consumption until the polyester double bonds are converted to 40%. On the other hand, SEM (scanning electron microscopy) shows phase separation of a UP-rich phase in the early stages of the polymerization. Most radicals are probably trapped in this phase, which explains the higher initial conversion of the UP double bonds than styrene double bonds. A kinetic model would have to take this observation into account. 

Azomethine ylids derived from amidines undergo internal [3 + 2]cyclization reactions with a styrenic double bond present in the starting amidine. This strategy has been applied to the synthesis of physostigmine and erythramine. 

Sparteine 10 is a powerful promoter of carbolithiation, and can lead to good levels of asymmetric induction in the addition step. Successful enantioselective carbolithiations have all involved functionalised styrenic double bonds. Potentially, two new stereogenic centres... 

Sparteine is such a powerful activator of organolithiums that it promotes carbolithiation even of unfunctionalised 1,2-disubstituted styrenic double bonds, giving for example 64.37... 

Cuparene 244 contains two adjacent quaternary centres, and by adding TMEDA Bailey was able to make it in 76% yield from 243.126 Although the standard conditions for halogen-metal exchange employ two equivalents of f-BuLi, in this case it turned out that the second equivalent added directly to the styrene double bond.127 One equivalent turned out to give acceptable yields. 

Show the intermediate that would result if the growing chain added to the other end of the styrene double bond. Explain why the final polymer has phenyl groups substituted on alternate carbon atoms rather than randomly distributed. 

Noting the clear difference in reactivity between the vinyl ether and the styrenic double bonds, Deffieux and his co-workers synthesized macrocyclic polymers via living cationic polymerization [123,162,226], Thus, the French group started the synthesis from an asymmetrically hexafunctional monomer, with three vinyl ether and three p-alkoxystyrenic groups (27, Fig. 11) [162]. Addition of three equivalents of hydrogen iodide selectively... 

Intramolecular addition of the secondary amine moiety to the styrene double bond in styryl kelobascs Ph—CH=CH—CO—CH2CH2—NH—Ar also produces piperidino derivatives. ... 

The problem with peroxides is that they are thermally unstable and do not survive the polymerization to reach the devolatilizer. Blakemore [23] solved this problem by using cyclic peroxides which have very high decomposition temperatures. Their thermal stability is due to the peroxide bond reforming once broken, because the two oxy radicals cannot escape each other so they recouple. If styrene happens to be in the vicinity of the cyclic peroxide while it is a dioxyradical, the diradical adds across the styrene double bond (Scheme 4.1). 

Phenols are normally resistant to reduction as a consequence of ionization. This property was put to good effect in the reduction of 7-methoxyfluoren-2-ol (59 Scheme 8). Because the phenoxide ring does not serve as an electron sink, the 5,8-dihydro derivative is formed, while the styrenic double bond is protected from reduction. However, phenols have been reduced with concentrated solutions of lithium... 

However, Sauer and coworkers have been able to reduce selectively the styrene double bond in ergoline derivative (17) without affecting the indole double bond (equation 50). ... 

Products of addition to styrene double bonds can arise as a result of light induced electron transfer reactions. Lewis has studied the intramolecular reaction of l-phenyl-w-amino alkenes (422) 289,290 products arise from electron transfer from the amine nitrogen to the excited state of the styryl group followed by intramolecular proton transfer in the radical ion pair produced. The resultant biradical then couples to yield the isolated products (423) and (424). Sensitisation of the intermolecular analogue of this reaction by 1,4-dicyanobenzene has been reported and is proposed to occur by electron transfer from the styrene to the excited state of the sensitiser followed by attack of an amine on the styrene radical cation. This ultimately leads to the product of anti-Markovnikov addition of the amine across the double bond of the styrene. This is similar to the sequence long since established by... 

In aliphatic amines, the free electron pair on N is placed in an sp orbital, whereas in pyridine it is in an sp orbital [91], The sp electrons are more strongly attracted by the nucleus. Thus pyridine and aromatic amines are less suitable for donor—acceptor interactions. Methyl methacrylate does not polymerize with these compounds. The electron density on the styrene double bond is higher than in methyl methacrylate under the described conditions, styrene does not polymerize even in the presence of aliphatic amines [92]. 

---