Styrenes with electrophilic carbon

Aryl and vinylic bromides and iodides react with the least substituted and most electrophilic carbon atoms of activated olefins, e.g., styrenes, allylic alcohols, a,p-unsaturated esters and nitriles. 

The reactive nature of compound 22 is illustrated by the series of transformations shown in Scheme 7.12, in which its Zr—C bond reacts selectively with electrophilic reagents to produce a-haloboronates 36—38. Compound 22 also catalyzes the polymerization of styrene. The polymers thus obtained had weight-average molecular masses in the range 75000—100000 with polydispersities of 1.8—2.1. An X-ray analysis of 22 confirmed it to be a four-coordinate Zr complex with two cyclopentadienyl rings, chlorine, and the aliphatic C-l carbon atom as the ligands (Fig. 7.4). 

What about a nucleophilic carbene, for which negative charge should build up on the olefinic carbon atoms during the carbene addition cf. 5 With ArCH=CH2 substrates, electron-withdrawing aryl substituents would stabilize such a transition state and the p value should be positive. There are several examples of this phenomenon. For example, cycloheptatrienylidene, 10, adds to / -substituted styrenes with p = -t-1.02 - 1.05 (vs. a) consistent with a nucleophilic selectivity that seems to implicate the aromatic resonance form 10a as an important feature of the carbene. [45] It is satisfying to compare this result with p = -0.76 (vs. a) or -0.46 (vs. a" ") for additions to styrenes of cyclopentadienylidene, 11, where contributions of the cyclopentadienide form (11a) would render the carbene electrophilic. [46] However, these conclusions are too facile. There is reason to believe that the chemistry attributed to 10 might in fact be due to its allenic isomer 12. [47] And the electronic structure of 11 is also more complicated than the simple depiction above. [48]... 

Hydroboration is highly regioselective and stereospecific. The boron becomes bonded primarily to the less-substituted carbon atom of the alkene. A combination of steric and electronic effects works to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value (-0.5).156 Compared with bromination (p+ = -4.3),157 this is a small substituent effect, but it does favor addition of the electrophilic boron at the less-substituted end of the double bond. In contrast to the case of addition of protic acids to alkenes, it is the boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is reinforced by steric factors. Hydroboration is usually done under conditions in which the borane eventually reacts with three alkene molecules to give a trialkylborane. The... 

Hydroboration is highly regioselective and is stereospecilic. The boron becomes bonded primarily to the less substituted carbon atom of the alkene. A combination of steric and electronic effects work together to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value... 

Nucleophile addition to styrene derivatives (e.g. 75) coordinated with Cr(CO)3 is another example of addition-electrophile trapping.23,128 Addition of reactive anions is selective at the 3-position of the styrene ligand, leading to the stabilized benzylic anion (76). The intermediate reacts with protons and a variety of carbon electrophiles to give substituted alkylbenzene ligands (in 77) (equation 52). 

A DIE of k 2o/k )2o = 3.2 has been found in the hydrolysis of a-benzoyloxystyrenes, 382, in concentrated perchloric acid solution450. The rate of hydrolysis was found to be linear with the acidity function, Ho- A small inverse deuterium KIE, A h /I d = 0.78, at the -carbon has been found in the oxidation of cinnamic and crotonic acids451 by quinolinium dichromate. The reaction involves electrophilic attack on the double bond and a carbonium-ion intermediate (p = —4.0). A small inverse D KIE (k /k ->) = 0.80, p = —4.0] has been also observed452 in the acid cleavage of substituted styrenes by quinolinium dichromate in DMF in the presence of acid452. 

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 classical Vilsmeier-Haack reaction - involves electrophilic substitution of a suitable carbon nucleophile with a chloromethyleneiminium salt, for example salt (1). Suitable carbon nucleophiles are generally electron-rich aromatic compounds such as V,N-dimethylaniline (2), alkene derivatives such as styrene (3) or activated methyl or methylene compounds such as 2,4,6-trinitrotoluene (4 Scheme I). These compounds (2-4) react with salt (1) giving, after loss of hydrogen chloride, the corresponding im-inium salts (5-7). Hydrolysis of iminium salt (5) affords aldehyde derivative (8) and this transformation (Ar—H - Ar—CHO) is the well-known Vilsmeier-Haack formylation reaction. Hydrolysis of iminium... 

In the above condensation resist designs, the phenolic resin offers a reaction site as well as base solubility. Self-condensation of polymeric furan derivatives has been utilized as an alternative crosslinking mechanism for aqueous base development (Fig. 126) [375]. The copolymer resist is based on poly[4-hydroxy-styrene-co-4-(3-furyl-3-hydroxypropyl)styrene], which was prepared by radical copolymerization of the acetyl-protected furan monomer with BOCST followed by base hydrolysis. The furan methanol residue, highly reactive toward electrophiles due to a mesomeric electron release from oxygen that facilitates the attack on the ring carbons, readily yields a stable carbocation upon acid treatment. Thus, the pendant furfuryl groups serve as both the latent electrophile and the nucleophile. Model reactions indicated that the furfuryl carbocation reacts more preferentially with the furan nucleus than the phenolic functionality. 

The [4+ + 2] cycloadditions proceed in a regiospecific manner with the nucleophilic carbon of the electron-rich dienophile attaching to the expected electrophilic site of the aromatic quaternary salt (e.g., C-6 for acridizinium salts, C-l for isoquinolinium salts) and the rate of reaction (ketene aminals, ketene acetals, enamines > cyclopentadiene > 2,3-di-methylbutadiene > styrene > dihydropyran > 2-butene > maleic anhydride) does increase as the nucleophilic character of the dienophile is enhanced [Eq. (66)].l48e 149... 

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