B-Bromostyrene

The classical Hunsdiecker conditions using Ag, and modifications using metals such as Hg, Tl, Pb, and Mn(II), are not very synthetically useful because of the use of toxic metals, requirement of high temperature, and poor yields. As a consequence, many variants of greener chemistry have been developed to replace heavy metals. In addition to Barton s radical approaches, Roy et al. developed a metal-free Hunsdiecker reaction where the acid was treated with A-bromosuccinamide (NBS) and a catalytic amount of LiOAc27 or the phase transfer catalyst (PTC) tetrabutylammonium trifluoroacetate (TBATFA).28-30 As shown below, cinnamic acid 26 was converted to (B-bromostyrene 27 in almost quantitative yield.28 The authors also found that a mixture of 93 7 MeCN/H20 was also a good solvent for the metal-free Hunsdiecker reaction.29 In place of TBATFA, another phase transfer catalyst Select flur was found to be an efficient catalyst for the metal-free Hunsdiecker reaction as well (e.g., 29—>30).31... 

Outline a synthesis of cis-B-bromostyrene from cinnamic acid (PhCH=CHCOOH). Which isomer of the acid is required, cis or trans. 

Benzyl tiglate Benzyl valerate Bicycle [2.2.2]-5-octene-2-carboxylic acid, 1(or 4)-methyl-4(or 1)-(1-methylethyl) methyl ester Bicyclopentylone Bicyclopentylyl butenoate y-Bisabolene Bitter almond (Prunus amybdalus) extract L-Borneol L-Bornyl acetate Bornyl isovalerate Bromo-D-camphor B-Bromostyrene Buchu leaves oil Butanol, 2-methyl-4-(2,2,3-trimethyl-3-cyclopentenyl)- p-t-Butyl acetophenone... 

Fig. 7a, b. PMIM-image of (a) a poly-p-bromostyrene surface [118], (b) a PS/polybutadiene diblock copolymer, PS-b-PB, at approximately 100 fold magnification. The lateral resolution is of the order of 1 pm while the height resolution is of the order of 0.6 nm. The root-mean-square roughness averaged over the area shown is 0.8 nm in (a) close to the resolution limit of the technique. It is much larger (10 nm) in (b) due to the formation of steps after annealing. The scale in z-direction in (a) and (b) is different by a factor of 7... 

The reaction of 1-phenylethyl-, 2-octyl-, and 2-butyl-magnesium chloride (36a, b, c) with vinyl bromide (37a), (E)-p-bromostyrene (37b), 2-bromopropene (37c), and bromobenzene (37d) was carried out in the presence of 0.5 mol- % of a nickel catalyst prepared in situ from nickel chloride and the chiral ligand (35). 

Experimental values are presented of the molar Kerr constants /x and dipole moments squared, lx, for the copolymers poly(styrene-co-p-bromostyrene), where x is the degree of polymerization. Some results are also presented for poly(styrene-co-p-chlorostyrene) and related polymers. The RIS model of Yoon etal. (Yoon, D. Y. Sundararajan, P. R. Flory, P. J. Macromolecules 1975, 8, 776) is used to calculate mK/x and /x values as a function of tacticity and composition. The statistical weight matrices are identical with those used by Saiz etal. (Saiz, E. Mark, J. E. Flory, P. J. Macromolecules 1977, 10, 967), with the following parameters h = 0.8 exp 397/RT), co = o = 1.3 exp - 1987/RT) and m,= 1.B exp -(2186/RT), where T = 298 K is the temperature. 

In water, ionization of the C-Br bond occurs first (Ei mechanism) to give the intermediate resonance-stabilized benzylic zwitterion C. After fast rotation about the C-C bond, carbon dioxide leaves conformer D perpendicularly to the plane of the car-benium ion, to give mainly the most stable ( )-isomer of / -bromostyrene. In butanone, after fast rotation about the C-C bond, elimination of CO2 and Br occurs in a concerted single-step (E2 mechanism) for stereoelectronic reasons (Br and C02 must be anti to one another) to give conformer B, which decomposes exclusively to the thermodynamically less stable (Z)-isomer. In more polar solvents, the partly zwitterionic activated complex, leading to zwitterion C in the rate-determining step, will clearly be more stabilized by solvation than the less dipolar activated complex leading directly to the (Z)-isomer of / -bromostyrene from conformer B [851]. 

Fig. 2-7. Data for in the region of relatively close to unity. Different symbols mark poly(chloroprene) [67], PS [68], poly(p-bromostyrene) [69], and PIB [47] in various poor solvents. Solid curve, reproduction of the line B in Fig. 2-6. 

The effect of molecular weight on the interfacial excess (z(B) ), tension jabc) and width (vv bc) in polystyrene/poly(d(8)-styrene-co-4-bromostyrene)/poly (styrene-co-4-bromostyrene) (AB C) system was studied by Genzer and Composto (1998). Low-energy forward recoil spectrometry (LE-FRES) was used to measure z(B) as a function of the B volume fraction in the B C blend. The experimental z(B) s were found to be in excellent agreement with those calculated using the... 

Reactions Catalytic in Tin Maleczka and Terstiege [72] have published several papers in this area. They first developed a one-pot hydrostannylation/Stille couphng protocol for reactions between 1-alkynes and bromostyrene to give 1,3-dienes. A mixture of BUjSnCl (catalytic amount), polymethylhydrosiloxane (PMHS) and aqueous sodium carbonate (to convert the chloride to bis(tri-n-butyltin)oxide that reacts with PMHS to give BUjSnH in situ) in combination with Pd2(dba)j and trifurylphosphine leads to diene formation. Use of MejSnCl instead of the tributyltin chloride [73] improves yields drastically (to up to 90%). Further improvement is obtained in the third protocol reported [74a,b], which was used to prepare... 

In Part B of the experiment, the solvent is changed from water (used in Part A) to a much less polar solvent, acetone. Under these nonaqueous conditions, elimination also occurs, but in this case the experimental data demand a different mechanism, one involving a smooth concerted electron flow without short-lived intermediate formation. Furthermore, when carried out in acetone, the reaction yields exclusively cis-2 -bromostyrene. Details of the mechanism follow ... 

A similar assembly starting with /3-bromostyrene and a 1,6-enyne gave rise to a five-membered ring-annelated cyclohexadiene formed by a subsequent b-TT-electrocycUzation of the intermediate ( , Z,Z)-conflgurated 1,3,5-hexatriene. The disiotatory electrocyclization placed the two substituents on opposite sides of the six-monbered ring (Scheme 9)3 ... 

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