2-Phenylethyl bromide

The same idea will explain an experiment in which a styrene phial (20 mmole) was broken at -10 °C into 100 ml of a 6 x 10 3 M solution (C104" content) of the coloured ionic reaction product between AgC104 and 1-phenylethyl bromide (see above). Because of the rather slow initial mixing, the colour of the solution was not completely discharged by the styrene, and a violent polymerisation ensued, at least 100 times faster than the reaction catalysed by an equivalent amount of perchloric acid would have been. 

Methylene dichloride [6] and perchloric acid [7] were purified and dosed as described. Silver perchlorate (BDH) was treated in vacuo for a few hours before use. 1-Phenylethyl bromide (Eastman-Kodak) was fractionally distilled under high vacuum and the middle fraction was collected into breakable phials since this compound undergoes a slow decomposition, yielding hydrogen bromide and styrene, when kept in bulk, solutions of it in methylene dichloride were prepared from the original phials by the tipping technique [7]. Styrene was purified [8], dried, and stored [9] as described. Shortly before use it was vacuum-distilled into breakable phials from a microburette. 

Since we could not prepare a stable solution of the ester, we attempted its preparation in the styrene solution to be polymerised. Silver perchlorate was dissolved in this and the reaction was started by the crushing of a phial containing 1-phenylethyl bromide (under our conditions styrene was not polymerised by the silver perchlorate alone). The solutions became cloudy because of the formation of colloidal silver bromide, but no colour formation could be observed until the end of the polymerisation then the solutions became yellow, very like the reaction mixtures in which perchloric acid had been used as catalyst. The ester was found to be as effective a catalyst as anhydrous perchloric acid. Equal concentrations of the ester and the acid produced very similar polymerisations as shown in the Figure. The accelerating parts of the curves obtained with the ester as catalyst are readily explained by the fact that the reaction between silver perchlorate and 1-phenylethyl bromide is not instantaneous and therefore a steady increase in catalyst concentration characterises the first part of the polymerisation. 

The ester hypothesis is supported by experiments in which the ester was formed in situ by the reaction of 1-phenylethyl bromide with silver perchlorate in the presence of styrene. These gave results in close agreement with those obtained with perchloric acid [6]. Propagation by the ester is envisaged as addition of the two parts of the ester across the double bond of the monomer in modern terminology this is an insertion . 

Fig. 3-21 Plots of ln([M]o/[M]) versus time for ATRP polymerizations of styrene at 110°C with CuBr, 1-phenylethyl bromide (I), and 4,4-di-5-nonyl-2,2 -bipyridine (L). Bulk polymerization ( ) [M] = 8.7 M, [CuBr]0 = [L]0/2 = [I] = 0.087 M Solution polymerization in diphenyl ether (o) [M] = 4.3 M, [CuBr]0 = [L]0/2 = [I] = 0.045 M. After Matyjaszewski et al. [1997] (by permission of American Chemical Society, Washington, DC) an original plot, from which this figure was drawn, was kindly supplied by Dr. K. Matyjaszewski.

Figures 3-21 and 3-22 show results in the ATRP polymerization of styrene using 1-phenylethyl bromide as the initiator, CuBr as catalyst (activator), and 4,4-di-5-nonyl-2,2 -bipyridine as ligand [Matyjaszewski et al., 1997]. Figure 3-21 shows the decrease in monomer concentration to be first-order in monomer, as required by Eq. 3-223. The linearity over time indicates that the concentration of propagating radicals is constant throughout the polymerization. The first-order dependencies of Rp on monomer, activator, and initiator and the inverse first-order dependence on deactivator have been verified in many ATRP reactions [Davis et al., 1999 Patten and Matyjaszewski, 1998 Wang et al., 1997].

ATRP of 5.0 M styrene is carried out at 110°C using 1-phenylethyl bromide (0.050 M) and CuBr (0.050 M) in the presence of 4,4 -di(5-nonyl)-2,2 -bipyridine (0.10 M). The number-average molecular weight at 72% conversion is 7150. Compare the observed molecular weight to the theoretical value expected for this polymerization. 

The necessary alkene, styrene, is available by dehydrohalogenation of the given starting material, 1-phenylethyl bromide. 

TABLE 2. Product yields (percent) from electroreduction of 1-phenylethyl bromide at different potentials... 

Boron trichloride and tribromide exchange ligands when used with alkyl halides and acetates initiators. For example, as shown in Eq. (35), 1-phenylethyl bromide and BBr2Cl are formed rapidly and quantitatively when 1-phenylethyl chloride is mixed with an equimolar amount of boron tribromide [59,67]. 

A later study by the same group showed that, as the initiator, not only PhEtCl but also its bromide counterpart (1-phenylethyl bromide) can be used that, as the added salt, not only the chloride but also the bromide and the iodide (nBi NY Y = Br, I) can be used (see Figure 23) [25], As in isobutene polymerization, the polymer s w-end (tail group) is a chloride... 

The first successful formation of an optically active Grignard reagent in which the magnesium is directly attached to the chiral center was reported in a short communication in 1961 [43], followed by a full paper in 1964 [44]. Previous attempts to prepare optically active Grignard reagents from acyclic halides such as (— )-2-iodobutane [45], ( —)-2-bromooctane [46, 47] and optically active 1-phenylethyl bromide [48], as well as a cyclic halide (— )-3,3-dimethylcyclohexyl chloride [48], all had given rise to racemic products. 

Problem 14.12 2-Phenylethyl bromide undergoes E2 elimination about 10 times as fast as 1-phenylethyl bromide, even though they both yield the same alkene. Suggest a possible explanation for this. 

To promote a polymerization, the newly formed carbon-halogen bond must be capable of being reactivated and the new radical must be able to add another alkene. This was accomplished for the radical polymerizations of St and methyl acrylate (MA), which were initiated by 1-phenylethyl bromide and catalyzed by a Cu(I)/2,2 -bipyridine (bpy) complex [42,79-81]. The process was called Atom Transfer Radical Polymerization (ATRP) to reflect its origins in ATRA. A successful ATRP relies on fast initiation, where all the initiator is consumed quickly, and fast deactivation of the active species by the higher oxidation state metal. The resulting polymers are well defined and have predictable molecular weights and low polydispersities. Other reports used different initiator or catalyst systems, but obtained similar results [43,82]. Numerous examples of using ATRP to prepare well-defined polymers can now be found [44-47,49]. Scheme 4 illustrates the concepts of ATRA and ATRP. To simplify schemes 3,4 and 5, termination was omitted. 

Sawamoto et aL used the RuCl2(PPh3)3/Al(OzPr)3 catalyst to prepare St/MMA copolymers [126]. They found that the polymerization proceeded well using 1-phenylethyl bromide as the initiator and that the composition of the copolymer matched the comonomer feed composition, or behaved azeotropically [126]. The polymers were well-defined, with predictable molecular weights and relatively low polydispersities (Mw/Mn<1.5). The reactivity ratios were similar to those determined from conventional free radical processes. Later work used a NiBr2(n-Bu3P)2 catalyst system for the ATRP of a 50/50 mixture of MMA/MA and MMA/nBA [127]. The results indicated that the copolymerization was controlled with copolymer Mn=ll,800 (Mw/Mn=1.47) and 12,500 (Mw/Mn=1.47),respectively. 

In cases where the more established kinetic approaches become insensitive, mixed solvent studies on rate of elimination may prove enlightening. Although p and knlko remain unchanged in the elimination of 2-arylethyl bromides with t-butoxide in t-butyl alcohol on the addition of dimethyl sulphoxide (0-2.23 M), a rate increase of 120 times is noted" . Smaller increases in rate are observed for similar variations in the concentration of added dipolar aprotic solvent in elimination from 1-phenylethyl bromide and 9-bromo-9,9 -bifluorenyP , but a steeper rise in rate is observed for 2,2-diphenylethyl tosylate. A cor-... 

The kinetic resolution of racemic 1-phenylethyl bromide was examined in the alkylation of benzoic acid using trisubstituted monocyclic guanidines as chiral sources [67]. (R)-Excess ester was obtained in 96% yield even with 15% ee, when the reaction was carried out in benzene with l,3-dimethyl-(45,55)-diphenyl-2-[(15)-phenylethylimino]imidazolidines [27a] (Scheme 4.25). 

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