3.5- Dibromo-4- cinnamic acid

Preparation 323.—Cinnamic Acid Dibromide Z-Phenyl-2 3-dibromo-propan acid. ... 

Examples of the solvent-dependent competition between nucleophilic substitution and / -elimination reactions [i.e. SnI versus Ei and Sn2 versus E2) have already been given in Section 5.3.1 [cf. Table 5-7). A nice example of a dichotomic y9-elimination reaction, which can proceed via an Ei or E2 mechanism depending on the solvent used, is shown in Eq. (5-140a) cf. also Eqs. (5-20) and (5-21) in Section 5.3.1. The thermolysis of the potassium salt of racemic 2,3-dibromo-l-phenylpropanoic acid (A), prepared by bromine addition to ( )-cinnamic acid, yields, in polar solvents [e.g. water), apart from carbon dioxide and potassium bromide, the ( )-isomer of l-bromo-2-phenylethene, while in solvents with low or intermediate polarity e.g. butanone) it yields the (Z)-isomer [851]. 

Hunsdiecker reaction. Cinnamic acids give P-bromostyrenes or a-(dibromo-methyl)benzenemethanols on reaction with NBS (one or two equivalents, respectively) in the presence of Mn(OAc),. 

Reaction of frans-cinnamic acid with Br2 in solution gives erythro-2,3-dihrom.o-3-phenylpropionic acid. Addition of bromine to frans-cinnamic acid that has been coordinated with a dicobalt species in an organic capsule" gives )-threo-2,3-dibromo-3-phenylpropionic acid, however. What do these results suggest about the differing mechanisms of the two addition reactions ... 

Dibromo- 2-phenylpropionic acid, cinnamic acid dibromide, i-phenyl- 2-dibromopropionic acid)... 

Dimethylaminopropyl) [134276-56-5]. 3,5-Dibromo-4-[3- dimethylamino)propoxy]cinnamic acid M 407.101... 

The second stage (Experiment [D2]) involves the addition of bromine to the intermediate formed in the first step, frans-cinnamic acid. The product of this halogenation is e T/fizro-2,3-dibromo-3-phenylpropanoic acid. The stereochemistry involved in the formation of this second intermediate is a result of the nature of the anti addition of molecular bromine to a trans alkene. The details are given in the discussion in Experiment [D2]. 

Bromination of trans-Cinnamic Acid erythro-2,3-Dibromo-3-phenylpropanoic Acid... 

Purpose. The bromination of frans-cinnamic acid is carried out to obtain en/flzro-2,3-dibromo-3-phenylpropanoic acid, the direct precursor to 2 -bromo-styrene, which is the synthetic target of Sequence D. You wiU review the stereospecificity of the addition of molecular bromine to an alkene. 

Because both cinnamic acid and molecular bromine are achiral, two enantiomeric bromonium ions are formed at equal rates. Ring opening of each bromonium ion may preferentially occur as shown here at the carbon bearing the phenyl group, since this carbon bears more fractional positive charge than the carbon adjacent to the carboxyl group. Because the two reaction pathways shown here are enantiomeric, they proceed at equal rates to produce racemic ery thro-1,3 - dibromo- 3 -phenylpropanoic add ... 

A more direct approach to acetylenes has been reported which utilizes phase-transfer generated tetrabutylammonium hydroxide to dehydrohalogenate a-halo-olefins or bis-dehydrohalogenate vicinal dihalides. An example of the latter is the reaction of 1,2-dibromo-l-phenylethane in pentane with 50% aqueous sodium hydroxide in the presence of tetrabutylammonium bisulfate. The product, phenylacetylene, is produced in 87% yield in less than an hour by this method (see Eq. 9.11) [27]. The advantage of this approach is apparent when one considers that in the traditional method, jS-bromostyrene (from the dehydrohalogenation and decarboxylation of cinnamic acid dibromide) is heated with KOH at over 200° and the phenylacetylene distills as formed in 67% yield [28]. 

Cognate preparations. Phenylpropynoic acid. Place a solution of 88 g (84 ml, 0.5 mol) of ethyl cinnamate (Expt 6.137) in 50 ml of carbon tetrachloride in a 500-ml round-bottomed flask. Immerse the flask in ice and add 80 g (25.5 ml, 0.5 mol) of bromine from a separatory funnel slowly with frequent shaking. The halogen will disappear rapidly at first, but more slowly towards the end of the reaction no hydrogen bromide is evolved and the time of the addition is about 20-25 minutes. Allow the mixture to stand for 1 hour, pour the solution into a large evaporating dish and permit the excess of bromine and the carbon tetrachloride to evaporate spontaneously in the fume cupboard. The crude ethyl 2,3-dibromo-3-phenylpropanoate will remain as a solid cake this... 

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