Trans-bromine atom

The influence that a possible chelation has on the stereochemistry of the bromine-lithium exchange in dibromoaUcenes was first studied in the chiral MEM (methoxyethoxymethyl) ether 40. When this compound is treated with 1.2 equivalents of n-butyllithium in tetrahydrofnran, the acids E- and Z-42 are obtained in a ratio of 32 68 after carboxylation. Obvionsly, a kinetically favored substitution of the more easily accessible trans bromine atom occurs. When, however, slightly less than one equivalent (0.95-0.98 equiv) of -bntyllithinm is slowly added to a solution of 40 in diethyl ether at — 105°C, the -confignrated carbenoid 41 forms almost exclusively, and the carboxylic acids E- and Z-42 are obtained after reaction with dry ice in a ratio of over 99 1 (Scheme 9) ,i93 ... 

The stereospedfic and regioselective hydrobromination of alkynes with chlorobis(T -cyclopentadienyl)hydrozirconium and NBS produces ( )-vinylic bromides in good yields. The bromine atom usually adds regioselectively to the carbon atom that bears the smaller substituent and stereoselectively trans to the larger substituent (D.W. Hart, 1975 M. Nakatsuka,... 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

How does the formation of a bromonium ion account for the observed anti stereochemistry of addition to cyclopentene If a bromonium ion is formed as an intermediate, we can imagine that the large bromine atom might "shield" one side of the molecule. Reaction with Br ion in the second step could then occur only from the opposite, unshielded side to give trans product. 

Polymerization of 4-bromo-6,8-dioxabicyclo[3.2.1 ]octane 2 7 in dichloromethane solution at —78 °C with phosphorus pentafluoride as initiator gave a 60% yield of polymer having an inherent viscosity of 0.10 dl/g1. Although it is not described explicitly, the monomer used seems to be a mixture of the stereoisomers, 7 7a and 17b, in which the bromine atom is oriented trans and cis, respectively, to the five-membered ring of the bicyclic structure. Recently, the present authors found that pure 17b was very reluctant to polymerize under similar conditions. This is understandable in terms of a smaller enthalpy change from 17b to its polymer compared with that for 17a. In the monomeric states, 17b is less strained than 17a on account of the equatorial orientation of the bromine atom in the former, whereas in the polymeric states, the polymer from 17b is energetically less stable than that from 17a, because the former takes a conformation in which the bromine atom occupies the axial positioa Its flipped conformation would be even more unstable, because the stabilization by the anomeric effect is lost, in addition to the axial orientation of the methylene group. 

The product obtained from this type of decarboxylation is reported to contain only about 5% of /ra s-stilbene.5 A sample made according to the above directions can be treated with bromine in carbon tetrachloride at room temperature in the dark to give an 80-85% yield of the d/-dibromide which arises from trans addition to cw-stilbene. The meso-dibromide, which is very soluble and easily separated, is obtained only to the extent of 10% or less. Part of this latter product may arise from the action of bromine atoms on cw-stilbene rather than from trans addition to tfnms-stilbene. The cu-stilbene prepared by this method is readily and completely soluble in cold absolute ethanol. It freezes solid at about —5°. Its ultraviolet absorption coefficient (8) is 1.10 X 104 at 274 mil and 8.7 X 103 at 294 mp, quite different from h-aws-stilbene. 

Secondly it is found—with those simple alkenes in which it can be detected, e.g. trans 2-butene (5)—that the two bromine atoms add... 

As shown in the synthetic sequence, to induce isomerisation of the c -B/C junction to a trans configuration, a double bond between C(6) and C(7), that can transmit the electronic effects of carbonyl group at C(5) must be created ex profeso. Yhis relatively simple structural modification requires no less than five different synthetic operations i) bromination at C(6) ii) substitution of the bromine atom by an OH group iii) oxidation iv) 0-methylation with dimethyl sulphate, and... 

Stereoselective bromine-lithium exchange of one of a geminal pair of vinylic bromine atoms is also possible. Compounds 142-145 all lead to predominant exchange at the bromine trans to Ph when they are added to a solution of BuLi, with greater selectivity the more hindered that bromine becomes.124 Selectivity does not change with time, but is inverted when the... 

Figure 6-11 shows a cyclic case where one of the faces of a cyclopentane ring has been labeled by a deuterium atom. Deuterium has the same size and shape as hydrogen and it undergoes the same reactions. It distinguishes between the two faces of the ring the bromine atom is cis to the deuterium in the reactant, so the nucleophile is cis to the deuterium in the retention product. The nucleophile is trans to the deuterium in... 

An E2 elimination can take place on this chair conformation only if the proton and the leaving group can get into a trans-diaxial arrangement. Figure 7-9 shows the E2 dehy-drohalogenation of bromocyclohexane. The molecule must flip into the chair conformation with the bromine atom axial before elimination can occur. 

In an E2 elimination, the hydrogen atom and the leaving group must have a trans-diaxial relationship. In this compound, only one hydrogen atom—the deuterium—is trans to the bromine atom. When the bromine atom is axial, the adjacent deuterium is also axial, providing a trans-diaxial arrangement. 

Rotating meso-2,3-dibromobutane into a conformation where the bromine atoms are anti and coplanar, we find that the product will be trans-but-2-ene. A similar conformation of either enantiomer of the ( ) diastereomer shows that the product will be cw-but-2-ene. Hint Your models will be helpful.)... 

A direct conversion of cis and trans isomers would indeed demand a very high activation energy. Thus the transformation in the case of dichloroethylene occurs by the catalytic action of bromine atoms, and the first step is addition of such an atom with the formation of a radical with a single rotatable bond. 

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