Bridging bromine

Further evidence for a bromine-bridged radical comes from radical substitution of optically active 2-bromobutane. Most of the 2,3-dibromobutane which is formed is racemic, indicating that the stereogenic center is involved in the reaction. A bridged intermediate that can react at either carbon can explain the racemization. When the 3-deuterated reagent is used, it can be shown that the hydrogen (or deuterium) that is abstracted is replaced by bromine with retention of stereochemistry These results are also consistent with a bridged bromine radical. 

Figure If shows the molecular structure of the [Tc6Br6(/t3-Br)5]2- anion. In general, the structure of this anion is similar to the structure of the well-known octahedral halogenide clusters of molybdenum and tungsten [M6X8]4 + (X = Cl,Br, I) [5,8]. The principal difference is that the eight equivalent positions of bridging bromine atoms in the technetium clusters are not fully occupied.

In all cases, the structure can be described as edge-shared bioctahe-dral and none of the interbond angles deviate from idealized values by more than a few degrees. The Bi-Br distances to the bridging bromines, in the examples given, are longer than those to the terminal bromines, as expected, but in all cases, the Bi-Br-Bi bridging units are quite symmetrical. 

Protactinium(V) oxytribromide possesses monoclinic symmetry (Table III). The structure (53, 54) comprises chains of protactinium atoms linked by bridging bromine atoms and cross-linked by 3-coordinate oxygen atoms. Each protactinium atom is 7-coordinate (Pig. 5) and protactinium-bromine bond lengths lie within the range 2.69-3.02 A. It would be interesting to have structural information on the relatively unstable uranium(V) oxytribromide since the limited X-ray powder... 

The bromide ion attacks the carbon from the opposite side to the bridged bromine atom, because the carbon/bromine antibonding orbital projects in this direction. This mode of addition is called anti-addition. This is clearly seen from the conformation of the final product before any rotation of the carbon/carbon bond has occurred. As the bromide ion attacks the 5+ carbon, that carbon/ bromine bond breaks in order that the octet of electrons is not exceeded on the carbon. This mechanism explains why the final product is the 1,2-dibromo adduct and not the 1,1-dibromo adduct. 

The reaction involves an intermediate bromonium ion, which can be ring-opened by attack of Br at either carbon atom of the 3-membered ring (as these are equivalent). The bridging bromine atom prevents rotation about the central C-C bond, and hence the Z-relationship of the ethyl groups in the alkene is conserved... 

Labels according to Terao et al sites 1,2 are terminal and site 3 is a bridging bromine Labels according to Terao and Okuda. Sites 1 and 2 are assigned to terminal and bridging Br atoms, respectively... 

Because the addition of Br2 to the cis alkene forms the threo enantiomers, we know that addition of Br2 is an example of anti addition because if syn addition had occurred to the cis alkene, the erythro enantiomers would have been formed. The addition of Bt2 is anti because the reaction intermediate is a cyclic bromonium ion (Section 4.7). Once the bromonium ion is formed, the bridged bromine atom blocks that side of the ion. As a result, the negatively charged bromide ion must approach from the opposite side (following either the green arrows or the red arrows). Thus, the two bromine atoms add to opposite sides of the double bond. Because only anti addition of Br2 can occur, only two of the four possible stereoisomers are obtained. 

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