Partially bridged intermediate

When the bromination of the unsubstituted P-methy 1-styrenes (ref. 19) is carried out in methylene chloride, the two diastereoisomeric dibromides are obtained in ratios of 72 threo/28 erythro and 20 threo/80 erythro for the cis and trans isomers, respectively. This result agrees fairly well with a partially bridged intermediate, since the corresponding benzylic carbocation leads to a 65 erythro/35 threo ratio (ref. 20). When the same reactions are carried out in methanol, the... 

Since the products are optically active, this excludes the symmetric, resonance-stabilized allylic ion intermediate. The main (treats) product arises from the preferential attack of Br- on the more stable, less strained, partially bridged 30 intermediate (as compared to 31). Similar conclusions were drawn from the results of the iodination and haloiodination of 2,3-pentadiene256 and from bromination of 1,2-cycloalkadienes.257... 

Although many overall rearrangements can be formulated as a series of 1,2-shifts, both isotopic tracer studies and con utational work have demonstrated foe involvement of other species. These are bridged ions in which hydride or alkyl groups are partially bound to two other carbons. Such structures can be transition states for hydride and alkyl-group shifts, but some evidence indicates that these structures can also be intermediates. 

The main difference between the structures of compound (IV) and compounds (I), (II), and (III) is that in compound (IV) half of the bridging and terminal bromine atoms, and all the axial bromine atoms, are substituted by iodine atoms, with partial substitution being statistical [75]. Thus, according to the X-ray diffraction data, the effective Tc-Tc distances are intermediate between analogous distances in bromide and iodide complexes of technetium(IV) (Table 1) [104,105]. We observed that the substitution of I for Br has almost no effect on the Tc-Tc distances. 

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