Carboranes, rearrangement

Dicarba-close-carboranes rearrangement-prone, 18 71 stable, 18 71... 

Using the above results, and the premiss that double DSD processes are less favorable than single DSD processes, and so on, (on the grounds that a single DSD process produces the smallest structural perturbation), Wales and Stone were able to rationalize the available experimental evidence for borane and carborane rearrangement rates. ... 

The C2B10H12 molecule is the most stable carborane and takes its common name from the name of the entire class. Only three positional isomers are possible, 1,2- 1,7-, and 1,12-, for which the notations of ortho, meta, and para (o-, m-, P-) are usually used. The least stable isomer (o-carborane) rearranges to m-carborane at a temperature of 450 °C (an activation barrier of 260 kJ mol has been reported for this process), while /n-carborane rearranges to p-carborane at the higher temperature of 600 °C. Various mechanism have been suggested (Table 5). In addition, at about 350 °C there is a process that scrambles boron vertices in o-carborane but not in m-carborane. 

Allylation of the 10-carborane 236 (pKa = 18-22) with diallyl carbonate is possible under neutral conditions to give 237[146], Allylation and rearrangement of the trialkylalkynylborane 238 affords the trisubstituted alkene 239 stereoselectively [ 147],... 

Other convenient routes to carboranes, selected from the growing number of recently reported syntheses, are as follows. Monocarbon carboranes can be prepared in good yield by the transition-metal catalysed hydroboration of alkenes followed by thermal rearrangement of the intermediate product, c.gP" ... 

Metallocarboranes and metalloboranes, discussed in 6.5.4, may be prepared from metal halides and carborane anions. Selected examples of complexes formed by thermal—or otherwise—induced polyhedral rearrangements of existing metallocarboranes are given in a few equations, and some other individual compounds are listed in tables. 

However, the duster compounds differ greatly in their water solubility. Whereas cioso-dodecaborate (as the sodium salt) is readily water soluble, o-carborane and its thermal rearrangement products m-carborane and p-carborane show poor water solubility. The nido-carborane system, because of its negative charge, is water soluble. The azanonaborane duster is neutral, but still shows a certain degree of water solubility. 

A one pot route to nido-1,6-diiodo-tctracarba-nuio-hcxaborane derivatives is depicted in Scheme 3.2-32. After iodoboration of the alkynes with BI3 the dehaloge-nation of the alkenyl derivatives presumably leads to dimerization and rearrangement to give structurally characterized 1,6-diiodo-nido-carboranes 56d [81]. 

Reduction of the pentaalkyl-1,5-dicarba-closo-pentaboranes(5) 44 with elemental K opens the cluster and subsequent reaction with I2 enforces oxidative fusion of two C2B3 units to give a C4B6 cage (Scheme 3.2-36). The peralkylated hexaboraadamantane derivatives 70 (R = Me, Et X-ray structure analysis for R = Me [89]) rearrange irreversibly into the carboranes 71 with a wido-C4B6 framework [89] (X-ray structural analysis for R = Et [90]). The structure of the nido-C Bf, cluster is fluxional in solution [91]. 

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