Boron, migration

There is evidence that boron migration occurs intramolecularly.177 A TS involving an electron-deficient tt complex about 20-25 kcal above the trialkylborane that describes the migration has been located computationally.178... 

There is also evidence that boron migration can occur intramolecularly.136 A transition state that could describe this process has been located computationally.137 It involves an electron-deficient rc-complcx about 20-25 kcal above the trialkylborane. 

A multistep pathway analogous to the mechanism of alkene hydrogenation has been shown to be operative in the rhodium-catalyzed hydroboration of alkenes.363 Deuterium labeling studies furnished evidence that the reversibility of the elementary steps is strongly substrate-dependent. The key step is hydride rather than boron migration to the rhodium-bound alkene. 

This reaction also proceeds well with a variety of other substituted stilbene derivatives. Thus, the boranes obtained by hydroboration of E and Z stilbenes 80 undergo the thermal boron migration with excellent diastereoselectivity leading to the anti and syn organoboranes 81. After amination of 81 the corresponding benzylic amines (anti and syn 82) were obtained in 66 and 40 % yield and with diastereoselectivity better than 90 10 between C(l) and C(2) (Scheme 16) [7, 8,13],... 

Microbes play an important role in biogeochemical boron migration in arid regions. In these conditions, from 0.018 up to 0.125% of B content in soils were found in microbial biomass. The content of boron in microbial plasma depends directly upon B content in soil and the adaptation to increasing content is very high. 

According to the reactive element effect, the reactive element ion, such as beryllium, diffuses in to the native oxide grain boundaries and prevents the outward diffusion of substrate metal cations (Czerwinski and Smeltzer, 1993 Czerwinski and Szpunar, 1998 Czerwinski, 2000, 2004). The inhibitory effect of boron on aluminum alloy oxidation is clearly a surface phenomenon given the effectiveness of very low levels of boron. This could occur through a combination of boron migration into the MgO lattice and/or boron bonding to the defect-rich MgO surface (Choudhary and Pandit, 1991). 

Both products 97 as well as 99 contain two well-defined chiral carbon atoms, but even three contiguous sp [Pg.80]

As with syn-eliminations 104 benefits from the perfect 1,2 overlap in cydopen-tanes. Of particular importance here is the activation of a C-H bond and the synthetic value of this stereospecific boron migration emerges clearly from the transformation of 103 into the acetylene compound 102 and of 105 into the amine 106, with standard boron chemistry. 

Both steps are complete in a few minutes at room temperature the reaction seems quite general. Since the methyl ketones will undergo a haloform reaction, a way is available for preparation of carboxylic acids having lengthened the chain by three carbons. 1-Alkylcyclohexenes which are hydroborated normally will (cb was shown some years ago in acyclic systems) isomerize with heat such that the boron migrates to the primary position of the alkyl substituent ... 

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