Carbon-boron bonds oxidative cleavage

Schuster and co-workers discovered that 1,4-dicyanonaphthalene solutions containing an alkyltriphenylborate salt, when irradiated, yield one-eleetron oxidation of the alkyltriphenylborate leading to carbon-boron bond cleavage and formation of free alkyl radicals [23]. In Gottschalk s hands [24, 25], it was shown that ionic salt pairs formed from cyanine dyes and alkyltriphenylborates (Figure 1) could be used as photoinitiators [26] that were active in the visible region of the spectrum. 

An efficient method for intramolecular direct arylation was employed on a doubly functionalized caUx[4]arene fixed in the cone conformation (Scheme 3.25). Dihydroxycalixarene 113 was obtained from dibromide 112 in 84% yield using lithiation, calixboronate formation and the final oxidative carbon-boron bond cleavage. The hydroxy groups of 113 were reacted with benzyl bromide (Williamson ether synthesis) and CS2CO3 in acetone to produce corresponding... 

Sequential oxidation of trialkylboranes takes place with trimethylamine N-oxide and oxidation of a chiral boronic acid by flavoenzyme cyclohexanone oxygenase proceeds with retention of configuration at the migrating centre in an analogous manner to peroxide oxidation. One electron oxidation of alkyltriphenyl borate anions leads to carbon-boron bond cleavage and the formation of free alkyl radicals. ... 

A carbon-heteroatom bond can likewise be fragmented under these conditions (see Scheme 6). Convenient substrates are silanes, germanes, and stannanes " as well as isoelectronic boron derivatives (as in the cleavage of benzyltriphenylborates after oxidation to the corresponding radical). ... 

The oxidative cleavage of the boron-carbon bond with radioiodide/Chloramine-T has been shown to be useftil for the radioiodination of various organic compounds (Kabalka et al. 1981). This method is characterized by the reaction sequence in O Fig. 44.4. 

The properties of diamond, cubic boron nitride, and conventional hard materials are summarized in Table 9.1. In addition to being the hardest known substance, diamond is chemically inert to essentially aU environments below a temperature of about 500°C and is therefore uniquely qualified for many applications. Diamond has a cubic structure, with each carbon atom bonded to four nearest neighbors. Cleavage normally occurs on one of four (111) planes. In addition to its intrinsic brittleness, diamond has two important limitations. Diamond begins to oxidize and/or graphitize rapidly at temperatures above 600-700°C in air or an oxidizing atmosphere. Diamond readily dissolves in and can be graphitized by ferrous metals such as iron, steels, nickel, and nickel-based superaUoys, and therefore abrasion resistance with these metals is poor. 

Such complexes possess sharply reduced chemical reactivity and consequently they often tend to stabilize the valence state of the acceptor metallic atom. Lithium tetraphenylboronate requires heating in acid solution in order to effect cleavage of the boron-carbon bonds and is quite stable in air toward oxidation. The acceptance of the phenyl anion has satisfied the electronic demands of boron. A direct preparation of analogous alkyl complexes has been realized by heating lithium aluminum hydride with ethylene under pressure 139) ... 

Scheme 6.25. A representation of the oxidative hydroboration of 1-methylcyclopentene (as a typical alkene). The addition of boron and hydride (H ) is suprafacial with the double bond attacking the boron so as to build up charge on that carbon of the double bond that would be the most stable carbocation it is to this carbon of the double bond that the hydride (H") adds. In the oxidation step, the anion of hydrogen peroxide (H02 ) attacks the boron. Subsequent rearrangement of carbon to oxygen with oxygen-oxygen bond cleavage produces the alcohol. Only one ligand to boron is shown to emphasize the oxidation process.

Compounds 66 and 67 typically reacted through the boron-boron multiple bond to afford n-complexes with coinage or alkali metals (68 and 69, Figure 5.10). Additionally, complexes 62, 66, and 67 were shown to undergo simultaneous B-B bond cleavage/cycloaddition reactions by oxidation with elemental sulfur or selenium (70 and 71). " Diboron complexes 66 and 67 also reacted with carbon monoxide and isonitriles, respectively, to afford NHC-stabilized bis(boralactone) 72 and bis(boraketenimine) 73. ... 

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