Oxygen-boron bonds

Step 4 Hydrolysis cleaves the boron-oxygen bond yielding the alcohol... 

The hydrated insular groups may polymerize in various ways by splitting out water this process may be accompanied by the breaking of boron-oxygen bonds within the polyanion framework. 

Organic compounds containing boron-oxygen bonds... 

Nucleophilic attack hy the hydroperoxide anion produces initially a tetrahedral borale (26) in which one alkyl group, here (-)-Ipc 31. migrates from boron to oxygen, in the course of which a hydroxide anion is eliminated and a stable boron-oxygen bond develops in compound 27. This step proceeds with retention of configuration at the migrating carbon atom. The same step are repealed with the second residue to give trialkoxyborane 28. Finally, hydrolysis results in release of alcohol 4, two molecules of (-)-Ipc alcohol 30. and one equivalent of borale 29. 

Even greater diastereoselectivity in die aldol reaction can be achieved using boron etiolates as the carbon nucleophile. Boron etiolates are easily prepared from aldehydes and ketones, and the syn and die anti isomers can be separated as pure compounds. They react with aldehydes and ketones to give aldol products by a similar transition state. The difference is fliat boron oxygen bonds are shorter than lidiium oxygen bonds, and thus steric interactions in the transition state are magnified and result in greater diastereoselectivity. 

The facile exchange of boron-carbon with boron-oxygen bonds has been applied to the synthesis of trialkylboroxines from trialkylboranes and anhydrous boric acid (115). 

Exchange involving boron-halogen bonds and boron-oxygen bonds has been observed in the synthetically important reactions for the preparation of alkyldihaloboranes from alkylboroxines and boron trihalides (165, 167, 169). 

Exchange of boron-oxygen bonds with boron-sulfur bonds has been investigated 178) in the systems described by Eqs. (86)-(89), where X and Y... 

Wells (433) has quoted ranges for the boron-oxygen bond lengths of trigonally and tetrahedrally coordinated boron as... 

Boron-oxygen bond lengths in tetrahedrally coordinated boron vary from about 1.4 to 1.55 A synthetic CatBsO OHlJ - 2H20 possesses... 

Boron-oxygen bond strengths (480-565 kJ mol 1) are greater than boron-carbon bond strengths (350-400 kJ mol 1). This reflects an interaction between the empty p orbital on boron and an electron pair in one of the oxygen s two filled non-bonding sp3 orbitals. 

In Figure B5.4, N(CH2CH2OH)3 cleaves the three boron-oxygen bonds in the addition product and forms IM(CH2CH20)3B. 

Aldol reactions of boron enolates are frequently more diastereoselective than aldol reactions of, for example, lithium or aluminium enolates. This is partly ascribed to the relatively short boron-oxygen bond length (B-O = 1.36-1.47 A, Li-0 = 1.92-2.00 A, Al-0 = 1.92 A) which exacerbates the unfavourable 1,3-diaxial interactions that occur between the boron substituent... 

Zirconium-aryne complexes have also found application in the regiose-lective synthesis of halophenols.62 Reaction of 101 with methyl diethylbori-nate or triethylborate leads to insertion of a boron-oxygen bond into a Zr-benzyne bond with formation of 102 and 104, respectively (Scheme 12). Further reactions of 102 and 104 with bromine or iodine followed by excess NaOH and 30% hydrogen peroxide gives halomethyl(methoxy)phe-nols 103 and 105 in good yields. In most cases only one regioisomer of the halophenol was observed. 

Elemental boron is a refractory material that is usually isolated either as a shiny black crystalline solid or a softer, browner, more impure amorphous solid. Reduction of readily available boron compounds containing boron oxygen bonds to elemental boron is energy intensive and costly. This has limited the extent of conunercial use of this material. Many related refractory boron compounds have been prepared and characterized including metal borides, boron carbides, boron nitrides, and various boron metal alloys. These refractory materials and elemental boron are also discussed in some detail in the article Borides Solid-state Chemistry. Other general references are available on elemental boron and other refractory boron compounds. " ... 

Table 3 The boron-oxygen bond distances (A) of five-membered ring internai esters of boronic acids, which have been characterized by X-ray crystaiiographic studies...

I. Bally and A. T. Balaban Stud. Cercet. Chim. 17. 431 (1969) Unsaturated organoboron heterocycles with boron-oxygen bonds (Rom.) 32 (164)... 

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