Boron redistribution

Boron in Organic Matter. Although much of the boron in soils is associated with minerals resistant to weathering (3), boron is also contained in the organic fraction of soils. Little is known, however, of the reactions and availability of boron in soil organic matter other than that the quantity is small and is restricted to the surface horizon of soils primarily. As this organic matter fraction mineralizes, the boron redistributes in the soil-water system, becoming available in part for plants (10). 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Monohalogenoboranes are conveniendy prepared from borane—dimethyl sulfide and boron trihahdes (BX where X = Cl, Br, I) by redistribution reaction, eg, for monochloroborane—dimethyl sulfide [63348-81-2] (9) (81—83). Other methods are also known (84—87). 

Dihalogenoboranes are conveniently prepared by the redistribution of borane—dimethyl sulfide with boron trihaUde—dimethyl sulfide complexes (82,83), eg, for dibromoborane—dimethyl sulfide [55671-55-1] (14). 

Some of the reactions (e.g., that of dimethylaluminum chloride in Table 2) involve redistribution of alkyl and halide groups between the metals. The boronic acids, ArB(OH>2, prepared by Sn/B transmetallation, have been used in Suzuki coupling reactions. It is remarkable that the bistributyltin derivative of 1,1 -binaphthyl undergoes... 

The boron trihalides can undergo redistribution reactions of the type... 

Above 100° C, thermal redistributions were observed in the trialkylborane systems involving also isomerization of primary alkyl groups to secondary alkyl groups and subsequent equilibration of the primary and secondary alkyl groups on the boron atoms (115,141,171). 

For R = C2H5 and X = C1, the values of the equilibrium constants were Kt = 5.3 and K2 = 1.5. For R = w-C4H9 and X = Cl, Kx = 5.3 and /with boron trifluoride etherate (176). The reaction of Eq. (76) has been utilized as a general method for the preparation of various alkylhaloboranes. Thermal disproportionation studies of these products have been reported (168). 

The redistribution reaction in lead compounds is straightforward and there are no appreciable side reactions. It is normally carried out commercially in the liquid phase at substantially room temperature. However, a catalyst is required to effect the reaction with lead compounds. A number of catalysts have been patented, but the exact procedure as practiced commercially has never been revealed. Among the effective catalysts are activated alumina and other activated metal oxides, triethyllead chloride, triethyllead iodide, phosphorus trichloride, arsenic trichloride, bismuth trichloride, iron(III)chloride, zirconium(IV)-chloride, tin(IV)chloride, zinc chloride, zinc fluoride, mercury(II)chloride, boron trifluoride, aluminum chloride, aluminum bromide, dimethyl-aluminum chloride, and platinum(IV)chloride 43,70-72,79,80,97,117, 131,31s) A separate catalyst compound is not required for the exchange between R.jPb and R3PbX compounds however, this type of uncatalyzed exchange is rather slow. Again, the products are practically a random mixture. 

Fluoride anion strongly interacts with various inorganic and organic boron and silicon compounds. These reactions are the basis for several fluoride sensors. Interaction of fluoride with boron compounds results in electron density redistribution and may also induce structural changes. Formation of fluoride complex by ferrocene derivative (Figure 16.19a) results in a decrease of oxidation potential by 200 mV... 

Redistribution of substituents tends to be especially facile for halides, hydrides, and alkyls of Groups I—III nontransition elements because these compounds are electron-deficient. Bridging groups are present in many of these compounds. Even in the boron trihalides that are not bridged, a bridged transition state making use of the empty valence shell orbitals is possible, so that redistribution can occur with a relatively low activation energy (113) ... 

Reaction of a donor molecule with a previously equilibrated mixture of free boron trihalides gives an initial adduct mixture corresponding to the equilibrium mixture of the free boron trihalides. This method should be suitable for all mixtures except BF3/BI3 (as noted previously). However, equilibria in the adducts can be quite different from the corresponding equilibria in the free boron trihalides. If halogen redistribution is fast and if the mixed adducts are discriminated against, then this method does not succeed (28). 

Although most known BH X3 adducts are stable and most known BX Y3 adducts are unstable to redistribution of substituents about boron, this need not reflect a major difference in behavior of the two types of adduct but may, instead, reflect the interests of the investigators. The BH X3 adduct studies have emphasized synthesis and, hence, systems that are stable to redistribution, whereas mixed boron trihalide adduct studies have emphasized redistribution reactions, which are easier to observe when the donor is weak and redistribution is rapid. 

The requirement for an initial dissociation step implies that halogen redistribution rates should parallel the ease of dissociation of one of the halogens, and this expectation is confirmed in fluorine - heavier halogen redistributions where the rates change from slow to very fast over the series Cl, Br, I as the B—X bond becomes weaker. Rates should also be related to stabilization of the residual boron trihalide formed on dissociation. The boron trihalide with the greatest number... 

In boron trihalide adducts and tetrahaloborate ions, halogen redistribution equilibria are reasonably close to this ideal random case when chlorine, bromine, and iodine are involved (27, 28, 80, 100, 112), as are equilibria in the uncomplexed heavier boron trihalides (111). 

In the uncomplexed boron trihalides, mixtures of fluorine with chlorine, bromine, and iodine are successively less favored 111), in accord with the symbiotic principle. This principle can rationalize halogen redistribution equilibria in adducts of the mixed boron trihalides as well. Incompatibility of fluorine with the heavier halogens is greatest with the softest donors. In the extreme case of PH3 as... 

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