Boron compounds reaction mechanisms

Many other compounds have been shown to act as co-catalysts in various systems, and their activity is interpreted by analogous reactions [30-33]. However, the confidence with which one previously generalised this simple picture has been shaken by some extremely important papers from Eastham s group [34], These authors have studied the isomerization of cis- and Zraws-but-2-ene and of but-l-ene and the polymerization of propene and of the butenes by boron fluoride with either methanol or acetic acid as cocatalyst. Their complicated kinetic results indicate that more than one complex may be involved in the reaction mechanism, and the authors have discussed the implications of their findings in some detail. 

Since the metal-alkene association preceding the peroxymetalation reaction in mechanism (B) is a pure Lewis acid/Lewis base interaction, it would be expected that compounds having alkylperoxy groups bonded to a Lewis acid center should be active for the epoxidation of alkenes. This is indeed found for boron compounds, which are active as catalysts for the epoxidation of alkenes by alkyl hydroperoxides.246,247 Isolated boron tris(alkyl peroxides), B(OOR)3, have been shown to epoxidize alkenes stoichiometrically, presumably through alkylperoxyboration of the double bond (equation 76).248... 

The influence of the metal atom on mechanism is particularly well illustrated by the pronounced tendency of the alkyls of boron and aluminium to react via coordination complexes in which the metal atom is four-co-ordinate. It is no coincidence that the prime examples of substitution by mechanism SE2(co-ord) occur in reactions of alkyl-boron compounds. 

The mechanism proposed for aromatic C-H borylation of aromatic compounds 1 by B2pin2 3 catalyzed by the Ir-bpy complex is depicted in Scheme 3 [6-9]. A tris(boryl)Ir (III) species [5, 6, 11] 6 generated by reaction of an Ir(I) complex 5 with 3 is chemically and kinetically suitable to be an intermediate in the catalytic process. Oxidative addition of 1 to 6 yields an Ir(V) species 7 that reductively eliminates an aromatic boron compound 4 to give a bis(boryl)Ir(III) hydride complex 8. Oxidative addition of 3 to 8 can be followed by reductive elimination of HBpin 2 from 9 to regenerate 6. 2 also participates in the catalytic cycle via a sequence of oxidative addition to 8 and reductive elimination of H2 from an 18-electron Ir(V) intermediate 10. Borylation of 1 by 2 may occur after consumption of 3, because the catalytic reaction is a two-step process - fast borylation by 3 then slow borylation by 2 [6],... 

Only a few reports deal with the interaction of boron compounds with HFA. No addition products of HFA with monoborane have been detected in thermal 101) or in photochemical 264) reactions. However, co-photolysis of HFA with pentaborane(9) and 2,4-dicarbapentaborane(7) results in the insertion of HFA into B—H bonds 16a). Mechanisms are discussed in detail. 

Boron cluster compounds have many electron deficient sites in them and they possess polyhedral skeleton. The procedure by which the boron cluster compounds are built from small polyhedral skeletons to a big polyhedral skeleton is in wide use for the synthesis of boron neutron capture therapy. The reactivity and reaction mechanism for the formation of the boron cluster compounds from a small cluster to a big cluster have been studied. 

Through this study we suggest a useful synthetic procedure for good candidate compounds of a new cage-cluster. The ort/io-carboranes are very stable chemicals in this research. Carboranyl acetic ester can be easily introduced to the ortho- carborane skeleton and then many types of cyclisation of carboranyl cluster can be made to occur. The reactivity and reaction mechanism for cyclisation of carborane cluster have been discussed. The mechanisms for boron cluster expansion reaction and cyclisation at carboranyl edge are summarized as follows. 

The coordination chemistry of boron was reviewed some time ago and the structure and properties of compounds of the general formula BX3 L, where X and L can be one of a wide variety of substituents and electron pair donors, respectively (15). Indeed, the reactions of tricoordinate boron compounds in general are thought to proceed via addition of the reaction partner in a Lewis acid-base reaction to yield a tetracoord-inate intermediate that then undergoes further reaction. Stable tetra-coordinate boron compounds are subject to ligand displacement reactions for which a variety of mechanisms obtain (16). The coordination chemistry of transition metals is vast and includes not only structimal facts (17) but considerable information on the mechanistic behavior of these species as well (18). In our brief comparison we will restrict ourselves to low oxidation state chemistry and group 16 metals (19). 

The dotted arrows on the transmetallation step 243 show only what joins to what and are not intended as a serious mechanism. Indeed a better mechanism might involve addition of RO to the boron atom before transmetallation. This process can be used to couple aryl to aryl, vinyl to vinyl, and aryl to vinyl (either way round ). As boron compounds are much less toxic than tin compounds, the Suzuki coupling is often preferred industrially. Because each partner in the coupling reaction is marked in a different way - one with a boron atom and one with a halide - we can be sure that we shall get cross coupling reactions only. 

Natural boron consists of two isotopes, I0B (19.6%) and UB (80.4%). Isotopically enriched boron compounds can be made and are useful in spectroscopic and reaction-mechanism studies. The boron nuclear spins, (10B, S= 3 nB, S = 3/2) are also highly useful in structural elucidation. For an example see page 244. 

Group III.—Boron. It has only recently been conclusively shown that the stoicheiometry of the boric acid-tartaric acid complex is 1 1. Kinetic results from T-jump experiments indicate attack by an alcohol-oxygen lone-pair at the electron-deficient boron. Other references to reaction mechanisms of three-co-ordinate boron compounds deal with hydrolysis and methanolysis of boron halides, with disproportionation of boranes, and with Sh2 radical reactions of alkylboranes. ... 

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