Boranes from alkanes

Complex 31 also catalyses the high-yield formation of linear alkylboranes from alkanes under thermal conditions [22]. Scheme 10 depicts the dehydro-genative coupling of pinacolborane 33 with n-octane 35. In this reaction, 1-oc-tylpinacolborane 36 was the only product to be detected. No borane derivatives from functionalization at the internal positions were observed. 2-Methylhep-tane and methylcyclohexane provided only alkylboranes from primary C-H bond functionalization. The high selectivity for the termini of alkanes is kineti-cally determined and results from a steric preference for formation of a linear metal-alkyl complex. 

In particular, iminoboranes (XBNR) are isoelectronic with alkynes (XCCR). Well-known comparable pairs of isoelectronic species are aminoboranes (X2BNR2) and alkenes (X2CCR2), amine-boranes (X3BNR3) and alkanes (X3CCR3), borazines [(XBNR)3] and benzenes [(XCCR)3], etc. The structure of aminoboranes, amine-boranes, and borazines is well known from many examples. It has turned out that these BN species are not only isoelectronic, but also have structures comparable with the corresponding CC species. In the case of borazines, the aromatic character was widely discussed on the basis of theoretical and experimental arguments. The structural and physical properties of... 

Rh-H / -elimination step in step E gives the vinyl-borane which then dissociates to generate the first intermediate. This cycle explains the production of equimolar quantities of alkane and -vinylborane with unreactive R2BH derived from ephedrines with catecholborane there is an additional shunt pathway — step C in competition with step E leading to the primary regio-isomer of alkylborane. 

Perhaps the most interesting development of these types of reactions, the formation of a single product from terminal borylation of linear alkanes by a transition metal catalyzed reaction, comes from Hartwig. Cp Rh(jj -C6Me6) catalyzes the high-yield formation of linear alkylboranes from commercially available borane reagents under thermal conditions. Iridium catalysts are also effective. ... 

Chiral alkynes, (Z)-alkenes and alkanes can be obtained from alkyl(l,l,2-trimelhyl-propyl)boranes by the following sequence of reactions9. 

Undoubtedly, the synthetic value of the Rh-catalyzed alkyl borylation reaction follows from the fact that it can selectively deliver a functional group, such as an alcohol, an amine or an alkene, at the terminus of an alkyl chain during the synthesis of more complex structures. With the proper substituent at boron, this process can extrude the same borane reagent that was used in the alkane functionalization step to allow recycling of the main group reagent for large-scale synthesis. 

The reactions of alkanes with borane reagents occurs in the presence of late transition metal catalysts to form products from the replacement of a terminal C-H bond by a boryl group, and these are the first reactions to form products with a functional group in the terminal position with such high selectivity. In 1999 and 2000, Hartwig and... 

Bis(boracyclyl)alkanes 23, 27), formed by the hydroboration of diolefins with B2H6 23) at room temperature or from trialkylamine boranes at high temperatures 27), isomerize in the same way as B-alkylboracyclanes when heated. The equilibrium mixture of B2(C H2 )3 (XVIII) ( = 5, 6) was determined, before and after thermal isomerization, by oxidation and hydrolysis to diols 2J). 

Boranes not only form interesting stmctures, but they are also valuable as catalysts in organic reactions. For example, adding an alkene to a diborane forms an alkane bonded to the boron atom. The alkane can be cleaved from the boron, resulting in a net hydrogenation reaction such as the one shown here that can be carried out under mild (lower temperature) conditions ... 

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