Borane activation

Bochmann has also produced a novel class of large dianionic borates XV based on the non-labile fefra-cyano Ni and Pd(II) metallates [K12 M(CN) i 2. 215a The metals in these WCAs are strictly square planar and as such the anion assumes a flat rather than spherical shape, with a torus of fluorination on the periphery of the plane. The bis-trityl salts of these dianions afford catalysts that are not as productive as those formed from the XIV class (X = CN), although they are still much more active than borane activated catalysts. The lower activity was attributed to the inherently greater cation/anion attraction in a monocation/dianion pair. 

As the reaction mixture is warmed to temperatures of about — 40 °C, product formation begins mainly via an [R3Sn]+ catalyzed mechanism as indicated in the scheme. Thus, the B(C6F5)3 functions here primarily as an initiator, while the majority of the catalysis is mediated by the tin cation generated upon initial attack of allyltributyltin on the borane activated substrate. 

Alcohols protected as methyl ethers can be retrieved by reaction with tribromo-borane. Activation of the methyl ether by co-ordination of the Lewis acidic tri-bromoborane followed by nucleophilic cleavage of the O—Me bond (with concomitant formation of bromomethane) is typical behaviour. However, the cleavage took a different course with 39.1 [Scheme L39] instead of the O—Me bond being cleaved, the alternative C—O bond cleaved owing to participation of the remote acetoxy group.72 The formation of bromide 39.4 with retention of configuration is circumstantial evidence implicating dioxonium ion intermediate 393. 

Scheme 11.17 Amino-borane activation of H2 molecular tweezers .

As for borane-activated catalyst systems, such as Cp TiMe3/B(QF5)3, the addition of TIBAis essential to increase the catalyst activity, syndioselectivity, and the molecular weight of the resultant sps 43-46,59 Herein, TIB A acts as a scavenger for impurities in the polymerization system. 

SCHEME 1 Formation of polymers with O- or N-func-tional groups by use of borane-activated zircononcene catalysts reported by Wa5miouth and co-workers (MR Kesti, GW Coates, RM Wa5mouth, J. Am. Chem. Soc. 114 9679,1992). R = NiPr2. 

TRANSITION METAL NANOPARTICLES CATALYST IN HYDROGEN GENERATION 173 TABLE 7.2 Transition Metal Nanoparticles Catalysts Used in Hydrogen Generation from the Hydrolysis of Ammonia Borane Activation Energy of the Catalytic Hydrolysis and Activity of Catalyst in Turnover Frequency (TOF) at 25 °C, —Not Reported ... 

Reactivation of this compound could be achieved through exchange with Al-cocatalysts the presence of borane activators (through alkene insertion into the Zr-allyl bond) or reaction with H2. DFT calculations suggested that reactivation by H2 was the lo west-energy path way NMR evidence has been... 

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