Computational Design Of A Novel B-B Bond Activation Mode And Its Synthetic Utilities

Diborane compounds,R2B-BR2,are highly useful synthetic modules for concise synthesis of organoboronates,which are versatile building blocks for the generation of new chemical bonds using the Suzuki-Miyaura crossing-coupling reactions.Due to the relative high bond dissociation energies of B-B bond in diboranes,transition-metal complexes are always required to effectively cleave the B-B bond of diborane and catalyze the corresponding borylation or diborylation reactions of unsaturated organic substrates.On the other hand,the Lewis bases(for example,Cs2CO3,t-BuOK)catalyzed diboration of alkenes have been reported recently.It has been shown that diborane compounds became a source of nucleophilic boryl moiety when one of the boron atoms of the diborane was coordinated to a strong base.In such cases,the(sp2)B-B(sp3)bonds are always cleaved heterolytically in these processes.Given the considerable utility of diboranes in synthetic chemistry,it is highly desirable to develop new strategies for the activation of B-B bond.It is well known that the computational chemistry has play a significant role in understanding the mechanism of chemical reactions and new reaction predictions.However,because of the complexity of chemical reactions,the theoretical predictions for developing new reactions are still rather limited.In this thesis,we carried out a combined computational and experimental investigations to design a novel strategy for boron-boron bond activation.Based on the newly established method,new chemical reactions were investigated both theoretically and experimentally.The main work of this thesis includes two parts:first,we have designed a novel B-B bond activation mode,which was further verified experimentally;second,based on density functional theory calculations,we have also proposed that the pyridine-bory radical generated in situ using 4-cyanopyridine and bis(pinacolato)diboron,exhibits a carbon radical character,and such a radical can be used as a bifunctional "reagent" to synthesize 4-substituted pyridine derivatives using?,?-unsaturated ketones(or aryl aldehydes)via a novel radical migration/coupling mechanism.The main contributions of the present work can be summarized as follows:1.In chapter 3,based on DFT calculations,we found that 4-cyanopyridine was capable of homolytically cleaving the B-B ?-bond of diborane via the cooperative coordination to the two boron atoms of the diborane to generate pyridine boryl radicals.Our experimental verification provides a supportive evidence for this new B-B activation mode.With this novel activation strategy,we have experimentally realized the catalytic reduction of azo-compounds to hydrazine derivatives,deoxygenation of sulfoxides to sulfides and reduction of quinones with B2(pin)2 at mild conditions.2.Pyridine core is an important class of structural unit found in pharmaceutical compounds and functional materials.Transition metal catalysts or organometallic reagents have played privileged roles in the synthesis of pyridine derivatives.In chapter 4,we have developed a metal-free approach for the synthesis of C-4 substituted pyridine derivatives.Our theoretical calculations suggested that the pyridine-boryl radical generated in situ using 4-cyanopyridine and B2(pin)2 can be used as a bifunctional "reagent",which serves as not only a pyridine precursor but also a boryl radical.The combined computational and experimental studies showed that the 4-substituted pyridine derivatives could be synthesized in moderate to good yield via a novel radical migration/coupling mechanism with readily available reagents:4-cyanopyridine,B2(pin)2 and enones(or aryl aldehydes).The reactions occur under simple reaction conditions without the use of transition-metal catalyst or organometallic reagent.In summary,our proposed strategy for the activation of B-B bond provides a new method for the activation and transformations of the diborane compounds.With this novel activation mode,we have experimentally realized the catalytic reduction of azo-compounds to hydrazine derivatives,deoxygenation of sulfoxides to sulfides,reduction of quinones with B2(pin)2 at mild conditions and a metal-free approach for the synthesis of 4-substituted pyridine derivatives.