Theoretical Study On Mechanism Of Selective Borylation Of Alkynes

Vinylboronate esters are versatile reagents in organic synthesis,which have wide applications in natural product total synthesis.To achieve the effective synthesis of vinylboronate esters,borylation of alkynes provides an atom economic strategy.In general,the cis-addition mode is strictly obeyed in the borylation of alkynes.During the past decades,pioneering works on trans-borylation of alkynes have also been achieved by some groups.However,relative mechanistic study on elucidating the origin of the trans-selectivity is very rare,especially for some recent work.In this dissertation,theoretical calculations are employed to clarify the mechanisms of two trans-borylation reactions.The first part of this dissertation reviews the advance of borylation reactions on alkynes,which includes hydroboration,diboration,carboboration and metalloboration.Furthermore,the density functional theory,basis set and solvation model employed in this dissertation are also introduced in this part.The second part conducts a mechanistic study on Pd-catalyzed trans-hydroboration of internal enynes.The calculation results show that the conventional inner-sphere mechanism is unfavorable for this reaction,in which the transition metal is directly participate in the H-B bond cleavage.In contrast,this reaction follows an outer-sphere mechanism,where the Pd center does not directly involved in the H-B bond cleavage.More specifically,the favorable outer-sphere mechanism consists three steps,C-B bond formation,intermolecular hydride transfer and reductive elimination.Meanwhile,further analysis showed that the unique steric effect and electronic effect introduced by the special ligand are the main reason for the special trans-selectivity.The third part turns to unravel the origin of regio-/stereo-selectivity for a transition-metal free diboration of alkynamides with mixed diboron reagents.The favorable pathway consists deprotonation,Bdan transfer and Bpin transfer.More specifically,the Bdan transfer step is the rate and regioselectivity-determining step,which undergoes a five-membered ring transition states.The rotation of the conjugated double bond in the Bpin transfer step is the stereoselecitivity-determining step.