Palladium-catalyzed Highly Selectivitive Addition Of Aryl Boronic Acid With Allenes Under Alkaline Conditions

Allenes are a class of unique compounds with two?-orbitals perpendicular to each other.Their special structure results in different reactivities of the two double bonds.Due to its special structure,it has a very unique response performance.In particular,the controlling of its chemical selectivity,regioselectivity and stereoselectivity is challenging.Electrophilic addition reaction of allenes catalyzed by the transition metal catalyst has been widely reported,such as the addition of allenes with arylboronic acid in the presence of palladium catalysts with acetic acid,triethylamine or potassium hydroxide.Recently,the synthesis of chiral multi-substituted alkenes with rhodium catalyst has also been achieved.Although the above reactions have been widely reported,the selective 1,2-addition of functionalized allenes with organoboranic acids to afford multi-substituted allenes using palladium as catalyst has not yet been reported so far.Inthiswork,thePd?II?-catalyzedhydroarylationof diphenylphosphinylallenes?via 1,2-addition of allenic double bond?with arylboronic acids in the presence of NaOH and O₂ is developed.The regioselectivitiesturnouttobewellcontrolled,affording2-aryl-3-?diphenylphosphinyl?alkenes as the only product.The paper includes the following sections.Part I:12 kinds of diphenylphosphinoallenes,which have different substituents in allene carbon position 1 and 3,including n-propyl,methyl,n-butyl alkyl substituent and aryl groups with electronic effect of substituents were synthesized.Part II:The reaction conditions of the Pd?II?-catalyzed hydroarylation of diphenylphosphinylallenes with arylboronic acids were fully explored,including the effects of solvent,temperature,catalyst and the amounts of base.Finally,the optimal reaction conditions were achieved.Part III:Under the optimal reaction conditions,the reaction of allenes and aryl boronic acid were deeply studied.First the reaction of4-methylbenzene boric acid with 12 kinds of different substituted allene compounds were investigated;then the reaction of eight kinds of aryl boronic acid bearing different substituents with one allene substrate were examined.Part IV:The possible reaction mechanism was discussed.Firstly,the transmetalation of the arylboronic acid with Pd?II?leads to the formation of an Ar-Pd intermediate.Subsequently 1,2-allenic double bond of allene is inserted into Pd-Ar bond of the Ar-Pd intermediate in high regioselectivity,and a further protonation leads to the formation of the desired final product.On the other hand,a second transmetalation of the arylboronic acid with Ar-Pd intermediate may also lead to the formation of Ar-Pd-Ar intermediate.Therefore,the byproduct Ar-Ar might be obtained through the reductive elimination,resulting in the formation of a Pd⁰ species.Finally,Pd⁰ species can be oxidized by O₂ to Pd?II?species.Multi-substituted funcationalized alkenes are important synthetic intermediates.This reaction protocol-the 1,2-addition of organoboronic acids to allenes-provides a very effective method to synthesis multi-substituted functionalized alkenes.This study further develops the allene chemistry,as well as the synthesis of natural products.It is important to note that this work has been successfully published in the international famous journal–Adv.Synth.Catal.?impact factor 6.453?.