Allylation And Propargylation Of Carbonyl Compounds Catalyzed By Titanium/Visible-light Cocatalysis

The allylation or propargylation of carbonyl compounds were an important method for the synthesis of high value-added fine chemicals,such as homoallyl alcohol and homopropargyl alcohol,which has been widely used in the synthesis of natural products and drug molecules.In traditional methods,the substrates were activated by ionic reaction pathway for the formation of allyl/propargyl-metal intermediates,which was a two-electron transfer process.However,the activation method via the pathway of radical was few reported owing to the high reactivity and instability of allyl/propargyl radical.Visible-light catalysis can utilize light energy to realize the organic transformation through the pathway of radical under mild conditions and effectively reduce the activation energy of the reaction.So,the key factor for the development of visible-light catalysis is the selective control of allyl/propargyl radicals.Based on this,we made an effort to design titanium/visible-light cocatalysis system by using low carbon olefins as the allyl/propargyl source.In the catalytic system,the allyl/propargyl radicals were captured selectively by active Cp₂TiCl species.Thus,the highly active radicals were transformed into more stable allyl/propargyl-titanium species,which can effectively solve the problem of low reaction efficiency caused by short lifetime and high reaction activity of radicals as well as realize the selective control of radicals.So as to improve the selectivity and efficiency of reaction and increase the diversity of reaction.The conclusions are summarized as follows:（1）The Barbier-type allylation of carbonyl compounds was realized by using allyl halides as the allyl source.The allyl radicals could be captured effectively by Cp₂TiCl and generate allyl-titanium species,which greatly improved the reaction efficiency and stereoselectivity.The catalytic system showed well compatibility and diastereoselectivity（d.r.≥10:1）for both aldehydes and aliphatic ketones.Hantzsch ester was used as both electron sacrificial agents and proton donors in the reaction.In addition,when both aldehydes and ketones were presented in the substrates,the catalytic system could preferentially react with aldehydes,while ketones were intact.（2）The allylation of carbonyl compounds in multi-component system was realized by using 1,3-butadiene as the allyl source.By regulating the production rate of radical,allyl radicals could be captured selectively by Cp₂TiCl in the presence of a variety of radicals.This catalytic system provided a more mild and efficient method for highly selective conversion of 1,3-butadiene into homoallyl alcohol.It’s worth mentioning that the system has excellent catalytic effect for different types of aliphatic and aromatic carbonyl substrates,and the diastereoselectivity was up to 20:1.（3）The propargylation of aldehydes in multi-component system was realized by using 1,3-butenynes as the propargyl source.The selective capture of propargyl radicals were achieved by Cp₂TiCl to generate the corresponding propargyl-titanium species in multi-component reaction system.It provides a new strategy for highly efficient and selective synthesis of homopropargyl alcohol.This catalytic system showed well catalytic effect for different types of aldehydes,1,3-butenynes,as well as radical precursors.And the diastereoselectivity of aliphatic aldehydes was up to 20:1.