Research On Cyclization Of ?,?-dienes With Benzyl C-H Bond By Organic Rare-Earth Catalysts

Cyclization of enynes,diynes,and dienes catalyzed by organometallics is an important reaction for producing carbon-and heterocyclic skeletons,thereby it also becomes a key step in the synthesis of fine chemicals including pharmaceuticals,dyes,and pesticides as well as natural products.Among theses reactions,catalytic cyclization of enynes,diynes and dienes with inert C-H bond features a green process to form substituted carbocycles by virtue of its 100%atom economy together with its step economy.In this paper,we synthesize a series of CGC rare-earth dialkyl complexes through a alkyl-elimination reaction between pyridine-based Cp-H ligand and LnR3.In addition,using borate compound as a co-catalyst,we investigated the cyclization of ?,?-dienes with benzyl C(sp3)-H bond by these rare-earth complexes.The details are as follows:(1)The synthesis and characterization of a series of novel CGC-CpLnR2 complexes.By equimolar acid-base reaction between LnR3 and C5HMe4-(C5H3NR-o),two kinds of CGC rare-earth complexes(?5:?-C5Me4-(C5H3NR-o))Ln(CH2SiMe3)2(THF)n(Ln=Sc,Y,Lu:R=Me,Br,Ph,Naphth)and(?5:?-C5Me4-(C5H3NR-o))Sc(CH2C6H4NMe2-o)2(Ln=Sc.Y.Lu;R=H,Me,Ph.Naphth)was obtained in high yields and these complexes were characterized by 1H,13C NMR.(2)A green process to producing five-membered and six-membered carbocycles through scandium-catalyzed cyclization of 1,5-and 1,6-dienes with benzylic C(sp3)-H bond was developed in this paper.Using the combination of(?5:?-C5Me4-(C5H3N-Me-o))Ln(CH2SiMe3)2/[Ph3C][B(C6F5)4](Ca.4.0mmol%)as catalyst at 70? in chlorobenzene,a wide range of substrates including various picolines and 2-methyl-anisoles as well as deverse 1,5-and 1,6-dienes were investigated,and the corresponding cyclization products were obtained in 60-95%yields upon isolation with excellent cis-selectivity.(3)A possible cyclization mechanism was proposed.On the base of the results of deuterium-labeling experiments and KIE experiments,a reaction pathway involving a benzyl C-H activation and subsequent alkene insertion sequence should accounts for such a unique ring-closure reaction.