ABSTRACT
An efficient and selective bis(amidate)bis(amido) titanium precatalyst for the anti-Markovnikov hydroamination of alkynes is reported. Hydroamination of terminal and internal alkynes with primary alkylamines, arylamines, and hydrazines is promoted by 5-10 mol % of Ti catalyst. Various functional groups are tolerated including esters, protected alcohols, and imines. The in situ generated complex shows comparable catalytic activity, demonstrating its synthetic versatility for benchtop application. Applications of this catalyst for the synthesis of amino alcohols and a one-pot procedure for indole synthesis are described. A mechanistic proposal that invokes turnover-limiting protonolysis is presented to rationalize the observed regioselectivities.
Subject(s)
Alkynes/chemistry , Imines/chemistry , Organometallic Compounds/chemical synthesis , Titanium/chemistry , Amination , Catalysis , Molecular StructureABSTRACT
Intermolecular hydroamination of heteroatom-substituted allenes with a bulky arylamine was carried out using a bis(amidate) bis(amido) titanium(IV) complex (1) as a precatalyst. The reaction of 2,6-dimethylaniline with oxygen-substituted allene 2c or 2d in the presence of complex 1 gives the ketimine regioisomer as the exclusive product. Reduction of such ketimine products resulted in the formation of amino ethers that were further employed as proligands for the formation of N,O-chelating five-membered titana- and zirconacycles. Such sterically demanding N,O-chelating ligands result in the high-yielding preparation of mono-ligated products. Solid-state molecular structures of all the complexes revealed distorted trigonal bipyramidal geometry about the metal centers, with a dative bond between the metal and the oxygen donor atom. These new complexes obtained using hydroamination as the key-step in ligand preparation were also shown to be useful cyclohydroamination precatalysts in their own right.
