Imidotitanium Complexes as Hydroamination Catalysts:  Substantially Enhanced Reactivity from an Unexpected Cyclopentadienide/Amide Ligand Exchange

Abstract

The addition of an N-H bond across a carbon-carbon double or triple bond is potentially one of the most direct and efficient methods of alkylating an amine. Considerable effort has been expended in the search for transition metal complexes that will catalyze this desirable transformation, and while notable successes have been realized, general solutions to the problem have proven elusive. 1 As a consequence, the discovery and development of hydroamination reactions remain important goals.Relatively few early transition metal complexes have been shown to be hydroamination catalyst precursors. Among these are the class of bis(cyclopentadienyl)zirconium and titanium alkyne hydroamination catalysts studied earlier by our group 2 and by Doye's 3,4 (eq 1). In both systems it has been assumed that the critical intermediates are the 16-electron imido complexes Cp 2 M=NR. The purpose of this communication is to report titanium-based imido complexes 5,6 that function as catalyst precursors for both allene and alkyne hydroamination reactions. 7-9 A detailed mechanistic investigation has revealed an unexpected cyclopentadienide/amide ligand exchange reaction that transforms the biscyclopentadienyltitanium precursors (Cp 2 TiL n ) into a monocyclopentadienyl titanium (amido) complex (Cp(ArNH)TiL n ) 10 exhibiting substantially higher reactivity than the Cp 2 TiL n precursor.(1).We became interested in catalytic allene hydroamination with the discovery that enantiopure (ebthi)zirconium imido complexes kinetically resolve chiral racemic allenes under stoichiometric conditions (ebthi = ethylene bis(tetrahydroindenyl)). 11 Experiments to explore the feasibility of metal-catalyzed allene hydroamination were undertaken with the eventual goal of developing a catalytic variant of this kinetic resolution. A brief screen of catalysts revealed that titanium complexes were more reactive than their zirconium counterparts, and that Cp 2 TiMe 2 12 was a convenient precursor to an undefined, but competent, catalyst species that hydroaminated 1,2-propadiene in good yields (eq 2). Aryl and alkylamines are both tolerated, as are hydrazines; bulky amines appear to react with greater facility than those with lessened steric demand.

Highly flexible synthesis of indenylethylamines as ligand precursors for titanium complexes

“…In search of a more convenient synthesis of aldehyde 5, we then found that lithiated indene (12) smoothly undergoes a nucleophilic substitution reaction with commercially available and inexpensive 2-bromo-1,1-diethoxyethane (13) to give the corresponding indenylethylacetal 14 in excellent yield of 98 % (Scheme 5). After aqueous work-up, acetal 14 was already obtained with high purity (97 % as determined by GC analysis) and therefore, the crude product could directly be used for the subsequent acetal cleavage.…”

“…However, it is well established that in the presence of amines, bis(η 5 -cyclopentadienyl)titanium precursors (Cp 2 TiL n ) undergo an unexpected cyclopentadienide/amide ligand exchange reaction that results in the formation of mono(η 5 -cyclopentadienyl)titanium(amido) complexes (Cp(RNH)TiL n , Scheme 2). 13 In analogy, it can be assumed that the catalyst precursor Ind 2 TiMe 2 used for hydroaminoalkylation reactions of alkenes undergoes a comparable ligand exchange reaction with the amine substrate and as a result, the expected catalytically active titanaaziridine formed from Ind 2 TiMe 2 possesses only one indenyl ligand and an amido ligand (Scheme 2). However, these two ligands are expected to remain bonded to the titanium center during the entire catalytic cycle and as a consequence, they both determine the activity of the catalytic species.…”

Is an MN σ‐Bond Insertion Route a Viable Alternative to the MN [2+2] Cycloaddition Route in Intramolecular Aminoallene Hydroamination/Cyclisation Catalysed by Neutral Zirconium Bis(amido) Complexes? A Computational Mechanistic Study

“…Keywords: allenes · density functional calculations · hydroamination · reaction mechanisms · zirconium ly proposed by Bergman and co-workers for intermolecular alkyne and allene hydroamination with primary amines mediated by zirconocene compounds [8] and was later extended to cyclohydroamination. [6] Various experimental studies, [9] of which the detailed kinetic investigation of Doye [9a] is most notable, and also computational [10] examination have substantiated this mechanism.…”

“…This scenario is similar to the mechanism established for the Zr-catalysed intermolecular hydroamination of alkynes and allenes with primary amines. [8] The kinetically most demanding step of the productive cycle (i.e., 9 A!12 A along the predominantly traversed branch) has a somewhat lower barrier (DG°= 28.8 kcal mol À1 , Scheme 10).…”

Synthesis, Characterization, and Catalytic Activity of Pd(II) Salen-Functionalized Mesoporous Silica

“…There has been much interest in using them as starting material for the synthesis of allylamines. Only a small number of transition-metal catalyzed intermolecular hydroamination reactions of allenes have been described [9][10][11][12][13][14]. Previously, we reported the addition of amines to allene using Pd(OAc) 2 , from which a mixture of the desired allylic amine and a minor amount of dienic amine was obtained [15].…”