Highly flexible synthesis of indenylethylamines as ligand precursors for titanium complexes

Abstract

Dedicated to Professor Jürgen Martens in honor of his outstanding contribution to synthetic organic chemistry DOI: http://dx.doi.org/10.3998/ark.5550190.p008.859 AbstractVarious indenylethylamines are synthesized for the first time by reductive amination of 2-(1H-inden-1-yl)acetaldehyde with commercially available primary amines. In addition, a new twostep synthesis of 2-(1H-inden-1-yl)acetaldehyde that uses inexpensive indene and 2-bromo-1,1-diethoxyethane as starting materials is presented. Finally, a selected indenylethylamine is used as a ligand precursor for the synthesis of a corresponding indenylethylamido titanium complex. The latter result paves the way for applications of corresponding complexes as catalysts for important chemical reactions.

Titanium Catalysts with Linked Indenyl–Amido Ligands for Hydroamination and Hydroaminoalkylation Reactions

“…Indenylethylamines 1a−e (Schemes 1 and 3) and the dichloro titanium complex 2d were synthesized according to previously published literature procedures. 22 Because the N-phenylsubstituted complex 2d is easily available from the dilithiated ligand precursor 1d, which reacts smoothly with titanium tetrachloride in diethyl ether, 22 the same approach was used to synthesize the additional dichloro titanium complexes 2a−c, which possess N-alkyl substituents (Scheme 1). In these cases, the amines 1a−c were initially treated with a 2-fold excess of nbutyllithium to generate the corresponding dilithium salts, which turned out to be only stable as suspensions in Et 2 O.…”

“…In this context, it must also be noted that we have already reported the structure of 2d elsewhere. 22 The central titanium atoms of complexes 2a−d 2). 23 With dichloro titanium complexes 2a−d in hand, attempts to synthesize the corresponding dimethyl complexes by methylation with 2 equiv of methyllithium in diethyl ether were carried out (Scheme 2).…”

“…Not surprisingly, the pseudotetrahedral coordination environment of the titanium center of 3d and the η 5 coordination of the indenyl ligand (Ω = 3.3°, Δ Ti−C = 0.11 Å) are similar to those for the dichloro complex 2d (Ω = 3.6°, Δ Ti−C = 0.08 Å). 22 In addition, in both cases, the coordination geometry of the nitrogen atom is trigonal planar (∑∠N1: 2d, 359°; 3d, 357°). However, the change from chloro (2d) to methyl (3d) ligands leads to a significant elongation of the Ti−N bond (Ti1−N1: 2d, 1.8935(6) Å; 3d, 1.9360(25) Å).…”

“…Amines and alkenes were distilled and degassed (freeze, pump, thaw) prior to use. Aminoalkenes (7 and 10) 27 and indenylethylamines (1a−e) 22 were synthesized according to appropriate literature procedures. 1 H and 13 C NMR spectra were recorded on a Bruker AVANCE III 500 MHz or a Bruker Avance DRX 500 MHz spectrometer ( 1 H, 500 MHz; 13 C, 126 MHz).…”

“…On the other hand, we have already described the synthesis of several indenylethylamines and one example of a dichloro indenylethylamido titanium complex. 22 Herein we describe the synthesis of new indenylethylamido dichloro and dimethyl titanium complexes and their catalytic reactivity in hydroamination and hydroaminoalkylation reactions.…”