Aromatic Imines in the Titanocene Coordination Sphere—Titanaaziridine vs 1-Aza-2-titanacyclopent-4-ene Structures

Abstract

Two independent synthetic routes to η2-imine titanocene complexes were developed. On one hand side, ligand exchange reactions of bis(trimethylsilyl)acetylene by (p-Tolyl)HCNPh (3) employing the Rosenthal reagent Cp2Ti{η2-C2(SiMe3)2} (1) lead to Cp2Ti{η2-(p-Tolyl)CHNPh} (5), exhibiting a titanaaziridine structure. On the other hand, the direct reductive complexation of 3 by using Cp2TiCl2 (2) and Mg as reducing agent leads also to 5, one of the rare known titanoceneaziridines without additional ligands. By using the ketimine (p-Tolyl)2CNPh (4) instead of the aldimine 3, an unexpected coordination mode was found by X-ray diffraction, exhibiting an azatitanacyclopent-4-ene structure involving one tolyl fragment. In such a way, via the reductive complexation of 4, employing 2 or Cp*TiCl3 (12), the 1-aza-2-titanacyclopent-4-ene complexes 6 and 13 are formed. Density functional calculations at the M06-2X level identify these new complexes 6 and 13 as 1-aza-2-titanacyclopent-4-enes, in agreement with an analysis based on the experimental structural parameters. A theoretical study of the bonding between the titanocene fragment and the imine ligand reveals that steric factors are more pronounced for titanaaziridines and disfavor their formation compared to azatitanacyclopentenes. This provides a rationalization for the preferred formation of titanoceneaziridines in the case of aldimine ligands and azatitanacyclopentenes when ketimines are applied. Whereas titanoceneaziridine 5 undergoes insertion reactions into the Ti–C carbon σ-bond with aldehydes, ketones, or carbodiimides to the five-membered titanacycles 20 and 21, complex 6 appears to be inert in comparable reactions.

Efficient Access to Titanaaziridines by CH Activation of N‐Methylanilines at Ambient Temperature

“…This elongation is analogous to that found in h 2 -aldimine titanocene complexes (1.403(2) ). [13] Thea ngle sum of the bonds at the nitrogen atom (3478 8)matches with the deflection of the Nphenyl ligand from the plane of the three-membered ring (238 8). According to the smallest possible substitution pattern present at the imine carbon atom, the compounds 3a-c constitute three new examples of the few isolated h 2 -complexes of N-methyleneaniline.O wing to spontaneous oligomerization or trimerization, N-methyleneaniline cannot be used directly as ligand.…”

“…Compound 6 is isolated, after the removal of surplus 1-hexene,i n9 0% yield without any by-products. [15] TheN MR spectra of 6 reveal that only one structural isomer is formed (shown in Scheme 4), with the C 4 H 9 -substituent in the position b to the titanium center as evident by the 13 Cchemical shift (d = 66.2 ppm) which is characteristic of the TiCH 2 -group. [27,28] Thet ertiary carbon atom (marked with a*i nS cheme 4)) as as tereogenic center, which is situated in the ring structure,i nduces the appearance of diastereotopic signals of the metallocene (eight signals), as .…”

Effizienter Zugang zu Titanaaziridinen durch C‐H‐Aktivierung von N‐Methylanilinen bei Raumtemperatur

“…Nach Entfernen des überschüssigen 1-Hexens wird 6 als brauner Feststoff mit einer isolierten Ausbeute von 90 %, ohne weitere Nebenprodukte,erhalten. [15] In den NMR-Spektren von 6 ( 1 H, 13 C, DEPT135) wird nur das gezeigte Regioisomer mit dem C 4 H 9 -Rest in b-Position zum Titan (Schema 4) detektiert. Dabei erweist sich die 13 C-chemische Verschiebung (d = 66.2) für die TiCH 2 -Gruppierung als besonders signifikant.…”

“…[15] In den NMR-Spektren von 6 ( 1 H, 13 C, DEPT135) wird nur das gezeigte Regioisomer mit dem C 4 H 9 -Rest in b-Position zum Titan (Schema 4) detektiert. Dabei erweist sich die 13 C-chemische Verschiebung (d = 66.2) für die TiCH 2 -Gruppierung als besonders signifikant. [27,28] Das im Ring enthaltene tertiäre C-Atom (*) induziert als Asymmetriezentrum das Auftreten diastereotoper Signale der Metalloceneinheit (acht Signale) sowie die Aufspaltung der Methylenringprotonen Ti-CH 2 -( je 1H,m ,d = 1.13;2 .62) bzw.N -CH 2 (je 1H,m ,d=2.89, 3.22).…”

Crystal structure of 1,1-bis(η 5-adamantylcyclopentadienyl)-3-phenyl-2-trimethylsilyl-2,3-dihydroisotitanazole, C42H55NSiTi

“…Titanaaziridines are used in a variety of organic synthesis methods [5,6]. In this context, the hydroaminoalkylation of alkenes, in which the insertion of the alkene into the Ti-C bond of the titanaaziridine is supposed to be the C-C forming step [7][8][9][10], is likely the most important one.…”