Unprecedented Spectroscopic and Computational Evidence for Allenyl and Propargyl Titanocene(IV) Complexes: Electrophilic Quenching of Their Metallotropic Equilibrium

Abstract

The synthesis and structural characterization of allenyl titanocene(IV) [TiClCp2 (CH=C=CH2 )] 3 and propargyl titanocene(IV) [TiClCp2 (CH2 -C≡C-(CH2 )4 CH3 )] 9 have been described for the first time. Advanced NMR methods including diffusion NMR methods (diffusion pulsed field gradient stimulated spin echo (PFG-STE) and DOSY) have been applied and established that these organometallics are monomers in THF solution with hydrodynamic radii (from the Stokes-Einstein equation) of 3.5 and 4.1 Å for 3 and 9, respectively. Full (1) H, (13) C, Δ(1) H, and Δ(13) C NMR data are given, and through the analysis of the Ramsey equation, the first electronic insights into these derivatives are provided. In solution, they are involved in their respective metallotropic allenyl-propargyl equilibria that, after quenching experiments with aromatic and aliphatic aldehydes, ketones, and protonating agents, always give the propargyl products P (when carbonyls are employed), or allenyl products A (when a proton source is added) as the major isomers. In all the cases assayed, the ratio of products suggests that the metallotropic equilibrium should be faster than the reactions of 3 and 9 with electrophiles. Indeed, DFT calculations predict lower Gibbs energy barriers for the metallotropic equilibrium, thus confirming dynamic kinetic resolution.

1-Zirconacyclobuta-2,3-dienes: synthesis of organometallic analogs of elusive 1,2-cyclobutadiene, unprecedented intramolecular C–H activation, and reactivity studies

“…Early transition metal complexes with CH 2 C CR units are known as the combination of η 3 -propargyl and η 3 -allenyl resonance structures. 27 In the herein reported CH(SiMe 3 )C CR structure, C1–C2 and C2–C3 bond lengths correspond to a triple and double bond, respectively, and the C 3 ligand unit is thus best described as a resonance form between η 3 -propargyl and allenyl structures. The Zr–C1–C2–C3 unit is planar (−1.5(5)°) and this is also in agreement with the planarity of such η 3 -propargyl/allenyl complexes.…”

The interpretation of small molecule diffusion coefficients: Quantitative use of diffusion-ordered NMR spectroscopy

“…While the SEGWE method was explicitly designed for small molecules, containing only lighter atoms such as C, H, and O, this has not stopped its use in the analysis of compounds and complexes containing aluminium [254][255][256], cobalt [257], copper [258,259], gold [260], heavy alkaline earth elements [261], all heavier Group 14 elements [262], iridium [263], nickel [264], palladium [236,265,266], samarium [267], selenium [268], silver [269,270], titanium [271], uranium [272], yttrium [273,274], and zinc [275,276]. While caution might be well advised in the extrapolated use of an effective density optimised using a training set consisting solely of small, organic molecules, it was noted in [259] that for the species studied therein, the ligand units used made up the bulk of the molecular weight, 540 g mol −1 , compared with the heavier Cu (64 g mol −1 ) atom and halides (35 g mol −1 for Cl, 127 g mol −1 for I).…”

Cp2Ti(III)Cl and Analogues as Sustainable Templates in Organic Synthesis

“…The allenyl-and propargyltitanocene(IV) complexes were characterized by spectroscopic techniques and DFT calculations. 63,66 In order to rationalize the different behaviour of aldehydes and ketones with internal propargylic halides, an equilibrium between the propargyl and allenyltitanium species was proposed (Scheme 38). Firstly, a propargyl radical is generated by Cp 2 Ti(III)Cl to give the propargyltitanium(IV) complex 69.…”

“…Consequently the aldehyde reacts with the propargyltitanium(IV) species 69 to afford the α-hydroxyallenes 78 whereas the coordination with ketones is slower and the propargylic alcohol 74 is the major product. 66 The intramolecular reaction can proceed with the propargylic complex 69 possibly due to steric hindrance.…”