Metallotropic Equilibrium and Configurational Stability of 3-Chloro-1-(trimethylsilyl)propargyl and -allenyl Metals: Comparative Study among Lithium, Titanium, and Zinc

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.

Section: Introductionmentioning

confidence: 99%

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

Ruiz-Muelle

Oña‐Burgos

Ortuño

et al. 2016

“…Products 2 c were obtained with some racemization, which is always expected under the reaction conditions, [17] because allenylzinc compounds have limited configurational stability at ambient temperature [32,33] and allenylpalladium compounds are also reported to undergo some racemization in the presence of Pd 0 catalysts. [30,34,35] Interestingly, as indicated in Scheme 7, the degree of racemization in cis-2 c was significantly higher than in the trans isomer.…”

Section: Comparison Between the Results In Entry 2 Ofmentioning

confidence: 99%

Catalyst‐ and Solvent‐Dependent Stereodivergence in the Intramolecular Et₂Zn/Pd⁰‐Promoted Carbonyl Propargylation: Mechanistic Implications

Arrate

Durana

Lorenzo

et al. 2013

Carbonyl-tethered propargylic benzoates undergo intramolecular carbonylpropargylation upon treatment with Et2 Zn in the presence of a catalytic amount of Pd(0) with the formation of 2-alkynylcyclopentanol products. A ligand/solvent effect on the cis/trans selectivity (referring to the relative positions of alkynyl and OH groups) of ring-closure has been found. In a non-coordinating solvent (benzene), increasing the electron-donating ability of the phosphine ligand (while decreasing its dissociation ability) leads to an increased tendency towards the trans product. On the other hand, the combination of a coordinating solvent (THF) and PPh3 , an easily dissociated phosphine, results in the exclusive formation of cis products. Experimental and computational results are compatible with a divergent behavior of an allenylethylpalladium intermediate that partitions between competitive carbonyl-addition and transmetalation pathways, each leading to a different diastereoisomer. These results also suggest that the dissociating ability of the phosphine regulates that behavior.

“…With a primary propargylic bromide, the in situ‐formed propargylic zinc reagent would react with CO 2 via a cyclic transition state IM‐1 to give the allenoic acids 2 ; for the secondary 2‐alkynyl bromide, a cyclic transition state involving allenyl intermediate IM‐2 would give the 3‐alkynoic acid 3 . The steric effect of the R 2 group determines the equilibrium between the propargyl zinc species and the allenyl zinc intermediate 17. 18…”

Section: Methodsmentioning

confidence: 99%

The Renaissance of an Old Problem: Highly Regioselective Carboxylation of 2‐Alkynyl Bromides with Carbon Dioxide

Miao

2015

A steric effect-controlled, zinc-mediated carboxylation of different 2-alkynyl bromides under an atmospheric pressure of CO2 has been developed by careful tuning of different reaction parameters, including the metal, solvent, temperature, and additive. 2-Substituted 2,3-allenoic acids were afforded from primary 2-alkynyl bromides, whereas the carboxylation of secondary 2-alkynyl bromides yielded 3-alkynoic acids in decent yields. A rationale for the observed regioselectivity has been proposed.