Acetylene−Dicobaltcarbonyl Complexes with Chiral Phosphinooxazoline Ligands:  Synthesis, Structural Characterization, and Application to Enantioselective Intermolecular Pauson−Khand Reactions

Abstract

The reaction of the phenylacetylene−dicobalthexacarbonyl complex (2) with the 4-R-2-(2-diphenylphosphinophenyl)oxazolines 1 (R = Ph) and 4 (R = CH2CH2SCH3) leads to the selective formation of the chelated complexes 3 and 5, respectively. On the other hand, the tert-butyl-substituted phosphinooxazoline 6 acts as a monodentate ligand, and its reaction with several 1-alkyne-derived complexes (2,7−10) affords readily separable mixtures of the diastereomer nonchelated complexes 11a,b−15a,b. The interconversion rate between diastereomeric pairs is dependent on the steric bulk of the alkyne substituent, and neither 3 nor 5 epimerize at room temperature. The structures of both kinds of complexes have been ascertained by a combination of spectroscopical (IR, NMR), X-ray diffraction, and chiroptical methods; this has allowed the development of a practical procedure for the establishment of the absolute configuration of the chiral alkyne−dicobaltcarbonyl complexes obtained by the selective substitution of a carbon monoxide on one of the diastereotopic cobalt atoms. The intermolecular Pauson−Khand reaction of the chelated complexes 3 and 5 with norbornadiene respectively affords the (+) and (−) enantiomers of expected enone adduct 25, but in low enantiomeric excesses. Contrary to that, the tertiary amine N-oxide-promoted intermolecular Pauson−Khand reactions of nonchelated complexes 11a,b−13a,b give the corresponding norbornadiene- or norbornene-derived adducts both in high yields (85−99%) and enantioselectivities (93−97% enantiomeric excess), in what constitutes a substantial improvement over preexisting procedures for this reaction. The possibility of achieving chiral induction in the Pauson−Khand reaction of symmetrical alkynes (via the corresponding dicobaltpentacarbonyl complexes with ligand 6) has been demonstrated for the first time. An enantioselectivity mnemonic rule and a mechanistic model that explains the observed asymmetric sense of induction have been developed, and have been found to be in agreement with the results of model semiempirical molecular orbital calculations.

A comparison between the reaction of P2Ph4 with [{Co2(CO)6}2(μ-η2:μ-η2-HOCH2CCCCCH2OH)] and the reaction of [Co2(μ-PPh2)2(CO)6] with HOCH2CCCCCH2OH

“…Likewise, C(14) and C (15) lie, respectively, 0.040 and 0.026 Å closer to Co(3) than to Co(4). It is known that monodentate phosphine ligands prefer the pseudoaxial sites of alkyne dicobalt carbonyl complexes; X-ray structures of both monosubstituted [4,[59][60][61][62][63][64] and disubstituted [65][66][67] phosphine complexes confirm this. All previous X-ray structures of diphosphine-bridged alkyne dicobalt complexes have, however, exhibited pseudoequatorial substitution [5,56,[68][69][70][71][72][73][74][75][76][77][78][79].…”

Dicobaltbisnorbornadienetetracarbonyl [Co2(Nbd)2(CO)4] Reacts with Oxygen-Containing Alkynes and Gives Crossover Products in the Presence of other Alkenes

“…2-(Methoxymethyl)-3a,4,7,7a-tetrahydro-4,7-methano-1H-inden-1-one (11): To a flame, dried (200 ml) Schlenk flask equipped with a reflux condenser was added DDTC (2.0 g, 4.72 mmol), methyl propargyl ether (0.44 ml, 5.19 mmol) and dry, degassed dichloromethane (20 ml) under an inert atmosphere. The reaction mixture was heated at reflux for 12 hours then filtered and evaporated to dryness (black/brown solid), dissolved in DCM (100 ml) treated with activated charcoal (0.5 g) silica (ca 0.5 g) and stirred under an atmosphere of air overnight.…”

Practical Gas Cylinder-Free Preparations of Important Transition Metal-Based Precatalysts Requiring Gaseous Reagents

“…54 In order to enable an enantioselective variation of this reaction, chiral cobalt acetylene complexes have also been investigated (Scheme 9). 55,56 For the synthesis of such catalysts, (μ-HCCH)−Co 2 (CO) 6 is commonly prepared by bubbling acetylene gas from a pressurized gas cylinder into a Co 2 (CO) 8 solution. We envisioned using acetylenegen, a solid and stable fluorenoneprotected acetylene that can release the desired gaseous reagent ex situ in the presence of catalytic amounts of base.…”