Intramolecular [2+2+2] Cycloaddition Reactions of Yne‐ene‐yne and Yne‐yne‐ene Enediynes Catalysed by Rh^I: Experimental and Theoretical Mechanistic Studies

Abstract

N-tosyl-linked open-chain yne-ene-yne enediynes 1 and 2 and yne-yne-ene enediynes 3 and 4 have been satisfactorily synthesised. The [2+2+2] cycloaddition process catalysed by the Wilkinson catalyst [RhCl(PPh(3))(3)] was tested with the above-mentioned substrates resulting in the production of high yields of the cycloadducts. Enediynes 1 and 2 gave standard [2+2+2] cycloaddition reactions whereas enediynes 3 and 4 suffered β-hydride elimination followed by reductive elimination of the Wilkinson catalyst to give cycloadducts, which are isomers of those that would be obtained by standard [2+2+2] cycloaddition reactions. The different reactivities of these two types of enediyne have been rationalised by density functional theory calculations.

A Computational Study of the Intermolecular [2+2+2] Cycloaddition of Acetylene and C60 Catalyzed by Wilkinson's Catalyst

“…As it can be seen, the C−C bond to be formed in this step has a bond length of 2.008 Å. This is the highest energy barrier throughout the catalytic cycle through path A and, therefore, oxidative addition is the rds for this path, similar to some other [2+2+2] cycloadditions . Oxidative addition affords the rhodacyclopentadiene species A4 by releasing 11.0 kcal mol −1 .…”

P‐Stereogenic Secondary Iminophosphorane Ligands and Their Rhodium(I) Complexes: Taking Advantage of NH/PH Tautomerism

“…However, after some experimentation, we found that the addition of a catalytic amount of acid enhanced the selectivity with this substrate. [21] Most remarkably, the selectivities found in this study of new SIP ligands are the highest reported for substrates that cyclize through pathway II, and thus highlight the utility of ligands with PH/NH tautomerism in catalysis. Preparation of neutral Rh I complexes with SIP ligands.…”

“…[18a] On the other hand, DFT calculations indicated that enediynes with terminal alkynes cyclize preferentially through initial alkyne-alkyne coupling (pathway II), usually leading to low enantiomeric excess. [21] Most remarkably, the selectivities found in this study of new SIP ligands are the highest reported for substrates that cyclize through pathway II, and thus highlight the utility of ligands with PH/NH tautomerism in catalysis.…”

Stereoselective Rhodium‐Catalysed [2+2+2] Cycloaddition of Linear Allene–Ene/Yne–Allene Substrates: Reactivity and Theoretical Mechanistic Studies

“…A comparative analysis can be performed at this point between linear allene–ene–allene and yne–ene–yne systems15d when submitted to a rhodium‐catalysed [2+2+2] cycloaddition reaction. Comparing the corresponding postulated mechanisms, several qualitative differences arise.…”