Correlation between Functionality Preference of Ru Carbenes and exo/endo Product Selectivity for Clarifying the Mechanism of Ring-Closing Enyne Metathesis

Abstract

Functionality preferences of metathesis Ru carbenes to various alkenes and alkynes with electronic and steric diversity were determined by using time-dependent fluorescence quenching. The functionality preferences depend not only on the properties of multiple bonds but also on the ligands on Ru. There was a good correlation between functionality preference and the metathesis reaction outcome. The correlation between functionality preference and exo/endo product ratio offers a solution to resolve the mechanistic issue related with alkene- vs alkyne-initiated pathway in ring-closing enyne metathesis. The correlation indicates the preference is likely to dictate the reaction pathway and eventually the outcome of the reaction. The Ru catalyst favoring alkyne over alkene provides more endo product, indicating that the reaction mainly initiates at the alkyne. By changing the substitution pattern, the preference can be reversed to give an exclusive exo product.

A General Approach to Sequence-Controlled Polymers Using Macrocyclic Ring Opening Metathesis Polymerization

“…Further encouragement derives from Choi’s polymerization of an unstrained cyclohexene using an alkyne unit as the relay (Scheme 1 B). 10 While NHC-ligated Grubbs catalysts are highly reactive toward terminal alkynes, 11 the resulting vinyl ruthenium carbenes are usually resistant to further metathesis chemistry. As a result, the key to this process is the close proximity of the cyclohexene olefin to the alkyne, permitting fast intramolecular cyclization and rapid polymerization.…”

Ring-closing metathesis of prochiral oxaenediynes to racemic 4-alkenyl-2-alkynyl-3,6-dihydro-2H-pyrans

“…In contrast to G-I, the presence of an ethene atmosphere was highly detrimental for the RCEYM catalyzed by the more stable and active G-II and HG-II precatalysts, as this led to the intermolecular cross-metathesis between the triple bonds of the substrates and ethene ( Figure 3) as was previously observed [24]. Thus, complex inseparable mixtures with not fully assignable 1 H NMR spectra were obtained in all cases.…”

“…As mentioned in the Introduction, the substrate structure [23,24,[34][35][36], the choice of the precatalyst [24], and the pres-…”

“…Both of these factors determine the metathesis mechanism: i.e., whether a double (ene-then-yne mechanism) or a triple (yne-then-ene mechanism) bond first enters the initiation step of the precatalyst activation. The group 6 metal-based precatalysts prefer the latter mechanism and yield the endoproducts, while Ru precatalysts enable both mechanisms and generally follow the ene-then-yne mechanism for substrates with a sterically unhindered double bond, yielding the exo-products (Scheme 1) [21][22][23][24][25][26]. The presence of ethene (Mori conditions) is often beneficial in RCEYM reactions, giving significantly better yields (Scheme 2).…”

Ring closing metathesis of prochiral oxaenediynes to racemic 4-alkenyl-2-alkynyl-3,6-dihydro-2H-pyrans

“…Both of these factors determine the metathesis mechanism: whether double (ene-then-yne mechanism) or triple (ynethen-ene mechanism) bond first enters the initiation step of the precatalyst activation. Group 6 metal based precatalysts prefer the latter mechanism and yield endoproducts, while Ru precatalysts allow both mechanisms and generally prefer the enethen-yne mechanism for substrates with sterically unhindered double bond, yielding exo-products (Scheme 1) [18][19][20][21]. The presence of ethene (Mori conditions) is often benefitial in RCEYM, giving significantly better yields.…”