Part 1. C?H Bond Activation Mechanism by Rh Catalyst Systems
In chapter 2, biarydiazoacetates undergo intramolecular aromatic carbenoid insertion to form fluorene carboxylates in high yields under very mild conditions when the reaction is performed in the presence of catalytic amounts of rhodium(??) acetate or copper(??) triflate. Whereas thermal conditions provide a mixture of two regioisomeric products when substituted biaryldiazoacetates are employed as a substrate, a wide range of susbtituted fluorene carboxylates were efficiently obtained with high selectivity under the catalytic conditions. The insertion mechanism was revealed to be electrophilic aromatic substitution, which was supported by the scrambling study, deuterium kinetic isotope and substituent effects of aromatic ring. Density functional calculations of the cyclization process suggest that the observed regioselectivity can be readily explained in terms of steric interactions between the substituents and sterically demanding rhodium metal species in addition to the electronic induction effects of those substituents.
Subsequent issue is a catalytic procedure to introduce organic functional groups into 2,2’-bipyridines and 2,2’-biquinolines, arguably the most representative organic chelating compounds, by using a rhodium catalyst system. The conceptual basis enabling this catalytic conversion is that an initially generated rhodium chelate becomes labile due to the presence of a N-heterocyclic carbene (NHC) ligand that exhibits a strong trans-effect to weaken a rhodium-pyridyl bond trans to NHC. A subsequent rollover cyclometalation process leads to the key C?H bond activation of substrates by the action of the NHC-bound rhodium catalyst. Density functional calculations were in full agreement with the experimental data, and provided additional mechanistic insights.
Part 2. Rearrangement Mechanism
In chapter 2, we study about new direct trifluoromethylthiolation reaction developed by Shibata ...