Electronic Effect on the Molecular Structure of Sterically Congested Triplet Diphenylcarbenes

Carbenes are important reactive intermediates in a variety of chemical reaction, and therefore have been extensively studies both experimentally and theoretically to explore their chemical properties and various parameters that affect their stabilities. In order to stabilize triplet carbenes kinetically to extent that they are able to survive under normal conditions, we attempt to protect the carbenic center of triplet diphenylcarbenes by introducing a series of substituents at the ortho position, and revealed that it is very effective on the stabilities of triplet diphenylcarbenes to introduce the big and unreactive substituents at ortho position. But this method has been limited that diazo compounds are used as precursor of carbenes and the preparation of diazo compound becomes more difficult as the bigger substituents are introduced in the ortho position. For this reason, the other approach, that the electronic (thermodynamic) effect of para substituents on the stabilities of triplet diphenylcarbenes, has been considered. To explore this effect, a series of triplet di(2,6-dimethylphenyl)carbenes (2) bearing symmetrical para strong electronic disubstituents have been generated by the irradiation of the corresponding diazo precursors and studied by means of electron paramagnetic resonance (EPR) spectroscopy. The zero field splitting parameters, DandE, were measured in matrices of different viscosities and were analyzed with molecular structures in terms of a scale of spin-delocalization substituent constants. And studied the stabilities of triplet di(2,6-dimethylphenyl)carbenes (2) by measuring the disappearance temperature of (2). Finally we have measured the life-time of the triplet di(2,6-dimethylphenyl)carbenes (2) by laser flash photolysis (LFP). It was showed that the triplet di(2,6-dimethylphenyl)carbene having stronger spin delocalized substituent have more line structure and show more stability.