Structure and reactivity of carbenes having aryl substituents

Department of Chemistry. University of Illinois, Urbana, Illinois U.S.A. 

4 Experimental investigation of the chemical and physical properties of carbenes 320 

Low temperature epr and optical spectroscopy 321 Room temperature spectroscopy on a short timescale 324 Reactivity as a probe of spin state 326 

Diphenylmethylene (DPM), and its derivatives, dimesitylcarbene (DMC), dibenzocycloheptadienylidene (DCH), and sila-anthrylidene (SA) 349 

One of the central objectives of the study of chemistry is sufficient knowledge to permit the forecast of the chemical and physical properties of a substance directly and reliably from its structure. This objective has been achieved to a remarkable degree for the host of isolable compounds that have been prepared and investigated. These findings have been codified in generalizing 

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Structure and reactivity of carbenes having aryl substituents, 22, 311... 

Structure and Reactivity of Carbenes having Aryl Substituents... 

Schuster, G.B., Structure and Reactivity of Carbenes Having Aryl Substituents, Adv. Phys. Org. Chem., 22, 311,1986. 

Carbene chemistry, structure and mechanism in, 7, 163 Carbenes generated within cyclodextrins and zeohtes, 40, 1, 353 Carhenes having aryl substituents, structure and reactivity of, 22, 311 Carhocation rearrangements, degenerate,... 

The subject of this chapter is carbenes with aryl substituents (aromatic carbenes). These materials are short-lived reactive intermediates in which the normal tetravalency of carbon is reduced by two. Carbenes have been the object of speculation and investigation for more than 80 years. Nevertheless, there still is considerable uncertainty about their chemical and physical properties. In the last five years the pace of research in carbene chemistry has quickened. This is a consequence of the development of high-speed pulsed lasers that permit, for the first time, direct observation of carbenes under the conditions in which they react. This research has provided new information on the effect of structure on the chemical and physical properties of carbenes. 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

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