Arynes representative

Cyclizations by formation of carbon—selenium bonds represent a modern method with a high synthetic potential in the chemistry of cyclophanes. Selenocyanates such as 16 are accessible usually in excellent yields through the reaction of bromides with KSeCN [27], The reaction with benzylic bromides under reductive conditions using the dilution principle results in good to excellent yields of [3.3]di-selenacyclophanes which can be deselenized photochemically, pyrolytically (without previous oxidation), or by reaction with arynes, Stevens rearrangement and subsequent reaction with Raney nickel. [2.2]Metacyclophane (18), for example, is accessible in 47% total yield by using this sequence of reactions starting with... 

Contrary to the majority of other reactions of A -aminoazoles, oxidation reactions only began to be studied much later, approximately from the 1960s. Since that time, interest has grown, and presently they represent one of the most intriguing and synthetically useful types of N-aminoazole conversions. With the help of these reactions, one can effectively generate cycloalkynes and arynes, synthesize the difficult-to-obtain 1,2,3-and 1,2,4-triazines, /V,N -azoazoles, and other important compounds. 

The intermediacy of aryne-metal complexes in many organometallic reactions has been postulated on the basis of the structures of the reaction products. This mechanistic hypothesis received strong support in 1979 with Schrock and co-workers isolation of complex 8 and its structural characterization by X-ray crystallography [ 10]. Since then, many aryne complexes of early transition metals have been prepared, including complexes of Ti, Zr, Nb, Mo, W, Re, V, and even the actinoid elements U and Th. Figure 3 shows some representative examples of aryne complexes of early transition metals that have been characterized by X-ray diffraction studies. 

Multicomponent coupling represents a synthetically attractive approach for the assembly of complex and diverse molecules by a cascade of elementary chemical reactions. Mulhcomponent couplings involving aryne components have recently attracted considerable attention due to the extraordinary reactivity of the aryne species. 

Benzyne (28) (a representative of the short-lived reactive intermediates of the aryne type) can be trapped by Diels-Alder reachon with furan to give the cycloadduct 29, which is isomerized to a-naphthol by acid ... 

This aryne annulation is likely to proceed via electrophilic attack of benzyne at the enamide 82 ( 86) followed by ring-closure and dehydration (86 87 83), thus representing a formal (4 + 2)-cycloaddition of 82 to the aryne. 

Type A of ring closure to acridines is represented by reactions of arynes with suitable partners. Recendy, 2-aminoaryl ketones 90 with arynes generated from 91 to give acridines 92 have been developed and optimized (Scheme 32). The major by-products are the corresponding (2-(phenylamino)phenyl) ketones and result by protonation after attack of the amino group on the... 

---