Didehydrobenzene

Other researchers have reported that the cyclization step is believed to be rate determining in the cycloaromatization (Bergman) reaction of aliphatic enediynes." It has been found that the rate-limiting step is hydrogen abstraction by benzannelation. This effect should be attributable to the faster rate of retro-Bergman cyclization from the aromatic ring-condensed 1,4-didehydrobenzene diradicals and/or the slower rate of hydrogen abstraction by them. 

Photolysis of cyclophanediones was applied to the generation of several derivatives of 13. Thus, 5-fluoro- and 5-methyl-l,3-didehydrobenzene were isolated in argon matrices and characterized by their IR spectra. The generation of 2-substi-tuted derivatives of 13 is not feasible with this method, since the corresponding cyclophanediones could not be synthesized. 

Figure 6.11 Structures of 1,3-didehydrobenzene (m-benzyne) from experiment and RHF calculations. Because of its tendency to overemphasize bonding interactions, RHF optimization results in a bicyclic structure. While the RHF error in bond length is very large, it should be noted that the bond-stretching coordinate is known to be very flat (for very detailed analyses on the sensitivity of this system to different theoretical levels, see Kraka et al. 2001 and Winkler and Sander 2001)...

Aza-enediynes are a new class of anti-neoplastic agents that produce a cytotoxic effect through an aza-Bergman cyclization (1) of an enediyne core to a 1,4-didehydrobenzene reaction intermediate. Both Bergman and benzothiazolium salt diradical cyclizations are illustrated in Eq. 1 and Eq. 2, respectively. 

Nicolaides, A. Borden, W. T. Cl calculations on didehydrobenzenes predict heats of formation for the meta and para Isomers that are substantially higher than previons experimental values, /. Am Chem. Soc. 1993, 775,11951-11957. 

Scheme 60. Photolysis of [2.2]metacyclophane-2,9-dione in Ar at 10 K yieldsp-xylylene, carbon monoxide and 1,3-didehydrobenzene [120]...

Bergman and coworkers initially studied the cyclization of enediynes more than two decades ago [5, 33], The parent compound of these systems, (Z)-hex-3-ene-l,5-diyne (100) cyclizes at elevated temperatures to afford the 1,4-didehydrobenzene diradical 6 (Fig. 8). 

Correlation of the normal modes of benzene (6) with those of 1,2-didehydrobenzene (o-benzyne, 3). 

The interconversion of the 1,2-, 1,3-, and 1,4-didehydrobenzenes has been studied. A 1,2- to 1,3-didehydrobenzene conversion has been postulated to occur during pyrolysis (900 °C) of the phenyl substituted aryne precursors as shown in Scheme 7.4. Extremely high temperatures are required for benzyne interconversion. 

Scheme 7.4 Interconversion of 1,2- and 1,3-didehydrobenzenes ortho and meta, respectively).

Although the first suggestion about the existence of o-benzyne (1,2-didehydrobenzene) (1) dates from 1927, the isotopic labeling experiments carried out by Roberts etal. in 1953 provided the first compelling experimental evidence for the intermediacy of 1 in the reaction of chlorobenzene with KNHj [2]. 

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