Cumulene carbenes

The next lower member, oxoethenylidene C2O (93), played already an important role in the history of matrix isolation of cumulenic carbenes. Jacox et al.130 generated oxoethenylidene 93 by reaction of carbon atoms obtained from... 

Because of its carbene functionality, our decision to consider only the parent hydrocarbons would mean that cyclopropenylidene would be ignored here even if monoolefins were of relevance. This philosophy accounts for our ignoring the energetics of the isomeric carbenes, propargylene (HCCCH) and propenylidene (CH2CC), in the earlier section on cumulenes in the current chapter. 

The majority of work has involved derivatives 369 with a double-bonded group at C-5 which provide access to either the cumulenes 370 or, by further loss of CO, the carbenes 371. FVP of the acenaphthylene compound 372 at 860 °C gives the vinylidene, which rearranges to ethynylacenaphthylene 311151. The formation of cyclopentylideneketene 374 by FVP of 373 at 500 °C is complicated by simultaneous production of the isomer 377... 

Coupling of the Carbene Ligand with Cumulenes and C—X tr-Bonds 1107... 

Cumulenes can be prepared by the Doering-Moore-Skattebol method using methylene-substituted dihalocyclopropanes. In principle, such reactions can be used in succession to produce extended cumulenes however, limitations include the stability of the cumulene and the non-regioselectivity of the carbene insertion reactions. 

Evidence for the formation of azirines of the type 162 by matrix photolysis of aryl azides is also beginning to appear.2203 In some cases it can be ascertained, however, that the seven-membered ring intermediates are more stable than the fused bicyclic azirines. Furthermore, in all cases uncovered to date it turns out that the seven-membered rings, when isolable, exist as cumulenes rather than carbenes. 

Turning to carbene related reactive species, alkylidene carbenoids like 26 (X = halogen, OR, NR2) are particularly valuable for preparative purposes since they can undergo cycloaddition reactions with olefins (to methylenecyclopropanes), isomerizations (to alkynes by the so-called Fritsch-Buttenberg-Wiechell rearrangement), and dimerization (to [3]cumulenes). Although carbenoids have been studied extensively by NMR spectroscopy [23], the first X-ray structural analysis of a stable carbenoid, 27, as a TMEDA 2THF complex has been reported only recently [24]. 

Linear C species may be represented simply as shown in Fig. 1-6. For an even number of carbons, the simplest electronic structure may be either a dicarbene-cumulene structure, or a diradical-polyyne . The corresponding cyclic structures will be nonlinear and strained, but formally possess closed-shell cumulene or polyyne structures. These differ by having all bond lengths equal, or alternating bond lengths, respectively. For it = odd, the linear structures may be of the dicarbene-cumulene or tetraradical-polyyne type. The cyclic isomers may be cumulene or carbene-polyyne . 

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