Carbene-carbene rearrangements

One hundred years of benzotropone chemistry

“…Benzo[7]annulenylidenes 73 – 75 and their rearrangements have attracted much interest due to their thermal and photochemical transformations ( Fig. 5 ) [ 74 – 76 ]. For the first time, Jones reported the chemical trapping of thermal and photochemical decomposition of the tosylhydrazone sodium salt of 4,5-benzotropone ( 11 ) and defined carbene–carbene rearrangements of 77 – 79 before finally it was verified by trapping of unstable intermediates 77 – 79 ( Scheme 14 ) [ 77 ].…”

The Fulvalenes

“…The dimerisation of cycloheptatrienylidenes (tropylidenes) to heptafulvalene has been recognised for many years with Jones and Ennis isolating up to 50 % of 7 from photolysis of the sodium salt of tropone tosylhydrazone. [194] Subsequent studies, [195] the inclusion of phenylcarbene within the C 7 H 6 manifold, [196] and recognition that cycloheptate-traene is an energetically viable valence bond isomer [197] served to delineate the reactivity of C 7 H 6 and demonstrate the involvement of substituents at the 1-and 1,6-positions. Thus, dehydrochlorination of 2-chloro-1,3-dimethylcycloheptatriene at 100°C gives 147 in 92 % yield.…”

1‐Phenyl‐1,2,4,6‐cycloheptatetraene: the missing intermediate of the diphenylcarbene to fluorene rearrangement

“…[1][2][3] Trapping experiments suggested an equilibrium between 1 and cycloheptatrienylidene 4, the latter dimerizing to heptafulvene. [3][4][5] Using matrix isolation spectroscopy, Chapman and coworkers demonstrated that carbene 1 is highly photolabile and in a reversibly rearranges to allene 2. [6,7] The strained cyclopropene 3 was not observed in these experiments and cycloheptatrienylidene 4 could also be excluded.…”

“…Gas phase pyrolysis of diphenyldiazomethane 6 produces fluorene 7 in 25-30% yield. [4] Conventional photolysis of 6 in solution yields only trapping products of 5 such as tetraphenylethene, but no products can be linked to rearrangements of 5. In contrast, high intensity photolysis using a pulsed excimer laser in solution [23] or in solvent drops [24] produces fluorene 7 among other products.…”

“…The rearrangement of phenylcarbene 1 to cycloheptatetraene 2 is a prototypical carbene rearrangement and has been investigated in great experimental and theoretical detail (Scheme ) . Trapping experiments suggested an equilibrium between 1 and cycloheptatrienylidene 4 , the latter dimerizing to heptafulvene . Using matrix isolation spectroscopy, Chapman and coworkers demonstrated that carbene 1 is highly photolabile and in a reversibly rearranges to allene 2 .…”