Cycloheptatrienylidenes

The IR and UV spectra of the triplet cycloheptatrienylidene [71] were recorded after the UV photolysis (A>574 nm) of diazocycloheptatriene [72] in an argon matrix (McMahon and Chapman, 1986). This carbene interacts with the CO-doped matrix, forming the ketene [73], and it also dimerizes with formation of heptafulvalene [74]. Experiments have shown that [71] cannot be converted into the cycloheptatetraene [48] either photochemically... 

The four-coordinate iron(II) complex of cycloheptatrienylidene is a rare example of a fully reversible singlet (5 = 0 at 6 K) to triplet (5 = 1 at 293 K) transition in the slow relaxation regime [159]. 

Although the observed products are consistent with initial formation of 4a, Untch proposed that they might arise via the intermediacy of cyclic allene 3a.23 Subsequent experiments by W.M. Jones and co-workers supported the initial formation of 3a in the dehydrohalogenation reactions, but left open the possibility of a rapid equilibrium between cycloheptatetraene (3a) and cycloheptatrienylidene (4a, Scheme 9).22,24 It was also found that fusion of benzene or naphthalene rings to different positions on the seven-membered ring strongly influenced whether a carbenic or allenic structure predominates.25... 

The early calculations on cycloheptatrienylidene (4a) uniformly found it to be higher in energy than allene 3a. INDO calculations by W.M. Jones and co-workers predicted carbene 4a to be 14 kcal/mol less stable than allene 3a.28a Waali s MNDO calculations predicted that singlet 4a is 23 kcal/mol less stable than 3a, and that 4a serves as a transition state for the enantiomerization of 3a.28c,e The HF/4-31G ab initio calculations of Radom et al. also found both singlet and triplet states of 4a to be less stable than allene 3a.29a... 

Azacycloheptatrienylidene (4b), the aza analogue of cycloheptatrienylidene (4a), has on occasion been postulated to be the product of the ring expansion of lb.la In contrast to the case with 34a,lla 34b has never been directly detected. However, Iwamura and co-workers have reported chemical trapping of 4b and its o-phenyl derivative (4c) by tetracyanoethylene (TCNE), as shown in Scheme 15 49 While the product in Scheme 15 is formally the product of reaction of 4b with TCNE, the authors noted that it could also arise from reaction of TCNE with ketenimine 3b. 

The nature and role of cycloheptatrienylidene (4a) has been a tantalizing puzzle to chemists since the 1960s. An appreciation for the reasons why begins with a consideration of the it,it, and carbenic nonbonding molecular orbitals of 4a, depicted in Fig. 8. 

Since the five spin states for cycloheptatrienylidene include both singlet... 

Figure 8. Schematic drawings of the n, n", and carbenic molecular orbitals of planar C2, cycloheptatrienylidene (4a). The bi and 02 orbitals in each horizontal pair are not strictly degenerate, as they are in D h cydoheptatrienyl cation. In 4a a pair of electrons must be distributed between the ai, 3bi, and 2a2 MOs.

Scheme 6.102 Calculated relative energies of 1,2,4,6-cycloheptatetraene (5), the closed-shell 2,4,6-cycloheptatrienylidene (5-Z,) and the open-shell 5-D.

Another paper64, which also contains a literature survey about the problem discussed, describes the rearrangements of C7H6 systems which can be generated by thermolysis of phenyldiazomethane. By using spectral methods and chemical reactions, the formation of bicyclo[4.1.0]hepta-2,4,6-triene (139), cycloheptatrienylidene (137a) and bicyclo[3.2.0] hepta-l,3,6-triene (135) was excluded, and evidence for the formation of intermediate cycloheptatetraene (137b) (see equation 45) was furnished. 

Answer to 4(d). We can consider the orbitals of cycloheptatrienylidene to arise from the interaction of the orbitals of hexatriene and the valence orbitals of a di-coordinated carbon atom (a 2p orbital and an sp" hybrid orbital). The orbitals of hexatriene may be obtained from an SHMO calculation. The interaction diagram is shown in Figure B7.2. The p orbital of the carbene site is raised as a result of the dominant interaction with 713 of hexatriene. The orbital 714, which is closest in energy to the carbene s p orbital, does not interact because of symmetry, and 5 interacts less strongly because the coefficients at the terminal positions of the hexatriene are... 

Figure B7.2. Electronic structure of cycloheptatrienylidene from the interaction of the orbitals of hexatriene and methylene. Symmetry labels refer to the p and n orbitals and a vertical mirror.

C NMR measurements of 6-dimethylamino-6-methylfulvene, synthesized from 13C-7 labeled phenyldiazomethane via fulvenallene and ethynylcyclopentadiene, supported the proposed mechanism of the ring contraction of phenylcarbene to fulvenallene. The uniform distribution of 13C in the product fulvenallene may be explained in terms of a preequilibrium Before ring contraction, phenylcarbene, a bicyclic intermediate and cycloheptatrienylidene interconvert rapidly [406] ... 

Cyclodecyl cation hydride bridge, 147 1,4-Cycloheptadiene, 170, 171 Cycloheptatriene, 281 Cycloheptatrienes rearrangements, 290 Cycloheptatrienylidene, 275 interaction diagram, 276... 

Conversion of the alkynylstannane within 149 into the corresponding alkynyliodonium salt 150 proceeded as expected, and this fragile intermediate was treated immediately with base at low temperature (Scheme 23). TLC analysis of the crude reaction mixture indicated only a single off-baseline product, quite visible as a bright purple spot. Isolation of this compound by chromatography and characterization by standard spectral techniques led to the realization that the desired cycloheptatrienylidene product 151 had been formed in good yield. Careful examination of the crude reaction mixture s H NMR spectrum did not provide any indication that a 1,6 C-H insertion-derived... 

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