1.2.4.6- cycloheptatetraene

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.

Scheme 6.104 Methods of generation of 1,2,4,6-cycloheptatetraene (5) for its direct observation by IR spectroscopy in an argon matrix at temperatures ranging from -263 to -258 °C, according to Chapman and co-workers.

Scheme 6.107 Cycloadducts of 1,2,4,6-cycloheptatetraene (S) to styrene, phenylacetylene, (Z)-l,3-pentadiene and ( )-l,3-pentadiene.

Scheme6.n0 Structure of the hemicarcerands 547 that Warmuth and Marvel used to prepare isolated molecules of 1,2,4,6-cycloheptatetraene (5) and its 5-methyl derivative (10) to make them observable at room temperature and to study their reactions with hydrogen chloride and triplet oxygen.

Scheme 6.112 The three methyl-1,2,4,6-cycloheptatetraenes (10,553,554) as key intermediates in the transformations of the tolylcarbenes in the gas phase.

Scheme 6.116 Substituted 1,2,4,6-cycloheptatetraenes that were observed or assumed in addition to those in Schemes 6.111—6.115.

Transition-metal complexes of the more highly unsaturated seven-mem-bered allene 1,2,4,6-cycloheptatetraene have been isolated in two forms depending on the metal and its ligands an allene form (329), which is a complex of the parent cycloheptatetraene 327,134 and a carbene form (330/331), which is a complex of the controversial cycloheptatrienylidene 328.4135136 The carbene form corresponds to the allyl cation (320) that was suggested as an intermediate/transition state for fluxionality of 318. Scheme 42 lists all such complexes that have been prepared to date. The allene form is the ground state for all Pt(0) complexes (343, 344, 345, 347)82,83,130 137 138 and one dibenzannelated W(II) complex139 (346), whereas the carbene form is the ground state for all Fe(II)+ complexes (332-336),140-143 all Ru(II)+ complexes (337-340),144 one Pt(II)+ complex (342),137 and one W(0) complex (341).139... 

The rearrangement of phenylcarbene (59) to 1,2,4,6-cycloheptatetraene (60) has been extensively studied (Scheme 12).158 Recently, Warmuth has also reported the generation of carbene 59 within a HC.159 Encapsulated allene 60 was protected from dimerization, so even the H 2-D ROESY NMR spectrum of 60 HC could be measured. 

The gas phase rearrangements of phenyl carbene, particularly the conversion to cycloheptatrienylidene, which is actually 1,2,4,6-cycloheptatetraene, and probably involving bicyclo[4.1.0]hepta-2,4-6(7)-triene, are particularly fascinating (Scheme 8.6). ... 

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