Isotoluenes

Strictly speaking, compounds such as fulvenes, isotoluenes and 3,4-dimethylenecyclo-butene also qualify as semicyclic trienes. However, they will be discussed in the following section of this chapter because of their relation to aromatic hydrocarbons. 

VII. CONJUGATED SPECIES WITH EXO-METHYLENE GROUPS FULVENES, ISOTOLUENES, XYLYLENES AND RELATED SPECIES... 

The compounds in this section—fulvenes, isotoluenes, xylylenes—are characterized by trivial names. Our various reference citations document their nontrivial chemistry isomerization, polymerization and oxidation befall the unwary experimentalist who would study them. 

Although 1,2,4,6-heptatetraene (22) and its derivatives have largely been studied because of the mechanism of their thermocyclization to alkyl aromatics - in the simplest case, 22 isomerizes to toluene (333) via isotoluene (332) as an intermediate (Scheme 5.50) [8, 79, 80, 83] - other reactions of this interesting combination of conjugated and cumulated p-systems are also known. 

The intermediacy of the radical cations of isotoluenes (methylenecyclohexadienes) and their derivatives is a common feature in organic mass spectrometry however, it is widely ignored because of the rather difficult experimental access to neutral isotoluenes. Again, the reader is referred to the discussion on methylenecyclohexadienes in the 1990 review on ionized alkylbenzenes7. An early paper by Lifshitz and Bauer150 on mass spectrometry of bicyclo[3.2.0]hepta-2,6-diene, another C7H8 isomer, as well as of bicyclo[3.2.0]hept-6-ene and one of its isomers, cyclohepta-1,3-diene, may also be mentioned in this context. 

Photolysis of 1-benzyl-l-methylsilacyclobutane and 1-benzyl-l-phenylsilacyclobutane also leads to formation of isotoluene derivatives as the major primary products (Scheme 5). The primary photolysis in both cases is dominated by reactivity specific to the benzyl chromophore rather than the SCB moiety, unlike the case with other SCBs. Silene formation competes significantly in the case of 1-benzyl-l-phenylsilacyclobutane, when the role of the primary chromophore is shared between the benzyl group and the SCB ring via a second phenyl group attached to silicon. 

Photolysis of 1 -benzyl- 1-methylsilacyclobutane, which proceeds via rearrangement to an isotoluene intermediate followed by ring opening, produces l-propyl-l-methyl-2,3-benzosilacyclobutene in quantitative yield <2000CJC1459>. 

Photochemistry of the isotoluene derivative of 1-benzyl-l-phenylsilacyclobutane is initiated by preferential cleavage of the weaker of the exocyclic silacyclobutyl Si-C bonds, yielding benzyl- and phenylsilacyclobutyl radicals. 

Cycloheptatriene, Norbomadiene, Methylenecyclohexadienes (Isotoluenes) and Bicy-clof3.2.0/heptadienes. The gas-phase ion chemistry of ionized 1,3,5-cycloheptatriene is closely related to that of ionized toluene, in particular, and to that of norbomadiene and other non-aromatic ( yllx isomers. This extensive body of work will not be discussed here since a detailed review on this topic has been published by one of these authors in the context of the gas-phase chemistry of the alkylbenzene radical cations This chemistry pertains also to the well-known isomerization of the even-electron CvHv ions and to their formation from the respective parents, e.g. CyHs" " . A related, albeit chemically different held concerns protonated cycloheptatriene, i.e. the even-electron C7H9+ ions , and alkylcycloheptatrienes, which are closely related to protonated toluene and higher alkylbenzenium ions. A parallel review by one of these authors on protonated alkylbenzenes has been pubUshed, and recent investigations on protonated alkylcycloheptatrienes have highUghtened the complexity of this gas-phase ion chemistry 42 jp a minor extent, ionized and protonated fulvenes have also been investigated with respect to their interconversion to their (mainly arene-derived) isomers. 

---