Progress toward the synthesis and characterization of 1,4-dehydroaromatic systems. 1,4-dehydrobenzene (para-benzene or butalene). 1,4-dehydrotropyl cation, anion and the bicyclo [3.2.0]heptatrien-3-one

Citation

Jones, Richard R. (1976) Progress toward the synthesis and characterization of 1,4-dehydroaromatic systems. 1,4-dehydrobenzene (para-benzene or butalene). 1,4-dehydrotropyl cation, anion and the bicyclo [3.2.0]heptatrien-3-one. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/QQ2T-5185. https://resolver.caltech.edu/CaltechETD:etd-04142004-135323

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. This thesis concerns the attempted synthesis of two strained nonbenzenoid, conjugated bicyclic systems, 1,4-dehydrobenzene (butalene or para-benzyne, 28) and the 1,4-dehydrotropylium ion (30c). Both 28a and 30c contain a fused cyclobutadiene ring with [...] its accompanying antiaromaticity but overall possess 4n+2 [pi] electrons. The concept of nonbenzenoid aromaticity as it applies to compounds such as these is reviewed. The synthetic approach to 28 and 30 systems is based on the pyrolytic cyclization of a 1,5-hexadiyne moiety to form a dimethylenecyclobutene, thereby introducing the strain and unsaturation of the fused four-membered ring in one step.

In Part I, the synthesis and pyrolysis of cis-1,6-dideutero-1,5-hexadiyn-3-ene (cis-60a) is described. By a combination of nmr and ms techniques, it is demonstrated that cis-60a undergoes a degenerate rearrangement at 200?C in the gas phase at atmospheric pressure, interconverting cis-60a with its 3,4-dideuteroisomer cis-60b with an equilibration half-life of 30 sec. None of the single exchange isomer cis-60c or any trans-60 was formed in this pyrolysis. These data require a transition state or intermediate between cis-60a and cis-60b with [...] symmetry, i. e., [...] 1,4-dehydrobenzene. Similarly prepared trans-60a did not undergo isomerization or rearrangement at these temperatures.

Pyrolysis of cis- and trans-60 in solution provided a high yield of benzene in the presence of hydrogen donors at the expense of cis-60. In carbon tetrachloride, para-dichlorobenzene was the major product. In methanol, a 10% yield of benzyl alcohol but no anisole was formed. Since these are typical products of radical abstraction, it is suggested that the symmetrical species implicated in the deuterium scrambling is a true intermediate and of 1,4-benzenediyl structure 28d or e (instead of 28a). [...].

While traces of benzene were seen in the atmospheric and vacuum pyrolysis of cis- and trans-60, no satisfactory gas phase trapping experiment has been performed. Neither have substantial levels of deuterium scrambling been demonstrated in liquid phase pyrolysis. These points together with some indication of the stereospecificity of the benzene forming reaction in solution remain to be answered. The added complication of a possible triplet ground state for the 1,4-benzenediyl (28d) must also be resolved before a truly consistent story can be told. A possible direct route to a triplet 28d, photosensitized irradiation of cis-60a, failed to produce scrambling in competition with a demonstrated approach to the photostationary state. No transfer of acetylenic label was observed also under direct (254 nm) irradiation of cis-60a.

In Part II, toward formation of the 1,4-dehydrotropylium ion (30c) as well as the 1,4-dehydrotropyl anion (30a) and the bicyclo [3.2.0]heptatrien-3-one (52), the synthesis of [...] bicyclo[3.2.0]hepta-1,4,6-trienes substituted in the 3-position is discussed. The required 3-substituted 1,2-diethynylcyclopropanes were approached via carbene addition to hexadiynenes, eneynes, and via y-elimination routes. Of these, only the addition of propargyl carbene to methoxybuten-3-yne (150) provided any cyclopropane product. The 3-methoxy-1,2-diethynylcyclopropane (61d) [...] prepared in < 1% isolated yield by this method readily isomerized to the air sensitive 3-methoxybicyclo[3.2.0]hepta-1,4,6-triene (62d) upon pyrolysis under atmospheric pressure flow conditions at 305?C (contact time 10 sec). Attempted formation of 30a by ether cleavage reduction of 62d with potassium led only to polymer without any evidence for formation of an anion. However, treatment of 62d with SbF5 containing a catalytic amount of HOSO2F in a soft matrix at -180?C gave evidence in the nmr of formation of 30c although at such low concentrations that a clear spectrum was not obtained. This interesting result is currently under reinvestigation.

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