C O M M U N I C A T I O N S
Scheme 2. Model Systems, Transitions States, and Intermediates
Used for Computing the Energy Barrier Associated with Phenyl
Shifting onto sp3 (a) and sp2 (b) Radical Centers (BLYP/6-31G*)
impact on the structures expected for polymers produced utilizing
phenyl-terminated enediyne monomers.27 These findings are also
relevant in formulating mechanisms for processes involving sp2
radicals and phenyl rings, such as combustion chemistry and soot
formation.28
Acknowledgment. The authors thank the ACS-PRF, the Oak
Ridge Associated Universities (Ralph E. Powe Award), and the
University of Michigan for funding.
Supporting Information Available: Synthetic procedures, tables
containing yields, and coordinate files and methodology for computa-
tions. This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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of such a transformation, calculations to determine the enthalpic
barrier for the facile process of a phenyl ring shifting onto an sp3
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shift onto an sp2 center. The barrier to formation of the spirocyclic
transition state for 2-phenylethyl radical is 14.5 kcal/mol21 (Scheme
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The high inherent barrier for phenyl shifting in an aryl system
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2,3-diphenylnaphthalene-1,4-diyl (17), in the cyclization of 12
experiences strong steric repulsion between adjacent phenyl rings.
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of 17 and p-naphthyne to 2-phenylnaphthalene-1,4-diyl (16) was
carried out to derive an energy of interaction between the phenyl
rings of 6.3 kcal/mol. Another driving force is the formation of
more energetically stable diradicals. Experimental determinations22
of the enthalpic heats of formation for isomeric benzynes indicate
that the meta and ortho isomers are more stable than p-benzyne by
15.3 and 30.7 kcal/mol, respectively. Similarly, theoretical studies23
predict the enthalpies of formation of 1,3- and 2,3-naphthyne to be
lower than that of 1,4-naphthyne by 16.0 and 30.3 kcal/mol,
respectively. The formation of more stable benzynes provides an
additional driving force for the formation of phenyl-shifted products.
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ization of a triple bond to a vinylidene carbene, followed by
cyclization, could be invoked to explain the occurrence of 14 in
the thermolysis of 12. However, heating (280 °C, 7 d, benzene) a
model compound, 1-methyl-2-(phenylethynyl)benzene, did not
result in formation of the expected phenylindenes,25 thereby
excluding vinylidene carbenes from potentially accounting for the
phenyl-shifted products. Alternatively, a multistep mechanism for
“phenyl walking” has been proposed for some isomerizations under
flash vacuum pyrolysis conditions.26 However, studies on a deriva-
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that no deuterium transfer to the naphthalene ring occurred, thus
ruling out this phenyl shifting mechanism (see Supporting Informa-
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(7) Semmelhack, M. F.; Neu, T.; Foubelo, F. J. Org. Chem. 1994, 59, 5038-
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(24) The barrier to phenyl shifting through a spirocyclic transition state to a
meta diradical from 17 is 24.6 kcal/mol and lies 62.4 kcal/mol above 12.
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In conclusion, the study of particularly hindered Bergman
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