An unusual, DDQ-promoted oxidative rearrangement of 9,9-disubstituted 1,4,5,8-tetrahydro-4a,8a-methanonaphthalenes

Article abstract of DOI:10.1246/cl.2002.64

The action of excess DDQ on 1,4,5,8-tetrahydro-4a,8a-methanonaphthalenes bearing a methyl group and one other substituent (cyclopropyl, methyl, phenyl) at the 9 position resulted in a peculiar oxidative rearrangement to 1-vinylnaphthalene derivatives. Two closely related mechanisms, both of which involve a 1,5-sigmatropic shift of the one-carbon bridge (Berson-Willcott rearrangement), are considered for this novel process.

Full text of DOI:10.1246/cl.2002.64

64
Chemistry Letters 2002
An Unusual, DDQ-Promoted Oxidative Rearrangement of 9,9-Disubstituted 1,4,5,8-Tetrahydro-
4a,8a-methanonaphthalenes
Jonah A. Barasz, Ali H. Ghaffari, Douglas A. Otte, and Dasan M. Thamattoorꢀ
Department of Chemistry, Colby College, Waterville, ME 04901 USA
(Received September 10, 2001; CL-010895)
The action of excess DDQ on 1,4,5,8-tetrahydro-4a,8a-
methanonaphthalenes bearing a methyl group and one other
substituent (cyclopropyl, methyl, phenyl) at the 9 position
resulted in a peculiar oxidative rearrangement to 1-vinylnaphtha-
lene derivatives. Two closely related mechanisms, both of which
involve a 1,5-sigmatropic shift of the one-carbon bridge (Berson-
Willcott rearrangement), are considered for this novel process.
More than four decades ago, Vogel announced that the
bromination of both double bonds in 1, followed by quadruple
dehydrobromination in base, led to the aromatic hydrocarbon 1,6-
methano[10]annulene (2).1 A few years following this remark-
able synthesis (Figure 1), Nelson and Untch demonstrated that the
conversion of 1 into 2 can be accomplished quite expeditiously by
Figure 2. The synthesis of 6a-c and their
DDQ-promoted rearrangement to 7a-c.
direct oxidation with 2,3-dichloro-5,6-dicyanobenzoquinone
(DDQ).2
Rearrangements by way of alkyl and aryl shifts are not
uncommon during DDQ oxidations.7 To the best of our knowl-
edge, however, the conversion of 6 into 7 appears to be a unique
process that raises intriguing mechanistic questions. One
possibility is that a Berson-Willcott rearrangement,8 exemplified
by the 1,5-sigmatropic carbon shift in equation 2, plays a key role
in the mechanism.
Figure 1. Vogel synthesis (method A) and Nelson
and Untch synthesis (method B) of 1,6-metha-
no[10]annulene.
In the course of our search for nonnitrogenous precursors to
carbenes, we became interested in the 1,6-methano[10]annulene
systems because of their known ability to extrude carbenes,3
probably via the ring-closed form 3 (equation 1). Specifically, we
sought to prepare a precursor to cyclopropylmethylcarbene (3,
R1 ¼ CH3 and R2 ¼ cyclopropyl) by oxidizing 9-cyclopropyl-9-
methyl-1,4,5,8-tetrahydro-4a,8a-methanonaphthalene (6a, R1 ¼
CH3 and R ¼ cyclopropyl) with DDQ. The synthesis of 6a was
carried out as shown in Figure 2. Unexpectedly, however, the
DDQ oxidation of 6a led to 1-(1-cyclopropylvinyl)naphthalene
(7a) in a yield of 75%.4 Puzzled by this outcome, we synthesized
6b and 6c and discovered that these two compounds also
rearranged to give the corresponding vinylnaphthalene deriva-
tives 7b and 7c when subjected to DDQ oxidation (Figure 2),
albeit in lower yields of 26% and 24% respectively. In all cases,
the by-products included naphthalene and other partially oxidized
substrates (vide infra).
Indeed, related shifts are known to occur in the methano[10]-
annulene systems. For example, a single 1,5-carbon shift occurs
in the parent hydrocarbon (3, R1 ¼ R2 ¼ H) upon flash vacuum
pyrolysis at 500 ꢁC,9 and in its 11,11-dicyanoderivative at room
temperature.10 In the latter compound, two 1,5-shifts of carbon
occur at 99 ꢁC.11 A similar ‘‘double shift’’ also accompanies the
reaction of the monoester derivative (3, R1 ¼ H, R2 ¼ CO2CH3)
under electrophilic acylation conditions.12 Given these prece-
dents, the two mechanistic routes from 6 to 7, outlined in Figure 3,
seem plausible.
Thus, in accord with what is known about DDQ
dehydrogenations,7 abstraction of a hydride ion from the allylic
position in 6 followed by deprotonation gives a mixture of
isomeric partially oxidized trienes 8. A 1,5-carbon shift in 8 leads
to 9 that can then lose a hydride ion to DDQ and open the
cyclopropyl ring to give cation 10. Orbital symmetry considera-
tions suggest that if the 1,5-shift were concerted, the exo and endo
dispositions of the two substituents on the migrating carbon (X
and Y in equation 2) will change.12 It is conceivable therefore that
hydride abstraction from 9 by DDQ might be influenced by steric
factors depending on whether the methyl group is endo or exo.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
65
Figure 4. DDQ oxidation of 14 gives the annulene 15
instead the rearranged product 17.
4). Thus it appears that two substituents are necessary at the 9
position to induce the oxidative rearrangement. It is yet to be
determined, however, whether one of the two groups is required to
be a methyl in order for the rearrangement to occur. Further
investigations into the scope and mechanism of this interesting
reaction are now underway in our laboratory as are computational
studies to model key steps in the proposed mechanisms.
We acknowledge Research Corporation and the Colby
College Division of Natural Sciences for funds, and the NSF for
NMR upgrade. We also thank Professor Maitland Jones, Jr. for
supporting portions of this work through a grant from the National
Science Foundation (Princeton, NSF CHE-9322579). DAO is
grateful to the Howard Hughes Medical Institute for a summer
research assistantship.
References and Notes
1
2
3
E. Vogel and H. D. Roth, Angew. Chem., 76, 145 (1964).
P. H. Nelson and K. G. Untch, Tetrahedron Lett., 1969, 4475.
V. Rautenstrauch, H.-J. Scholl, and E. Vogel, Angew. Chem.,
Int. Ed. Engl., 7, 288 (1968).
4
5
6
D. M. Thamattoor, Ph. D. Dissertation, Princeton University,
Princeton, NJ, U.S.A., 1997.
P. R. Spurr and D. P. G.Hamon, J. Am. Chem. Soc., 105, 4734
(1983).
a) K. Kitatani, T. Hiyama, and H. Nozaki, J. Am. Chem. Soc.,
98, 2362 (1976). b) K. Kitatani, T. Hiyama, and H. Nozaki,
Bull. Chem. Soc. Jpn., 50, 1600 (1977).
Figure 3. Two related mechanistic routes for the oxida-
tive rearrangement 6 to 7.
Finally, deprotonation of 10 followed by another DDQ dehy-
drogenation completes the sequence to afford the observed
naphthalene derivative 7 (Figure 3).
Alternatively, further oxidation of 8 gives the methano[10]-
annulene derivative 11 which undergoes a 1,5-carbon shift to
produce 12. The cation 13 could then issue from 12 by a process
that is analogous to the conversion of 9 into 10. In the last step,
deprotonation of 13 leads to 7.
7
8
9
a) P. P. Fu and R. G. Harvey, Chem. Rev., 78, 317 (1978). b)
D. Walker and J. D. Hiebert, Chem. Rev., 67, 153 (1967).
a) J. A. Berson and M. R. Willcott, J. Am. Chem. Soc., 88,
2494 (1966). b) J. A. Berson, Acc. Chem. Res., 1, 152 (1968).
E. Vogel, Special Publ. No. 21, Chem. Soc. (London), 1967,
113.
10 E. Vogel, T. Scholl, J. Lex, and G. Hohlneicher, Angew.
Chem., 94, 878 (1982).
We have also determined that the monomethyl substrate 14
gives the annulene derivative 15 without rearrangement to
vinylnaphthalene (16) when subjected to DDQ oxidation (Figure
11 E. Ciganek, J. Am. Chem. Soc., 89, 1454 (1967).
12 D. G. Barrett and S. H. Gellman, Tetrahedron Lett., 35, 2299
(1994).