J . Org. Chem. 1998, 63, 3769-3771
3769
Ch a r t 1
Tr a p p in g th e 3-Ha lobicyclo[1.1.1]p en t-1-yl
Ca tion . Mech a n istic Im p lica tion s a n d
Syn th esis of Mixed
1,3-Dih a lobicyclo[1.1.1]p en ta n es
Ian R. Milne and Dennis K. Taylor*,†
Department of Chemistry, The University of Adelaide,
Australia, 5005
Received December 16, 1997
Solvolysis of bridgehead-substituted bicyclo- and poly-
cycloalkanes generally proceed at a much reduced rate
compared to that of their open-chain analogues.1 This
has been ascribed to the increased difficulty of the system
in achieving a planar configuration about the developing
positive charge. As a result, the usual stabilizing factors,
in particular â-C-H hyperconjugation, are no longer
available to the cation. Mu¨ller and his associates dem-
onstrated this by showing that there is a linear correla-
tion between the free energy of activation of solvolysis
(∆Gq) of numerous caged substrates and the calculated
strain energy difference between the intermediate bridge-
head cation and the parent hydrocarbon (∆ES(R+-R-
H)).2 In addition, recent correlations between MP2/6-
31G* cation energies of related caged cations and the
solvolysis rates of their precursor triflates also support
the above argument.3
The 1-bromobicyclo[n.1.1]alkanes (n ) 1-3) represent
a family of bicycloalkanes which display aberrant sol-
volytic behavior. Thus, the 1-bicyclo[3.1.1]heptyl 1,
1-bicyclo[2.1.1]hexyl 2, and 1-bicyclo[1.1.1]pentyl 3 bro-
mides (Chart 1) undergo SN1 processes at extraordinary
rapid rates, the substrates 1 and 3 reacting even faster
than the classical system, tert-butyl bromide.4-6 These
three systems have been the focus of attention over recent
years. For example, 1-bromobicyclo[3.1.1]heptane and
3-methoxycarbonylbicyclo[2.1.1]hexyl triflate have been
shown unequivocally to solvolyze via the corresponding
cations 4a (R ) H) and 4b (R ) CO2CH3) and the
remarkably fast reaction has been attributed to the
enhanced stability of 4a (R ) H) and 4b (R ) CO2CH3),
which is accounted for by the phenomenon of homohy-
perconjugation as depicted in 5a (R ) H) and 5b (R )
CO2CH3).4,5
but one of the 3-R-substituted bicyclo[1.1.1]pent-1-yl
derivatives studied has remained contentious. Recently,
two groups have reported observations which strongly
suggest the intermediacy of the substituted species,
3-iodobicyclo[1.1.1]pent-1-yl cation 6. Thus, Wiberg re-
ports that treatment of 1,3-diiodobicyclo[1.1.1]pentane 7
with methanolic KOH leads to 3-methoxybicyclo-
[1.1.1]pent-1-yl iodide 8 while similar treatment of 7 in
the presence of azide ion afforded 3-iodobicyclo[1.1.1]pent-
1-yl azide (9),7 and Adcock and associates have isolated
the pyridinium salt 10 from treatment of the diiodide 7
with the weak nucleophile, pyridine.8
We report now the successful trapping by external
nucleophiles of the 3-iodo- and 3-bromobicyclo[1.1.1]pent-
1-yl cations (6 and 11) and its application to the synthesis
of several mixed dihalides which have been otherwise
difficult, if not impossible, to generate by other means.
The trapping of the 3-bromobicyclo[1.1.1]pent-1-yl cation
11 is significant as this represents the second example
of the intermediacy of a discrete 3-substituted bicyclo-
[1.1.1]pent-1-yl cation.
It is known that treatment of [1.1.1]propellane 12 with
iodine results in quantitative diiodide 7 formation. No
rearrangement products resulting from ring opening of
cation 6 with concomitant formation of cation 13 are
detectable.9 This was originally taken as evidence that
iodine adds to 12 in a radical manner;9 however, recent
studies7 clearly contradict this. To gauge the extent of
[1.1.1]propellane production in our experiments, we first
allowed an aliquot of our stock [1.1.1]propellane solution
(in ether/pentane), prepared in the usual fashion10 from
1,1-dibromo-2,2-bis(chloromethyl)cyclopropane, to react
with excess iodine. As expected, no rearranged products
were detectable and 7 was formed in 67% yield from the
propellane precursor as depicted in Table 1. Formation
of the dibromide (14) by the addition of bromine to 12 is
The formation and reactions of various 3-R-substituted
bicyclo[1.1.1]pent-1-yl cations have been the subject of
several major studies;6,7 however, the intermediacy of the
parent bicyclo[1.1.1]pent-1-yl cation 4c (R ) H) and all
† Tel: +(61 8) 8303 5494. Fax: +(61 8) 8303 4358. E-mail:
dtaylor@chemistry.adelaide.edu.au.
(1) Mu¨ller, P.; Mareda, J . In Cage Hydrocarbons; Olah, G. G. A.,
Ed.; Wiley: New York, 1990; Chapter 6. Della, E. W.; Schiesser, C. H.
In Advances in Carbocations; Coxon, J . M., Ed.; J AI Press: Greenwich,
CT, 1995; p 91.
(2) Mu¨ller, P.; Blanc, J .; Mareda, J . J . Helv. Chim. Acta 1986, 69,
635. Mu¨ller, P.; Mareda, J . J . Helv. Chim. Acta 1987, 70, 1017. Also
see: Della, E. W.; Gill, P. M. W.; Schiesser, C. H. J . Org. Chem. 1988,
53, 4354.
(3) Della, E. W.; J anowski, W. K. J . Org. Chem. 1995, 60, 7756.
(4) Della, E. W., Elsey, G. M. Aust. J . Chem, 1995, 48, 867 and
references therein.
(7) Wiberg, K. B.; McMurdie, N. J . Am. Chem. Soc. 1994, 116, 11990
and references therein.
(8) Adcock, J . L.; Gakh, A, A. Tetrahedron Lett. 1992, 33, 4878.
Adcock, J . L.; Gakh, A, A. J . Org. Chem. 1992, 57, 6206.
(9) Wiberg, K. B.; Waddell, S. T.; Laidig, K. Tetrahedron Lett. 1986,
27, 1553.
(5) Della, E. W.; J anowski, W. K. J . Chem. Soc., Chem. Commun.
1994, 1763 and references cited therein.
(6) Della, E. W.; Grob, C, A.; Taylor, D. K. J . Am. Chem. Soc. 1994,
116, 6159 and references cited therein.
(10) Belzner, J .; Bunz, U.; Semmler, K.; Szeimies, G.; Opitz, K.;
Schlu¨ter, A.-D. Chem. Ber. 1989, 122, 397.
S0022-3263(97)02276-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/08/1998