Bicyclo[2.2.2]octylidene

Article abstract of DOI:10.1016/S0040-4039(01)01462-9

Bicyclo[2.2.2]octylidene is formed in four quite different ways. Reactions of the precursors do not complicate the chemistry of the carbene. The products are tricyclo[2.2.2.0^2,6]octane and bicyclo[2.2.2]octene, formed in approximately a 70:30 r

Full text of DOI:10.1016/S0040-4039(01)01462-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6979–6981
Bicyclo[2.2.2]octylidene
Qing Ye,a Maitland Jones, Jr.,a,* Ting Chenb and Philip B. Shevlinb
aDepartment of Chemistry, Princeton University, Princeton, NJ 08544, USA
bDepartment of Chemistry, Auburn University, Auburn, AL 36849, USA
Received 28 June 2001; accepted 6 August 2001
Abstract—Bicyclo[2.2.2]octylidene is formed in four quite different ways. Reactions of the precursors do not complicate the
chemistry of the carbene. The products are tricyclo[2.2.2.02,6]octane and bicyclo[2.2.2]octene, formed in approximately a 70:30
ratio. © 2001 Elsevier Science Ltd. All rights reserved.
In this letter we describe the reactions of bicy-
clo[2.2.2]octylidene produced in four ways. Although
this carbene has been described only once before, and
there is reasonable doubt that the method reported
almost 40 years ago actually made the carbene,1 it is
surely legitimate to ask why we bother to reinvestigate
this seemingly straightforward bicyclic alkylcarbene.
and co-workers have cleverly used bridged bicy-
clo[2.2.1]heptylidenes to investigate the effect of the
extent of overlap of a carbonꢀhydrogen bond with the
carbenes empty 2p orbital on the ease of migration of
adjacent hydrogens.4 Very recently, Creary and
Butchko used the bicyclo[2.2.2]octane framework to
investigate the effect of spectator substituents on the
various carbonꢀhydrogen insertion reactions of several
bicyclo[2.2.2]octylidenes.5 In this latter paper, the inter-
vention of precursor chemistry was explicitly discussed
and deemed unlikely. It is our aim in this letter to put
the notion that bicycloalkylidenes are truly generated in
Nickon and Creary’s work to the test.
In Grob and Hostynek’s paper of 1963,1 it was demon-
strated that decomposition of the tosylhydrazone of
bicyclo[2.2.2]octanone in a solution of sodium in acet-
amide at 160°C (Bamford–Stevens conditions) led to
tricyclo[2.2.2.02,6]octane (1) and bicyclo[2.2.2]octene (2)
in a 70:30 ratio. A carbene was the presumed
intermediate.
Why should one be uncomfortable with the idea that
thermal or photochemical decomposition of alkyl diazo
compounds, surely the most widely used of all sources
of carbenes, might not give pure carbene chemistry? If
one takes the decompositions of dimethyl diazirine or
tert-butyl diazomethane as exemplars, it is clear that
there is a lot to worry about.6,7 In each case, much of
the chemistry attributed to carbenes is in fact the result
of rearrangement of the precursor as the nitrogen
leaves. Nor is it safe to assume that only photochemical
decompositions are bedeviled by precursor chemistry.5
There are examples of thermal reactions in which the
diazo compound leads to products on its own.8
In recent years, the intramolecular chemistry of alkyl-
carbenes has been shown to be accompanied by sub-
stantial amounts of precursor chemistry. Indeed, this
problem was recognized decades ago.2,3 Therefore, it is
of interest to uncover the real chemistry of this model
for many bicyclic carbenes; to see what the unpolluted
carbene chemistry really is. Much more important,
various bi- and polycycloalkylidenes have served as
frameworks that enforce specific geometries on
cycloalkylidenes and allow mechanistic details of car-
bene reactions to be uncovered. For example, Nickon
Na, 160 °C
O
+
N
NHTs
1, 70%
2, 30%
H3C
NH2
* Corresponding author. Fax: 609 258-2383; e-mail: mjjr@princeton.edu
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01462-9

6980
Q. Ye et al. / Tetrahedron Letters 42 (2001) 6979–6981
It is true that the dimethyl and tert-butyl systems differ
from the bicyclic molecules in question here. As noted
by Creary and Butchko,5 the acyclic precursors can
easily adopt conformations in which migration of car-
bon or hydrogen can take place as nitrogen departs,
whereas the bicyclic molecules can not. Therefore, it
does not seem that concerted migration in either a 1,2
or 1,3 fashion should be particularly favorable.
Nonetheless, rather than rely on the assumption that
carbenes are involved, we thought it worthwhile to
attempt to put the mechanistic experiments on a firmer
reactions gave the same ratio of 1:2 within experimental
error.
It would seem that both thermal and photochemical
decomposition of bicyclic tosyl hydrazone salts do give
pure carbene chemistry. The precursor chemistry that
intrudes in the acyclic examples does not appear here,
presumably because the geometry for concerted migra-
tion is unfavorable. That 1 is the major product fits well
with Creary’s calculation that the activation energy for
formation of 1 is 2.1 kcal/mol lower than that for
formation of 2.5
basis.
We
compared
the
ratio
of
tricy-
clo[2.2.2.02,6]octane (1) and bicyclo[2.2.2]octene (2)
formed from ‘bicyclo[2.2.2]octylidene’ made in four
ways, two suspect and two not.
Of course, one cannot simply dismiss the possibility of
precursor chemistry in all polycyclic molecules. When
the geometry of a polycyclic molecule is favorable, as in
homocubylidene for example,11 precursor chemistry
completely dominates carbene chemistry. Again, it is a
question of overlap; when a bond can align with the
breaking carbonꢀnitrogen bond, as in the acyclic
molecules, or is held in the correct position, as in
homocubylidene, it participates in nitrogen loss. When
For the two ‘non-suspect’ methods we take reactions of
tert-butylcarbene as a model. Both carbon atom-medi-
ated deoxygenation of the related ketone7,9 and passage
of the geminate dihalo compound through Udo
Brinker’s ‘methyllithium tube’10 led to pure reactions of
tert-butylcarbene, at least as judged by comparison to
reactions of an unimpeachable hydrocarbon carbene
source.7 The ratio of 1:2 from these two reactions was
compared to that from two ‘suspect’ sources, photolysis
and flash vacuum pyrolysis (FVP) of the tosyl hydra-
zone salt. We are happy to report that all four new
a
bond is not properly aligned, as in bicy-
clo[2.2.2]octylidene, carbene chemistry is observed.
Nitrogenous precursors should be avoided when possi-
ble in all cases in which neighboring groups can posi-
tion themselves to assist nitrogen loss.
+
Reaction Conditions
Reference
%1
%2
Na, 160 °C
1
70
30
acetamide
N
NHTs
34a
31
66a
69
FVP, 170 °C
hv, 25 °C
this work
"
"
this work
this work
C, 77 K
75
25
O
CH3Li, 65 °C
this work
70a
30a
Br
gas phase
Br
a
Average of several runs. All runs within ± 5%

Q. Ye et al. / Tetrahedron Letters 42 (2001) 6979–6981
6981
Acknowledgements
P.; Hudson, C. W. J. Org. Chem. 1977, 42, 1935 and (d)
Freeman, P. K.; Hardy, T. A.; Balyeat, J. R.; Wescott,
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examples.
This work was supported by the National Science
Foundation through grants CHE-9901068 (Auburn)
and CHE-0073373 (Princeton). Q.Y. thanks Atofina
Chemicals, Inc. for a Fellowship.
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