Unexpected interconnection of the 7-norbornenyl and 3-nortricyclcyl/5- norbornen-2-yl cations

Article abstract of DOI:10.1016/j.tetlet.2004.04.134

Fragmentation of syn-7-norbornenyloxychlorocarbene provides products derived from both 7-norbornenyl cation-chloride anion pairs and from 3-nortricyclyl/2-norbornen-5-yl cation-chloride anion pairs. A 5,7-hydride shift within the 7-norbornenyl cation is proposed to interconnect the two cation systems.

Full text of DOI:10.1016/j.tetlet.2004.04.134

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 5321–5324
Unexpected interconnection of the 7-norbornenyl and
3-nortricyclcyl/5-norbornen-2-yl cations
Robert A. Moss* and Xiaolin Fu
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, USA
Received 26 March 2004; accepted 19 April 2004
Abstract—Fragmentation of syn-7-norbornenyloxychlorocarbene provides products derived fromboth 7-norbornenyl cation-
chloride anion pairs and from3-nortricyclyl/2-norbornen-5-yl cation-chloride anion pairs. A 5,7-hydride shift within the 7-nor-
bornenyl cation is proposed to interconnect the two cation systems.
ꢀ 2004 Elsevier Ltd. All rights reserved.
The 7-norbornenyl cation 1 is the prime exemplar of
homoallylic delocalization in organic chemistry.1;2 Its
formation via acetolysis of the anti-tosylate 2 is ꢀ1011
times faster than the acetolysis of (saturated) 7-nor-
bornyl tosylate, and the anti-7-norbornenyl acetate (3)
reaction product is formed stereospecifically by the
capture of 1.1;2
more slowly than 3 is formed from 2.3 Not only tosylate
solvolyses, but also amine deaminations follow this
scenario: the aqueous HNO2/HOAc deamination of
anti-7-norbornenylamine gives only 3 and anti-7-nor-
bornenol, whereas deamination of syn-7-norbornenyl-
amine affords only 5 and the corresponding alcohol (as
an epimeric mixture).4
Apparently unrelated are the solvolyses of 3-nortricyclyl
brosylate (6a) and exo-norbornenyl brosylate (7a) in
MeOH, HOAc, or HCOOH, which afford mixtures of
6b and 7b, 6c and 7c, and 6d and 7d, containing >90% of
nortricyclyl products.5;6 Similarly, diazonium ions 6e
and 7e decompose in water to alcohols 6f and 7f, with 6f
strongly dominant.7 These results can be attributed to
the intermediacy of the 3-nortricyclyl and 2-norbornen-
5-yl cations, 8 and 9, which can be regarded as canonical
forms of a resonance hybrid whose structure (in super
acid solution) closely resembles 8.8
H
H
OTs
OAc
+
1
3
2
H
OTs
OAc
4
5
X
+
X
When the double bond is not in position to assist the
tosylate’s departure, as in the acetolysis of syn-tosylate,
4, the reaction takes a very different course. Now,
anchimeric assistance involves an anti r bond, 1,2-car-
bon migration is concerted with tosylate loss, and the
product is rearranged acetate 5, formed from 4 107 times
+
6
8
9
7
a, X=OBs; b, X=OMe; c, X=OAc;
d, X=OOCH; e, X=N2+; f, X=OH
We have undertaken an extensive study of carbene
fragmentation, as nonsolvolytic entry to notable carbo-
cations of organic chemistry; cf., Eq. 1.9 Studies of the
2-norbornyl10 and 3-bicyclo[3.1.0]hexyl11 cations, for
* Corresponding author. Tel.: +1-732-445-2606; fax: +1-732-445-5312;
e-mail: moss@rutchem.rutgers.edu
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.04.134

5322
R. A. Moss, X. Fu / Tetrahedron Letters 45 (2004) 5321–5324
example, gave insight into the cation/anion/solvent
equilibration of short-lived ion pairs.
Conversion of diazirine 12 to chloride 15 via carbene 14
was quite clean; about 3% of dichloride (ROCHCl2) and
4% of formate (ROOCH), trapping products20 of 14 by
HCl or H2O, respectively, accompanied 93% of 15.
Photolyses of 12 in cyclohexane-d12 or CD3CN gave
79% or 86% of 15, respectively, with the balance as
dichloride and formate.
ROCCl ! ½R OC ClÀŠ ! RCl þ CO
ð1Þ
þ
€
Here, we report the existence of a ‘wormhole’ connect-
ing the 7-norbornenyl and 3-nortricyclyl/2-norbornen-5-
yl cation manifolds. This unanticipated nexus can be
accessed by the fragmentation of syn-7-norbornenyl-
oxychlorocarbene.
Fragmentation of 14 in the presence of excess
Bu4NþClÀin dichloroethane led only to anti-chloride 15;
there was no evidence for the formation of the syn-
chloride isomer by a SN2 reaction21 at C-7. Thus, the
fragmentation of anti-carbene 14 to chloride 15 is ste-
reospecific, joining the acetolysis of tosylate 2,1 and the
deamination of anti-7-norbornenylamine,4 as reactions,
which funnel through the delocalized 7-norbornenyl
cation, 1.
anti-7-Norbornenol was prepared in two steps from
norbornadiene by literature methods,12–14 while syn-7-
norbornenol was obtained fromnorbornene in two steps
by the method of Baird.15 These alcohols were converted
to anti- and syn-isouroniumsalts 10 and 11 by reaction
with cyanamide and methanesulfonic acid.16 The salts
were obtained as oils admixed with urea. They were not
purified, but directly converted to diazirines 12 and 13
by oxidation with NaOCl.17 Both diazirines were
unstable on silica, but could be purified by rapid chro-
matography on short, neutral alumina columns, eluted
with very cold pentane. The diazirines were character-
ized by 1H and 13C NMR,18 as well as IR and UV
spectroscopy. In particular, there were characteristic17
N@N bands in their IR (1540 cmÀ1) and UV (349 nm,
pentane) spectra.
The photolysis of syn-diazirine 13 in CDCl3, however, is
considerably more complicated, affording 4-chlorobicy-
clo[3.2.0]hept-2-ene (17), anti-7-chloronorbornene (15),
3-chloronortricyclene (6-Cl), and exo-5-chloro-2-nor-
bornene (7-Cl); cf., Scheme 2. Not shown in the scheme
are the dichloride (ROCHCl2) and formate (ROOCH)
trapping products of carbene 16, which account for
ꢀ22% and ꢀ4%, respectively, of the product mixture.22
1
Product identities were established by GC–MS and H
and 13C NMR comparisons (including spiking experi-
ments) with authentic materials. Chloride 1723 was
prepared by reaction of anti-bicyclo[3.2.0]hept-2-en-4-
ol3 with SOCl2 and pyridine in ether. anti-7-Chloro-
norbornene (15) is described above.19 Nortricyclcyl
+
++
NH2
NH2
OCNH2, OMs-
OMs--
H
H
OCNH2,
24
chloride (6-Cl) was prepared fromnortricyclanol and
SOCl2 in THF.25 exo-5-Chloro-2-norbornene (7-Cl) was
made from exo-5-norbornen-2-ol26 and SOCl2 in pyr-
idine/ether.27 Importantly, the key vinyl and C–Cl re-
10
11
1
gions of the H and 13C NMR spectra of the product
mixtures from carbene 16 did not show signals (<5%)
that could be attributed to syn-7-chloronorbornene (18).
By capillary GC, the normalized product distribution in
Scheme 2 was 17 (74%), 15 (3.6%), 6-Cl (8.6 %) and 7-Cl
(13.7%).
N
N
N
N
H
H
O
O
Cl
Cl
13
12
In that bicyclic chloride 17 is the major product from
syn-oxychlorocarbene 16, the fragmentation reaction
follows the pattern established in the acetolysis of syn-
tosylate 4,3 and the deamination of syn-7-aminonor-
bornene:4 1,2-alkyl migration anti to the leaving group is
the dominant process in each case. In contrast to the
earlier reactions, however, 1,2-alkyl migration (to 17) is
not the exclusive process, in the fragmentation of 16:
appreciable quantities of nortricylclyl chloride 6-Cl and
5-norbornenyl chloride 7-Cl are also formed.
Photolysis of anti-diazirine 12 in CDCl3 at 350 nmand
25 ꢁC gave anti-7-norbornenyl chloride (15), most rea-
sonably understood as arising via the fragmentation of
carbene 14 to (ion pair) 1, followed by chloride capture;
see Scheme 1. The identity of 15 followed fromits NMR
spectra, and GC and NMR spiking experiments with
authentic 15 prepared from anti-7-norbornenol and
SOCl2 in ether.19
..
H
H
C
OCCl
Cl-
l
+
hν
-CO
12
1
15
14
Scheme 1.

R. A. Moss, X. Fu / Tetrahedron Letters 45 (2004) 5321–5324
5323
Cl
17
..
OCCl
H
H
H
Cl
Cl
hν
13
16
15
18
Cl
+
Cl
6-Cl
7-Cl
Scheme 2.
H
OC
+
Cl-
+
+
19
20
9
This unprecedented outcome must, in part, reflect the
low activation energy required for the fragmentation of
carbene 16. Laser flash photolysis21 of diazirines 12 and
13 in dichloroethane, with pyridine ylide visualization
of the carbene,28 gave20 kfragð14Þ ¼ 1:7ðꢁ0:3Þ ꢂ 105 sÀ1
and kfragð16Þ ¼ 1:5 ꢁ 0:2 ꢂ 106 sÀ1. For syn-carbene 16,
an Arrhenius study from )30 to 30 ꢁC gave Ea ¼
deamination of syn-7-aminonorbornene to give nortri-
cyclyl and 5-norbornenyl products (which it does not).30
In summary, fragmentation of syn-7-norbornenyloxy-
chlorocarbene provides products derived fromboth
7-norbornenyl cation-chloride anion pairs and (sur-
prisingly) from3-nortricyclyl/2-norbornen-5-yl cation-
chloride anion pairs. A 5,7-hydride shift within the 7-
norbornenyl cation is postulated to interconnect the two
cation systems.
7:4 kcal molÀ1 29
.
The low Ea required for the fragmentation of 16 obviates
the need for extensive anchimeric assistance from a
migrating ethano carbon, as occurs in the acetolysis of 4.
The carbene fragmentation is ‘disconnected’ from alkyl
group migration, and other processes can compete. A
similar disconnection occurs in the fragmentation of
3-bicyclo[3.1.0]hexyloxychlorocarbene.11
Acknowledgements
We are grateful to Professor Ronald R. Sauers for
helpful discussions and to the National Science Foun-
dation for financial support.
How does carbene 16 access the products of Scheme 2?
We suggest that fragmentation of 16 initially affords
short-lived ion pair 19, fromwhich 1,2-ethano migration
affords the bicyclo[3.2.0]hept-2-ene-4-yl cation, 20, and
then chloride 17 by collapse with a chloride counterion.
A small quantity of 19 evidently also undergoes intra-
ion pair reorganization, ultimately leading to anti-7-
norbornenyl chloride, 15.
References and notes
1. (a) Winstein, S.; Shavatsky, M.; Norton, C.; Woodward,
R. B. J. Am. Chem. Soc. 1955, 77, 4183; (b) Winstein, S.;
Shavatsky, M. J. Am. Chem. Soc. 1956, 78, 592.
2. For a discussion and leading references, see: March, J.
Advanced Organic Chemistry; 4th ed.; Wiley: New York,
1992, p 314f.
3. Winstein, S.; Stafford, E. T. J. Am. Chem. Soc. 1957, 79,
505.
4. Tanida, H.; Tsuji, T.; Irie, T. J. Org. Chem. 1966, 31, 3941.
5. Cristol, S. J.; Seifert, W. K.; Johnson, D. W.; Jurale, J. B.
J. Am. Chem. Soc. 1962, 84, 3918.
Further in Scheme 2, we note that the simplest con-
nection between the 7-norbornenyl cation of 19 and the
2-norbornen-5-yl cation (9) involves a 5,7-hydride shift,
which would directly transform 19 into 9. Chloride
collapse with 9 then affords 7-Cl, while 8 also provides
nortricyclyl chloride, 6-Cl. The postulated hydride shift
that converts 19 to 8/9 must compete with the ethano
shift that transforms 19 to 20. We suspect that
the chloride anion of 19 plays an important part in the
19 fi 9 conversion; otherwise we would also expect the
6. Roberts, J. D.; Lee, C. C.; Saunders, W. H., Jr. J. Am.
Chem. Soc. 1955, 77, 3034.
€
7. Kirmse, W.; Knopfel, N. J. Am. Chem. Soc. 1976, 98,
4672.

5324
R. A. Moss, X. Fu / Tetrahedron Letters 45 (2004) 5321–5324
8. Jarret, R. M.; Veniero, J. C.; Byrne, T. P.; Saunders, M.;
Laidig, K. E. J. Am. Chem. Soc. 1988, 110, 8287, and
references cited therein.
9. Moss, R. A. Acc. Chem. Res. 1999, 32, 969.
10. Moss, R. A.; Zheng, F.; Sauers, R. R.; Toscano, J. P.
J. Am. Chem. Soc. 2001, 123, 8109.
20. Moss, R. A.; Ge, C.-S.; Maksimovic, L. J. Am. Chem.
Soc. 1996, 118, 9792. Traces of water are responsible
for carbene trapping, with the ultimate formation of for-
mates.
21. Moss, R. A.; Johnson, L. A.; Merrer, D. C.; Lee, G. E., Jr.
J. Am. Chem. Soc. 1999, 121, 5940.
11. Moss, R. A.; Fu, X. Org. Lett. 2004, 6, 981.
12. Weiss, R. G.; Snyder, E. I. J. Org. Chem. 1970, 35, 1627.
13. Tanida, H.; Tsuji, T. J. Org. Chem. 1964, 29, 849.
14. Franzus, B.; Snyder, E. I. J. Am. Chem. Soc. 1965, 87, 3423.
15. Baird, W. C., Jr. J. Org. Chem. 1966, 31, 2411.
16. Moss, R. A.; Kaczmarczyk, G. M.; Johnson, L. A. Synth.
Commun. 2000, 30, 3233.
22. The dichloride is suppressed when the reaction is run in
the presence of pyridine, which scavenges HCl.
1
23. Key resonances (d, CDCl3): H NMR: 6.10, 5.97 (m, 2H,
vinyl H), 4.74 (m, 1H, H–Cl–Cl). 13C NMR: 139.4, 131.2
(vinyl C), 69.8 (C–Cl).
24. Brown, H. C.; Peters, E. N. J. Am. Chem. Soc. 1975, 97,
1927.
25. Trecker, D. J.; Henry, J. P. J. Am. Chem. Soc. 1963, 85,
17. Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396.
18. Diazirine 12: 1H NMR (d, CDCl3): 6.01 (q, J ¼ 4 Hz,
2H, vinyl H), 3.92 (s, 1H, H–C–O), 2.82 (m, 2H, bridge-
1
3204, Key resonances (d, CDCl3): H NMR: 3.89 (s, 1H,
H–C–Cl). 13C NMR: 65.8 (C–Cl).
head H), 1.57 and 1.00 (m’s, 2H each, C-5 and C-6 H). 13
C
26. Posner, G. H.; Ting, J.-S.; Lentz, C. M. Tetrahedron 1976,
32, 2281.
NMR (d, CDCl3): 133.6, 86.3, 68.2, 43.8, 21.3. Diazirine
13: 1H NMR (d, CDCl3): 5.93 (m, 2H, vinyl H), 4.10 (s, 1H,
H–C–O), 3.03 (m, 2H, bridgehead H), 1.77 and 1.04 (m’s,
2H each, C-5 and C-6 H). 13C NMR (d, CDCl3): 131.0,
92.0, 68.2, 44.7, 21.6. The diazirine carbon resonances at
68.2 ppmare very weak.
27. For 1H NMR, see: Steele, W. C.; Jennings, B. H.; Botyos,
G. L.; Dudek, G. O. J. Org. Chem. 1965, 30, 2886, Key 13
C
NMR resonances (d, CDCl3): 140.8, 133.7 (vinyl C), 58.9
(C–Cl).
28. Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu, M. T.
H. J. Am. Chem. Soc. 1988, 110, 5595.
29. We were unable to complete an Arrhenius study of
carbene 14 due to weak carbene–pyridine ylide signals.
30. Ea for the decomposition of the syn-7-norbornenyldi-
azoniumion is likely to be quite low.
19. Tanida, H.; Hata, Y. J. Org. Chem. 1965, 30, 977, Key
NMR resonances (d, CDCl3): 1H: 6.07 (dd, J ¼ 2:0,
1.2 Hz, 2H, vinyl H), 3.70 (d, J ¼ 1:2 Hz, 1H, H–C–Cl),
2.75 (m, 2H, bridgehead H). 13C: 135.3 (vinyl C), 68.1 (C–
Cl), 47.0 (C-l, C-4), 21.3 (C-5, C-6).