C. A. Ramsden, M. M. Shaw / Tetrahedron Letters 50 (2009) 3321–3324
3323
H
O
O
O
+
O
O
H
O
XeF
XeF
O
or
H
-
Xe + F
Xe + F
H+
O
15
24
14
O
H
O
O
O
O
O
+
H+
-
24
16
Scheme 3.
reaction in CH2Cl2/PTFE gives a high yield of 1-fluoroadamantane 3
(76%), together with 1-chloroadamantane (14%) and a trace of
diadamantyl. When increasing amounts of carbon tetrachloride
(5–20%) were added to the solvent, the yield of the chloroalkane
increased proportionally with a corresponding decrease in 1-flu-
oroadamantane and no trace of diadamantyl. These observations
are consistent with product formation via an adamantyl radical
(Scheme 2). The small amount of 1-adamantanol formed under
these conditions probably arises from a rearrangement sequence
initially catalysed by HF.
Reaction of diphenylacetic acid 4 (entry 2) with XeF2 in CH2Cl2/
PyrexÒ gave an 82% yield of di(diphenylmethyl) ether 5. The for-
mation of this product can be rationalised by rearrangement, lead-
ing to diphenylmethanol, together with formation of the relatively
stable cation. This combination of the rearrangement and decar-
boxylation pathways (Scheme 1) leads to the ether product. The
protonated alcohol may also be a source of the cation. Diphenylflu-
oromethane 6 was obtained in 85% yield when the reaction was
carried out in CH2Cl2/PTFE, again emphasising the profound influ-
ence of the vessel.
via the primary carbenium ion. Two other products that formed
in comparable yields were identified as -phenyl- -butyrolactam
15 (18%) and 4,5-dihydro-1-benzoxepin-2(3H)-one 16 (24%). The
formation of these cyclic products can be rationalised in terms of
alternative modes of reaction of the fluoroxenon ester 24 (Scheme
3), which avoid formation of the unstable primary carbenium ion.
c
c
Thus, elimination of xenon and fluoride, transfer of the c-hydrogen
(either by formation of a benzyl cation or via a three-centre bonded
species formed by direct C–H bond insertion of the electrophilic
oxygen15,16) and subsequent loss of a proton give product 15. Alter-
natively, electrophilic attack of the phenyl group (C=C bond inser-
tion by the oxygen electrophile), accompanied by loss of xenon and
fluoride, gives an arenium ion that can deprotonate to give product
16. There was no evidence of rearrangement products, which is
consistent with primary alkyl groups having poor migratory
aptitude.
Reaction of the acid 14 in PTFE gave a relatively poor yield of 3-
fluoro-1-phenylpropane 18 (32%) together with 1-phenylpropane
19 (18%). The hydrocarbon 19 is almost certainly formed via the al-
kyl radical, and this is supported by the observation of 1,6-diphe-
nylhexane (11%). Since 1,1,2,2-tetrachloroethane (14%) was also
detected, the alkyl radical is presumed to abstract hydrogen from
the solvent; no 3-chloro-1-phenylpropane was detected. Second-
ary products resulting from fluorination of the benzylic position
of products 18 and 19 were also detected. The primary alkyl substi-
tuent in the acid 14 probably disfavours pathway (b) shown in
Scheme 2, and this leads to mainly radical products. This is in
accordance with observations of Patrick and co-workers,8 and
may account for the low yield relative to the acids 1, 4, 7 and 11.
Finally, we investigated styrylacetic acid 20 (entry 6). As with
the other primary acids (entries 4 and 5), in a PyrexÒ flask this gave
a moderate yield of the corresponding ester 22 (24%) suggesting
some participation of the primary carbenium ion. The major prod-
In PyrexÒ, the reaction of the dicyclohexylacetic acid 7 (entry 3)
gave alkenes as the only identifiable products. The major product
was cyclohex-1-enylcyclohexylmethane 8 (19%) together with a
small amount of the cyclohexylmethylene isomer 9 (3%). These
E1 elimination products probably arise from initial formation of
the secondary carbenium ion (Scheme 1), which deprotonates to
give the minor product 9. Reprotonation, or rearrangement of the
initial cation, may then give a tertiary carbenium ion leading to
the isomeric alkene 8.13,14 There was no evidence of fluoroformate
or alcohol formation, but these may also be precursors of elimina-
tion products, which cannot be formed from substrates 1 and 4. As
expected, in PTFE the fluoro derivative 10 was the major product
(42%), but the alkenes 8 and 9 were also present in significant
amounts, supporting the view that carbocations can form under
these conditions (Scheme 2).
uct (45%) was the b-fluoro-c-butyrolactone 21, which was ob-
tained as a diastereomeric mixture. This probably arises from
XeF2 reacting with the double bond to give a benzyl cation that
cyclises leading to the product 21. Using XeF2 in PTFE the only sig-
nificant product was the fluoride 23 (42%) with no evidence of rad-
ical derived products.
In conclusion, we have found that whereas reaction of carbox-
ylic acids with XeF2 in CH2Cl2 and a PTFE flask consistently results
in fluorodecarboxylation (RCO2H?RF), use of a PyrexÒ flask catal-
yses alternative modes of reaction, including rearrangement and
cyclisation, which are consistent with the formation of intermedi-
ate fluoroxenon esters (Scheme 1). This behaviour is analogous to
that of other hypervalent intermediates including those formed
by (diacetoxyiodo)benzene.17 It is well known that Lewis acids
can catalyse reactions of XeF2.18 Using CH2Cl2 as solvent, a PyrexÒ
The benzylic carboxylic acid 11 (entry 4) reacted in PyrexÒ to
give a 56% yield of the ester 12. A low yield (12%) of di(4-isopro-
pyl)benzyl ether was also detected, indicating that some C–O rear-
rangement occurred to form the benzyl alcohol. The formation of
the ester 12 and the dibenzyl ether implies that a benzyl cation
intermediate probably formed via a decarboxylation mechanism
(Scheme 1), followed by rapid reaction with the precursor acid
11, or to a lesser extent with the benzyl alcohol. In PyrexÒ there
was no evidence of fluoro-products, but in a PTFE flask a good yield
of the benzyl fluoride 13 was obtained.
An interesting variation in behaviour was observed when 4-
phenylbutanoic acid 14 (entry 5) was reacted with XeF2 in
CH2Cl2/PyrexÒ. The ester 17 was formed in 20% yield presumably