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Mechanistic and Preparative Aspects of Vinyl Cation Chemistry.

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Mechanistic and Preparative Aspects of Vinyl Cation Chemistry
By Michael Hanack"]
Compelling evidence for the intermediacy of vinyl cations has accumulated in recent years.
Vinyl cations can be generated by electrophilic additions to alkynes and allenes, and on participation of such functions in solvolysis reactions. Heterolysis of vinyl compounds likewise affords
vinyl cations. Syntheses of substituted indenes, cyclobutanones, and cyclopropyl ketones illustrate
preparative applications.
1. Introduction
Few areas of organic chemistry have been studied so intensively over a period of 75 years as has the chemistry of carbenium ions. The term carbenium ion was long applied only
to those intermediates in which the positively charged carbon
atom is linked to three substituents (trisubstituted or saturated
carbenium ions) ( I ) .
Only comparatively recently have disubstituted carbenium
ions, of which the vinyl cation (2) is an important representative, been studied in detail; thus in 1969 W M . Jones was
at last able to write: "Vinyl cations have finally become acceptable members of the reactive intermediate community"['].
Apart from vinyl cations, acylium (3) and nitrilium ions
( 4 ) also number among the disubstituted carbenium ions.
A further disubstituted carbenium ion, i. e. the phenyl cation
( 5 ) , has so far been unequivocally detected in only few reactions.
to the same mechanism, vinyl cations were formulated only
reluctantly as intermediates in electrophilic additions.
The chemistry of vinyl cations has now come very much
to the fore. This is partly a result of more accurate mechanistic
studies on electrophilic additions to acetylenes and allenes.
However, the observation that vinyl cations can also be
generated under certain conditions by simple solvolysis reactions led to a breakthrough in the chemistry of these intermediates. Subsequent developments in vinyl cation chemistry
also led to interesting preparative applications.
Direct detection of vinyl cations by 'H- and I3C-NMR
spectroscopy has also been attemptedl4I.
Vinyl cations occur as intermediates in electrophilic additions and in solvolysis reactions which proceed with neighboring group participation. These include electrophilic additions
to CC triple bonds [eq. (a)],
E\ C=?-R'-
+ Nue
E@= e.g. H@, c a r b e n i u m ions; Nu@ = e.g. halide
solvolysis reactions involving these bonds [eq. (b)],
The monosubstituted ethynyl cation (6) corresponds to
the disubstituted vinyl cation ( 2 ) . Calculations have shown
(6) to have a particularly high energy; however, experimental
evidence has yet to be obtained for its existencer2,31.
Textbooks often state that vinyl cations (2) occur only
rarely as intermediates owing to their low stability, but recent
studies have revealed a large number of reactions proceeding
via vinyl cations. In some reactions, vinyl cations qualitatively
resemble saturated carbenium ions.
One of the reasons for rejecting vinyl cations as intermediates was considered to be the modest reactivity of vinyl
halides in solvolysis reactions, i. e. it appeared unlikely that
these species could react via an SN1 mechanism. Although
it was generally accepted that, e. g. electrophilic additions
to a double and a triple bond should proceed according
R = H, Alkyl, Aryl; Xo = Halide, Sulfonate
electrophilic additions to allenes [eq. (c)],
+ E@
and solvolysis reactions involving this group [eq. (d)].
Heterolysis of vinyl compounds [eq. (e)] likewise leads to
vinyl cations. Among the special reactions, mention should
be made of photochemical processes.
[*] Prof. Dr. M. Hanack
Institut fur Organische Chemie der Universitat
Lehrstuhl fur Organische Chemie I1
Auf der Morgenstelle 18, D-7400Tiibingen
Angew. Chem. Int. Ed. Engl. 17,333-341 (1978)
In the present article, examplesofthese reactions for generating vinyl cations will be considered from a present-day
mechanistic vantage point and preparative applications will
be described. Comprehensive surveys of vinyl cation chemistry
have been published['], and reviews have appeared on such
detailed aspects as the chemistry of stabilized vinyl cations[61
or solvolyses of a-arylvinyl derivatives['!
2. Electrophilic Additions and Solvolyses Proceeding
with Neighboring Group Participation
2.1. Electrophilic Additions to CC Triple Bonds
E m= e.g. H@, carbenium ions; Nu' = e.g. halide
Like electrophilic additions to CC double bonds, additions
to CC triple bonds can also proceed stepwise, as shown in
eq. (a) (AdE2mechanism). The linear vinyl cation (7) formed
transiently in the slow step is attacked from both sides
by nucleophile Nue, yielding the products in a non-stereospecific reaction. This simple addition scheme, however, is of
limited validity and applies only to simple acetylenes[*, "I.
Subsequent studies have shown that the direction of nucleophilic attack on the vinyl cation (7) depends upon the relative
size and upon the electronic character of the P-substituents
R and EI'o-'21. Ow'ing to ion-pair formation on addition,
the cis isomer can predominate. The intermediate cation is
not necessarily an open vinyl cation. Depending upon the
structure of the acetylene and the attacking electrophile, an
"onium" cation ( 8 ) (o bridged) or a n-bridged cation ( 9 )
may also act as intermediate. Attack of ( 9 ) by nucleophile
would preferentially yield the trans product.-Electrophilic
addition to acetylenes may also follow an AdE3 mechanism,
instead of the AdE2mechanism.
The most thoroughly studied electrophile is the proton;
however, other species such as carbenium ions may also be
involved. Among all electrophilic additions to alkynes, addition of He shows the greatest variation; its mechanism is
a function of the reaction conditions. In the case of highly
polar, strongly acidic, but only weakly nucleophilic acids such
as trifluoroacetic acid, a vinyl cation is formed preferentially
as intermediate; with less polar and more strongly nucleophilic
reactants, e. g. acetic acid, other addition mechanisms prevailt5'J.Arylacetylenes can react via different mechanisms from
alkylacetylenes.Some examples are presented in Section 2.1.1.
2.1.1. Addition of Acids to Aryl- and Alkyl-acetylenes
Acid-catalyzed hydration of variously substituted arylacetylenes ( 1 0) gives the vinyl cation ( 1 1 ) as intermediate stabilized
by the a-aryl substituent (Markovnikov ~rientation)['~!Aryl
methyl ketones (12) are obtained quantitatively as the sole
products. Kinetic studies, especially the large solvent isotope
effects measured, demonstrate protonation of the triple bond
in the rate-determining step.
X = H, m- and p - halogen, m - and p-CH,, CH,O
Phenylacetylene reacts with HCI in glacial acetic acid at
25°C to give a-chlorostyrene and acetophenone in the ratio
of 12 : 1. Detailed mechanistic studies showed that under these
conditions the addition proceeds exclusively by an AdE2
mechanism, i. e. via a vinyl cation['4! Under different reaction
conditions, the effect of solvent and added catalysts can cause
the AdE3mechanism to predominate. Thus, for instance, addition of HCl to deuterated phenylacetylene in CH2C12 with
ZnCI2 as catalyst gives mainly syn addition products[", "I.
The stabilizing effect of a-substituents on the intermediate
vinyl cation is apparent from the acid-catalyzed hydration
Fc-C SC-Ph
of ferrocenyl-phenylacetylene ( 1 3)[16]. Benzyl ferrocenyl
ketone ( 1 5 ) is formed quantitatively, thus demonstrating the
greater stability of the a-ferrocenylvinylcation (1 4 ) as against
the analogousa-phenylvinyl cation. Accordingly, the acid-catalyzed hydration of (16) is lo5 times faster than that of (17);
protonation of the triple bond is the rate-determining step
of this reaction.
Detailed studies on the kinetics and products of addition
of carboxylic acids, especially trifluoroacetic acid, to alkylacetylenes convincingly demonstrate the intermediacy of vinyl
cations in these reactions. Thus 3-hexyne (18a) reacts nonstereospecifically with trifluoroacetic acid to form the vinyl
trifluoroacetates (2)-(19a) and (E)-(19a),in the approximate
ratio of 1 :1, together with other product^^'^! This result,
along with specific substituent effects, was considered to constitute proof for the formation of vinyl cations on addition
of trifluoroacetic acid. Other dialkylacetylenes, e. g. 2-butyne
(18b), preferentially afford the ( Z ) isomer ( Z ) - ( 1 9 b )on addition of trifluoroacetic acid. This is explained in terms of attraction between the P-methyl group in the intermediate vinyl
cation and the attacking nucleophile[18!
Addition of anhydrous HCI and HBr to alkylacetylenes
(20) in the liquid phase leads to interesting [,2,+ ,2,] cycloaddition reactions, product formation being formulated via the
vinyl cation (21)[191. Reaction of (21) with the alkyne (20)
gives the stereoisomeric cyclodimers (23) in good yields, presumably via the resonance-stabilized 1,3-disubstitutedcyclobutenyl cation (22). The normal mono- and di-adducts to the
triple bond are also obtained. It recently also proved possible
to prepare 1,3-dialkyl-l,3-dichlorocyclobutanes
(23) containAngew. Chem. I n t . Ed. Engl. 17, 333-341 ( 1 9 7 8 )
attacked from the least hindered side by the nucleophile Xe,
giving products ( 3 6 ) with a high degree of stereoselectivity.
The same additions to mono- and di-substituted alkylacetylenes do not proceed in such a clear-cut fashion. The reaction
products can also arise by pathways other than that involving
a vinyl cation[26!
ing two different alkyl groups by reaction of mixtures of
two alkynes['gcl. Alkylating cycloadditions proceeding via a
vinyl cation intermediate also occur during the reaction of
gaseous HC1 with propene and propyne in the molar ratio
2: 1 :
In the presence of Lewis acids, I-alkynes ( 2 4 ) react
with alkenes ( 2 5 ) by cycloaddition to give cyclobutenes (26).
The catalytic effect of Lewis acids is probably due to complexation of the alkyne which leads to polarization in the sense
of a vinyl catiod2'!
t- B u
X = C1, Br; R = A l k y l
Addition of tert-butyl cation to 2-butyne ( 3 7 ) leads to
an interesting rearrangement which was interpreted as a double
1,2-methyl shiftfz71:the initially formed vinyl cation ( 3 8 )
undergoes a methyl shift across the double bond to give the
1.1- R'n
t-BuCI, SbF,
\ o
s q ,- 7 5 "C
R = Alkyl
A detailed study of the addition of fluorosulfuric acid to
alkynes in S02ClF at - 120°C or in SOz at -78°C furnished
the following picture["]: terminal alkynes such as 1-butyne
or I-hexyne give solvent-separated ion pairs which react preferentially by syn addition to give vinyl fluorosulfates. In aryl-substituted alkynes ( 2 7 ) , stabilization via the adjacent aryl group
leads to the vinyl cation (28), which does not exist as an
ion pair. The species (28) reacts with a second molecule
of alkyne ( 2 7 ) to give cyclobutenyl cations (29), which thus
become readily accessible[z31.
vinyl cation (39) which affords the allyl cation by a second
methyl shift towards the double bond. Cation ( 4 0 ) was
detected by NMR spectroscopy; a 1,3-methyl shift was ruled
out by D-labeling.
(41a). R = P h
(41b), R = H
u Ph
R = Ph, CH,, t - B u
R' = P h , CH,
Protonation of alkynes, e . g . (30), with magic acid
(FS03H-SbF5) in SOzCIF or SOz at -78°C leads merely
to oligomers; i. e. the intermediate vinyl cation ( 3 1 ) is insufficiently stable to be detected under these conditions. At higher
temperatures, e.g. -2O"C, it rearranges to the allyl cation
(32)' 241.
R = H, Alkyl: R'= A l k y l
Reaction of diphenylmethyl chloride (41 a ) and benzyl
chloride (41 b ) with phenylacetylenes ( 4 2 ) in the presence
of AlC13 (in CHZC1,) illustrates the preparative utility of the
addition of carbenium ions to triple bonds['*]. Substituted
indenes ( 4 5 ) are obtained. The initially formed vinyl cation
( 4 3 ) cyclizesto (44), yielding ( 4 5 ) on proton loss. Compound
( 4 5 ) is accompanied by the normal products of addition
to the triple bond.
2.2. Participation of CC Triple Bonds in Solvolyses
2.1.2. Addition of Carbenium Ions
An AdE2 mechanism, i.e. vinyl cation formation, has also
been demonstrated on addition of tert-butyl halides ( 3 3 ) to
phenylacetylenes (34)"'. '53251.
The tert-butyl cation formed by the action of the Lewis
acid on ( 3 3 ) serves as electrophilic reagent. A linear vinyl
cation ( 3 5 ) is formed as intermediate; it is preferentially
Angew. Chem. Int. Ed. Engl. 17,333-341 ( 1 9 7 8 )
R = H, Alkyl, A r y l ; Xo = H a l i d e , S u l f o n a t e
Reactions of carbenium ions involving participation of CC
double bonds have been known for several decades; the result335
ing cyclizations and kinetic effects have been established by
a wealth of experimental material[29].We first employed the
CC triple bond as neighboring group in 1965 [see eq. (b)If3O1:
solvolysis of reactive homopropargyl derivatives (46) in solvents of high ionizing power and low nucleophilicity gives
cyclopropyl ketones ( 5 1 ) and cyclobutanones (52). In-depth
mechanistic studies showed that the stabilized vinyl cations
( 4 7 ) and (48) are formed as intermediates, and react via
the enol ethers ( 4 9 ) and (50) to give the ketones ( 5 1 ) and
(52)C3 ‘1.
R-C- C-CH2-CH2-X
(51) R - C q
( a ) , R = H; (b), R
id), R
= Alkyl; (c),
in anhydrous trifluoroeth-
cyclic products?
Cyclic products [%]
50 (Six-membered rings, e. g . i-cyclohexenyl
trifluoroethyl ether)
15 (Six-membered rings)
72 (Five-membered rings)
22 (Seven-membered rings)
The triflates (55 a) and (55 b) solvolyze without cyclizing.
Participation of the triple bond would lead to a cyclopentenyl
cation (58), whose formation in solvolysisreactions is unlikely.
However, the triflate (56 a ) affords six-membered ring derivatives via the cyclohexenyl cation ( 5 9 ) ; (56b) solvolyzes to
give a mixture of six- and five-membered ring compounds
formed from the vinyl cations ( 6 0 ) and (61). The greater
R = Aryl;
= Cyclopropyl
X = -0-SOz
CH,(OTs), -O-S02-CF,(OTf)
The product pattern obtained from this “homopropargyl
rearrangement” depends upon the substituent R in (46). Compounds (46a), R=H, and (46b), R=alkyl, give mainly the
four-membered rings (50a) and (52a), and ( 5 0 b ) and (52b).
In compounds (46c), R = aryl, and (46d), R =cyclopropyl,
the stabilizing effect of these substituents on the intermediate
cyclopropylidenemethyl cation (47c) and (47d) favors the
three-membered ring derivatives ( 4 9 c ) and (51 c), and ( 4 9 d )
and (51 d), respectively[31!
The homopropargyl rearrangement is of preparative significance as a facile method for the synthesis of cyclobutanones,
as will be illustrated by an example. The readily accessible
compound 3-butynyltriflate (46a), X=OTf, is heated to 50°C
in trifluoroacetic acid for 24 h; essentially pure cyclobutanone
(52a) (yield 60%) arises owing to the greater stability of
(48a) compared with the primary vinyl cation (47a)[321.
2-Alkylcyclobutanones (52 b ) are readily obtained in similar
manner in high yield starting from (46 b)L3Ia, 321. The relatively
high stability of cyclobutenyl cations (48) endows the homopropargyl rearrangement with considerable scope for the synthesis of cyclobutanones. Thus, starting from the corresponding homopropargyl derivatives, disubstituted cyclobutanones
( 5 3 ) and, e. g., also condensed cyclobutanones ( 5 4 ) are accessible[31a.331
R = Alkyl
Cyclization reactions involving more distant triple bonds
are also feasible. Table 1 lists the solvolysis products of some
acyclic alkynyl triflates, showing how they depend upon the
position of the triple bond relative to the functional carbon
Table 1. Solvolysis of alkynyl triflates (55)--(57)
stability of the linear vinyl cation (61) compared to the cyclic
vinyl cation (60) leads to preferential formation of products
derived from (61). Compound ( 5 7 ) still solvolyzes with participation of the triple bond; however, only 22 % of seven-membered ring derivatives are formed owing to the length of
the chain[351.
Transannular participation of a triple bond in medium-sized
rings was first observed by us on solvolysis of 5-cyclodecynyl
derivatives (62)[361.These esters ( 6 2 ) also rearrange on solvol-
ysis in comparatively nucleophilic solvents, e.g. in ethanol/
water mixtures, to give mainly cis- and trans-I-decalone ( 6 5 )
while bicyclo[5.3.0]decan-2-one ( 6 6 ) is formed only in small
quantities. The predominant formation of ( 6 5 ) is explained
by the greater stability of the vinyl cation ( 6 3 ) compared
with (64)f3’1.
The proclivity of the triple bond to participate increases
drastically in the 5-cyclononynyl derivative ( 6 7 ) ; it solvolyzes
100 times as fast as ( 6 2 ) in 75 % aqueous ethanol, rearranging
quantitatively to cis-4-hydrindanone (68) in spite of the
pronounced nucleophilic nature of the solvent[381.
Syntheses of steroids and triterpenes by biomimetic cyclizati on^'^^] have recently also been performed with systems
involving triple-bond participation and vinyl cation intermeAngew. Chem. Int. Ed. Engl. 17, 333-341 ( 1 9 7 8 )
cation. After attack of the electrophile E' on the central
carbon atom, a vacant p orbital is oriented perpendicularly
on the remaining double bond as shown in (77). Resonance
diates. Thus, treatment of the dienynol ( 6 9 ) with formic acid
in pentane gives the enol formate (72) in 90%
The acid attacks the tertiary OH group of (69); the resulting
carbenium ion cyclizes with participation of the double bond
giving a new carbenium ion via formation of a six-membered
ring. This ion is then transformed, with triple-bond participation, into the linear vinyl cation (70).
In the preparation of DL-progesterone (75), the trienynol
( 7 2 ) is converted into the vinyl cation ( 7 3 ) in the final cyclization step involving the triple bond; (73) affords the ketone
(74)l4O]. Progesterone (75) can be obtained from ( 7 4 ) in
two further steps.
(82 u), R = R = alkyl
(82 b ) , R=phenyl, tolyl; R ' = H
0 '
( 75)
Other comparable cyclization reactions in which vinyl
cations are formed have provided an entry to testosterone
benzoate[4'], and also to longif~lene[~~!
The high stereospecificity of the cyclization reaction has also been exploited for
asymmetric synthesesf39!
butene (80). In contrast, only partial protonation of the terminal carbon atom occurs on reaction of 1,3-dialkyl-allenes
(82 a). The ratio of terminal to central carbon protonation
depends upon the reaction condition~[~~].
no longer forms vinyl cation intermediates but, as expected,
reacts via protonation of the central carbon atom to give
a tertiary carbenium ion[481.
stabilization can only occur after a 90" rotation about the
single bond in (77). Additions to unsubstituted allene therefore
proceed exclusively according to eq. (c)~"~].
Thus acid-catalyzed
hydration of allene gives only acetone, and addition of HCl
and HBr to allene leads to the same products, including
the cyclobutane derivatives, as are obtained on reaction of
propyne with HCI under the same conditions. This suggests
formation of the vinyl cation [eq. ( c ) ] [ ~ ~ - ~ ~ !
Electrophilic addition is not so clear-cut with substituted
allenes; intermediate formation of vinyl cations depends upon
the number and nature of substituents and upon the electrophile. This situation is illustrated by a few examples of reactions between allenes and HCI: 1,2-butadiene ( 7 8 ) reacts
at - 78 "C with HCI with intermediate formation of the vinyl
cation (79)[461,giving exclusively (2)- and (E)-2-chloro-2-
Addition of HCI to arylallenes (826) in glacial acetic acid
also Proceeds via Protonation of the central carbon atom
to form the more stable benzyl cation'491.
2.4. Participation of Allenic Groups in Solvolyses
2.3. Electrophilic Additions to Allenes
The cumulated double bonds of allenes permit electrophilic
additions to follow various pathways. Additions to allene
itself and to substituted allenes may proceed differently: addition to allene can give a vinyl cation, according to eq. (c),
which has added the electrophile at the terminal carbon atom.
In contrast, attack at the central carbon atom of the allene
( 7 6 ) would lead to a carbenium ion ( 7 7 ) which is formally
an allyl cation and therefore appears preferentially. The carbenium ion (77), however, is not a resonance-stabilized allyl
Angew. Chem. I n t . Ed. Engl. 17,333-341 (1978)
Equation (d) shows the participation of allenic groups in
the generation of a carbenium ion. This reaction was first
observed by us[s01,and independently also by Bertrand and
'I, in the solvolysis of homoallenyl derivatives (82).
Thus (82), R = H, CH3;R'= R" = H, gives mainly alkyl cyclopropyl ketones ( 8 4 ) in various solvents. Preferred cyclization
to the ketones was rationalized in terms of intermediate formation of the stabilized cyclopropylvinyl cation (83), owing
to participation of the allenic bond in the solvolysis of (82).
This reaction is known as homoallenyl rearrangement because
of its resemblance to the homoallyl rearrangement.
Subsequent systematic studies on the homoallenyl rearrangement showed that product formation depends upon the
structure of the homoallenyl compound. While an alkyl
attached to the functional carbon atom in (82), R‘=alkyl,
R = H, still leads to solvolysis products, among which the
cyclopropyl alkyl ketone (84) predominates, doubly substituted homoallenyl derivatives (82), R = R = alkyl, solvolyze
preferentially to give cyciobutane derivatives[52! Homoallenyl
rearrangement to cyclopropyl ketones (84) has also been
The kinetics and products of solvolysis of cyclopropylvinyl
derivatives (halides and tosylates) and isomeric homoallenyl
derivatives led to the conclusion that stabilized cyclopropylvinyl cations are the initial intermediates in the solvolysis
of homoallenyl derivatives.Thus cyclopropylvinyliodide ( 8 5 )
and the isomeric homoallenyl iodide ( 8 2 a ) react under the
same conditions to give the same solvolysis products, which
contain more than 65% of cyclopropyl deri~atives”~].
Solvolysis of the tosylates (86), (87), and ( 8 8 ) in acetic acid gives
products of very similar composition and stereochemistry
whose formation is best explained by way of the cyclopropylvinyl cation (89)[551.
Participation of more distant allenic groups in solvolysis
reactions also leads to cyclic products, which are not obtained,
however, via vinyl cations as intermediates[561.
3. Formation of Vinyl Cations by Heterolysis
The reluctance of vinyl halides to undergo SN1 reactions
has a twofold explanation: it is attributed partly to the compar338
ative instability of vinyl cations and partly to the ground
state stabilization of vinyl halides. Stabilization of the ground
state is a consequence of the higher s character of the carbonhalogen bond in vinyl halides (sp’) compared with alkyl halides
(sp3)[57. 581,
Solvolytic generation of vinyl cations [see eq. (e)] is feasible
only if one of the following conditions is fulfilled: 1) Especially
good leaving groups must be used for solvolysis, e.g. the
“super-leaving groups” trifluoromethanesulfonate (triflateY5]
or the even more reactive nonafluorobutanesulfonate (nonaflateY5’1. 2) On use of leaving groups less prone to undergo
SNl reactions, e.g. halides, then vinyl cations will only be
formed if they are stabilized by a neighboring group with
electron-donating ability.
In any case, it is necessary to establish that solvolysis of
the vinyl compounds (90) investigated do actually proceed
via vinyl cations. The pertinent mechanistic criteria can be
briefly summarized as follows16o1:if a vinyl cation is formed
in the rate-determining step of the solvolysis of (90) then
the solvolysis must obey first-order kinetics. The rate of solvolysis is independent of solvent pH and of the concentration
of added base, but dependent upon the ionizing strength of
the solvent. The Winstein-Grunwald rn value[611should lie
between 0.5 and 1.O. If the solvolysis is performed in deuterated
solvents, then only a very slight solvent isotope effect, if any,
is to be expected. Moreover, effects characteristic of carbenium
ion mechanisms, such as salt effects, ion-pair formation, and
phenomena of internal return have been observed. The linear
geometry of the intermediate vinyl cation ( 9 1 ) suggests that
the reaction products (92) and ( 9 3 ) are formed in equal
quantities. Finally, the formation of rearranged products is
a good mechanistic criterion for the course of the solvolysis
reaction via vinyl cations.
Alternatively, the solvolysis products can be formed by,
say, addition-elimination mechanisms; the initial step may
comprise electrophilic addition of solvent,or possibly a nucleophilic addition of solvent to the double bond[601.On solvolysis
of a vinyl sulfonate, the product can also arise by nucleophilic
attack of the solvent on the sulfonyloxy sulfur atom. If R
or R ’ = H in the vinyl derivative (90), concerted fl elimination often yields the alkyne.
3.1. Solvolysis of Simple Vinyl Derivatives with Particularly
Good Leaving Groups
On replacement of halide as leaving group by the tosyloxy
group, which has been thoroughly studied in the chemistry
of saturated carbenium ions, intermediate formation of a vinyl
cation can be accomplished by solvolysis of simple alkyl-substituted vinyl derivatives.Thus, for example, (E)-I-methyl-1-propenyl tosylate (E)-(94), X = OTs, is solvolyzed in 50 % aqueous
methanol via the vinyl cation ( 9 5 ) to give a mixture of 2-butyne
and 2-butanone (96); in other solvents, such as acetic acid
or formic acid, another mechanism is operative[621.
Angew. Chem. I n t . Ed. Engl. 17, 333-341 11978)
( E 1-194)
The search for an even better leaving group led to the
t r i f l a t e ~ ~and
~ ~ nl o n a f l a t e ~ ~which
~ ~ ] react some lo4 times
faster than tosylates. Acyclic secondary alkyl-substituted vinyl
triflates and nonaflates solvolyze with few exceptions via intermediate vinyl cation formation: thus the vinyl triflates (E)-(94)
and (2)-(94), X = OTf, react in ethanol/water mixtures, even
at relatively low temperatures (76"C),considerably faster than
the tosylates via (95) to give the products [e.g. ketone
Systematic studies on the solvolysis reactions of the triflates
( 9 7 ) in solvents of various ionizing strengths and nucleophilicity revealed a pronounced carbenium ion character and no
indication of a SN2reaction. The triflates ( 9 7 ) can solvolyze
via ion-pair formation. This can lead to preferential formation
of one stereoisomer in spite of expected formation of a linear
vinyl cation intermediate'651.
R = CH,, i - P r , t-Bu; R' = H, CH,; R" = H, CH3
group participates in the rearrangement; under certain structural conditions, vinylenebenzonium ions ( 1 05) may be
formed which correspond to the benzonium ions of saturated
carbenium ion chemistryr6'].
7 0 ,
The driving force of the rearrangements is the formation
of more stable carbenium ions. The rearrangements can involve
shifts of alkyl and aryl groups; hydride shifts have also been
observed. A few typical examples have been selected from
the wealth of known rearrangements:
The vinyl triflate ( 9 8 ) solvolyzes in aqueous ethanol to
give the vinyl cation (99), which affords the saturated carbenium ion (100) by migration of a methyl group towards
the double bond. The solvolysis products result mainly from
(100) and only to a slight extent from the vinyl cation (99)[661.
Rearrangement across the double bond was observed on
solvolysis of the vinyl triflate (101). The initially formed vinyl
cation (102) undergoes a phenyl shift, giving the vinyl cation
(103) almost quantitatively. The positive charge in (103)
is stabilized by the neighboring phenyl group; the ketone
(104) is obtained on reaction with the solventr67! The 0-aryl
Angew. Chern. Znr. Ed. Engl. 17,333-341 (1978)
* -+
Hydride shifts towards the double bond have been found
in the solvolysis reactions of the vinyl triflate (1061, and
across the double bond in those of the vinyl triflates (107).
These processes afford a more stable carbenium ion and a
more stable vinyl cation, respectively[69!
As in the solvolysis to give saturated carbenium ions, the
solvolysis of vinyl derivatives is accompanied by rearrangements which can serve as evidence for an intermediate vinyl
cation (seeSection 3). Both rearrangements towards the double
bond [eq. (f)] and rearrangements across the double bond
are possible [eq. (g)].
Cyclic vinyl triflates also solvolyze with formation of vinyl
cations. Table 2 shows examples of solvolysisrates as a function
of ring sizer7']. The cyclic vinyl triflates (108) solvolyze faster
with increasing ring size, reaching a maximum rate with the
nine-membered ring. The comparatively low reaction rate
of I-cyclohexenyl triflate is attributed to the impossibility
of achieving the linear geometry required in the transition
Table 2. Rates of solvolysis of 1-cycloalkenyl triflates (108) in 50 % aqueous
ethanol at 75°C and of (E)-t-methyl-1-propenyl triflate ( E ) - ( 9 4 a ) [70].
( C s o T f
k [s-'1
1.2 x 10-8
4.2 x
4.5 x to-4
2.3 x t O - '
3.3 x 10-3
1.1 x 1 0 - 2
6.8 x 1 O - j
7.1 x 1 0 - ~
2.5 x t o - '
state for a vinyl cation with a ring of this size. In contrast,
the I-cyclooctenyl cation and cyclic vinyl cations having even
more ring members can adopt a linear geometry as in the
vinyl cation arising from (E)-(94 a ) . That the reaction rates
of the cyclic vinyl triflates with more than eight ring members
are higher than that of(E)-(94 a ) is probably due to a reduction
of the I-strain in the transition state on formation of the
vinyl cation. Qualitative comparison with the rates of solvolysis of I-methylcycloalkyl chlorides appears in
maximum rate of solvolysis was also found in the range of
medium ring size which could be correlated with the strain
energy of the medium-sized rings.
Rearrangements typical of a vinyl cation mechanism are
again found in the solvolysis of cyclic vinyl triflates: 2-methylI-cyclohexenyl triflate ( 1 0 9 ) gives mainly the ring-contracted
cyclopentyl methyl ketone on solvolysis in aqueous trifluoroethanol, the initially formed cyclic vinyl cation rearranging
to the more stable linear vinyl cation (l10)[721.
Cyclic vinyl cations are more stable than primary linear
vinyl cations, as demonstrated by the solvolysis of the vinyl
triflate ( 1 1 1 ) in aqueous trifluoroethanol: cycloheptanone
( 1 13) is formed almost quantitatively via the cycloheptenyl
cation ( 1 12). Rearrangement to the cyclic vinyl cation ( 1 1 2 )
proceeds via a concerted ionization-alkyl shift without formation of a primary vinyl cation[73!
3.2. Stabilized Vinyl Cations
Vinyl cations ( 2 ) are disubstituted carbenium ions. Stabilizing C-located substituents can be linked to C, and C,. Substituents on C, give secondary vinyl cations. If CB is formally
part of the substituent group then especially stabilized cations
are obtained, e . g . the allenyl cation (125) or the cyclopropylidenemethyl cation (1 28).
In contrast to cyclohexenyl triflate (1 08), n = 4, cyclopentenyl triflate fails to react by a vinyl cation mechanism in
solvents of high ionizing strength, e. g . trifluoroethan~l[~~I.
A cyclopentenyl cation (58) would be highly strained; it
appears that the deviations from linear geometry no longer
permit solvolytic generation of this intermediate.
The reactions of simple vinyl triflates described thus far
are qualitatively reminiscent of the behavior of saturated carbenium ions. Two other reactions typical of saturated carbenium ion were also observed on solvolysis of vinyl triflates:
thus solvolysis of the (Z)- and (E)-dienyl triflates (114) in
trifluoroethanol gives not only acyclic products but also 20
to 35 % of the cyclic trifluoroethyl ethers (115), (116), and
( 1 17), respectively, by participation of the double bond and
formation of homoallyl-, allyl-, and cyclopropylmethyl
The enynyl triflate (118) solvolyzes in solvents of various
ionizing strength with participation of the triple bond via
the linear vinyl cation ( 1 1 9 ) giving mainly the ketone (121).
The cyclic vinyl cation ( 1 2 0 ) is only formed in minor amounts,
as indicated by the low yields of ketone (122)[761.
Any substituents of proven efficacy in the case of saturated
carbenium ions can be used to stabilize vinyl cations. Use
of the aryl group as substituent gives the vinyl cation ( 1 23).
The stabilized cation (1 24) is obtained with a vinyl group.
The effect of a neighboring triple bond in (126) has not
yet been examined.
Vinyl cations are stabilized particularly well by nonclassical
interaction with a cyclopropyl group as in (127) and (128).
The favorable geometry for overlapping of the orbitals
involvedconfersaparticularly high stability upon cyclopropylidenemethyl cations (128).
With the exception of (126), the vinyl cations ( 1 2 3 ) to
( 1 28) have been detected as intermediates in solvolysis reactions of the corresponding vinyl halides. Owing to the stabilizing neighboring groups, halide suffices as leaving group for
the solvolytic generation of these vinyl cations (for a survey,
see ref. [Q).
Other reactions involving intermediate formation of a vinyl
cation by heterolytic bond rupture cannot be dealt with here;
they are only of minor importance compared with the solvolyses of vinyl perfluoroalkylsulfonates described. For example,
an attempt has been made to produce vinyl cations via diaAngew. Chem. Int. Ed. Engl. 17,333-341 ( 1 9 7 8 )
zonium ions. A detailed account will be found in the reviews
4. Generation of Vinyl Cations by Special Reactions
In this connection, mention should be made of, e. g., photochemical reactions of vinyl iodides in which vinyl cations
have been postulated as inte~mediates~~’].
The reader is
referred to the literature regarding this and a series of other
reactions proceeding via vinyl cations1781.
Support of our own experimentai work cited in this article
by the Deutsche Forschungsgerneinschaft, the Fonds der Chemischen Industrie, and the Volkswagen-StSftung is gratefully acknowledged. Particular thanks are due to my co-workers who
have pursued these studies with such great dedication.
Received: May 3, 1977 [A 211 IE]
German version: Angew. Chem. 90, 346 (1978)
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chemistry, preparation, mechanistic, aspects, vinyl, cation
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