L. R. Subramanian and M. Hanack Fachbereich 14 University of Saorlond D 66 SoarbrGcken, Germany
Generation of Vinyl cations by Solvolysis Reactions
I
A lesser known genre o f carbenium ions
Among the reactive intermediates in organic chemistry, carbenium ions are the most important and extensively studied (I). Although they were discovered shortly after 1900 (Z), carhenium ions have been well recognized only since Ingold's definition of S N reactions ~ (3). Since then great progress and an expansion of ideas in this field have given carbenium ions a dominant role in mechanistic as well as in preparative organic chemistry (4). The positive charge in a "normal" saturated carbenium ion is located on a carhon atom which is connected to three suhstituents and is known as the trisuhstituted carhenium ion (I). Another species of carbedium ion, that canying the positive charge on a carbon atom connected to only two suhstituents, was almost unknown until recently. These are the so-called disuhstituted carbenium ions of which the vinyl cation (U) is an important representative1
Structure and Geometry of Vinyl Cations
Two structures which appear most likely on chemical grounds can he wtitten for a vinyl cation. These are the r-protonated acetylene in which the proton attaches itself symmetrically to both the carbon atoms utilizing one of the r-electron pairs of acetylene for bonding, and the classical unbridged structure. From molecular orbital calculations (7) there is a general agreement that the vinyl cations prefer the unbridged structure. I t is then important to decide whether the carbon atom carrying the charge is sp-hybridized having a linear structure or spz-hybridized with a bent structure. MO calculations again point out that the most stable arrangement is a linear structure (7, 8)
m,
(I1
Before the exact concept of spatial orientation of o- and r-electron clouds was available, it was not thought conceivable that a positive charge could exist on the same carhon atom as the double bond. One can write a formal mesomeric structure for the vinyl cation (U), hut there is no experimental evidence for its presence
>=;-
There are indications of the intermediacy of vinyl cations when aryl vinyl fluorides are reacted with ShFs in SOzClF (6).
-
;L-
The main reason for the view that vinyl cations are extremely unstable has t o he traced t o the scant reactivity of the vinyl halides in solvolysis reactions. Simple vinyl halides can not be brought to reaction even in the presence of silver nitrate under solvolysis conditions (5). However, through extensive studies on the S N reactions ~ of suitable vinyl derivatives in the last five years a wealth of experimental data has been assembled which indirectly shows the existence of vinyl cations. From this it became clear that vinyl cations are not basically different from normal trisuhstituted carhenium ions. They are also capable of undergoing rearrangement and elimination reactions, and their behavior towards nucleophiles is quite comparable to that of saturated carhenium ions. Recently efforts have been made towards the direct ohsewation of vinyl cations by 1H and '3C-nmr spectroscopy.
Modes of Generation of Vinyl Cations
Vinyl cations (11)can he generated mainly in three ways 1) Heterolvtie cleavaae . of vinyl substrates containing suitable
leaving groups (eqn. (1))
(11
2) Electmphilie additionsto triple bonds (eqn. (2))
3) Electmphilic additions to allenes (eqn. (3))
We consider here only the recently developed heterolytic solvolysis reactions. More elaborate reviews of the chemistry of vinyl cations including the history of the concept and methods of formation have already appeared (9). Heterolysis of Vinyl Substrates
Analogous to the vinyl cations (INare the acylium-, nitrilium-, ((m) and (IV)), the alkynyl cations (V), and the aryl cations (VI).While there is evidence for the intermediacy of the disuhstituted cations (m), (IV), and (V) in organic reactions, the existence of the aryl cation (VI) is shrouded in controversy. 1
80
/ Journal of Chemical Education
The generation of vinyl cations by simple solvolysis reactions of suitable vinyl derivatives is one of the more exciting reactions in vinyl cations chemistry. While solvolysis reactions leading to trisuhstituted carhenium ions as intermediates have been extensively examined, the studies of the same type of reactions of vinyl derivatives have only recently been very successful. I t has been inferred in organic chemistry textbooks, that simple vinyl halides are unreactive and do not undergo SN1-type solvolysis reactions (5). The lack of reactivity of vinyl halides has been ascribed not only t o transition state destabilization due to
the instability of vinyl relative t o aliphatic cations but also to lowered ground state energies of vinyl halides. The latter has been attributed to the increased a-bond strength (10) of the carhon-halogen bond due to the change in hybridization of carbon from sp3-in alkyl halides to +-in vinyl halides, as well as to the partial double bond character of the bond in the mesomeric form of the vinyl halides (11).
pam-a
\
A nucleophilic addition-elimination mechanism is very Using as a working hypothesis the consideration that the meager reactivity of vinyl halides is due to the low stability of the intermediate vinyl cation as well as to the greater bond strength of the vinyl-halogen bond, the generation of vinyl cations (II) in solvolysis reactions could be easier if one fullfils the following two criteria: 1) In place of the slweish halides. better leavine erouns which are more proneto S N 1reactions should beu:ed,'2) The intermediate vinvl cations should be stabilized bv suitable substituents. In conformint! to only one of the above conditions the pursuance of vinyl cations generated by solvolysis reactions was rewarded. Mechanism of Solvolysis of Vinyl Substrates (9, 12)
likely only if a strong nucleophile is used and can be ruled out in neutral media if the rate of the reaction did not increase with added base (15, 16). Synchronous Elimination The synchronous elimination mechanism with solvent acting as the base is applicable only for vinyl substrates containing a p-hydrogen atom. However, this is easily recognized, as the rate of reaction should increase with added base if this mechanism is operative (14).
Nucleophilic Attack on Sulfur
Five important alternative mechanisms can be considered. Vinyl Cation Mechanism Analogous to the heterolytic cleavage of a C-X bond in saturated systems, the loss of both bonding electrons from the sp2 carbon atom carrying the leaving group will lead to a vinyl cation as the intermediate. If this is the ratedetermining step, then the reaction should obey firstorder kinetics under certain conditions. The reaction rate should he independent of solvent p H as well as not dependent upon base concentration. The solvent deuterium isotope effect should be small or non-existent when the solvolysis is carried out in deuterated solvents
rearranged products
R~-C-C-R~,,
The measure of the sensitivity of the reaction to solvent ionizing power, the Winstein-Grunwald rn value (13). should be of reasonable value indicating *at the rate of reaction is dependent on the ionizing power of the solvent system as is the case for S N reactions. ~ Moreover, formation of rearranged products is also a fair indication of the intermediacy of vinyl cations. Acetylenic products also will be obtained if P-hydrogens are present in the vinyl substrate. Addition-Elimination Mechanism Both electrophilic (Path A) and nucleophilic (Path B) addition-elimination mechanisms are possible. The reaction rate in these cases will be dependent upon the p H of the media. Analysis of the reaction products obtained by conducting the solvolysis in deuterium-labeled solvents like CHBCOOD or CzHaOD will also provide additional evidence if an electrophilic addition-elimination mechanism is operative (14), while there will be a reasonably large solvent deuterium isotope effect
&-OH
This possihility exists when the leaving group is a sulfonate and the formation of the vinyl cation is questionable. If such a mechanism is operative, then the rate of the reaction 9hould increase with added base. When the solvolysis is carried out in marked solvent systems such as CzH60H-H2180,no incorporation of '80 in the product will point to an oxygen-sulfur cleavage. Direct S w 2Displacement a t the Vinylic Carbon
The other alternative to the two-step process of substitution a t a vinylic carbon atom through the intermediate vinyl cation, is the one-step direct SNZ displacement by nucleophiles a t the vinylic carbon atom carrying the functional leaving group. Although many examples of these processes are available in the saturated analogs, such direct displacements are energetically unfavorable a t vinyl carbons and have so far not been observed (27). The different mechanisms mentioned above are elaborated upon in several cases to substantiate the intermediacy of vinyl cations in solvolysis reactions. Criteria I: Soivolysis ot Vinyl Substrates Possessing Better Leaving Groups The first trial in using a better leaving group in solvolysis reactions with a view towards generating vinyl cations would be to try the tosylate and the brosylate anions instead of halide ions as the leaving groups. The former have long been used in studying the chemistry of tri-substituted carbenium ions. E- and Z- 2-butene-2-yltosylates ( ( E - W ) and ( Z - W ) ) can be considered as gwd examples (18) which solvolyze to give the corresponding ketone, 2butanone, in solvents of high ionizing power and low nucfeophilicity, e.g., formic acid. It was demonstrated in this example that the product analysis does not necessarily indicate the intermediacy of the vinyl cation Volume 52, Number 2, February 1975 / 81
vinyl cation. This practically quantitative rearrangement leads to the ketone
In fact the kinetic data clearly showed that a vinyl cation is not responsible for the production of the ketone. When the butenyl tosylates were solvolyzed in a non-acidic solvent of high ionizing power, e.g., 50% methanolwater, at 130°C, a mixture of 2-butyne and 2-hutanone was obtained (B).The tosylate ( Z - W ) reacted ten times faster than the ( E - W ) isomer and gave more of the acetylenic compound as the product. It is then clear that the tosylate containing the H atom trans to the leaving mu^ reacted with a svnchronous elimination mechanism withbut involving the h n y ~cation, while the tosylate (EW)underwent heterolvtic cleavage via the vinvl cation. The search for still better leaving groups than the t a y l ate led to the already known trifluormethanesulfonates (triflates), which have been shown to react -1W times faster than the tosylates (20). This leaving p u p is now used successfully to generate vinyl cations in solvolysis reactions. Thus the corresponding triflate derivative of (E-VII) reacts in 80% ethanol-water even a t 76°C with a reaction rate of 2.25 x 10-5 s-1 (21) which is much faster than the tosylate (E-VII) (k = 0.71 X 10-6 s-1 in 50% methanol water a t 130°C). Mechanistic studies using the corresponding deutrated triflates have shown that the solvolysis occurs via a vinyl cation also in the above mentioned triflate. As in the case of saturated carbenium ions, indications of the reactive intermediacy of vinyl cations can be followed by their characteristic reactions. I t is well known that carhenium ions are prone to rearrangements. These are seen also in the case of vinyl cations. In vinyl cations two types of rearrangements are possible: to the double bond (eqn. 4) and across the double bond (eqn. 5)
The t-hutyl vinyl triflate (Vm) solvolyzes in aqueous ethanol preferably with the formation of rearranged products. The vinyl cation first formed undergoes a 1,2-methyl shift to form the more stable carbenium ion, from which the products are derived (22)
-
C"
aq ethanol
CH-C-C=CH
I I
CH,
I + I CH,
CH,-C-C=CHZ
80%
CH, OTf
---t
Vlll
CH,
CH, I
I
t
CH,=C--C=CH,
I CH,
+ CHI CH,
F"
Ho-C+-CHz
I
I
The first example of a rearrangement across the double bond was noticed during the solvolysis of 1-methyl-2,2diphenylvinyltriflate (IX) (23). One of the phenyl groups migrates acmss the double bond to give the more stable 82
/ Journal of Chemical Education
8-Aryl participation in the solvolysis of aryl substituted vinyl derivatives is a much studied process (24). Theparticipation of 8-aryl groups and the secondary kinetic deuterium isotope effects in the solvolysis of vinyl sulfonates are now reported (25). E- and Z-isomers of 3-phenyl-2-huten2-yl triflates (X) and their a- and 8-CD3 analogs ((Xh) and (Xc)) were prepared and their solvolyses were investigated. The E-isomer was found to react 20 times faster than the 2-isomer in aqueous ethanol and scrambling of methyl groups was observed in the products of CD8 analog compounds (Xb) and (Xc). An ion pair mechanism involving a vinylidene phenonium ion originating from the participation of the 8-aryl group is proposed t o explain the results. The 8-aryl participation assisting anchimerically in the removal of the leaving group in the E-isomer was found toplay a smaller role in the Z-isomer
E-X
The CD3 group in the Z-isomer is a to the leaving group having a 8-isotopic substitution on a saturated carbon atom as compared to the case where the isotopic suhstitution 8 to the leaving group is on an unsaturated carbon atom (see above). A number of examples of the latter have recently been reported (21a, 26). The 8-deuterium isotope effect of the former kind is exemplified in (E-X) and (Z-X). The 8-deuterium isotope effect in the Z-isomers ((Z-Xb) and (Z-Xc)) was found to he much larger ( k ~ / k=~1.47) than for the E-isomers ((E-Xa) and (EXh)) with k ~ l k o= 1.16. The formation of a bridged cation in the E-isomers is a corollarv deduced from the lesser isotope effect due to the considerable charge delocalization into the phenvl rinz. In contrast, the Z-isomer ionizes to an essentially open iinear vinyl cation and there is a greater charge concentration in the a position and hence a greater isotope effect. The studies on the occurrence of vinyl cations during solvolysis reactions are not limited only to the acyclic vinyl derivatives. By use of the super leaving groups triflate anion and the even better leaving group nonafluorohutanesulfonate anion (nonailate) (27) it became possihle to investizate the es~eciallvinterestine cvclic vinvl compounds. 1; has been pointed out earlier-thk the vihyl cation prefers the enerpeticallv more favorable linear geomet r y . ~ e n c ei t could i;e expected that the rates of soivolysis of cyclic vinyl triflates should increase with increasine ringsize, the intermediate vinyl cation prefering the more linear geometry available as the ring becomes larger. The relative rates of the cvclic vinvl triflates from cvclopentenyl to cyclodecenyl 'to that bf the open c h a k 2huten-2-vl-triflate are assembled below.
-m
R\ ,,C=C--CH, +
CH,
-
R\
C , -C CH ,
pH' W ' H
CF,
508 EtOH 1
1.1 X 10-
3
X
10.'
0.32
It can he seen that the cyclic vinyl triflates solvolyze faster with increasing-ring . size, the maximum reaction rate being reached by the nine-membered ring derivates (28, 29). The high reaction rate of eight-, nine-, and ten-memhered rings to that of the open chain vinvl triflate is nrobably d u e l o the special con~ormationaleifects of the &dium rings. A rearrangement during solvolysis is demonstrated in the 2-methylcyclohexenyl triflate (XI) (28). Here the cyclic vinyl cation rearranges to the more stable linear vinyl cation.
The kinetic studies on cyclic vinyl triflates (see above) pmvide an insight into the structure of vinyl cations. On the other hand the stereochemistry of the solvolysis products will also throw some light on the stability and stmct u r e of vinyl cations. A direct S Ndisplacement ~ at a vinylic carbon should give a product of complete inversion (30) which was never found in the solvolvsis of vinvl com~ounds. Moreover such a backside displacement is n i t f e a s i h here according to theoretical calculations (17). Solvol$tic reactions at vinyl centen proceed mostly with complete randomization of stereochemistry (31). The stereoisomeric mixtures ohtained from E- and Z-isomers of vinyl derivatives are mostly equal, especially if the intermediate vinyl cation is stabilized and possesses a linear structure. Attack of solvent on the open chain linear vinyl cation gives the same ratio of E- and Z-products from either E- and 2-precursor (see below).
Recently, stereochemical inversion of products in the solvolysis of simple vinyl triflates was ohsewed (32, 33). The solvolysis of 3-methyl-2-heptenyl-triflates ((Z-XII) and (E-XII)) in dry trifluoroethanol buffered with 2,6-lutidine gave the trifluoroethvl ethers torrether with some 1methyl-1-n-hutylallene f o A e d by elimination. The different E/Z trifluoroethylether ratios ohtained from E- and 2-isomers showed that there is a significant inversion of configuration at the vinylic center. The data have been tentatively interpreted in terms of ion p a i n (Z-XIlI) and (E-XIII), where the side of the molecule from which the triflate group is departing is shielded to some extent from attack by solvent. From a study of a series of simple vinyl triflates, i t was concluded that the ion pair S N mecha~ nism with the leaving group blocking the front side, probably best accounts for the results.
IE-XlI)
R,
,c=c
CH,
/OCHCF,
\CH,
1E.XIIII R = CH,--CH,--CHl-CH,-
The applicahility of the new fast leaving group, nonaflate ion (27) was used to find out if the parent vinyl cation can he generated by solvolysis of the vinyl nonaflate (XIV) (34). However this unstahilized primary vinyl cation could not he formed under solvolysis conditions, while due to the trans-hydrogen atom present in the molecule, a synchronous elimination of nonafluorohutanesulfonic acid takes place to give acetylene. The rate of reaction of (XIV) increased on added base thus providing evidence for a synchronous elimination mechanism. The lack of success in formation of the parent vinyl cation is in agreement with the theoretical calculations (8). aqEtOH1TEA Nf
-HONE
HC-CH
Criteria 2: Stabilized Vinyl Cations The effect of stabilization of the reactive intermediate. carhenium ion, by neighboring electron donating groups is another convenient was of generating these species. All the electron donating snbstitients which are u k d to stabilize normal carhenium ions are also suitable for the stabilization of vinyl cations. Phenyl and vinyl groups provide stabilization in the classical way by the overlap of their occupied p-orbitals with the vacant p-orbital of the vinyl cation. By using these suhstituents four types of structures of stabilized vinyl cations (XV-XVIII) can he written as follows. Except for the vinyl substituted allenyl cation (XVIII) all others are known
The vinyl cation can also he stabilized in non-classical way by the overlap of a vacant p-orbital of the vinyl cation with the ir-bonds of the stabilizing groups used. I t has been amply demonstrated that cyclopropane rings next to a carbon atom carrying the positive charge can stabilize it well by non-classical interaction (35). Two forms of stabilization of vinyl cations by cyclopropyl groups are possible. In stmcture (XIX), the cyclopropane ring is depicted as an m-substituent on the carbon atom carrying the positive charge, while in (XX), the cyclopropane ring is more intimately connected to the vinylic cation center.
& (XMI
p=b IXX)
R ,Q+ IXXI)
In the cyclohutenyl system (XXI), the cation is also specially stable due to non-classical distrihution of positive charge to two other carbon atoms. The a-Aryl Vinyl System: The first examples of such a derivative giving a vinyl cation on solvolysis were studied by Groh and Cseh (36). They showed that a series of ahromostyrenes (XXII) carrying electron releasing substitVolume 52, Number 2, February 1975
/
83
uents in the p-position followed 9 ~ reaction 1 kinetics in aqueous ethanol.
The vinyl cation mechanism was established after careful study of the solvolysis in dioxane-water a t different p H and buffer concentrations especially in the case of pamino-N-bromostyrene ((XXII), X -. NHz)) to exclude the addition-elimination mechanism (37). The rate of solvolysis was shown to be constant in a pH-range of 3-13, which removes any suspicion of the alternative addition-elimination mechanism. The steric and polar effects of a @-methylsubstituent in a-hromo-p-methoxystyrene were studied (.?8), and the results were internreted bv formulatine intimate ion nairs. Stable vinyl cations have also been p&tulated as intermediates in the solvolvsis of triawl vinvl halides and sulfonates (72, 9 ) Solvolyses of trianisyl derivatives have been extensively investigated by Rappoport and coworkers. Common ion rate depression and independency of the rate of reaction on added base and nucleophile (sodium p-toluenethinlate), indicate the intermediacy of a vinyl cat.inn for the eolvolysis of (XXIIT) in ethanol (40)
The ratios of the substitution products obtained in the solvolysis of the isomers (XXIV) and (XXV) were equal, pointing to a linear vinyl cation intermediate (31~).It was shown in the aryl vinyl syst,ems, that the change of substituents at the 8-carbon atom did not affect the reaction significantly except in the case of a 0-t-hutyl derivative (24~1A . 1.2 rearraneement across the double bond was reportkd recently d u k g the acetolysis of 8,P-dianisyl-aphenvlvinyIbmmide (XXVI). Reflox of (XXVI) with silver icetite i n dry acetic acid for one hour gave the acetates (XXVII) and (XXVIIT) (41) in a ratio of 1:l. This argues stmnglg that the products are derived from the linear cation which is formed by the migration of the anisyl group acmss the double hond as depicted below. This cation is the same as t,hat obtained from ( X X V ) and (XXV).
,
(XXXI
(XXIXI
The requirement of non-coplanarity in conjugated bromodienes for facile solvolysis reactions was demonstrated by the lack of reactivity of 2-bromocyclohexa-1,3-diene (XXX) (43). However, when the 1,3-diene systems were introduced in seven- and eight-membered rings, they were found to undergo solvolysis reactions. 1,3-Cycloheptadiene-2-yl (XXXI) and 1,3-cyclooctadiene-2-yl triflates (XXXII) were prepared and their products and rates of solvolysis were investigated (44). The higher rates observed for the dienyl triflates are explained by an increasing stabilization of the intermediate vinyl cations in which the vacant p-orbital can overlap with the n-orbitals of the allylic double bond. The flexibility of the seven- and eight-membered rings contributes to this overlap, thereby stabilizing the vinyl cation intermediate.
CXXXJII
tXXXI1
The Allenyl System The solvolysis reactions of the simple allenyl and pmpargyl bromides were investigated by Stang, et al. (45). The pmpargyl bromide reacted 4 X 103 faster than the allenyl bromide. While the simple vinyl halides do not undergo solvolysis reactions even with aqueous silver nitrate solution a t high temperatures (5), the allenyl bromides do solvolyze, albeit slowly, obeying first-order kinetics. Alt h o u ~ hthe ~ m d u c t swere not able to be detected due to polykerizatfon, the kinetic data suggest that the intermediate is a resonance stabilized allenyl cation CH,-C&
-
+
CH,--c-CH
The solvolysis of triphenylallenylchloride (XXXllI) and other substituted allenyl bromides in acetone-water mixture gave excellent first-order kinetics and activation parameters for an Sw1 reaction. The data are interpreted in terms of a unimolecular reaction proceeding via a chargedelocalized allenyl cation (46).
The a-Cyclopropyl System: During the solvolytic studies on homoallenyl derivatives (XXXIV) a vinyl cation intermediate was proposed (47) to explain the products of the reaction. This cation (XXXV) arises by the participation of the allenyl double bond on the developing carbeni u m ion to produce cyclic products. It is then logical to assume that vinyl derivatives of structure (XXXVI) with an N-cyclopropyl substituent will solvolyze via the vinyl cation.
The Dienyl System: The delocalizing ability of vinylic double bonds can also he used to stabilize an intermediate vinyl cation as demonstrated in the study of conjugated bromodienes (42). Substituted bromodienes were found to solvolvze readilv in aaueous ethanol comnared to the narent chmpound,- 2-broko-1,3-hutndiene (XXIX) which is known t,o lack solvolvtic reactivitv even a t elevated tempcreturea. The kine& results and product studies are consistent with the formulation of a non-coplanar mesomeric vinyl cation 84
/
.burnal of Chernica! Education
l-Cyclopropyl-l-chloroethylene((XXXVI), R=H, X=CI)) was prepared and found to react immediately at room temperature with AgC104 in unbuffered acetic acid to give mainly the cyclopropyl methyl ketone (48aJ. The formation of the stabilized vinyl cation ((XXXV), R=H)) is very likely in this reaction as shown by product studies
and kinetic data. Analogous results were obtained for the iodide ((XXXVI), R=H, X=I)) (486). Detailed studies of the solvolytic reaction of E- and Z-isomen of l-cyclopmpyl-l-iodo-propanes ((XXXVI), R=CH$, X=I)) were reported (49). The investigation~of the stereochemistry of the products formed have shown that a vinyl cation is involved in the reaction. Molecular orbital calculations (17) suggest that the linear l-cyclopmpyl cation is more stable in the "bisected" conformation than in the "perpendicular" one. in analom with cvclopropvl . . .. carbinvl system. The bisected' conformkon is more favorable because of the possibilitv of maximum overlap of the vacant p-orbital of the cation with the orbitals of the cyclopropane ring
"
/
R
prpendicdar
The Cyclopropylidene System: The ~yclopropylidene methyl cation (XX) is stabilized due to its fixed geometry and the short C-C distance which allows the orbitals of cyclopropane to overlap well with the vacant p-orhital of the vinyl cation. The simple bromometbylenecyclopropane (XXXW) solvolyzes to give the ring enlarged pmduct, cyclobutanone, in comparison to the simple vinyl bromides which are inactive (50). The initially formed primary vinyl cation rearranges to the more stable cyclobutenyl cation
1XXXWI
A series of bromomethylene cyclopropanes (XXXVIW (XL) were synthesized and their solvolytic behavior studied with the aim of observing the intermediate vinyl cation. The substitution of a phenyl group on the a-carboo atom makes the system more reactive and the 1bmmo-l-phenylmethylene cyclopropane (XXXIX) solvolyzes faster than the bromometbylene cyclopmpane in aqueous ethanol (51). The kinetics of the solvolysis reaction of (XXXIX) are first-order and the main product is phenylcyclopmpylketone. l-Bromo-l-metbyl-methylenecyclopropane (XXXVIII) also solvolyzed faster than the simple l-bmmomethylenecyclopropane ($52). The 1bmmo-l-cyclopmpylmethylene cyclopropane (XL) represents a special system in which the corresponding vinyl cation is stabilized by two cyclopropyl groups.
This compound solvolyzes very fast in aqueous ethanol and gives only dicyclopropylketone as the reaction product (53). The relative rates of these compounds are compared below
The Cyclobutenyl System: The cyclobutenyl cation (XI) has been proposed as an intermediate in the homopmpargyl rearrangement being formed by the participation of the triple bond during the solvolysis of these deriv-
atives (54). This could easily explain the products of the reaction, but a four-membered vinyl cation is hard
1XLII
to presume because of the difficulty of formatinxi of bent vinyl cations (see above for solvolysis rates of cyclic vinyl triflates). However, molecular orbital calculations support a non-classical stabilization for the cyclobutenyl cation (55). If this is tme, then one must be able to find a high reaction rate for cyclobutenyl triflate/nonaflate in solvolysis reactions. As shown below, the cyclobutenyl nonaflate solvolyses 73,700 times faster (56) than the cvclo~entenvlnonaflate, which can only be explained
k,lllOOoCl EtOH 73700
1
10
ZWXO
by postulating a nou-classical structure (XLI) for the cyclobutenyl cation. Further proof of a uon-classical stabilization was obtained from the solvolysis of 2- and 3-methyl cyclobutenyl nonaflates. They were found to solvolyze 140 times and 10 times, respectively, faster than the cyclobutenyl nonaflate (57), in agreement with the MO calculations showing a considerable amount of positive charge on the Cz and the C j carbon atoms. 'The literature on vinyl cations is increasing rapidly and it is no longer a specialized field. We believe that the time is ripe now for vinyl cations t o be treated at textbook level. Literature Cited Inc.. NrwVork. 1913. (2) Nenilzwcu, C. D.;.*~'~Carbonium loni." "01. 1, iEdi'an: olah ii. ond Slhleyq P. v. R.1 Wiley-lnfemeieoce.New York; 1968, p. 2 if. (3) Gleave, 1.L., Hugh-, E I)., and Ingold. C K.. J. Chem. Sue.. 238 1193Si. (4) kth.1, I).. and Cold. V.. "Csrboniun lorn: An Intmduefiori." Acndmlic Press. Ine.. New York. 1967: Oloh, .,and Cererio. M. C., "Rasic Pnncipia. of o,sanic ChemisLw. w. A. Bel,,amin, I""., i\'w Yark. ,966, p. 821: Allinger, N. L., Caua, M. P., Dc Jmll. 11. C.. Johnron. C. H.. Iekl, N. A , and Stwens. C. L., "Orgaganir rhemiriri.l'Worth Pnhiishon, Inc.. NewYork, 197L.pM3. " (6) Sichl. H.~".. Camahun. J L J. C.. Eckrs. L.. md Hanack. M.. 4nyel.i. Chrm..XR. 677 11974).However see aiw, Hogween. ki.. bed., C. F., litml~adroa 3343 (19711. (7) Suatman. R., Williamo. .I. E., Dewar, M J. S.. AIlm I.. C.. and Schlrver, 1'. u. R.. J, Amer Cham. S o c . 91. 5850 lL969): Hopkinaon, A. C., Ynlrr, I < , nnd Cdsmadis,l G.J. ChemPhys..56.3835(197LI. (8) lafhsn. W. A,. Hehre, W. J., and Pople, J. A.. J Amcr. Chrm Soc.. $1. 808 (L97L):Rsdom, L., Hariharm, P. C., Popie, J. A,, and Schlever. P,u. H.. J. Amer Chem Sm., 9%.€531(1978):~ (9) Hsnack. M., Accounts Chem. R e 3, 209 (1970):M0ds.a. O . . and 'I.ondini9.U.. Ad". Phys Org. Chm. 9, 186. 119711: Slang, P, J., Prwr Phy~018 Chem.,
.\.
,n wK,,m>3 .","".,~.",",.
(101 Moffif, W., Pmr. Hay. Sor. Ser. A . 202,548 i1Wl). (111 Reberrs. J.D.,sndChambers. V. C.,J.Amcr.Chrm. So< 71,5O?C(L9511. (121 Rappopon,2.. andKaspi. J.,J Chem Sos, Perkinll. llli?119721. 1131 Gxunwald, E.. and Winstein, S., J. A m e r Chcm. So
