The vinyl cation is a carbocation with the positive charge on an alkene carbon. Its empirical formula of the parent ion is . Vinyl cation are invoked as reactive intermediates in solvolysis of vinyl halides, as well as electrophilic addition to alkynes and allenes.
History Vinyl cations have long been poorly-understood and were initially thought to be too high energy to form as reactive intermediates. Vinyl cations were first proposed in 1944 as a reactive intermediate for the acid-catalyzed hydrolysis of alkoxyacetylenes to give alkyl acetate. In the first step of their facile hydration reaction, which was the rate limiting step, a vinyl cation reactive intermediate was proposed; the positive charge was believed to formally lie on a dicoordinate carbon. This is the first time such a transition state can be found in the literature.
In 1959, Grob and Cseh detected vinyl cations during solvolysis reactions of alpha-vinyl halides. Indeed, for this contribution, Grob has been called “the father of the vinyl cation”. The 1960s saw a flurry of vinyl cation-related research, with kinetics data driving the argument for the existence of the species. Noyce and coworkers, for example, reported the formation of a vinyl cation in acid-catalyzed hydration of phenylpropiolic acid. The authors note that in the rate limiting step, a large positive charge develops on the benzylic carbon, indicating that the reaction proceeds through a vinyl cation transition state. Hyperconjugation and hydrogen bonding was evoked to explain the accessibility of the vinyl cation described by Noyce.
Generation
Vinyl cations have been observed as reactive intermediates during solvolysis reactions. Consistent with SN1 chemistry, these reactions follow first order kinetics. Generally, vinylic halides are unreactive in solution: silver nitrate does not precipitate silver halides in the presence of vinyl halides, and this fact was historically used to dispute the existence of the vinyl cation species. The introduction of “super” leaving group in the 1970s first allowed for the generation of vinyl cation reactive intermediates with appreciable lifetimes. These excellent leaving groups, such as triflate (trifluoromethanesulfonate) and nonaflate (nonafluorobutanesulfonate), are highly prone to SN1 reactivity. Utilization of these super leaving groups allowed researchers for the first time to move beyond speculation about the existence of such vinyl cations.
Other leaving groups, such as hypervalent iodine moities (which are 1 million fold better leaving groups than the classic triflates), have been utilized to such end as well. Hinkle and coworkers synthesized a number of alkenyl(aryl)iodonium triflates from hypervalent phenyliodo precursors. In the scheme shown, the E- and Z-vinyl triflates form after heterolytic carbon-iodine bond cleavage and subsequent trapping of the cation by triflate. The presence of both E- and Z-vinyl triflate products offers support for the formation of a primary vinyl cation reactive intermediate; through SN2 chemistry, both only one isomer would form.
Vinyl cation reactive intermediates have been generated in photochemical solvolysis reactions. The figure to the right depicts photochemical solvolysis of vinyl iodonium salt, through heterolytic carbon-iodine bond cleavage, to generate a vinyl carbocation and iodobenzene. The reactive intermediate is prone to either nucleophilic attack by the solvent to yield E- and Z-enol ether isomers, or beta hydrogen elimination.
Generation of cyclic vinyl cations The ease of generating cyclic vinyl cations depends on the size of the ring system, with vinyl cations residing on smaller rings being more difficult to produce. This trend is supported by calculations showing that the vinyl cation prefers a linear arrangement. Due to the high degree of strain in 3-membered ring systems, the generation of the smallest cyclic vinyl cation, cycloprop-1-enyl cation, remains elusive. The SN1 solvolysis chemistry used to produce other vinyl cations has not proven facile for the cycloprop-1-enyl cation. This is a chemical challenge that remains unsolved.
Structure
Two possible structures can be envisioned for , the simplest vinyl cation: a classical linear or a non-classical bridged structure. Ab initio calculations favor the bridged structure vs the classical by 5.0 kcal/mol. For substituted vinyl cations, however, the linear structure is supported by 13C and 1H NMR measurements. NMR spectroscopy of β-silyl vinyl cations exhibited a single 29Si NMR signal which implies that the two Si are equivalent. The vinyl cation has an intense IR peak at 1987 cm−1 for the C=C+ stretching. The crystallography reveals the bond angles between the vinyl cation carbons and the first carbon of the alkyl substituted to be near 180o.
Stability
Initially it was believed that the existence of vinyl cations was questionable because of the large energy difference between it and its vinyl precursor. Once it was established that stable vinyl cation intermediates can be attained through the solvolysis of vinyl compounds with good leaving groups like triflate and nonaflate and stabilized by electron-donating groups, a significant amount of progress as taken place and produced a field of stable vinyl cations.
One of the earliest vinyl cations studied had aryl substituents with an electron-donating moiety. Arylvinyl compounds are stabilized by resonance. Upon the removal of the leaving group, the empty p-orbital is perpendicular to the conjugated system of the phenyl ring, so it can only achieve resonance stabilization in its transition state when the vinyl empty p-orbital is coplanar with the p system of the phenyl ring. Adding steric bulk to the ortho-positions improve conjugation as it makes the phenyl ring orthogonal to the vinyl carbons but coplanar with the empty p-orbital.
Like arylvinyl cations, dienyl and allenyl cations are also stabilized by conjugation. Once again, double bonds in the conjugated system must be coplanar to the empty p-orbital to achieve resonance stabilization. In allenyl cations, the positive charge is well-distributed across the whole structure.
Cyclopropylvinyl cations exhibit a non-classical approach to stabilization. When it is in its bisected structure, there is suitable overlap between its empty p-orbital and the cyclopropyl ring that stabilization is achieved. In its other form, the perpendicular structure, the empty p-orbital is perpendicular to the ring system. The stabilizing power of the cyclopropyl ring is so great that it has become a driving thermodynamic force in rearrangements like 1,2-hydride shifts in (E)- and (Z)-3-cyclopropyl-2-propenyl triflate solvolysis.
Substituent effects on vinyl cation stability SubstituentStabilization#Electronic effect from α-substituentInduction^π-donationHyperconjugation-CH˭CH2+-+*-CH3++-Cl+-+*-Br+-+*-I+-+*-F--*+-NH2+-+*-OH+-+*-SH+-+*-C6H5++*-CF3---CH2F---NO2---C≡N+-+*-CH2Y***+-+*-Si(CH3)3++-C(O)H-+/-**-COOH-+/-**-C(CH3)2OH+--C≡CH+-+* Table 1: Electronic effects responsible for stabilization of vinyl cation at the α-position
^ ‘-’ electron-withdrawing, ‘+’ electron-donating
# ‘+’ indicates stabilization and ‘-‘ indicates destabilization of the substituted vinyl cation with respect to neutral alkene equivalent
*indicates the strongest factor responsible for (de)stabilization for substituents that exhibit more than one electronic effect
** the substituent is inductively withdrawing at the carbonyl carbon and also exhibits small electron delocalization from the carbonyl oxygen
*** Y = -F, -Cl, -Br, -I, -OH, -CN, -CF3
The presence of an empty p-orbital perpendicular to the p-bond imparts unwanted destabilization onto the vinyl cation. This inherent instability can be diminished through favorable interactions with a-substituents that reduce the charge at the carbocation. Ab initio computational methods have been used to show stabilizing or destabilizing effects of substituents by monitoring changes in the enthalpies, bond lengths, bond order, and charges in the structures.
There are three possible electronic effects that a substituent may exhibit to influence the stability of the vinyl cation. It may either destabilize the cation by drawing even more electron density from the carbon or stabilizing by contributing more electron density. The carbocation positive charge can be relieved by an unsaturated carbon-based or heteroatomic substituent through p-donation and/or C-H hyperconjugation by methylene/methyl substituents. In addition, inductive effects can either stabilize or destabilizing depending on whether the substituent is electron-donating or –withdrawing. Individual electronic effects are not isolable from the others as all three work together to influence the overall stability of the cation.
For vinyl cations, relative stabilities can be compared with respect to their neutral alkene analogs. To obtain the stabilization properties of a-substituents, the isodesmic reaction was used to calculate enthalpy differences between the substituted vinyl cation and its neutral alkene precursor by getting its reaction enthalpy. This method is advantageous as it can be benchmarked against experimentally-determined thermochemical values. Calculations are initialized from the bridged, nonclassical structure of vinyl cations as it is the global minimum.
In a preliminary work, 4 substituents (-CH=CH2, -F, -Cl, -CH3) were initially studied to investigate electronic effects on vinyl cation stability. The a-substituents induce structural changes in the vinyl cation when compared to its neutral alkene counterpart. These changes can be attributed to the electronic effects present. In vinyl cations, there is a marked decrease in the C-R and C=C bond lengths, indicative of electron donation or induction between Ca and R, and Cb and Ca. On the other hand, the increase in the Cb-H bond length implies a strong hyperconjugative effect that is inversely related to the thermodynamic stability of the cation. Stabilization is possible because of a good overlap between the C-H bond and the empty p-orbital at Ca. Hyperconjugation is evident in all structures because of the adjacent Cb-H bond and in the –CH3 substituent.
Enthalpy calculations obtained from the isodesmic reaction are fair accurate and shows good correlation with experimental data. Stabilization is ranked the order, -F < -Cl < -CH3 < -CH=CH2. All substituents impart stability except for fluorine which destabilized the vinyl cation by 7 kcal/mole. This phenomenon can be explained by comparing a-fluorine substituent effects on vinyl and ethyl cations. In ethyl cations, fluorine stabilizes the carbocation. The stark difference in the stabilizing capabilities of fluorine in the vinyl and ethyl cation is due to the difference in the hybridization of the a-carbons. Because the vinyl cation has a more electronegative sp-hybridized carbon, inductive effects will be more prominent. Having electronegative sp-hybridized carbon interact with fluorine significantly destabilizes the structure. This phenomenon is also apparent in a lesser extent when comparing –CH3 and –CH=CH2 substituents, where -CH=CH2 is less stabilizing.
Heteroatoms like fluorine and chlorine, can exhibit both inductive (electron-withdrawing) and p-donation electronic effects because of their high electronegativities and p-electrons. Stabilization then depends on the balance between the two electronic effects. For fluorine, destabilization via induction is dominant and resonance is significantly weaker. While for chlorine, resonance is sufficient to counteract induction so that overall the effect is stabilizing.