Vinyl cation

The vinyl cation is a carbocation with the positive charge on an alkene carbon. Its empirical formula is C2H+3. More generally, a vinylic cation is any disubstituted, trivalent carbon, where the carbon bearing the positive charge is part of a double bond and is sp hybridized. In the chemical literature, substituted vinylic cations are often referred to as vinyl cations, and understood to refer to the broad class rather than the C2H+3 variant alone. The vinyl cation is one of the main types of reactive intermediates involving a non-tetrahedrally coordinated carbon atom, and is necessary to explain a wide variety of observed reactivity trends. Vinyl cations are observed as reactive intermediates in solvolysis reactions, as well during electrophilic addition to alkynes, for example, through protonation of an alkyne by a strong acid. As expected from its sp hybridization, the vinyl cation prefers a linear geometry. Compounds related to the vinyl cation include allylic carbocations and benzylic carbocations, as well as aryl carbocations. The vinyl cation is a carbocation with the positive charge on an alkene carbon. Its empirical formula is C2H+3. More generally, a vinylic cation is any disubstituted, trivalent carbon, where the carbon bearing the positive charge is part of a double bond and is sp hybridized. In the chemical literature, substituted vinylic cations are often referred to as vinyl cations, and understood to refer to the broad class rather than the C2H+3 variant alone. The vinyl cation is one of the main types of reactive intermediates involving a non-tetrahedrally coordinated carbon atom, and is necessary to explain a wide variety of observed reactivity trends. Vinyl cations are observed as reactive intermediates in solvolysis reactions, as well during electrophilic addition to alkynes, for example, through protonation of an alkyne by a strong acid. As expected from its sp hybridization, the vinyl cation prefers a linear geometry. Compounds related to the vinyl cation include allylic carbocations and benzylic carbocations, as well as aryl carbocations. Compared to other reactive intermediates such as radicals and carbanions, the vinyl cation long remained 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 diicoordinate carbon. This is the first time such a transition state can be found in the literature. It wasn’t until the fifteen years later that this idea was revisited, with Grob and Cseh detecting vinyl cation formation during solvolysis reactions of alpha-vinyl halides in their seminal work. 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 phenylporopiolic 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. Generating Vinyl Cations 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 groups 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. Recently, 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. 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.