Additions of acids
to 1,3-dienes are conventionally understood
as involving discrete intermediates that undergo an ordinary competition
between subsequent pathways to form the observed products. The combined
experimental, computational, and dynamic trajectory study here suggests
that this view is incorrect, and that solvation dynamics plays a critical
role in the mechanism. While implicit solvent models were inadequate,
QM/QM′ trajectories in explicit solvent provide an accurate
prediction of the experimental selectivity in the addition of HCl
to 1,3-pentadiene. Trajectories initiated from a protonation saddle
point on the potential of mean force surface are predominantly unproductive
due to a gating effect of solvation that allows diene protonation
only when the incipient ion pair is neither too solvent-stabilized
nor too little. Protonation then leads to relatively unsolvated ion
pairs, and a majority of these collapse rapidly to the 1,2-product,
without barrier and without achieving equilibrium solvation as intermediates.
The remainder decay slowly, at a rate consistent with equilibrium
solvation as true intermediates, affording a mixture of addition products.
Overall, an accurate description of the nature and pathway selectivity
of the ion pair intermediates in carbocation reactions must allow
for species lacking equilibrium solvation. Potential reinterpretations
of a series of historically notable observations in carbocation reactions
are discussed.
Díaz-Urrutia and Ott (Reports, 22 March 2019, p. 1326) report a selective conversion of methane to methanesulfonic acid that is proposed to occur by a cationic chain reaction in which CH3+ adds to sulfur trioxide (SO3) to form CH3–S(O)2O+. This mechanism is not plausible because of the solvent reactivity of CH3+, the non-nucleophilicity of the sulfur atom of SO3, and the high energy of CH3–S(O)2O+.
A new class of bipolar redox active molecules with enhanced voltages is reported via the electronic coupling of phthalimide anolytes and phenothiazine catholytes. Their performance is tested under relevant nonaqueous redox flow battery conditions.
The carboborative ring contraction of cyclohexenes exhibits
an
abnormal selectivity pattern in which a formally concerted double
migration gives rise to predominant but not exclusive inversion products.
In dynamic trajectories, the inversion and retention products are
formed from the same transition state, and the trajectories accurately
account for the experimental product ratios. The unusual origin of
the selectivity is the dynamically retained non-equivalence of newly
formed versus pre-existing bonds after the first bond migration.
A combination of experimental C kinetic isotope effects (KIEs) and high-level density functional theory (DFT) calculations is used to distinguish between "enamine" and "enol" mechanisms in the Michael addition of acetone to trans-β-nitrostyrene catalyzed by Jacobsen's primary amine thiourea catalyst. In light of the recent findings that the widely used O-incorporation probe for these mechanisms is flawed, the results described in this communication demonstrate an alternative probe to distinguish between these pathways. A key advantage of this probe is that quantitative mechanistic information is obtained without modifying experimental conditions. This approach is expected to find application in resolving mechanistic debates, while providing valuable information about the key transition state of organocatalyzed reactions involving the α-functionalization of carbonyls.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.