Alkene metathesis is part of a new generation of synthetic reaction in which a double C=C bond is formed from appropriate reactants where no bond initially exists (another example is the Wittig reaction), with the involvement† of a 4-membered-ring metallacyclobutane ring 1 (again, very similar to the Wittig). I thought it might make a good addition to my collection of reaction mechanisms and so as the first step I set about locating the transition state (TS or TS’) for the reaction, using in this case a model for Grubbs’ catalyst. I have located a fair few transition states in my time, and was frankly not expecting a surprise. This is the story that showed otherwise …
Oxime formation from hydroxylamine and ketone. Part 2: Elimination.
September 25th, 2012This is the follow-up to the previous post exploring a typical nucleophilic addition-elimination reaction. Here is the elimination step, which as before requires proton transfers. We again adopt a cyclic mechanism to try to avoid the build up of charge separation during those proton movements.
The ten-electron homologue of semibullvalene.
September 21st, 2012Semibullvalene is a molecule which undergoes a facile [3,3] sigmatropic shift. So facile that it appears this equilibrium can be frozen out at the transition state if suitable substituents are used. This is a six-electron process, which leads to one of those homologous questions; what happens with ten electrons?
The direct approach is not always the best: butadiene plus dichlorocarbene
September 19th, 2012The four-electron thermal cycloaddition (in reverse a cheletropic elimination) of dichlorocarbene to ethene is a classic example of a forbidden pericyclic process taking a roundabout route to avoid directly violating the Woodward-Hoffmann rules. However, a thermal six-electron process normally does take the direct route, as in for example the Diels-Alder cycloaddition as Houk and co have recently showed using molecular dynamics[cite]10.1073/pnas.1209316109[/cite]. So can one contrive a six-electron cycloaddition involving dichlorocarbene?
Predicted properties of a candidate for a frozen semibullvalene.
September 17th, 2012I am following up on one unfinished thread in my previous post; a candidate was proposed in which the transition state for [3,3] sigmatropic rearrangement in a semibullvalene might be frozen out to become instead a stable minimum.
Frozen Semibullvalene: a holy grail (and a bis-homoaromatic molecule).
September 15th, 2012Semibullvalene is an unsettling molecule. Whilst it has a classical structure describable by a combination of Lewis-style two electron and four electron bonds, its NMR behaviour reveals it to be highly fluxional. This means that even at low temperatures, the position of these two-electron bonds rapidly shifts in the equilibrium shown below. Nevertheless, this dynamic behaviour can be frozen out at sufficiently low temperatures. But the barrier was sufficiently low that a challenge was set; could one achieve a system in which the barrier was removed entirely, to freeze out the coordinates of the molecule into a structure where the transition state (shown at the top) became instead a true minimum (bottom)? A similar challenge had been set for freezing out the transition state for the Sn2 reaction into a minimum, the topic also of a more recent post here. Here I explore how close we might be to achieving inversion of the semibullvalene [3,3] sigmatropic potential.
What is the range of values for a (sp3)C-C(sp3) single bond length?
September 12th, 2012Here is a challenge: what is the longest C-C bond actually determined (in which both carbon termini are sp3 hybridised)? I pose this question since Steve Bachrach has posted on how to stabilize long bonds by attractive dispersive interactions, and more recently commenting on what the longest straight chain alkane might be before dispersive interaction start to fold it (the answer appears to be C17).
The Sn2 reaction: followed up.
September 12th, 2012An obvious issue to follow-up my last post on the (solvated) intrinisic reaction coordinate for the Sn2 reaction is how variation of the halogen (X) impacts upon the nature of the potential.