I return to this reaction one more time. Trying to explain why it is enantioselective for the epoxide product poses peculiar difficulties. Most of the substituents can adopt one of several conformations, and some exploration of this conformational space is needed.
Posts Tagged ‘energy’
Sharpless epoxidation, enantioselectivity and conformational analysis.
Thursday, January 3rd, 2013The mechanism of the Birch reduction. Sequel to benzene reduction.
Wednesday, December 5th, 2012I noted briefly in discussing why Birch reduction of benzene gives 1,4-cyclohexadiene (diagram below) that the geometry of the end-stage pentadienyl anion was distorted in the presence of the sodium cation to favour this product. This distortion actually has some pedagogic value, and so I elaborate this here.
The mechanism of the Birch reduction. Part 3: reduction of benzene
Tuesday, December 4th, 2012Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this?
The mechanism of the Birch reduction. Part 1: reduction of anisole.
Saturday, December 1st, 2012The Birch reduction is a classic method for partially reducing e.g. aryl ethers using electrons (from sodium dissolved in ammonia) as the reductant rather than e.g. dihydrogen. As happens occasionally in chemistry, a long debate broke out over the two alternative mechanisms labelled O (for ortho protonation of the initial radical anion intermediate) or M (for meta protonation). Text books seem to have settled down of late in favour of O. Here I take a look at the issue myself.
Secrets of a university tutor. An exercise in mechanistic logic: second dénouement.
Monday, October 29th, 2012Following on from our first mechanistic reality check, we now need to verify how product A might arise in the mechanism shown below, starting from B.
Text-books and the bromination of ethene.
Sunday, October 14th, 2012There is often a disconnect between how a text-book (schematically) represents a reaction and a more quantitive “reality” revealed by quantum mechanics. Is the bromination of ethene to give 1,2-dibromoethane one such example?
Ring-flipping in cyclohexane in a different light
Friday, October 12th, 2012The conformational analysis of cyclohexane is a mainstay of organic chemistry. Is there anything new that can be said about it? Let us start with the diagram below:
Frozen Semibullvalene: a holy grail (and a bis-homoaromatic molecule).
Saturday, 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.