December 9th, 2012
Sharpless epoxidation converts a prochiral allylic alcohol into the corresponding chiral epoxide with > 90% enantiomeric excess[1],[2]. Here is the first step in trying to explain how this magic is achieved.
Read the rest of this entry »
References
- J.M. Klunder, S.Y. Ko, and K.B. Sharpless, "Asymmetric epoxidation of allyl alcohol: efficient routes to homochiral .beta.-adrenergic blocking agents", The Journal of Organic Chemistry, vol. 51, pp. 3710-3712, 1986. https://doi.org/10.1021/jo00369a032
- R.M. Hanson, and K.B. Sharpless, "Procedure for the catalytic asymmetric epoxidation of allylic alcohols in the presence of molecular sieves", The Journal of Organic Chemistry, vol. 51, pp. 1922-1925, 1986. https://doi.org/10.1021/jo00360a058
Tags: animation, asymmetric epoxidation, catalysis, Enantioselective, free energy, lower energy conformations, Reaction Mechanism, Tutorial material
Posted in Interesting chemistry | 1 Comment »
December 4th, 2012
Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this?
Read the rest of this entry »
Tags: Birch reduction, energy, eventual product, less stable diene product, Reaction Mechanism, Tutorial material
Posted in Uncategorised | 4 Comments »
December 3rd, 2012
I promised that the follow-up to on the topic of Birch reduction would focus on the proton transfer reaction between the radical anion of anisole and a proton source, as part of analysing whether the mechanistic pathway proceeds O or M.
Read the rest of this entry »
Tags: Birch reduction, dielectric, energy, free energy, free energy barrier, Hammonds postulate, proton transfer, Reaction Mechanism, Tutorial material
Posted in Uncategorised | 2 Comments »
December 1st, 2012
The 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.
Read the rest of this entry »
Tags: dielectric, energy, free energy, Reaction Mechanism, Tutorial material
Posted in Uncategorised | 2 Comments »
November 30th, 2012
A dichotomy is a division into two mutually exclusive, opposed, or contradictory groups. Consider the reaction below. The bicyclic pentadiene on the left could in principle open on heating to give the monocyclic [12]-annulene (blue or red) via what is called an electrocyclic reaction as either a six (red) or eight (blue) electron process. These two possibilities represent our dichotomy; according to the Woodward-Hoffmann (WH) pericyclic selection rules, they represent contradictory groups. Depending on the (relative) stereochemistry at the ring junctions, if one reaction is allowed by the WH rules, the other must be forbidden, and of course vice-versa. It is a nice challenge to ask students to see if the dichotomy can be reconciled.
Read the rest of this entry »
Tags: Möbius, pericyclic, pericylic, positive energy activation barrier, Reaction Mechanism, Tutorial material
Posted in Uncategorised | No Comments »
November 26th, 2012
I was intrigued by one aspect of the calculated transition state for di-imide reduction of an alkene; the calculated NMR shieldings indicated an diatropic ring current at the centre of the ring, but very deshielded shifts for the hydrogen atoms being transferred. This indicated, like most thermal pericyclic reactions, an aromatic transition state. Well, one game one can play with this sort of reaction is to add a double bond. This adds quite a twist to this classical reaction!
Read the rest of this entry »
Tags: free energy, Möbius knot, Möbius link, pericyclic, Reaction Mechanism
Posted in Interesting chemistry | No Comments »
November 25th, 2012
Not a few posts on this blog dissect the mechanisms of well known text-book reactions. But one reaction type where there are few examples on these pages are reductions. These come in three types; using electrons, using a hydride anion and using di-hydrogen. Here I first take a closer look at the third type, and in particular di-hydrogen as delivered from di-imide.
Read the rest of this entry »
Tags: free energy discrimination, pericyclic, Reaction Mechanism, Tutorial material
Posted in Uncategorised | 2 Comments »
November 20th, 2012
More than 60 million molecules are known, and many are fascinating. But beauty is in the eye of the beholder. Thus it was that I came across the attached molecule[1]. It struck me immediately as, well, beautiful!
Read the rest of this entry »
References
- S. Lai, T. Lin, Y. Chen, C. Wang, G. Lee, M. Yang, M. Leung, and S. Peng, "Metal String Complexes: Synthesis and Crystal Structure of [Ni<sub>4</sub>(μ<sub>4</sub>-phdpda)<sub>4</sub>] and [Ni<sub>7</sub>(μ<sub>7</sub>-teptra)<sub>4</sub>Cl<sub>2</sub>] (H<sub>2</sub>phdpda = <i>N</i>-Phenyldipyridyldiamine and H<sub>3</sub>teptra = Tetrapyridyltriamine)", Journal of the American Chemical Society, vol. 121, pp. 250-251, 1998. https://doi.org/10.1021/ja982065w
Tags: metal wire
Posted in Interesting chemistry | 2 Comments »
November 18th, 2012
A game chemists often play is to guess the mechanism for any given reaction. I thought I would give it a go for the decomposition of the tris-peroxide shown below. This reaction is known to (rapidly, very rapidly) result in the production of three molecules of propanone, one of ozone and a lot of entropy (but not heat).[1]
Read the rest of this entry »
References
- F. Dubnikova, R. Kosloff, J. Almog, Y. Zeiri, R. Boese, H. Itzhaky, A. Alt, and E. Keinan, "Decomposition of Triacetone Triperoxide Is an Entropic Explosion", Journal of the American Chemical Society, vol. 127, pp. 1146-1159, 2005. https://doi.org/10.1021/ja0464903
Tags: free energy, lower energy saddle point, Reaction Mechanism
Posted in Interesting chemistry | No Comments »