This is a follow-up to one aspect of the previous two posts dealing with nucleophilic substitution reactions at silicon. Here I look at the geometries of 5-coordinate compounds containing as a central atom 4A = Si, Ge, Sn, Pb and of the specific formula C34AO2 with a trigonal bipyramidal geometry. This search arose because of a casual comment I made in the earlier post regarding possible cooperative effects between the two axial ligands (the ones with an angle of ~180 degrees subtended at silicon). Perhaps the geometries might expand upon this comment?
An alternative mechanism for nucleophilic substitution at silicon using a tetra-alkyl ammonium fluoride.May 27th, 2016
In the previous post, I explored the mechanism for nucleophilic substitution at a silicon centre proceeding via retention of configuration involving a Berry-like pseudorotation. Here I probe an alternative route involving inversion of configuration at the Si centre. Both stereochemical modes are known to occur, depending on the leaving group, solvent and other factors.,,
- L. Wozniak, M. Cypryk, J. Chojnowski, and G. Lanneau, "Optically active silyl esters of phosphorus. II. Stereochemistry of reactions with nucleophiles", Tetrahedron, vol. 45, pp. 4403-4414, 1989. http://dx.doi.org/10.1016/S0040-4020(01)89077-3
- L.H. Sommer, and H. Fujimoto, "Stereochemistry of asymmetric silicon. X. Solvent and reagent effects on stereochemistry crossover in alkoxy-alkoxy exchange reactions at silicon centers", J. Am. Chem. Soc., vol. 90, pp. 982-987, 1968. http://dx.doi.org/10.1021/ja01006a024
- D.N. Roark, and L.H. Sommer, "Dramatic stereochemistry crossover to retention of configuration with angle-strained asymmetric silicon", J. Am. Chem. Soc., vol. 95, pp. 969-971, 1973. http://dx.doi.org/10.1021/ja00784a081
The substitution of a nucleofuge (a good leaving group) by a nucleophile at a carbon centre occurs with inversion of configuration at the carbon, the mechanism being known by the term SN2 (a story I have also told in this post). Such displacement at silicon famously proceeds by a quite different mechanism, which I here quantify with some calculations.
I occasionally post about "RDM" (research data management), an activity that has recently become a formalised essential part of the research processes. I say recently formalised, since researchers have of course kept research notebooks recording their activities and their data since the dawn of science, but not always in an open and transparent manner. The desirability of doing so was revealed by the 2009 "Climategate" events. In the UK, Climategate was apparently the catalyst which persuaded the funding councils (such as the EPSRC, the Royal Society, etc) to formulate policies which required all their funded researchers to adopt the principles of RDM by May 2015 and in their future researches. An early career researcher here, anxious to conform to the funding body instructions, sent me an email a few days ago asking about one aspect of RDM which got me thinking.
I have previously commented on the Bürgi–Dunitz angle, this being the preferred approach trajectory of a nucleophile towards the electrophilic carbon of a carbonyl group. Some special types of nucleophile such as hydrazines (R2N-NR2) are supposed to have enhanced reactivity due to what might be described as buttressing of adjacent lone pairs. Here I focus in on how this might manifest by performing searches of the Cambridge structural database for intermolecular (non-bonded) interactions between X-Y nucleophiles (X,Y= N,O,S) and carbonyl compounds OC(NM)2.
- G. Klopman, K. Tsuda, J. Louis, and R. Davis, "Supernucleophiles—I", Tetrahedron, vol. 26, pp. 4549-4554, 1970. http://dx.doi.org/10.1016/S0040-4020(01)93101-1
Earlier, I constructed a possible model of hydronium hydroxide, or H3O+.OH– One way of assessing the quality of the model is to calculate the free energy difference between it and two normal water molecules and compare the result to the measured difference. Here I apply a further test of the model using isotopes.
I want to describe a recent attempt by a group of collaborators to share the research data associated with their just published article.
- C. Romain, Y. Zhu, P. Dingwall, S. Paul, H.S. Rzepa, A. Buchard, and C.K. Williams, "Chemoselective Polymerizations from Mixtures of Epoxide, Lactone, Anhydride, and Carbon Dioxide", J. Am. Chem. Soc., vol. 138, pp. 4120-4131, 2016. http://dx.doi.org/10.1021/jacs.5b13070
Scientists are familiar with the term data, at least in a scientific or chemical context, but appreciating metadata (meaning "after", or "beyond") is slightly more subtle, in the sense of using it to mean data about data. The challenge lies in clarifying where the boundary between data and its metadata lies and in specifying and controlling the vocabulary used for these metadata descriptions. Items in a chemical metadata dictionary might include e.g. subject classifications such as Organic Molecular Chemistry or identifiers such as InChIkey. But what could metametadata be? Here I briefly show some examples by way of illustration.