Posts Tagged ‘Anomeric effect’

A periodic table for anomeric centres, this time with quantified interactions.

Monday, August 8th, 2016
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The previous post contained an exploration of the anomeric effect as it occurs at an atom centre X for which the effect is manifest in crystal structures. Here I quantify the effect, by selecting the test molecule MeO-X-OMe, where X is of two types:

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A periodic table for anomeric centres.

Saturday, August 6th, 2016
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In the last few posts, I have explored the anomeric effect as it occurs at an atom centre X. Here I try to summarise the atoms for which the effect is manifest in crystal structures.

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Anomeric effects at carbon involving lone pairs originating from one or two nitrogens.

Friday, July 8th, 2016
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The previous post looked at anomeric effects set up on centres such as B, Si or P, and involving two oxygen groups attached to these atoms. Here I vary the attached groups to include either one or two nitrogen atoms.[1]

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References

  1. Henry Rzepa., "Anomeric effects at carbon, involving lone pairs originating from one or two nitrogens", 2016. http://dx.doi.org/10.14469/hpc/936

Anomeric effects at boron, silicon and phosphorus.

Friday, July 1st, 2016
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The anomeric effect occurs at 4-coordinate (sp3) carbon centres carrying two oxygen substituents and involves an alignment of a lone electron pair on one oxygen with the adjacent C-O σ*-bond of the other oxygen. Here I explore whether other centres can exhibit the phenomenon. I start with 4-coordinate boron, using the crystal structure search definition below (along with R < 0.1, no disorder, no errors).[1]anomeric-bo-sq

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References

  1. Henry Rzepa., "Anomeric effects at boron, silicon and phosphorus.", 2016. http://dx.doi.org/10.14469/hpc/696

The geometries of 5-coordinate compounds of group 14 elements.

Monday, May 30th, 2016
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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?

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A visualization of the anomeric effect from crystal structures.

Thursday, August 27th, 2015
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The anomeric effect is best known in sugars, occuring in sub-structures such as RO-C-OR. Its origins relate to how the lone pairs on each oxygen atom align with the adjacent C-O bonds. When the alignment is 180°, one oxygen lone pair can donate into the C-O σ* empty orbital and a stabilisation occurs. Here I explore whether crystal structures reflect this effect.

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The formation of tetrahedral intermediates.

Friday, June 12th, 2015
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In the preceding post, I discussed the reaction between mCPBA (meta-chloroperbenzoic acid) and cyclohexanone, resulting in Baeyer-Villiger oxidation via a tetrahedral intermediate (TI). Dan Singleton, in whose group the original KIE (kinetic isotope measurements) were made, has kindly pointed out on this blog that his was a mixed-phase reaction, and that mechanistic comparison with homogenous solutions may not be justified. An intriguing aspect of the (solution) mechanism would be whether the TI forms quickly and/or reversibly and what the position of any equilibrium between it and the starting ketone is. This reminded me of work we did some years ago,[1] and here I discuss that.

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References

  1. A.M. Lobo, M.M. Marques, S. Prabhakar, and H.S. Rzepa, "Tetrahedral intermediates formed by nitrogen and oxygen attack of aromatic hydroxylamines on acetyl cyanide", The Journal of Organic Chemistry, vol. 52, pp. 2925-2927, 1987. http://dx.doi.org/10.1021/jo00389a050

Tunable bonds looked at in a different way

Sunday, July 11th, 2010
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The title of this post merges those of the two previous ones. The tunable C-Cl bond brought about in the molecule tris(amino)chloromethane by anomeric effects will be probed using the Laplacian of the electronic density.

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