I have written a few times about the so-called “anomeric effect“, which relates to stereoelectronic interactions in molecules such as sugars bearing a tetrahedral carbon atom with at least two oxygen substituents. The effect can be detected when the two C-O bond lengths in such molecules are inspected, most obviously when one of these bonds has a very different length from the other. The effect originates when one of the lone pair of electrons on one oxygen atom uniquely overlaps with the C-O antibonding σ* on another oxygen, thus shortening the length of the donating oxygen-carbon length and lengthening the length of accepting C-O bond. Here I take a look at tetra-substituted versions of this (C(OR)4), where in theory there are up to eight lone pairs, interacting with any of three C-O bonds, giving a total of 24 possible anomeric effects in one molecule.
Posts Tagged ‘Interesting chemistry’
Detecting anomeric effects in tetrahedral carbon bearing four oxygen substituents.
Monday, March 18th, 2024Scholarly journals vs Scholarly Blogs.
Friday, January 12th, 2024First, a very brief history of scholarly publishing, starting in 1665[cite]10.1098/rstl.1665.0001[/cite] when scientific journals started to be published by learned societies. This model continued until the 1950s, when commercial publishers such as Pergamon Press started with their USP (unique selling point) of rapid time to publication of ~3 months,[cite]10.1016/0040-4020(57)85003-0[/cite] compared to typical times for many learned society publishers of 2 years or longer. Fast forward another 50 years or so, and the commercial publishers were now dominating the scene, but the business model was still based on institutional subscriptions, whereby the institution rather than authors paid the costs of publication. As the number of journals expanded, even well-off institutions had to make difficult decisions on which subscriptions to keep and which to cancel. By the late 1990s the delivery model was changing from print to online, but the overall issue was that many scientists around the world no longer had access to many journals.
More examples of “double-headed” curly arrows: S and C Nucleophiles attacking acetyl chloride
Thursday, October 12th, 2023In an earlier post on this topic,[cite]10.59350/f00wf-5tq46[/cite]‡ I described how the curly-arrows describing the mechanism of a nucleophilic addition at a carbonyl group choreograph in two distinct ways, as seen in red or blue below. The arrows in red can be described as firstly addition to the carbonyl group to form either a transient intermediate (a two-step process) or instead a formal transition state state as a concerted single-step mechanism. The blue arrows do the reverse; firstly elimination and then followed by addition. I will use the shorthand AE for the first type and EA for the second type. Here I explore some more nucleophiles to see which of these two mechanisms they follow. Data for these results can be found at 10.14469/hpc/13171
N- carbon ylid: This is a very facile (low-barrier) reaction with a C-O bond length response that initially increases steeply, followed by a more modest decline and hence corresponds to an AE mechanism.
Blue blood.
Monday, August 7th, 2023Respiratory pigments are metalloproteins that transport O2, the best known being the bright red/crimson coloured hemoglobin in human blood. The colour derives from Fe2+ at the core of a tetraporphyrin ring. But less well known is blue blood, and here the colour derives from an oxyhemocyanin unit based on Cu1+ (the de-oxy form is colourless) rather than iron. See below for the carapace of a red rock crab.
One vs two bond rotation – An example using Acyl amides
Monday, April 3rd, 2023One of the important aspects of chemical reaction mechanisms is the order in which things happen. More specifically, the order in which bonds make or break when there are more than two involved in undertaking a reaction. So we have:
Derek Lowe asks “What’s a Journal For?” – Knowledge graphs?
Friday, October 21st, 2022What’s a Journal For? This debate has been raging ever since preprint servers were introduced as far back as 1991! Indeed, during my recent submission of a journal article, one of the questions asked was whether the article was already deposited in such a preprint server (in a positive sense, and not one excluding further submission progress). Since my previous comment on this theme was made more than three years ago, I thought I might update it.
Nitroaryls- A less-toxic alternative reagent for ozonolysis: modelling the final step to form carbonyls.
Saturday, October 8th, 2022Sometimes you come across a reaction which is so simple in concept that you wonder why it took so long to be accomplished in practice. In this case, replacing toxic ozone O3 as used to fragment an alkene into two carbonyl compounds (“ozonolysis”) by a relatively non-toxic simple nitro-group based reagent, ArNO2 in which the central atom of ozone is substituted by an N-aryl group. As reported by Derek Lowe, two groups have published[cite]10.1021/jacs.2c05648[/cite], [cite]10.1038/s41586-022-05211-0[/cite] details of such a reaction (Ar = 4-cyano or 3-CF3,5-NO2). But there are (at least) two tricks; the first is to use photo-excitation using purple LEDs (390nm light) to activate the nitro group. The second is to establish the best aryl substituents to use for achieving maximum yields of the carbonyl compounds and the best conditions for achieving the cyclo-reversion reaction, shown below as TS1. That step requires heating the cyclo-adduct up to ~80° in (aqueous) acetonitrile for anywhere between 1-48 hours. Here I take a computational look at that last step, the premise being that if such a model is available for this mechanism, it could in principle be used to optimise the conditions for the process.
What is the largest angle possible at 4-coordinate carbon – 180°?
Sunday, September 11th, 2022Four-coordinate carbon normally adopts a tetrahedral shape, where the four angles at the carbon are all 109.47°. But how large can that angle get, and can it even get to be 180°?
Unexpected Isomerization of Oxetane-Carboxylic Acids – catalyst design.
Saturday, August 13th, 2022Previously, a mechanism with a reasonable predicted energy was modelled for the isomerisation of an oxetane carboxylic acid to a lactone by using two further molecules of acid to transfer the proton and in the process encouraging an Sn2 reaction with inversion to open the oxetane ring. 
Dioxane tetraketone – an ACS molecule of the week with a mystery.
Wednesday, June 22nd, 2022I have long been fascinated by polymers of either carbon dioxide,† or carbon monoxide, or combinations of both. One such molecule, referred to as dioxane tetraketone when it was featured on the ACS molecule-of-the-week site and also known as the anhydride of oxalic acid, or more formally 1,4-dioxane-2,3,5,6-tetraone, has been speculated upon for more than a century.[cite]10.1002/cber.19080410335[/cite]