{"id":22407,"date":"2011-11-16T11:32:08","date_gmt":"2011-11-16T11:32:08","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=5500"},"modified":"2011-11-16T11:32:08","modified_gmt":"2011-11-16T11:32:08","slug":"the-oxidation-of-alkynes-things-are-not-always-what-they-seem-2","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=22407","title":{"rendered":"The peroxidation of alkynes: things are not always what they seem."},"content":{"rendered":"
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The epoxidation of an alkene<\/em> to give an oxirane is taught in introductory organic chemistry. Formulating an analogous mechanism for such reaction of an alkyne<\/em><\/strong>\u00a0sounds straightforward, but one gradually realises that it requires raiding knowledge from several other areas of (perhaps slightly more advanced) chemistry to achieve a joined up approach to the problem. I had indeed hinted in a\u00a0previous post<\/a> that the mechanism for oxidation of acetylene to ketene might be an interesting arrow pushing challenge to set a bright tutorial group, and it was that self-hint that has led me to here.\u00a0I now explore how my “arrow pushing” intuition stands up to a computational examination.<\/p>\n

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The products of the reaction of an acetylene with mCPBA (m-chloroperbenzoic acid) are set out[1]<\/a><\/span> and the primary step of the mechanism[1]<\/a><\/span>\u00a0(this article declines to discuss the subsequent steps).\u00a0The primary product is the formation of oxirene 1<\/strong>, a 4\u03c0-annulene which then undergoes an electrocyclic pericyclic ring opening to give the formyl carbene 4<\/strong> (or 2<\/strong> or 3<\/strong>). This presumed intermediate is also presumed to be highly reactive, and likely to undergo a variety of reactions (for example it will insert into a C-H bond if given one). I concentrate initially on just one path, the Wolff rearrangement (a [1,2] sigmatropic pericyclic reaction) to give the ketene 7<\/strong>. The scheme above constitutes one of those mandatory mechanistic challenges that organic chemists, almost without exception, cannot resist trying to solve, in the same way that some people may be addicted to Sudoko puzzles! So now for the reality check.<\/p>\n

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  1. The intrinsic reaction coordinate (IRC) for the reaction to form oxirene 1<\/b>\u00a0is shown below (DOI: 10.14469\/ch\/10244<\/a>). Its uneventful profile is deceptive.
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  2. Look how delightfully non-linear the motions of the atoms are; one of the new C-O bonds clearly forms long before the second, in what is termed an asynchronous exothermic reaction. This is almost certainly due to the forming anti-aromaticity of the product, which tends to favour such asymmetry.
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    IRC for per oxidation of acetylene. Click for 3D<\/p><\/div>\n<\/li>\n

  3. Easily overlooked however would be the nature of the resulting product, which is actually a hydrogen-bonded complex of the oxirene and the acetic acid. Oxirene is a planar ring with two \u03c0-electrons from the double bond, and two \u03c0Lp<\/sub> electrons from the oxygen. This makes it an anti-aromatic annulene, just like say cyclobutadiene<\/a>. One aspect of this anti-aromaticity which is not often noted is that the anti-aromaticity makes these\u00a0\u03c0 electrons especially basic, and so it is with oxirene. The oxygen forms a remarkably short\u00a0(1.673\u00c5) hydrogen bond to the H-O part of the carboxylic acid (in the process slightly attenuating its otherwise unremitting anti-aromaticity).<\/li>\n
  4. However, the IRC for the next stage is unexpected (DOI: 10.14469\/ch\/10245<\/a>). According to the scheme above, the oxirene would ring open to give a carbene 3<\/strong>, at which point this species might be expected to steady\u00a0itself before deciding what pathway it\u00a0will undertake. Shown below is the route 1-3-6<\/strong>.\"\"<\/a>\"\"<\/a>\n