{"id":7027,"date":"2012-07-09T20:58:16","date_gmt":"2012-07-09T19:58:16","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=7027"},"modified":"2012-07-09T20:58:16","modified_gmt":"2012-07-09T19:58:16","slug":"joining-up-the-pieces-peroxidation-of-ethyne","status":"publish","type":"post","link":"https:\/\/www.rzepa.net\/blog\/?p=7027","title":{"rendered":"Joining up the pieces. Peroxidation of ethyne."},"content":{"rendered":"
\n

Sometimes, connections between different areas of chemistry just pop out (without the help of semantic web tools, this is called serendipity). So here, I will try to join up some threads which emerge from previous posts.<\/p>\n

    \n
  1. I had noted<\/a> that antiaromaticity in cyclopropenium anion is lessened by the system adopting gross geometric distortions, which take the anionic lone pair out of conjugation from the ring.<\/li>\n
  2. Similarly, cyclobutadiene can form a complex<\/a> with the guanidinium cation in which the anti-aromaticity is reduced by the formation of strong C…H-N hydrogen bonds.<\/li>\n
  3. Unhappy with modelling a cation without a counter-ion, I added one<\/a>. I noted that the cyclobutadiene+ ion pair was more stable in this more complete form.<\/li>\n
  4. My next connection is to a post on how ethyne reacts with peracetic acid<\/a>. The initial product of this reaction is oxirene, which like cyclobutadiene or cyclopropenium anion is anti-aromatic. This time, the liberated acetic acid forms a remarkably strong hydrogen bond to the oxygen of the antiaromatic ring as a way of reducing the antiaromaticity.\u00a0<\/li>\n
  5. Particularly noteworthy was that the initial attack of oxygen on the alkyne was very asymmetric. This reminded of another post<\/a> on the reaction of dichlorocarbene with ethene, which too is asymmetric, yet again to avoid an antiaromatic transition state. However, as the hydrogen bond\u00a0in 4<\/strong> above get stronger, the antiaromatic oxirene becomes symmetrical again. It is as if the hydrogen bond had replaced the need for asymmetry (as with 2 above).<\/li>\n
  6. Another asymmetric example<\/a> is the 2+2 closed shell cycloaddition of two ethenes, which adopt a different form of distortion.<\/li>\n<\/ol>\n

    The original alkyne+peracid study was conducted using a gas phase model. I decided to revisit it now, but to change the modelled medium from the gas phase to continuum water. I show the IRC (intrinsic reaction coordinates) for this reaction<\/a> in continuum water followed by the gas phase below (click on the animations to see the transition state model).<\/p>\n

    \"\"<\/p>\n

    \"\"<\/a><\/p>\n

    \"\"<\/p>\n

    \"\"<\/a><\/p>\n

    I want to compare the difference that introducing a model solvent (water) has made to the appearance of the reaction path.<\/p>\n

      \n
    1. In water, the symmetry of the forming antiaromatic oxirene ring is always maintained. There is no distortion; the combination of hydrogen bond, developing ionicity and its stabilization by the model solvent, appears to eliminate the need for such distortion. The free energy barrier, \u0394G\u2021<\/sup> (\u03c9B97XD\/6-311G(d,p) is 32.2 kcal\/mol, outside of a room temperature reaction.<\/li>\n
    2. In water, the proton transfer step comes much later, and is visible in the RMS gradient norm at +1.4.<\/li>\n
    3. In the gas phase, the IRC is much more complex (as previously noted). Pronounced asymmetry develops, and this only resymmetrises late on, when the hydrogen bond forms.<\/li>\n
    4. In the gas phase, the proton transfer occurs relatively early, and it cannot be found as a discrete feature in the RMS gradient norm plot.\u00a0<\/li>\n
    5. If a more acidic peracid is introduced, say\u00a0CF3<\/sub>CO3<\/sub>H, and the reaction is again simulated in water, the proton transfer is further delayed (below), and the barrier drops to\u00a0\u0394G\u2021<\/sup> 25.9 kcal\/mol, an entirely viable thermal reaction. I do not believe this particular variation has ever been tested experimentally;\u00a0anyone up for it?\u00a0\"\"<\/a><\/li>\n
    6. The product of the CF3<\/sub>CO3<\/sub>H reaction is shown below. It has a remarkably short predicted hydrogen bond of\u00a01.55\u00c5\u00a0between the oxirene and the trifluoracetic acid.\"\"<\/a><\/li>\n<\/ol>\n

      The take home message is that the very nature of a reaction, the geometry (symmetry) of the molecules taking part, and the timing of the changes can be very visibly changed by simulating the event with a solvent. In the past of course, all such computational studies were conducted purely as a gas phase model.<\/p>\n

      Postscript:<\/strong>The above shows how even a change in continuum solvent can affect the features of the reaction path. A rather greater perturbation is to change e.g.<\/em> the substituents on the alkyne. I have tried replacing one H with t-butyl, and the other with OH. The rationale for the former is that t-butyl acetylene is actually the substrate that this reaction has been performed on, and for OH that it pushes electrons into the oxirene, making is more anti-aromatic and hence more liable to avoid that antiaromaticity. Animation of the IRC for this combination<\/a> is shown below. Notice how the reaction now proceeds in a concerted manner directly from the alkyne to the hydroxy-carbene, without any sign of an intervening oxirene.\u00a0\"\"<\/a><\/p>\n

      The energy and gradient profiles for this variation are shown below. Notice in particular how the barrier has dropped; it is now a much easier reaction.\"\"<\/a><\/p>\n

      \"\"<\/a><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

      Sometimes, connections between different areas of chemistry just pop out (without the help of semantic web tools, this is called serendipity). So here, I will try to join up some threads which emerge from previous posts. I had noted that antiaromaticity in cyclopropenium anion is lessened by the system adopting gross geometric distortions, which take […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[2336,1085],"tags":[839,169,171,872,1530,882],"class_list":["post-7027","post","type-post","status-publish","format-standard","hentry","category-curl-arrows","category-reaction-mechanism-2","tag-alkyne","tag-gas-phase","tag-gas-phase-model","tag-perepoxidation","tag-reaction-mechanism","tag-semantic-web-tools"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p1gPyz-1Pl","jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7027","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=7027"}],"version-history":[{"count":0,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7027\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7027"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7027"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rzepa.net\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7027"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}