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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head profile="http://gmpg.org/xfn/1"> <title>Computational Organic Chemistry » 2018 » December</title> <meta name="google-site-verification" content="g1Myv4tUVAmqRbwZeBi7IPuSZpP64RWjVJ6itIoouCo"> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <style type="text/css" media="screen">@import url( /blog/wp-content/themes/comporg/style.css);</style> <link rel="stylesheet" id="wp-block-library-css" href="/blog/wp-includes/css/dist/block-library/style.min.css?ver=5.6.1" type="text/css" media="all"> <script type="text/javascript">  </script> </head> <body> <div id="header"> <div id="header_img"></div> </div> <div id="link_section"> <div style="float:left"> <a href="/blog/about">About this Blog</a> | <a href="/">Book Homepage</a> | <a href="http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471713422.html">Purchase the Book</a> </div> </div> <div id="after_links"></div> <div id="content"> <div id="main"> <h2 class="post-title">Archive for December, 2018</h2> <div class="box"> <h2><a href="/blog/archives/4271" rel="bookmark" title="Permanent Link: Electrocycic reactions of cethrene derivatives">Electrocycic reactions of cethrene derivatives</a></h2> <div class="post-content"> <p>Pericyclic reactions remain a fruitful area of research despite the seminal publication of the Woodward-Hoffmann rules decades ago. Here are two related papers of pericyclic reactions that violate the Woodward-Hoffmann rules.</p> <p>First, Solomek, Ravat, Mou, Kertesz, and Jur铆cek reported on the thermal and photochemical electrocyclization reaction of diphenylcetherene <b>1a</b>.<a href="#cethreneR1"><sup>1</sup></a> Though they were not able to directly detect the intermediate <b>2</b>, through careful examination of the photochemical reaction, they were able to infer that the thermal cyclization goes via the formally forbidden conrotatory pathway (see Scheme 1).</p> <p><b>Scheme 2</b>.</p> <p align="center"><img src="/blog/wp-content/cethreneImg1.png"></p> <p>Kinetic studies estimate the activation barrier is 14.1 kcal mol<sup>-1</sup>. They performed DFT computations of the parent <b>1b</b> using a variety of functionals with both restricted and unrestricted wavefunctions. The allowed pathway to <b>2syn</b> is predicted to be greater than 27 kcal mol<sup>-1</sup>, while the formally forbidden pathway to <b>2anti</b> is estimated to have a lower barrier of about 23 kcal mol<sup>-1</sup>. The two transition states for these different pathways are shown in Figure 1, and the sterics that force a helical structure to <b>1</b> help make the forbidden pathway more favorable.</p> <table align="center" border="0" cellspacing="0" cellpadding="3"> <tr align="center" valign="bottom"> <td> <p><img src="/blog/wp-content/cethreneTSsyn.png"><br><b>TS(1b→2b-syn)</b></p> </td> <td> <p><img src="/blog/wp-content/cethreneTSanti.png"><br><b>TS(1b→2b-anti)</b></p> </td> </tr> </table> <p align="center"><b>Figure 1</b>. (U)B3LYP/6-31G optimized geometries of the transition states taking <b>1</b> into <b>2</b>.</p> <p>Nonetheless, all of the DFT computations significantly overestimate the activation barrier. The authors make the case that a low-lying singlet excited state results in an early conical intersection that reduces the symmetry from <i>C<sub>2</sub></i> to <i>C<sub>1</sub></i>. In this lower symmetry pathway, all of the states can mix, leading to a lower barrier. However, since DFT is intrinsically a single Slater configuration, the mixing of the other states is not accounted for, leading to the overestimated barrier height.</p> <p>In a follow up study, this group examined the thermal and photo cyclization of 13,14-dimethylcethrene <b>4</b>.<href><sup>2</sup> The added methyl groups make the centhrene backbone more helical, and this precludes the formal allowed disrotatory process. The methyl groups also prohibit the oxidation that occurs with <b>1</b>, driven by aromatization, allowing for the isolation of the direct product of the cyclization <b>5</b>. This <i>anti</i> stereochemistry is confirmed by NMR and x-ray crystallography. The interconversion between <b>4</b> and <b>5</b> can be controlled by heat and light, making the system an interesting photoswitch.</href></p> <p align="center"><img src="/blog/wp-content/cethreneImg2.png"></p> <p>Also of interest is the singlet-triplet gap of <b>1</b> and <b>4</b>. The DFT computed Δ<i>E<sub>ST</sub></i> is about 10 kcal mol<sup>-1</sup> for <b>4</b>, larger than the computed value of 6 kcal mol<sup>-1</sup> for <b>1b</b>. The EPR of <b>1b</b> does show a signal while that of <b>4</b> has no signal. To assess the role of the methyl group, they computed the singlet triplet gaps for <b>1b</b> and <b>4</b> at two different geometries: where the distance between the carbons bearing the methyl groups is that in <b>1b</b> (3.03 Å) and in <b>4</b> (3.37 Å). The lengthening of this distance by the methyl substituents is due to increased helical twist in <b>4</b> than in <b>1b</b>. For <b>1b</b>, the gap increases with twisting, from 7.1 to 8.3 kcal mol<sup>-1</sup>, while for <b>4</b> the gap increases by 1.8 kcal mol<sup>-1</sup> with the increased twisting. This change is less than the effect of methyl substitution, which increases the gap by 2.2 kcal mol<sup>-1</sup> at the shorter distance and 2.8 kcal mol<sup>-1</sup> at the longer distance. Thus, the electronic (orbital) effect of methyl substitution affects the singlet-triplet gap more than the geometric twisting.</p> <h3>References</h3> <p><a name="cethreneR1"></a></p> <p>1) 艩olomek, T.; Ravat, P.; Mou, Z.; Kertesz, M.; Jur铆ček, M., "Cethrene: The Chameleon of Woodward鈥揌offmann Rules." <i>J. Org. Chem.</i> <b>2018</b>, <i>83</i>, 4769-4774, DOI: <a href="https://doi.org/10.1021/acs.joc.8b00656">10.1021/acs.joc.8b00656</a>.</p> <p><a name="cethreneR2"></a></p> <p>2) Ravat, P.; 艩olomek, T.; H盲ussinger, D.; Blacque, O.; Jur铆ček, M., "Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch." <i>J. Am. Chem. Soc.</i> <b>2018</b>, <i>140</i>, 10839-10847, DOI: <a href="https://doi.org/10.1021/jacs.8b05465">10.1021/jacs.8b05465</a>.</p> <h3>InChIs</h3> <p><b>1b</b>: InChI=1S/C28H16/c1-5-17-7-3-11-23-25(17)19(9-1)15-21-13-14-22-16-20-10-2-6-18-8-4-12-24(26(18)20)28(22)27(21)23/h1-16H<br>InChIKey=GBMHAGKZRAVBDO-UHFFFAOYSA-N</p> <p><b>4</b>: InChI=1S/C30H20/c1-17-9-11-19-5-3-7-21-15-23-13-14-24-16-22-8-4-6-20-12-10-18(2)26(28(20)22)30(24)29(23)25(17)27(19)21/h3-16H,1-2H3<br>InChIKey=MXTVFWTUCPRNIW-UHFFFAOYSA-N</p> <p><b>5</b>: nChI=1S/C30H20/c1-29-13-11-17-5-3-7-19-15-21-9-10-22-16-20-8-4-6-18-12-14-30(29,2)28(24(18)20)26(22)25(21)27(29)23(17)19/h3-16H,1-2H3/t29-,30-/m0/s1<br>InChIKey=SUMMGEBJORQMAI-KYJUHHDHSA-N</p>  </div> <p class="bottom"> <span class="cat"><a href="/blog/archives/category/qm-method/dft" rel="category tag">DFT</a> &<a href="/blog/archives/category/reactions/electrocyclization" rel="category tag">electrocyclization</a></span> <span class="user">Steven Bachrach</span> <span 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2013</a></li> <li><a href="/blog/archives/date/2013/10">October 2013</a></li> <li><a href="/blog/archives/date/2013/09">September 2013</a></li> <li><a href="/blog/archives/date/2013/08">August 2013</a></li> <li><a href="/blog/archives/date/2013/07">July 2013</a></li> <li><a href="/blog/archives/date/2013/06">June 2013</a></li> <li><a href="/blog/archives/date/2013/05">May 2013</a></li> <li><a href="/blog/archives/date/2013/04">April 2013</a></li> <li><a href="/blog/archives/date/2013/03">March 2013</a></li> <li><a href="/blog/archives/date/2013/02">February 2013</a></li> <li><a href="/blog/archives/date/2013/01">January 2013</a></li> <li><a href="/blog/archives/date/2012/12">December 2012</a></li> <li><a href="/blog/archives/date/2012/11">November 2012</a></li> <li><a href="/blog/archives/date/2012/10">October 2012</a></li> <li><a href="/blog/archives/date/2012/09">September 2012</a></li> <li><a href="/blog/archives/date/2012/08">August 2012</a></li> <li><a href="/blog/archives/date/2012/07">July 2012</a></li> <li><a href="/blog/archives/date/2012/06">June 2012</a></li> <li><a href="/blog/archives/date/2012/05">May 2012</a></li> <li><a href="/blog/archives/date/2012/04">April 2012</a></li> <li><a href="/blog/archives/date/2012/03">March 2012</a></li> <li><a href="/blog/archives/date/2012/02">February 2012</a></li> <li><a href="/blog/archives/date/2012/01">January 2012</a></li> <li><a href="/blog/archives/date/2011/12">December 2011</a></li> <li><a href="/blog/archives/date/2011/11">November 2011</a></li> <li><a href="/blog/archives/date/2011/10">October 2011</a></li> <li><a href="/blog/archives/date/2011/09">September 2011</a></li> <li><a href="/blog/archives/date/2011/08">August 2011</a></li> <li><a href="/blog/archives/date/2011/07">July 2011</a></li> <li><a href="/blog/archives/date/2011/06">June 2011</a></li> <li><a href="/blog/archives/date/2011/05">May 2011</a></li> <li><a href="/blog/archives/date/2011/04">April 2011</a></li> <li><a href="/blog/archives/date/2011/03">March 2011</a></li> <li><a href="/blog/archives/date/2011/02">February 2011</a></li> <li><a href="/blog/archives/date/2011/01">January 2011</a></li> <li><a href="/blog/archives/date/2010/12">December 2010</a></li> <li><a href="/blog/archives/date/2010/11">November 2010</a></li> <li><a href="/blog/archives/date/2010/10">October 2010</a></li> <li><a href="/blog/archives/date/2010/09">September 2010</a></li> <li><a href="/blog/archives/date/2010/08">August 2010</a></li> <li><a href="/blog/archives/date/2010/07">July 2010</a></li> <li><a href="/blog/archives/date/2010/06">June 2010</a></li> <li><a href="/blog/archives/date/2010/05">May 2010</a></li> <li><a href="/blog/archives/date/2010/04">April 2010</a></li> <li><a href="/blog/archives/date/2010/03">March 2010</a></li> <li><a href="/blog/archives/date/2010/02">February 2010</a></li> <li><a href="/blog/archives/date/2010/01">January 2010</a></li> <li><a href="/blog/archives/date/2009/12">December 2009</a></li> <li><a href="/blog/archives/date/2009/11">November 2009</a></li> <li><a href="/blog/archives/date/2009/10">October 2009</a></li> <li><a href="/blog/archives/date/2009/09">September 2009</a></li> <li><a href="/blog/archives/date/2009/08">August 2009</a></li> <li><a href="/blog/archives/date/2009/07">July 2009</a></li> <li><a href="/blog/archives/date/2009/06">June 2009</a></li> <li><a href="/blog/archives/date/2009/05">May 2009</a></li> <li><a href="/blog/archives/date/2009/04">April 2009</a></li> <li><a href="/blog/archives/date/2009/03">March 2009</a></li> <li><a href="/blog/archives/date/2009/02">February 2009</a></li> <li><a href="/blog/archives/date/2009/01">January 2009</a></li> <li><a href="/blog/archives/date/2008/12">December 2008</a></li> <li><a href="/blog/archives/date/2008/11">November 2008</a></li> <li><a href="/blog/archives/date/2008/10">October 2008</a></li> <li><a href="/blog/archives/date/2008/09">September 2008</a></li> <li><a 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