{"id":22774,"date":"2020-10-08T10:06:25","date_gmt":"2020-10-08T09:06:25","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=22774"},"modified":"2020-10-08T11:33:22","modified_gmt":"2020-10-08T10:33:22","slug":"trimerous-pericyclic-reactions","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22774","title":{"rendered":"Trimerous pericyclic reactions."},"content":{"rendered":"
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I occasionally spot an old blog that emerges, if only briefly, as “trending”. In this instance, only the second blog<\/a> I ever wrote here, way back in 2009 as a follow up to this article.[1]<\/a><\/span> With something of that age, its always worth revisiting to see if any aspect needs updating or expanding, given the uptick in interest. It related to the observation that there can be more than one way of expressing the “curly arrows” for some pericyclic reactions. These alternatives may each represent different types of such reactions, hence leading to a conundrum for students of how to label the mechanism. I had noted in that blog that I intended to revisit the topic and so a mere eleven years later here it is!<\/p>\n

Annulenes, or cyclic conjugated polyenes can (hypothetically) indulge in transannular cyclisations which can be regarded as either two electrocyclic reactions, or one cycloaddition reaction, or perhaps a chimera of all three which here I describe as trimerous. Locating the transition states for such a trimerous reaction is quite straightforward and here I give the FAIR data DOI for all the examples shown below (DOI: 10.14469\/hpc\/7440<\/a>).\u2021<\/sup> The calculations are all at the B3LYP+GD3BJ\/Def2-TZVPP level.<\/p>\n

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The central reaction can be represented as a cycloaddition in which two new bonds form between the termini of two conjugated alkenes. The stereochemistry for each alkene component is defined as either suprafacial<\/em><\/strong> (the two new bonds form to the same face of that alkene) or antarafacial<\/strong><\/em> (the two new bonds form on opposite faces of that alkene). Another way of representing the curly arrow mechanism is to draw to separate electrocyclic reactions, in which one new bond is formed between the termini of a conjugated alkene. If this bond connects at each end to the same face of that alkene, the bond forms suprafacially<\/em><\/strong>, if it connects opposite faces of that alkene it forms antarafacially<\/strong><\/em>. One must also count the number of cyclic curly arrows used in each representation; the examples above illustrate two, three or four curly arrows, representing four, six or eight electrons. One can now combine these attributes to form some selection rules.<\/p>\n

For thermal reactions, one can state that if the total electron count represented by an odd number of curly arrows corresponding to the formula 4n+2<\/strong>\u00a0(n = 0,1,2, etc, the n is NOT the same as that shown in the scheme above) and there are either no (or an even number of)\u00a0antarafacial<\/em><\/strong> components, the reaction will be “allowed<\/strong>” and proceed through a “Huckel\u00a0aromatic transition state<\/strong>“. Alternatively, if the total electron count corresponds to an even number of curly arrows matching the formula 4n<\/strong> (n=1,2, etc) and there is one (or an odd number of)\u00a0antarafacial<\/strong> <\/em>components, the reaction will this time proceed through a “Mobius aromatic transition state<\/strong>“. These are in fact a concise alternative statement of the Woodward-Hoffmann selection rules for thermal pericyclic reactions.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Entry #<\/th>\nn (scheme only)<\/th>\n[ ]-Annulene
\nTS<\/th>\n		\u00a0electron count<\/span>
\nin each ring<\/span><\/th>\n		s<\/i>\/a<\/i>
\ncomponents<\/th>\n		NICS for each
\nring centroid<\/th>\n<\/tr>\n
1<\/td>\n0<\/td>\n10 endo<\/a><\/td>\n4,6,4<\/td>\na,s+s,a<\/span><\/td>\n-3.5<\/span>,-15.3<\/span>,-3.5<\/span><\/td>\n<\/tr>\n
2<\/td>\n0<\/td>\n10 exo<\/a><\/td>\n4,6,4<\/td>\na,s+s,a<\/span><\/td>\n-2.8<\/span>,-11.6<\/span>,-2.8<\/span><\/td>\n<\/tr>\n
3<\/td>\n0<\/td>\n12, saddle=1<\/a>\u2660<\/sup><\/td>\n4,8,4<\/td>\ns,s+a,*<\/span><\/td>\n-0.1,-10.3<\/span>,-2.0<\/td>\n<\/tr>\n
4<\/td>\n0<\/td>\n12, saddle=2<\/a><\/td>\n4,8,4<\/td>\n*,s+a,*<\/span><\/td>\n-0.9,-10.7<\/span>,-0.9<\/td>\n<\/tr>\n
5<\/td>\n1<\/td>\n14 endo<\/a><\/td>\n6,6,6<\/td>\na,s+s,a<\/span><\/td>\n+2.2<\/span>,-17.1<\/span>,+2.2<\/span><\/td>\n<\/tr>\n
6<\/td>\n1<\/td>\n14 exo<\/a><\/td>\n6,6,6<\/td>\na,s+s,a<\/span><\/td>\n+9.7<\/span>,-11.1<\/span>,+9.7<\/span><\/td>\n<\/tr>\n
7<\/td>\n1<\/td>\n16<\/a><\/td>\n6,8,6<\/td>\na,s+a,a<\/span><\/td>\n+9.1<\/span>,-9.6<\/span>,+9.1<\/span><\/td>\n<\/tr>\n
8<\/td>\n2<\/td>\n18 endo, saddle=1<\/a><\/td>\n8,6,8<\/td>\ns,s+s,a<\/span><\/td>\n-2.0,-15.0,-\u2665<\/sup><\/span><\/span><\/td>\n<\/tr>\n
9<\/td>\n2<\/td>\n18 endo, saddle=2<\/a><\/td>\n8,6,8<\/td>\n*,s+s,*<\/span><\/td>\n-0.5,-17.1,-0.5<\/span><\/td>\n<\/tr>\n
10<\/td>\n2<\/td>\n18 exo,saddle=1<\/a><\/td>\n8,6,8<\/td>\na,s+s,a<\/span><\/td>\n-6.2,-4.5,-3.7<\/span><\/td>\n<\/tr>\n
11<\/td>\n2<\/td>\n18 exo,saddle=2<\/a><\/td>\n8,6,8<\/td>\na,s+s,a<\/span><\/td>\n-8.2,-1.4,-8.2<\/span><\/td>\n<\/tr>\n
12<\/td>\n2<\/td>\n20,saddle=3<\/a><\/td>\n8,8,8<\/td>\na,s+*,a<\/span><\/span><\/td>\n-8.5<\/span>,+0.3,-8.5<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

\u2665<\/sup>No ring centroid in AIM analysis.\u2660<\/sup>The symmetrical geometry has two negative force constants (saddle=2), representing an asymmetric distortion to a true transition state (saddle=1).<\/small><\/p>\n

For the reactions shown in the scheme above, we will determine the NICS (Nucleus independent chemical shift) value at the ring centroid of each reaction to ascertain the aromaticity in that ring. The effective ring centroid in turn is located by performing an AIM analysis of the topology of the electron density and locating the RCP (ring critical points in that density;\u00a0a critical point itself is one where the first derivatives of the density with respect to the three cartesian coordinates is zero) or in several examples the CCP (Cage critical point). I will discuss some of these systems individually, but in fact there is a wealth of information available for each one and to discover it all, you should go to the data files and inspect all the structures for yourself. Firstly, the colour code in the table above:<\/p>\n