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Issue 7, 2019
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Pushing the limits of concertedness. A waltz of wandering carbocations

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Abstract

Among the array of complex terpene-forming carbocation cyclization/rearrangement reactions, the so-called “triple shift” reactions are among the most unexpected. Such reactions involve the asynchronous combination of three 1,n-shifts into a concerted process, e.g., a 1,2-alkyl shift followed by a 1,3-hydride shift followed by a second 1,2-alkyl shift. This type of reaction so far has been proposed to occur during the biosynthesis of diterpenes and the sidechains of sterols. Here we describe efforts to push the limits of concertedness in this type of carbocation reaction by designing, and characterizing with quantum chemical computations, systems that could couple additional 1,n-shift events to a triple shift leading, in principle to quadruple, pentuple, etc. shifts. While our designs did not lead to clear-cut examples of quadruple, etc. shifts, they did lead to reactions with surprisingly flat energy surfaces where more than five chemical events connect reactants and plausible products. Ab initio molecular dynamics simulations demonstrate that the formal minima on these surfaces interchange on short timescales, both with each other and with additional unexpected structures, allowing us a glimpse into a very complex manifold that allows ready access to great structural diversity.

Graphical abstract: Pushing the limits of concertedness. A waltz of wandering carbocations

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Supplementary files

Article information


Submitted
10 Aug 2018
Accepted
10 Dec 2018
First published
11 Dec 2018

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2019,10, 2159-2170
Article type
Edge Article

Pushing the limits of concertedness. A waltz of wandering carbocations

M. Castiñeira Reis, C. S. López, O. Nieto Faza and D. J. Tantillo, Chem. Sci., 2019, 10, 2159
DOI: 10.1039/C8SC03567A

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