Three concomitant C–C dissociation pathways during the mechanical activation of an N-heterocyclic carbene precursor

Abstract

Chemical reactions usually proceed through radical, concerted, or ionic mechanisms, yet transformations in which these three mechanisms take place at the same time are rare. In polymer mechanochemistry a mechanical force, transduced along polymer chains, is used to activate covalent bonds in mechanosensitive molecules (mechanophores). Cleavage of a C-C bond often follows a homolytic pathway but some mechanophores have also been designed that react in a concerted or, more rarely, a heterolytic manner. Here, using 1 H-and 19 F-nuclear magnetic resonance spectroscopy in combination with deuterium labelling, we show that the dissociation of a mechanophore built around an Nheterocyclic carbene precursor proceeds with the rupture of a C-C bond through concomitant heterolytic, concerted, and homolytic pathways. The distribution of products likely arises from a posttransition-state bifurcation in the reaction pathway, and their relative proportion is dictated by the polarisation of the scissile C-C bond.Mechanical scission of covalent bonds usually occurs in a homolytic fashion 1,2 although mechanophores can be designed to react in a concerted 3 or, more rarely, heterolytic fashion [4][5][6][7][8][9][10] . Perhaps the most interesting aspect of polymer mechanochemistry comes from the fact that molecules under tension follow reaction pathways that can significantly deviate from their zero-force mechanism and lead to unexpected outcomes 2 . This deviation can be expressed in the nature of the product formed, such as in the electrocyclic ring openings of strained cycles along a symmetry forbidden pathway, 11 12 and/or in the nature of the reactive intermediates 13,14 , or even in the change of the rate-determining step itself 15,16 17 .Here we show that the ultrasound-induced dissociation of a neutral N-heterocyclic carbene precursor proceeds with the rupture of a single C-C bond via concomitant heterolytic, concerted, and homolytic dissociation pathways.

Force-modulated reductive elimination from platinum(ii) diaryl complexes

“…[1][2][3] Mechanochemical strategies continue to evolve, including their recent use in biasing and probing reaction pathways, 4,5 the release of small molecules and protons, [6][7][8] stress reporting, [9][10][11][12][13] stress strengthening, [14][15][16] degradable polymers, 17,18 and fundamental studies of polymer behaviour under load. 19 In organic reactions, mechanochemical coupling has been investigated in simple bond dissociation reactions [20][21][22][23][24] and in a wide variety of reaction classes with respect to regiochemistry, [25][26][27][28] orbital symmetry, 25,[29][30][31] stereochemistry, 32,33 supramolecular architecture, 34,35 dynamic effects, 36,37 and the alignment and/or loading of scissile bonds with applied tension. 38,39 Unlike their organic counterparts, however, reported mechanochemical reactions in organometallic complexes involve almost entirely the direct, forced dissociation of a ligand.…”

Isotope Effect in the Activation of a Mechanophore

“…The importance of the concerted and heterolytic pathways for the other mechanophore were determined in the same way (see SI) and are reported in Table 1. Two trends emerge from these results (Table 1): the proportion of concerted dissociation increases as the polarization of the scissile bond increases in the protonated series (1Ha-c), 13 while the heterolytic scission is always favored in the deuterated series (1Da-c). This latter trend culminates with the mono-fluoro adduct, which displays an almost complete reversal of selectivity upon deuteration.…”

“…34 We have previously shown that the mechanistic divergence was the result of a post-transition-state bifurcation on the force-modified potential energy surface (PES). 13 Upon activation of the mechanophore, the C1-C2 bond connecting the aromatic leaving group to the imidazolidine unit elongates until it reaches the transition state (TS, Figure 2a). Further elongation of this bond leads to the heterolytic cleavage of the mechanophore to produce an imidazolinium cation and an aryl anion (Phetero) while concomitant elongation of C1-H and C1-C2 brings the reactive intermediate onto a concerted trajectory that ultimately delivers a carbene and a neutral aromatic unit (Pconc).…”

“…1,2 Since the emergence of the mechanophore concept, 3 chemists have been able to explore and control reactivity under tension with clever molecular designs. 4 Several factors affecting the reactivity under tension have been uncovered such as: the polymer architecture, [5][6][7] the nature of the linker, [8][9][10][11][12] substituents, [13][14][15][16] or dependence on their stereo-, [16][17][18][19][20][21][22] regio-, 18,[23][24][25][26][27] or topochemistry. [28][29][30] These factors usually affect the rate of activation or, more rarely, the outcome of the reaction itself.…”

“…13,21,31,32 The latter case often being the result of a post-transition-state bifurcation on the force modified potential energy surface (PES). 13,32,33 We have recently shown that the mechanical activation of 1H proceeds via three concomitant dissociation pathways: homolytic, heterolytic, and concerted. 13 Interestingly, the polarization of the scissile bond (i.e.…”