The addition of alkyl groups to aromatic systems is a vital transformation in both academic and industrial contexts, providing access to a wide variety of synthetic intermediates, fine chemicals and feedstocks. As benzene does not typically undergo nucleophilic substitution (SN1 or SN2), this is conventionally achieved by the electrophilic aromatic (Friedel-Crafts) substitution of a benzene C-H bond using a strong Lewis acid catalyst (Figure 1, A).1, 2 These reactions rely on using reactive alkylating agents such as alkyl halides and, while efficient, their applications are inevitably hampered by their cost and the generation of stoichiometric by-products.
In contrast, hydroarylation, the addition of an arene C-H bond across olefins, offers significant advantages over classical Friedel-Crafts alkylation as the reaction is by-product free and the olefin starting materials are generally cheaper and more readily available compared to the corresponding alkyl halide. While a number of potent nucleophilic main group species have very recently been shown to activate and functionalise aryl C-H bonds,3-9 only transition metal complexes have, thus far, provided an alternative catalytic pathway to new alkylated arene products (Figure 1, B).10-14 Hydroarylation of olefins catalysed by Ru, Rh, Ir, Pt, Ni, Co and Fe has been extensively studied and, in each case, transition metal alkyl complexes have been identified as catalytically important intermediates which operate via a non-Friedel-Crafts mechanism.14 These transformations, however, are heavily reliant on directing groups to achieve appreciable levels of site selectivity and suffer from poor specificity toward the formation of branched vs. linear (i.e. Markovnikov vs. anti-Markovnikov) arene products (Figure 1, B).10-12 The selective hydroarylation of simple arenes, such as benzene, to linear (anti-Markovnikov) alkyl arene products, therefore, remains a significant challenge.
Figure 1: (A) Friedel-Crafts alkylation. (B) transition metal-catalysed hydroarylation of olefins. (C) σ-bond metathesis of a lanthanide alkyl with a benzene C-H bond.
Despite the first reports of the successful isolation of lanthanide(III) organometallics now dating from nearly 70 years ago15-19 there are, hitherto, no reports of their ability to achieve the hydroarylation of olefins. These complexes are, however, highly active for a number of catalytic olefin-based transformations, including cyclization/functionalisation, hydrosilylation, hydroboration, hydrogenation and polymerisation.20, 21 Although lanthanide(III) alkyls can also facilitate the C-H activation of benzene and other heteroarenes, these processes invariably ensue with the formation of new lanthanide-C(aryl) bonds concomitant with the formation of the respective alkane, via a conventional 4-membered σ-bond metathesis transition state (Figure 1, C).19, 22-25
In our recent work published in Nature Communications, we demonstrate the facile synthesis of a highly reactive low-valent and low-coordinate ytterbium(II) hydride, [BDIDippYbH]2 (BDI = CH[C(CH3)NDipp]2, Dipp = 2,6-diisopropylphenyl), which can sequester both ethene and propene to give the low-coordinate ytterbium(II) n-alkyls [BDIDippYbR]2 (R = Et or Pr). Both the ytterbium(II) ethyl and n-propyl derivatives react with either protio- or deuterobenzene at room temperature and density functional theory (DFT) calculations are supportive of a nucleophilic substitution reaction mechanism at benzene reminiscent of SN2 type reactivity. Under select conditions, the ytterbium(II) hydride mediates the exclusively selective, catalytic anti-Markovnikov hydrophenylation of ethene and propene (Figure 2).
Figure 2: Proposed mechanism for catalytic hydrophenylation of ethene and propene with benzene.
We believe that these investigations mark a milestone for lanthanide(II) catalysis and open a new avenue of exploration on the direct alkylation/functionalization of arenes.
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