Unexpected linkage: The use of dicationic RuIV complexes in the Friedel–Crafts‐type allylation of aromatic alcohols leads unexpectedly to the formation of a new CC bond and not to the phenolation product (see scheme). NMR spectroscopy, diffusion data, DFT calculations, and X‐ray diffraction analysis provide new insights with respect to the mechanism of this reaction.

X-ray, 13 C NMR, and DFT studies on the cationic Ru(IV) allyl complex Ru(Cp*)Cl(CH 3 CN)(η 3 -PhCHCHCH 2 ), as a PF 6 salt, have revealed a marked asymmetry in the bonding of the allyl ligand, which can be interpreted as arising from differences in π-bonding from the metal center to the two terminal allyl carbons. This asymmetry in the bonding is offered as an explanation for the observed control of regioselectivity in the Rucatalyzed allylic alkylation reaction.

C NMR studies have shown that in both Pd(II)-and Pt(II)-allyl (modified-MOP) (MOP ) (S)-2-diarylphosphino-1,1′-binaphthyl) complexes the substituent on the MOP auxiliary can affect how the naphthyl backbone interacts with a metal center. With the MeO-MOP analogue, the metal binds the carbon in a weak η 1 -fashion, whereas with H-MOP it prefers an η 2 -binding mode. For the Pt complexes, the 1 J( 195 Pt, 13 C) values proved to be diagnostic tools. Both modes of bonding afford relatively weak bonds to the metal. Modifying the MOP ligand structure from a PPh 2 to a P(3,5-di-tert-butylphenyl) 2 analogue can markedly affect the bond distances within the coordination sphere, as indicated by the X-ray structural data for PdCl(η 3 -C 3 H 5 )(modified-MOP). 2-D NMR exchange spectroscopy can be used to recognize and distinguish between the two most common types of η 3 -η 1 -η 3 isomerization process, i.e., rotation around the allyl C-C bond versus rotation around the allyl M-C bond. For the complex PdCl(η 3 -C 3 H 5 )(H-MOP), the fastest isomerization process involves rotation around the allyl C-C bond.Supporting Information Available: Text giving experimental details and a full listing of crystallographic data for 13, including tables of positional and isotropic equivalent displacement parameters, anisotropic displacement parameters, calculated positions of the hydrogen atoms, bond distances, and bond angles. ORTEP figure showing the full numbering schemes. X-ray data are also available as a CIF file. This material is available free of charge via the Internet at http://pubs.acs.org. OM049039T

Transition-metal-catalyzed allylic alkylation reactions continue to attract attention. This chemistry can be accomplished using a variety of (primarily) late-transition-metal complexes [1][2][3] and affords high yields of product. In contrast to palladium(ii)-catalyzed allylic alkylation reactions, [1] which often afford the linear (less substituted) product, the use of ruthenium-based catalysts affords primarily branched organic products; that is, the nucleophile prefers to attack at the more substituted carbon, [4] immediately adjacent to the phenyl group [Eq. (1) Cp* = C 5 Me 5 ; X = halide or carbonate].Trost et al. [4] have shown that the cationic tris(nitrile) complex [Ru(Cp*)(CH 3 CN) 3 ]PF 6 (1), is an excellent catalyst for the chemistry shown in Equation (1) and that carbonate is the preferred leaving group, X, in terms of the observed regioselectivity.[4] Bruneau et al. [5] have recently reported that the bipyridine derivatives, 2, are also efficient catalysts. Further, we note that conventional wisdom suggests that an oxidative addition mechanism is operating in this allylation chemistry, [4,6,7] although no explanation for the role of the anion (chloride or carbonate) has been forthcoming.We have recently reported [8] that the source of the observed high branched-to-linear regioselectivity (when X = Cl), has an electronic origin. These findings are based on Xray crystallography and NMR spectroscopy data, together with DFT calculations. In an extension of this study we show herein that a) the carbonate anion can remain in the coordination sphere of the ruthenium and b) an isolated, well characterized Ru IV allyl carbonate complex is a better catalyst than 1.Reaction of the branched tert-butyl carbonates 4 a and 4 b (Scheme 1) with 1 in DMF at ambient temperature affords the Ru IV cationic complexes 3 a and 3 b, respectively, in excellent yield.[9]The solid-state structure [10] of 3 b (Figure 1) reveals the expected distorted piano-stool arrangement of the various ligands in this coordinatively saturated complex. The tertbutyl carbonate ligand clearly coordinates in a bidentate fashion through two oxygen atoms, and the allyl ligand shows an endo configuration such that the phenyl substituent is remote to the bulky Cp* ligand. Thus the oxidative addition affords the monocationic carbonate and not the bis(acetonitrile) dicationic complex.The characteristic chemical shifts in the 1 H and 13 C NMR spectra [9] of pure 3 a have been recorded and compared to the analogous data from the in situ measurement involving stoichiometric amounts of 1 and the branched tert-butyl carbonate. These results show that 3 a is formed immediately on mixing, which indicates that a) the oxidative addition is not likely to be the rate-determining step in the catalysis and b) the isolated carbonate complex is not formed during the work-up procedure.The reaction between a DMF solution of 3 a and one equivalent of the dimethyl malonate anion (DMM) leads to the formation of a mixture of branched and linear organic products [Eq. (2)...

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