International audienceTriarylsulfonium salts are prompted to undergo efficient homolytic reduction by single electron transfer under mild photocatalytic conditions. The liberated aryl radical can then participate in carbon-carbon bond formation processes with allyl sulfones and activated olefins. Triarylsulfonium salts emerge as a valuable and alternative source of aryl radicals for synthesis

A variety of fragmentations and rearrangements can follow Bergman cyclization in enediynes equipped with acetal rings mimicking the carbohydrate moiety of natural enediyne antibiotics of the esperamicine and calchiamicine families. In the first step of all these processes, intramolecular H-atom abstraction efficiently intercepts the p-benzyne product of the Bergman cyclization through a six-membered TS and transforms the p-benzyne into a new more stable radical. Depending on the substitution pattern and reaction conditions, this radical follows four alternative paths: (a) abstraction of an external hydrogen atom, (b) O-neophyl rearrangement which transposes O- and C-atoms of the substituent, (c) fragmentation of the O-C bond in the acetal ring, or (d) fragmentation with elimination of the appended acetal moiety as a whole. Experiments with varying concentrations of external H-atom donor (1,4-cyclohexadiene) were performed to gain further insight into the competition between intermolecular H-abstraction and the fragmentations. The Thorpe-Ingold effect in gem-dimethyl substituted enediynes enhances the efficiency of fragmentation to the extent where it cannot be prevented even by a large excess of external H-atom donor. These processes provide insight into a possible mechanism of unusual fragmentation of esperamicin A(1) upon its Bergman cycloaromatization and lay foundation for a new approach for the conformational control of reactivity of these natural antitumor antibiotics. Such an approach, in conjunction with supramolecular constraints, may provide a plausible mechanism for resistance to enediyne antibiotics by the enediyne-producing microorganisms.

Using DFT calculations we identify a low-energy reaction path connecting methyl acetate and Milstein's trans-[Ru(H) 2 (PNN)(CO)] catalyst directly with acetaldehyde and trans-[Ru(H)(OMe)(PNN)(CO)]. The transformation represents a metathesis in which a hydride and an alkoxide are swapped between a metal center and an acyl group. The reaction leads to a simple mechanism systematically applicable to the diverse hydrogenation and dehydrogenative coupling chemistry that can be achieved by the given catalyst.

Milstein and co-workers have reported that the pincer complexes trans-[Ru(H) 2 (PNN)(CO)] catalyze the unprecedented homogeneous hydrogenation of dimethyl carbonate to methanol. A mechanism for this reaction was proposed on the basis of (i) carbonyl group insertion into one of the Ru−H bonds to produce the six-coordinate trans-[Ru(OCH(OMe) 2 )(H)(PNN)(CO)] intermediate and (ii) a metal−ligand cooperative transformation, involving proton transfer from the phosphine arm of the PNN ligand to a methoxy group of the Ru-coordinated [OCH(OMe) 2 ] − anion along with cleavage of a C−OMe bond, to produce methanol and an O-bound methyl formate complex of the dearomatized square-pyramidal form of the catalyst, [Ru(H)(PNN)(CO)]. We investigate herein the possibility of an alternative reaction pathway proceeding as (i) an outer-sphere hydride transfer from [Ru(H) 2 (PNN)(CO)] to the carbonyl of dimethyl carbonate to give an ion pair of the cationic metal fragment and the [OCH(OMe) 2 ] − anion in which the C−H bond is facing the metal center, (ii) reorientation of the [OCH(OMe) 2 ] − anion within the intact ion pair to coordinate a methoxy group to the metal, and (iii) C−OMe bond cleavage (methoxide abstraction by the cationic ruthenium center) to yield methyl formate and trans-[Ru(H)(OMe)(PNN)(CO)]. Using DFT calculations applied at the M06 and ωB97X-D levels with a polarizable continuum representing THF as solvent, we calculate the energy profile of this pathway to be significantly lower than the metal−ligand cooperative pathway. The analogous pathway is also favored for the reaction of [Ru(H) 2 (PNN)(CO)] with methyl formate. The new mechanism corresponds to a direct metathesis transformation in which a hydride and an alkoxide are exchanged between a metal center and a carbonyl group via an outer sphere ion pair formation and reorientation of the alkoxide anion. The calculations also indicate that the metathesis can proceed indirectly via outer sphere ion pair mediated carbonyl insertion of dimethyl carbonate and methyl formate to give [Ru(H)(OCH(OMe) 2 )(PNN)(CO)] and [Ru(H)(OCH 2 OMe)(PNN)(CO)], respectively, as intermediates, followed by ion pair mediated deinsertion of methyl formate or formaldehyde. Inclusion of one methanol molecule as an explicit H-bond donor solvent does not change the main conclusions of the study.

We report a metal-free procedure for transformation of phenols into esters and amides of benzoic acids via a new radical cascade. Diaryl thiocarbonates and thiocarbamates, available in a single high-yielding step from phenols, selectively add silyl radicals at the sulfur atom of the C═S moiety. This addition step, analogous to the first step of the Barton-McCombie reaction, produces a carbon radical which undergoes 1,2 O→C transposition through an O-neophyl rearrangement. The usually unfavorable equilibrium in the reversible rearrangement step is shifted forward via a highly exothermic C-S bond scission in the O-centered radical, which furnishes the final benzoic ester or benzamide product. The metal-free preparation of benzoic acid derivatives from phenols provides a potentially useful alternative to metal-catalyzed carbonylation of aryl triflates.