Tandem isomerization/telomerization of long chain dienes.

The first example of a tandem reaction involving double-bond migration in combination with telomerization is reported. Homogeneous and heterogeneous Ru catalysts were employed as isomerization catalysts, and telomerization was realized using a homogeneous Pd(0) precursor complex with a N-heterocyclic carbene (IMes) ligand. Overall conversions approaching 60% were achieved with the best selectivity to telomerization products of 91% attained at 11% conversion. Conversion was markedly higher in the presence of longer-chain alcohol (1-butanol) as the nucleophile (telogen).Financial support from Engineering and Physical Sciences Research Council (EPSRC, UK) via grant GR/S86112/01 is gratefully acknowledged.This is the final published version, which can also be found on the publisher's website at: http://journal.frontiersin.org/Journal/10.3389/fchem.2014.00037/ful

Twisting the arm: structural constraints in bicyclic expanded-ring N-heterocyclic carbenes

A series of diaryl, mono-aryl/alkyl and dialkyl mono- and bicyclic expanded-ring N-heterocyclic carbenes (ER-NHCs) have been prepared and their complexation to Au(I) investigated through the structural analysis of fifteen Au(NHC)X and/or [Au(NHC)2]X complexes. The substituted diaryl 7-NHCs are the most sterically encumbered with large buried volume (%VB) values of 40–50% with the less flexible six-membered analogues having %VB values at least 5% smaller. Although the bicyclic systems containing fused 6- and 7-membered rings (6,7-NHCs) are constrained with relatively acute NCN bond angles, they have the largest %VB values of the dialkyl derivatives reported here, a feature related to the fixed conformation of the heterocyclic rings and the compressional effect of a pre-set methyl substituent

Expanded ring N-heterocyclic carbenes: A comparative study of ring size in palladium (0) catalysed Mizoroki-Heck coupling

The synthesis and characterisation of a range of new 6- and 7-membered expanded ring N-heterocyclic carbene (NHC) complexes of zero valent palladium dvtms (divinyltetramethyldisiloxane) 8–14 is reported, with the first comparative catalytic study probing the effects of large ring N-heterocyclic carbene ligands (6-Mes and 7-Mes) with their analogous 5-membered derivative (IMes) on catalytic performance. The catalytic performances of the [Pd(NHC)(dvtms)] complexes are evaluated in the Mizoroki–Heck coupling of 4-bromoacetophenone and n-butyl acrylate, providing a ready and effective comparison into the influence of ring size and steric congestion on catalytic activity in carbon–carbon cross coupling reactions

Expanded ring N-heterocyclic carbene complexes of zero valent platinum dvtms (divinyltetramethyldisiloxane): Highly efficient hydrosilylation catalysts

The synthesis and characterisation of a series of new 6- and 7-membered NHC (N-heterocyclic carbene) complexes 7–12 of zero valent platinum dvtms (divinyltetramethyldisiloxane), [Pt(NHC)(dvtms)] are reported. A number of the complexes were investigated as catalyst precursors in the hydrosilylation of a range of unsaturated substrates, including alkynes, alkenes and a ketone with triethylsilane and bis(trimethylsiloxy)methylsilane (MD’M), at low catalyst loading (0.005 mol %); in general, the activity, and the selectivity for 1-functionalised product was found to be high

Pd–PEPPSI-Type Expanded Ring N‑Heterocyclic Carbene Complexes: Synthesis, Characterization, and Catalytic Activity in Suzuki–Miyaura Cross Coupling

The synthesis and characterization of the new six- and seven-membered Pd–PEPPSI-type N-heterocyclic carbene (NHC) complexes <b>3</b> and <b>4</b> is described. Complexes of the general formula [Pd­(NHC)­(3ClPy)­Cl₂] (NHC = 6- or 7-Mes, 3ClPy = 3-chloropyridine) are accessed via the oxidation of the well-defined parent palladium(0) complexes <b>1</b> and <b>2</b>. Complexes <b>3</b> and <b>4</b> have been employed in Suzuki–Miyaura cross-coupling and catalytic dehalogenation of a range of aryl halide substrates

Rhodium-catalyzed C-C coupling reactions: Mechanistic considerations

An alternative mechanism for intramolecular C−C coupling between heterocycles and alkenes with rhodium phosphine catalysts is presented involving oxidative addition, alkene insertion, and reductive elimination (route 2), as described previously for similar group 10 reactions by Cavell and McGuinness. Computational studies indicate that the rate-determining step is associated with reductive elimination of the product and overall barriers indicate this mechanism would be competitive with an alternative involving formation of a carbene complex derived from mechanistic work by Bergman, Ellman, and associates (route 1). Activation of the reacting azole through inclusion of an acid catalyst appears to support the route 2 mechanism. A much lower activation barrier is observed under acidic conditions, a result consistent with that found under experimental conditions

Telomerisation of long-chain dienes with alcohols using Pd(IMes)(dvds) catalyst

Several homogeneous palladium catalysts based on PPh3 and nucleophilic carbene (NHC) ligands were screened in the telomerisation reaction of 1,3-pentadiene with methanol. A Pd(acac)(2)-3PPh(3) system showed the highest activity; initially, poor activity was observed using the Pd(0)-NHC catalysts. However, when methanol solvent/nucleophile was replaced by longer chain alcohols 1-propanol and 1-butanol, improved activity and selectivity was observed. Similarly, 1,3-hexadiene was telomerised with 1-propanol and 1-butanol with good selectivity using Pd(IMes)(dvds) as the catalyst. Pd(II)-PPh3 systems were ineffective as catalysts for the telomerisation of 1,3-hexadiene