Air-stable 18-electron adducts of Schrock catalysts with tuned stability constants for spontaneous release of the active species.

Schrock alkylidenes are highly versatile, very active olefin metathesis catalysts, but their pronounced sensitivity to air still hinders their applications. Converting them into more robust but inactive 18-electron adducts was suggested previously to facilitate their handling. Generating the active species from the inactive adducts, however, required a high-temperature Lewis acid treatment and resulted in an insoluble by-product, limiting the practicality of the methodology. Herein, we introduce an approach to circumvent the inconvenient, costly, and environmentally taxing activation process. We show that 18-electron adducts of W- and Mo-based Schrock catalysts with finite stability constants (typically K = 200-15,000 M-1) can readily be prepared and isolated in excellent yields. The adducts display enhanced air-stability in the solid state, and in solution they dissociate spontaneously, hence liberating the active alkylidenes without chemical assistance.

Metal-free borylative ring-opening of vinyl epoxides and aziridines.

A rational approach towards the borylative ring-opening of vinylepoxides and vinylaziridines, by the in situ formed MeO(-)→bis(pinacolato)diboron adduct, has been developed. The enhanced nucleophilic character of the Bpin (sp(2)) moiety from the reagent favours the SN2' conjugated B addition with the concomitant opening of the epoxide and aziridine rings. The reaction proceeds with total chemoselectivity towards the polyfunctionalised (-OH or -NHTs) allyl boronate. Theoretical calculations have determined the transition states that come from the reaction of the vinylic substrates with the activated MeO(-)→bis(pinacolato)diboron adduct, and a plausible mechanism for the organocatalytic borylative ring opening reaction has been suggested.

Essential role of phosphines in organocatalytic ß-boration reaction.

The use of phosphines to assist the organocatalytic ß-boration reaction of α,ß-unsaturated carbonyl compounds has been demonstrated with a selected number of substrates. The new method eludes the use of Brönsted bases to promote the catalytic active species and PR(3) becomes essential to interact with the substrate resulting in the formation of a zwitterionic phosphonium enolate. This species can further deprotonate MeOH when B(2)pin(2) is present forming eventually the ion pair [α-(H),ß-(PR(3))-ketone](+)[B(2)pin(2)·MeO](-) that is responsible for the catalysis.

Asymmetric organocatalytic diboration of alkenes.

The use of chiral alcohols to form the Lewis acid-base *RO(-)→ bis(pinacolato)diboron adduct, in situ, provides an opportunity to induce asymmetry in the organocatalytic diboration of alkenes and complements the well established transition metal-mediated enantioselective diboration.

Asymmetric synthesis of α-amino boronate esters via organocatalytic pinacolboryl addition to tosylaldimines.

Organocatalytic nucleophilic pinacolboryl addition from in situ generated MeO(-)→ B(2)pin(2) to C=N double bond can be performed enantioselectively with the aid of chiral phosphines, which promote enantiofacial differentiation in the course of the C-B bond formation.

Trivalent boron nucleophile as a new tool in organic synthesis: reactivity and asymmetric induction.

Boron compounds have been traditionally regarded as "Lewis acids" preferring to accept electrons rather than donate them in the course of their reactions but current examples of unusual reactivity between tricoordinated boranes and electrophilic sites suggest a new conceptual context for the boryl moieties, based on their nucleophilic character which can be enhanced depending on the substituents on boron.

Activation of diboron reagents with Brønsted bases and alcohols: an experimental and theoretical perspective of the organocatalytic boron conjugate addition reaction.

Bases play an important role in organocatalytic boron conjugate addition reactions. The sole use of MeOH and a base can efficiently transform acyclic and cyclic activated olefins into the corresponding ß-borated products in the presence of diboron reagents. Inorganic and organic bases deprotonate MeOH in the presence of diboron reagents. It is concluded, on the basis of theoretical calculations, NMR spectroscopic data, and ESI-MS experiments, that the methoxide anion forms a Lewis acid-base adduct with the diboron reagent. The sp(2) B atom of the methoxide-diboron adduct gains a strongly nucleophilic character, and attacks the electron-deficient olefin. The methanol protonates the intermediate, generating the product and another methoxide anion. This appears to be the simplest method to activate diboron reagents and make them suitable for incorporation into target organic molecules.

Catalytic 1,3-difunctionalisation of organic backbones through a highly stereoselective, one-pot, boron conjugate-addition/reduction/oxidation process.

A simple one-pot, three-step synthetic route to chiral 1,3-amino alcohols and 1,3-diols has been established. Considering the overall stereocontrol of the synthetic protocol, the first and key step is an enantioselective ß-boration of α,ß-unsaturated imines and ketones, respectively. The enantioselectivity provided by the Cu(I) catalyst modified with Josiphos- and Mandyphos-type ligands has been examined. The oxidative substitution of the boryl unit with a hydroxyl group proceeds with complete retention of configuration at the C(ß)-atom. In parallel, the stoichiometric reduction of the imino or carbonyl group provides a second stereogenic centre. Depending on the nature of the reducing reagent, exceptionally high diastereoselectivity is achieved, especially for syn-1,3-amino alcohols and 1,3-diols.

Zn(II) Robson macrocycles as templates for chelating diphosphines.

Chelating diphosphines were constructed using dinuclear Zn(II) complexes of Robson macrocycles (Zn-RMCs) as templates. The assembly process is driven by the interaction between the metal centers (Lewis acids) with anionic and neutral Lewis base-functionalized monophosphines. The stability of the final structure depends on the geometry and the affinity of the functional groups of the ditopic phosphines and on the structure of the RMC. In the free ligand the ditopic phosphines coordinate at opposite faces of the pseudo-planar macrocycle as is shown in the molecular structure of several of the assemblies, according to X-ray diffraction. Pre-organization of the system by coordinating the phosphorus atoms to a transition metal center enforced coordination of the functional groups at the same face of the RMC. For several templated diphosphines cis-PtCl(2) complexes were identified by NMR. The in situ assembled diphosphines showed a chelating effect in the rhodium catalyzed hydroformylation of 1-octene. Combination of Zn-RMC 3 and phosphine A gave the highest l/b ratio (13) in acetonitrile.

A new context for palladium mediated B-addition reaction: an open door to consecutive functionalization.

Palladium metal-catalyzed boron addition to unsaturated carbon-carbon bonds provides an efficient and convenient route for the preparation of organoboranes, which are versatile intermediates for organic synthesis. Palladium complexes are responsible for the exclusive catalytic performance and eventually allow access to selectively functionalized molecules by catalytic consecutive tandem sequences. The final objective is to find suitable palladium complexes that make it possible to perform a one-pot sequential reaction (B-addition/functionalization) by means of a multifaceted palladium catalyst. Mechanistic insights into the concatenated reactions through B chemistry are required if one wants to understand the experimental results.

Tandem beta-boration/arylation of alpha,beta-unsaturated carbonyl compounds by using a single palladium complex to catalyse both steps.

Diphenyl(3-methyl-2-indolyl)phosphine (C(9)H(8)NPPh(2), 1) gives stable dimeric palladium(II) complexes that contain the phosphine in P,N-bridging coordination mode. On treating 1 with [Pd(O(2)CCH(3))(2)], the new complexes [Pd(mu-C(9)H(7)NPPh(2))(NCCH(3))](2) (2) or [Pd(mu-C(9)H(7)NPPh(2))(mu-O(2)CCH(3))](2) (3) were isolated, depending on the solvent used, acetonitrile or toluene, respectively. Further reaction of 3 with the ammonium salt of 1 led to the substitution of one carboxylate ligand to afford [Pd(mu-C(9)H(7)NPPh(2))(3)(mu-O(2)CCH(3))] (4), in which the bimetallic unit is bonded by three C(9)H(7)NPPh(2)(-) moieties and one carboxylate group. Using this methodology, [Pd(2)(mu-C(6)H(4)PPh(2))(2)(mu-C(9)H(7)NPPh(2))(mu-O(2)CCX(3))] (X=H (7); X=F (8)) were synthesised from the ortho-metalated compounds [Pd(C(6)H(4)PPh(2))(mu-O(2)CCX(3))](2) (X=H (5); X=F (6)). Complexes 3, 4, 7, and 8 have been found to be active in the catalytic beta-boration of alpha,beta-unsaturated esters and ketones under mild reaction conditions. Hindrance of the carbonyl moiety has an influence on the reaction rate, but quantitative conversion was achieved in many cases. More remarkably, when aryl bromides were added to the reaction media, complex 7 induced a highly successful consecutive beta-boration/cross-coupling reaction with dimethyl acrylamide as the substrate (99% conversion, 89% isolated yield).

Ionic interaction as a powerful driving force for the formation of heterobidentate assembly ligands.

An ionic interaction has been used for the first time to assemble monophosphane ligands. NMR spectroscopy and X-ray studies show that cationic and anionic triphenylphosphane derivatives form ion pairs and subsequently act as a ligand in various transition-metal complexes. The position of the ionic functional groups allows both cis and trans coordination of the novel assembly ligand in square-planar transition-metal complexes.