Stannylenes and Germylenes Stabilized by Tetradentate Bis(amidine) Ligands with a Rigid Naphthalene Backbone.

An unusual series of germylenes and stannylenes stabilized by new tetradentate bis(amidine) ligands RNC(R')N-linker-NC(R')NR with a rigid naphthalene backbone has been prepared by protonolysis reaction of Lappert's metallylenes [M(HMDS)2] (M = Ge or Sn). Germylenes and stannylenes were fully characterized by NMR spectroscopy and X-ray diffraction analysis. DFT calculations have been performed to clarify the structural and electronic properties associated with tetradentate bis(amidine) ligands. Stannylene L1Sn shows reactivity through oxidation, oxidative addition, and transmetalation reactions, affording the corresponding gallium and aluminum derivatives.

Synthesis and structures of homoleptic germylenes and stannylenes with sulfonimidamide ligands.

An unusual series of germylenes and stannylenes with homoleptic symmetric and unsymmetric N-substituted sulfonimidamide ligands PhSO(NiPr)(NHiPr) 1 and PhSO(NMes)(NHiPr) 2 have been prepared by protonolysis reaction of Lappert's metallylenes [M(HMDS)2] (M = Ge or Sn) with two equivalents of the appropriate sulfonimidamide. The homoleptic germylenes [PhSO(NiPr)2]2Ge 3 and [PhSO(NMes)(NiPr)]2Ge 4, and stannylenes [PhSO(NiPr)2]2Sn 5 and [PhSO(NMes)(NiPr)]2Sn 6 were fully characterized by NMR spectroscopy and by X-ray diffraction analysis. DFT calculations have been performed to understand the electronic properties brought by the sulfonimidamide ligand.

New Insight into the Reactivity of S,S-Bis-ylide.

The present work focuses on the reactivity of S,S-bis-ylide 2, which presents a strong nucleophilic character, as evidenced by reactions with methyl iodide and CO2, affording C-methylated salts 3 and betaine 4, respectively. The derivatization of betaine 4 affords the corresponding ester derivative 6, which is fully characterized by using NMR spectroscopy and X-ray diffraction analysis. Furthermore, an original reaction with phosphenium ions leads to the formation of a transient push-pull phosphino(sulfonio)carbene 8, which rearranges to give stabilized sulfonium ylide derivative 7.

Imine-stabilized silylium ions: synthesis, structure and application in catalysis.

Novel norbornene-based imine-stabilized silylium ions 2 have been synthesized via the simple reaction of sulfide-stabilized silylium ion 1 with carbonyl derivatives. Those silylium ions were fully characterized in solution and in the solid state by NMR spectroscopy and X-ray diffraction analysis as well as DFT calculations. Unlike the previously reported phosphine-stabilized silylium ion VI, behaving as a Lewis pair, calculations show that 2 have a strong Lewis acid character. Indeed, imine-stabilized silylium ions 2 are able to activate Si-H bonds and catalyzed the hydrosilylation of carbonyl derivatives under mild conditions.

Labile Base-Stabilized Silyliumylidene Ions. Non-Metallic Species Capable of Activating Multiple Small Molecules.

Several base-stabilized silyliumylidene ions (2 and 3) with different ligands were synthesized. Their behaviour appeared strongly dependent on the nature of ligand. Indeed, in contrast to the poorly reactive silyliumylidene ions 3 c,d stabilized by strongly donating ligands (DMAP, NHC), the silylene- and sulfide-supported one (2-H and 3 a) exhibits higher reactivity toward various small molecules. Furthermore, their capability to successively activate multiple small molecules was clearly demonstrated by processes involving successive reactions with silane/formamide, CO2 and H2 . Moreover, HBPin adduct of 3 a (8-C) catalyzes the hydroboration of pyridine. Of particular interest, silylene-supported silyliumylidene complex 2-H is one of the rare species able to activate two H2 molecules.

Norbornene based-sulfide-stabilized silylium ions: synthesis, structure and application in catalysis.

A norbornene-based sulfide stabilized silylium ion 4 has been synthesized. The S-Si interaction was studied in solution and in the solid state by NMR spectroscopy and X-ray diffraction analysis as well as DFT calculations. Unlike the previously reported phosphine-stabilized silylium ion VII, behaving as a Lewis pair, calculations predict that 4 should behave as a Lewis acid toward acrylate derivatives. Indeed, the base-stabilized silylium ion 4 has emerged as an easy-to-handle silylium ion-based Lewis acid catalyst, particularly for the Diels-Alder cycloaddition, with poorly reactive dienes, and hydrodefluorination reactions.

Complexes of Dichlorogermylene with Phosphine/Sulfoxide-Supported Carbone as Ligand.

Due to their remarkable electronic features, recent years have witnessed the emergence of carbones L2C, which consist in two donating L ligands coordinating a central carbon atom bearing two lone pairs. In this context, the phosphine/sulfoxide-supported carbone 4 exhibits a strong nucleophilic character, and here, we describe its ability to coordinate dichlorogermylene. Two original stable coordination complexes were obtained and fully characterized in solution and in the solid state by NMR spectroscopy and X-ray diffraction analysis, respectively. At 60 °C, in the presence of 4, the Ge(II)-complex 5 undergoes a slow isomerization that transforms the bis-ylide ligand into an yldiide.

Phosphine/Sulfoxide-Supported Carbon(0) Complex.

A new carbon(0) complex 2 with two different L ligands, a phosphine and a sulfoxide, was synthesized and fully characterized. This new type of carbone exhibits excellent coordination ability, in contrast to the related phosphine/sulfide-supported carbon(0) complexes. Several organometallic complexes were isolated and, of special interest, the νav (CO) value of RhI -dicarbonyl complex indicates that 2 has a donor capability superior to classical NHCs.

Azavinylidenephosphoranes: a class of cyclic push-pull carbenes.

The synthesis of a novel family of cyclic push-pull carbenes, namely, azavinylidene phosphoranes, is described. The methodology is based on a formal [3+2] cycloaddition between terminal alkynes and phosphine-imines followed by an oxidation/deprotonation step. Carbenes 6, obtained by simple deprotonation, exhibit typical transient carbene reactivity like the intramolecular CH insertion reaction and a pronounced ambiphilic character exemplified by [2+1] cycloaddition with electron-poor methyl acrylate. Owing to the cyclic structure, carbenes 6 also exhibit an excellent coordination ability toward transition metals. Rh(I) complex 10 was obtained in excellent yield and was fully characterized by multinuclear NMR spectroscopy and X-ray crystallography. The corresponding Rh(I) -carbonyl complex was also prepared; this indicates that carbenes 6 belong to the strongest σ-donating ligands to date. DFT calculations confirmed the high σ-donation ability of 6 and their classification as push-pull carbenes with a relatively small singlet-triplet energy gap of 23.2-24.3 kcal mol(-1) .

Chemical behaviour of a prototype boryl(phosphino)carbene.

We recently disclosed the synthesis of a novel "push-pull" boryl(phosphino)carbene. To determine the influence of this substitution pattern on the chemical behaviour, a study into the reactivity of the prototype (1) of this new family of B(sp(2))-substituted phosphinocarbenes was undertaken. Carbene 1 exhibits one of the most common intramolecular rearrangements of singlet carbenes, involving a 1,2-mesityl shift, and typical [2+1] cycloaddition reactions with electron-poor acrylonitrile. A pronounced α,ß-ambiphilic character was also shown by the reaction of 1 with benzaldehyde, leading to phosphorylalkene 4. Due to its specific electronic properties, carbene 1 also exhibits unprecedented reactivity with chloroacrylonitrile, enabling the formation of bicyclo[1.1.0]phosphetanium salt 6 and borylcyclopropene 9, which have been fully characterised by NMR spectroscopy and X-ray crystallography.

Borylated methylenephosphonium salts: precursors of elusive boryl(phosphino)carbenes.

The synthesis of a new family of boryl-substituted methylenephosphonium derivatives, the phosphorus analogues of iminium salts, has been developed. They were used in the preparation of the first stable boryl(phosphino)carbene, which has been fully characterized by NMR spectroscopy and X-ray crystallography. Density functional theory calculations indicate that these carbenes can be classified as push-pull carbenes with a relatively small singlet-triplet energy gap.

Organocatalytic asymmetric direct phosphonylation of alpha,beta-unsaturated aldehydes: mechanism, scope, and application in synthesis.

The first direct organocatalytic enantioselective phosphonylation of alpha,beta-unsaturated aldehydes with phosphite, in combination with a Brønsted acid and a nucleophile, is presented. Mechanistic investigations have revealed that the first step in the catalytic process, after the formation of the iminium intermediate, is the addition of phosphite to the beta-carbon atom, leading to the phosphonium ion-enamine intermediate. The rate-determining step for the reaction is the transformation of P(III) to P(V), which occurs via a nucleophilic SN2-type dealkylation, and a screening of various nucleophiles shows that soft nucleophiles in combination with a Brønsted acid improve the reaction rate and enantioselectivity. The reaction conditions developed show that the use of 2-[bis(3,5-bistrifluoromethylphenyl)trimethylsilanoxymethyl]pyrrolidine as the catalyst and tri-iso-propyl phosphite as the phosphonylation reagent, in the presence of stoichiometric amount of benzoic acid and sodium iodide, gave the beta-phosphonylation of aromatic and aliphatic alpha,beta-unsaturated aldehydes in good yields and enantioselectivities. The products formed by this new reaction have been used for the synthesis of a number of biologically important compounds, such as optically active hydroxyl phosphonate esters, phosphonic acids, and especially glutamic acid and fosmidomycin precursors, of which the two latter are showing important properties for the treatment of central nervous system diseases and as anti-malarial compounds, respectively. DFT calculations have been applied to explain the approach of the phosphite to the reactive carbon atom in the iminium intermediate in order to account for the observed absolute enantioselectivity in the reaction.

Asymmetric synthesis of highly functionalized tetrahydrothiophenes by organocatalytic domino reactions.

A simple approach for the formation of optically active highly functionalized tetrahydrothiophenes, which might find important use in biochemistry, pharmaceutical science, and nanoscience is presented. Development of new organocatalytic Michael-aldol domino reactions is outlined, and with the appropriate choice of additives it is possible to control the regioselectivity of these domino reactions, yielding diastereomerically pure (tetrahydrothiophen-2-yl)phenyl methanones or tetrahydrothiophene carbaldehydes in good yields and with excellent enantioselectivities up to 96% ee. The stereochemical outcome of these reactions is investigated, and the mechanism of these organocatalytic domino processes is presented.