Diastereoselective and intramolecular cycloadditions of asymmetric p-nitroso phosphine oxides

Benzyl phenyl P-nitroso phosphine oxide (5) reacts as an N-O heterodienophile with 1,3-cyclopentadiene to give the diastereomeric cycloadducts 6a,b in a ratio of 1.5:1 (6a:6b). The same reaction in the presence of tin tetrachloride produces 6a,b in a ratio of 2.9:1 (6a:6b). Cycloaddition of the structurally modified P-nitroso phosphine oxide (18) with 1,3-cyclopentadiene forms the diastereomeric cycloadducts 16a,b in a ratio of 3.1:1 (16a:16b). These results suggest the reactions of these P-nitroso phosphine oxides and 1,3-cyclopentadiene occur through a transition state where the heterodienophile adopts an s-cis conformation and approaches the diene in an exo fashion syn to the phenyl group. This model resembles those proposed for the cycloadditions of the structurally similar asymmetric vinyl phosphine oxides. Reaction of 18 with 1,3-cyclopentadiene in the presence of a Lewis acid produces cycloadducts 16a,b in a ratio of 7:1 (16a:16b), which approaches synthetic utility. Similar experiments show that 1,3-cyclohexadiene likely reacts with P-nitroso phosphine oxides through a different transition state, limiting current predictions regarding the diastereoselectivity of these reactions. The intramolecular cycloaddition of an asymmetric P-nitroso phosphine oxide (19) for the first time produces a unique phosphorus-containing heterocyclic compound (20).     

N,C-bonded beta-diketiminato phosphenium cations

The first examples of N,C-bonded beta-diketiminato phosphenium cations have been isolated as their triflate or tetrachloroaluminate salts, both of which have been structurally characterized.     

Pentamethylcyclopentadienyl rhodium(III) trifluorovinyl phosphine complexes and attempted intramolecular dehydrofluorinative coupling of pentamethylcyclopentadienyl and trifluorovinyl phosphine ligands

The trifluorovinyl phosphine complexes [Cp*RhCl₂{PR_3−x(CFCF₂)_x}] (1x = 1, a R = Ph, b Prⁱ, c Et; 2x = 2, R = Ph) have been prepared by treatment of [Cp*RhCl(μ-Cl)]₂ with the relevant phosphine. The salt [Cp*RhCl(CNBu^t){PPh₂(CFCF₂)}]BF₄, 3, was prepared by addition of Bu^tNC to 1a in the presence of NaBF₄. The salt [Cp*RhCl{κP,κS-(CF₂CF)PPh(C₆H₄SMe-2)}]BF₄ was prepared as a mixture of cis (5a) and trans (5b) isomers by treatment of [Cp*RhCl(μ-Cl)]₂ with the phosphine-thioether (CF₂CF)PPh(C₆H₄SMe-2), 4, in the presence of NaBF₄. The structures of 1a-c and 5a have been determined by single-crystal X-ray diffraction. Intramolecular dehydrofluorinative carbon-carbon coupling between pentamethylcyclopentadienyl and trifluorovinylphosphine ligands of 1a, 3 and 5 has been attempted. No reaction was observed on treatment of the neutral complex [Cp*RhCl₂{PPh₂(CFCF₂)}], 1a, with proton sponge, however, 5a underwent dehydrofluorinative coupling to yield [{η⁵,κP,κS-(C₅Me₄CH₂CFCF)PPh(C₆H₄SMe-2)}RhCl]BF₄, 6. Other reactions, in particular addition of HF across the vinyl bonds of 5, occurred leading to a mixture of products. The cation of 3 underwent similar reactions.     

New intramolecular exciplexes between polynuclear aromatic hydrocarbons and head-to-head vs head-to-tail photodimerization of mesosubstituted anthracenes.

New dissymetrical non conjugated bichromophores linked by an oxygenated chain: (9-anthrylmethyloxymethyl)arenes[arenes:benzene(I_a),2-naphthalene(I_b),9-phenanthrene (I_c) and 1-pyrene (I_d) and (9-phenanthrylmethyloxymethyl)-1-pyrene (II)] have been prepared. I_c–d and II exhibit intramolecular exciplex fluorescence in methylcyclohexane at room temperature. Intramolecular photoadducts were not found but I_a–d yield a mixture of head-to-head and head-to-tail anthracenic photodimers.     

On the relationship between fluorescence and molecular geometry solvent-assisted formation of luminescent intramolecular exciplexes from anthronyl-substituted anthracenes

The fluorescence efficiency of 1′,2′-dihydro-spiro[anthracene-9(10H),3′-[3H]benz[de]anthracen]-10-one in 10 solvents of varying dielectric constant has been investigated. The observed exciplex luminescence is attributed to a solvent-assisted intramolecular charge-transfer interaction between the ground-state anthrone and excited-state anthracene moietis. The geometry of the exciplex is suggested to resemble that of a σ-π complex.     

Aspects of organochalcogen (S, Se, Te) compounds stabilized by intramolecular coordination

The application of intramolecular coordination in the isolation of novel diaryl diselenides and their derivatives, monomeric chalcogenolato complexes of group 12 metals, glutathione peroxidase mimics, hybrid bi-, tri- and multidentate ligands and selenium-containing azamacrocycles is described.     

Hypervalent organoselenium compounds stabilized by intramolecular coordination: synthesis and crystal structures

Two novel hypervalent selenium(IV) compounds stabilized by intramolecular interactions, namely 6‐phenyl‐6,7‐dihydro‐5H‐2,3‐dioxa‐2aλ⁴‐selenacyclopenta[hi]indene, C₁₄H₁₂O₂Se, 14 , and 5‐phenyl‐5,6‐dihydro‐4H‐benzo[c][1,2]oxaselenole‐7‐carbaldehyde, C₁₄H₁₂OSe₂, 15 , have been synthesized by the reaction of 2‐chloro‐1‐formyl‐3‐(hydroxymethylene)cyclohexene with in‐situ‐generated disodium diselenide (Na₂Se₂). The title compounds were characterized by FT–IR spectroscopy, ESI–MS, and single‐crystal X‐ray diffraction studies. For 14 , there is whole‐molecule disorder, with occupancies of 0.605 (10) and 0.395 (10), a double bond between C and Se, and the five‐membered selenopentalene rings are coplanar. The packing is stabilized by π–π stacking interactions involving one of the five‐membered Se/C/C/C/O rings [centroid–centroid (Cg…Cg) distance = 3.6472 (18) Å and slippage = 1.361 Å], as well as C—H…π interactions involving a C—H group and the phenyl ring. In addition, there are bifurcated C—H…Se,O interactions which link the molecules into ribbons in the c direction. For 15 , the C—Se bond lengths are longer than those of 14 . The two five‐membered rings are coplanar. There are no π–π or C—H…π interactions; however, molecules are linked by C—H…O interactions into centrosymmetric dimers, with graph‐set notation R₂²(16).     

Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity

Enhanced rates and selectivity in enzymes are enabled in part by precisely tuned electric fields within active sites. Analogously, the use of charged groups to leverage electrostatics in molecular systems is a promising strategy to tune reactivity. However, separation of the through space and through bond effects of charged functional groups is a long standing challenge that limits the rational application of electric fields in molecular systems. To address this challenge we developed a method using the phosphorus selenium coupling value (J_P–Se) of anionic phosphine selenides to quantify the electrostatic contribution of the borate moiety to donor strength. In this analysis we report the synthesis of a novel anionic phosphine, PPh₂CH₂BF₃K, the corresponding tetraphenyl phosphonium and tetraethyl ammonium selenides [PPh₄][SePPh₂CH₂BF₃] and [TEA][SePPh₂CH₂BF₃], and the Rh carbonyl complex [PPh₄][Rh(acac)(CO)(PPh₂(CH₂BF₃))]. Solvent-dependent changes in J_P–Se were fit using Coulomb''s law and support up to an 80% electrostatic contribution to the increase in donor strength of [PPh₄][SePPh₂CH₂BF₃] relative to SePPh₂Et, while controls with [TEA][SePPh₂CH₂BF₃] exclude convoluting ion pairing effects. Calculations using explicit solvation or point charges effectively replicate the experimental data. This J_P–Se method was extended to [PPh₄][SePPh₂(2-BF₃Ph)] and likewise estimates up to a 70% electrostatic contribution to the increase in donor strength relative to SePPh₃. The use of PPh₂CH₂BF₃K also accelerates C–F oxidative addition reactivity with Ni(COD)₂ by an order of magnitude in comparison to the comparatively donating neutral phosphines PEt₃ and PCy₃. This enhanced reactivity prompted the investigation of catalytic fluoroarene C–F borylation, with improved yields observed for less fluorinated arenes. These results demonstrate that covalently bound charged functionalities can exert a significant electrostatic influence under common solution phase reaction conditions and experimentally validate theoretical predictions regarding electrostatic effects in reactivity.

A detailed analysis of donor properties in solution reveals a major, even dominant, electrostatic effect from charged substituents.     

A mechanistic alternative for the intramolecular hydroboration of homoallylic amine and phosphine borane complexes

Intramolecular hydroboration is demonstrated starting from homoallylic amine boranes upon activation by iodine. The process involves a B-iodoborane complex as the intermediate and may occur via internal displacement of iodide by the alkene to generate a cationic borane-alkene pi-complex on the way to hydroboration products. The reaction can be carried out using a catalytic amount of iodine.     

Halogenations of anthracenes and Dibenz

Halogenation of dibenz[a,c]anthracene (1) by NBS in CCl(4) affords the products of 9- and 10-monobromination in the ratio of 9:1. The reaction is accelerated by iodine, and HBr effects rearrangement of 9-bromo product to the sterically less crowded 10-bromo isomer. The mechanism is proposed to involve reversible addition of Br(2), followed by elimination of HBr. Reaction of NCS with 1 in CCl(4) requires addition of HCl and affords exclusively 9-chlorination. The different reactivities of NBS and NCS are ascribed to the relative amounts of free halogen produced (due to differences in N-X bond strengths involving Br and Cl), and the different sizes of the halogens. Under similar conditions, NCS chlorinates 9-bromoanthracene (2a) to afford 9,10-dichloroanthracene and 9-bromo-10-chloroanthracene in the ratio of 65:35. This reaction ostensibly occurs by addition of Cl(2) to 2a, followed by preferential loss of HBr rather than HCl. 9-Methylanthracene (3) affords exclusively 9-(bromomethyl)anthracene with NBS in the absence of iodine, but mainly (67%) 9-bromo-10-methylanthracene in the presence of iodine. Chlorination of 3 with NCS in the presence of HCl also affords mostly (65%) nuclear halogenation. Nuclear bromination of anthracene, 9-methylanthracene, and dibenz[a, c]anthracene by NBS in the absence of added HBr is accelerated by iodine. This effect is probably due to an increase in the amount of bromine produced from NBS in the presence of iodine.     

Synthesis and characterization of gold nanoparticles stabilized by palladium(II) phosphine thiol

This work is focused on the synthesis of innovative hybrids made by linking gold nanoparticles to protected organometallic Pd(II) thiolate. The organometallic protected Pd(II) thiolate, i.e. trans-thioacetate-ethynylphenyl-bis(tributylphosphine)palladium(II) has been synthesized, in situ deprotected and linked to Au nanoparticles. In this way new hybrid, with a direct link between Pd(II) and Au nanoparticles through a single S bridge, has been isolated. The combination of the organometallic Pd(II) thiol with gold nanoparticles allows the enhancement and tailoring of electronic and optical properties of the new organic-inorganic nano-compound. Single-crystal gold nanoparticles, uniform in shape and size were obtained by applying a modified two-phase method (improved Brust-Schiffrin reaction). In addition, the chemical environment of the Au nanoparticles was investigated and a covalent bonding between Au nanoparticles and the organometallic thiols was observed.     

A nonpeptidic reverse-turn scaffold stabilized by urea-based dual intramolecular hydrogen bonding

A novel nonpeptidic reverse-turn scaffold containing urea fragments that are connected by a conformationally constrained D-prolyl-cis-1,2-diaminocyclohexane (D-Pro-DACH) linker is reported. The scaffold adopts a well-defined reverse-turn conformation that is stabilized by dual intramolecular hydrogen bonding in both solution and solid states.     

Photoinduced intramolecular charge separation in donor/acceptor-substituted bicyclohexylidene and bicyclohexyl

The photophysical properties of a bicyclohexylidene (1DA) and a bicyclohexyl (2DA) substituted with an anilino electron donor and a dicyanoethylene electron acceptor have been studied. Quenching of local donor emission is observed for these compounds as well as quenching of the "pseudo-local" acceptor emission. Transient absorption spectra show dialkylanilino-type radical-cation and dicyanoethylene-type radical-anion absorptions. These results show that intramolecular charge separation takes place in 1DA and 2DA. This was corroborated by time-resolved microwave conductivity measurements from which large excited-state dipole moments were found for both 1DA and 2DA. Time-resolved fluorescence spectroscopy revealed that in the charge-separated state in cyclohexane for 2DA, molecular folding takes place on a nanosecond timescale. For 1DA in cyclohexane, either charge separation takes place in a (fully) folded conformation or very rapid (subnanosecond timescale) folding takes place subsequent to charge separation. In addition to this difference in conformational behavior, the presence of the exocyclic double bond between the cyclohexyl-type rings results in efficient quenching of the anilino donor triplet state and acceleration of the charge recombination rate by a factor of 20.     

Cycloaddition of phosphenium ions to norbornadiene and quadricyclane

Bis(dimethylamino)-, bis(diethylamino)-, and chloro(diisopropylamino)phosphenium ions (1a–c, respectively) react cleanly with norbornadiene (N) to yield the homo-1,4-cycloadducts (2a–c). In addition, 1c reacts cleanly with quadricyclane (Q) to yield the same product.     

Ab initio evaluation of intramolecular electron transfer reactions in halobenzenes and stabilized derivatives

The potential energy surfaces for the fragmentation of the radical anions of p-nitrochlorobenzene and p- and m-chloroacetophenones were explored using first principle methods. The behavior of these compounds, stabilized by pi acceptors, is compared to that shown by the unsubstituted halobenzenes (PhX, X = F, Cl, Br, I). The presence of pi and sigma radical anions was inspected as well as the intramolecular electron transfer (intra-ET) from the pi to the sigma surface, responsible for the dissociation of these intermediates. The profiles obtained with the B3LYP functional in the gas phase and in the presence of a polar solvent are in agreement with the spectroscopic evidence and with the experimentally observed reactivity of the compounds under study. The stability of the radical anion of p-nitrochlorobenzene and the adiabatic and endothermic nature of its dissociation are explained. The order of the rate constants for dissociation m-chloroacetophenone < p-chloroacetophenone is interpreted on the basis of the differences in the adiabatic character of the intra-ET of both isomers which is ascribed to the nodal properties of their SOMOs. In the halobenzene family, the electronic factors responsible for the intra-ET are analyzed. The stabilization of the sigma surface exerted by the different halogens and its effect on the rate constants for dissociation are explained.     

Microwave-assisted synthesis of substituted phenanthrenes, anthracenes, acenaphthenes, and fluorenes

Rapid coupling reactions of polycyclic aromatic halides with various N-, S-, and Se-nucleophiles under focused microwave irradiation are described. Using this method, the desired products are obtained with good to excellent yields in a short reaction time.     

Biradicals stabilized by intramolecular charge transfer: properties of heterosubstituted pentalene and cyclooctatetraene biradicals

Intramolecular charge transfer can lead to substantial stabilization of singlet ground state and a corresponding increase of the singlet-triplet gap for molecules isoelectronic with the dianions of antiaromatic hydrocarbons. The formal biradicals 2,5-di-heterosubstituted-pentalenes and 1,5-di-heterosubstituted-cyclooctatetraenes are theoretically predicted to have the potential to be stable, persistent non-Kekulé molecules, as supported by high-level quantum chemical calculations. The singlet-triplet energy gaps and the S(0)-S(1) excitation energies of these molecules are similar to those of aromatic molecules rather than standard biradicals. These formal biradicals have a pronounced zwitterionic character, having a singlet ground state. The marked stabilization of the ground-state singlet for these non-Kekulé molecules is accompanied by a significant destabilization of the highest occupied molecular orbital (HOMO), leading to a low ionization potential (IP). This apparent inconsistency is explained by analyzing the electronic structure of the molecules. In the case of di-aza-pentalene, the energy of the first electronic excited state is only slightly lower than the ionization potential, making it a candidate for molecular autoionization.     

Reactions of phosphine oxides with bromophosphoranimines; synthesis and unusual rearrangements of O-donor stabilized phosphoranimine cations

Reaction of phosphine oxides R(3)P═O [R = Me (1a), Et (1c), (i)Pr (1d) and Ph (1e)], with the bromophosphoranimines BrPR'R'P═NSiMe(3) [R' = R' = Me (2a); R' = Me, R' = Ph (2b); R' = R' = OCH(2)CF(3) (2c)] in the presence or absence of AgOTf (OTf = CF(3)SO(3)) resulted in a rearrangement reaction to give the salts [R(3)P═N═PR'R'O-SiMe(3)]X (X = Br or OTf) ([4]X). Reaction of phosphine oxide 1a with the phosphoranimine BrPMe(2)═NSiPh(3) (5) with a sterically encumbered silyl group also resulted in the analogous rearranged product [Me(3)P═N═PMe(2)O-SiPh(3)]X ([8]X) but at a significantly slower rate. In contrast, the direct reaction of the bulky tert-butyl substituted phosphine oxide, (t)Bu(3)P═O (1b) with 2a or 2c in the presence of AgOTf yielded the phosphine oxide-stabilized phosphoranimine cations [(t)Bu(3)P═O·PR'(2)═NSiMe(3)](+) ([3](+), R' = Me (d), OCH(2)CF(3) (e)). A mechanism is proposed for the unexpected formation of [4](+) in which the formation of the donor-stabilized adduct [3](+) occurs as the first step.