Twofold π-Extension of Polyarenes via Double and Triple Radical Alkyne peri-Annulations: Radical Cascades Converging on the Same Aromatic Core.

A versatile synthetic route to distannyl-substituted polyarenes was developed via double radical peri-annulations. The cyclization precursors were equipped with propargylic OMe traceless directing groups (TDGs) for regioselective Sn-radical attack at the triple bonds. The two peri-annulations converge at a variety of polycyclic cores to yield expanded difunctionalized polycyclic aromatic hydrocarbons (PAHs). This approach can be extended to triple peri-annulations, where annulations are coupled with a radical cascade that connects two preexisting aromatic cores via a formal C-H activation step. The installed Bu3Sn groups serve as chemical handles for further functionalization via direct cross-coupling, iodination, or protodestannylation and increase solubility of the products in organic solvents. Photophysical studies reveal that the Bu3Sn-substituted PAHs are moderately fluorescent, and their protodestannylation results in an up to 10-fold fluorescence quantum yield enhancement. DFT calculations identified the most likely possible mechanism of this complex chemical transformation involving two independent peri-cyclizations at the central core.

Radical Alkyne peri-Annulation Reactions for the Synthesis of Functionalized Phenalenes, Benzanthrenes, and Olympicene.

Radical cyclization reactions at a peri position were used for the synthesis of polyaromatic compounds. Depending on the choice of reaction conditions and substrate, this flexible approach led to Bu3 Sn-substituted phenalene, benzanthrene, and olympicene derivatives. Subsequent reactions with electrophiles provided synthetic access to previously inaccessible functionalized polyaromatic compounds.

Synthesis and oxidative reactivity of 2,2'-pyridylpyrrolide complexes of Ni(II).

Synthesis and characterization of divalent nickel complexed by 2-pyridylpyrrolide bidentate ligands are reported, as possible precursors to complexes with redox active ligands. Varied substituents on the pyrrolide, two CF3 (L(2)), two (t)Bu (L(0)), and one of each type (L(1)) are employed and the resulting Ni(L(n))2 complexes show different Lewis acidity toward CO, H2O, tetrahydrofuran (THF), or MeCN, the L(2) case being the most acidic. Density functional theory calculations show that the frontier orbitals of all three Ni(L(n))2 species are localized at the pyrrolide groups of both ligands and Ni(L(n))2(+) can be detected by mass spectrometry and in cyclic voltammograms (CVs). Following cyclic voltammetry studies, which show electroactivity primarily in the oxidative direction, reactions with pyridine N-oxide or Br2 are reported. The former yield simple bis adducts, Ni(L(2))2(pyNO)2 and the latter effects electrophilic aromatic substitution of the one pyrrolide ring hydrogen for both chelates, leaving it brominated.

Reactivity of the terminal oxo species (((t)Bu2PCH2SiMe2)2N)RhO.

Reactivity of the 4-coordinate molecule (PNP)RhO (PNP is ((t)Bu2PCH2SiMe2)2N) towards CO proceeds stepwise, first forming an η(2)-CO2 complex, by a mechanism which involves a preliminary adduct of CO on Rh, then a second CO displaces CO2. Reaction of the oxo complex with CO2 occurs in time of mixing even at low temperature to form (PNP)Rh(η(2)-CO3), with no intermediate detectable. DFT calculations indicate an initial bond formation between the oxo center and the CO2 carbon. Reaction of (PNP)RhO with H2 occurs only at a 1 : 2 molar stoichiometry, to ultimately form (PNP)Rh(H)2 and free H2O. No intermediate reaches detectable population even at -60 °C, but DFT mapping of various possible mechanisms on the singlet energy surface shows that the nearly equi-energetic (PNP)Rh(H2O) and (PNP)RhH(OH) are formed, but only the latter readily adds the second molecule of H2 to proceed to the observed products; these reactions thus both involve heterolytic splitting of H2.

A ring fragmentation approach to medium-sized cyclic 2-alkynones.

Bicyclic γ-silyloxy-ß-hydroxy-α-diazoketones, in which the Cß-Cγ bond is the ring fusion bond, productively fragment when treated with tin(IV) chloride to provide medium-sized cyclic 2-alkynones. This method provides good to excellent yields of 10-, 11-, and 12-membered alkynone products.

Assessment of the electronic structure of 2,2'-pyridylpyrrolides as ligands.

The ligand class 2,2'-pyridylpyrrolide is surveyed, both for its structural features and its electronic structure, when attached to monovalent K, Cu, Ag, Au, and Rh. The influence of pyrrolide ring substituents is studied, as well as the question of push/pull interaction between the pyridyl and pyrrolide halves. The π donor ability of the pyrrolide is found to be less than that of an analogous phenyl. However, in contrast to the phenyl analog, the HOMO is pyrrolide π in character for pyridylpyrrolide complexes of copper and rhodium, while it is conventionally metal localized for planar, d(8) rhodium pyridylphenyl. Monovalent three-coordinate copper complexes show great deviations from Y-shaped toward T-shaped structures, including cases where the pyridyl ligand bonds only weakly.

Three-coordinate NiII: tracing the origin of an unusual, facile Si-C(sp3) bond cleavage in [(tBu2PCH2SiMe2)2N]Ni+.

All attempts to synthesize (PNP)Ni(OTf) form instead ((t)Bu(2)PCH(2)SiMe(2)NSiMe(2)OTf)Ni(CH(2)P(t)Bu(2)). Abstraction of F(-) from (PNP)NiF by even a catalytic amount of BF(3) causes rearrangement of the (transient) (PNP)Ni(+) to analogous ring-opened [((t)Bu(2)PCH(2)SiMe(2)NSiMe(2)F)]Ni(CH(2)P(t)Bu(2)). Abstraction of Cl(-) from (PNP)NiCl with NaB(C(6)H(3)(CF(3))(2))(4) in CH(2)Cl(2) or C(6)H(5)F gives (PNP)NiB(C(6)H(3)(CF(3))(2))(4), the key intermediate in these reactions is (PNP)Ni(+), [(PNP)Ni](+), in which one Si-C bond (together with N and two P) donates to Ni. This makes this Si-C bond subject to nucleophilic attack by F(-), triflate, and alkoxide/ether (from THF). This σ(Si-C) complex binds CO in the time of mixing and also binds chloride, both at nickel. Evidence is offered of a "self-healing" process, where the broken Si-C bond can be reformed in an equilibrium process. (PNP)Ni(+) reacts rapidly with H(2) to give (PN(H)P)NiH(+), which can be deprotonated to form (PNP)NiH. A variety of nucleophilic attacks (and THF polymerization) on the coordinated Si-C bond are envisioned to occur perpendicular to the Si-C bond, based on the character of the LUMO of (PNP)Ni(+).

Ligand influence on metal aggregation: a unique bonding mode for pyridylpyrrolides.

The synthesis and characterization of a Cu(I) complex with a cis-bidentate monoanionic nitrogenous ligand, 2-pyridylpyrrolide, L, is reported. This shows binding of one base B = MeCN or CO per copper in a species LCu(B), but this readily releases the volatile ligand under vacuum with aggregation of transient LCu to a mixture of two enantiomers of a chiral trimer: a zwitterion containing inequivalent Cu(I) centers, possible via a new bonding mode of pyridylpyrrolide, and one with nitrogen lone pairs donating to two different metals. Density functional theory calculations show the energetics of both ligand binding and aggregation (including dimer and monomer alternatives), as well as the ability of this ligand to rotate away from planarity to accommodate a bridging structural role. The trimer serves as a synthon for the simple fragment LCu.

Mechanism of heterolysis of H2 by an unsaturated d8 nickel center: via tetravalent nickel?

Collision of H(2) with the unusual nickel complex, (PNP)Ni(+), where PNP = ((t)Bu(2)PCH(2)SiMe(2))(2)N, forms a rare dihydrogen complex of the d(8) configuration which then rearranges to heterolytically cleave the H-H bond. Experimental studies support a short H/H distance in the coordinated diatomic, and DFT calculations show that the transition state for heterolysis, in spite of the fact that this involves an amide nitrogen located trans to the H(2), has the H/H bond fully split, and has all the geometric features of Ni(IV), but this is a local maximum, not a minimum.

Unusual selectivity of a (pincer)Ni-hydride reacting with CO2.

The reaction of (PNP)NiH, where PNP is ((t)Bu2PCH2SiMe2)2-N(-1), with CO2 occurs over a period of hours at 25 degrees C to form (POP)NiH(NCO), which involves transposition of the initial amide nitrogen and one oxygen of CO2.