Electrophilic substitution reactions of metallabenzynes

The electrophilic substitution reactions of metallabenzynes Os(≡CC(R)═C(CH(3))C(R)═CH)Cl(2)(PPh(3))(2) (R = SiMe(3), H) were studied. These metallabenzynes react with electrophilic reagents, including Br(2), NO(2)BF(4), NOBF(4), HCl/H(2)O(2), and AlCl(3)/H(2)O(2) to afford the corresponding bromination, nitration, nitrosation, and chlorination products. The reactions usually occur at the C2 and C4 positions of the metallacycle. These observations support the notion that metallabenzynes exhibit aromatic properties.     

Osmabenzenes from the reactions of a dicationic osmabenzyne complex

Treatment of the osmabenzyne Os([triple bond]CC(SiMe(3))=C(Me)C(SiMe(3))=CH)Cl(2)(PPh(3))(2) (1) with 2,2'-bipyridine (bipy) and thallium triflate (TlOTf) produces the thermally stable dicationic osmabenzyne [Os([triple bond]CC(SiMe(3))=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)](OTf)(2) (2). The dicationic osmabenzyne 2 reacts with ROH (R = H, Me) to give osmabenzene complexes [Os(=C(OR)CH=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)]OTf, in which the metallabenzene ring deviates significantly from planarity. In contrast, reaction of the dicationic complex 2 with NaBH(4) produces a cyclopentadienyl complex, presumably through the osmabenzene intermediate [Os(=CHC(SiMe(3))=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)]OTf. The higher thermal stability of [Os(=C(OR)CH=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)]OTf relative to [Os(=CHC(SiMe(3))=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)]OTf can be related to the stabilization effect of the OR groups on the metallacycle. A theoretical study shows that conversion of the dicationic osmabenzyne complex [Os([triple bond]CC(SiMe(3))=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)](OTf)(2) to a carbene complex by reductive elimination is thermodynamically unfavorable. The theoretical study also suggests that the nonplanarity of the osmabenzenes [Os(=C(OR)CH=C(Me)C(SiMe(3))=CH)(bipy)(PPh(3))(2)]OTf is mainly due to electronic reasons.     

Palladium-catalyzed sequential alkylation-alkenylation reactions: new three-component coupling leading to oxacycles

A new three-component domino reaction catalyzed by palladium was devised, producing polysubstituted bicyclic molecules in good yields from readily available substrates. The reaction conditions and the scope of the process were examined, and a possible mechanism is proposed. [reaction: see text]     

Tandem amine propargylation-Sonogashira reactions: new three-component coupling leading to functionalized substituted propargylic amines

Three-component coupling reactions of propargyl halides, amines, and organic halides in the presence of palladium-copper catalysis produced efficiently highly functionalized propargylic amines in good to excellent yields at room temperature. Extension to the synthesis of 2-(dialkylaminomethyl)benzo[b]furan or indole derivatives is described.     

Conversions of osmabenzyne and isoosmabenzene

We report herein the first example of the conversion of metallabenzyne II and isometallabenzene III. The osmium hydride vinylidene complex 1 reacts with HC?CCH(OEt)(2) to give osmabenzyne 3 via isoosmabenzene 2. Compound 3 exhibits high thermal stability in air. Nonetheless, nucleophilic attack at 3 provides isoosmabenzenes 4?a and 4?b, or opens the ring to produce 5?a and 5?b. We propose mechanisms to disclose the intrinsic connection between the six-membered metallacycles, and carry out DFT calculations to rationalize the regioselectivity of the nucleophilic addition reactions.     

Conjugation and cyclization—two strong driving forces leading to the formation of new chromophores

Novel chromophores formed in the solvent reactions of α-amino acids and small peptides were identified by crystal structure analysis and characterized by UV absorption. The formation of these chromophores in basic solutions was attributed to two strong driving forces—conjugation and cyclization. The discussion of possible reaction pathways could benefit the future design of α-amino acid-based chromophores.     

A DFT investigation of alkyne bromination reactions

[Structure: see text]. A DFT calculation study of the addition reaction between molecular bromine and the number of symmetrical or unsymmetrical substituted alkynes 1 (R-CC-R'), where R = R' = H (1a), Me (1b), t-Bu (1c), or Ph (1d), or R = H and R' = Me (1e), t-Bu (1f), or Ph (1g), was performed. Two possible reactions were checked: (a) the reactions suitable for the gas-phase interactions, which start from a 1:1 Br2-alkyne pi-complex and do not enter into a 2:1 Br2-alkyne pi-complex; and (b) the processes passing through a 2:1 Br2-alkyne pi-complex, which look more realistic for the reactions in solutions. The structures of the starting reactants and the final products as well as the possible stable intermediates have been optimized. The transition states of the predicted process have been found. Both trans- and cis-dibromoalkenes (2 and 3) may ensue without the formation of ionic intermediates from a pi-complex of two bromine molecules with the alkyne (solution reactions). The geometry around the double bond forming in dibromoalkenes strongly depends on the nature of the substituents at the triple bond. The "cluster model" was also used for the prediction of solvent influence on the value of the activation barrier of the but-2-yne (1b) bromination reaction.     

Reactions leading to the formation of atomic phosphorus

Conclusions By resonance fluorescence in a jet at 300–670°K, the rate constants of the reactions of atomic hydrogen with phosphorus trichloride and P₄, are determined. In the products of these reactions, atomic phosphorus is registered in concentrations comparable with the concentrations of expended hydrogen atoms.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 22–25, January, 1982.     

Protonation, deprotonation, and protonation: conjugated photochemical reactions of ferrocenylazophenol

4-Ferrocenylazophenol (FAP) and 8-(4-ferrocenylazophenoxy)octanyl-1-bromide (FAOB) have been synthesized and their protonation and photochemical behavior have been investigated. FAP showed reversible protonation and deprotonation behavior in the presence of trifluoroacetic acid and potassium tert-butylate, respectively. FAP and FAOB exhibit high photoreactivity, which was induced through excitation of not only the pi-pi* band by UV light but also the d-pi* band by green light. The photoreaction of FAP and FAOB in ethanol or acetonitrile with a trace amount of water afforded phenylhydrazonocyclopentadiene (PHP) derivatives, indicating that hydrolysis of the photoexcited species occurs.     

1,3-Diaryl-2,2-dihaloaziridines in nitration and bromination reactions

In the nitration of 3-(4-nitrophenyl)-1-phenyl-2,2-dichloroaziridine in acetic acid, 1-(o- and p-nitrophenyl)-derivatives are formed in a 35:65 ratio. 1,3-Di-phenyl-2,2-dichloroaziridine undergoes opening of the three-membered ring under the same conditions, forming a mixture of o- and p-nitroanilides and 2-nitro-4-chloroanilides of 2-acetoxy (or 2-chloro)-2-phenylacetic acids. The bromination of 3-(4-nitrophenyl)-1-phenyl-2,2-dichloroaziridine in aqueous acetic acid leads to 1-(4-bromophenyl)-3-(4-nitrophenyl)-2,2-dichloroaziridine, while in a mixture of acetic acid and acetic anhydride it leads to the anilide of 2-bromo-2-phenyl-acetic acid and 2-bromo-N-(2,4-dibromophenyl)-1-(4-nitrophenyl)-2,2-dichloro-ethylamine.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 912–916, July, 1984.     

Chemical reactions of double bonds in activated carbon: microwave and bromination methods

The reaction of localised C[double bond, length as m-dash]C bonds on the surface of activated carbons has been shown to be an effective method of chemical modification especially using microwave-assisted reactions.     

Catalytic direct arylation with aryl chlorides, bromides, and iodides: intramolecular studies leading to new intermolecular reactions

A catalyst for the intramolecular direct arylation of a broad range of simple and heterocyclic arenes with aryl iodides, bromides, and chlorides has been developed. These reactions occur in excellent yield and are highly selective. Studies with aryl iodides substrates revealed that catalyst poisoning occurs due to the accumulation of iodide in the reaction media. This can be overcome by the addition of silver salts which also permits these reactions to occur at lower temperature. The utility of the methodology is illustrated by a rapid synthesis of a carbazole natural product and by the synthesis of sterically encumbered tetra-ortho-substituted biaryls via ring-opening reactions of the direct arylation products. Mechanistic investigations have provided insight into the catalyst's mode of action and show the presence of a kinetically significant C-H bond cleavage in palladium-catalyzed direct arylation of simple arenes. Knowledge garnered from these studies has led to the development of new intermolecular arylation reactions with previously inaccessible arenes, opening the door for the development of other new direct arylation processes.     

Associations leading to formation of reversible networks and gels

One of the most important properties of associative polymers is their ability to form reversible networks and gels. The issue of whether or not the reversible gelation of associative polymers is a thermodynamic transition has not been settled yet. At the same time the understanding of different mechanisms governing the dynamics of associative polymers has been significantly advanced. © 1999 Elsevier Science Ltd.     

Photodecarbonylation of alpha-diketones: a mechanistic study of reactions leading to acenes

Poly(acene)s are significant compounds for various electronic applications. A clean, one-step synthesis involves alpha-diketones (2-4), which undergo facile Strating-Zwanenburg photodecarbonylation producing the corresponding poly(acene)s (i.e., anthracene, hexacene, and heptacene, respectively). Compounds 2-4 show weak fluorescence (lambdaF=approximately 525-530 nm and PhiF=approximately 0.1-0.4%) and phosphorescence (lambdaPh=approximately 565-570 nm) and have a small singlet-triplet energy gap (S1-T1 gap, approximately 4 kcal/mol) that facilitates rapid intersystem crossing from the singlet to the triplet state. Both the singlet states (tauS=approximately 20-218 ps) and the triplet states (tauT=approximately 370 ps to <7 ns) of 2-4 are short-lived, while the decarbonylation of 2-4 is a rapid process occurring within 7 ns from both the singlet and the triplet manifolds. The nanosecond laser flash photolysis of 4 also reveals the T-T absorption of heptacene (580 nm, tau=approximately 11 micros).     

Generation of model reactions leading to limit cycle behavior

The system of differential equations by Feistel and Ebeling has been generalized. Some new formal kinetic reactions with two internal components, which may exhibit limit cycle behavior have been studied. Based upon the numerical integration of the deterministic models of these reactions the oscillatory character of the systems has been confirmed.

---

. , . .

     

A new mild regioselective bromination of arylamines

A new efficient regioselective boron-assisted mono bromination methodology for arylamines is described. Our one-pot, three-stage approach (lithiation, boron amide formation and bromination) proved to be highly useful in mildly brominating a variety of arylamines in up to 94% yields.     

Ring closing and opening reactions leading to aza-polycyclic aromatic compounds

A series of functionalized aza-polycyclic aromatic compounds were prepared by a superacid-promoted ring closing and opening reaction cascade. A reaction mechanism is proposed, which involves reactive dicationic intermediates. A key step in the conversions involves ipso protonation of an aryl group and elimination of an alkyl phenyl group.