Cascade radical reactions via alpha-(arylsulfanyl)imidoyl radicals: competitive [4 + 2] and [4 + 1] radical annulations of alkynyl isothiocyanates with aryl radicals

Aryl radicals react with 2-(2-phenylethynyl)phenyl isothiocyanate through a novel radical cascade reaction entailing formation of alpha-(arylsulfanyl)imidoyl radicals and affording a new class of compounds, i.e. thiochromeno[2,3-b]indoles. These derivatives are formed as mixtures of substituted analogues arising from competitive [4 + 2] and [4 + 1] radical annulations. The isomer ratio is strongly dependent on the aryl substituent and is correlated to its capability to delocalize spin density. The presence of a methylsulfanyl group in the ortho-position of the initial aryl radical results in complete regioselectivity and better yields, as the consequence of both strong spin-delocalization effect, which promotes exclusive [4 + 1] annulation, and good radical leaving-group ability, which facilitates aromatization of the final cyclohexadienyl radical. Theoretical calculations support the hypothesis of competitive, independent [4 + 2] and [4 + 1] annulation pathways. They also suggest that rearrangement onto the sulfur atom of the [4 + 1] intermediate does not occur via a sulfuranyl radical but rather through either a transition state or a sulfur-centered (thioamidyl) radical; the latter is possibly the preferred route in the presence of an o-methylsulfanyl moiety that can act as a leaving group in the final ipso-cyclization process.     

Decarboxylative Conjunctive Cross-coupling of Vinyl Boronic Esters using Metallaphotoredox Catalysis

The synthesis of complex alkyl boronic esters through conjunctive cross-coupling of vinyl boronic esters with carboxylic acids and aryl iodides is described. The reaction proceeds under mild metallaphotoredox conditions and involves an unprecedented decarboxylative radical addition/cross-coupling cascade of vinyl boronic esters. Excellent functional-group tolerance is displayed, and application of a range of carboxylic acids, including secondary α-amino acids, and aryl iodides provides efficient access to highly functionalized alkyl boronic esters. The decarboxylative conjunctive cross-coupling was also applied to the synthesis of sedum alkaloids.     

Novel intramolecular [4 + 1] and [4 + 2] annulation reactions employing cascade radical cyclizations

Tributyltin hydride and tris(trimethylsilyl)silane promote sequential/cascade free radical cyclization reactions of dienoate tethered vinyliodides or alkynes. These processes produce [4 + 1] and [4 + 2] annulated products. In contrast, the electrochemical reductions of the vinyliodides afford monocyclic compounds. Both the regiochemical and stereochemical courses of the sequential radical cyclizations strongly depend on substrate structure. Especially important is the balance between steric and stereoelectronic (Baldwin's rules) factors that serve to control cyclization regiochemistry.     

Decarboxylative Conjunctive Cross‐coupling of Vinyl Boronic Esters using Metallaphotoredox Catalysis

The synthesis of complex alkyl boronic esters through conjunctive cross‐coupling of vinyl boronic esters with carboxylic acids and aryl iodides is described. The reaction proceeds under mild metallaphotoredox conditions and involves an unprecedented decarboxylative radical addition/cross‐coupling cascade of vinyl boronic esters. Excellent functional‐group tolerance is displayed, and application of a range of carboxylic acids, including secondary α‐amino acids, and aryl iodides provides efficient access to highly functionalized alkyl boronic esters. The decarboxylative conjunctive cross‐coupling was also applied to the synthesis of sedum alkaloids.     

Total synthesis of (+/-)-strychnine via a [4 + 2]-cycloaddition/rearrangement cascade

A new strategy for the synthesis of the Strychnos alkaloid (+/-)-strychnine has been developed and is based on an intramolecular [4 + 2]-cycloaddition/rearrangement cascade of an indolyl-substituted amidofuran. The critical D-ring was assembled by an intramolecular palladium-catalyzed enolate-driven cross-coupling of an N-tethered vinyl iodide. [reaction: see text].     

Palladium-catalyzed alkenylation and alkynylation of ortho-C(sp2)-H bonds of benzylamine picolinamides

An efficient functionalization of ortho-C(sp(2))-H bonds of picolinamide (PA)-protected benzylamine substrates with a range of vinyl iodides as well as acetylenic bromide is reported. ortho-Phenyl benzoic acid (oPBA) acts as an effective promoter in this reaction system. This method provides a practical strategy to access highly functionalized benzylamine compounds for organic synthesis.     

Reaction of the i-C4H5 (CH2CCHCH2) radical with O2

The resonantly stabilized radical i-C(4)H(5) (CH(2)CCHCH(2)) is an important intermediate in the combustion of unsaturated hydrocarbons and is thought to be involved in the formation of polycyclic aromatic hydrocarbons through its reaction with acetylene (C(2)H(2)) to form benzene + H. This study uses quantum chemistry and statistical reaction rate theory to investigate the mechanism and kinetics of the i-C(4)H(5) + O(2) reaction as a function of temperature and pressure, and unlike most resonantly stabilized radicals we show that i-C(4)H(5) is consumed relatively rapidly by its reaction with molecular oxygen. O(2) addition occurs at the vinylic and allenic radical sites in i-C(4)H(5), with respective barriers of 0.9 and 4.9 kcal mol(-1). Addition to the allenic radical form produces an allenemethylperoxy radical adduct with only around 20 kcal mol(-1) excess vibrational energy. This adduct can isomerize to the ca. 14 kcal mol(-1) more stable 1,3-divinyl-2-peroxy radical via concerted and stepwise processes, both steps with barriers around 10 kcal mol(-1) below the entrance channel energy. Addition of O(2) to the vinylic radical site in i-C(4)H(5) directly forms the 1,3-divinyl-2-peroxy radical with a small barrier and around 36.8 kcal mol(-1) of excess energy. The 1,3-divinyl-2-peroxy radical isomerizes via ipso addition of the O(2) moiety followed by O atom insertion into the adjacent C-C bond. This process forms an unstable intermediate that ultimately dissociates to give the vinyl radical, formaldehyde, and CO. At higher temperatures formation of vinylacetylene + HO(2), the vinoxyl radical + ketene, and the 1,3-divinyl-2-oxyl radical + O paths have some importance. Because of the adiabatic transition states for O(2) addition, and significant reverse dissociation channels in the peroxy radical adducts, the i-C(4)H(5) + O(2) reaction proceeds to new products with rate constant of around 10(11) cm(3) mol(-1) s(-1) at typical combustion temperatures (1000-2000 K). For fuel-rich flames we show that the reaction of i-C(4)H(5) with O(2) is likely to be faster than that with C(2)H(2), bringing into question the importance of the i-C(4)H(5) + C(2)H(2) reaction in initiating ring formation in sooting flames.     

Photoinduced Palladium-Catalyzed Dicarbofunctionalization of Terminal Alkynes

Herein, a conceptually distinct approach was developed that allowed for the dicarbofunctionalization of alkynes at room temperature using simple, bench-stable alkyl iodides and a second molecule of alkyne as coupling partner. Specifically, the photochemical activation of palladium complexes enabled this strategic dicarbofunctionalization via addition of alkyl radicals from secondary and tertiary alkyl iodides and formation of an intermediate palladium vinyl complex that could undergo subsequent Sonogashira reaction with a second alkyne molecule. This alkylation–alkynylation sequence allowed the one-step synthesis of 1,3-enynes including heteroarenes and biologically active compounds with high efficiency without exogenous photosensitizers or oxidants and now opens up pathways towards cascade reactions via photochemical palladium catalysis.     

SYNTHESIS OF 3-PER(POLY)FLUOROALKYLPYRAZOLES

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Thermal decomposition of 1,2-dimethyl-3(5)-(4-R-styryl)-pyrazolium iodides

The reaction of 1,2,3(5)-trimethylpyrazolium iodide with aromatic aldehydes yielded the series of 1,2-dimethyl-3(5)-(4-R-styryl)pyrazolium iodides (R = H, NMe₂, NO₂). The quantitative ratio of 1-methyl-3- and -5-(4-R-styryl)pyrazoles formed in the thermal decomposition of the above-mentioned pyrazolium iodides depends on steric factors in the initial compounds and on the electronic influence of the substituent R. A simple method was proposed for the production of trans,trans-1,4-bis(1-methyl-3-pyrazolylvinyl)benzene and 1-methyl-3,5-distyrylpyrazole, which possess fluorescent properties.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1533–1536, November, 1984.     

Pressure-dependent OH yields in alkene + HO2 reactions: a theoretical study

The major bimolecular product of alkyl + O(2) reactions is alkene + hydroperoxyl radical (HO(2)), but in the reverse direction, the reactants are reformed to a very limited extent only. The most important products of the alkene + HO(2) reactions are alkylperoxy radical (ROO(?)), hydroxyl radical (OH) + cyclic ether, and the corresponding hydroperoxyalkyl ((?)QOOH) species. Moreover, abstraction of allylic hydrogens can compete with the addition, further complicating the possible outcome of this reaction type and its effect on low-temperature combustion chemistry. In this paper, six alkene + HO(2) reactions and the reaction between an unsaturated oxygenate and HO(2) are studied based on previously established potential energy surfaces. The studied unsaturated compounds are ethene, propene, 1-butene, trans-2-butene, isobutene, cyclohexene, and vinyl alcohol. Using multiwell master equations, temperature- (300-1200 K) and pressure-dependent rate coefficients and branching fractions are calculated for these reactions. The importance of this reaction type for the combustion of unsaturated compounds is also assessed, and we show that, to get reliable results, it is important to include the pressure-dependence of the rate coefficients in the calculations.     

Palladium(0)-promoted cross-coupling of allenylmetal compounds with aryl and vinyl iodides. A novel route to aryl- and vinyl-substituted allenes

The reaction of allenylmetal compounds, RR{′}CCCHM (I), with aryl and vinyl iodides, R I, have been studied. With Pd(PPh₃)₄ as catalyst a highly regioselective formation of allenes, RR{′}CC CHR″ (II), has been observed for MgCl, Cu, Cu_1/2,Li_1/2, Ag, Ag_1/2Li_1/2, or ZnCl, but not Li. The reaction of the iodides proceeds with retention of configuration.     

4 + 1] cycloaddition of bis(alkylthio)carbenes with vinyl isocyanates. Total synthesis of (+/-)-mesembrine

[reaction: see text] The Sceletium alkaloid mesembrine has been synthesized in 13% overall yield by a sequence featuring a [4 + 1] cycloaddition of a bis(alkylthio)carbene with a functionalized vinyl isocyanate.     

Reactions of ethyl bromodifluoroacetate in the presence of copper powder

We have examined the reaction of ethyl bromodifluoroacetate (1) in the presence of copper powder as a procedure for the synthesis of compounds containing a CF₂ group. The complex formed in the above reaction reacted with vinyl or aryl iodides to give cross-coupling products, with Michael acceptors to give 1,4-addition products and with olefins to give radical addition products. The cross-coupling reaction was applied to the synthesis of 4,4-difluoro-α-tocopherol.     

Sulfonium Ylides by (3+2) Cycloaddition of Arynes with Vinyl Sulfides: Stereoselective Synthesis of Highly Substituted Alkenes

The reaction of in situ generated arynes with vinyl sulfides provides benzannulated sulfonium ylides in a (3+2) cycloaddition. Trapping of the intermediate ylides with electrophiles (proton transfer or a second aryne addition) and subsequent β‐elimination give rise to di‐, tri‐, or tetrasubstituted alkenes with high stereoselectivity. Experimental studies and DFT calculations provide insight into the mechanisms of these cascade reactions.     

Iron-catalyzed reductive radical cyclization of organic halides in the presence of NaBH4: evidence of an active hydrido iron(I) catalyst

Iron made'em: iron(II) complexes such as FeCl(2) and [FeCl(2)(dppe)(2) ] (dppe=1,2-bisdiphenylphosphinoethane) are efficient precatalysts for the radical cyclization of unsaturated iodides and bromides in the presence of NaBH(4). Cyclic voltammetry studies suggests that the reaction occurs through a radical mechanism via an anionic hydrido iron(I) species as the key intermediate for the activation of the substrates by electron transfer.     

Modular Assembly of Spirocarbocyclic Scaffolds through Pd0‐Catalyzed Intermolecular Dearomatizing [2+2+1] Annulation of Bromonaphthols with Aryl Iodides and Alkynes

A novel palladium(0)‐catalyzed dearomatizing [2+2+1] spiroannulation of 1‐bromo‐2‐naphthols with aryl iodides and alkynes was developed for the rapid assembly of spiro[indene‐1,1′‐naphthalen]‐2′‐ones. This three‐component cascade reaction was realized through consecutive Catellani‐type C−H activation, unsymmetrical biaryl coupling, alkyne migratory insertion, and arene dearomatization. The potential utility of our method is illustrated by the one‐step construction of the polycyclic skeletons of dalesconols A and B from alkyne‐tethered aryl iodides and 1‐bromo‐2‐naphthol.     

Electrochemical Studies of 1,4-Bis[2-(2-pyridyl)-vinyl] Benzene and 1,4-Bis[2-(4-pyridyl) vinyl] Benzene Laser Dyes via Cyclic Voltammetry, Convolutive Voltammetry and Digital Simulation Methods

Electrochemical properties of two diolefinic laser dyes namely 1,4-bis[2-(2-pyridyl)-vinyl] benzene (2PVB) and 1,4-bis[2-(4-pyridyl) vinyl] benzene (4PVB) have been investigated using cyclic voltammetry and convolutive voltammetry combined with digital simulation at a platinum electrode in 0.1 mol/L tetrabutyl ammonium perchlorate (TBAP) in the two different solvents acetonitrile (CH3CN) and dimethylformamide (DMF). The species were reduced via consumption of two sequential electrons to form radical anion and dianion. In switching the potential to positive direction, the two compounds were oxidized by loss of one electron, which was followed by a fast isomerisation process. The electrode reaction pathway and the electrochemical parameters of the investigated compounds were determined using cyclic voltammetry. The extracted electrochemical parameters were verified and confirmed via digital simulation and convolutive voltammetry methods.     

Reactions of tetrachlorodiborane(4) with fluoroolefins

Tetrachlorodiborane(4) reacts with trifluoroethylene to give dichloro-2,2- difluorovinylborane and both isomers of dichloro-2-chloro-2-fluorovinylborane. These compounds can be converted to the corresponding difluoro (halovinyl)boranes by treatment with SbF₃. Reaction of B₂Cl₄ with vinyl fluoride gives 1,1,2-tris(dichloroboryl)ethane. A facile halogen exchange with B₂Cl₄, was observed for a number of fluoroolefins, includin vinyl fluoride, 1,1-difluoroethylene, 2-fluoropropene, and 3,3,3-trifluoropropene. No reaction was observed with 2,3,3,3-tetrafluoropropene.     

Cascade cyclization: an easy access to highly unsaturated polycyclic ring systems through a tandem stille/[4 + 2] reaction under mild conditions

The synthesis of several polycyclic compounds 1a-c, 2, and 3 has been performed through a tandem Stille/[4 + 2] cascade reaction from cyclic bis(enoltrifluomethanesulfonate) 4a-c, 5, and 6, respectively. The reaction proceeds very efficiently in a one-pot operation at roomtemperature in DMF in the presence of a catalytic amount of Pd(CH(3)CN)(2)Cl(2) and LiCl.[reaction: see text]