Parent Vinyl Research Articles

Reaction Pathways and Dynamics Effects for Ionizations of Vinyl Diazonium Ions

Abstract The denitrogenation reactions of fifteen differently substituted vinyl diazonium (ethenediazonium) ions were examined by MO method at MP2/6-31G*. The parent vinyl diazonium ion (1, CH2=CH–N2+) was shown to initially give the H-bridged cation via the anti-H migration transition state (TS) or the syn-H migration TS. The relative barrier for the two TSs is very small, being 1.0 kcal mol−1 favoring the anti-H migration TS in free energy. The primary open cation is located on a very shallow minimum, and is 4.4 kcal mol−1 less stable than the bridged cation in free energy. The reactions of Me- and Ph-substituted vinyl diazonium ions showed characteristic features depending on the substituent. Molecular dynamics simulations for the reactions of 1 revealed that trajectories from the reactant state mostly gave the bridged cation as expected from the MO result, but there were trajectories that directly led to the open cation without H-participation, a clear manifestation of a dynamics effect. A dynamics effect on the relative population of the bridged versus the open cation is also presented.

Infrared fluorescence from multiphoton dissociation of vinyl bromide: Emission from the products and the parent molecule

Abstract A time-resolved study of the infrared fluorescence emission produced in the infrared multiphoton dissociation of vinyl bromide at different experimental conditions has been carried out. Two different regimes, namely unimolecular and collisional, have been identified and characterized through the parent vinyl bromide and the HBr and C 2 H 2 photoproducts emissions. In the pressure interval studied, the formation of HBr has been determined to be mainly unimolecular, while the C 2 H 2 is produced through a collisionally assisted photodecomposition process. The collisional regime is explained in terms of the 3-centre elimination channel while the unimolecular regime is ascribed to the 4-centre mechanism. Relaxation rate constants for both the vibrationally excited HBr by vinyl bromide and by Ar and for vibrational excited VBr by unexcited VBr, are reported.

Vinyl Cations Substituted with β π-Donors Have Triplet Ground States

Computations at the CASPT2, CBS-QB3, and B3LYP levels of theory demonstrate that beta-substitution of vinyl cations with pi-donors switches the ground state of these ions from the familiar closed-shell singlet state to a carbene-like triplet state similar to the electronic state of triplet phenyl cations. Although the parent vinyl cation is a ground-state singlet species with a very large energy gap to the lowest energy triplet state, substituting the beta hydrogens with just one strong pi-donor (e.g., NH(2), NMe(2), OMe) or two moderate pi-donors (e.g., F, OH, Ar, vinyl) makes the triplet state the computed ground electronic state. In many cases, the singlet states for these beta pi-donor-substituted vinyl cations are prone to rearrangements, although such rearrangements can be inhibited through incorporation of the pi-donors into rings. For example, a vinyl cation based on 1,3-dimethyl-2-methylene imidazolidine (32) is predicted to show a substantial barrier to singlet state rearrangement as well as possess a triplet ground state with a large energy gap. In contrast to the singlet states, the stabilized triplet states appear to be well behaved and more immune to rearrangements. These triplet ions may exhibit substantially different properties and reaction chemistry than those seen for typical closed-shell vinyl cations.

Trisilyl‐Substituted Vinyl Cations

A series of beta,beta-disilyl-substituted vinyl cations were prepared by intramolecular addition of transient silylium ions to C[triple bond]C triple bonds. The vinyl cations were isolated from hydrocarbon solutions as tetrakis(pentafluorophenyl) borates at room temperature. The substituents directly attached to the positively charged carbon atom were varied from tert-butyl- to trialkylsilyl- to trialkylgermyl groups. The cations were identified by their characteristic (13)C and (29)Si NMR data. The NMR investigations revealed for beta,beta-disilyl-alpha-germyl- and trisilyl-substituted vinyl cations a highly dynamic structure in which both vinylic carbon atoms undergo an intramolecular exchange process which is fast on the NMR time scale at room temperature. NMR studies using a doubly (13)C-labeled vinyl cation suggest as mechanism for this exchange process a rotation of the dicarbyne C[triple bond]C(2-) unit within the triangle defined by the three cationic silyl fragments. Therefore the dynamic structure indicated for trisilyl- or disilylgermyl-substituted vinyl cations parallels the situation found for the parent vinyl cation, the protonated acetylene.

Choice of Bond Dissociation Enthalpies on Which To Base the Stabilization Energies of Simple Radicals: DH(R−H) Is Preferred Because DH(R−Me) and DH(R−R) Are Perturbed by Changes in Chain Branching

The relative stabilization energies of radicals, SE(R*), along the simple series methyl/ethyl/isopropyl/tert-butyl are known to vary in spread and even direction dependent on which dissociation enthalpies, DH(R-X), are used for their definition. Using a highly electronegative X is recognized as unwise, but it is not clear that using X = Me or X = R itself might not be preferred over the almost universal use of X = H. The enthalpies of formal isomerization of C4 radical pairs that vary only in the substitution pattern at the radical center but not in carbon skeleton confirm that X = H is indeed the better choice. Comparisons in the context of recent predictive models for alkane and radical stability indicate that, while relative DH(R-H) values highlight the desired difference in substitution pattern at the radical center, relative DH(R-Me) values are perturbed by differences in skeletal branching or protobranching which are well-known to affect thermochemistry. As a result, SE(R*) values derived from relative DH(R-Me) values are consistently too small. The same pattern is illustrated for prim, sec, and tert allylic and benzylic radicals (larger SE(R*)) and for the parent vinyl, phenyl, and ethynyl radicals (negative SE(R*)).

Carbonylative Approaches to α,β-Unsaturated Acyl Radicals and α-Ketenyl Radicals. Their Structure and Applications in Synthesis

Abstract The carbonylation of vinyl radicals gives α,β-unsaturated acyl radicals. This transformation was successfully applied to tandem radical reactions, resulting in assembling three and four components. In these reactions, both halogen abstraction from vinyl halides and hetero atom radical additions to alkynes are used to generate the parent vinyl radicals. Ab initio calculations and density functional methods predict that α,β-unsaturated acyl radicals and the isomeric α-ketenyl radicals are not canonical forms, but are isomeric species that undergo interconversion. Calculations also indicate that α,β-unsaturated acyl radicals are more stable than α-ketenyl radicals, whereas α-ketenyl radicals containing a heteroatom, such as Si, Ge, and Sn, at α-position are more stable than the corresponding α,β-unsaturated acyl radicals. This represents a promising resource for developing new synthetic applications that involve the use of the α-ketenyl radicals. Indeed, following the prediction by calculation, we succeeded in trapping of a tin-attached α-ketenyl radical by imines and amines in an intramolecular fashion. We were also able to achieve the intermolecular trapping of α-ketenyl radicals, providing a new method for alkyne carbonylation by hybrid radical/ionic reactions.

The reaction products of the 193 nm photolysis of vinyl bromide and vinyl chloride studied by time-resolved Fourier transform infrared emission spectroscopy

Time-resolved Fourier transform infrared (TRFTIR) emission spectroscopy has been used to study the 193 nm photolysis of vinyl bromide (C(2)H(3)Br) and vinyl chloride (C(2)H(3)Cl). Time-resolved IR emission was analysed to obtain nascent vibrational state populations of two primary photolysis products: HBr (v = 1-7) and HCl (v = 1-6). In both cases the nascent vibrational state populations monotonically decrease with increasing v and are in excellent agreement with previously published data. Time-resolved populations were analysed to yield rate constants for vibrational relaxation of HBr (v = 1-3) and HCl (v = 1-4) by parent vinyl bromide and vinyl chloride, respectively. In both cases the rate constants were found to increase with increasing vibrational quantum number, in agreement with a single quantum de-excitation via vibrational to vibrational energy transfer. Butadiene (C(4)H(6)) was identified as a secondary product of the photolysis of both vinyl halides, and shown to be formed from the reaction of parent vinyl halide with the vinyl radical. The presence of a buffer gas was found to produce a strong emission feature centred at 2,200 cm(-1), the intensity of which was dependent on the pressure of the buffer gas used, and whose kinetics are indicative of a secondary reaction product. We propose that this emission is from the vibrational progression of the electronic transition A(0, v, 1) --> X(0, v, 2) in the secondary reaction product C(2)H, whose formation route is favoured by the presence of buffer gas.

Nucleophilic Vinylic Substitution and Vinyl Cation Intermediates in the Reactions of Vinyl Iodonium Salts

AbstractFor Abstract see ChemInform Abstract in Full Text.

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A time-resolved study of the infrared fluorescence emission produced in the infrared multiphoton dissociation of vinyl bromide at different experimental conditions has been carried out. Two different regimes, namely unimolecular and collisional, have been identified and characterized through the parent vinyl bromide and the HBr and C2H2 photoproducts emissions. In the pressure interval studied, the formation of HBr has been determined to be mainly unimolecular, while the C2H2 is produced through a collisionally assisted photodecomposition process. The collisional regime is explained in terms of the 3-centre elimination channel while the unimolecular regime is ascribed to the 4-centre mechanism. Relaxation rate constants for both the vibrationally excited HBr by vinyl bromide and by Ar and for vibrational excited VBr by unexcited VBr, are reported.

ChemInform Abstract: Synthons for the Parent Vinyl Carbene Complex in the Benzannulation Reaction.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Synthons for the Parent Vinyl Carbene Complex in the Benzannulation Reaction

The benzannulation reaction of Fischer carbene complexes with alkynes to produce phenols was found not be synthetically useful for the parent vinyl carbene complex 6a. The β-silylated vinyl carbene complex 6c is more stable than the parent vinyl complex 6a and can be effectively used as a synthon for 6a since during the benzannulation the silicon migrates to the phenol function preferentially over hydrogen to give the arene chromium tricarbonyl complexes of the type 19. This reaction is general for a number of alkynes that have incorporated important functionality and is more useful for terminal alkynes than for internal alkynes. The migration of the silicon is responsible for the fact that stable arene chromium tricarbonyl complexes can be isolated from these reactions and makes possible transformations that take advantage of the activating ability of the chromium tricarbonyl group. This is demonstrated in intramolecular aromatic nucleophilic additions reactions. It is also shown in a single example that the α-silylated vinyl carbene complex 10b can in principle also serve as a synthon for the parent vinyl carbene complex 6a, since the benzannulated product can be disilylated with trifluoroacetic acid

Synthesis of Ethenyl(phenyl)iodonium Triflate, [H2CCHIPh][OSO2CF3], and Its Application as a Parent Vinyl Cation Equivalent

Synthesis of Ethenyl(phenyl)iodonium Triflate, [H₂CCHIPh][OSO₂CF₃], and Its Application as a Parent Vinyl Cation Equivalent

Radicals formed from acetylenes by high energy radiation and hydrogen atom bombardment: an electron spin resonance study

A variety of acetylenic compounds, including acetylenedicarboxylic acid, its salts, and its dimethyl ester, propiolic acid and its salt, mono- and di-phenylacetylene, phenylpropiolic acid, propargyl chloride and bromide, and 1,4-dichlorobut-2-yne have been exposed to 60Co γ-rays at 77 K, and the radical products at 77 K and during annealing have been studied by e.s.r. spectroscopy. Several of these compounds were also exposed to beams of hydrogen or deuterium atoms and again examined by e.s.r. spectroscopy at 77 K. Radicals thus identified include hydrogen atom addition products, R1ĊHCR2, several of which are thought to be linear at the radical centre, rather than being bent as in the parent vinyl radical. Curiously, dipotassium acetylenedicarboxylate gave, on exposure to γ-rays at 77 K, H2CĊCO2– radicals. On annealing, new radicals were detected which in several instances were probably dimers formed by addition of the parent radicals to neighbouring acetylene monomers. Other radicals identified include HCCĊH2, H2CĊPh, HCCĊHCl, and HCCĊHBr.