Free radical additions to the C=O bond of formaldehyde

The reactions between formaldehyde and n-propyl radicals were studied at 333 and 363 K. Addition to the C=O bond was found to be several times faster than abstraction of the formyl hydrogen atom. With a TST estimate of log(A/dm³ mol^–1 s^–1)=7.9±0.5, 28.0±2.1 kJ mol^–1 was obtained for the activation energy of the addition reaction.

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- 333 363 . C=O . 28.0±2.1 ^–1 , lg(A/ ^{–1 –1})=7.9±0.5 .

     

Free radical fluoroalkylation of terminal alkenes and alkynes with iododifluoromethanesulfonamides

Free radical fluoroalkylation of terminal alkenes and alkynes with iododifluoromethanesulfonamides has been successfully achieved.It was found that both the catalytic amount of sodium dithionite (Na2S2O4) and the stoichiometric amount of triethylborane (Et3B)/air can efficiently initiate the current free-radical atom transfer reaction.     

Activation parameters for additions of ambiphilic methoxychlorocarbene to alkenes

Activation parameters are determined for the additions of methoxychlorocarbene (MeOCCl) to four alkenes, including tetramethylethylene (TME). MeOCCl is the first carbene to exhibit appreciable activation energy in addition to TME (5.8 kcal/mol), although this reaction is still "dominated" by entropy (-TDeltaS(double dagger) approximately 7 kcal/mol).     

Intramolecular radical additions to pyridines

Intramolecular 6-exolendo-trig and 5-exo-trig cyclisations of aryl radical intermediates to the alpha-, beta- and gamma-carbons of pyridine have been shown to be facile processes at neutral pH. The tether conjoining the radical donor to the pyridine plays an important role in determining the outcome of the reaction. When a Z-alkene is used as a tether, ortho-cyclisation proceeds in good yield. A more complex course is followed when a saturated two carbon tether is employed, leading to products derived from hydrogen atom abstraction, ipso-cyclisation and ortho-cyclisation pathways. All attempts to effect 5-exolendo-trig cyclisations failed. Tributyltin hydride, tris(trimethylsilyl)silane, tris(trimethylsilyl)germane and, in part, samarium(II) iodide can each be employed as mediators of the reaction.     

Carbamoyl radicals from Se-phenylselenocarbamates: Intramolecular additions to alkenes

A series of 5 exo-trig cyclizations of carbamoyl radicals generated from readily available Se-phenylselenocarbamates is reported. Kinetic studies indicate that the rate constant of this cyclization exceeds 1×10⁸s⁻¹ in reveral cases.     

Cycloaddition of bis(trifluoromethyl)diazomethane to activated alkenes and alkynes

Bis(Trifluoromethyl)diazomethane undergoes [3+2] cycloaddition with activated alkenes and alkynes regiospecifically to give substituted pyrazolines and pyrazoles.A. N. Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 452–455, February, 1992.     

Rate constants for radical recombination. III. The trifluoromethyl radical

Products of the radical reactions arising from t-Bu₂O₂, CF₃I, and CH₃I at 146°C in the vapor phase have been measured over a 33-fold range of CH₃I/CH₃I ratios and shown to be governed by the rapidly established equilibrium Together with K estimated by thermochemical methods, the results yield, for the rate of recombination for CF₃· radicals, k_r = 10^{9.7 ± 0.5} M^?1 sec^?1.     

Solid-phase intermolecular radical reactions 1. Sulfonyl radical addition to isolated alkenes and alkynes

The addition of toluenesulfonyl radicals to solid-supported alkenes and alkynes gives bromo-sulfonyl alkenes and alkanes in good yields.     

Phospha-Michael additions to activated internal alkenes: steric and electronic effects

The addition of P(O)-H bonds to internal alkenes has been accomplished under solvent-free conditions without the addition of a catalyst or radical initiator. Using a prototypical secondary phosphine oxide, a range of substrates including cinnamates, crotonates, coumarins, sulfones, and chalcones were successfully functionalized. Highly activated acceptors such as isopropylidenemalononitrile and ethyl 2-cyano-3-methyl-2-butenoate underwent the phospha-Michael reaction upon simple trituration of the reagents at room temperature, whereas less activated substrates such as ethyl cinnamate and methyl crotonate required heating (>150 °C) in a microwave reactor to achieve significant consumption of the starting alkenes. For the latter alkenes, a competing reaction involving disproportionation of the ditolylphosphine oxide into ditolylphosphinic acid and ditolylphosphine was observed at the high temperatures needed to promote the addition reaction.     

Ligand-switchable directing effects of tethered alkenes in nickel-catalyzed additions to alkynes

Nickel-catalyzed reductive couplings of aldehydes with alkynes that contain tethered olefins are described, in which the degree and sense of regioselectivity are controlled by the length of the tether and the presence or absence of an additive. When the alkyne and alkene are separated by four bonds, very high (>95:5) regioselectivities are observed. Use of a monodentate phosphine as an additive leads to formation of the opposite regioisomer in equal and opposite selectivity (5: >95). These results provide strong evidence for an interaction between the remote alkene and the metal center during the regioselectivity-determining step and suggest that reactions with and without an additive proceed via fundamentally distinct mechanisms.     

Intramolecular homolytic displacements 7 — Free radical additions to unsaturated peroxy compounds : Synthesis of oxygenated heterocycles

Free radical additions of hydrogen donor solvents to tert-butyl perpent-4-enoate have given 4-substituted γ-butyrolactones, with yields of 50 % or greater. Such reactions applied to allyl-tert-butyl peroxide have produced 2,3-epoxypropanation of these solvents. Similar addition-elimination processes occurred with tert-butyl 4-methyl-perpent-4-enoate, tert-butyl 2,2-dimethyl-perpent-4-enoate and tert-butyl-methallyl peroxide, but they failed with tert-butyl 5-methyl-perhex-4-enoate and tert-butyl-3-methyl-but-2-enyl peroxide.     

Iron-mediated radical halo-nitration of alkenes

Radical halo-nitration of alkenes using iron(III) nitrate nonahydrate and halogen salt has been developed. The present reaction proceeds by radical addition of nitrogen dioxide generated by thermal decomposition of iron(III) nitrate nonahydrate and subsequent trapping of the resultant radical by a halogen atom in the presence of halogen salt. Application of this method to synthesis of nitroalkenes is also described. The practicality of the present method using nontoxic and inexpensive iron reagents has been shown by the application to broad alkenes.     

Diastereoselective tin-free tandem radical additions to 3-formylchromones

A tin-free tandem radical addition methodology onto 3-formylchromones is presented. This radical process yields functionalized chromone structures via two carbon-carbon bond-forming steps. These products contain up to three contiguous stereocenters with good to excellent dr's and yields.     

Using correlations to compare additions to alkenes: homogeneous hydrogenation by using Wilkinson's catalyst

Plots of the logarithms of relative rates of homogeneous catalytic hydrogenation of alkenes (log k(rel) values) by using Wilkinson's catalyst versus their ionization potentials (IPs) and versus their lowest unoccupied molecular orbital energy levels (LUMOs) display good-to-excellent correlations. The correlations indicate that the rate-determining step of this reaction is a nucleophilic addition to the alkene double bond, which is dependent upon both electronic effects and steric effects. This conclusion is in agreement with only two of three previously proposed mechanisms for the reaction, effectively ruling out one in which the rate-determining step involves electrophilic addition to the alkene. Characteristics of the analysis using these correlations are compared and contrasted with other additions to alkenes, such as the Wacker oxidation, to probe patterns in transition state characteristics.     

A convenient AIBN-initiated radical addition of ethyl iododifluoroacetate to alkenes

The AIBN-initiated addition of ethyl 4-iodo-2,2-difluoroacetate to a variety of alkene substrates is described. The addition generally led to the corresponding addition products in good to excellent yields and various functional groups could be tolerated under the reaction conditions.     

Pyrones to pyrans: enantioselective radical additions to acyloxy pyrones

This paper describes a highly site-, diastereo-, and enantioselective intermolecular radical addition/hydrogen atom transfer to hydroxypyrone pyromeconic and kojic acids. The methodology can be extended to the formation of chiral quaternary centers. The products obtained are densely functionalized pyran moieties. The products contain structural features amenable for the introduction of additional substituents.     

Iron-catalyzed sulfonyl radical formations from sulfonylhydrazides and oxidative addition to alkenes

Generation of sulfonyl radicals from sulfonylhydrazides has been achieved in the presence of a non-toxic iron catalyst and oxygen. The intermolecular addition of resultant sulfonyl radicals to alkenes affords β-hydroxysulfone compounds.     

Synthesis of dihydrofurans containing trifluoromethyl ketone and heterocycles by radical cyclization of fluorinated 1,3-dicarbonyl compounds with 2-thienyl and 2-furyl substituted alkenes

Manganese(III) acetate based radical cyclization of various fluorinated 1,3-dicarbonyl compounds with 2-thienyl and 2-furyl substituted alkenes produced 3-trifluoroacetyl and 2-trifluoromethyl-dihydrofurans in good yields. The radical cyclizations of 2-methyl-5-[(E)-2-phenylvinyl]furan 2b and 2-[(E)-2-phenylvinyl]thiophene 2c led to the formations of 5-(5-methyl-2-furyl)-4,5-dihydrofuran and 5-(2-thienyl)-4,5-dihydrofuran, respectively. In the reactions of 1,3-dicarbonyls with alkenes, 2-thienyl substituted alkenes formed 4,5-dihydrofurans in higher yields than 2-furyl substituted alkenes.