A novel and highly efficient synthesis of gem-difluorocyclopropanes

[reaction: see text] A new and highly versatile source of difluorocarbene is reported. Trimethylsilyl fluorosulfonyldifluoroacetate (TFDA) undergoes decomposition in the presence of catalytic fluoride to form difluorocarbene under conditions that allow its addition to relatively electron deficient alkenes in high yield. For example, unprecedented CF2: addition to n-butyl acrylate proceeded in 73% yield.     

A simple theoretical model for predicting the effects of benzannelation and bismethylenation on the rates of four-membered ring-opening reactions

A model based on simple Hückel MO theory is applied to the ring-openings of several cyclobutene derivatives and their benzo- and bis-methylene-analogs. The calculated changes in activation enthalpy upon benzannelation and bismethylenation are found to be in qualitative accord with experimental results. Application of the model to Dewar benzene and bicyclo[4.2.0]octa-2,4,7-triene ring-opening is discussed.     

Use of gas-phase (1)H NMR to determine the kinetics of unimolecular rearrangement of 2,2-dichloro-1-methylenecyclopropane and the bimolecular dimerization of (dichloromethylene)cyclopropane

Gas-phase (1)H NMR analysis has been applied to investigate the kinetics of the unimolecular rearrangement of 2,2-dichloro-1-methylenecyclopropane (1) to (dichloromethylene)cyclopropane (2) [k(1) = 7.9 x 10(12) exp(-34.4 +/- 0.6 kcal mol(-1)/RT)], as well as for the subsequent second-order dimerization of 2 [k(2) = 2.4 x 10(6) exp(-18.5 +/- 1.1 kcal mol(-1)/RT)] to form 7,7,8,8-tetrachlorodispiro[2.0.2.2]octane (3)     

Reactions of TFDA with ketones. synthesis of difluoromethyl 2,2-difluorocyclopropyl ethers

The sequential reaction of 2 equiv of difluorocarbene (generated from trimethylsilyl fluorosulfonyldifluoroacetate (TFDA) by treatment with catalytic fluoride ion) with a series of electron-rich aromatic ketones and alpha,beta-unsaturated ketones leads to the formation of difluoromethyl 2,2-difluorocyclopropyl ethers in good yield.     

Absolute rate constants of alkene addition reactions of a fluorinated radical in water

Absolute rate constants of *R(f)SO(3)(-) radical addition to a series of water-soluble alkenes containing ionic, carboxylate substituents were measured by laser flash photolysis experiments in water. The observed rate constants were all considerably larger than those of structurally similar analogues in a nonpolar organic solvent, with rate factors of 3-9-fold being observed. It is concluded that such rate enhancements derive at least in part from stabilization of the polar transition state for addition of the electrophilic fluorinated radical to alkenes by the polar solvent water.     

Competitive kinetic processes in the thermal rearrangement of 1,1-difluoro-2-(dideuteriomethylene)-cyclopropane

The synthesis of 1,1-difluoro-2-(dideuteriomethylene)-cyclopropane is reported along with the kinetics of its competing, reversible, first order rearrangements: (a) the degenerate rearrangement to 1,1-difluoro-3,3-dideuterio-2-methylene-cyclopropane, and (b) its rearrangement to 2,2-dideuterio-1-(difluoromethylene)-cyclopropane. The observed experimental ratio of these two rate constants, 2.3, is consistent with Borden's theoretically predicted ratio of 3.     

Mono‐ and difluorinated 1,1,2,2,9,9,10,10‐octafluoro[2.2]paracyclophanes (AF4s)—A 1H and 19F NMR study

Complete assignment of the ¹H and ¹⁹F chemical shifts in 4‐fluoro‐AF4 (1) were based on the nOes seen in its ¹⁹F‐¹H HOESY spectrum. This allowed for identification of features which can further be applied to the assignment of the regiochemistry of substituted perfluoroparacyclophanes (PCPs) and AF4s: (i) an aromatic fluorine couples with the two fluorines in the closest bridge that are syn to it, with constants of ca. 20 Hz; (ii) an aromatic fluorine couples with the bridge fluorine five bonds away that is anti to it in the same paraphenylene moiety, with a constant of ca. 3.5 Hz; (iii) the geminal coupling of the bridge fluorines is 246 Hz if they have an ortho fluorine and 238 Hz if they do not; (iv) a bridge fluorine couples with those aromatic protons in the same paraphenylene moiety that are four or five bonds away and anti. These features have been used to assign the regiochemistry of the pseudo‐ortho, pseudo‐meta and pseudo‐para‐difluoro AF4s 2–4. It has also been demonstrated that SCS for the bridge fluorines can be used as well for this assignment. Copyright © 2009 John Wiley & Sons, Ltd.     

The thermal unimolecular isomerization of 1,1-divinylcyclopropane

The thermal isomerization of 1,1-divinylcyclopropane has been investigated in the gas phase in the temperature range of 238 to 288°C and for pressures in the range of 4 to 12 torr. The isomerization to 1-vinylcyclopentene is homogeneous and kinetically first order and almost certainly unimolecular. The rate constants yield the Arrhenius equation or The assumption of a similar transition state for this isomerization with that of l-methyl-l-vinylcyclopropane leads to a value for an alkylpentadienyl radical stabilisation energy of 19.2 ± 1.6 kcal/mole (80.4 ± 6.7 kJ/mole), and this value is compared with other estimates.     

Rate constants for hydrogen abstraction from alkoxides by a perfluoroalkyl radical. An oxyanion accelerated process

A combination of laser flash photolysis and competitive kinetic methods has been used to measure the absolute bimolecular rate constants for hydrogen atom abstraction in water from a series of fluorinated alkoxides and aldehyde hydrates by the perfluoroalkyl radical, *CF2CF2OCF2CF2SO3- Na+. The bimolecular rate constants observed for the beta-fluorinated alkoxides were in the 10(5) M(-1) s(-1) range, such rates representing enhancements (relative to the respective alcohols) of between 100 and almost 1000-fold, depending on the reactivity of the alkoxide. Likewise, the monobasic sodium salts of chloral and fluoral hydrate exhibit similar rate enhancements, relative to their respective hydrates.