J. Michael Birchall

Cyclopropane chemistry. Part 5 [1,2]. Hexafluorocyclopropane as a source of difluorocarbene

Abstract Thermolysis of hexafluorocyclopropane in the presence of ethylene, propene, vinyl chloride, and vinyl bromide gives good yields of the corresponding 1,1-difluorocyclopropanes, formed by addition of difluorocarbene to the olefin. The tetrafluoroethylene formed dimerises to octafluorocyclobutane, co-dimerises with the olefin, or survives, depending on the reaction conditions. With allene, hexafluorocyclopropane gives 1-(difluoromethylene)cyclopropane, 2,2,3,3-tetrafluorospiropentane, and products derived from tetrafluoroethylene and allene.

Cyclopropane chemistry. Part III. Thermal decomposition of some halogenopolyfluorocyclopropanes

Thermal decomposition of pentafluoro-, perfluoro(methyl)-, 1,1-dichlorotrifluoro(trifluoromethyl)-, bromopentafluoro-, iodopentafluoro-, 1,2-dichlorotetrafluoro-, 1-chloro-1,2,2,3-tetrafluoro-, and 1,1,2-trifluoro-2-trifluoromethyl-cyclopropane, at temperatures in the range 170–250° in glass vessels, is described. The first three compounds decompose exclusively via the elimination of difluorocarbene, the last undergoes exclusive rearrangement to an olefin, and the remainder display both types of behaviour. The cyclopropanes were prepared from the appropriate olefins and either difluorotristrifluoromethylphosphorane or trifluoro(trichloromethyl)silane, or by reduction of a dichlorocyclopropane with tributyltin hydride. Application of this last method to 1,2-dichloro-tetrafluorocyclopropane gave cis- and trans-1,1,2,3-tetrafluorocyclopropane; the preparation of 3-chloro-1,1,2-trifluoro-2-trifluoromethylcyclopropane from 1,1-dichlorotrifluoro(trifluoromethyl)cyclopropane is also described.

Reactions of trifluoromethyl radicals. Part I. The photochemical reactions of trifluoroiodomethane with benzene and some halogenobenzenes

The photochemical reaction of trifluoroiodomethane with benzene in the presence of mercury gives good yields of benzotrifluoride, and similar reactions occur with fluoro-, chloro-, and bromo-benzene to give mixtures of the corresponding halogenobenzotrifluorides. Detailed analysis of the gaseous and liquid products of these reactions is described and the thermal reaction of trifluoroiodomethane with bromobenzene has been re-examined.

Carbene chemistry. Part IX. Some C-H insertion reactions of dichlorocarbene in the gas phase

Dichlorocarbene, from the thermal decomposition of trifluoro(trichloromethyl)silane at 140–180 °C in the gas phase, gives good yields of α-C–H insertion products with ethylbenzene, cumene, diethyl ether, 2-methoxypropane, and tetrahydrofuran. The tertiary C–H bond of isobutane is 70 times more reactive than a C–H bond of cyclohexane at 140 °C, and no products of insertion into primary C–H bonds have been identified with certainty. Dichlorocarbene reacts with cyclopentadiene at 180 °C to give chlorobenzene (58 %) and with indene to give 2-chloronaphthalene (69 %).

Polyfluoroarenes. Part XX. Some homolytic arylations with pentafluorophenylhydrazine

Oxidation of pentafluorophenylhydrazine in benzene by aqueous reagents (CuSO4, NaOCl, H2O2, and H2O2–FeSO4) gives 2,3,4,5,6-pentafluorobiphenyl. Use of solid potassium permanganate in pyridine or silver oxide in a solution of naphthalene in carbon tetrachloride gives low yields of the corresponding pentafluorophenylarenes, but a 57% yield of 2,3,4,5,6-pentafluoro(phenoxy)biphenyls is obtained from the hydrazine and silver oxide in molten diphenyl ether. The isomer ratio for the last reaction has been determined. Unambiguous syntheses of the two pentafluorophenylnaphthalenes and the three 2,3,4,5,6-pentafluoro(phenoxy)biphenyls are described.

Cyclopropene chemistry. Part 1. Preparation and some reactions of 3,3-dichloro-1,2-bistrifluoromethyl- and 1,3-dichloro-2,3-bistrifluoromethyl-cyclopropene

Both the title compounds may be obtained from the reaction of dichlorocarbene [generated by thermal decomposition of trifluoro(trichloromethyl)silane] with hexafluorobut-2-yne; the 3,3-dichloro-compound is isomerised to the 1,3-dichloro-isomer by heat, light, or chemical catalysis, but conditions are described for the preparation of either pure cyclopropene. Treatment of either isomer with antimony pentafluoride gives solutions containing the chlorobistrifluoromethylcyclopropenium ion. Both isomers undergo free-radical addition of halogens, but the slow reaction of the 1,3-dichloro-compound with trifluoromethyl radicals gives a mixture of products. Diels–Alder reactions of both cyclopropenes with cyclopentadiene or with furan are described, and the 3,3-dichloro-compound gives a high yield of ‘ene’-reaction products with (E)-but-2-ene.

Nitroxide chemistry. Part 15. Reactions between bistrifluoromethyl nitroxide and benzyl alcohol, azide, chloride, and cyanide

Bistrifluoromethyl nitroxide readily attacks benzyl cyanide and benzyl chloride at room temperature to give the α-bistrifluoromethylamino-oxy-derivatives PhCHRCN and PhCHRCl [R =–ON(CF3)2] respectively; treatment of the former product with the nitroxide yields benzoyl cyanide and the di-substituted derivative PhCR2CN. Abstraction of a benzylic hydrogen atom by the nitroxide from benzyl alcohol and benzyl azide leads, respectively, to the formation of benzaldehyde (and hence PhCOR) and benzonitrile.

Nitroxide chemistry. Part VIII. Abstraction of allylic hydrogen from isobutene by bistrifluoromethyl nitroxide

Vapour-phase reaction of isobutene with bistrifluoromethyl nitroxide at 200 °C and low pressure leads to the formation of the 1 : 2 olefin–nitroxide adduct Me2R·CH2R and, mainly, the product arising from free-radical abstraction of allylic hydrogen, RCH2·CMe:CH2[R =(CF3)2N·O]. The former product is best prepared on a large scale by passing the nitroxide into the liquid olefin at ca.–60 °C. Acid-catalysed reaction of NN-bistrifluoromethylhydroxylamine with isobutene constitutes a convenient laboratory route to the amino-oxypropane Me3CR.

Carbene chemistry. Part VIII. The thermal decomposition of trichloromethyltrifluorosilane: a kinetic investigation

The gas-phase thermal decomposition of trichloromethyltrifluorosilane (CCl3·SiF3) at 100–160 °C and 10–100 Torr initial pressure in the presence of alkenes (2,3-dimethylbut-2-ene, cis-, or trans-but-2-ene) is first order in silane; the rate constant is independent of the olefin used and the surface to volume ratio and is given by (i). log10(k/s–1)= 12.794 ± 0.110 –(126.0 ± 2.6) kJ mol–1/2.303RT(i) These results provide strong evidence for the participation of free dichlorocarbene in these reactions.

Carbene chemistry. Part V. A route to dichlorocarbene in the vapour phase

The thermal decomposition of trifluoro (trichloromethyl) silane in the vapour phase at 140 °C in the presence of the olefins cyclohexene, CMe2:CMe2′, CMe2:CHMe, CMe2:CH2′, CH2:CH2′, CH2:CF2′, and CCl2:CCl2 leads to high yields (>85% in most cases) of the corresponding gem-dichlorocyclopropanes. The reaction is stereospecific with cis- and trans-but-2-ene, and an almost quantitative breakdown of the silane into dichlorocarbene and chlorotrifluorosilane is inferred.