Cyclopropane ring opening by photolytically generated bromine atoms

Abstract

Bromine atoms, generated by irradiation by a mercury lamp bearing a filter to assure that only wavelengths >310 nm passed, were permitted to react with a series of 1,2-diarylcyclopropanes in carbon tetrachloride solution.The major product in all cases (>80%) was the 1,3-dibromo-1,3-diarylpropane. Kinetic data a t approximately M gave a rate expression dBrddt = -h(cyclopropane)(bromine)'/*. A Hammett treatment of the data gave p = -0.5 for both the cis series and the trans series. Synthesis of several possible minor products and their comparison with the reaction mixture by TLC is reported. Possible reactions such as induced isomerization are thus eliminated from consideration.The addition of halogens to cyclopropanes to give ring opening products is generally accepted as one of the unusual properties af the three-membered ring. In studies of the 1,2-diphenylcyclopropanes and phenylcyclopropane with bromine, LaLonde' delineated three reaction modes: type A, photocatalyzed, nonpolar solvents, free-radical-like ring opening; type B, electrophilic ring opening; and type C, aromatic substitution. This paper addresses itself to type A reactions in the 1,2-diarylcyclopropane series. ResultsIt is important to assure that the reactions which we have studied were the same as those that LaLonde observed. Our work was aimed a t determining something about the reaction's sensitivity to electron density of the cyclopropane ring and, hence, used a number of compounds not used in LaLonde's work. We shall return to examine those that do overlap after considering the general character of our study.We synthesized a series of 1,2-diarylcyclopropanes by standard methods3p4 and characterized them by elemental analysis, molecular weights, and spectral examinations (cf. Table I).Carbon tetrachloride solutions of these compounds were subject to irradiation from a medium-pressure mercury arc (GE-AH-4) equipped with a Corning no. 5031 filter. This filter was effective below 310 nm as seen by the failure of this assembly to expose any malachite green leucocyanide after 1 hr of irradiation.Irradiation of the carbon tetrachloride solutions of the cyclopropanes in the absence of bromine gave no rearrangement or ring opening products after several hours with this system. Mixtures of the cyclopropane solutions with equimolar bromine under dark conditions showed no detectable reaction after 48 hr. For reference, the photo-3005 chemical studies were 90% complete on the order of 120 min.The reaction was shown to utilize 1 mol of bromine per mole of cyclopropane and to produce no HBr, except in the cases of the p-methyl and p-dimethylamino compounds which were not included in the kinetic study as a result.Major products were isolated by chromatography and characterized by spectral means. In two cases, p-chloro and unsubstituted, the 1,3-diaryl-1,3-dibromopropanes were synthesized and the compounds compared with the isolated material and with the TLC and GLC of the reaction mixtures. Several minor product candidates, 1,2-diphenyl-1,3-dibromopropane,...

Metal‐Free, Selective Alkane Functionalizations

“…[130] The C À H brominations of strained hydrocarbons are usually even more problematic than the chlorinations because the bromine radical typically attacks carbon atoms and breaks the C À C bonds (cage-opened, less strained products result). This was shown for a number of strained hydrocarbons: cyclopropane, [131] cubane [132] and some others. [133] The direct iodinations of unactivated C À H bonds of alkanes with I are not possible, and all approaches are based on indirect methods (vide infra).…”

A Method for the Synthesis of Carbene Precursors from Aldehydes and the Metalates (η5-C5H5)(CO)2Fe- and (η5-C9H7)(CO)2Fe-

“…cis - and trans -1-( p -methoxyphenyl)-2-phenylcyclopropanes generated from 4c or from 7c and styrene were isolated by Kugelrohr distillation or by column chromatography on alumina (activity III). cis -1-( p -Methylphenyl)-2-phenylcyclopropane and cis- 1-( p -chlorophenyl)-2-phenylcyclopropane 25 generated from styrene and 4d and 4e , respectively, were isolated by Kugelrohr distillation. The yields and isomeric ratios are listed in Tables and .…”

Kinetic study of the homolytic brominolysis of 1,2-diarylcyclopropanes

“…19-2.46). An analytical sample was recrystallized from 10% ethanol and dried over phosphorus pentoxide for 12 h at 2.3 mmHg and 56 °C: mp 77.0-78.0 °C; NMR (CDCI3) 1.20-1.43 (m, 2 ), 2.02 (t, 2 H, J = 7 Hz), 3.54 (br s, 2 ), 6.61 (d, 2 H, J = 9 Hz), 6.86-7.31 (m, 6 H).…”

“…From 4-methoxy-4,-aminochalcone was obtained a mixture of cis and trans isomers of l-(p-aminophenyl)-2-(p-methoxyphenyflcyclopropane (86%) as a yellow-brown oil: NMR (CDC13) 1.03-1.43 (m, 2 ), 1.98 (t, 1.18 H, J = 7 Hz), 2.27 (t, 0.82 H, =7 = 7 Hz), 3.48 (br s, 2 H), 3.65 (s, 1.23 H), 3.73 (s, 1.77 H), 6.30-7.10 (m, 8 H). The oil was used in subsequent transformations without further purification.…”