Cyclopropanes difluoro

The behavior of strained,/Zuorimiret/ methylenecyelopropanes depends upon the position and level of fluorination [34], l-(Difluoromethylene)cyclopropane is much like tetrafluoroethylene in its preference for [2+2] cycloaddition (equation 37), but Its 2,2-difluoro isomer favors [4+2] cycloadditions (equation 38). Perfluoromethylenecyclopropane is an exceptionally reactive dienophile but does not undergo [2+2] cycloadditions, possibly because of stenc reasons [34, 45] Cycloadditions involving most possible combinations of simple fluoroalkenes and alkenes or alkynes have been tried [85], but kinetic activation enthalpies (A/f j for only the dimerizations of tetrafluoroethylene (22 6-23 5 kcal/mol), chlorotri-fluoroethylene (23 6 kcal/mol), and perfluoropropene (31.6 kcal/mol) and the cycloaddition between chlorotnfluoroethylene and perfluoropropene (25.5 kcal/mol) have been determined accurately [97, 98] Some cycloadditions involving more functionalized alkenes are listed in Table 5 [99. 100, 101, 102, 103]... 

The strength of the carbon-carbon bond adjacent to a difluoromethylene group IS hardly affected by the presence of fluorine on the cyclopropyl ring When heated, 2,2 difluoromethylenecyclopropane undergoes methylene-cyclopropane rearrangement [/2 ] Under kinetic control, 2,2 difluoro 1 methylenecyclopropane and (difluoromethylene)cyclopropane are formed in a 2 1 ratio, although the latter IS slightly more stable [129] (equation 27)... 

In theoretical work, the initial steps in the polymerization of 1,1-dicyano-, 1,1-difluoro-, and 1,1-dimethyl-cyclopropanes by reaction with H, OH, and Me have been modelled by ab initio methods. " Other ab initio MO calculations for the reactions of H, Me, Ft, j-Pr, and r-Bu with a variety of silanes and germanes have been carried out. The results indicate that the attacking and leaving radicals adopt an almost co-linear arrangement. Bond distances and energy barriers were predicted for the reactions studied. 

As carbenes to afford O- and 5-difluoromethyl derivatives and difluoro cyclopropanes. 

The contents of the — 140 C trap were purified further by another trap-to-lrap distillation through traps cooled to —95. — 116, and — 196 C. The contents of the — 95 C trap were found to be m-2,3-difluorooxirane(n.v-44), ci.s- and ow -3.5-difluoro-1,2,4-trioxolane(43), and small amountsofrK,m-l,2,3-trifluorocyclopropane and HC02H. As previously mentioned the ci.v.fM-cyclopropane is formed only on ozonolysis of the Z-alkene. The — 116 C trap contained Pw v-2.3-difluorooxirane (lrans-44), m.trcms-1,2,3-trifiuorocyclopropane. and small amounts of the cw-epoxide. The — 196 C trap contained mostly HCOF and small amounts of SiF4. The contents of each trap were further purified by preparative GC (17 ft x 0.25 in. Teflon column packed with 10% Halocarbon oil on 40 60 mesh Chromosorb T at 22 C). 

The chemical effect of a gem-difluoro group on cyclopropane is to facilitate the thermal cleavage of the distal ring bond. This is illustrated by the facile endojexo isomerization of tricyclic cTO-difluorocyclopropane 1, which occurs at 60 C.4... 

The cis/trans isomerization of l,l-difluoro-2,3-dimethylcyclopropane (2) has an activation energy about lOkcal mol-1 lower than that of the parent hydrocarbon.5 Unlike the parent hydrocarbon, competing cyclopropane to propene rearrangement is not observed, a generalization that extends to other gem-difluorocyclopropanes studied in the Dolbier laboratories.1... 

The increasing tendency toward thermal extrusion of CF2 with increasing number of fluorine substituents on the cyclopropane ring has been demonstrated in studies of 1,1-difluorocyclo-propane, 1,1,2-trifluorocyclopropane, 1,1.2,2-tetrafluorocyclopropane,19 and perfluorocyclo-propane.20 Studies of the thermolysis of 1,1,2,2-tetrafluorospiropentane 8 21 and 1,1.2,2-tetra-fluoro-3-methylenecyclopropane (25),22 however, demonstrated that the CF2-CF2 bond in a tetrafluorocyclopropane is substantially weakened. In the case of 25, rearrangement to 2-(di-fluoromethylene)-l,l-difluorocyclopropane (26) occurs irreversibly at 150°C at a rate that was calculated to be 7850 times faster than that of the gem-difluoro analog 21. 

Cyan-2.2-difluoro-l -(2-inethoxy-carbonyl-elhenyl)- E17b. 1053 (subsi. 1-Nj — cyclobuten -> 1-CN —cyclopropan)... 

Dihalocydopropanes readily undergo reductive dehalogenation under a variety of conditions. Suitable choice of reagents and reaction conditions will allow the synthesis of monohalocyclopropanes or the parent cyclopropanes.19 " The ease of reduction follows the expected order I > Br > Cl > F. In general, complete reduction of dibromo and dichloro compounds is accomplished by alkali metal in alcohol,99-102 liquid ammonia103 or tetrahydrofuran (equations 28 and 29).104 The dihalocydopropanes can be reduced conveniently with LAH (equation 30).105 LAH reduction is particularly suited for difluoro compounds which are resistant to dissolving metal reductions.19 106 It is noteworthy that the sequence of dihalocar-bene addition to an alkene followed by the reduction of the dihalocyclopropyl compounds (equation 31) provides a convenient and powerful alternative to Simmons-Smith cyclopropanation, which is not always reliable. 

It is abundantly clear from the preceding discussion that dihalocyclopropanes are versatile intermediates in organic synthesis. Although a wealth of chemistry has already been uncovered, prospects remain bright for interesting developments in the future. Areas such as the application of dihalocyclopropanes in heterocyclic synthesis via carbene insertion into C—H bonds adjacent to heteroatoms, reactions of dihalocyclopropanes with organometallics and the synthetic applications of metallated derivatives deserve further exploration. The chemistry of difluoro-, diiodo- and mixed dihalo-cyclopropanes can be expected to attract some attention. Finally, other heteroatom-substituted cyclopropanes derived ftom dihalocyclopropanes will also invoke further investigation. 

Thermolysis of l,l-difluoro-2,3-diphenylcyclopropane in supercritical CO2 has allowed the rate of geometrical isomerization [i.e. cis-( 109) to /ra/M-(109)] and racemization [i.e. (/< )-( 109) to (S)-( 109)] to be determined from O2 dependence of the trapping rate of the postulated intermediate 1,3-biradical.246 Above 150 °C, the formation of 2,2-difluoroindane and its decomposition products is reported. A similar thermally induced equilibrating series of stereomutations has been observed with the analogous non-fluorinated cyclopropane in which rate constants and deuterium exchange isotope effects are reported.247 Theoretical studies of this isomerization have focused on classical248 and quasi-classical trajectories.249... 

Difluoro(methylene)cyclopropanes have been obtained in modest yields from allene by difluorocarbene addition.61 The difluorocyclopropane moiety proved stable upon... 

Craig, N. C. Piper, L. G. Wheeler, V. L. Thermodynamics of cis-trans isomerizations. II. The l-chloro-2-fluoroethylenes, 1,2-difluoro-cyclopropanes, and related molecules. J. Phys. Chem. 1971, 75, 1453-1460. 

Regioselective cleavage of gem-difluoropropanes. l,l-Difluoro-2-(bromo-methyl)cyclopropane is cleaved by Bu3SnH/AIBN to 3,3-difluoro-l-butene as the... 

Cyclopropane 3-( 1 -Acetoxy-ethyl)-l,l-difluoro-2-phenyl- E10 (F2ClC-COOM + Ar-En-R)... 

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