Thermolysis cyclopropanes

Dolbier WR, Sellers SF (1982) J. Am. Chem. Soc. 104 2494. A review of the authors works on thermal rearrangements of gem-difluorocyclopropanes covering, inter alia, cyclopropane thermolysis, methylene cyclopropane and spiropentane rearrangements, vinyl-cyclopropane and cyclopropylcarbinyl isomerizations has been published Dolbier WR (1981) Acc. Chem. Res. 14 195... 

Finally, A -pyrazolines lose N2 (73KGS64) but it is possible that this takes place thermally before ionization, since thermolysis of A -pyrazolines usually gives cyclopropanes (Section 4.04.2.2.2(iv)). 

Thermolysis of (24) gives 4-methylcholest-4-ene-3,6-dione (25) as the sole product,whereas photolysis provides the 4,5-cyclopropane (26) in addition to (25).2°2... 

Biradicals have also been encountered as intermediates in the Mg reduction of ketones to pinacols (p. 218) and, as radical anions, in the acyloin condensation of esters (p. 218). The thermolysis of cyclopropane (131) to propene (132) at 500° is also believed to involve... 

The course of decomposition of confirmed or presumed metallocyclo-butane intermediates is important, but most results reported deal with stoichiometric rather than catalytic processes. Retention of the 3-carbon skeleton via pathways d or f in Eq. (26) occurs much more frequently than does cleavage to metathesis-related products. For example, thermolysis of phenyl-substituted platinocyclobutanes yields propenylben-zenes and phenyl-cyclopropane, but no styrene or ethylene (77). Similarly, the decomposition of tantalum carbene adducts (8) with olefins... 

Certain transition metal complexes catalyze the decomposition of diazo compounds. The metal-bonded carbene intermediates behave differently from the free species generated via photolysis or thermolysis of the corresponding carbene precursor. The first catalytic asymmetric cyclopropanation reaction was reported in 1966 when Nozaki et al.93 showed that the cyclopropane compound trans- 182 was obtained as the major product from the cyclopropanation of styrene with diazoacetate with an ee value of 6% (Scheme 5-56). This reaction was effected by a copper(II) complex 181 that bears a salicyladimine ligand. 

Thermolysis of (cycloheptatrienylmethyl)carbene complexes 554 [toluene, 1-2 h, 80-100°C MLn = Cr(CO)5, W(CO)5] affords an equilibrium mixture of 4,5-homotropilidenes 555 and 556. According to the NMR data and the results of AMI calculations, the formation of isomer 556 (equation 218) is strongly favored277. This course of events was called intramolecular cyclopropanation , and it was shown that the equilibrium between the 4,5-homotropilidene complexes is significantly different from that of the metal-free ligands. By reaction of the latter (555 and 556) with bis(ethylene)rhodium 1,3-pentanedionate 557, the complexes 558 and 559 of both 4,5-homotropilidenes were obtained in a 1 3 ratio. These complexes are non-fluxional and are configurationally stable at room temperature (equation 219)277. 

Unlike many carbenes, pyrrolylidene 54 does not add to olefins to give cyclopropanes. Thus, thermolysis or photolysis of 45 in cyclohexene. 

Thermal rearrangement of 0-acyl Af-hydroxycarbamates carrying a cyclopropane substituent was reported (equation 255). When subject to flash vacuum thermolysis at 500 °C the carbamate 573 generates the Af-acyl imine 574 that rearranges to pyrroline 577 in 21-37% yield. The formation of a biradical intermediate 575 or a polar zwitterionic structure 576 was proposed. 

Small ring hydrocarbons have a wide range of thermal reactivity, with cyclopropane and cyclobutane being quite stable thermally. With these compounds, the thermolysis is known to proceed via initial cleavage of one C—C bond giving a diyl, which has a relatively high energy. 

The thermolysis of cyclopropane, cyclobutane and their derivatives has received considerable attention. The thermal rearrangement of cylcopropane to propene is a clean, first-order process.79 Information concerning the course of the reaction was provided by a study of the thermal isomerization of cis- and mmr-1,2-dideuteriocyclopropane (18).80 The process occurs significantly faster than conversion to propene, suggesting a propane-1,3-diyl 19 as an intermediate. 

In support of the above mechanism, spiro[cyclopropane-l,2 -(6 -methyienebicycIo[3.1.0]hexane)] (10) is indeed isolated from thermolysis of the sodium salt of 7-oxospiro[bicyclo[3.2.0]hept-3-ene-2,1 -cyclopropane] tosylhydrazone (9), presumably due to the fact that the corresponding homofulvene would be too strained to form.8 Additional support of the proposed mechanism is also provided by the pyrolysis of the sodium salt of [7,7-2H2]bicyclo[3.2.1]hept-2-en-6-one tosylhydrazone (11), which gives [7,7-2H2]-4-methylenebicyclo[3.1.0]hex-2-ene (12).8... 

From the correspondingly substituted A -pyrazolines (18), 1-alkoxy- and l-acetoxy-l,2,2,3,3-pentasubstituted cyclopropanes were obtained in good yields by thermolysis in cyclohexane solution under nitrogen using a high-pressure vessel (91JHC1773). 

The imine 26 shows a similar behavior (90CB1161). In this case, too, the reaction products (2,3-dimethyl-2-butene and methyl isocyanide) are the expected thermolysis products of a cyclopropane derivative, and it is therefore safe to postulate the latter as the ring contraction product that is formed by cyclization of an intermediate diradical evolving from 26. 

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. 

We have mentioned that the structural parameters of C2H4 bridged compounds can vary over a wide range. Whereas most examples reported do not have metal-metal bonds, there is one conspicuous exception. Theopold and Bergman succeeded in synthesizing the propane-1,3-d iyl cobalt derivative 125 from the radical anion [(t) ,-C5H5)Co(/z-CO)12 and 1,3-dibromopropane (98, 295) in 40 5 yield. This compound is best described as a dimetallacyclopentane, and its chemistry (thermolysis and reaction with CO and phosphines Scheme 34) supports this view. Formation of cyclopropane (100°C or I2/25°C) is probably the most remarkable feature of this cyclic system. Simple C—C bond formation has never been observed before in ligand-induced or thermal reactions of either mono- or binuclear cyclopentadienylcobalt complexes. The architectural details of... 

Some recent studies have shown that the enolate of the cyclopropane (91) was readily formed, and the resulting aldol product (92) underwent a remarkable diversity of ring-opening reactions, as shown in Scheme 18.39 Treatment with acid generated the p.y-unsaturated ketone (93), which under more vigorous conditions cyclized to the furanone (94). On treatment of (92) with acidic methanol the tetrahydrofu-ran (95) was formed, which on thermolysis was converted to the dihydrofuran (96). 

An elegant application of this chemistry to the formal synthesis of ( )-quadrone has been reported by Piers and coworkers.78a l2la Decomposition of ethyl diazoacetate in the presence of the bicyclic structure (119) resulted in selective cyclopropanation to form (120). Further modification of (120) generated the /ram-divinylcyclopropane (121), which on thermolysis followed by desilylation produced the tricyclic system (122). Conversion of (122) to the ketone (123) completed the formal synthesis because (123 Scheme 24) had been previously converted to ( )-quadione.l2lb... 

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