Methylenecyclopropanes

MCPs have served as excellent three-carbon units in various annulation reactions (Table 2.4). The nickel-catalyzed intramolecular [3-f2] annulation 

In the presence ofnickel(O) catalysts, ethyl cyclopropylideneacetate participated in [3+2+2], [4+3], and [4+3+2] annulation to furnish medium-sized carbocy-cles (Table 2.5) [116]. Synthesis of seven-membered ring compounds by formal o-bond metathesis between the MCP and four-membered ring compounds was also achieved [117]. 

DFT calculations of nickel-catalyzed [3+2+2] annulation were performed [118]. The lowest-energy pathway involves (i) formation of nickelacyclopentadi-ene (ii) regioselective insertion of a C=C bond to the more sterically accessible 

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An interesting synthetic method for the [3.3.3]propellane 74 by intramolecular cycloaddition of a disubstituted methylenecyclopropane with an iinsa-... 

Cycloaddition of COj with the dimethyl-substituted methylenecyclopropane 75 proceeds smoothly above 100 °C under pressure, yielding the five-membered ring lactone 76. The regiocheraistry of this reaction is different from that of above-mentioned diphenyl-substituted methylenecyclopropanes 66 and 67[61], This allylic lactone 76 is another source of trimethylenemethane when it is treated with Pd(0) catalyst coordinated by dppe in refluxing toluene to generate 77, and its reaction with aldehydes or ketones affords the 3-methylenetetrahy-drofuran derivative 78 as expected for this intermediate. Also, the lactone 76 reacts with a, /3-unsaturated carbonyl compounds. The reaction of coumarin (79) with 76 to give the chroman-2-one derivative 80 is an example[62]. 

Caution The preparation of methylenecyclopropane must be carried out in an efficient hood because ammonia is evolved. The preparation and handling of oxaspiropentane should be carried out behind a safety screen. 

Note 9) in 500 ml. of dry tetrahydrofuran i.q added to the gtirred basic mixture heated to 65° over a period of approximately 8 hours a light nitrogen stream is used to carry the methylenecyclopropane into the cold trap. After the addition is complete, the reaction mixture is stirred and heated to 65° for 3 more hours (Note 10). The trap flask contains 58 g. (43%) of methylenecyclopropane (Note 11). 

A final distillation at 760 mm. through a 50-cm. stainless-steel spinning band column yields 41 g. (64% from methylenecyclopropane) of pure cyclobutanone (b.p. 100-101°) (Notes 19, 20). 

The procedure described for the synthesis of methylenecyclopropane is patterned after the method reported by Caubere and Coudert. Methylenecyclopropane is also available from the stepwise method described by Kdster and co-workers. ... 

Methylenecyclopropane, b.p. 11° (760 mm.), is volatile at room temperature all adapter fittings must be carefully checked. The checkers recommend the use of two cold traps in series. 

The yield is determined by weighing the cold trap before and after distillation of methylenecyclopropane. Any small amounts of tetra-hydrofuran carried into the methylenecyclopropane trap are eliminated in a subsequent distillation. By proton magnetic resonance analysis the checkers found that no tetrahydrofuran reached the cold traps the spectrum (dichloromethane) shows a triplet at S 1.00 and a quintuplet at S 5.35 in the ratio 4 2. 

Caution The. yield isolated from this reaction depends on the efficiency of this condenser the epoxidation is exothermic and methylenecyclopropane is volatile. 

High-temperalure carbene additions to allenes are especially prone to give rearranged methylenecyclopropanes [42, 43, 44] (equations 14 and 15), and there-... 

Similarly, partially fluorinated and perfluorinated methylenecyclopropanes [57, 52], cyclopropenes [55, 84, 55], cyclobutenes [75, 56], and bicychc alkenes [57, 55, 59, 90] apparently denve dienophilic reactivity from relief of their ground-state strain during reaction Thus 2,2-difluoromethylenecyclopropane and perfluoromethylenecyclopropane undergo exclusive [244] cycloadditions [57, 52] (equations 72 and 73), whereas (difluoromethylene)cyclopropane undergoes only [24-2] cycloadditions [57]... 

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)... 

Reaction of the cyclopropyl-substituted pivalate (25) with dimethyl benzylidenema-lonate in the presence of a palladium catalyst gave a mixture of alkylidenecyclo-propane (26) and vinylcyclopropane (27). The ratio of these two adducts is found to be quite sensitive to the choice of ligand and solvent. While triisopropyl phosphite favors the formation of the methylenecyclopropane (26), this selectivity is completely reversed with the use of the bidentate phosphite ligand dptp (12). Interestingly there was no evidence for any products that would have derived from the ring opening of the cyclopropyl-TMM intermediate (Scheme 2.8) [18]. 

Methylenecyclopropane, 48, 194 bond lengths, 38 rotational barrier, 38 Methylenimine, 83 MINDO A 54 Molecular geometries, 287 Molecular orbitals, 57, see also individual molecules... 

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