Cyclopropane-1,2-diyl

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. 

Calculations and Experiments on the Stereomutation of Cyclopropane. In 1965, Hoffmann published a seminal paper on trimethylene, another name for propane-1,3-diyl (8). He used extended hiickel (EH) calculations and an orbital interaction diagram to show that hyperconjugative electron donation from the central methylene group destabilizes the symmetric combination of 2p-n AOs on the terminal carbons in the (0,0) conformation of this diradical. Hoffmann s calculations predicted that the resulting occupancy of the antisymmetric combination of 2p-n AOs in 8 should favor conrotatory opening of cyclopropane (7), as depicted in Figure 22.8. 

Figure 22.8. Conrotatory ring opening of cyclopropane (7) to what Hoffmann called the (0,0) conformation of propane-1,3-diyl (8). The in-phase combination of Ip-n AOs in the LUMO is destabilized by an antibonding interaction with the re combination of C—H bonding orbitals at C2. A lower energy MO, which is not shown, is stabihzed by h3fperconjugative electron donation that is, the bonding version of this interaction. The out-of-phase combination of 2p-n AOs in the HOMO has a node at C2 hence, it does not mix with the C—H orbitals at this carbon.

Structures of protonated cyclobutanes have been studied in the same fashion (see Figure 9 B). In the corner-protonated cyclobutane, the structure corresponds essentially to a methyl cation interacting with a trimethylene diyl, and is much less favorable than that for cyclopropane. Similarly, for the edge-protonated ion, the proton must come much closer to the carbons to form a bond than for cyclopropane, and as a result, cyclobutane is much less basic. 

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. 

Attempts to observe circumambulatory rearrangement in the 2,6-unri-tricyclo [5.1.0.03,5]octane-2,6-diyl dication 178 have been unsuccessful.397 The dication 178 would appear to rearrange instantaneously to the homotropylium ion 179 by proton elimination. However, substituted dications of type 178 (e.g., 180) are quite stable they are static, and a substantial part of the charge is delocalized into the cyclopropane rings. 

Cyclopropanes Cyclopropene 1,3-propane-diyl ketal. Trimethylstannylmethyllithium. 

The cyclopropane ring is known to be susceptible to insertion reactions when treated with metal complexes. Aryl-substituted cyelopropanes were shown to displace ethene from the di-chloro(ethene)platinum dimer with formation of substituted (propane-1,3-diyl)platinum dichlo-ride. The cleavage site was the bond next to the aryl substituent. 

Table 1. Formation of PtX2(PROPANE-l,3-DiYL)L2 Complexes from Cyclopropane Derivatives via the Tetrameric PtX2 (Propane-1,3-diyl) Compounds (X = Cl OR Br) ...

Clearly, this indicates that the 3,3-shift transition state has little cyclohexane-1,4-diyl character. This is not unreasonable considering that bond formation in this case must occur with generation of a strained cyclopropane bond so the transition state, no doubt, has much more bisallyl radical character (see Chapter 7, Section 4.1). Indeed, radical stabilizing substituents on the cyclopropane ring bond being broken dramatically increase the rate of the 3,3-shift to the point where the transition state is nearly equi-energetic with starting materials (Scheme 10.35). ... 

Reaction of linear compounds containing a ketone and diene functional groups with TMSC(Li)N2 produces alkylidene carbenes that undergo intramolecular cyclopropanation followed by formation of trimethylenemethane diyls and then [3 + 2]cyclo-addition, giving linearly fused triquinanes (eq 31). ... 

Though it was ideal to introduce the methyl group prior to the formation of tetraquinane skeleton, the tandem cycloaddition reaction of the methyl-containing substrate 36 did not produce any cycloaddition product. While the preliminary interpretation of this failure was that the TMM diyl cycloaddition reaction would be sensitive to the steric environment, it turned out that the initial cyclopropanation reaction was sensitive to the substitution patterns of the olefin in the substrate, as the alternative cycloaddition reaction route via TMM yielded the tetraquinane product (Scheme 9). 

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