Methylenecyclopropanes structure

The etiolate of cyclopropanecarboxylic acid and its derivatives is an important building block for organic synthesis.However, the strained methylenecyclopropane structure of the enolate renders this species not only difficult to prepare, but also unstable. A rather simple method for the preparation of substituted 2-cyclopropylidene-5,5-dimethyl-l,3-dioxanes 15 is the thermolysis of the corresponding 2-alkylidenecyclopropanone acetals 14 which rearrange at 150 °C to the hydrolytically unstable ketene acetals. 

When a partially deuterium-labeled starting material was used it became evident that the isomerization of 1 (R = R = H) to 3-methylenecyclopentene was accompanied by a competing methylenecyclopropane rearrangement that gave rise to an equilibrium between two equivalent methylenecyclopropane structures. 

Whereas the parent MCP (1) is not reported to give [4 + 2] cycloadditions, bicyclopropylidene (3) has been shown to give Diels-Alder adducts. Bicyclo-propylidene (3) is a unique olefin, combining the structural features of a tetra-substituted ethylene and two methylenecyclopropane units. The central... 

The reactivity of methylenecyclopropanes with olefins, as exemplified by the following examples, is also governed by subtle structural factors, which are able to steer the outcome of the reaction towards different products arising from alternative mechanistic pathways. 

Recently, the successful generation of PCU-8-vinylidenecarbene (4a) via reaction of 8-(dibromo-methylene)-PCU (3) with n-BuLi hs been reported [15]. When this reaction is performed in the presence of an alkene trapping agent (i.e., cyclohexene), a cage-functionalized erro-methylenecyclopropane, 5, is the only product. Compound 5 subsequently was characterized via conversion to the corresponding substituted dichlorospiro(cyclopentane), 6 (Scheme 2) the structure was established unequivocally via single-crystal X-ray structural analysis [15]. 

More recently, we have investigated the corresponding reaction of 8-(di-bromomethylene)-11-methylene PCU (7) with n-BuLi. Once again, a cage-functionalized erro-methylenecyclopropane (i.e., 9) was obtained as the only product. Compound 9 subsequently was characterized via conversion into 10 (Scheme 3), the structure of which was established unequivocally via singlecrystal X-ray structural analysis [17]. 

The reaction of 4-(dibromomethylene)pentacyclo[6.3.0.0 .0 °.0 ]undecane (11) with n-BuLi resulted in the corresponding vinylidenecarbene, 12a, which could be trapped in situ by cyclohexene to obtain the corresponding cage-functionalized exo-methylenecyclopropane (i.e., 13, Scheme 4). Compound 13 subsequently was characterized via conversion into 14 (Scheme 4), which structure was established unequivocally via single-crystal X-ray structural analysis [18]. 

The methylenecyclopropane derivative 3-SPh with its capto-dative substitution pattern has demonstrated essentially the same reaction mode and underwent dimerization to afford a mixture of E) and (Z)-17 (ratio 1.3 1) upon attempted cycloaddition of 3-SPh onto bicyclopropylidene [7h, 291 (Scheme 5). The assignment of these diastereomers was secured by an X-ray crystal structure analysis of E) and (Z)-16 [11c, 30] as well as E)-17 [29]. 

In some cases, the structures of oxygenation products have been crucial for assigning the structures of unusual radical cations. Eor example, the endo-peroxides (83 and 85) support the structures assigned to radical cations (24 and 84 ) derived from l,l-diaryl-2-methylenecyclopropane (23) and 2,5-diaryl-l,5-hexadiene, respectively.Time-resolved spectroscopic data suggest that 83 is generated by coupling of triplet biradical (24 ) with (triplet) molecular oxygen. 

Notably, the lithium enolates have the planar methylenecyclopropane-type structure56, but give C-alkylation products49"52. X-ray structure analysis of the lithium enolate56 and bicyclobutyllithium57 TMEDA complexes revealed that both crystallize as lithium bridging dimers. 

X-ray structural analysis of 2,2-dimethyl-3-phenyl-l-methylenecyclopropane tungsten pentacarbonyl reveals an octahedral complex with characteristic W—C bond distance of 238 pm. The typical bond distances within the organic ligand are 138 (complexed C=C), 148 (proximal C—C), 154 (distal C—C) pm, compared e.g. with 140, 148 and 154 pm, respectively, for the Feist s ester iron complex analogue (see above). 

Interestingly, chloropalladation reaction of the more constrained cu-7-methylenebicy-clo[4.1.0]heptane did not afford the expected dis-in kinetic product but rather the rearranged to >/ to rf ) thermodynamic isomer whose structure (as the acac mononuclear complex) was confirmed by X-ray crystallographic analysis (equation 327)394. More recently, 1-aryl-substituted derivatives of this bicyclic methylenecyclopropane (equation... 

Triphenylphosphonium cyclopropylide (279), although not a carbanion , is of interest because its structure has been determined by X-ray crystallography by Schmidbaur and coworkers. The most important feature is the pyramidal configuration of the ylidic C-atom the P-atom is bent out of the plane of the cyclopropyl carbon atoms by 58° There is no analogy to planar methylenecyclopropanes like 280 nor to other ylids all of which are planar As Schmidbaur points out the description of the ylid double bond is becoming a problem. 

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