Intermediates, reactive trimethylenemethane

The TT-allylpalladium complexes 241 formed from the ally carbonates 240 bearing an anion-stabilizing EWG are converted into the Pd complexes of TMM (trimethylenemethane) as reactive, dipolar intermediates 242 by intramolecular deprotonation with the alkoxide anion, and undergo [3 + 2] cycloaddition to give five-membered ring compounds 244 by Michael addition to an electron-deficient double bond and subsequent intramolecular allylation of the generated carbanion 243. This cycloaddition proceeds under neutral conditions, yielding the functionalized methylenecyclopentanes 244[148], The syn-... 

As a final suggestion for future research, cyclobutanones have also provided the organic photochemist with the opportunity of investigating the existence of unusual and reactive intermediates oxacarbenes, trimethylene biradicals, trimethylenemethane biradicals, acyl alkyl biradicals, and ketenes. Evidence for the intervention of oxacarbenes in the ring-expansion reaction is quite compelling however, their unusual behavior relative to "typical" carbenes (e.g., failure to form cyclopropane adducts with some olefinic substrates) makes them prime subjects for further study and characterization. Unlike oxacarbenes, the existence of acyl alkyl biradicals (e.g., [30]) is tenuous at best. Ideally,... 

Trimethylenemethane has been postulated to be the reactive intermediate in a number of reactions involving the ring-opening and rearrangement of methylenecyclopropanes In fact, methylenecyclopropanes are known to undergo degenerate thermal rearrangements. An early example, the ethylidenecyclopropane-methyl methylenecyclopropane interconversion, is shown in equation 20. ... 

Theoretical studies 225>226> as well as preparative work strongly indicate that the reactive palladium organic intermediate in Reaction 115b and 115c is an unsym-metrical, zwitterionic trimethylenemethane-palladium (TMM-Pd) complex, as formulated in Eq. 117. Moreover, cycloaddition with a cyclic TMM-Pd-precursor revealed that the electron-deficient olefin attacks the TMM-Pd unit from the side away from the metal. This demonstrates that complexation of the olefin with the metal does not occur prior to C—C bond formation 183>. 

The oxyallyl system, another reactive intermediate usually written with two charges 6.370 instead of as a diradical, has a similar conjugated system, except that the coefficients will be different, and the central carbon atom, although close to a node in ip2 and u>3, will not have a node exactly through it. When generated on its own, it dimerises with different regioselectivity from trimethylenemethane, giving the 1,4-dioxan 6.371.875... 

Since the time when the thermally induced methylenecyclopropane rearrangement (A - B) of Feist s esters was first observed by Ullman in 1959, rearrangements of a large number of methylenecyclopropane derivatives have been subjected to kinetic, stereochemical, and theoretical studies. The main objectives of these efforts were to understand the role and nature of the trimethylenemethane biradical intermediate (C ) in the rearrangement process. Considerable attention has focused on the theoretical and experimental elucidation of the relationship among structure, spin state, and reactivity of this simple non-Kekule molecule as well as on its applications as a synthetic and practicaP" intermediate. 

Many other species are stabilized in 18-electron organometallic complexes car-benes and carbynes, enyls and polyenyls (XL ligands), o-xylylene (o-quinodime-thane), trimethylenemethane, benzyne, norbornadiene-7-one, cyclohexyne, 1,2-di-hydropyridines (intermediates in biological processes), thermodynamically unfavorable organic tautomers such as vinyl alcohols [less stable by 14 kcafrmol (58.5 kJ mol ) than their aldehyde tautomers], aromatic anions resulting from deprotonation in juxta-cyclic position such as tautomers of phenolates and benzylic carbanions. All these species have a specific reactivity that can lead to synthetic applications in the same way as cyclobutadiene above. 

There have been reports of a number of reactions of CPNA 73 that result in cleavage of the strained C—C o-bond under thermal conditions. The formed reactive intermediate 74 undergoes insertion and cheletropic [1+2]-, [3+2]-, and [3-1-4] cycloaddition reactions under thermal conditions (Scheme 6.13a). The reactivity profiles reported to date are consistent with such a a-delocalized singlet species 74 that can react either as a 1,1- or as a 1,3-dipole. Moreover, the 2-alkylidenecyclopropanone acetal 75 derived from a CPNA 76 is a useful precursor of dialkoxy trimethylenemethane (TMM) 77. MUd thermolysis of 75 in the presence of an electrophile generates 77, which undergoes a [3+2] cycloaddition to form cyclopentane derivative 78 (Scheme 6.13b). These results were reviewed by Nakamura and coworkers [32]. 

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