Methylene and ring

Organometallic catalysis insertion and ring opening mechanism of methylene-cyclopropane over Cp2LaH and Cp2LuH catalysts... 

A multiplet centered at 8 4.90 was assigned to the C-16 proton, which exhibited COSY interactions with the C-15 methylene and C-17 methine protons. Since an acetoxy group was inferred from IR absorptions and the presence of a singlet at 8 1.90 in the ID H-NMR spectrum, C-16 seemed to be a plausible site of its substitudon in ring D of the skeleton (fragment III). 

Experimental observations,23 supported by high-level ab initio calculations, 24 indicate that two extreme resonance forms contribute to the general energy of the benzyl cation the aromatic form A, in which the positive charge is concentrated at the methylene group, and the nonaromatic, methylene arenium form B with a sp2 ipso-carbon atom and ring-localized charge (Scheme 3.13). Unlike benzyl cations of the form A, which were isolated and studied, especially by Olah and coworkers,23 compounds represented by the form B remained elusive. Thus, metal complexation... 

The IR spectra of the polymer (P) contained two sharp absorptions near 1000 and 1100 cm-- -, indicative of the presence of unsubstituted cyclopentadienyl rings in the products. The 250-MHz - -H-NMR spectrum, shown in Figure 3, contained the expected peaks for the methyl, methylene, and cyclopentadienyl protons, respectively, at 61.52, 1.57 and 4.04 ppm. No olefinic proton resonances were present, and all of the samples of the polymer in Table II exhibited the same - -H-NMR spectrum. 

The two articles in this current volume describe recent developments with small ring compounds which have not teen compiled in such a context before. T. Hirao discusses selective transformations initiated by transition derivatives in the construction of functionally substituted five-, six- and seven-membered rings as well as open-chair compounds. Cycloadditions onto methylene- and alkylidene-cyclopropane derivatives, described by A. Goti, F. M. Cordero and A. Brandi, not only yield products with spirocyclopropane moieties which can be desirable as such or as potential mimics of gem-dimethyl groupings, but also intermediates which can undergo further transformations with ring-opening of the cyclopropane units. 

This chapter, therefore, ends the monograph with a potpourri of reactions all of which occur without a change in oxidation state. In many cases, the reaction is one of nucleophilic attack at an electrophilic C-atom. The result is often hydrolytic bond cleavage (e.g., in carbohydrate conjugates, disubstitut-ed methylene and methine groups, imines, oximes, isocyanates, and nitriles, and various ring systems) or a nucleophilic substitution (e.g., hydrolytic de-halogenation of halocarbons and chloroplatin derivatives, and cyclization reactions). The formation of multiple bonds by dehydration is a special case to be discussed separately. 

The reaction mixture is dehydrated at temperatures as high as 160°C (higher temperatures can be tolerated than with resoles). The prepolymer is cooled, crushed, blended with 5-15% hexamethylenetetramine, (CH2)6N4, and sold to the fabricator. Hexamethylenetetramine, referred to as hexa, is the product of the reaction of 6 mol of formaldehyde and 4 mol of ammonia. Curring occurs rapidly on heating with the formation of both methylene and ben-zylamine crosslinking bridges between benzene rings. The crosslinked network is pictured as... 

NMR spectrum showing the presence of a 3-substituted pyridine with four nonequivalent methylene units in the substituent, and by its conversion to 2,3-bi-pyridyl with chloranil (26). Its synthesis was made by condensation of A -ben-zoylpiperidone (258) with ethyl nicotinate (259) followed by heating with concentrated hydrochloric acid, resulting in hydrolysis, decarboxylation, and ring closure (Scheme 18) 401). Application of the Mundy A-acyllactam rearrangement to A-nicotinoylpiperidone (261) has also led to a synthesis of anaba-seine (8) (Scheme 18) 402). 

Heteromethylene-5(4//)-oxazolones 439 show similar behavior to that described above. Hydrolysis or alcoholysis of 439 provides the corresponding p-substituted acrylic acids or acrylates 440 (Scheme 7.145). In this context, ring-opening reactions of 4-bis [(methylthio)methylene]- and 4-bis[(benzylthio)-methylene]-, 4-(aIkylaminomethylene)-, 4-(dialkylaminomethylene)-, and other 4-heteromethylene-5(4//)-oxazolones have recently been described. 

The decay of the tetramethylene diradical derived from 2,2,5,5-t/4-cyclopenta-none is much slower than seen for the C4Hg diradical. Both principal decay modes, fragmentation to two ethylenes and ring-closure to cyclobutane, may be dependent dynamically on torsional motions of the terminal methylene groups. 

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