Transient Three-membered-ring Compounds

Transient Three-membered Ring Compounds.—The existence of stable protonated alkylcyclopropanes as reactive intermediates has been discussed in detail. The collisional activation spectra of the gaseous ions formed upon protonation of pro-pene and cyclopropane are identical and the low activation energy for the conversion of protonated cyclopropane into the 2-propyl carbocation thus indicated is inconsistent with MINDO/3 calculations. Protonated cyclopropanes have been implicated in the reduction of (225) in the presence of strong acids and in the decomposition of cyclopropane on Zeolite HY. The involvement of the 3-methyl function in the deamination of 3-methyl-2-aminobutane has been evident for some time. However, studies with optically active and labelled substrates have now confirmed 

The thermal decomposition of tropone to benzene and carbon monoxide has to proceed, of necessity, via norcaradienone and a derivative of this species has been invoked as a reaction intermediate.  

The electrochemical reduction of ethyl a-butylacetoacetate yields products the nature of which implies that the Tafel rearrangement proceeds via a cyclopropane intermediate. A theoretical study of the rearrangement (378)- (380) indicates that (379) could be on the energy surface. Further studies on the interconversion 

Bertrand, J. P. Dulcere, G. Gil, J. Grimaldi, and P. Sylvcstre-Panthet, Tetrahedron Letters, 1976,3305. 

Nakazawa, and K. Okayama, Bull. Chem. Soc. Japan, 1976,49,1671. 

Transient Three-membered-Ring Compounds.—A review on protonated cyclopropanes as reactive intermediates has appeared. The reaction of methylcyclo-propane with proton is predicted, by EH-MO calculations, to involve two dominant paths (i) removal of hydride ion to generate cyclopropylcarbinyl cation, and (ii) 

A further suggestion that the oxidation of trisubstituted resorcinols proceeds via a cyclopropanone has been advanced, and a similar claim for its involvement in the oxidation of P-substituted a-keto-y-lactones by periodate has been made. The decomposition of cyclic sulphites (404) proceeds to (405), which can be trapped 

Mass spectral and low-temperature (4K) e.s.r. data suggest that the phenacyl radical is best represented as spirocyclopropanone radical (409). Mass spectral fragmentation patterns have also involved cyclopropanones.  

Discussions of the automerization of pyridylcarbenes and of the CgHjN energy surface have been presented in some detail. Labelling studies show that decomposition of (410) does not lead to carbene insertion into a methyl C—H bond, as previously suggested. Instead, carbene insertion into an adjacent aryl site occurs, with subsequent automerization and production of (411).  

A new and detailed mechanism for the acyloin condensation encompasses many previously inexplicable results 2Uid reactions. Included is the metal-ammonia reduction of dimethyl malonate, for which a series of cyclopropane and cyclopropene epoxide intermediates have been suggested.  

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The possibility of the transient formation of 1,2-siloxetane, thietane, and selenetane in the reactions between silylene (H2Si ) and the corresponding three-membered ring compounds (Scheme 4) has been discussed on the basis of calculated enthalpies and energy barriers for the elementary reactions <2000CJC1496>. 

Photochemical or thermal extrusion of molecular nitrogen from ot-diazocarbonyl compounds generates a-carbonylcarbenes. These transient species possess a resonance contribution from a 1,3-dipolar (303, Scheme 8.74) or 1,3-diradical form, depending on their spin state. The three-atom moiety has been trapped in a [3 + 2] cycloaddition fashion, but this reaction is rare because of the predominance of a fast rearrangement of the ketocarbene into a ketene intermediate. There are a steadily increasing number of transition metal catalyzed reactions of diazocarbonyl compounds with carbon-carbon and carbon-heteroatom double bonds, that, instead of affording three-membered rings, furnish hve-membered heterocycles which... 

Treatment of P3C3Bu 3 with [(CO)sW <—PMe], generated / si/u by thermal decomposition of 56b (R = Ph) at 110°C, furnished a 2 1 mixture of quadricyclane 58b and the tricyclic compound 59, which were separated by fractional crystallization as colorless or yellow crystals in 39% and 25% yield, respectively. The presence of a three-membered ring in 59 agrees with an initial attack of the phosphinidene complex at a P=C bond of the triphosphinine to give transient 60. An intramolecular electrophilic aromatic substitution furnished product 59, whereas rearrangement of 60 to tetraphosphanorbornadiene 57a and its intramolecular [2+2] cycloaddition would rationalize the formation of 58b (Scheme 22) <2001CEJ3545>. 

Crystal structure analyses of fourteen aluminum(I) compounds have uncovered three structural motifs (1) four-membered AI4 rings for solvated aluminum(I) halides (AlX Do)4, (2) AI4 tetrahedra for aluminum(I) compounds with organic substituents (RA1)4, and (3) a monomer for the nacnac derivative. In addition, a tetramer-monomer equilibrium for (Cp Al)4 was established through VT NMR studies, and the gas-phase structure of Cp Al was determined by electron diffraction [20]. A dimeric structure similar to that of (Dipp Ga)2 [33, 34] was suggested for the transient Dipp Al species, a proposal based on the structure of the isolated cycloaddition product [27]. 

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