- cis-2,3-Di-tert-butylcyclopropanones
-
The preparation of the strained cis-2,3-di-tert-butylcyclopropanone 2 from the acyclic compound, α,α′-dibromodineopentyl ketone 1, using a previously reported methodology, is dramatic evidence of both the existence of oxyallyl intermediates in the mechanism of this reaction, and of the integrity with which oxyallyls ring-close to cyclopropanones by a disrotatory route. Because of the bulky cis substituents, cyclopropanone 2 exhibits a number of unusual spectroscopic features (as compared to the trans isomer 5). With the aid of ab initio calculations on 2 and 5, it can be shown that the C2 - C3 bond in 2 interacts with the carbonyl π-orbitals, thus causing the carbonyl oxygen to bend 12° out of the plane; this interaction is absent in 5 and the latter has a planar carbonyl group. As with other cyclopropanones, 2 can be photochemically decarbonylated. This process itself appears to be stereospecific even though highly strained alkenes are produced. Cyclopropanone 2 is thermally rearranged to the trans isomer 5 and the kinetics for this are reported; our favoured mechanism involves oxyallyl intermediates. Other reactions of 2 also appear to proceed through these oxyallyl species; for example, alcohols initially add to 2 to give α-alkoxy enols, solutions of 2 enter into very facile diene cycloadditions, and the dimerization of neat 2 also appears to involve these oxyallyl species.
- Sorensen,Sun
-
-
Read Online
- Effect of Metal Loading and Triphenylphosphine on Product Selectivities in the Hydrogenation of Di-tert-butylacetylene and 3-Hexyne over Palladium/Alumina
-
The effect of triphenylphosphine and metal loading and/or dispersion on the product distributions from di-tert-butylacetylene indicates that the surface structure of the metal particles also may affect stereospecificities by promoting different catalytic mechanisms at different sites.
- Siegel, Samuel,Hawkins, James Adrian
-
p. 1638 - 1640
(2007/10/02)
-
- trans-2,3-Di-tert-butylthiirane 1,1-Dioxide and 2,5-Di-tert-butyl-2,5-dihydro-1,3,4-thiadiazole 1,1-Dioxides from tert-Butyldiazomethane and Sulfur Dioxide. Solvent Effects and Mechanisms of the Staudinger-Pfenninger-Reaction
-
When allowed to react with sulfur dioxide tert-butyldiazomethane formed not only trans-di-tert-butylthiirane 1,1-dioxide (3) but also the diastereomeric 2,5-dihydro-1,3,4-thiadiazole 1,1-dioxides 7e which under base-catalysis rearranged to the 2,3-dihydro-1,3,4-thiadiazole 1,1-dioxide 11.The structures were deduced from IR, UV, and 1H-NMR-spectra, and from the products obtained after loss sulfur dioxide.Whereas increase of the solvent polarity favoured 3 over 7e, the diastereomer ratio cis-7e/trans-7e remained virtually unaffected.The ratio of thiirane 1,1-dioxide 3 to dihydrothiadiazole 1,1-dioxides did not depend on whether the intermediate tert-butylsulfene 6e had been generated from 5e and sulfur dioxide or from 2,2-dimethylpropane sulfonylchloride (14) and triethylamine.In contrast to reports in the literature, the reaction of the phenyldiazoalkanes 5f and 5h with sulfur dioxide afforded cis/trans ratios of the thiirane 1,1-dioxides 9f and 9h, respectively, which were only slightly influenced by the solvent.Apparently, the dihydrothiadiazole 1,1-dioxides 7 and hence their decomposition products, the azines 8 (as far as these do not arise from the direct decomposition of the diazo compounds), are the result of a cycloaddition between the diazoalkane 5 and the sulfene 6.In contrast, the thiirane 1,1-dioxides 9 seem to originate via the zwitterions 20 the formation of which is favoured over the cycloaddition by solvents of high polarity, by the presence of aryl substituents at the sulfene, as well as by the lack of alkyl groups.The dichotomy in the behaviour of sulfenes towards diazoalkanes can be traced back to the existence of two low-lying sulfene MO's, only one of which exhibits ?-symmetry.
- Quast, Helmut,Kees, Frieder
-
p. 787 - 801
(2007/10/02)
-
- Photochemistry of Alkenes. 8. Sterically Congested Alkenes
-
A study of photobehavior of the sterically congested tri- and di-tert-butylethenes 9, 21, and 25 has afforded additional insights into the excited singlet state behavior of alkenes.In pentane solution the carbene-derived products 11, 12, 24, and 32 were formed, respectively.In methanol tri-tert-butylethene (9) afforded additionally ether 14 and the rearranged alkenes 15 and 16 derived from protonation of carbene intermediate 10. 1,1-Di-tert-butyl analogue 21 afforded no detectable ether or rearranged alkene products.However, irradiation in methanol-O-d resulted in incorporation of deuterium at the vinyl positions of 21 recovered after partial conversion, apparently from deuteration-deprotonation of carbene intermediate 22.By contrast, 1,2-di-tert-butyl isomer 25 afforded no products attributable to protonation of the corresponding carbene intermediate 23 but gave ether 35b via 1,2 addition of methanol across the double bond of 25, perhaps involving protonation of the highly polarizable orthogonal 1(?,?*) intermediate.
- Kropp, Paul J.,Tise, Frank P.
-
p. 7293 - 7298
(2007/10/02)
-