Polar stepwise

The thermal addition of dimethyl acetylenedicarboxylate to indoles, unlike the photocycloaddition (see Section 3.2.1.4.1.1.), proceeds via a polar stepwise process to yield, initially, 3,4-benzo-2-azabicyclo[3.2.0]hepta-3,6-dienes which in some cases are isolable,13 141 but which, in general, ring open in situ to give the indolylacrylates 3 and/or undergo electrocyclic ring expansion to 1-benzazepines.21... 

Scheme 1.8. Polar stepwise vs. concerted interactions of Cp2Zr complexes with n-bonds.

In the Barton reaction, an alkyl nitrite is converted into an alcohol-oxime. The nitrite is photoexcited to a 1,2-diradical. It fragments to NO and an alkoxy radical (RO-), and the latter abstracts H- from the nearest C-H bond. The resulting alkyl radical then combines with NO to give a nitroso compound, which then tautomerizes to the oxime, probably by a polar stepwise mechanism. The Barton reaction has been used for the remote functionalization of hydrocarbons, especially steroids. 

Olefinic, acetylenic, and typical electron-deficient dienophiles participate in apparent concerted [4 + 2] cycloadditions with the thioacylketene thioacetals (Scheme 8-V), while enamines provide modest yields of [4 + 2] cycloadducts via a polar, stepwise addition-cyclization.22c... 

A net [4+2] cycloaddition of benzyne to vinyl isocyanates allows one to construct phenanthridinones and benzphenanthradinones in one step and 40-50% yield . Thus lead tetraacetate oxidation of aminobenztriazole 66 in the presence of429 gave 430 in good yield. Whether the mechanism involves a concerted [4+2] addition to the 2-azadiene moiety of the isocyanate or a polar stepwise addition remains for further study. 

After eleaning and removal of corrosion products the samples are weighed (both before and after exposure). During a test lasting six hours, linear polarization measurements are carried out after two and four hours. At the end of the test, the samples are polarized stepwise in the cathodic and anodic directions, and the current at the various potential steps is recorded. 

PROBLEM 12.55 Although the Diels-Alder reaction is concerted, an analysis of a polar, stepwise mechanism can explain the regiochemical preferences often observed. Use such an analysis to explain the following data ... 

Isoxazole (418) was obtained from a stepwise polar addition of diethylaminopropyne to the 1,4,2-dioxazolone (417) which provided the (NOC) synthon (73TL233). 

The reactions of bis(trifluoromethyl)-subsatuted hetero 1,3 dienes are predomi nantly LLJMO controlled processes [238] With polar or highly polarizable dieno philes, the tendency to undergo stepwise cycloaddition reactions is considerable Notably these hetero-1,3-dienes react with a,(l unsaturated hetero multiple bond systems across the hetero multiple bond exclusively [243, 246 248] (equation 53)... 

In non-polar solvents, the reaction with piperidine is best represented by a two-term kinetic form indicating a mixed 2nd- and 3rd-order reaction. Also, base catalysis by tri-ri-butylamine was observed. This kinetic pattern is strongly reminiscent of the results obtained with nitro-activated benzenes.Another interesting result is that stepwise replacement of chlorine atoms by amino groups results in marked... 

Figure 8.13 Schematic diagram of 2-D planar cliromatographic separation using "D. In the first dimension (a and b) the total solvent strength was reduced stepwise at constant selectivity (Sx, Svi Sx Syi) to acliieve differences in polarity. In the second dimension (c and d), the...

For benzoyl and acetyl peroxides, loss of carbon dioxide occurs in a stepwise process. Estimates of the rate constants for step c in Scheme 1 are 7 x 10 sec (benzene, 60°). The corresponding process for acetyl peroxide has k = 2x 10 sec (n-hexane, 60°), so that the lifetime of radical pairs containing acetoxy radicals is comparable to the time necessary for nuclear polarization to take place (Kaptein, 1971b Kaptein and den Hollander, 1972 Kaptein et al., 1972). Propionoxy radicals are claimed to decarboxylate 15-20 times faster than acetoxy radicals (Dombchik, 1969). 

Systems with different selectivity were nsed for the separation of 10-deacetyl-baccatin III (10 DAB 111) from yew extracts [69]. A silica column with stepwise gradient elution with aqneous methanolic mobile phases can be nsed for separation of the taxoid fraction from nonpolar materials, partial separation of the taxoid fraction into a polar one (containing 10-DAB 111), and for a medinm polarity taxoid fraction (containing paclitaxel and cephalomannine). Most polar material (tannins... 

Multiple development techniques using stepwise solvent gradients enable a subset of optimal separation conditions to be used to separate a mixture of wide polarity that cannot be separated using a single mobile phase (117,119,120,125). As an example of this approach the separation of 20 common protein amino acid PTH-derivatives is shorn in Figure 7.12 (126). Five... 

Additional reports of multifunctional kalihinols implicate a polar and stepwise biosynthetic cationic mechanism [21, 45]. 

Benzylideneeyclopropane (156) and diphenylmethylenecyclopropane (157) reacted rapidly with TCNE to afford 159 and 160, respectively, in good yields. Since the reaction rate is highly dependent on the solvent polarity, the cycloadditions of 156 and 157 with TCNE were rationalized as stepwise reactions involving the dipolar ions 158 (Scheme 23) [37],... 

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