Rearrangement Temperature

Selenium. The substance is heated with a large excess of selenium at 280-350° for 36-48 hours. Better yields (and less side reactions) are usually obtained than with sulphur, but, owing to the higher temperature, rearrangements are more likely. Oxygen-containing groups are particularly prone to elimination. 

The behaviour of /S-oxovinylazides is quite similar to those above. The Z isomer (556), formed from the /S-halo carbonyl compound and sodium azide, is unstable losing N2 and forming the isoxazole (557) in an anchimerically assisted concerted reaction (75AG(E)775, 78H(9)1207). At moderate temperatures (50-80 °C) the E isomer formed acylazirines which at higher temperatures rearranged to oxazoles and isoxazoles. 

Interestingly, it was possible to probe the spin-forbidden component of the tunneling reaction with internal and external heavy atom effects. Such effects are well known to enhance the rates of intersystem crossing of electronically excited triplets to ground singlet states, where the presence of heavier nuclei increases spin-orbit coupling. Relative rates for the low-temperature rearrangements of 12 to 13 were... 

Spirocyclic oxindole 60 was synthesized by [3,3]-sigmatropic rearrangement of the Af-phenyl-O-acylhydroxamic acid 58 (equation 19). The potassium enolate formed by treatment of 58 with potassium hexamethyldisilazide at low temperature rearranged to 59, which easily cyclized to the spirocyclic oxindole 60. Spirooxindoles were previously synthesized by Wolff and Taddei. The spirooxindole 60 was formed in 51% yield from cyclohexanecarboxylic acid after heating the preformed lithium salts of phenyl hydrazide 61 to 205-210 °C. 

Benzo[6]furan and 2-methylbenzo[6]furan fail to undergo photooxygenation but 2,3-dimethyIbenzo[6]furan yields 2-acetoxyacetophenone (364). At -78 °C the peroxide (363) is produced which at higher temperature rearranges to the acetophenone (364 Scheme 98). 2-Vinylbenzo[6 Jfurans, e.g. trans- 2- styrylbenzo[6 Jfuran (365 Scheme 99), yield photooxides which may be isolated. Compound (366) on treatment with a catalytic amount of triethylamine yields the lactone (367) (77BCJ3026). 

Whatever the case, the formation of the cyclooctenone 320 from the cyclobutanone 317 constituted the first example of a one-step C4 — C8 ring expansion. In the same way, the spirocyclobutanone 266 63,73), was treated with vinylmagnesium bromide to generate a mixture of the diastereomeric cyclobutanols 322 and 323 in 78% yield (ratio 21/79). Each diastereomer, separated by HPLC, individually subjected to KH in THF at room temperature, rearranged cleanly to bicyclo[5.3. l]undec-l(l l)-en-4-one in 80% yield, Eq. (87)161>. 

The first isolated example of a bis(olefin)zirconocene has been prepared by this method. Photolysis of 10 in the presence of 2,3,5,6-tetrakis(methylene)bicyclo[2.2.2]octane yields 11, which at elevated temperature rearranges to the (conjugated-diene)zirconocene complex 12... 

Bicychbataaes buUuBenes. Bicyclobutane (I) upon trcabnenl with silver perchlorate at room temperature rearranges to butadiene (2) in about 90% yield. 

Phosphorylation of the imidoyl halide (233) by reaction with dialkyl fluorophosphite results in the formation of the rearrangement product (234 X = F) which, at a higher temperature, rearranges prototropically and irreversibly to the phosphonate (235 ... 

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