Isomerization thermal, ring contraction

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

The 2a,7b-dihydrocyclobuta[7>]benzofuran structure, obtained by addition of activated acetylenes to benzofurans (see Houben-Weyl, Vol.E6b, p 148), isomerizes thermally to a 1-benz-oxepin182 184 185 which reacts further to 1-naphthol derivatives at higher temperature182,185 (see Section 1.2 ). Under photochemical conditions, the 1-benzoxepins undergo ring contraction to the starting material.182 184... 

The non-planar polyene nature of azepines renders them susceptible to a variety of intra-and inter-molecular pericyclic processes. The azepine-benzeneimine valence isomerization has been discussed in Section 5.16.2.4, and the ring contractions of azepines to benzenoid compounds in the presence of electrophiles is covered in Section 5.16.3.3. In this section the thermal and photochemical ring contractions of azepines to bicyclic systems, their dimerizations and their isomerizations via sigmatropic hydrogen shifts are discussed. Noteworthy is a recent comprehensive review which compares and contrasts the many and varied valence isomerizations, dimerizations and cycloadditions of heteroepins (conjugated seven-membered heterocycles) containing one, two and three heteroatoms (81H(15)1569). 

Reaction of (9) with A1H((-Bu)2 liberates cyclooctene, but at 40 °C it slowly isomerizes thermally in solution to the stable CpCo derivative (11) (Scheme 18). In the presence of excess cod, this latter complex is converted catalytically to tetrahydropentalene. Ring contraction to the pentalenyl system can be understood as an intraligand reductive elimination see Reductive Elimination), the driving force of which may be coordination of cobalt to the double bond of free cod in solution. In the catalytic cycle some (10) is formed. 

Thermal or photochemical treatment of isoxazoies 851 has been found to result in a ring-contraction reaction to produce acyl 277-azirines 852, which sometimes rearrange to form other heterocycles like oxazoles 853. This ring-contraction reaction can also be promoted by iron(ii) catalysts. Thus, 5-alkoxy- and 5-aminoisoxazoles isomerize to 27/-azirine-2-carboxylic esters and 2/7-azirine-2-carboxamides, respectively, in nearly quantitative yield by reaction with catalytic FeCb (Scheme 212) <1997T10911>. 

The reaction between a diazocarbonyl compound and an electron-deficient alkene is carried out in the presence of an additive that suppresses the 4,5-dihydro-3 f-pyrazole to 4,5-dihydro-l/f-pyrazole isomerization, thus allowing the ring contraction of the 4,5-dihydro-3//-pyrazole by thermal nitrogen elimination to occur under the reaction conditions. 

Related carbenes, i.e. 3//-benzocycloheptatrien-3-ylidene and 5/7-dibenzo[a,c]cycloheptatrien-5-ylidene generated from the corresponding tosylhydrazones, thermally or photolytically, isomerize to benzo-12 or dibenzobicyclo[4.1.0]hepta-2,4,6-trienes, respectively. In contrast to cycloheptatrienylidene, 5/7-dibenzo[a,c]cycloheptatrien-5-ylidene underwent ring contraction to form a cyclopropene derivative 13 before reaction with alkenes. 

We took advantage of this valence isomerization method we prepared the precursor 8b efficiently by using enone-olefin photocycloaddition and subsequent ring contraction. Thermal isomerization of 8b proceeded quantitatively, giving methoxycarbonyl-substituted cyclophane lb (Scheme 3) [5]. Bis(methoxycarbonyl) derivative Ic [6] and diketo derivative le (Structures 2)... 

Without additional reagents Stereospecific ring contraction by thermal isomerization... 

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