Z/£-isomerization

The spectroscopic properties of the /V-nitrosamines, especially the nmr and mass spectra, vary widely depending on the substituents on the amine nitrogen (44—47). The nmr spectra are affected by the E—Z isomerism around the N—N partial double bond and by the axial—equatorial geometry resulting from conformational isomerism in the heterocycles (44,45). Some general spectral characteristics for typical dialkylnitrosamines and simple heterocycHc nitrosamines are given in Table 1. 

Asano and co-workers have reported die kinetic effects of pressure, solvent, and substituent on geometric isomerization about die carbon-nitrogen double bond for pyrazol-3-one azomethines 406 (R = H), 406 (R = NO2) and 407, (Scheme 93). The results demonstrate the versatility of die inversion mechanism. The rotation mechanism has been invalidated. First-wder rate constants and activating volumes for diermal E-Z isomerization for 406 (R = H) and 406 (R = NO2) are given at 25°C in benzene and methanol (89JOC379). 

Cyclocondensation of 2-iminopiperidine hydrochloride with an E Z isomeric mixture of ethyl 2-cyano-3-methylsulfanyl-3-( 1,2,4,5-tetrahydro-3/f-benzo[r/ azepin-3-yl)acrylate in DMF in the presence of DBU at 100°C gave 2-substituted 6,7,8,9-tetrahydro-4//-pyrido[l, 2-n]pyrimidine-3-carbo-nitrile (01EUP1074549). 

In the coupling with vinyl groups, the olefin geometry is usually retained E/Z-isomerization is only rarely observed. 

The rapid formation of the (Z)-diazoate is followed by the slower (Z/J )-isomeri-zation of the diazoate (see Scheme 5-14, reaction 5). Some representative examples are given in Table 5-2. Both reactions are first-order with regard to the diazonium ion, and the first reaction is also first-order in [OH-], i.e., second-order overall. So as to make the rate constants k and k5 directly comparable, we calculated half-lives for reactions with [ArNj ]0 = 0.01 m carried out at pH = 9.00 and 25 °C. The isomerization rate of the unsubstituted benzenediazonium ion cannot be measured at room temperature due to the predominance of decomposition (homolytic dediazoniations) even at low temperature. Nevertheless, it can be concluded that the half-lives for (Z/ )-isomerizations are at least five powers of ten greater than those for the formation of the (Z)-diazohydroxide (reaction 1) for unsubstituted and most substituted benzenediazonium ions (see bottom row of Table 5-2). Only for diazonium ions with strong -M type substituents (e.g., N02, CN) in the 2- or 4-position is the ratio r1/2 (5)/t1/2 (1) in the range 6 x 104 to 250 x 104 (Table 5-2). 

Arenediazoisocyanides (Ar —N2NC) were synthesized by Ignasiak et al. (1975) by reaction of arenediazonium ions with formamide and dehydration of the resulting l-aryl-3-formyltriazene with thionyl chloride (Scheme 6-19). These compounds are interesting in the context of the (Z)/( )-isomerism of diazocyanides (see Sec. 6.6). 

The arenediazocyanides have been known since 1879. They played an important role in the Hantzsch-Bamberger debate on the (Z)/( ,)-isomerism of diazo compounds (see Sec. 7.1). When an aqueous solution of a diazonium salt is added to a solution of sodium or potassium cyanide, both in relatively high concentration, at a temperature below 0°C, a yellow to red (Z)-arenediazocyanide starts to crystallize. Hantzsch and Schulze (1895 a) found that these compounds rearrange into the (ii)-isomers, which have a bathochromically shifted visible absorption (see Sec. 7.1). Under strongly alkaline conditions a 1 2 adduct is formed, to which Stephenson and Waters (1939) assigned the structure 6.36. It was never corroborated, however, by modern instrumental analysis. 

More recently, Scaiano et al. (1991) observed (Zs)->(Z)-isomerization of 1,3-di-phenyltriazene also in methanol by using flash photolysis, transient spectroscopy, and laser-induced optoacoustic calorimetry (LIOAC). The interpretation of the data is consistent with the mechanism shown in Scheme 13-4, involving two solvent molecules. 

The stereochemistry of the addition of amines to acetylenic sulfones has been investigated by Truce and coworkers74. Reaction of phenyl 1-propynyl sulfone with ethylamine gives a mixture of ( )- and (Z)-isomeric adducts (equation 90)75. 

In Ghosh s enantioselective total synthesis of the cytotoxic marine macrolide (+)-amphidinolide T1 (318) [143], the C1-C10 fragment 317 was constructed by CM of subunits 315 and 316 (Scheme 62). The reaction mediated by catalyst C (5 mol%) afforded in the first cycle an inconsequential 1 1 mixture of (E/Z)-isomeric CM products 317 in 60% yield, along with the homodimers of 315 and 316. The self-coupling products were separated by chromatography and exposed to a second metathesis reaction to provide olefins 317 in additional 36% yield [144]. 

Isomerization of ( /Z) isomers is another important transformation. Isomerization of ( ) and (Z-) conjugated amides is effected photochemically " (photo-isomerization " ). There is a rather high energy barrier for the excited state required for (E/Z) isomerization. Isomerization of the C=C units in dienes is also induced photochemically. " Isomerization of cyclic alkenes is more difficult but cyclooctene is isomerized photochemically. " Conjugated aldehydes have been isomerized... 

Clough, J.M. and Pattenden, G., Stereochemical assignment of prolycopene and other poly-z-isomeric carotenoids in the fruits of the tangarine tomato Lycopersicum esculentum var. Tangella, J. Chem. Soc. Perkin Trans. I, 3011, 1983. 

The reactions are accelerated by bromide salts, which are thought to exchange for acetate in the ir-allylic complex. The reactions of acyclic compounds occur with minimal E Z isomerization. This result implies that the TT-allyl intermediate is captured by carbonylation faster than E Z isomerization occurs. 

The reaction conditions were optimized to afford clean coupling of enol tosylate 32 using only a slight excess of amide 24 (1.05equiv) at 100 °C, 5mol% Pd2(dba)3/ dppb catalyst, and a toluene/tert-amyl alcohol solvent system. Even under the harsh reaction conditions required for complete conversion of the tosylate (100 °C, 20 h) no detectable E/Z isomerization was seen, providing further proof that the hindered nature of the enamide aids stability to isomerization. Treatment of the mixture with activated carbon (Darco KB-B) at the end of the reaction followed by isolation of the product by crystallization, afforded enamide 22 in 92% isolated yield. 

The aim of the reaction of 15a with Cl—P=C(SiMe3)Ph was to synthesize the 2,3,1-diphosphasilabuta-1,3-diene derivative 42 (Scheme 11). However, it turned out that the Cl—P bond of the phosphaalkene exclusively adds to the Si=P bond, forming the addition product 43, which probably has the ( >isomeric structure.45 The corresponding (Z)-isomer 43 was not formed. On heating of 43, even for 8 hours at 110°C, no elimination of /Pr3SiCl was observed. Instead, ( )/(Z)-isomerization simply occurred (43 and 43 in the molar ratio of 1 0.8). 

Information on the E-Z isomerization of the disilenes sheds light on the nature of the chemical bonding of the silicon-silicon double bond. Kinetic parameters, which have been reported for cis-trans isomerization of five disilenes, are listed in Table IV. 

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