E — Z Isomerization

In comparison to the conventional two-way isomerization (Z E) of stilbene and other aryl alkenes 12 (Scheme 5), a novel one-way isomerization (Z - E) of C=C double bonds was achieved upon replacing a phenyl group of stilbene by a 2-anthryl group9a,c. Tokumaru and coworkers found in isomerization studies on substituted anthracenes9 that substitution at the C=C bond resulted in complete isomerization of the Z-isomer to the corresponding /i -isomer upon irradiation, via a quantum chain process. Interestingly, the isomerization takes place as an adiabatic process in the triplet manifold on both direct and triplet sensitized irradiations. 

Rate Ratios kz/kB for Electrophilic Attack on Isomeric Z/E-Alkenes R-CH=CH-R (94)... 

Prolonged reaction time causes the isomerization of (Z)-e-[2-(N,N-dimethylam1no)phenyl]styrene to the (E)-isomer. 

Predict the preferred conformation of the isomeric (Z- and E-) 3-penten-2-ones, A R — CH3. How would you expect the conformational picture to change as R becomes progressively larger ... 

The cyclization product is thermally unstable relative to Z-stilbene and reverts to starting material unless trapped by an oxidizing agent. The extent of eyclization is solvent-dependent, with nonpolar solvents favoring cyclization more than polar ones. ° Whereas the quantum yield for Z-E isomerization is nearly constant at about 35%, the cyclization... 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

The structure of the excited state of 1,3-dienes is also significant with respect to Z E isomerization. If the excited state is an allylmethylene diradical, only one of the two double bonds would be isomerized in any single excitation event ... 

On direct irradiation of 1,3-pentadiene, Z-E isomerization is accompanied by cyclization to 1,3-dimethylcyclopropene and 3-methylcyclobutene ... 

Bamberger proposed the constitutional isomerism 1.3-1.4 for syn- and anti-diazoates, and Hantzsch the cis/trans [ Z)/(E) -isomerism 1.5-1.6 (Table 1-1). 

We will introduce the (Z)/(E) isomeric equilibria of diazohydroxides and diazoates in Section 5.2. In the present section the formulas ArN2OH and ArN20 in Schemes 5-1 to 5-12 refer only to the (Z) isomers that are formed initially. 

Luchkevich et al. (1986, Table 6) demonstrated that for the three isomeric nitro-benzenediazonium ions and their (Z)-diazohxydroxides the acidity constants can be determined by ultraviolet spectrophotometry, by potentiometry, from the kinetics of reaction with hydroxide ions, from the (Z) (E) isomerization kinetics, and from the kinetics of azo coupling reactions. These independent methods gave surprisingly consistent results. ... 

In this and the following section we discuss the combination of the acid-base and (Z/E) equilibria and their kinetics. The experimental evidence for the stereochemistry of the isomeric diazoates and related compounds will be given later (Sec. 7.1). 

Scheme 5-14 may be called a two-dimensional system of reactions, in contrast to Scheme 5-1 which consists of a one-dimensional sequence of two acid-base equilibria. In Scheme 5-14 the (Z/E) configurational isomerism is added to the acid-base reactions as a second dimension . The real situation, however, is yet more complex, as the TV-nitrosoamines may be involved as constitutional isomers of the diazohydroxide. In order not to make Scheme 5-14 too complex the nitrosoamines are not included, but are shown instead in Scheme 5-15. The latter also includes the addition reactions of the (Z)- and ( )-diazoates (5.4 and 5.5) to the diazonium ion to form the (Z,Z)-, (Z,E)- and (2 2i)-diazoanhydrides (5.6, 5.7 and 5.8) as well as proto-de-nitrosation reactions (steps 10, 11 and 12). This pathway corresponds to the reverse reaction of diazotization, as amine and nitrosating reagent (nitrosyl ion) are formed in this reaction sequence.

Basically the kinetic results are consistent with the first (rapid) reaction being the addition of a hydroxide ion to the diazonium ion followed by the very fast deprotonation of the (Z)-diazohydroxide to give the (Z)-diazoate (steps 1 and 2 in Scheme 5-14). In addition, however, the stopped-flow experiments showed that the diazonium ion also reacts with the water molecule, initially forming the conjugate acid of the (Z)-diazohydroxide (ArN2OH2), which is then very rapidly deprotonated (reaction 1 in Scheme 5-14). The rate of the relatively slow (Z/E)-isomerization (reaction 5 in Scheme 5-14) can in general be measured by conventional spectrophotometry. 

They found no evidence of (Z)/(E)-isomerism in the diazosulfones formed. This may be due to the lowering of the barrier of rotation about the NN double bond by the contributions of expanded octet structures such as 6.17 b. It is therefore likely that the observed diazosulfones are ( -compounds. 

The products of reactions generating double bonds can exhibit positional isomerism, as rotation of the moieties, given before the reaction, is now prevented. This is usually referred to as cis/trans or, as a complementary description, Z/E isomerism. There are first hints that cis/trans or ZjE ratios of the products with micro-reactor processing differ from the corresponding data for conventional processing. 

In several cases of syntheses of highly functionalized molecules, use of CH3Li-LiBr for ylide formation has been found to be advantageous. For example, in the synthesis of milbemycin D, Crimmins and co-workers obtained an 84% yield with 10 1 Z E selectivity.251 In this case, the more stable E-isomer was required and it was obtained by I2-catalyzed isomerization. 

Disilenes readily add halogens14,66 and active hydrogen compounds (HX), such as hydrogen halides,63,66 alcohols, and water,27 63 as well as hydride reagents, such as tin hydride and lithium aluminum hydride.66 These reactions are summarized in Scheme 9. The reaction of the stereo-isomeric disilene (E)-3 with hydrogen chloride and alcohols led to a mixture of E- and Z-isomers, but the reaction with chlorine gave only one of the two possible stereoisomers, thus indicating that the former two reactions proceed stepwise while the latter occurs without Si—Si rotation. 

With the availability of stable geometric isomers of doubly bonded germanium compounds, experimental determinations of the 7r-bond strength can be made. The enthalpy of activation for double bond isomerization in Mes(Tip)Ge=Ge(Tip)Mes (Tip = 2,4,6-triisopropylphenyl) has been determined for the Z-E conversion, 22.2 . 3 kcal/mol and for the E-Z conversion, 20.0 0.3 kcal/mol.15 These values agree well with recent theoretical estimations.7 The isomerization barrier in germaphos-... 

Another possible source of modification of the HBI optical properties arises from cis-trans (or, more properly, Z-E) isomerization around its exocyclic ethylene bridge (dihedral angle x as depicted in Fig. 3a) [74, 75]. The absorption spectrum of trans HBI in different solvents is red-shifted by 5-10 nm compared to that of the cis conformation [76]. While the trans conformation is thermodynamically unfavorable and contributes only a minor population at room temperature, cis-trans isomerization seems to take place regardless of the chromophore ionization state, and involves a relatively low energy barrier of about 50 kJ/mol [75], a value that appears significantly lower than initially predicted from quantum mechanics [77, 78]. 

Azidofurans lose nitrogen giving products reminiscent of those formed from furyl carbenes and biradicals.278 External nitrenes apparently add to furan double bonds just as carbenes do, and again the initial products collapse very readily to give nonheterocyclic products. The nitrene produced oxidation of N-aminophthalimide reacts as in Scheme 54. The products are relatively stable derivatives of but-2-endial and are therefore of potential synthetic value, initially the butene link is Z but easily isomerizes to E on silica columns.279... 

The photochemical isomerization of E-stilbenes has been applied in the preparation of phenanthrenes, as Z-stilbenes undergo electrocyclie ring closure (cf. chapter 3.1.3) to dihydrophenanthrenes which in turn are easily oxidized to phenanthrenes (3.1) 305). This sequence has also been employed in the synthesis of benzoquinolines 306) or of benzoquinolizines (3.2) 307). 

The enone system itself is usually part of a five- or six membered ring, although acyclic a,(3-unsaturated ketones and enols of P-diketones are also found to undergo cycloadditions under certain conditions. For seven- and higher membered rings the primary photochemical event is Z—E isomerization around the C—C double bond, the E-isomer then eventually undergoing further thermal reactions. 

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