Stilbene formation

Skraup quinoline synthesis, 443 Smiles rearrangement, phenothiazine, 534 Spiroalkylation, 222, 280 Spirocyclization, conjugate addition, 386 Spiroimidazolone formation, 335 Spiropyrazolopiperidine, 375 Stannylation, alkyne, 15 Stereoselective dehydration, 198 Grignard addition, 198, 199 reduction, 129, 226 hydroxyketone, 400 iminoketone beta, 553 oxazaborohydride, 585 transfer chirality, 321 Stilbene formation, self alkylation, 525 Stobbe condensation, benzophenone, 103... 

The preparation of arylcyclopropanes from arylhalomethanes and aryidihalomethanes is usually accompanied by formation of various byproducts. Stilbene formation is generally observed and compounds resulting from C-H insertion are often formed. In some cases a formal byproduct may become predominant and even be formed in good yield. This is the case when benzal chloride is treated with methyllithium. Thus, treatment of a mixture of 2,3-dimethylbut-... 

Monoaryldiazomethanes, readily prepared by a number of methods, are the carbene precursors most frequently used for the synthesis of arylcyclopropanes. When such diazo compounds are decomposed photochemically, thermally, or by using various transition-metal salts in the presence of an alkene, arylcyclopropanes are formed. The yield is often quite high, but in a number of cases cyclopropane formation has been hampered by competing reactions, of which, disregarding intramolecular reactions, azine formation, stilbene formation, and hydrogen abstraction followed by dimerization are the most predominant. Many aspects related to the use of diazomethane derivatives as carbene precursors have been thoroughly discussed by Wentrup. ... 

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) we o-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42) It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,100), stilbene formation from aryl diazomethanes and carbene insertion into aliphatic C—H bonds , intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

Stilbene formation by the Wittig reaction usually gives a mixture (here 7 3 Z E) of E- and Z-isomers 280. Irradiation of a heptane solution containing a small amount of iodine in sunlight isomerises the mixture to the -stilbene 280 which crystallises from solution in 68% yield.57... 

The same acetates give products labelled vice versa when anti-elimination is induced with potassium t-butoxide. The great preference for trani-stilbene formation is caused by the necessity to avoid eclipsing of the phenyl groups in the transition states. Similar "syn" stereospecificity is demonstrated for pyrolysis of 2-butyl-3-phenylxanthates , the threo isomer yielding t/j-di-methylstyrene (137) while the erythro isomer gives the tran -olefin (138). 

In the less basic medium of aqueous acetone or aqueous dioxan containing hydroxide ion, a carbene mechanism has been suggested to explain the quantitative conversion of 4-nitrobenzyl chloride and 4-nitrobenzyl dimethyl-sulphonium ion into cis- and /ra/is-4,4 -dinitrostilbenes . In both cases, the rate of elimination follows first-order kinetics in both the substrate and hydroxide ion, and deuterium exchange of labelled substrates occurs more rapidly than stilbene formation. In this aqueous medium, a rate-determining ion-pair dissociation is unlikely. However, again the selectivity of the carbene is astonishing and it is difficult to explain the lack of insertion products with the reaction medium or the failure to isolate cyclopropane adducts when the elimination reactions are performed in the presence of added olefins. 

Preliminary experiments show that or//io-dinitrobenzene almost completely prevents stilbene formation from 4-nitrobenzyl dimethylsulphonium ion with hydroxide and the major product, p- nitrobenzyl alcohol, is formed at a much slower rate than is the stilbene in the absence of the inhibitor . ... 

The lowest energy reaction path for E-stilbene formation (path C, Scheme 10.9) is the only thermally accessible catalytic cycle calculated for 10 and hence the... 

Using cresyl violet perchlorate (CVP) as the electron-transfer sensitizer, photo-induced electron transfer of stilbene was also studied (Fig. 6.13). As expected, isomerization of franx-stilbene in ethanol resulted in fran -isomer as main product (Table 6.4). Interestingly, in aqueous media containing dendrimers, CM-isomers were obtained as main products. Additionally, like direct irradiation, dendrimers with smaller cavity size favored c/x-stilbene formation. This observation can confirm the role of dendrimer cavity size. 

Keywords alkylidene ligands, arene ligands, cycloisomerization, homogeneous catalysis, stilbene formation... 

Ester pyrolysis has been shown to be a syn elimination by use of deuterium labels. In the case of stilbene formation, deuterium was introduced by stereospecific reduction of cis- and trans-stilhene oxide by LiAlD4. The syn elimination is demonstrated by retention of deuterium in the product from the trans epoxide and its elimination in the product from the cis epoxide. 

Note that the rathenium p-cymene complex with NHC ligands, as well as the following alkylidene complexes, can also induce metathesis reactions, which indicates that the olefin metathesis reaction of the vinyl monomers should be considered as a competitive process, for example, stilbene formation during styrene polymerization. 

Scheme 2.4 Stilbene formation from the reaction of a PdCVstyrene complex with aromatics.

Table 2.1 Effect of NaOAc on stilbene formation from 1 and benzene.

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