Diaryl olefins

In all cases studied the E,Z-isomerization has the higher quantum yield, so it is of no importance from which isomer the photoreaction starts. In analogy to the primary product from stilbene the cyclization products from other diaryl olefins will be denoted by DHP. The lifetime of such DHP s can vary from some milliseconds to several hours I2). 

The photocyclodehydrogenation of thienyl ethylenes is well-defined when both thiophene rings are bound via a C(2) atom to the ethylenic bond as in (70). In other cases, however, more cyclization products are possible. To predict the photocyclization mode for heterohelicenes the F s rule fails in many cases, because correction factors for the hetero atoms in the Huckel MO calculation have to be introduced and the systems are not well comparable with carbocyclic diaryl ethylenes. A better reaction parameter in these cases is the Mulliken overlap population (nrs)51), introduced by Muszkat52) for these cases. The overlap populations of the atoms r and s in ground and excited state (nIiS and n s), are calculated using the extended Huckel method. Cyclizations should not occur when nr>s and An s (= nr>s — n s) have negative values. (This method can also be used for diaryl olefins, but in these cases calculation of F s is more simple.). 

The reaction gives poor yields of ethers with secondary and tertiary alcohols dehydration to form the corresponding olefin is a more favorable reaction. The reaction fails for the production of diaryl ethers from phenols. 

Wahrend Dichlor-aryl-methane zu 1,2-Dichlor- 1,2-diaryl-athane dimerisiert werden (Bd. XIII/2a, S. 443) erhalt man aus Dialkyl- bzw. Diaryl-dichlor-methanen unter Dimerisierung die entsprechenden Olefine (s. Bd. V/lb, S. 424f.) ... 

Due to their weak P-H bonds (-370 kj mol"0 [2] and the high rate constants for the transfer of the P-H hydrogen [3] (/c=1.5 10 L rnoL s" for Ph2PH and k=5,0 10 L mol s for (c-hexyl)2PH), diaryl and dialkyl phosphines present a high interest as H-donors. Since the corresponding phosphinyl radicals are good chain carriers [4,5], diaryl and dialkyl phosphines can be added to olefinic or acetylenic compounds through radical chain reactions. Simpkins et al. [6] used... 

Phosphinyl radicals, obtained by hydrogen abstraction from dialkyl phosphites, have been trapped with t-nitrosobutane and the resultant nitroxyl radicals examined by e.s.r, The reaction of phosphinyl radicals, e.g. (5) and (6), with olefins has been shown to occur with retention of configuration at phosphorus. " These radicals have also been postulated as intermediates in the reactions of dialkyl disulphides and diaryl disulphides with phosphinates. " From the reaction of diphenyl disulphide... 

With this encouraging result from the model system, a gram quantity of the racemic sulfoxide 40 was prepared by oxidation of benzoxathiin 16 with mCPBA and a small amount of chiral sulfoxide (A)-40 with 94% ee was isolated by subsequent chiral HPLC separation (Scheme 5.12). When chiral sulfoxide (S)-40 was treated with borane-dimethylsulfide, a clean reduction of the olefin and the sulfoxide was observed. More surprisingly, only the desired cis-diaryl dihydrobenzoxathiin 12 was observed in high yield and unchanged 94% ee. No trans-isomer or 16 was observed. With this proof of concept in hand, an efficient... 

Michael addition in the absence of any catalytic agent has been reported for dialkyl and diaryl phosphites and thiophosphites with a-cyanoacrylate esters and a-cyanoacrylic acid.444 Yields of the conjugate addition products were moderate to good. The regiochemistry of this process is the opposite of that previously reported for similar additions to ketene acetals, the latter presumably proceeding by initial protonation of the distal olefinic carbon site.445... 

While Table 1 is an important tool for many of the studies, the current trend is to elaborate more the increasingly difficult substrates within a particular class of olefins. Andersson and coworkers have published several variations of the 1, 2-diaryl substrate 25a for the fairly more difficult 1,1 diaryl class of trisubstituted olefins (Scheme 3) [45]. The authors note that the products form an important class of compounds that are pharmaceutically relevant and difficult to prepare by other available methods. 

Reduction of all of the reported geminal diaryl substrates was completely under the control of the third stereo defining R-group. Particularly difficult were the sterically demanding 29 and the electron poor olefin 30, both of which required heating to produce poor to moderate yields. 

Hydrosilylation of monosubstituted and. em-disubstituted olefins (Reactions 5.3 and 5.4) are efficient processes and have been shown to occur with high regioselectivity (awti-Markovnikov) in the case of both electron-rich and electron-poor olefins [25]. For cis or trans disubstituted double bonds, hydrosilylation is still an efficient process, although it requires slightly longer reaction times and an activating substituent (Reaction 5.5) [25]. Any hydrosilylation product has been observed with 1,2-dialkyl-and 1,2-diaryl-substituted olefins, due to the predominant reversible addition of (TMS)3Si radical to the double bond [19]. 

The heteroaryl Heck reaction is an efficient tool for the introduction of other five membered heteroaromatic systems too (for more details see Chapter 6.4.). Chloropyrazines reacted readily with oxazole to give the coupled product (7.57.), consisting solely of the 5-oxazolyl isomer.77 Extension of the reaction to imidazole led to a similar observation,78 and the expected 5-pyrazyl-imidazole derivative was isolated in acceptable yield. Reactions using thiophene as the masked olefin gave similar results. Under forcing conditions the 2,5-diarylation of fiiran was also observed.77... 

A number of related couplings have been reported during the synthesis of the alkaloid ( )-cryptopleurine149 and also intramolecular coupling of diaryl amides to dibenzazepine and dibenzazodne structures.150 A versatile method for the preparation of tetrahydroquinolines and jololidines has been developed.151 The method involves the anodic oxidation of AT,AT-dimethyl-aniline in methanol to afford a-methoxylated or a, a -dimethoxylated compounds and subsequent treatment of products with Lewis acids in the presence of olefins. 

The indium hydride compound, generated in situ from sodium borohydride and a catalytic amount of indium(m) chloride, selectively reduces carbon-carbon double bonds in conjugated alkenes such as a,a-dicyano olefins, a,/3-unsaturated nitriles, cyano esters, cyanophosphonates, diesters, and ketones (Scheme 107).372 This combined reagent system in acetonitrile reduces exclusively the a,/3-carbon-carbon double bond in a,/3,7,<5-unsaturated diaryl ketones, dicarboxylic esters, cyano esters, and dicyano compounds (Scheme 108).373... 

Diaryl ditellurium compounds catalyzed the oxidation of aryl-substituted olefins by /-butyl hydroperoxide, hydrogen peroxide, 3-chloroperbenzoic acid, pure oxygen, or air in the presence of sulfuric acid. When the reactions were carried out in refluxing methanol, the vic-dimethoxy compounds were obtained. With acetic acid as the reaction medium, the vic-diacetoxy derivatives were isolated, text.-Butyl hydroperoxide was the best oxidizing agent with yields approaching 100%. ... 

In addition, diaryl tellurium dichlorides were used to arylate olefins. 

Diaryl tellurium dichlorides react with olefins in the presence of palladium(II) chloride to give arylated olefins in moderate yields. The reactions are catalytic with respect to palladium chloride when copper halides are added as oxidants. An aryl palladium compound was postulated as the intermediate5 ... 

Many fewer photocatalytic organic reductions have been reported. Reductions of organic substrates are less thoroughly studied, largely because of the early emphasis on the use of organic compounds as oxidizable source for the production of hydrogen gas. Nonetheless, some examples do exist, such as the hydrogenation of olefins, vinyl ethers, and a, S-unsaturated enones and alkynes [166, 167]. Similarly, other multiple bonds can be reduced, e.g., the N=N double bond of diaryl azo compounds [168] or carbonyl C=0 bonds [169, 170]. 

Sodium cyclopentadienyl-dicarbonylferrate 96 and tetrafluoroboric acid react with oxiranes to give olefins with the same stereostructure, after decomposition of the iron-alkene complex. With 96, diaryl and dialkyloxiranes are transformed to the olefins with inversion, by thermal decomposition of the intermediate alkoxide. Oxiranes interact with CF3C(0)I to yield olefins with the same geometry via a 3-iodotrifluoroacetate. ... 

The Beckmann rearrangement of ketoximes to the corresponding amides (31), the Fischer indole cyclization, isomerization of epoxides to the corresponding aldehydes, ketones, or alcohols, hydration and ammo-nolysis of epoxides, oxygen-sulfur interchange, formation of diaryl-ureas and -thioureas from condensation of aniline and carbonyl sulfide, and olefin carbonylation occur over zeolite catalysts (62). The oxo reaction over rhodium and cobalt containing zeolites recently has been claimed (22). 

Several larger membered ring azocompounds have also been synthesized and their decomposition kinetics studied . Reported kinetic data are collected in Table 8. The decomposition products are the corresponding diaryl cycloparaffins and isomeric olefins. The cis isomers decompose with considerably lower activation energies than the trans isomers. This feature of the reaction has been attributed to the greater ease with which both aryl substituents achieve coplanarity with the C-N=N-C linkage in the transition state-and thereby stabilize the incipient diradical — in the cis than in the trans isomer. A similar effect was noted for the... 

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