Isomerization of unsymmetrical

The stereoselective isomerization of unsymmetrical diallyl ethers to allyl ( )-vinyl ethers was also carried out in the presence of a cationic iridium(l) catalyst. The catalyst prepared in situ by treating [Ir(cod)(PPh2Me)2]PF6 with hydrogen was found to be an excellent catalyst for the selective isomerization of a less substituted allyl group to an ( )-vinyl ether (Scheme 44).72... 

Metal-catalyzed isomerization of unsymmetrical diallyl ethers is a unique route to synthesize allyl vinyl ethers for the Claisen rearrangement. In 1977, Reuter and Salomon reported that heating the diallyl ether 23 in the presence of a catalytic amount of tris(triphenylphosphine)ruthenium(II) dichloride resulted in the... 

Isomerization. Cationic iridium complex effects selective isomerization of unsymmetrical diallyl ethers and conjugated boronates containing an allylic ether group, including an access to 7-(siloxy)allylboronic esters. The conversion of allyl homoallyl ethers to -y,8-unsaturated carbonyl compounds is promoted by [(cod)IrCl]2. 

Miyaura has reported the iridium-catalyzed isomerization of unsymmetrical dia-Uyl ethers to ( )-allyl vinyl ethers [20]. The iridium catalyst 12 active for the isomerization of diallyl ethers was prepared in situ by bubbhng hydrogen into a red solution of [Ir(cod)(PPh2Me)2]PF(5 by the method reported by FeUdn (Eq. 3.1.11)... 

Scheme 12.6 Isomerization of unsymmetrical 1,3-disubstituted Jt-allyl complexes [13].

A mild procedure which does not involve strong adds, has to be used in the synthesis of pure isomers of unsymmetrically substituted porphyrins from dipyrromethanes. The best procedure having been applied, e.g. in unequivocal syntheses of uroporphyrins II, III, and IV (see p. 251f.), is the condensation of 5,5 -diformyldipyrromethanes with 5,5 -unsubstituted dipyrromethanes in a very dilute solution of hydriodic add in acetic acid (A.H. Jackson, 1973). The electron-withdrawing formyl groups disfavor protonation of the pyrrole and therefore isomerization. The porphodimethene that is formed during short reaction times isomerizes only very slowly, since the pyrrole units are part of a dipyrromethene chromophore (see below). Furthermore, it can be oxidized immediately after its synthesis to give stable porphyrins. 

Nitrobenzofuroxan (355) undergoes a rearrangement (recognizable as an isomerization in unsymmetrically substituted derivatives) which is an example of this general rearrangement (Scheme 45) (64AG(E)693) see Table 10. 

The oxidation of unsymmetrical tertiary amines with mercuric acetate may also lead to isomeric enamines. In such cases, structures can often be established by NMR and IR spectra of the enamines and their corresponding imonium salts, through comparison with model systems (202-205). 

BFO reacted readily with 1,3-diketones to give 2,3-disubstituted quinoxaline 1,4-dioxides. In the case of unsymmetrical 1,3-diketones, mixtures of isomeric quinoxaline dioxides were obtained, and the ratio of isomers was influenced by the steric bulk of the carbonyl substituent. When BFO 1 was combined with 1,3-diketone compounds 18 in the presence of triethylamine, the isomeric quinoxaline 1,4-dioxides 19 and 20 were obtained. When R = Me, 19 was the only product observed. As the steric bulk of R increased, increasing amounts of isomer 20 were observed. When R = tBu, 20 was the only product detected in the reaction. 

Group VI Donors. Oxygen donor ligands. A series of unsymmetrical tris-(P-diketonato)M " complexes (17) has been investigated in order to establish CIS—trans isomer distributions, and relative rates of isomerization. Included in this study was the preparation and separation of the geometric isomers of [Ru(tfac)3] and [Ru(bzac)3] (tfac = trifluoroacetylacetone, bzac = benzoyl-acetylacetone). In weakly polar media, the trans-isomer of [Ru(tfac)3] was found to be more stable, whereas the isomer distribution of similar complexes with R, R = alkyl or Ph, is statistical. ... 

SrnI processes have been shown to be relevant to the synthesis of simple thianthrenes which is done by irradiating the disodium salt of 4-methylbenzene-1,2-dithiol in the presence of 1,2-bromochlorobenzene (55%) or 1,2-di-iodobenzene (64%). More complex, fused thianthrenes result from l-bromo-2-iodonaphthalene (24%, 2 isomeric products) and 2,3-dichloroquinoxaline (100%). These clearly hold considerable promise for the controlled construction of unsymmetrical thianthrenes (87JOC1089). 

The high yields and enantioselectivities recorded in the Pd(0)/BPA-catalyzed reaction of rac-27a-27c with water and KHCO3 show that not only a highly enan-tioselective alkylation but also an efficient dynamic kinetic resolution either via racemization of the more slowly reacting enantiomer of the substrate or isomerization of the diastereomeric jt-allyl-Pd complexes (see Scheme 2.1.4.29) had occurred. Experiments with other racemic unsymmetrical allylic carbonates revealed a dependence of the enantioselectivity on the concentration of the Pd(0)/... 

Zirconocene diiodide can promote the addition of diallylzinc to a,/1-disubstituted unactivated alkynes. Thus, in the case of 5-decyne, a 94/6 mixture of the two isomeric alkenyl iodides (derived respectively from syn and anti additions to the triple bond) was obtained after iodinolysis (equation 74)108. However, the stereoselectivity was lower for 2-butyne (80/20) and the case of unsymmetrical alkynes was not mentioned. 

Allene ketene cycloadditions are of greater synthetic utility than cither mixed allene dimerization or mixed ketene dimerization. In this class of reaction the ketene is the more reactive species and homodimerization of ketene can be minimized by use of excess allene. Such cycloadditions always result in 2-alkylidenecyclobutanones with the sp carbons of both moieties forming the initial bond. In substituted allenes and ketenes, mixtures of stereoisomers of 2-alkylidenecyclobutanones are obtained with very little stereoselectivity, the stereoisomers arise from cisUrcins isomerism in the cyclobutane ring and EjZ isomerism of the exocyclic double bond. In unsymmetrically substituted allenes some regiochemical preference for ketene cycloaddition is observed. Examples of dimethylketene allene cycloadditions are summarized in Table 1,2... 

Isomerization of a it-allyl species can arise from a bimolecular reaction as shown in Scheme 8E.5 [44], In the case of unsymmetrically substituted allyl systems, this process constitutes... 

Trans-to-cis photoisomerization of unsymmetrical 4,4/-disubstituted trans-1,3-diphenyltriazenes gives a non-equilibrium mixture of two isomeric cis pairs, which, as in the case of symmetrical 1,3-diphenyltriazenes, undergo thermal cis-to-trans isomerization by means of 1,3-prototropic rearrangements catalysed by general acids and general bases (Scheme 124).187... 

Palladium-catalyzed oxidative cyclization of aryl homoallyl ethers affords 4-methyl-2//-chromenes in moderate yield. The reaction is proposed to proceed via activation of the alkene by coordination to Pd(ll) followed by intramolecular nucleophilic attack by the arene. Subsequent [1-hydride elimination and isomerization then affords 4-methyl-27/-chromenes (Scheme 13). Electron-rich aryl homoallyl ethers give the best yield and good regio-selectivity is observed for the reaction of unsymmetrical arenes <2005OL3355>. 

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