Isomerization phase-transfer catalysts

Shioiri and co-workers developed a catalytic asymmetric synthesis of allenes by isomerization of the alkyne 240 to allene 242 under the control of a chiral phase-transfer catalyst 241 (Scheme 4.62) [98], Although the enantiomeric excess is not high (35% ee), this is the first example of the asymmetric isomerization of alkynes under phase-transfer catalyzed conditions. 

Scheme 4.62 Asymmetric allene synthesis via isomeration of alkyne 240 with the chiral phase-transfer catalyst 241.

Alkynes react with haloethenes [38] to yield but-l-en-3-ynes (55-80%), when the reaction is catalysed by Cu(I) and Pd(0) in the presence of a quaternary ammonium salt. The formation of pent-l-en-4-ynes, obtained from the Cu(I)-catalysed reaction of equimolar amounts of alk-l-ynes and allyl halides, has greater applicability and versatility when conducted in the presence of a phase-transfer catalyst [39, 40] although, under strongly basic conditions, 5-arylpent-l-en-4-ynes isomerize. Symmetrical 1,3-diynes are produced by the catalysed dimerization of terminal alkynes in the presence of Pd(0) and a catalytic amount of allyl bromide [41]. No reaction occurs in the absence of the allyl bromide, and an increased amount of the bromide also significantly reduces the yield of the diyne with concomitant formation of an endiyene. The reaction probably involves the initial allylation of the ethnyl carbanion and subsequent displacement of the allyl group by a second ethynyl carbanion on the Pd(0) complex. 

Cyclodextrins are often used as inverse phase transfer catalysts [11-14]. They are able to intercalate hydrophobic substances and to transport them into a polar phase like water, where the reaction takes place. To study the influence of cyclodextrins on the isomerizing hydroformylation of frans-4-octene in the biphasic solvent system propylene carbonate/dodecane, the concentration of methylated /3-cyclodextrin was varied from 0.2 up to 2.0 mol.-% relative to the substrate frans-4-octene [24]. The results are given in Table 7. 

Keywords eugenol KOH, phase transfer catalyst, microwave irradiation, isomerization, isoeugenol... 

When allylic alcohols were used as substrates for dehydrogenation catalyzed by rhodium(I) complexes, isomerization to carbonyl compounds occurred instead. The product yields were excellent (Table IV) and the remarkably mild reaction could be effected in the absence of the phase-transfer catalyst with nearly as good results (55). Hydroxy-ir-allylrhodium complexes are probable reaction intermediates. 

The reaction proceeds almost exclusively by direct substitution (ipso), as shown by reactions of isomeric chlorotolnene complexes (Scheme 38). While polar protic solvents, snch as MeOH, strongly retard reaction,phase transfer catalysis (see Phase Transfer Catalyst) nsing benzene or addition of Crown Ethers to potassium alkoxides in benzene allows reaction at 25 °C. Even with strong electron donors such as alkyl, methoxy, or dialkylamtno in the ortho, meta, or para positions, substitution for chloride by potassium methoxide proceeds smoothly using the crown ether activation in benzene (eqnation 96). ... 

Reduction of ketones (8, 757). Reduction of methylcyclohexanones with sodium dithionite in a two-phase benzene-H20 mixture with Adogen 464 as a phase-transfer catalyst affords isomeric mixtures of methylcyclohexanols iq somewhat higher yield (70-82%) than that obtained when the reduction is conducted in aqueous solution only (15-68%). The stereoselectivity observed in the phase-transfer procedure is comparable to that reported with sodium borohydride. 

Thioxopyrido[3,2 4,5]thieno[3,2-J]pyrimidin-4(3//)-ones 17 isomeric to compound 16 were prepared by cyclocondensation of 2-ethoxycarbonyl-4-phenyl-6-sub-stituted thieno[2,3-6]pyridines 18 with isothiocyanates in the presence (or in the absence) of a phase-transfer catalyst (1997JHC937). 

Alkyl allylphosphonates were dichlorocyclopropanated using chloroform/base/phase-transfer catalyst to give the products 3 in good yield.Evidently, this reaction proceeds much faster than the base-catalyzed isomerization of allyl- to propenylphosphonates. In fact none of the latter are found in the reaction mixtures. 

Dibromocarbene undergoes addition to alkenes in a stereospecific manner. The sole case of nonstereospecific dibromocyclopropanation using bromoform/base/phase-transfer catalyst concerns ( )-cyclooctene, and is explained by isomerization of this cycloalkene caused by reversible addition of tribromomethyl or ethoxide anion the latter is formed from the ethanol present in bromoform (see also ref 2 and Houben-Weyl, Vol. El 9b, p 1617 for stereomutation in the reactions of dibromocarbene, generated from organomercury reagents, with low-active alkenes, see Section 1.2.1.4.3.1.5.1. and Vol. E19b, pp 1615 1616). 

Numerous examples of dibromocarbene adducts 3 of cycloalkenes, obtained by bromo-form/base/phase-transfer catalyst method have been described.Starting from the mixtures of isomeric cycloalkenes, in some cases, pure cw-isomers of adducts were isolated for the first time. 

As already mentioned, isomerization (double bond shift) is also a side reaction, which reduces the selectivity of the oxidation of higher olefins. High selectivity with respect to the formation of methylketones is obtained with a system consisting of t-butyl hydroperoxide as the oxidant, acetonitrile as the solvent, and P-cyclodextrine as a phase transfer catalyst in the oxidation of 1-dodecene [80] and of higher olefins C22-C20 [81]. Also, modified P-cyclodextrines increase the selectivity and activity of a PdSO /CuSO /phosphomolybdovanadic acid catalyst in biphasic oxidation of 1-decene to 2-decanone [82,83]. 

Barak. G. Sasson, Y. P-Cyclodextrin as a reverse phase transfer catalyst in the isomerization of 4-allylanisole. Bull. Soc. Chim. Fr. 1988. 584. 

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