Dehydration migration

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

Medroxyprogesterone acetate (74) is stmcturaHy related to and has been prepared from hydroxyprogesterone (39) (Fig. 10). Formation of the bis-ketal accomplishes the protection of the ketones and the required migration of the double bond. Epoxidation with peracetic acid produces a mixture of epoxides (75), with a predominating. Treatment of the a-epoxide with methyl magnesium bromide results in diaxial opening of the epoxide. Deprotection of the ketones provides (76), which is dehydrated to (77) by treatment with dilute sodium hydroxide in pyridine. Upon treatment with gaseous hydrochloric... 

The 2eohte sodium X (type 13X) has a crystallographic aperture of 0.74 nm. This compares well with the adsorbate value of 0.81 nm. ZeoHte calcium X exhibits a smaller apparent pore si2e of 0.78 nm (lOX). This difference is probably due to some distortion of the aluminosihcate framework upon dehydration and calcium ion migration. 

Without exception the 17-secondary alcohols give three products, the dehydration products with migration of the 13j5-methyl group, the 17a-fluoro derivatives and the 17/5-chlorofluoroacetates. ... 

No matter which of the electrophilic methods of double-bond shifting is employed, the thermodynamically most stable alkene is usually formed in the largest amount in most cases, though a few anomalies are known. However, there is another, indirect, method of double-bond isomerization, by means of which migration in the other direction can often be carried out. This involves conversion of the alkene to a borane (15-16), rearrangement of the borane (18-11), oxidation and hydrolysis of the newly formed borane to the alcohol (12-28), and dehydration of the alcohol (17-1) ... 

In dehydrated CoAF the migration of Co2+ ions from Co2 to Co2a sites on heating (Fig. lb) allows the occupancy of the most active a-sites where Co2+coordinates four framework oxygens. The irreversible and progressive decline of catalytic activity during dry-wet-dry cycles is likely to be due to cation hydration and movement from those sites, thus, decreasing their occupancies. 

Anhydro-D-mannitol is also obtained by a novel route from 1,6-dibenzoyl-D-mannitol. When this compound is heated with p-tolu-enesulfonic acid in acetylene tetrachloride, it suffers partial dehydration and one of the products is a dibenzoyl derivative of mannitan. On debenzoylation of this substance it yields crystalline 1,4-anhydro-D-mannitol.67 The formation of dibenzoyl-1,4-anhydro-D-mannitol has involved the migration of at least one benzoyl group and Hockett and coworkers6 believe that the initial product is either 2,6- or 3,6-dibenzoyl-1,4-anhydro-D-mannitol. 

Thus the foregoing analysis clarifies the two main observations of ECT, namely, the dehydration and acidity at the anode, and, excess hydration and alkalinity at the cathode. It will also explain the relative higher concentrations of Na+ and K at the cathode since CAT ions must have migrated to the anode and gotten discharged there, leaving a relative excess of Na+ and K in the vicinity of the cathode at the end of the ECT. 

ESR and ESEM studies of Cu(II) in a series of alkali metal ion-exchanged Tl-X zeolites were able to demonstrate the influence of mixed co-cations on the coordination and location of Cu(II) (60). The presence of Tl(l) forces of Cu(II) into the -cage to form a hexaaqua species, whereas Na and K result in the formation of triaqua or monoaqua species. In NaTl-X zeolite, both species are present with the same intensity, indicating that both cations can influence the location and coordination geometry of Cu(II). The Cu(II) species observed after dehydration of Tl-rich NaTl-X and KT1-X zeolites was able to interact with ethanol and DMSO adsorbates but no such interaction was observed with CsTl-X zeolites. This interaction with polar adsorbates was interpreted in terms of migrations of the copper from the -cages. 

The surface processes may comprise adsorption, surface migration (across terraces or along steps), dehydration of ions, and integration in the growth sites which are assumed to be kinks in surface steps. Any of these processes may be rate controlling, either alone or several together (10-11). 

The dehydration of menthols over alumina, prepared from aluminum isopropoxide and having intrinsic acidic sites, was accompanied by double bond migration of the cycloalkenes produced. The isomerization was, however, suppressed by the preferential neutralization of the strong acid sites with bases. The neutralization of acidic sites thus preventing isomerization was confirmed by von Rudloff (61) who evacuated pyridine-treated alumina for 6 hours, when about 0.8% base was retained. 

The retention of optical activity of camphene rules out methyl migration (Nametkin rearrangement) (71) or a symmetrical intermediate. On the acidic alumina at low contact time the retention of optical activity is high, about 80%. At longer contact time, however, there is essentially complete racemization. Hence, the dehydration mechanism seems to be the same on the acidic and on the base-modified alumina. The acidic alumina, however, causes the readsorption of the dehydration product leading to isomerization and equilibration. 

Pines and Herling 83) dehydrated the title alcohols over alumina B (Footnotes, Table V). The dehydration was made at 350° and the contact time was adjusted in order to obtain 50-60% styrenes. The dehydration was accompanied by a 6% of the phenyl and 9% of the tolyl migration from carbon atom 2 to carbon atom 1 and can be explained as follows ... 

The dehydration of neopentyl alcohol can best be explained by a concerted mechanism involving the removal of the proton from the y-carbon atom by the basic sites and of the hydroxyl group by the acid sites of the alumina, with migration of the methyl group ... 

The mechanism of dehydration of alcohols over acidic and non-acidic alumina is the same. In the presence of the acidic alumina, however, readsorption of the dehydrated product can occur, leading to either double bond migration or skeletal isomerization, depending on the strength of the acid sites, the structure of the olefins produced, and the experimental conditions. 

It has previously been shown that the oxazines (169) can isomerize by ringopening to the Schiff bases (170). The reactions of the oxazines with Ni , Co", and Cu" have now been examined in the hope of yielding chelates of the Schiff bases. However, the products were not the expected ones in the case of nickel, two kinds of chelate were obtained. Those formed in the absence of air were (171), in which proton migration from the original ligand had occurred, and, in the presence of air, (172) was formed in which dehydration had taken place on the original oxazine. The Co" and Cu" chelates are structurally analogous to the nickel complexes. ... 

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