Acid-catalysed rearrangement

The mechanism of formation of 12"-alkoxy derivatives is the same as the one involved in the u ci-rearrangement—acid-catalysed hydroxyl elimination in position 12", here followed by an alcohol addition (Schneider etai, 1977 Rudman et ai, 1991). Similar to the u ci-derivatives, also the 12"-alkoxy derivatives can be present as impurities in the salts of ergopeptines. 

Davies and Warren" found that when 1,4-dimethylnaphthalene was treated with nitric acid in acetic anhydride, and the mixture was quenched after 34 hr, a pale yellow solid with an ultraviolet spectrum similar to that of a-nitro-naphthalene was produced. However, if the mixture was allowed to stand for 5 days, the product was i-methyl-4 nitromethylnaphthalene, in agreement with earlier findings. Davies and Warren suggested that the intermediate was 1,4-dimethyl-5 nitronaphthalene, which underwent acid catalysed rearrangement to the final product. Robinson pointed out that this is improbable, and suggested an alternative structure (iv) for the intermediate, together with a scheme for its formation from an adduct (ill) (analogous to l above) and its subsequent decomposition to the observed product. 

Polyether Polyols. Polyether polyols are addition products derived from cyclic ethers (Table 4). The alkylene oxide polymerisation is usually initiated by alkah hydroxides, especially potassium hydroxide. In the base-catalysed polymerisation of propylene oxide, some rearrangement occurs to give aHyl alcohol. Further reaction of aHyl alcohol with propylene oxide produces a monofunctional alcohol. Therefore, polyether polyols derived from propylene oxide are not truly diftmctional. By using sine hexacyano cobaltate as catalyst, a more diftmctional polyol is obtained (20). Olin has introduced the diftmctional polyether polyols under the trade name POLY-L. Trichlorobutylene oxide-derived polyether polyols are useful as reactive fire retardants. Poly(tetramethylene glycol) (PTMG) is produced in the acid-catalysed homopolymerisation of tetrahydrofuran. Copolymers derived from tetrahydrofuran and ethylene oxide are also produced. 

The above mechanism involves a-opening of the epoxysilane, followed by a 1,2-elimination of a /3-hydroxysilanc (Peterson olefination, Chapter 10). However, it has recently been shown that aj8-dihydroxysilanes, particularly t-butyldimethylsilyl species, undergo an acid-catalysed sila-pinacol rearrangement to produce /J-aldehydo- and /i-kctosilancs (5) ... 

Recently it has been shown (4) that cr/3-dihydroxysilanes. particularly t-butyldimethyl species, undergo an acid-catalysed silapinacol rearrangement to /3-aldehydo- and / -ketosilanes, in most respectable yields. The implications of this rearrangement on the acid-catalysed rearrangement of u/i-epoxysilanes to carbonyl compounds are discussed in Chapter 4. 

In this, as in many catalysed reactions, the protonated substrate is postulated as an intermediate, and although the proposed reaction scheme in fact accords with all the known experimental facts it perhaps would be instructive to determine the dependence of the rate coefficient on the Hammett acidity function at high acid concentration and also to investigate the solvent isotope effect kD2JkH20. Both these criteria have been used successfully (see Sections 2.2-2.4) to confirm the intermediacy of the protonated substrate in other acid-catalysed aromatic rearrangements. 

It is important for acid-catalysed reactions to determine whether the reaction is specifically catalysed by hydrogen ions or whether general acid catalysis takes place. Specific acid catalysis has been conclusively demonstrated for the benzidine rearrangement by three different sorts of kinetic experiments. In the first, it has been shown41 by the standard test for general acid catalysis (by measuring the rate of reaction in a buffered solution at constant pH over a range of concentration... 

White et al.1A have obtained similar kinetic results for the acid-catalysed rearrangement of N-nitro-N-methylaniline, i.e. a first-order dependence on the nitroamine with a linear H0 plot of slope 1.19 for phosphoric acid, and a deuterium solvent isotope effect of about three, although the results have only been presented in preliminary form. Further, an excellent Hammett a+ correlation was claimed for thirteen para substituted nitroamines which gave a p value of —3.9. Since it is expected that the rate coefficients would correlate with a (rather than different basicities of the amines, the a+ correlation implies that the amino nitrogen is electron-deficient in the transition state,... 

M. O Sullivan, The acid-catalysed rearrangements of hydrazobenzenes. M. Sc. Thesis, London, 1966. 

Cyclobuta[fc]chroman-4-ols, derived from chromones by a [2+2] photocycloaddition to ethylene, are prone to acid-catalysed rearrangements. Elaboration of the parent system prior to rearrangement has enabled the marine sesquiterpene filiformin <96JOC4391>, the henzo-1,3-dioxan nucleus of averufin <96JOC9164> and cyclobuta[h][l]benzoxepin-8,9-diones <96CC1965> to be synthesised. 

Some insect pheromones are internal ketals. We have already mentioned multistriatin (pp T 2 and 99) and frontalin p 193). Brevicomin (22) is another example. Disconnection of the ketal gives (23) containing a 1,2-diol. Among other syntheses, hydroxy-lation of protected enone (24) by epoxidation and acid catalysed rearrangement gives brevicomin stereo-specifically,... 

Acid-catalysed rearrangement of epoxides is another widely used reaction in the fine chemicals industry. Here again the use of solid acid catalysts such as zeolites is proving advantageous. Two examples are shown in Fig. 2.25 the isomerization of rsophorone oxide (Elings et al., 1997) and the conversion of a-pinene oxide to campholenic aldehyde (Holderich et al., 1997 Kunkeler etal., 1998). Both products are fragrance intermediates. 

This relatively ready rearrangement can be a nuisance in the preparative addition of acids, e.g. hydrogen halides (p. 184) to alkenes, or in their acid-catalysed hydration (p. 187) mixed products that are difficult to separate may result or, in unfavourable cases, practically... 

Another example of migration of a group, in the original case Me, to a cationic carbon atom occurs in the acid-catalysed rearrangement of 1,2-diols, e.g. pinacol (cf. p. 218) Me2C(OH)C(OH)Me2 (34) to ketones, e.g. pinacolone, MeCOCMe3 (35) ... 

A very similar rearrangement takes place during the acid-catalysed decomposition of hydroperoxides, RO—OH, where R is a secondary or tertiary carbon atom carrying alkyl or aryl groups. A good example is the decomposition of the hydroperoxide (84) obtained by the air-oxidation of cumene [(l-methylethyl)benzene] this is used on the large scale for the preparation of phenol and acetone ... 

Autoxidation may in some cases be of preparative use thus reference has already been made to the large-scale production of phenol+ acetone by the acid-catalysed rearrangement of the hydroperoxide from 2-phenylpropane (cumene, p. 128). Another example involves the hydroperoxide (94) obtained by the air oxidation at 70° of tetrahydro-naphthalene (tetralin) the action of base then yields the ketone (a-tetralone, 95), and reductive fission of the 0—0 linkage the alcohol (a-tetralol, 96) ... 

Beckmann rearrangement of oxime is an acid catalysed reaction. The environmental problems associated with the use of sulphuric acid instigated interest to use number of solid acid catalysts [1], There are only scanty references about Lewis acid ion-exchanged MeAlPOs. Beyer et al. [2], Mihalyi et al. [3] and Mavrodinova et al. [4] already suggested the presence of Lewis acid metal ions as MO+ species in zeolites. The present study focussed the synthesis and characterisation of Fe3+, La3+ and Ce3+ ion-exchanged MAPO-36. The catalytic results of Beckmann rearrangement of cyclohexanone oxime over ion-exchanged catalysts are delineated in this article. 

Acid catalysed rearrangement of the tetrahydro 1-benzazepine sulfonamides 52 gave the 9-substituted sulfone derivatives 53 plus, in the case of 52b, the 7-substituted isomeric derivative... 

Cyclopenta-l,4-dioxanes 95 are formed in high yields through the acid-catalysed rearrangement of the dioxolanes 94 in which electrocyclisation of a hydroxypentadienyl carbocation, akin to a Nazarov cyclisation, is involved (Scheme 62) <00CEJ4021>. 

Aldehyde 54 and the hydroxamic acids 55 were generated together in an acid-catalysed elimination reaction (Scheme 7 pathway (ii)). A crossover experiment indicated that esters are formed in a concerted rearrangement concomitant with the likely formation of the hydroxynitrene 57 (Scheme 7 pathway (iii)) while there is no evidence to date for the formation of hydroxynitrene, joint solvolysis of equimolar quantities of /V-acetoxy-/V-butoxy-/>-chlorobenzamide 26e and N- acetoxy-/V-benzyloxybenzamide 27a afforded significant quantities of butyl p-chlorobenzo-ate (36%) and benzyl benzoate (54%) as the only esters. This is an example of a HERON reaction, which has been identified in these laboratories as a characteristic rearrangement of bisheteroatom-substituted amides.32,33,42 43 155 158 Since ester formation was shown to prevail in neutral or low acid concentrations, it could involve the conjugate anion of the hydroxamic acid (vide infra).158... 

The use of a rotating reactor to provide efficient mixing has been proposed and tested for viscous polymerizations and acid-catalysed rearrangement reactions [6], The principle of these spinning disc reactors is that the centrifugal force... 

As mentioned earlier, products of disproportionation often accompany the rearrangement products. This reaction is also acid-catalysed and it is a reasonable assumption that reaction proceeds via the protonated species. Experiments with the 4,4 -diiodohydrazobenzene (19) showed that there were significant nitrogen and para-carbon kinetic isotope effects23. This implies that disproportionation must take place after C—C bonding has occurred, i.e. that the intermediate must be the quinonoid form 20 (and cannot, for example, be a jr-complex), which is then believed to react with another reactant molecule to give the disproportionation products (Scheme 4). 

The benzidine rearrangements can also be brought about thermally, but very few mechanistic studies have been carried out. One set of heavy-atom KIE measurements has been made in the reaction of 2,2 -hydrazonaphthalene (18)21. Substantial nitrogen (1.0611 for the [15N, 15N ]) and carbon (1.0182 for the [1,1 -13C2]) KIE values were obtained showing that, just as for the acid catalysed reaction, this is a [3,3]-sigmatropic rearrangement, this time presumably of the non-protonated reactant. 

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