Dehydration base-catalysed

In liquid phase reactions the base catalysed reaction may only proceed as far as its self-condensation to DAA with minimal dehydration to MO, however when operating in the gas phase at elevated temperatures, secondary reactions can take place (4). 

In general these reactions are base-catalysed in that it is necessary to remove a proton from HCXYZ in order to generate the carbanion, eCXYZ, the effective nucleophile one or more of X, Y and Z are usually electron-withdrawing in order to stabilise it. The initial adduct (84) acquires a proton from the solvent (often H20 or ROH) to yield the simple addition product (85). Whether or not this undergoes subsequent dehydration (86) depends on the availability of an H atom, either on an a-carbon or where X, Y or Z = H, and also on whether the C=C so introduced would, or would not, be conjugated with other C=C or C=0 linkages in the product ... 

Base-catalysed dehydrations are relatively unusual, and that one occurs here stems from the facts (a) that (89) contains a-H atoms removable by base to yield an ambient concentration of the carbanion (97), and (b) that this carbanion possesses a goodish leaving group—... 

A good example of elimination promotion is by the CHO group in aldol (58) making possible a base-catalysed dehydration to an... 

A simple two-step protocol for the generation of a terminal diene is to add allyl magnesium bromide to an aldehyde or a ketone and subsequent acid or base catalysed dehydration (equation 34)72. Cheng and coworkers used this sequence for the synthesis of some indole natural products (equation 35)72a. Regiospecific dienones can be prepared by 1,2-addition of vinyllithium to a,/l-unsaturated carbonyl compounds and oxidative rearrangement of the resulting dienols with pyridinium dichromate (equation 36)73. 

Base-catalysed cyclization of proximate diacetyl aromatics [e.g. o-diaccty I benzene (36)] gives the corresponding enone (37). Relative rates, activation parameters, and isotope effects are reported for (36), and also for 1,8-diacetylnaphthalene, 4,5-diacetylphenanthrene, and 2,2/-diacetylbiphenyl, in aqueous DMSO.61 Reaction proceeds via enolate formation (rate determining for the latter three substrates), followed by intramolecular nucleophilic attack [rate determining for (36)], and finally dehydration. 

Studies of relative rates, activation parameters, kinetic isotope, and solvent isotope effects, and correlation of rates with an acidity function, have elucidated the mechanisms of cyclization of diacetyl aromatics (23-26) promoted by tetramethyl-ammonium hydroxide in DMSO.32 Rate-determining base-catalysed enolate anion formation from (24-26) is followed by relatively rigid intramolecular nucleophilic attack and dehydration whereas the cyclization step is rate determining for (23). 

In the base-catalysed dehydration for oxime formation a significant development of the carbon-nitrogen double bond in the TS is assumed. 

The forward synthetic reaction is a base-catalysed condensation reaction between two carbonyl compounds, the aldol condensation leading to -hydroxy-aldehydes or / -hydroxyketones followed by dehydration. This sequence is one of the most important carbon-carbon bond forming reactions, and aldol-type condensation reactions are considered in a number of other sections of the text, for example, the Doebner reaction (Section 5.18.3, p. 805), the Knoevenagel reaction (Section 5.11.6, p. 681), the Perkin reaction (Section 6.12.3, p. 1036) and the Robinson annelation reaction (Section 7.2). 

On polarographic curves two waves are observed the height of both waves increases with increasing pH (Fig. 23) and depends on buffer composition, as the dehydration is general base-catalysed. At more positive potentials, the bond C—X is cleaved in the form, where the polarizibility of this bond is enhanced by the vicinity of the free CO group. The half-wave potentials of the more negative wave are identical with... 

Because the potential is more positive than E i, the formation of only one wave is observed. In acidic and alkaline media where the rate of the acid-base catalysed reaction (35 b) is fast and during the reaction all the phenylhydroxylamine derivative is transformed to quinoneimine, the height of the single wave corresponds to a transfer of six electrons [(35 a) plus (35 c)]. Because the life-time of the quinoneimine intermediate is short, its hydrolysis to form quinone does not affect the electrode process. In the medium pH range where the rate of dehydration is slow, the wave-height corresponds to a four-electron process. A theoretical... 

In practice, the Konstanecki-Robinson synthesis of chromones commences with O-benzoylation not C-benzoylation, to afford ester 9.24. Base-catalysed rearrangement produces the required 1,3-diketone 9.21, via intramolecular benzoylation of the intermediate enolate. Acid-catalysed dehydration then affords flavone 9.19. 

A widely studied example of this kind is the synthesis of methyl isobutyl ketone (MIBK, used as a solvent for inks and lacquers) from acetone. The former was previously prepared from the latter through a catalytic three-step process base-catalysed production of 4-hydroxy-4-methylpentan-2-one, acid dehydration into mesityloxide (MO), then hydrogenation of MO on a Pd catalyst. Since acetone aldolization occurs through acid catalysis as shown over a H-MFI zeolite at 433 K (MO is the main reaction product, the aldolization product being rapidly dehy-drated[5]), it is possible, by associating with the acid catalyst a hydrogenation phase,... 

Base-catalysed aldol reactions may give the aldol product, or may give the V dehydrated enone or enalby an ElcB mechanism... 

The catalysts were evaluated in two reactions - the base catalysed epoxidation of electron deficient alkenes),[15] and in the Linstead variation of the Knoevenagel condensation to give 3-nonenoic acid. This reaction utilises malonic acid, and leads to an unusual dehydration, giving the P/y-unsaturated acid, rather than the more typical a,P-enoic acid.[21-24] The product can be used as a precursor to the lactone, which is a flavour component of coconut oil. 

Examples of reactions in which acid-base catalysed dehydration was combined with acid-base equilibria either preceding or subsequent to the dehydration process are quoted for the waves of pyridine aldehydes (Tirouflet and Laviron, 1959 Volke, 1958 ManouSek and Zuman, 1964) and of glyoxalic acid (Kuta, 1959). Different dehydration rates were found for pyridinium ions and free pyridine derivatives, as well as for the free glyoxalic acid and its anion. For numerous aldehydic substances the effect of hydration has been observed but a quantitative treatment has not yet been applied. 

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