Isomeric alkoxides

Other variations of the 3,3-rearrangement have been reported where the vinyl unit is incorporated into a ring. Addition of vinyllithium to a cyclohexanespirocyclobutanone produces a mixture of two stereo-isomeric alkoxides, one of which on warming to room temperature undergoes a smooth anionic oxy-Cope rearrangement to the octahydrobenzocyclooctene, while the other suffers ring fission (Scheme 55). Interestingly, if the carbinols are isolated fu-st by a low temperature quench and then transformed... 

The mechanism of Payne rearrangement involves intramolecular nucleophilic attack by an alkoxide on an adjacent epoxide to form an isomeric alkoxide. This isomerisation results an equilibrium mixture of the two epoxides, the position of which is controlled by the relative thermodynamic stabilities of the two compounds. 

The synthesis of vitamin Dj from a sensitive dienone was another etu-ly success of phosphorus ylide synthesis (H.H. Inhoffen, 1958 A). This Wittig reaction could be carried out without any isomerization of the diene. An excess of the ylide was needed presumably because the alkoxides formed from the hydroxy group in the educt removed some of the ylide. 

Esterification. Both mono- and dialkyl maleates and fumarates are obtained on treatment of maleic anhydride or its isomeric acids with alcohols or alkoxides (25). An extensive review is available (59). Alkyl fumarates (18) often are made from isomeri2ation of the corresponding maleate (19) (60). 

KTB and KTA are superior to alkaU metal hydrides for deprotonation reactions because of the good solubiUties, and because no hydrogen is produced or oil residue left upon reaction. Furthermore, reactions of KTA and KTB can be performed in hydrocarbon solvents as sometimes requited for mild and nonpolar reaction conditions. Potassium alkoxides are used in large quantities for addition, esterification, transesterification, isomerization, and alkoxylation reactions. 

Solvent for Base-Catalyzed Reactions. The abihty of hydroxide or alkoxide ions to remove protons is enhanced by DMSO instead of water or alcohols (91). The equiUbrium change is also accompanied by a rate increase of 10 or more (92). Thus, reactions in which proton removal is rate-determining are favorably accompHshed in DMSO. These include olefin isomerizations, elimination reactions to produce olefins, racemizations, and H—D exchange reactions. 

In contrast to alkoxides, thiolates react with 3,4-bis(arylsulfonyl)furoxans to give both isomeric products 283 and 284 (Scheme 185) (96JHC327, 97JMC463). 

With alkoxide and phenoxide ions diazo ethers are formed. The latter played a considerable part in the classic controversy about diazo isomerism between Hantzsch and Bamberger at the turn of the 19th century (see Sec. 1.1). Von Pechmann and Frobenius (1894 a) showed that the most convenient synthesis for preparative pur-... 

Germanium, eyclopentadienyl-, 3, 218 Germanium, tris(acetyiacetone)-structure, 1, 65 Germanium alkoxides physical properties, 2, 346 Germanium complexes, 3,183-223 acetylacetone isomerization, 2,383 bivalent... 

The catalytic cycles that have been documented, namely alkyne eyelotrimerization and olefin isomerization, demonstrate that addition and elimination from dimetal centers can occur readily in the presence of metal-metal bonds and alkoxide 1igands. 

If hydrogen gas is added to the reaction mixture of J, and 11 the hydrogenolysis reaction of thorium-to-carbon sigma bonds (J-1 22) allows interception of species 13 and thus, catalytic hydrogenation of the inserted carbon monoxide functionality. At 35 C under 0.75 atm initial H2 pressure with [JJ =9.0 x 10" M and [ 1JJ = 6.5 x 10" M, hydrogenation and isomerization are competitive and both the enolate and the alkoxide reduction product 14 are produced (eq.(13)). Under these conditions, turnover fre-... 

The isomerization of an O-silyl ketene acetal to a C-silyl ester is catalyzed by a cationic zirconocene—alkoxide complex [92], This catalysis was observed as a side reaction in the zirconocene-catalyzed Mukaiyama aldol reactions and has not yet found synthetic use. The solvent-free bis(triflate) [Cp2Zr(OTf)2] also catalyzes the reaction in nitromethane (no reaction in dichloromethane), but in this case there may be competitive catalysis by TMSOTf (cf. the above discussion of the catalysis of the Mukaiyama aldol reaction) [91] (Scheme 8.51). 

Ketone homoenolatesAddition of BuLi to the potassium alkoxide (2) of a 3-hydroxy-l-alkene (1) results in partial isomerization via a to the corresponding... 

Table 9.1). The rate of dehydrobromination from the intermediate bromoalkenes follows the pattern 2-bromoalkenes > Z-l-bromoalkenes > E- -bromoalkenes the corresponding chloro derivatives react more slowly. For optimum yield, the reaction temperature should be <100°C to reduce decomposition of the catalyst, and the concentration of base should be kept low to prevent isomerization of the resulting alkynes. [3-Elimination of HBr from 1,2-dibromo-1 -phenylethane can be controlled to yield 1-bromo-l-phenylethene in 83% yield [15]. The addition of alcohols and diols have a co-catalytic effect on the elimination reaction, as the alkoxide anions are transferred more effectively than the hydroxide ions into the organic phase [13]. 

Contrary to the statements of Schulman etal. (81) and Taft et al. (77), there is very little similarity between thermal decomposition of aluminum alkoxides and dehydration of alcohols over aluminas. The thermal decomposition mechanism would not explain the skeletal isomerization occurring during the dehydration of 2-methyl-1-propanol (82). 

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