Ylides potassium tert-butoxide

R. The use of butyllithium in tetrahydrofuran or ether-hexane affords the triene 1n only 50-60% yield. When the ylide was generated with sodium hydride or potassium tert-butoxide in dimethyl sulfoxide by the submitter, the Wittig reaction gave triene containing 10-20% of the Z isomer. Part C illustrates the selective hydroboration of a diene with disiamylborane.1 The reaction is best carried out by adding preformed disiamylborane to the triene. Lower yields of homogeraniol were obtained by the submitter when the triene was added to the borane reagent. 

In all cases, the ylides 3 are easily generated from the corresponding phosphonium salts 2 by reaction with one equivalent of potassium tert-butoxide in ether at room temperature. 

The normal preference for (Z) alkenes in reactions of non-stabilized phos-phoranes can be reversed by employing the Schlosser modification of the Wittig reaction (Scheme 6).19 Here, equilibration of the initially formed erythro and threo betaine intermediates is achieved by reaction with additional strong base, usually an alkyl lithium. The resulting betaine ylide then gives the (E) alkene on treatment with a proton source followed by potassium tert-butoxide. 

Treatment of the phosphine resin 33 with alkyl bromides, e.g. 34, gave the corresponding phosphonium bromide 35 in excellent yields. The formation of the phosphorane was described as a cmdal step, for which the transport of the base to the phosphonium site within the polymer was essential. Various bases, such as sodium hydride, potassium tert-butoxide in THF, and n-butyllithium in dioxane, have been employed. Good results for the formation of the ylide were obtained with a mixture of sodium methoxide, methanol, and THF, and more recently with sodium bis(trimethyl)silylamide in THF [34, 35]. After the addition of aldehyde 36... 

Reaction of 2-oxoalkyltriphenylbismuthonium salts (82) with a base such as potassium tert-butoxide led to triphenylbismuthonium 2-oxoalkylides (86). These compounds showed the behaviour of moderately stabilized ylides towards various nucleophiles. 5-67... 

When a suspension of (3,3-dimethyl-2-oxobutyl)triphenylbismuthonium tetrafluoro-borate (126 mg, 0.2 mmol) in THF (5 ml) was mixed with potassium tert-butoxide (24 mg, 0.2 mmol) at — 78°C under argon, (3,3-dimethyl-2-oxobutylidene)triphenyl-A -bismuthane was formed as a yellow solution. Attempts to isolate this ylide under ambient conditions were unsuccessful due to its thermal instability and moisture sensitivity [94JCS(P1)2703]. 

Syntheses of Alkylidene cyclopropanes Via the Selenonium route The selenonium route proved to be more valuable. It has been specifically designed by us to replace the deficient selenoxide route (Scheme 38). It was expected to produce alkylidene cyclopropanes by a mechanism which mimics the selenoxide elimination step but which involves a selenonium ylide in which a carbanion has replaced the oxide. Cyclopropyl selenides are readily transformed to the corresponding selenonium salts on reaction with methyl fluorosulfonate or methyl iodide in the presence of silver tetrafluoroborate in dichloromethane at 20 °C and, as expected, methylseleno derivatives are more reactive than phenyl-seleno analogs. Alkylidene cyclopropanes are, in turn, smoothly prepared on reaction of the selenium salts at 20 °C with potassium tert-butoxide in THF (Scheme 38). Mainly alkyl cyclopropenes form at the beginning of the reaction. They then slowly rearranges, in the basic medium, to the more stable alkylidene cyclopropanes( 6 kcal/mol). In some cases the complete isomerisation requires treatment of the mixture formed in the above reaction with potassium fcrt-butoxide in THF. The reaction seems to occur via a selenonium ylide rather than via a P-elimina-tion reaction promoted by the direct attack of the /crt-butoxide anion on the P-hydrogen of the selenonium salt, since it has been shown in a separate experiment that the reaction does not occur when a diphenylselenonium salt (imable to produce the expected intermediate) is used instead of the phenyl-methyl or dimethyl selenonium analogs. It has also been found that the elimination reaction is the slow step in the process, since styrene oxide is formed if the reaction is performed in the presence of benzaldehyde which traps the ylide intermediately formed... 

In analogy to the properties of ordinary sulfonium salts, reaction of disulfonium dications with bases may lead to either a- or p-deprotonation. Deprotonation of the S-S dication formed during the Pummerer rearrangement of monosulfoxides of certain bis-sulfides 13, 59, 56 is a key step of the overall process. Furukawa and co-workers " also suggested that direct formation of disulfonium dication ylides occurred upon treatment of sulfinyli-mines of 14 and 41 with potassium tert-butoxide. Reaction of triflic anhydride with sulfoxide 100 affords sulfonium salt 102 through deprotonation of the corresponding S-S dication 101 (Scheme 39). ... 

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