Alkenes tert-butoxide

Compound A (C4H10) gives two different monochlondes on photochemical chlorination Treatment of either of these monochlondes with potassium tert butoxide in dimethyl sulfoxide gives the same alkene B (CaHg) as the only product What are the structures of compound A the two monochlondes and alkene B2... 

Compound A (CgHi4) gives three different monochlondes on photochemical chlonnation One of these monochlondes is inert to E2 elimination The other two monochlondes yield the same alkene B (CgHi2) on being heated with potassium tert butoxide in tert butyl alcohol Men tify compound A the three monochlondes and alkene B... 

Compounds A and B are isomers of molecular formula C9Hi9Br Both yield the same alkene C as the exclusive product of elimination on being treated with potassium tert butoxide in dimethyl sulfoxide Hydrogenation of alkene C gives 2 3 3 4 tetramethylpentane What are the structures of compounds A and B and alkene C2... 

Trialkylboranes react rapidly and in high yields with a-halo ketones,a-halo esters, a-halo nitriles, and a-halo sulfonyl derivatives (sulfones, sulfonic esters, sulfonamides) in the presence of a base to give, respectively, alkylated ketones, esters, nitriles, and sulfonyl derivatives. Potassium tert-butoxide is often a suitable base, but potassium 2,6-di-tert-butylphenoxide at 0°C in THF gives better results in most cases, possibly because the large bulk of the two tert-buXy groups prevents the base from coordinating with the R3B. The trialkylboranes are prepared by treatment of 3 mol of an alkene with 1 mol of BH3 (15-16). With appropriate boranes, the R group transferred to a-halo ketones, nitriles, and esters can be vinylic, or (for a-halo ketones and esters) aryl. " °... 

We can control which product we get by carefully choosing our base. If we use a strong base (like methoxide or ethoxide), then we will get the more substituted alkene. However, if we use a strong, sterically hindered base, such as tert-butoxide, then we will get the less substituted alkene. 

In the last step, tert-butoxide was used to favor elimination over substitution (see Section 10.10). hi summary, we have seen that we can use either an El process or an E2 process to convert an alcohol into an alkene. 

A bulky base such as potassium tert-butoxide in tert-butyl alcohol favors the formation of the less substituted alkene in dehydrohalgenation reactions. 

Scheme 6.58 Generation of 3<52-chromene (258) and its interception by the tert-butoxide ion and various activated alkenes.

Attempts to liberate l-methyl-l-aza-2,3-cyclohexadiene (329) from 3-bromo-l-methyl-l,2,5,6-tetrahydropyridine (326) by KOtBu in the presence of [18]crown-6 and furan or styrene did not lead to products that could have been ascribed to the intermediacy of 329 (Scheme 6.70) [156], Even if there is no doubt as to the allene nature of 329 on the basis of the calculations on the isopyridine 179 and 3d2-lH-quinoline (257), it is conceivable that the zwitterion 329-Za is only a few kcal mol-1 less stable than 329. This relationship could foster the reactivity of 329 towards the tert-butoxide ion to an extent that cycloadditions to activated alkenes would be too slow to compete. On the other hand, the ultimate product of the trapping of 329 by KOtBu could have been an N,0-acetal or a vinylogous N,0-acetal, which might not have survived the workup (see, for example, the sensitivity of the N,0-acetal 262 [14], Scheme 6.57). 

Dehydrohalogenation of 1-bromo-1-methylcyclohexane with a strong base (NaOH or potassium tert-butoxide) would also provide the desired alkene. This reaction, however, would be complicated by the formation of methylcyclohexene as the major product. 

Alkenes lacking hydrogen atoms at allylic positions are much less acidic than ordinary unsaturated hydrocarbons. Superbases regioselectively exchange allylic protons in alkenes whenever there is a choice. However, a few examples of metallation of alkenic C-H bonds with superbases are known and a compilation of them is reported in Table 1. Ethylene itself has been deprotonated by the superbasic mixture constituted by butyllithium, potassium tert-butoxide, and TMEDA.41... 

Formation of the Hofmann Product Bulky bases can also accomplish dehydrohalo-genations that do not follow the Zaitsev rule. Steric hindrance often prevents a bulky base from abstracting the proton that leads to the most highly substituted alkene. In these cases, it abstracts a less hindered proton, often the one that leads to formation of the least highly substituted product, called the Hofmann product. The following reaction gives mostly the Zaitsev product with the relatively unhindered ethoxide ion, but mostly the Hofmann product with the bulky tert-butoxide ion. 

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. 

In zeolites, this barrier is even higher. As discussed in Section II.B, the lower acid strength and the interaction between the zeolitic oxygen atoms and the hydrocarbon fragments lead to the formation of alkoxides rather than carbenium ions. Thus, extra energy is needed to transform these esters into carbonium ionlike transition states. Quantum-chemical calculations of hydride transfer between C2-C4 adsorbed alkenes and free alkanes on clusters representing zeolitic acid sites led to activation energies of approximately 200 kJ/mol for isobutane/tert-butoxide (29), 230-305 kJ/mol for propane/sec-propoxide, and 240 kJ/mol for isobutane/tert-butoxide (32), 130-150 kJ/mol for ethane/ethene (63), 95-105 kJ/mol for propane/propene, 88-109 kJ/mol for isobutane/isobutylene, and... 

The discovery that sodium in the presence of small amounts of organosodium compounds, produced in situ or deposited on alumina, acts as an effective catal for double bond isomerization of alkenes and cyclenes U) triggered much research in this field (2). It was subsequently discovered that base-catalyzed isomerization of olefins may proceed in homogeneous solutions using lithium ethylenediamine (3) or potassium tert-butoxide (UBuOK) in dimethyl sulfoxide (DMSO) (4). 

The existence of a palladium carbenoid has been shown by trapping the vinyl species 4 with an alkene. It is believed that treatment of an -acetylenic compound 3 with potassium tert-butoxide and a palladium(O) catalyst generated the carbenoid 4, which can either react intra-molecularly, yielding the cyclopropane 5 as only a minor component of the product mixture, or intermolecularly with norbornadiene to form two isomeric tricycles 8 and 9 as major products. 

Chloro-l-cyclopropylcyclopropanes 1 were also prepared from 1,1-dichloromethylcyclo-propane, potassium tert-butoxide and alkenes via the corresponding carbene (carbenoid), albeit in low yields. 

Reaction of bromofluorophenylmethane (for a preparation see ref 118) and potassium tert-butoxide with an alkene afforded l-fluoro-l-phenylcyclopropanes (Houben-Weyl, Vol.4/3, p233 Vol. E19b, p980). To identify the reacting species, e.g. carbene or carbenoid, the reaction was carried out without and with an equimolar amount of 18-crown-6. ° Alternatively, chlorofluorophenylmethane, in place of the bromo derivative, can be used. Reactions with bromofluorophenylmethane were performed at 25 °C (sealed tube), whilst those with chlorofluorophenylmethane at 60-80 °C. Addition of fluoro(phenyl)carbene to alkenes is at least 98% stereospecific e.g. formation of . ... 

General procedures for the preparation of 1-aryl-l-chlorocyclopropanes from alkenes, aryldi-chloromethanes and potassium tert-butoxide or methyllithium -are described... 

According to literature data, the reaction of dibromochloromethane with potassium tert-butoxide and an alkene results in the exclusive formation of 1-bromo-l-chlorocyclopropanes 1 (Houben-Weyl, Vol. El9b, pp 1589-1591). 

The base-induced monodehydrochlorination reaction was originally introduced as the second step of a convenient two-step synthesis of methylenecyclopropanes from alkenes. The first step involves carbene-type cyclopropanation of the alkene with a 1,1-dichloroalkane and either butyllithium or sodium bishexamethyldisilazanide as the base. The dehydrochlorination is then carried out by reacting the intermediate 1-alkyl-1-chlorocyclopropane with potassium tert-butoxide in dimethyl sulfoxide. For ordinary unhindered chlorocyclopropanes this procedure gives from about 60% to nearly quantitative yields of products (Table 1). The ready availability of the starting materials and reagents makes the base-induced dehydrochlorination a most useful 1,2-elimination reaction for preparation of methylenecyclopropanes. The procedure is illustrated by the synthesis of l,l-dimethyl-2-methylenecyclopropane (3) from 2-methylpropene ( ) ... 

Bromo-l-(2,5-dibromo-3-thienyl)cyclopropaneswere prepared via the addition of bromo(2,5-dibromo-3-thienyl)carbene, generated from 2,5-dibromo-3-dibromomethylthiophene and potassium tert-butoxide, to an alkene.178... 

Yet another base, potassium tert-butoxide, induces oc-elimination of dichlorofluoromethanc to form 1-chioro-l-fluorocyclopropanes with alkenes.5... 

Problem 7.45 Potassium tert-butoxide, K+OCMe3, is used as a base in E2 reactions, (a) How does it compare in effectiveness with ethylamine, CH3CH2NH2 (ft) Compare its effectiveness in the solvents te/7-butyl alcohol and dimethylsulfoxide (DMSO). (c) Give the major alkene product when it reacts with (CH3)2CC1CH2CH3. -4... 

In some elimination reactions, the less stable alkene is the major product. For example, if the base in an E2 reaction is sterically bulky and the approach to the alkyl halide is sterically hindered, the base will preferentially remove the most accessible hydrogen. In the following reaction, it is easier for the bulky tert-butoxide ion to remove one of the more exposed terminal hydrogens, which leads to formation of the less substituted alkene. Because the less substituted alkene is more easily formed, it is the major product of the reaction. 

Several 3-(2H)-pyridazinones have been prepared from monophenyl hydrazones of 1,2-dicarbonyl compounds and a variety of active methylene compounds within 1-20 min without solvent under focused irradiation conditions in the presence of carefully adjusted amounts of piperidine or solid potassium tert-butoxide (isolated yields 50-89%), in accordance with Scheme 10.109 [216]. In the synthesis of the pyridazinone 44, microwave irradiation has no specific effect, because the result (72%) was identical with that obtained by use of classical heating under the same conditions. With the dry media procedure it was possible to isolate the intermediate alkene, which was not obtained in the previously reported procedure. When the active methylene compound is a keto ester, the reaction follows a different pathway [216b]. 

Thus, hydrogenolysis of 63 affords the pyranose tautomer 64. Wittig reaction of 64 using propyltriphenylphosphonium bromide and potassiiun tert-butoxide in dry THF at room temperature for 30 min and then at 80 °C for 20 min affords an /Z mixture of the alkenes 65 in 58% yield. Hydrogenation of 65 followed by Swern oxidation of the reduced product affords the lactol 67 which has been previously transformed in three steps to canadensohde (68) (Scheme 14) [40]. 

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