Lead tert-butoxide

However, thermolysis of the phosphonium salts (X=+PPh3) leads directly to the indolic products without need of acid catalyst or PPh3, and thus may not proceed via a normal Wittig pathway. Alternatively, Hughes has effected a solid-phase version of this reaction employing a polymer-hound phosphonium salt and potassium tert-butoxide as base <96TL7595>. In this case, the phosphine oxide by-product remains bound to the polymer resin. 

This is manifest in the reactivity of 180/180-Z1 which was generated from 3-bromo-41-f-pyran (283) by /3-elimination of hydrogen bromide with KOtBu (Scheme 6.61). Whether or not this reaction was conducted in the presence of styrene or furan, the only product identified was tert-butyl 4H-pyran-4-yl ether (284). This is in line with the relationship of the intermediate to a pyrylium ion. Thus, the addition of the tert-butoxide ion to 180/180-Zj has to be expected at the 4-position with formation of the vinyl anion 285, which is then protonated to give 284. Likewise, the attack of the nucleophile is predicted at C2and C6 leading to the vinyl anions 286, which... 

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). 

The cydization can also be carried out on a-tosylaminoallenes, in which case the choice of reaction conditions determines whether the product is the N-tosyl-3-pyrro-line or whether elimination of toluenesulfonic acid acid gives the pyrrole. For example, in the presence of catalytic silver nitrate, allene 141 (Eq. 13.47) rearranges to N-tosylpyrroline 142 in excellent yield, whereas when 141 is treated with potassium tert-butoxide in DMSO, pyrrole 143 is formed in 71% yield [54]. Warming the lithium salt of 141 in DMSO also leads to 143. The rearrangement of 141 to 143 may be mechanistically related to the conversion of 130 to 131 (Eq. 13.42). 

Similarly, the intramolecular alkylation of lithium 3-(4-camphorsuIfonyloxybutyl)-2,4,6-trimeth-ylphenolate using potassium tert-butoxide as base, leads under reflux to bicyclo[4.4.0]decadiene derivatives5,6. 

Reaction of that with potassium tert-butoxide affords the corresponding carbanion this is thought to first add to the enone in (5-3). The anion from the reaction with a second equivalent of base then adds to the enone function to form the spiw ring. The fact that the product from this reaction has the same relative stereochemistry as the natural product is attributed to the better overlap of the enolate with the triple bond in the transition state leading to that isomer. The product from the reaction is thus + griseo-fulvin (5-6) [5]. 

The aniline nitrogen is then converted to the para-toluenesulfonamide (4-3). Reaction of this intermediate with ethyl co-chlorobutyrate in the presence of potassium carbonate then gives the alkylation product (4-4). Potassium tert-butoxide-catalyzed Claisen condensation of this diester leads to azepinone (4-5) as a mixture of methyl and ethyl esters resulting from alternate cyclization routes. A strong acid leads to the transient keto-acid, which then decarboxylates the toluensulfonyl group is lost under reaction conditions as well as affording the benzazepinone (4-6). This last intermediate is then acylated with the benzoyl chloride (4-7) to afford amide (4-8). 

Triazenes have been prepared by the treatment of resin-bound aromatic diazonium salts with secondary amines (Figure 3.27). Regeneration of the amine can be effected by mild acidolysis (Entry 1, Table 3.23). Triazenes have been shown to be stable towards bases such as TBAF, potassium hydroxide, or potassium tert-butoxide [454], and under the conditions of the Heck reaction [455]. Primary amines cannot be linked to supports as triazenes because treatment of triazenes such as R-HN-N=N-Ar-Pol with acid leads to the release of aliphatic diazonium salts into solution [373]. Triazenes derived from primary amines can, however, be used for the preparation of amides and ureas (see Section 3.3.4),... 

The Buchwald-Hartwig reaction was performed under a variety of conditions starting with the model compounds and leading up to target substrate. The results are shown in Table 3. There was a 70 % yield obtained in the reaction of triflate 35 with the cyclic secondary amine morpholine using condition A (entry l),32 however the reaction fails with diphenethylamine (entry 2). Changing the base to sodium tert-butoxide (condition B) or the catalyst to Pd2(dba)3 and ligand (condition C) also resulted in no reaction (entries 3 and 4). 

The addition of a selenium-stabilized carbanion to an electrophile can be followed by another reaction as selenones are good leaving groups. a-Selenonylalkyl compounds 111 can be deprotonated using potassium tert-butoxide. Reactions with a,/ -unsaturated / r/-butyl esters lead to cyclopropane derivatives 112 in good yields (Scheme 26).197... 

In trans-1 -bromo-2-fluorocyclohexane, E2 elimination occurs in the treatment of the compound with sodium methoxide or potassium tert-butoxide. There is only one hydrogen antiperiplanar to bromine, and its elimination leads to 3-fluorocyclohexene W. On the other hand, when, sodamide is used as a base, hydrogen fluoride is eliminated, not by the E2 mechanism but by a cA-elimination leading to X, 1-bromocyclohex-ene, probably by Elcb (carbanion) mechanism [132. ... 

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. 

Treatment of certain 2-(hydroxymethyl)aziridines with base can lead to an intramolecular ring-opening reaction to yield an cc-amino epoxide, a reaction analogous to the Payne rearrangement of (hydroxymethyl)epoxides. For example, treatment of the aziridine 109 with potassium tert-butoxide provides the epoxide 110 in good yield. 

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... 

For the case of tri(o-tolyl)phosphine-ligated catalysts, the upper pathway appears to predominate. Oxidative addition occurs first via loss of a ligand from the bisphosphine precursor to form the oxidative adduct, which exists as a dimer bridged through the halogen atoms (equation 33). This dimer is broken up by amine, the coordination of which to palladium renders its proton acidic. Subsequent deprotonation by base leads to the amido complex, which can then reductively eliminate to form the product. When tert-butoxide is used as the base, the rate is limited by formation of and reductive elimination from the amido complex, while for the stronger hexamethyldisilazide, the rate-determining step appears to be oxidative addition. ... 

Experimental conditions leading to good yields of olefins comprise heating the hydrazone or semicarbazone of the ketone with potassium tert-butoxide in anhydrous toluene/7]. The reason for the product difference is not known, but may be associated with preferential protonation at C(S) arising from the particular charge distribution and geometrical relationship in a specifically solvated ion-pair of the type (9), which would be favoured by the non-polar solvent. 

The reaction of a phosphonate ester, DBU, Nal, and HMPA with an aldehyde leads to a conjugated ester with excellent (Z)-selectivity. A (Z)-selective reaction was reported using a trifluoroethyl phosphonate in a reaction with an aldehyde and potassium tert-butoxide. ... 

If extreme steric hindrance is present in the ot-halo ketone substrate, base treatment can, however, lead to the isolation of a bulky cyclopropanone. The classic example in this area is the 1,3-dehydrobromination of a-bromo(dineopentyl) ketone 4 with potassium tert-butoxide in tert-butyl alcohol or diethyl ether, or with potassium 4-chlorophenyldimethylcarbinolate, ° giving ra i-2,3-di-rert-butylcyclopropanone 5 in 30-75 and 20-40% yields, respectively. In this case, it is crucial to use slightly less than one equivalent of base (max 0.95 equiv) since even a small excess results in complete conversion into the Favorskii ester. ... 

The corresponding reaction of the larger dichloro homologs with potassium tert-butoxide leads to didehydrochlorination (see Section 5.2.2.1.2.). 

There is one known example in which a monodehydrobromination reaction leads to the formation of an alkynylcyclopropane. When 4-bromo-l,l-dimethylspiropentane (1) was treated with excess potassium tert-butoxide in dimethyl sulfoxide at 25 °C, a mixture of hydrocarbons was obtained from which 2-ethynyl-l,l-dimethylcyclopropane (4) was isolated in 30% yield. 

Reaction of the j8-hydroxysilanes with thionyl chloride in triethylamine followed by treatment with fluoride, or with potassium hydride or tert-butoxide, leads to elimination of both the hydroxy and silyl groups and to the formation of alkylidenecyclopropanes (see Section 52.2.1.2). 

In contrast, reaction of the dichlorocarbene adduct to 9-mcthoxyphenanthrene with potassium tert-butoxide did not lead to the cycloproparene, but proceeded by dehydrochlorination to 1 -chloro-9-methoxy-laf/-cyclopropa[/]phenanthrene (16) as a reactive intermediate. This then rearranged to a vinylcarbene which, in turn, underwent intramolecular C —H bond insertion and aromatization to phenanthro[9,10-h]furan (17). ... 

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