Tert-butyl anion

In contrast, for />-fluoro carbanions, negative hyperconjugation [35] has a stabilizing effect. For example, in the perfluoro-tert-butyl anion the negative charge is not centered completely at the carbon atom, but comparably high partial charges are located on all the j5-fluorine atoms [33] (Scheme 2.110). 

Methyl anion Ethyl anion sec-Propyl anion tert-Butyl anion... 

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

FIGURE 4 10 Potential en ergy diagram for reaction of tert butyl cation with chio ride anion... 

FIGURE 4 11 Combi nation of tert butyl cation and chloride anion to give tert butyl chloride In phase overlap between a vacant p orbital of (CHbIbC and a filled p orbital of Cr gives a C—Cl (T bond... 

Is tot-butoxide anion a strong enough base to react wi th water In other words, can a solution of potassium tot-butoxide be prepared in water The pKa of tert-butyl alcohol is approximately 18. 

The reaction is believed to proceed via a six-membered cyclic transition state, analogously to the carbonyl addition of enolates, but the energy differences between boat- and chair-like arrangements are lower for x-sulfinyl carbanions69. Tor tert-butyl sulfoxides only anti- and, vn-products are obtained, arising from the approach onto the same diastereotopic face of the anion, but with different relative topicity. The exchange of lithium by zinc causes an increase of the anft-produci, but attempts to titanate the anion failed (see Table 3)69. 

Lithium and zinc tert-butyl phenylmethyl sulfoxide (1) and A-phenyl imines 2, in which the substituent R is alkenyl or aryl, react at —78 °C over 2 hours with high anti diastereoselection (d.r. >98.5 1.5)6. Shorter reaction times result in poorer yields, due to incomplete reaction. In contrast, the reaction of the sulfoxide anion with benzaldehyde is complete after 5 seconds, but shows poor diastereoselection. When the substituent R1 or R2 of the imine 2 is aliphatic, the substrates exhibit poor chemical reactivity and diastereoselection. The high anti diastereoselection suggests that if a chelated cyclic transition state is involved (E configuration of the imine), then the boat transition state 4 is favored over its chair counterpart 5. 

The anions derived from racemic alkyl and benzyl tert-butyl sulfoxides undergo 1,4-addition to a,/J-unsaturated esters to give adducts with high product diastereoselection (>5 1)10,11. Alkyl 4-methylphenyl sulfoxides were found to be less diastereoselective. In the case of 2-methyl-2-(methylsulfinyl)propanc the highly hindered 2,6-di-rer7-butyl-4-methylphenyl ester was required to prevent a competing acylation reaction. 

Analogous investigations on the reactivities of the acid and base forms were carried out with 2-methylindole (12.131) and its 4,6-di-tert-butyl derivative (Challis and Rzepa, 1975), and with pyrrole (12.132 Mitsumura et al., 1973). In the cases of the indoles the anions are about 2 x 108 times more reactive than the neutral com-... 

In the presence of a strong base, the ot carbon of a carboxylic ester can condense with the carbonyl carbon of an aldehyde or ketone to give a P-hydroxy ester, which may or may not be dehydrated to the a,P-unsaturated ester. This reaction is sometimes called the Claisen reaction,an unfortunate usage since that name is more firmly connected to 10-118. In a modem example of how the reaction is used, addition of tert-butyl acetate to LDA in hexane at -78°C gives the lithium salt of ferf-butyl acetate, " (12-21) an enolate anion. Subsequent reaction a ketone provides a simple rapid alternative to the Reformatsky reaction (16-31) as a means of preparing P-hydroxy erf-butyl esters. It is also possible for the a carbon of an aldehyde or ketone to add to the carbonyl carbon of a carboxylic ester, but this is a different reaction (10-119) involving nucleophilic substitution and not addition to a C=0 bond. It can, however, be a side reaction if the aldehyde or ketone has an a hydrogen. 

Carbo- and heterocyclic nitroarenes react with anions of tert-butyl and cumyl hydroperoxides in the presence of strong bases to form substituted o- and p-nitrophenols. Regiochemistry of the hydroxylation can be controlled to a substantial extent by selection of proper conditions (Eq. 9.52).85... 

The crystal structure of the compounds [Fe(R-Cp)(R -Cp)] [Ni(mnt)2] (R = H, R = ra-butyl, tert-butyl and R = R = diethyl, diisopropyl) consists of isolated chains of anions that are surrounded by chains of cations. In the anionic chains the [Ni(mnt)2] units are dimerized through S-S or Ni-S short contacts, the dimmers interacting through slightly longer (S—S or Ni-S) contacts. yT decreases with cooling, which is consistent with dominant AF interactions [71]. 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

FIGURE 3.23. Variation of the rate constant with the driving force in homolytic cleavage of various types of anion and cation radicals. The open symbols refer to bibenzyl derivatives and the stars to cation radicals to the tert-butyl derivatives of synthetic analogs of NADH. Adapted from Figures 8 and 10 of reference 34, with permission from the American Chemical Society. 

Tri- tert-butyl-1,3,5 -triphosphabenzene (128, Scheme 53) undergoes a ring contraction over a potassium mirror,159 with the elimination of potassium phosphine and formation of the aromatic 2,4,6-tri-tot-butyl-1,3-diphosphacyclopentadienide anion 129 with three bulky substituents (Scheme 53). The driving force of this reaction could be the higher aromaticity of the five-membered ring. 

The poly(vinylpyridine) and poly(tert-butyl methacrylate) copolymers can easily be converted to either cationic or anionic polyelectrolytes by protonation of the pyridine rings or by base hydrolysis of the tert-butyl ester units, respectively. The highly branched structure of the molecules, in combination with the polyelectrolyte effect, should confer useful properties to these materials in solution for applications such as pH-sensitive reversible gels. 

A variety of optically active 4,4-disubstituted allenecarboxylates 245 were provided by HWE reaction of intermediate disubstituted ketene acetates 244 with homochiral HWE reagents 246 developed by Tanaka and co-workers (Scheme 4.63) [99]. a,a-Di-substituted phenyl or 2,6-di-tert-butyl-4-methylphenyl (BHT) acetates 243 were used for the formation of 245 [100]. Addition of ZnCl2 to a solution of the lithiated phos-phonate may cause binding of the rigidly chelated phosphonate anion by Zn2+, where the axially chiral binaphthyl group dictates the orientation of the approach to the electrophile from the less hindered si phase of the reagent. Similarly, the aryl phosphorus methylphosphonium salt 248 was converted to a titanium ylide, which was condensed with aromatic aldehydes to provide allenes 249 with poor ee (Scheme 4.64) [101]. 

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... 

FIGURE 3.9 Deprotection of carboxyl groups by base-catalyzed hydrolysis of (A) esters and (C) trifluoroacetamides, involving direct attack by the hydroxide anion. (B) tert-Butyl esters are resistant to saponification. 

Trifluoroacetic acid removes tert-butyl-based protectors by the S vl mechanism, with the cation being trapped by the trifluoroacetate anion however, the tert-butyl trifluoroacetate produced is an alkylating agent, and the acid is not strong enough to protonate the side chains of methionine, tryptophan, and cysteine, so these are acceptors of tert-butyl. A scavenger is required to prevent their alkylation. Anisole... 

Prior to the actual metathesis event, coupling of 13 and 28 via an ester linkage was required (Scheme 2.3). Two methods were employed in this connection. The first involved the aforementioned two-carbon expansion of aldehyde 28. Thus, condensation of 28 with Rathke anion (lithiated tert-butyl acetate) generated a mixture of dia-stereomeric alcohols the major product was shown to have the requisite 3S configuration. TBS protection of ester 29 and subsequent ester hydrolysis generated the desired add, 31, which could be further esterified with alcohol 13 in 78 % yield. 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

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