Hydrides with bases

Experimental evidence exists for the direct reaction of some nonclassic hydrides with bases the nonclassical hydride (rather than the classical hydride) of [IrH(H2)(bq)L2]" is deprotonated by RLi and [CpRufdmpeXHz)] is deprotonated by Et3N in preference to its equilibrium dihydride form . However, direct involvement of nonclassical hydrides in stoichiometric or catalytic reduction of substrates is still not demonstrated a stoichiometric hydrogenation of 1-hexene has been reported using a dinuclear Ru-molecular hydrogen complex. ... 

In a similar manner it was shown (47) that the enamino ketone (54) on treatment with ethyl iodide gave an intermediate hydroiodide from which 7-ethoxy-2,3,4,5,6,10-hexahydroquinoline (55) was obtained on treatment with base. The corresponding N-alkylated product was obtained by alkylation of (54) in the presence of sodium hydride. 

Important and widely used variants of the Wittig reaction are based on carbanionic organophosphorus reagents, and are known as the Wadsworth-Emmons reaction, Wittig-Horner reaction or Horner-Wadsworth-Emmons reaction. As first reported by Horner, carbanionic phosphine oxides can be used today carbanions from alkyl phosphonates 13 are most often used. The latter are easily prepared by application of the Arbuzov reaction. The reactive carbanionic species—e.g. 14 —is generated by treatment of the appropriate phosphonate with base, e.g. with sodium hydride ... 

Substituted dibenz[6,/]oxepin-10(l l//)-ones can be reduced with sodium borohydride or lithium aluminum hydride and treated with thionyl chloride to afford the chloro derivatives which undergo dehydrohalogenation when heated or treated with base to give products... 

A polymer containing side-chain benzylphosphonium residues has been prepared and used in olefin synthesis. A suspension in THF was treated with base and benzaldehyde overnight and the polymeric phosphine oxide was then removed by filtration. The yields of stilbenes, 40% with potassium t-butoxide and 60% with sodium hydride, were not improved by using an excess of base or of aldehyde. 

As a practical matter, the alkoxide used as the base must be the same as the alcohol portion of the ester to prevent product mixtures resulting from ester interchange. Sodium hydride with a small amount of alcohol is frequently used as the base for ester condensation. The reactive base is the sodium alkoxide formed by reaction of sodium hydride with the alcohol released in the condensation. 

Many acylation reactions of esters using sodium hydride as base appear auto-catalytic, with considerable potential for runaway, since the active base in solution is an alkoxide and the alcohol is a product of reaction [4], A safe form of sodium hydride (as a solid solution in a halide) for large-scale industrial use has been claimed [3],... 

With Pd(0) generated in situ, the oxidative addition of aryl bromide 102 to Pd(0) proceeds to form Pd(II) intermediate 104. Migratory insertion of 104 then occurs to furnish the cyclized indoline intermediate 105. Subsequent reductive elimination of 105 takes place in a cis fashion, giving rise to exo-cyclic olefin 107, which then tautomerizes spontaneously to the thermodynamically more stable indole 103. The reductive elimination by-product as a palladium hydride species 106 reacts with base, regenerating Pd(0) to close the catalytic cycle. 

The reaction starts with the oxidative addition of an aryl halide (Cl, Br or I) to palladium zero. The next step is the insertion of an alkene into the palladium carbon bond just formed. The third step is (3-hydride elimination giving the organic product and a palladium hydrido halide. The latter reductively eliminates HX, which reacts with base to give a salt (Figure 13.15). 

Pines and Kolobielski (18) have shown that phenylcyclohexene, although it is not a cyclic diolefin, will also undergo reactions similar to those that cyclic diolefins undergo when treated with base catalysts. When heated to 200-220 with a sodium-benzyl-sodium catalyst, it underwent a hydrogen transfer reaction resulting in the formation of biphenyl and of phenyl-cyclohexane molecular hydrogen was not produced. The mechanism of this reaction may be pictured as an elimination of sodium hydride from one molecule with the hydride ion being accepted by another molecule (A"-E"). 

Let US use a systematic approach to consider what product is most likely to result when a mixture of an ester and a ketone, both capable of forming enolate anions, is treated with base. For example, consider an ethyl acetate-acetone mixture treated with sodium hydride in ether solution. 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

Reaction of the products of reduction with lithium aluminum hydride with water is very exothermic. This is especially true of reductions in which an excess of lithium aluminum hydride has been used. In such cases it is advisable to decompose the unreacted hydride by addition of ethyl acetate (provided its reduction product - ethanol - does not interfere with the isolation of the products). Then normal decomposition with water is carried out followed by acids [5i] or bases [121]. 

An important complement to the Wittig reaction is the reaction of phosphonate carbanions with carbonyl compounds.151 The alkylphosphonate esters are made by the reaction of an alkyl halide, preferably primary, with a phosphite ester. Phosphonate carbanions are more nucleophilic than an analogous ylide, and even when R is a carbanion-stabilizing substituent, they react readily with aldehydes and ketones to give alkenes. Phosphonate carbanions are generated by treating alkylphosphonate esters with bases such as sodium hydride, w-butyllithium, or sodium ethoxide. Alumina coated with KF or KOH has also found use as the base.152... 

C-5 substituent either a pyrimidine (98) (when = NR2) or an isothiazole (99) (when R = Ar) are finally obtained. Sodium hydride as base resulted in ring fragmentation to give the open-chain adduct (100). Multistep mechanisms are proposed for these transformations (89BCJ1086>. The behavior of methylthio and dialkylamino salts (5 R = SMe and NRj, respectively) is quite similar and both give with carbon nucleophiles (e.g. Meldrum s acid) the same alkylidene derivative (97) (X , = Meldrum s acid residue) <(85CC696,88JCS(P1)899>. 

The base employed by Brown and Yamashita was the potassium salt of 1,3-diaminopropane, prepared by reaction of potassium hydride with the solvent of the reaction, 1,3-diaminopropane. The reagent is very effective, and yields of isomerically pure products are high, but potassium hydride is hazardous, expensive and difficult to handle. 

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