Hydride substitution reactions

The formation of the above anions ("enolate type) depend on equilibria between the carbon compounds, the base, and the solvent. To ensure a substantial concentration of the anionic synthons in solution the pA" of both the conjugated acid of the base and of the solvent must be higher than the pAT -value of the carbon compound. Alkali hydroxides in water (p/T, 16), alkoxides in the corresponding alcohols (pAT, 20), sodium amide in liquid ammonia (pATj 35), dimsyl sodium in dimethyl sulfoxide (pAT, = 35), sodium hydride, lithium amides, or lithium alkyls in ether or hydrocarbon solvents (pAT, > 40) are common combinations used in synthesis. Sometimes the bases (e.g. methoxides, amides, lithium alkyls) react as nucleophiles, in other words they do not abstract a proton, but their anion undergoes addition and substitution reactions with the carbon compound. If such is the case, sterically hindered bases are employed. A few examples are given below (H.O. House, 1972 I. Kuwajima, 1976). 

Heterocyclic structures analogous to the intermediate complex result from azinium derivatives and amines, hydroxide or alkoxides, or Grignard reagents from quinazoline and orgahometallics, cyanide, bisulfite, etc. from various heterocycles with amide ion, metal hydrides,or lithium alkyls from A-acylazinium compounds and cyanide ion (Reissert compounds) many other examples are known. Factors favorable to nucleophilic addition rather than substitution reactions have been discussed by Albert, who has studied examples of easy covalent hydration of heterocycles. 

Another example of a nucleophilic acyl substitution reaction, this one a substitution by hydride inn to effect partial reduction of a thioester to an aldehyde, occurs in the biosynthesis of mevaldehyde, an intermediate in terpenoid... 

Substitution reactions at the transition metal M involve, displacement of neutral (CO) or anionic (halide, hydride, or metallate) ligands, as summarized in Eqs. (48)-(52). 

These nano-objects display an organometalhc surface chemistry comparable to usual organometalhc moieties and which can be studied by classical spectroscopic methods substitution reactions leading to structural changes in the particles, the fluxional or non-fluxional behavior of surface hgands, the formation and observation of surface hydride species, the monitoring of catalytic reactions etc. 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

The decomposition of peroxides in the presence of stannum hydride is accompanied by the chain decomposition of peroxide [56]. Chain propagation occurs by the substitution reaction. 

On the basis of these results we embarked on a systematic study on the synthesis of vinyl cations by intramolecular addition of transient silylium ions to C=C-triple bonds using alkynyl substituted disila alkanes 6 as precursors.(35-37) In a hydride transfer reaction with trityl cation the alkynes 6 are transformed into the reactive silylium ions 7. Under essentially nonHnucleophilic reaction conditions, i.e. in the presence of only weakly coordinating anions and using aromatic hydrocarbons as solvents, the preferred reaction channel for cations 7 is the intramolecular addition of the positively charged silicon atom to the C=C triple bond which results in the formation of vinyl cations 8-10 (Scheme 1). 

A series of a-aryl-substituted vinyl cations were synthesized at ambient temperatures according to the reactions summarized in Scheme 1.(35, 37) In all cases the hydride transfer reaction proceeded rapidly at room temperature and the resulting vinyl cation TPFPB salts were isolated as red to brown oils or... 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

If secondary isotope effects arise strictly from changes in force constants at the position of substitution, with none of the vibrations of the isotopic atom being coupled into the reaction coordinate, then a secondary isotope effect will vary from 1.00 when the transition state exactly resembles the reactant state (thus no change in force constants when reactant state is converted to transition state) to the value of the equilibrium isotope effect when the transition state exactly resembles the product state (so that conversion of reactant state to transition state produces the same change in force constants as conversion of reactant state to product state). For example in the hydride-transfer reaction shown under point 1 above, the equilibrium secondary isotope effect on conversion of NADH to NAD is 1.13. The kinetic secondary isotope effect is expected to vary from 1.00 (reactant-like transition state), through (1.13)° when the stmctural changes from reactant state to transition state are 50% advanced toward the product state, to 1.13 (product-like transition state). That this naive expectation... 

This is not strictly correct, in that hydride, from say sodium hydride, never acts as a nucleophile, but because of its small size and high charge density it always acts as a base. Nevertheless, there are a number of complex metal hydrides such as lithium aluminium hydride (LiAlHj LAH) and sodium borohydride (NaBH4) that deliver hydride in such a manner that it appears to act as a nucleophile. We have already met these reagents under nucleophilic substitution reactions (see Section 6.3.5). Hydride is also a very poor leaving group, so hydride reduction reactions are also irreversible (see Section 7.1.2). 

Reaction between acetonitrile and the radical-cations of secondary alkyl halides is almost entirely S l in character. Both direct substitution and 1,2-hydride shift reactions occur and the products from a chiral alkyl halide such as 2-iodooctane, are almost totally racemised [25]. 

Many boron hydrides, especially the higher boranes, undergo halogenation, alkylation and other substitution reactions when treated with electrophiles. Such reactions are catalyzed by acids, yielding a variety of stable products. 

Benzofuran in boiling dioxane and acetic acid, in the presence of equimolar proportions of styrene and palladium(II) aeetate, yields 2,3-diphenyl-dibenzofuran (26%) as well as 2-styryl- (16%) and 3-styrylbenzofuran (3%). The reaction presumably involves electrophilic substitution at the 2-position of benzofuran by palladium(II) acetate, followed by addition to the alkene and loss of palladium hydride. Further reaction at the 3-position of the resultant 2-styrylbenzofuran would yield an intermediate that could undergo... 

Abstraction of a hydride from carbon is almost invariably an endothermic process. The rate of the reaction depends on the stability of the transition structure which closely resembles the product carbocation and is expected to be stabilized by the same factors, among them, substitution by X and C substituents. Nevertheless, initial interactions set the trajectory for the hydride abstraction reaction. The interaction of a C—H bond with a C substituent is shown in Figure 10.7. The feature relevant to the present discussion is that the HOMO which involves some admixture of the C—H bond has been raised in energy. Therefore, attack by electrophiles, while most likely at the n bond of the C substituent, is also possible at the C—H bond. The interaction of an X substituent with a CH bond is shown in Figure 10.7a. In general a single X or C substituent is not sufficient to activate the C—H bond toward hydride abstraction. 

Reaction of trialkoxyboranes with metal alcoholates, alcoholysis or hydride transfer reactions of tetrahydroborates with aldehydes or ketones all result in the formation of tetraalkoxobor-ates. Steric factors play an important role in these reactions. As a consequence, sec-alcohols react very slowly and tetra-r-alkoxoborates in general cannot be obtained by any of the reactions above. At elevated temperatures the tetraalkoxoborates revert to the trialkoxyborane and metal alkoxide.75 Thioalcoholysis of tetrahydroborates can also be effected but, in contrast to the situation in alcoholysis, the last hydrogen atom is more difficult to substitute, probably for steric reasons.119 Tetraalkoxoborates and tetramercaptoborates are readily hydrolyzed by water or moist air. 

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