Hydride displacement

Nucleophilic substitutions of simple aromatic compounds which formally involve a hydride displacement are difficult to achieve because of the poor leaving group and the high electron density of the aromatic nucleus which repels approach of a nucleophile. However, rc-electron deficient aromatic compounds such as metal carbonyl complexes are susceptible to attack by certain carbon nucleophiles. Studies of this chemistry have shown [16] an opposite jegioselectivity to the corresponding electrophilic substitutions, in agreement with the polarity alternation rule. 

To suppress enamine-derived side products, we explored addition of benzotriazole (BtH) to the reaction mixture. The premise behind these experiments was the ability of BtH to form stable adducts with imines,23,24 thereby blocking tautomerization of 19 to 20 through in situ formation of the benzotriazolyl derivative 21. It was hoped that subsequent hydride displacement of the Bt moiety would afford the desired mono alkylated products 23. Indeed, analytical high-performance liquid chromatography (HPLC) revealed a remarkable improvement in terms of product purity, especially for reactions carried out at room temperature, with the desired secondary anilines 23 being essentially the only products detected. In... 

Mean cobalt-cobalt and nickel-nickel distances observed in these complexes are very close to interatomic distances determined at ambient temperatures in cobalt and nickel metals (Co-Co 2.489(7) A vs. 2.507 A in a-cobalt (33) Ni-Ni 2.469(6) A vs. 2.492 A in the metal (39)). The mean M-H bond lengths, as well as hydride displacements from M3 faces, are less for nickel in H3Ni4(Cp)4 than for cobalt in HFeCo3(CO)9(P(OMe)3)3. Although the differences are marginally significant within error limits (Ni-H 1.691(8) A vs. Co-H 1.734(4) A displacements from plane Ni3 0.90(3) A vs. Co3 0.978(3) A), they are in the expected direction since the covalent radius should vary inversely with atomic number within a transition series. However, other effects such as the number of electrons in the cluster also can influence these dimensions. 

We recognized that the isoelectronic hydrides H2Fe(CO)4 and HCo(CO)4 behaved as pseudo-nickel tetracarbonyls (hydride displacement principle). Moreover, manganese pentacarbonyl hydride (VII, 11), on which we later worked in Munich, was similar in many physical properties to iron pentacarbonyl, leading us to postulate a Drawing-in of the hydrogen... 

Reactions in which complex metallic hydrides (LiAlH4, etc.) act as sources of nucleophilic hydride are of considerable synthetic value (Gaylord, 1956 Wiberg and Amberger, 1971). In the case of nitro-aromatics, some interesting hydride displacements [e.g. (38)] have... 

The ammonium ion NH4+ has a tetrahedral structure and is slightly acidic (p/fa = 9.25). If no strong hydrogen bonds are present, like in NH4F, the properties of ammonium salts, for example, the solubility and the structure, are similar to the corresponding potassium and rubidium salts. The reasons are similar ionic radii of the aimnonium (143 pm), potassium (137 pm), and rubidium ions (148 pm). The oxonium salts H30+X , which have lower melting points, are also similar to the ammonium salts. The similar properties of element-hydrogen compounds have been rationalized by the hydride displacement law shown in Table 12. 

Alkylation of (a-tetralone)chromiumtricarbonyl 37 with MeLi and subsequent ionic hydrogenolysis with excess EtsSiH and CF3CO2H gave (1-endo-methyltetralin)-chromiumtricarbonyl (39) via stereoselective exo-hydride displacement on 38. On the other hand, the endo-acstate complex 40 was converted into (1-exo-methyltetralin)-chromiumtricarbonyl (41) via exo-methyl attack to the carbocation by treatment with MejAl (Sch. 23) [47]. 

Hydride Displacement. The first preparation of H3BNH3 was effected by hydride displacement using BH4 and an NH4" salt 19). The appropriate equation is BH4 + NH4+ H3BNH3 + H2. Further replacement of H to produce cations can be effected by carrying out the process at a higher temperature H) ... 

More Complete Hydride Displacement. Miller and Muetterties (14) found that complete hydride displacement could be effected by several different anions. Typical reactions are summarized below ... 

Cyclopentene oxide is very unreactive (k < l( itcyclohexene oxide), an observation clearly not in accord with expected Sn2 reactivity it gives cyclopentanone as the only observed product. This ketone cannot be formed by antiperiplanar hydride displacement of bromide from the tra/is-bromohydrin, whereas the c/5-bromohydrin could easily adopt the necessary conformation for hydride migration. Almost certainly the latter is formed by slow, perhaps rate-determining 5n2 attack by bromide on the initially formed trans-bromohydrin salt, as outlined in equation (121). This secondary reaction may be more important with Lil than with LiBr, although this has not been proven (see also equation 83 and discussion of MgX2-catalyzed reactions). 

The Hydrolysis of cis and trans Cyclohexene Chlorohydrin. When trans cyclohexene chlorohydrin is treated with strong alkali, trans cyclohcxene glycol is obtained. Thus inversion does not appear to have occurred during the displacement. When the reaction is carried out with the cis isomer, cyclohexanone is formed.27 These facts can be explained if it is assumed that the first step in the reaction is the removal of a proton from the hydroxyl group (XXI to XXIII and XXII to XXIV), followed by the displacement of chloride ion by the alkoxide ion of the trans form (XXII1 to XXV) and by a hydride ion in the cis form (XXIV to XXVII). Displacement by alkoxide leads to cyclohexene oxide (XXV), and this intermediate can be further attacked by hydroxide ion to give the dl trans glycol (XXVI). Cyclohexanone is produced by the hydride displacement (XXIV to XXVII) ... 

The concept of isosters was then broadened by Grimm in 1925 with the statement of Hydride Displacement Law, and, further on, Erlenmeyer extended Grimm s... 

V TABLE 15.2 Hydride Displacement Law In Each Vertical Column the Atom is Followed by its Pseudoatoms ... 

Later, in 1925, Grimm formulated the hydride displacement law , according to which the addition of hydrogen to an atom confers on the aggregate the properties of the atom of the next highest atomic number. An isoelectronic relationship exists among such aggregates which were named pseudoatoms. Thus, when a proton is added to the 0 ion in the nuclear sense, an isotope of fluorine is obtained (Fig. 13.1). 

Table 13.1 Hydride displacement law in each vertical column the atom is followed by its pseudoatoms...

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