Intermediate hydrides

As shown in Figure 16.10, this reaction mechanism involves nucleophilic attack by —SH on the substrate glyceraldehyde-3-P to form a covalent acylcysteine (or hemithioaeetal) intermediate. Hydride transfer to NAD generates a thioester intermediate. Nucleophilic attack by phosphate yields the desired mixed carboxylic-phosphoric anhydride product, 1,3-bisphosphoglycerate. Several examples of covalent catalysis will be discussed in detail in later chapters. 

Assuming equilibrium between B2H6 and BH3 and a steady-state concentration for the intermediate hydrides , this gives a rate expression... 

Intermediate hydrides is used here instead of B3H7 by the authors. 

BH3 + B2H6 -> intermediate hydrides intermediate hydrides+B2H6 - BjHn-(-2 H2 B5Hu - b5h9 + h2 BjHn -v higher hydrides+B2H6... 

Deactivation can take place if the intermediate hydride or alkyl species react with HCN. The result is the formation of nickel dicyanides, which cannot be reconverted to active species. We think that Lewis acids, in addition to promoting reductive elimination, may also have a retarding effect on this reaction, see Figure 11.4, as a more positively charged nickel species will react more slowly with protons. On the other hand, though, stronger acids are formed as a result of the interaction of HCN with the Lewis acids. 

The distinction between the two representations not a rigid one, hinging on the lifetime of the intermediate hydride ion. If the latter is very short (of the order of the time of a single vibration or collision), the two mechanisms clearly become equivalent. 

A further novel method for demetallation provides even higher yields. Hieber-type reaction of the tricarbonyl(ri4-cydopentadienone)iron complex with sodium hydroxide to the corresponding hydride complex followed by ligand exchange with iodo-pentane affords an intermediate iodoiron complex, which is readily demetallated in the presence of air and daylight at room temperature (Scheme 1.4) [9]. Combining steps a-c in a one-pot procedure without isolation of the intermediate hydride complex gave yields of up to 98%. 

As a side product in these reactions a novel bridged steroid alkaloid 230 is formed, presumable by an intermediate hydride shift in 227 from the benzylic position to the iminium ion to give a secondary amine, which then attacks the formed cationic benzylic position. This reaction becomes the main reaction with aniline, p-bromoaniline or nitroaniline in the presence of BF3 -OEt2 if a derivative of 224 is used containing a propyl instead of a propenyl side chain [61]. 

Hydride reduction of methyl 3-deoxy-3- dimethylamino)-2,4,6-tri-O-p-tolylsulfonyl-a-D-rtho-hexopyranoside in tetrahydrofuran gave the corresponding 6-deoxy sugar which, on hydrolysis, afforded 3,6-dideoxy-3-(dimethylamino)-j8-D-riho-hexose this was found to be identical witii mycaminose, a component of an antibiotic substance. Syntheses of 4,6-dideoxy-3-0-methyl-D-xyi o-hexose (chalcose) have been reported by the intermediate hydride reduction, in tetrahydrofuran, of 6-p-toluenesulfonates methyl 4-deoxy-6-0-p-tolylsulfonyl-a-D-xy/o-hexopyranoside afforded, by such treatment, the corresponding 6-deoxy glycoside, and methyl 4-deoxy-3-0-methyl-2,6-di-O-p-tolylsulfonyl-a-D-xi/Zo-hexopyranoside gave a mixture of five products, from which methyl a-D-chalcoside was isolated in 26% yield. 

Evidence later became available that indicated that the lack of isotope incorporation from solvent in enzymic experiments is misleading. Trapping experiments have established quite conclusively that an enediol (38) is a kinetically competent intermediate. Hydride transfer occurring through a transition state like (39 equation 22) is therefore not required. ... 

To identify the hydride source, we used per-deutero naphthalene in place of per-protio naphthalene. If the intermediate hydride existed on naphthalene, one might expect a statistical mixture of HD and D2. In fact. 

Since Pd(II)-hydride is generally not stable to decomposition to Pd(0) and the intermediate hydride has not been isolated. Stable Pd(II) hydrides have been reported, but this is not direct evidence they are intermediates in the oxidation. However, Maitlis, in his model system for Pd(II) catalysis, has been able to demonstrate Pd(II)-hydride elimination to give a stable hydride (133, 182) (see Section V, A). 

Similar intermediate hydrides are represented by examples 6 and 7 above (9). The names suggested show clearly their relationship to 4, 5, and 8. 

The hydrides of V, Nb, and Ta probably come closest to the idea of an interstitial compound and are discussed later. In other cases where the arrangement of metal atoms in the hydride is the same as in (one form of) the metal there may be a discontinuous increase in lattice parameter when the hydride is formed (Pd) or there may be an intermediate hydride with a different metal arrangement. For example, the h.c.p. 4f metals Gd-Tm form hexagonal trihydrides, but the intermediate dihydrides have the fluorite structure with c.c.p metal atoms. 

A) than the metal (5-49 A) and there is an intermediate hydride YbH2 with the quite different (CaH2) structure already noted. 

The reactions outlined above and in Table 1 can be generalized to other lone-pair (n-donor) ligands. ir-Donor ligands such as olefins, also behave in an analogous fashion, but Mo(CO) CjHj-h , produced by normal photosubstitution, yields an intermediate hydride upon irradiation ... 

Cyclooctadiene complexes XLVI yield 7t-allyl complexes XLVII when reacted with an isopropyl organomagnesium halide reagent in the presence of 1,3-pentadiene. Rapid p elimination gives an intermediate hydride, which adds to the diene. With isoprene, an... 

---