P-Hydride shift reactions

A. Heterolysis of the Metal-Carbon a-Bond Homolysis of the Metal-Carbon a-Bond Oxidation of Lm iM +1-R Followed by Homolysis P-Hydride Shift Reactions P-Elimination Reactions P-Elimination of Carboxylates CO Insertion/Methyl Migration... 

However, the same reaction applied to 2-methyl-2-butanol gave no 32, which demonstrated that 35 was not formed from 34. The conclusion was thus made that 35 was formed directly from 33. This experiment does not answer the question as to whether 35 was formed by a direct shift or through a protonated cyclopropane, but from other evidence" it appears that 1,3 hydride shifts that do not result from successive 1,2 migrations usually take place through protonated cyclopropane intermediates (which, as we saw on p. 1382, account for only a small percentage of the product in any case). However, there is evidence that direct 1,3 hydride shifts by way of A may take place in superacid solutions." ... 

Hydride and 1,2-alkyl shifts represent the most common rearrangement reactions of carbenes and carbenoids. They may be of minor importance compared to inter-molecular or other intramolecular processes, but may also become the preferred reaction modes. Some recent examples for the latter situation are collected in Table 23 (Entries 1-10, 15 1,2-hydride shifts Entries 11-15 1,2-alkyl shifts). Particularly noteworthy is the synthesis of thiepins and oxepins (Entry 11) utilizing such rearrangements, as well as the transformations a-diazo-p-hydroxyester - P-ketoester (Entries 6, 7) and a-diazo-p-hydroxyketone -> P-diketone (Entry 8) which all occur under very mild conditions and generally in high yield. 

The above is a typical example of a transannular hydride shift. The 1,2-diol is formed by a normal epoxide hydrolysis reaction (0-7). For a discussion of 1,3 and longer hydride shifts, see p. 1062. 

Hydride shifts can take place directly, without the intervention of a carbonium ion intermediate, if the geometry of the system is favorable. For example, in the solvolysis of cyclohexyl-2,6-d2 tosylate in 97 percent acetic acid, 1,3-hydride shifts have been reported to account for 33 percent of the product.124 If this is so, it must be because the reaction is made facile by the proximity of the 3-axial hydrogen to the empty p orbital. 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

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]. 

The mechanism of the Cannizzaro reaction involves a hydride shift (an example of mechanism type 2, p. 1706). First OH adds to the C=0 to give 55, which may lose a proton in the basic solution to give the diion 56. 

This homologation reaction most likely proceeds via nucleophilic addition of the diazo compound to the Lewis acid complexed carbonyl, followed by 1,2-alkyl migration with concomitant loss of N2. Application of this reaction to an aldehyde (168) gives, via 1,2-hydride shift, the corresponding P-keto ester (169 equation 70). ... 

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