Hydrogen Migration Reactions

Compound 46 (phosphine ligand = Ph2PCH2CH2CH2PPh2) maintains the same structure in solution as in the crystal but undergoes a rapid exchange of the bridging hydride ligands (H with H ) without exchange of either with the 

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Studies in stereochemistry. VI. The mechanisms of the E-1 and hydrogen migration reactions in the 3-phenyl-2-butanol system. J. Amer. chem. Soc. 74, 2137 (1952). 

Styrene derivatives may be obtained directly from the carbene, by internal hydrogen migration (reaction 3), or from the complex59 (25). 

Hydrogen migration reactions during the irradiation of (167) have been studied. Several examples of this process have been described previously. " A detailed study of the cycloreversion reaction of the tetraphenylanthracene endopteroxide (168) has been reported. ... 

Two other hydrogen migration reactions have been observed. One of these leads to ethylene and acetylene ... 

If the reactant is a mixture of cis- and frans-l,3,5-hexatriene, all the products mentioned above are readily formed, no induction period being observed. In addition, a hydrogen migration reaction is believed to originate in the triene. 

The hydrogen migration reaction to give 1,2,4-hexatriene is observed in the gas phase as well as in solution. It is not quenched by an increase in pressure, nor is it observed during pyrolysis of 1,3,5-hexatriene. It has hence been taken to be a reaction which proceeds from the electronically excited singlet state. Based on exactly the same reasoning, the process... 

The isomerization of c -l-methyl-2-vinylcyclopropane (1) to (Z)-hexa-1,4-diene (3) at 180 C in the gas phase, first described in 1964, is the prototype for all homo [1,5] hydrogen migration reactions of substituted cyclopropanes. 

In summary, the pyridine ylide method has told us quite a lot about 1,2-hydrogen migration reactions of invisible carbenes. We learned that simple aUcyl and dialkyl carbenes are true intermediates with nanosecond lifetimes. The pyridine ylide method revealed that the rate of rearrangement of alkylcarbenes are influenced by the cationic stabilizing power of X k. increases as X = FI < CH ... 

Scheme 43. Chemoselectivity of palladium catalysts in carbene dimerization and hydrogen migration reactions.

H2C=Os=CH2 < 0=0s=0 (Table 2). The calculations also showed that hydrogen migration reactions and rearrangements may compete with the [2-1-2] and [3-1-2] addition reactions of ethylene to 0s02(CH2)2 [143]. 

Ninomiya et al. [81 ] investigated the formation of 2-, 3-, and 4-pyridyl radicals by C—H bond fission from pyridine. The calculation results show that the heats of formation are 532.20, 551.87, and 550.61 kJ/mol, which correspond to 2-, 3-, and 4-pyridyl radicals, respectively. It can be seen that the energy of hydrogen migration reaction is smaller than that of C—H bond dissociation in pyridine and the sequence of energy does not change. 

There will be circumstances other than those I have described here in which "high oxidation state" organometallic chemistry of rhenium in a catalytic reaction will be viable, although it is becoming clear that the balance necessary to achieve this feat is more difficult to maintain as one moves to the right in the transition metal series, and that some of the d rhenium chemistry in fact may look like chemistry of dP osmium species. On this basis it would seem unlikely that the principles that have been used to prepare Re(VII) alkylidyne and alkylidene complexes (a hydrogen migration reactions) can be extended further (to technetium, or especially osmium or ruthenium), at least in a routine fashion. 

The diene epoxides having a transferable hydrogen atom in the C-2 alkyl substituent undergo type A reaction by hydrogen migration reaction. Some examples are shown in Scheme 2. 

The appearance of the complex (XXXIV) is interesting in that its formation must involve hydrogen transfer from the free diene or its complexed form. Although intramolecular hydrogen migration reactions are frequently encountered, such intermolecular hydrogen transfer reactions are most uncommon. 

Shapiro, R.H., and Djerassi, C. (1964) Mass spectrometry in structural and stereochemical problems. L. Fragmentation and hydrogen migration reactions of a,P-unsaturated 3-keto steroids. Journal of the American Chemical Society, 86,2825-2832. 

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