Hydrogen rearrangements

Under Favorskii conditions o. o -dihalo ketones 7, as well as a,a -dihdlo ketones, bearing one a -hydrogen, rearrange to give a ,/3-unsaturated esters 8 ... 

With secondary amines, cleavage of the bond j8 to the nitrogen atom occurs preferentially at the shortest hydrocarbon chain. If the shortest hydrocarbon chain has three or more carbon atoms, a, /3-cleavage occurs with a hydrogen rearrangement. 

A loss of 55 is possibly the loss of C4H7 from esters (double hydrogen rearrangement). The loss suggests a butyl or isobutyl group, especially when m/z 56 is also present. 

Benzoates (by double hydrogen rearrangement) (The ester must be ethyl or higher.)... 

Numerous other reactions can be used to access phosphaalkenes. For example, treating the primary phosphine Mes PH2 with CH2CI2 in the presence of KOH gives Mes P=CH2 [54]. In addition, interesting reactions of tantalum-or zirconium-phosphinidenes with aldehydes have afforded phosphaalkenes [55, 56]. The 1,3-hydrogen rearrangement of secondary vinylphosphines to phosphaalkenes has also been used to prepare phosphaalkenes [57,58]. 

Figure 1. Hydrogen rearrangment and acid-dissociated reactions of nitromethane...

In either neutral molecules or acid-dissociated anions, the nitro-type species are more stable than the aci-nitro-type species. The 1,3-intramolecular hydrogen rearrangment is a high barrier process. In the tautomeric system formed via the 1,3-hydrogen shift, the equilibrium is therefore strongly displaced to the side of nitro-type species. 

Schwarz, H. Some Newer Aspects of Mass Spectrometric Ortho Effects. 73, 231-263 (1978). Schwarz, H. Radical Eliminations From Gaseous Cation Radicals Via Multistep Pathways — The Concept of Hidden Hydrogen Rearrangements, 97, 1-31 (1981). 

The similarity of the MS spectra of isoterpinolene (11), terpinolene (12), a-terpinene (13) and the alio-ocimene (7) is striking. Whereas the hydrogen rearrangements suggested to explain this similarity might be speculative, they offer a reasonable explanation for the almost identical MS of the open and closed diene structures with that of the triene (7) spectrum. 

The above studies are important in providing a start towards an understanding of energy barriers involved in hydrogen rearrangements in tetranuclear hydrido clus-... 

As hydrogen rearrangements prior to dissociation are prevalent in alkenes, the radical site migrates along the chain, thereby obscuring the location of the double bond. [60]... 

Djerassi, C. Fenselau, C. Mass Spectrometry in Structural and Stereochemical Problems. LXXXIV. The Nature of the Cyclic Transition State in Hydrogen Rearrangements of Aliphatic Ethers. J. Am. Chem. Soc. 1965, 87, 5747-5762. 

Kingston, E.E. Eichholzer, J.V. Lyndon, P. McLeod, J.K. Summons, R.E. An Unexpected y-Hydrogen Rearrangement in the Mass Spectra of Di-Ortho-Suhstituted Alkylbenzenes. Org. Mass Spectrom. 1988, 23,42-47. 

Duffield AM, Beugelmans R, Budzikiewicz H, Lightner DA, Williams DH and Djerassi C (1965) Mass spectrometry in structural and stereochemical problems. LXIII. Hydrogen rearrangements induced by electron impact on N-n-butyl- and N-n-pentylpyrroles. J Am Chem Soc 87, 805-810. 

A contribution of QMT in solution at room temperature is also observed in 1,2-hydrogen rearrangement of a singlet carbene. ° ° ... 

While geometrical isomers usually give very similar MS, this is not always the case. An interesting example is that of cis- and trans- 2,4-dimethyloxetane (71ACS763). The principal fragment ion from the trans isomer (16) was the propene radical cation (m/e 42), while that from the cis isomer (17) was protonated acetaldehyde (m/e 45), apparently a hydrogen-rearrangement product. 

The base peak of straight-chain nitriles between C4 and C9 is m/z 41. This peak is the ion resulting from hydrogen rearrangement in a six-membered transition state. 

Minachev, Eidus et al. (25) found that Ca, Ni, Co, and NdY zeolites were active in the disproportionation of propylene to ethylene and butenes. The process was accompanied by hydrogen rearrangement to form saturated hydrocarbons and condensation products. The selectivity in this reaction depends on the composition of the catalysts, their pretreatment, and the experimental conditions (26). 

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