L/-»3/Hydrogen-migration

Mechanistic study revealed that the reaction proceeds through intramolecular 1,3-hydrogen migration, and the chiral rhodium catalyst differentiates the enantiotopic C-l hydrogens of allylic alcohols (Scheme 29).53... 

The actual reported results bear out this analysis. Thus a thermal [1,3] migration is allowed to take place only antarafacially, but such a transition state would be extremely strained, and thermal [l,3]-sigmatropic migrations of hydrogen are unknown. " On the other hand, the photochemical pathway allows suprafacial... 

In [l,2,3,4]-eliminations one substituent migrates from 2 to 3 and displaces a substituent from there to 4, from where the electronegative leaving group departs. The acid catalyzed dehydration of 242 (28% + 40% of the acetal of 243 with 242)1311 corresponds to this type unless a [l,2,3]-elimination would yield intermediately the enol of 243 with a [l,3]-hydrogen-shift to follow. 

Fig. 10.33. Nuclear positions and for antarafacial [l,3]-sigmatropic migration of hydrogen and for suprafacial [l,3]-sigmatropic migration of mediyl with inversion (B3LYP/6-3 ++G d p) Reproduced from J. Phys. Chem. A, 106, 11533 (2002), by permission of die American Chemical Society.

A modern method of expanding medium and large rings by two carbon atoms uses flash vacuum pyrolysis (FVP) of aUyl alcohols. Under the reaction conditions, only traces of the expected [l,5]-hydrogen migration are observed the main product arises presumably from a [l,3]-carbon shift, a process which involves a diradical intermediate. This ring-expansion sequence may also be repeated successfully. [181]... 

The migration of hydrogen through an allyl system is rearrangement of order 1, 3 because the univalent hydrogen move from C-l to C-3. 

The flash vacuum thermolysis of l-(isopropylamino)methylenemalonate (1713) at 600°C yielded iminoacrolein (1715) via imidoylketene (1714). 1,3-Methyl migration, similar to 1,3-hydrogen shift between 1711 and 1712, could not be detected giving cyanoacetone by elimination of propene (see Scheme 60). 

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