Hydride Shifts

[1,3]-Hydride Shift

     NOTE: The R subgroups in the following reaction schemes are not the same as to shown chirality.

Figure 1:) A [1,3]-Hydride Shift occuring suprafacially.

Figure 2:) A [1,3]-Hydride Shift occuring anterafacially.

     NOTE:  A reaction occuring in a superfacial stance causes the migrating atom to stay above the molecular plane, as an anterafacial stance causes the migrating atom to move behind the plane, inverting the stereochemistry of the chiral center.

     Under thermal conditions, in the [1,3]-Hydride Shift above in prop-1-ene, the H migrates from C₃ to C₁ as the electrons in the C₃-H bond attack C₁, causing the a carbocation to form at C₃.  Due to C₁ now having five bonds and C₃ now having a carbocation_, the pi electrons within the double bond concertedly shift from between C₁ and C₂ to between C₂ and C₃.  This reaction is expected to occur on the antera-face, however, this shift in prop-1-ene is geometrically impossible because of a lack of a favorable(cyclic) transition state and the required molecular orbital overlaps.

[1,5]-Hydride Shift

Figure 3:) A [1,5]-Hydride Shift

     NOTE:  Since the hydride is a terminal atom with the only other terminal atoms also being hydrogens, there is no chiral center involved.

Figure 4:)  [1,5]-Hydride Shift occuring suprafacially and anterafacially.

     This reaction follows the same type of reaction as the previous [1.3]-Hydride Shift, except it follows the [1,5] pattern instead.

[1,7]-Hydride Shift 

     An example of a [1,7]-Hydride Shift occurs when 7-dehyrdocholesterol gets exposed to UV light, causing an electrocyclic ring opening to occur.  After this ring opening, a hydride shift can occur to yield Vitamin D₃.  Vitamin D₃, also called Cholecalciferol, is one of the sterols of the body than helps maintain calcium and phosphorous intake.

Figure 5:) Electrocyclic ring opening of 7-dehydrocholesterol

Figure 6:) [1,7]-Hydride Shift yielding Vitamin D