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stereochemistry of the exocyclic olefin in the triene 104 and regenerating the active catalyst.
Scheme 3.8 Mechanism of the Rh(I)-catalyzed allenic Alder-ene reaction.
E
E
D D
104 63%
E
D
CD3 E
D
D D
103
Rh(I)L n
Rh(III)DLn CD3 C
E
E
E
E
•
100 CD3 D 3C
D D
CD 3
Rh(III)L n
D
Rh(I)Ln A
B
•
Rh(I)L n
A: complexation; B: oxidative addition; C: β-deuteride elimination; D: reductive elimination.
According to the proposed mechanism, the stereochemistry of the appending olefin in the
resulting triene is set during the β-hydride elimination step. Brummond and coworkers have
shown that the selectivity for the E-isomer in the reaction of 98 (Scheme 3.7) can be drastically
increased (>100 : 1) by utilizing an in situ generated cationic Ir(I) catalyst ([Ir(COD)Cl]2,
AgBF4). This protocol, however, was limited to substrates containing a silicon group on the
terminus of the alkyne.
102
The scope of the [Rh(CO)2Cl]2-catalyzed reaction has been demonstrated on carbon,
nitrogen and oxygen tethered allenynes. With most substrates, the reaction proceeds at rt
affording a variety of trienes in good yields (Scheme 3.9).
35
E
E
101
CD 3
CD 3