ca01 only detailed ToC 1..24

74 Science of Synthesis 1.1 Organometallic Complexes of Nickel
It is unclear whether the hydroalumination proceeds by hydronickelation or aluminonickelation,
and the mechanistic details may vary according to substrate and catalyst
structure. However, the pathways shown in Scheme 75 are likely mechanistic pathways
based on the data obtained.
Scheme 75 Mechanism of the Hydroalumination–Ring-Opening Sequence [169–171]
O
H Ni AlR2 2
O
H Ni AlR 2 2
O
R 2 2Al Ni H
O
H AlR 2 2
O
product
A related procedure for the cleavage of allyl ethers has been developed by Ogasawara.
Treatment of an allyl ether with diisobutylaluminum hydride and catalytic [1,3-bis(diphenylphosphino)propane]dichloronickel
leads to facile cleavage of the allylic C-O
bond (Scheme 76). [172,173] A mechanism similar to that proposed for the hydroalumination–ring
opening of oxabicyclic alkenes is suggested.
Scheme 76 Nickel-Catalyzed Allyl Ether Cleavage [172,173]
R 1 O
95
DIBAL-H (1.5 equiv)
1 mol% NiCl2(dppp) R 1 O
AlBui 2
H
R 1 = 4-MeOC6H4 90%
R 1 OH +
Reductive Ring Opening; General Procedure: [171]
The reaction scale was typically 0.3–0.5 mmol. [Ni(cod) 2](2; 0.1 equiv) in toluene (1.0 mL)
was added to dppb (0.2 equiv), and the resulting light brown soln was stirred at rt for 1 h.
Substrates bearing a free hydroxy moiety were premixed with 1.0 M DIBAL-H in hexanes
(1.1 equiv) in toluene (1.0 mL) to form the aluminum alkoxide. Protected alcohols were
dissolved in toluene (1.0 mL) and added directly to the flask containing [Ni(cod) 2] and
dppb. DIBAL-H (1.1–1.9 equiv) was added via a syringe pump over the indicated time. The
reaction was quenched by the addition of sat. aq NH 4Cl, and sufficient 10% HCl was added
to make the aqueous layer transparent. The organic layer was separated, and the aqueous
layer was extracted with Et 2O (3 ”). The combined organics were dried (MgSO 4). Removal
of the solvent in vacuo yielded a product mixture that was purified by chromatography. If
the reaction product was a diol, the reaction was quenched by 1.1 M potassium sodium
tartrate soln (1.0 mL). The resulting suspension was stirred at rt for 4 h and filtered, and
the white gel was washed with hot EtOAc several times. The combined filtrate was dried
(Na 2SO 4) and removal of the solvent in vacuo yielded a product mixture that was purified
by chromatography.
96