ca01 only detailed ToC 1..24

46 Science of Synthesis 1.1 Organometallic Complexes of Nickel
with CH 2Cl 2 (3 ” 35 mL). The combined organic layers were dried (MgSO 4). Removal of the
drying agent through filtration, followed by solvent evaporation in vacuo, afforded a yellow
oil. Chromatography (silica gel, hexanes/EtOAc 15:1) afforded 38 as a colorless oil;
yield: 69 mg (73%).
1.1.2.7.2 Variation2:
Enal-Derived ð-Allyl Complexes
Enal-derived nickel–ð-allyl complexes are efficient partners in cross-coupling reactions
with alkenyl- and arylstannanes leading to products such as 39 (Scheme 23). [39] As described
for the allylic ether derived complexes, the mechanism of the coupling process
involves transmetalation followed by reductive elimination. The reaction is catalytic in
nickel, unlike couplings of enals with alkyl halides which require stoichiometric quantities
of nickel. Many other variants of nickel-catalyzed conjugate additions may involve ðallyl
complexes, but these are treated separately in a section on alkenes (Section 1.1.4.2)
because the mechanism is poorly defined in most nickel-catalyzed conjugate additions.
Scheme 23 Addition of Organotins to Enal-Derived ð-Allyl Complexes [39]
H
O
Bu3SnCH CH2 TBDMSCl, Ni(cod)2 2
TBDMSO
Cl
Ni Ni
Cl
75%
OTBDMS
39
OTBDMS
(E)-1-(tert-Butyldimethylsiloxy)penta-1,4-diene (39): [39]
A 25-mL Schlenk tube equipped with a stirring bar was sequentially charged with propenal
(74.0 ìL, 1.11 mmol), (MeO)(TMSO)C=CMe 2 (30.0 mg, 0.148mmol), TBDMSCl (168mg,
1.12 mmol), and CH 2Cl 2 (1.5 mL). The soln was stirred for 5 min, treated with [Ni(cod) 2](2;
27 mg, 0.098mmol), stirred for an additional 5 min, and then treated with tributyl(vinyl)stannane
(293 ìL, 1.00 mmol) to give a clear red soln. This was stirred at 258C for
48h, near the end of which time the mixture turned black and deposited a metallic nickel
precipitate. The mixture was diluted with pentane (25 mL) and washed with aq 0.1 M
KH 2PO 4/NaOH buffer (2 ” 25 mL, pH 7). The organic layer was separated and the product
was dried (MgSO 4), filtered, and concentrated under reduced pressure (15 Torr) to afford
an oil [(E/Z) 19:1] which was chromatographed (silica gel, 35 g, hexane/EtOAc 98:2) to give
39 as a clear, colorless oil; yield: 149 mg (75%); (E/Z) > 50:1 by 1 H NMR analysis.
1.1.2.7.3 Variation3:
Allylic Alcohol Derived ð-Allyl Complexes
Allylic alcohols readily undergo nickel-catalyzed allylation with Grignard reagents
(Scheme 24). [52–55] Both isomers of 40 produce similar product ratios, and the reaction presumably
proceeds via nickel–ð-allyl intermediates. Only simple Grignard reagents that
lack â-hydrogens such as methylmagnesium bromide and phenylmagnesium bromide
are reported, but considerable variation of the allylic alcohol is tolerated.