ca01 only detailed ToC 1..24

44 Science of Synthesis 1.1 Organometallic Complexes of Nickel
pH 7), and shaken to give an emulsion that was filtered with suction through Celite to
achieve separation. The organic phase was washed with H 2O (3 ” 300 mL), dried (MgSO 4),
filtered, and concentrated under reduced pressure (15 Torr) to afford a mixture of 34 and
cycloocta-1,5-diene. Distillation through a 20-cm Vigreux column under reduced pressure
afforded 34 as a clear, colorless oil; yield: 3.62 g (72%); bp 80–82 8C/3 Torr.
1.1.2.7 Method 7:
Coupling of Nickel–Allyl Complexes with Main Group Organometallics
Nickel–ð-allyl complexes generated by a variety of methods undergo efficient coupling
with nonstabilized main group organometallic reagents. Magnesium reagents have been
most extensively investigated as the main group organometallic, [43] although organostannanes
[39] and organoborates [44] have also been reported to participate in catalytic allylations.
In some instances, efficient allylic reductions may be observed if the main group
organometallic possesses a â-hydrogen. The mechanism typically proceeds by oxidative
addition to afford the nickel–ð-allyl complex, transmetalation of the main group organometallic
to produce an alkyl/allylnickel species, and reductive elimination to afford the
coupled product (Scheme 20). The reductive elimination step has been studied in detail
by Kurosawa who demonstrated that the rate of reductive elimination from an 18-electron
(ç 3 -allyl)nickel complex is significantly greater than reductive elimination from either
the ç 3 or ç 1 16-electron complexes. [45,46] Although nickel(0) and nickel(II) complexes
are commonly invoked in this reaction class, the involvement of paramagnetic nickel
species is well documented in related reaction classes and cannot be ruled out here.
Scheme 20 Nickel-Catalyzed Allylation of Allylic Ethers and Enals [39,43,44]
allylic ether or enal
M = main group metal
+
NiLn 1.1.2.7.1 Variation1:
Allylic Ether Derived ð-Allyl Complexes
In contrast to the very widely used palladium-catalyzed allylations involving allylic acetates,
the corresponding nickel-catalyzed reactions commonly employ allylic ethers. The
nickel-catalyzed allylation of main group organometallics normally favors the more-substituted
regioisomer, and the reaction proceeds with overall inversion of configuration of
the allylic ether (e.g., 35 fi 36; Scheme 21). [43] Hoveyda has demonstrated that substratedirected
alkylations are possible by employing allylic ethers such as 37 with a tethered
phosphine moiety. [47,48] Phosphine-containing substrates direct alkylation to the distal position
of the allyl unit, giving products such as 38. Reductions employing ethylmagnesium
bromide, however, are directed to the proximal position of the allyl unit (Scheme
21). [49] Several asymmetric variants of allylations of Grignard reagents are utilized in relatively
simple systems (Scheme 22). Interestingly, whereas the Hoveyda studies on directed
reductions lead cleanly to hydrogen-atom incorporation in the presence of ethylmagnesium
bromide and dichlorobis(triphenylphosphine)nickel(II) (3), [49] the asymmetric variants
with chiral chelating phosphines give high yields of ethyl-group incorporation
with ethylmagnesium bromide. [50,51] Related additions to unsaturated acetals are discussed
in Section 1.1.4.9.
R 1 M
R 1
NiL n
R 1