ca01 only detailed ToC 1..24

2.6.4 Metal–s-Alkyl and –s-Aryl Non-homoleptic Complexes 119
2.6.4.5.3 Variation 3:
Catalytic Nozaki–Hiyama–Kishi Reaction (The Fürstner Procedure)
The examples outlined in Section 2.6.4.5.2 are stoichiometric in chromium(II) chloride
and generally employ a large excess of this reagent. As shown in Scheme 30, reaction of
the organic halide with two equivalents of chromium(II) halide yields the desired organochromium
species 72 and one equivalent of chromium(III) halide. The nucleophile then
adds to the aldehyde with formation of a chromium alkoxide species. The high stability of
the oxygen-chromium(III) bond serves as the thermodynamic sink; the alcohol product
is recovered by hydrolysis in the stoichiometric process. The use of halosilane, however,
forces an exchange by virtue of the higher oxophilicity of silicon, producing an additional
equivalent of chromium(III) halide. At this point the reaction can be made catalytic in
chromium by simply using a reagent capable of reducing chromium(III) to chromium(II),
e.g. metallic manganese. This modification does not compromise the scope, practicability,
efficiency, and chemo- and diastereoselectivity of the C-C bond formation. [130,132,133] In
addition, it reduces the consumption of the rather high-cost and toxic chromium reagent
and paves the way for potential applications in enantioselective syntheses using chromium
catalysts with chiral ancillary ligands. [134 ]
Scheme 30 The Catalytic Nozaki–Hiyama–Kishi Reaction
R 1 X
2 CrX 2
MnX 2
CrX 3
Mn
CrX 3
CrR1X2 72
OTMS
R 1 R 2
OCrX 2
R 1 R 2
TMSX
R 2 CHO
2-Butyl-1-(4-methoxyphenyl)prop-2-en-1-ol: Typical Procedure: [130]
A soln of 4-methoxybenzaldehyde (340 mg, 2.5 mmol), 2-[(trifluoromethyl)sulfonyloxy]hex-1-ene
(1.06 g, 4.6 mmol), and TMSCl (0.75 mL, 6.0 mmol) in DMF (1.5 mL) and DME
(5 mL) was dropped into a suspension of Mn powder (230 mg, 4.2 mmol), CrCl 2 (46 mg,
0.38 mmol), and NiCl 2 (10 mg, 0.07 mmol) in DME (5 mL) at 508C. After being stirred for
5 h at that temperature, the mixture was quenched with H 2O (15 mL) and extracted with
EtOAc (3 ” 50 mL), and the combined organic layers were washed with brine. Aq TBAF
(75% w/w) was added, and the soln was stirred at rt until TLC showed complete desilylation
of the crude product. Standard workup followed by flash chromatography (hexane/
EtOAc 15:1) afforded the product as a colorless syrup; yield: 420 mg (76%).
2.6.4.6 Method 6:
Additive–Reductive Carbonyl Dimerization
In this reaction an alkyl group R 3 is transferred from a suitable metal–alkyl complex to
the electrophilic carbon of a carbonyl substrate 73 (Scheme 31), resulting in the deoxygenation
and dimerization to product 74 in a single step. [135] The substrate 73 can be an
aromatic aldehyde or ketone, a conjugated enone, or a benzoic acid derivative. The alkyl
transfer reagents [R 3 M] are tungsten(V) compounds formulated as dialkyldipropoxo(ìpropoxo)tungsten(V)
dimers 75. They are obtained in situ by alkylation of the correspond-
for references see p 135