ca01 only detailed ToC 1..24

1.1.1 Nickel Complexes of 1,3-Dienes 35
(1R*,5S*,7aR*)-4-[2-(Methoxymethoxy)ethyl]-5-(4-methoxyphenyl)-7amethyl-1-(trimethylsiloxy)-2,3,5,7a-tetrahydro-1H-indene
(13): [26]
To an acid-washed, base-washed, 200-mL Schlenk flask was added dienyne 12 (665 mg,
1.595 mmol, 1.0 equiv). Under a positive N 2 flow, freshly distilled cyclohexane (160 mL)
and tris[2,2,2-trifluoro-1-(trifluoromethyl)ethyl] phosphite (170 mg, 0.319 mmol,
0.2 equiv) were added, followed by 0.075 M [Ni(cod) 2](2) in THF (2.13 ìL, 0.160 mmol,
0.1 equiv) and the reaction was stirred at rt for 1 h. The clear soln slowly changed to a golden
yellow soln which was then warmed to 808C and stirred for 17.5 h. The reaction was
quenched by opening to air and stirring for 30 min. Purification by flash filtration
through a 2.5-cm plug of silica gel (Et 2O/hexanes 1:4) followed by flash chromatography
(silica gel, Et 2O/hexanes 1:7) gave the cyclohexa-1,4-diene 13; yield: 600 mg (90%).
1.1.1.4 Method 4:
Diene–Aldehyde Reductive Cyclizations
Additions of dienes to aldehydes have emerged as a synthetically useful reaction class.
The reaction is formally a reductive coupling and may produce either ª,ä-unsaturated or
ä,å-unsaturated alcohols as products (Scheme 7). [27–31] Two mechanisms have been proposed,
and it is likely that the reaction is initiated either by formation of an oxametallacycle
or by hydrometalation of the diene.
Scheme 7 Diene–Aldehyde Reductive Coupling [27–31]
O
R 1 H
R2 +
Ni catalyst
reducing agent
1.1.1.4.1 Variation1:
Triethylsilane-Mediated Reactions
OH
R 1 R 2
or
OH
R 1 R 2
Studies by Mori demonstrate that triethylsilane and dienals undergo reductive cyclization
in the presence of bis(ç 4 -cycloocta-1,5-diene)nickel(0) (2) and triphenylphosphine (1:2) to
produce the silyl ether of cycloalkanols; [27–30] in this instance, ª,ä-unsaturated products
are obtained. However, if the reaction is carried out in the presence of cyclohexa-1,3-diene,
an analogous reaction proceeds to give ä,å-unsaturated products. This effect is reported
to be derived from selective diene hydrometalation followed by addition of the organonickel
intermediate to the tethered aldehyde. The reaction proceeds with five-, six-,
and seven-membered ring formation and with heterocyclic substrates. Several synthetic
applications of this cyclization methodology are reported (Scheme 8). Intermolecular
processes with simple dienes and aldehydes to afford ª,ä-unsaturated silyl ethers are
also possible.
Scheme 8 Reductive Coupling with Triethylsilane [27–30]
O
H
N
CHO
20 mol% Ni(cod)2 2
40 mol% Ph3P Et3SiH (5 equiv), THF
O
H
N
40%
OTES
+
O
H
N
38%
OTES
for references see p 79