ca01 only detailed ToC 1..24

68 Science of Synthesis 1.1 Organometallic Complexes of Nickel
Ethyl (5R)-5-[(1R,3aS,4E,7aR)-4-{[(4S,6R)-4,6-Bis(tert-butyldimethylsiloxy)-2,2-dioxo-
1,3,4,5,6,7-hexahydrobenzo[c]thiophen-1-yl]methylene}-7a-methyloctahydro-1Hinden-1-yl]hexanoate
(80): [145]
A mixture of pulverized NiCl 2 •6H 2O (2.38g, 10 mmol), Zn powder (3.27 g, 50 mmol), and
ethyl acrylate (79; 4.88 mL, 45 mmol) in pyridine (20 mL) was stirred at 608C under argon
for 30 min. The resulting dark red, heterogeneous mixture was cooled to 23 8C and treated
with a soln of 78 (7.49 g, 10 mmol) in pyridine (20 mL), producing a slight exotherm (23–
288C). After being stirred at rt for 2.5 h the mixture was worked up to give a pale yellow
foam (7.22 g), which was purified by flash chromatography (silica gel, 95 g, EtOAc/hexane
3:97; then EtOAc/hexane 5:95) to give, after evaporation of the solvents, 80; yield: 5.28g
(73%).
(€)-19,20-Didehydrotubifoline (82): [150]
A soln of the vinyl iodide 81 (47 mg, 0.11 mmol), 0.5 M LiCN in DMF (2.15 mL, 1.1 mmol),
and Et 3N (45 ìL, 0.32 mmol) in MeCN (5 mL) was added at rt to [Ni(cod) 2](2; 195 mg,
0.71 mmol). The resulting mixture was stirred at rt for 2.5 h and filtered through Celite,
washing carefully with Et 2O. The filtrate was sequentially washed with sat. aq Na 2CO 3
and brine. The dried organic phase was concentrated and chromatographed (Florisil,
CH 2Cl 2/MeOH 95:5) to give 82; yield: 11 mg (40%).
1.1.4.3 Method 3:
Coupling of Two Alkenes
The oxidative coupling of two unsaturated components coordinated to nickel(0) is a common
mechanistic theme throughout nickel chemistry (see also Houben–Weyl, Vol. E 18,
p 865). In the case of nickel–bis(alkene) complexes, a saturated nickel metallacyclopentane
is produced from such an oxidative cyclization. The interconversion of nickel(0)–
bis(alkene) complexes and nickel(II) metallacycles has been documented and carefully
studied (Scheme 55). However, most catalytic applications of this process have not been
rigorously defined from a mechanistic perspective. One case, however, that has been rigorously
studied is the formal [2+2+1] cycloaddition of an oxygen atom with two alkenes
to produce tetrahydrofurans. [153] The reaction proceeds by oxidative cyclization of a nickel(0)–bis(alkene)
complex to give a metallacycle 83, insertion of an oxygen atom from dinitrogen
monoxide into a C-Ni bond of metallacycle 83 to produce 84, and then oxidatively
induced reductive elimination with diiodine to produce the substituted tetrahydrofuran
85 (Scheme 64). Each of the intermediates shown was isolated and characterized.
Only strained alkenes were shown to participate in the process.
Scheme 64 [2+2+1] Cycloaddition of an Oxygen Atom and Two Alkenes [153]
Ni(cod)(bipy)
(bipy)Ni
83
N2O
44%
O
(bipy)Ni
The involvement of metallacycles has been proposed for the [3+2] cycloaddition of methylenecyclopropanes
with alkenes to produce methylenecyclopentanes. [154–156] Oxidative
cyclization of a methylenecyclopropane and an electron-deficient alkene produces a spirocyclic
metallacyclopentane 86. Cyclopropane ring opening followed by reductive elimination
affords the observed methylenecyclopentane products 87 and 88 (Scheme 65). Another
report describes the novel use of nanostructured nickel clusters as catalysts. [157]
84
I2
47%
O
85