· (MW 635.31)
(nickel(II) complex useful for the oligomerization of norbornadiene2 and the isomerization of alkenes3)
Physical Data: mp 212 °C (dec).
Solubility: only sparingly sol most organic solvents; most sol CH2Cl2.
Preparative Method: dry nickel(II) cyanide (11 g) was refluxed in 1 L ethanol with 78 g Triphenylphosphine for 48 h. The yellowish-gray Ni(CN)2 turned to a light yellow coarse crystalline mass. The solvent was decanted and the solid was rinsed with ethanol to remove unreacted PPh3. The remaining solid product was dried, extracted with CH2Cl2, and filtered to exclude insoluble Ni(CN)2. Removal of solvent provided 39 g (62%) of light yellow crystals.4
Handling, Storage, and Precautions: air stable. Store at rt.
Oligomerization of Norbornadiene.
Dicyanobis(triphenylphosphine) nickel(II) catalyzes the [2 + 2 + 2] cycloaddition between norbornadiene and electron-deficient alkenes and alkynes. Thus norbornadiene (1) reacts with acrylonitrile in the presence of the nickel catalyst to give a 93% yield of the homo-Diels-Alder adduct (2) as a mixture of epimers (eq 1).4 This catalyst is similar to Dicarbonylbis(triphenylphosphine)nickel(0), which gives 86% yield in the same reaction, but is easier to handle due to its lack of oxygen sensitivity (see also Bis(acrylonitrile)nickel(0) and Bis(1,5-cyclooctadiene)nickel(0).5 Alkynes also form adducts with norbornadiene; however, the yields are lower than with alkenes.
Isomerization of Alkenes.
When this complex is combined with Sodium Borohydride, it can be used as a catalyst for the isomerization of alkenes. The mechanism occurs through nickel hydride addition across the alkene and subsequent b-hydride elimination. Acceptance of another hydride by the organonickel intermediate followed by reductive elimination leads to overall hydrogenation of the alkene. In this way, dimethyl maleate is converted to a 95:5 mixture of the isomerized product, dimethyl fumarate, and the hydrogenated product, dimethyl succinate (eq 2).6 Other nickel(II) cyanide complexes such as Ni(CN)2(phen) and Ni(CN)42- give rise to greater proportions of hydrogenated product. With Ni(CN)2(PPh3)2, 1,5-cyclooctadiene can be isomerized to a mixture of 1,4- and 1,3-cyclooctadiene with no trace of the hydrogenated byproduct.
A cis selectivity was observed for the isomerization of 1-butene to 2-butene.3 With Ni(CN)2(PPh3)2, the cis isomer was favored by a factor of 2:1. Greater cis selectivities were observed for some cobalt complexes and other nickel(II) catalysts. These selectivities are dependent upon the cone angle of the phosphorus ligand, the size of the anionic ligands, and the steric congestion in the organometallic complexes.
- 1. Jolly, P. W.; Wilke, G. The Organic Chemistry of Nickel; Academic: New York, 1974, 1975; Vols. I and II.
- 2. Jolly, P. W. In Comprehensive Organometallic Chemistry; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: Oxford, 1982; Chapter 56.2, p 615.
- 3. Kanai, H.; Sakaki, S.; Sakatani, T. BCJ 1987, 60, 1589.
- 4. Schrauzer, G. N.; Glockner, P. CB 1964, 97, 2451.
- 5. Lautens, M.; Edwards, L. G. JOC 1991, 56, 3761 and references therein.
- 6. Samejima, H.; Mizuta, T.; Yamamoto, H.; Kwan, T. BCJ 1969, 42, 2722.
Paul A. Wender & Thomas E. Smith
Stanford University, CA, USA
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