[14167-20-5]  · C16H14N2NiO2  · N,N-Bis(salicylidene)ethylenediaminenickel(II)  · (MW 324.99)

(catalyst for alkane chlorination,2 alkene epoxidation,4 DNA oxidation,5 Michael addition,7 and radical cyclization8)

Alternate Name: Ni(salen).

Physical Data: mp 337 °C, d 1.325 g cm-3.

Solubility: insol H2O; slightly sol benzene and ether; sol acetone, acetic acid, pyridine, CHCl3, and CH2Cl2.

Form Supplied in: commercially available as an orange powder.

Preparative Method: easily and cheaply prepared from ethylenediamine, salicylaldehyde, and Ni(OAc)2.1

Handling, Storage, and Precautions: low sensitivity to air and water; cancer suspect agent; handle with care.

Alkane Chlorination.

Ni(salen) catalyzes the chlorination of saturated hydrocarbons, such as adamantane and cyclohexane, and the benzylic position of arenes.2 Reactions are carried out under phase-transfer conditions using aqueous Sodium Hypochlorite adjusted to pH 11 and CH2Cl2 solutions of the alkane (0.4 M) and catalyst (0.01 M). An 8.5:1 preference for chlorination at tertiary compared to secondary positions has been observed. Small amounts of ketone side products arise from hydrolysis of gem-dichloro compounds.

Alkene Epoxidation.

Ni(salen) is a highly efficient catalyst for alkene epoxidation using aqueous NaOCl adjusted to pH 9.3 under phase-transfer conditions (eq 1).3,4 As little as 0.013 mol % catalyst can be used with respect to alkene. The reaction is highly sensitive to pH. Epoxidation and alkene chlorination are both enhanced at lower pH, and pH 9.3 was found to give an optimal epoxidation rate while minimizing chlorination.4 Arylalkenes (styrenes, stilbenes) are the best substrates for this reaction, giving virtually quantitative conversion to epoxides, while alkylalkenes give relatively large percentages of chlorinated products. Use of optically active salen analogs fails to give enantioselectivity in epoxidation.3

DNA Oxidation.

Ni(salen) is among the most active of a series of nickel(II) complexes as a catalyst for DNA oxidation. Form I supercoiled plasmid DNA is readily converted to Form II nicked DNA by single-strand cleavage in the presence of Iodosylbenzene or magnesium monoperoxyphthalate (MMPP).5 Reactions typically involve use of 100 mM Ni(salen) and 1 mM oxidant at pH 6.8-7.0 for 5 min (PhIO) to 2.5 h (MMPP) at 25 °C.

A derivative of Ni(salen) bearing water-solubilizing groups (1) is an excellent reagent for oxidative alkylation of guanine residues in accessible sites of DNA structures.6 In the presence of KHSO5 as oxidant, a hairpin deoxyoligonucleotide showed specific reactivity at the G's located in the hairpin. The alkylation reaction gives rise to site-specific strand scission upon treatment with piperidine. Since the cleavage reaction is presumed to give clean phosphate ends, this method can be used for site-specific cleavage and synthesis of altered oligonucleotides.

Michael Addition.

Ni(salen) and related Schiff base complexes catalyze the Michael addition of b-keto esters to methyl vinyl ketone.7 For example, use of 1 mol % nickel catalyst in benzene gave excellent yields (84-96 %) of addition products when methyl vinyl ketone was used as the enone with various b-dicarbonyl compounds (eq 2). However, other Michael acceptors such as cyclohexenone gave considerably lower yields. Use of optically active Schiff base ligands gave low enantioselectivity (6% ee).

Radical Cyclization.

NiII(salen) undergoes reversible reduction to a nickel(I) species at a carbon cathode in DMF at a potential where 6-halo-1-hexynes are not electroactive. Thus Ni(salen) can be used as an electrocatalyst for the intramolecular cyclization of haloalkynes.8 Reactions are carried out in DMF with 0.1 M tetraethylammonium perchlorate as electrolyte. Controlled potential electrolysis at -1.00 V gave excellent conversions of 6-halo-1-phenyl-1-hexyne to the corresponding benzylidenecyclopentane (eq 3). Halide reduction was a minor competing process (6-17%).

1. Goedken, V. L.; Weiss, M. C. Inorg. Synth. 1980, 20, 115.
2. Querci, C.; Strologo, S.; Ricci, M. TL 1990, 31, 6577.
3. Yoon, H.; Burrows, C. J. JACS 1988, 110, 4087.
4. Yoon, H.; Wagler, T. R.; O'Connor, K. J.; Burrows, C. J. JACS 1990, 112, 4568.
5. Morrow, J. R.; Kolasa, K. A. ICA 1992, 195, 245.
6. Muller, J. G.; Paikoff, S. J.; Rokita, S. E.; Burrows, C. J. J. Inorg. Biochem. 1993, 47, 199.
7. Botteghi, C.; Schionato, A.; Rosini, C.; Salvadori, P. J. Mol. Catal. 1990, 63, 155.
8. Mubarak, M. S.; Peters, D. G. J. Electroanal. Chem. 1992, 332, 127.

Cynthia J. Burrows & Shiow-Jyi Wey

State University of New York at Stony Brook, NY, USA

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