Nickel alkoxides

Other reactive forms of nickel including nickel boride283 and nickel alkoxide complexes284 can also be used for desulfurization. Tri-n-butyltin hydride is an alternative reagent for desulfurization.285... 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Alkoxides of nickel(II) are conveniently prepared according to equation (177) in anhydrous conditions.1487 1488 All of these compounds are insoluble in the common organic solvents. Complexes with primary alkoxides are green and six-coordinated complexes with secondary and tertiary alkoxides are tetrahedral with colours ranging from blue to violet. All of the complexes decompose at about 90-100°C. The complexes with secondary and tertiary alkoxides undergo alcoholysis reactions when dissolved in primary alcohols. An interesting insertion reaction occurs when nickel alkoxide reacts with some isocyanates (equation 178).1489... 

Two possible mechanisms are outlined in Scheme 2. Insertion in the allylic C-O bond to form a 7t-allylic nickel alkoxide would be followed by reduction of the carbon nickel bond (path a). An alternative sequence involves a hydronickela-tion of the complexed olefin followed by a P-elimination of the oxygen bridge (path b). 

Further studies on the controlled oligomerization of isoprene (5) have shown that treatment with a nickel alkoxide catalyst provides tran5-/3-farnesene (6) in -60% yield. ... 

Coordination of an aldehyde molecule to the nickel center, followed by insertion into the nickel-carbon bond, affords a nickel-alkoxide intermediate. Attack of the coordinated alkojqr group at the ester moiety of the... 

Several sol-gel methods are described in the literature. Nickel alkoxides [(Ni(OR)2]n are polymeric and generally insoluble in alcohols at room temperature (Mehrotra, 1989), making them unsuitable for this purpose. Nickel oxide films were howevCT successfully dqtosited from mixtures of nickel nitrate, [Ni(N03)2(H20)6] in alcohol (Moser, 1990) or ethylene glycole (Wang. 1993 Mild, 1995) solutions. The stability of Ni(NOs)2 during thermal processing are likely to restrict its use to small-scale application. 

Nickel alkoxides undergo reactions with aryl isocyanates to give the expected mono- 332 and di-insertion products 333 . ... 

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