Nickel complexes alkoxides

Nickel(II) alkoxides have recently been reviewed1485,1486 and we will report here a few examples of these complexes. 

Ni(0) catalyst. A radical 5-exo cyclization to the potentially zinc or nickel-complexed ketone provides an alkoxyl radical that combines with the co-produced Ni(I) species. A transmetalation to a zinc alkoxide regenerates the catalyst and forms the zinc cyclopentoxide, from which products 79 are liberated on hydrolysis. A bimetallic Cu(I)-Mn(II) system provided similar results (see Sect. 8.4). Analogous samarium diiodide-mediated reactions require in contrast stoichiometric amounts of the reducing agent and are less diastereoselective [26, 27],... 

U. Schubert, S. Tewinkel and E Moller, Metal complexes in inorganic matrixes. 13. Nickel complexes with lysinate-substituted titanium alkoxides as ligands. X-ray structure analysis of [(ETO)jTi(glycinate)]2, Inorg. Chem., vol. 34, no. 4, pp. 995-997,1995. 

The most widely used cross-coupling arylation reaction is the Suzuki-Miyaura reaction (Scheme 1.10), which was first reported in 1979 [12, 26, 27]. The reaction involves transmetallation with an organoboron reagent that is usually a boronic acid or ester. No transmetallation occurs under neutral conditions only in the presence of a base, which is usually an alkaline earth metal alkoxide, although weak bases such as K COj can be used [12] (Scheme 1.10b). Pd(OAc)2 or Pdj(dba)3 are the common sources of Pd(0). In some circumstances, arenediazonium tetrafluoroborates have been used [12e]. Nickel complexes can be used under some circumstance instead of Pd. 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

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... 

Other reactive forms of nickel including nickel boride and nickel alkoxide complexes can also be used for desulfurization. Tri-w-butyltin hydride is an alternative reagent for desulfurization.204... 

As indicated in Scheme 27, indoles may be alkylated by their acid-catalyzed reaction with alcohols. Similarly, r-butylation of pyrroles has been effected by the acid-catalyzed reaction with t- butyl acetate (B-77MI30502), and the diarylmethylation of 1-methylpyrrole from the acid-catalyzed reaction with the chromium trichloride complex of the diarylcarbinol has been described (78JA4124). The alkylation of indoles by alcohols in the presence of the aluminum alkoxide and Raney nickel appears to be efficient for the synthesis of 3-substituted indoles, but is less successful in the alkylation of 2-methylindole (79JHC501). The corresponding isopropylation of pyrrole produces 2,5-diisopropylpyrrole and 1-isopropylpyrrolidine, as the major products (79JHC501). 

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... 

New chelate rings can be formed by the nucleophilic addition of alcohols to imine complexes. For example, the nickel(II) TAAB complex is susceptible to attack by bis-alkoxides (equation 31).127 It is not clear whether or not a kinetic template effect operates by prior coordination of the central oxygen or sulfur atom. However, such an effect is not necessary, as simple alkoxides undergo a similar addition reaction.128... 

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). 

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