Nickel bulky substituents

Nickel complexes prove to be the most effective catalysts for hydroboration of thioalkynes. The bidentate phosphine systems [(P-P)NiCl2] (P-P = dppf, l,3-bis(diphenylphosphino)propane(dppp), dppe) all displayed high activity, even with bulky substituents on the alkyne (Equation (4)) 43 44... 

Figure 21 Chain growth termination by fi-H transfer and displacement of unsaturated chain end from nickel centre by a monomer, hindered by bulky substituents R.

One of the more extraordinary recent developments in nickel and palladium polyalkene catalysis has been the development of a-diimines with bulky substituents as ligands in nickel and palladium complexes. When bulky aryl groups are used (R = isopropyl), these catalysts polymerize ethylene with high activities to high molecular weight highly branched... 

The selectivity of butadiene cyclooligomerization in the presence of bis(l,5-cyclooctadiene)-nickel(0) varies with the phosphane added, e.g., monophosphanes, such as (S)-ter/-butyl(iso-propyl)phenylphosphane and (- )-dimethyl(phenyl)phosphane, or diphosphanes, such as Diop. In all cases, 1,5-cyclooctadiene is preferentially formed, along with 4-vinylcyclohexene, ill a 4.5-6 1 ratio. The optical purity of the 4-vinylcyclohexene reaches 12% at a monophosphane/ nickel ratio of about 8 1 and is much lower with Diop57. The use of various 1,3,2-dioxaphospho-lanes with bulky substituents leads to a significant improvement in product selectivity (favoring 4-vinylcyclohexene over 1,5-cyclooctadiene in a ratio of up to 1 0.3) and in the enantioselectiv-ity. The best Optical yield (35% ee, later corrected to 24% ee58) was obtained with a nickel complex of diethyl 2-toT-butyl-l,3,2-dioxaphospholane-4,5-dicarboxylate (1) at 20 C57. 

Several nickel(II) salicylaldiminato systems have been developed which are highly active for the polymerization of ethylene (Fig. 3.7). Furthermore, bulky substituents in the 3-position of the salicylaldiminate ring were found to enhance... 

G. Britovsek, S. Baugh, O. Hoarau, V. Gibson, D. Wass, A. White, D. Williams, The role of bulky substituents in the polymerization of ethylene using late transition metal catalysts a comparative study of nickel and iron catalyst systems. Inorg. Chimica Acta 345, 279-291 (2003)... 

Bis(allyl)nickel (Figure 24.21) is one of the best known allyl complexes, but it is pyrophoric, and decomposes above 293 K. Bulky substituents can be used to stabilize analogues of Ni(ri -C3H5)2. Thus, Ni ri -l,3-(Me3Si)2C3H3 2 is kineti-cally stable in air for up to several days, and decomposes only when heated above 373 K. 

Possibly the very much reduced air-sensitivity is due to shielding of the metal atom by the bulky substituents. The presence of five free double bonds is confirmed by hydrogenation over Raney nickel, which gives a similar air-stable complex C2oH26Fe. Neither iron complex forms an isolable cation on oxidation. 

Unless retarded by repulsion between bulky substituents in the more crowded planar isomer, the tetrahedral-to-planar isomerization has a low enthalpy of activation, AH 10 4 kcal/mol [24]. Its entropy of activation is ordinarily quite negative for a unimolecular isomerization AS < —10 kcal/mol K) but is substantially less so - occasionally approaching zero - in nickel(II) complexes with halogen atoms as coordinating ligands [25]. This pattern of Arrhenius parameters, characteristic of reactions that occur with spin inversion (see Chapter 9), is hardly surprising in view of the fact that the tetrahedral complex is high-spin (5 = 1) whereas the square-planar complex is low-spin S = 0). 

Giacomelli and his co-workers have described a further application of the bis-(N-methylsalicilaldimine)nickel, [Ni(mesal)2], complex as a catalyst. This complex effects the head-to-tail dimerization of terminal acetylenes in the presence of stoicheiometric amounts of di-isobutylzinc to give conjugated enynes (102). The conversion and yield are both reduced in the presence of bulky substituents on the acetylene, but two equivalents of PhaP added to the nickel complex can improve the yield in these cases. 

Nickel and palladium react with a number of olefins other than ethylene, to afford a wide range of binary complexes. With styrene (11), Ni atoms react at 77 K to form tris(styrene)Ni(0), a red-brown solid that decomposes at -20 °C. The ability of nickel atoms to coordinate three olefins with a bulky phenyl substituent illustrates that the steric and electronic effects (54,141) responsible for the stability of a tris (planar) coordination are not sufficiently great to preclude formation of a tris complex rather than a bis (olefin) species as the highest-stoichiometry complex. In contrast to the nickel-atom reaction, chromium atoms react (11) with styrene, to form both polystyrene and an intractable material in which chromium is bonded to polystyrene. It would be interesting to ascertain whether such a polymeric material might have any catal3dic activity, in view of the current interest in polymer-sup-ported catalysts (51). 

The cyclic voltammogram of complex 3 B"-Ni shows two quasi-reversible Ni(0)/(I) and Ni(I)/(II) redox waves at —2.5 V and —I.IV vs. Fc/Fc, respectively. Neither of the two oxidized complexes was isolable, most likely due to the masking of the low-valent nickel(O) center by the three ter -butyl substituents on the carbene ligand. The search for alternative routes to these complexes using less sterically bulky NHC ligands, such as the TIMEN isopropyl derivative, is under investigation. 

The salen-Ni(II) complex 39a derived from (lR,2R)-[N,N -bis(2 -hydroxybenzyl-idene)]-l,2-diaminocyclohexane was also equally effective (Table 7.3, entry 4). In contrast to earlier reports on salen-metal complexes, where the introduction of a bulky tert- butyl substituent increased enantioselectivity [31], the use of complex 39b exhibited a significant decrease in enantioselectivity (entry 5). The presence of a bulky tert-butyl group obstructed the chelation of alkali metal ions by phenolic oxygen atoms. A dramatic increase in selectivity could be achieved when nickel was replaced with copper, and a salen-Cu(II) complex 39c afforded 85% ee (entry 6). Although screening of other bases or 50% NaOH were not advantageous, the use of 3 equiv. NaOH improved the enantiomeric excess to 92% (entry 9) and after recrystallization of a-methylphenylalanine optical purity was increased to 98% ee. 

Acid chlorides are suitable electrophiles. As aromatic and 1-alkenyl halides, bromides and iodides are generally good substrates. Until quite recently, the use of the corresponding chlorides, which are cheaper and often more readily accessible, had been limited to that bearing a strongly electron-withdrawing substituent at a proper position. The use of nickel catalyst [47], bulky phosphine [119], and heterocyclic carbene ligand (Scheme 23, Table 1) [116] enabled aryl chlorides to take part in the reaction. 

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