Isomerization nickel-catalyzed

Homogeneous nickel complexes proved to be versatile catalysts in dimerization and trimerization of dienes to yield different oligomeric products.46-55 Depending on the actual catalyst structure, nickel catalyzes the dimerization of 1,3-butadiene to yield isomeric octatrienes, and the cyclodimerization and cyclotrimerization to give 1,5-cyclooctadiene and all-trans-l,5,9-cyclododecatriene, respectively46 56 [Eq. (13.13)]. Ziegler-type complexes may be used to form cis,trans,trans-1,5,9-cyclododecatriene37,57 58 [Eq. (13.14)], which is an industrial intermediate ... 

The regiospecificity of DCN addition using ZnCl2, and the fact that very little deuterium is found in recovered 3PN, strongly suggest that, at least in this system, the back reactions in steps 3, 6, 14, and 16 are slow and olefin isomerization is catalyzed by cationic nickel hydrides, as shown in Fig. 6. This may also occur with other Lewis acids. 

The nickel-catalyzed hydrocyanation of butadiene is a two-step process (Figure 3.32). In the first step, HCN is added to butadiene in the presence of a nickel-tetrakis(phosphite) complex. This gives the desired linear product, 3-pente-nenitrile (3PN), and an unwanted branched by-product, 2-methyl-3-butenenitrile (2M3BN). The products are separated by distillation, and the 2M3BN is then isomerized to 3PN. In the second step, 3PN is isomerized to 4PN (using the same nickel catalyst), followed by anti-Markovnikov HCN addition to the terminal double bond. The second step is further complicated by the fact that there is another isomerization product, CH3CH2CH=CHCN or 2PN, which is thermodynamically more stable than 4PN. In fact, the equilibrium ratio of 3PN/2PN/4PN is only 20 78 1.6. Fortunately, the reaction kinetics favor the formation of 4PN [95],... 

Formation of the DVCB derivatives is complicated by the nickel-catalyzed isomerization of m-piperylene into traw.v-piperylene which proceeds through the formation of the cyclobutane (LIII). The rate of isomerization is dependent on the nature of the ligand attached to the metal, and increases in the series tri(o-phenylphenyl)phosphite < triphenyl-phosphine < tricyclohexylphosphine. In the case of the nickel-tricyclo-hexylphosphine catalyst, the rate of isomerization is faster than the cyclization reaction. 

Another factor must be considered. With increasing conversion, the rate of the nickel-catalyzed cleavage of methyl-substituted DVCB to piperyl-ene [Eq. (56)] increases. This reverse reaction is dependent on the ligand in the same sense as the isomerization of cw-piperylene. If the diene is removed as soon as it is formed (by working in a partial vacuum), no... 

Finally, the mechanism in Scheme 3 bears a resemblance to that presented above for the nickel-catalyzed reaction of methylmagnesium bromide and aryl bromides. However, there are outstanding differences between iron and nickel in their abilities to effect cross coupling reactions. Iron is a catalyst which is effective at lower temperatures and concentrations than used with nickel. Even more importantly, cross coupling can be effected completely stereospecifically with an iron catalyst and no alkyl isomerization of the Grignard component has been observed, in contrast to the nickel-catalyzed reactions. 

Interesting similarities can be advanced between nickel catalyzed hydrocyana> tion and hydroboration [181 reactions (the isomerization step being related to borane isomerization througli elirrunation-addition reactions), the drift of nickel toward the less hindered terminal position being genuinely reminiscent of organoborane chemistry. 

The synthesis of hexa-1,4-diene has been achieved by the nickel-catalyzed homogeneous addition of ethylene to butadiene. The nickel is introduced in the form of the complex Ni[P(OEt)3]4. The reaction is carried out in acid media, and the active catalyst is the cationic complex NiH[P(OEt)3]3 which is a 16-electron molecule. In Fig. 7 the sequence of reactions that leads to the catalytic formation of isomeric hexadienes is... 

The nickel-catalyzed hydrovinylation of bicycloheptene has been used as a standard reaction to test the efficacy of a new ligand. The reaction occurs with complete diastereoselectivity to give exo-2-vinylbicycloheptane (16) and none of the endo-isomer is formed. The same species, however, catalyze the isomerization of the primary product to cis- and franv-2-ethylidenebicycloheptane (17) and the codimerization with further ethylene to the butenyl derivatives 18 and 19. The product distribution is dependent upon the nature of the ligand [3, 8 c, 40]. 

It is generally accepted that the nickel-catalyzed hydrovinylation of cyclic 1,3-dienes proceeds in an analogous manner to that discussed for styrene with an initial 1,2-addition of the Ni-H species. However, it should be stressed that an initial 1,4-addition has not been excluded. The observation of two isomeric products from the reaction involving hexadeuterocyclopentadiene suggests that the... 

In line with this observation, l,l,2,2-tetramethyl-3-methylenecyclopropane (4) undergoes a highly selective nickel-catalyzed reaction with methyl acrylate to yield a [2 + 2] cycloadduct 5 as the sole cycloaddition product in a yield of 75%. An isomerization product, [2,3,3-trimethyl-pen ta-1,4-diene (6), 14.6%], of 4 is the main side product. 

Whereas palladium(0)-catalyzed reactions of dialkyl fumarate and dialkyl maleate yield reaction products identical to those obtained from the phosphane-modified nickel-catalyzed reactions (vide supra), analogous palladium(0)-catalyzed reactions with ( )-but-2-enoic or (E)-cinnamic acid derivatives lead to different products to the nickel-catalyzed reactions, i.e. in the palladium-catalyzed reactions formal distal cleavage of but-2-enoic MCP occurs to provide methyl tra i-2-methyl-4-methylenecyclopentanecarboxylate (12, R = Me) and methyl trans-4-methylene-2-phenylcyclopentanecarboxylate (12, R = Ph), respectively." Yields and stereoselectivities are slightly higher with palladium(O) catalysts. When R = Me, 7.4% of the C-C double bond isomerization product, methyl traM -2,4-dimethylcyclopent-3-enecarboxylate (13, R = Me), is additionally obtained, raising the combined yield of cyclocodimers to 49.9%. With methyl (jE )-cinnamate, analogous isomerization only occurs upon workup, i.e. distillation of the crude product. 

Among early reported Pd-catalyzed reactions, the Mori-Ban indole synthesis has proven to be very useful for pyrrole annulation. In 1977, based on their success with the nickel-catalyzed synthesis of indole from 2-chloro-A -allylaniline, the group led by Mori and Ban disclosed Pd-catalyzed intramolecular reactions of aryl halides with pendant olefins [111]. Compound 117, easily prepared from 2-bromo-A-acetylaniline and methyl bromocrotonate, was adopted as a cyclization precursor. Treatment of 117 with PdiOAc), (2 mol%), PhjP (4 mol%), and NaHCOj in DMF provided indole 118 via an intramolecular Heck reaction followed by olefin isomerization to afford the fully aromatic product. Although yields fr om the initial report were moderate, they have been greatly improved over the last two decades [112]. 

We have previously used this method to establish allyl isomerization events in nickel catalyzed cyclodimerization of butadiene, and describe here our application to the polymerization mechanism. 

Furthermore, a vast number of organometallic catalyzed reactions can be performed in a biphasic manner thus proving that also uncommon reactions may be worth to be investigated in liquid/liquid systems. For instance, Braddock describes the atom economic nitration of aromatics in a two-phase process [192], Nitration of aromatics leads usually to excessive acid waste streams and the classical Lewis acid catalysts such as boron trifluoride are destroyed in the aqueous quench after the reaction thus making any recycle impossible. In the method of Braddock the ytterbium triflate catalyst is solved in the aqueous phase and can be recycled by a simple evaporative process. Monflier and Mortreux [193] investigated the nickel catalyzed isomerization of olefins, for instance allylbenzene, in a two phase system yielding good yields of cis- and trans-methylstyrene. 

Recently, the nickel-catalyzed isomerization of geraniol and prenol has been investigated in homogeneous and two-phase systems. The best results with respect to activity and selectivity have been obtained in homogeneous systems with a bis(cy-cloocta-l,5-diene)nickel(0)/l,4-bis(diphenylphosphanyl)butane/trifluoroacetic acid combination. Catalyst deactivation occurs in the course of the reaction owing to coordination of the aldehyde group that is formed to the nickel species or as a result of protonolysis of hydrido or (jr-allyl)nickel complexes [2],... 

The dimeric azaphospholines derived from (/ )-myrtenal and (J )-l-phenylethylamine are also active ligands in the nickel catalyzed hydrovinylation of cyclopentadiene with ethylene to give exclusively 3-vinylcyclopentene (7) with 100% chemo- and regioselectivity and up to 76% ee5 (according to recent reports even higher inductions are obtainable53). No isomerization and dimerization of the products is observed. Previously, with other catalytic systems the same... 

In earlier investigations, asymmetric nickel-catalyzed isomerization and cyclodimerization of methylenecyclopropane was found to give l-methylene-2-vinylcyclopentane44. The nickel catalyst system was prepared by reduction of NiBr2L2 with butyllilhium. With tributylphosphane as ligand a 91% yield of the dimerization product was obtained. With dibromobis(( - )-methyl(phenyl)propylphosphane]nickel and butyllithium, optically active (no enantiomeric excesses given) l-methylene-2-vinylcyclopentane of unknown absolute configuration was obtained in 30% yield. Involvement of a n-allyl intermediate is proposed (loc. cit. 141 in ref 45). 

Lewis acid catalyzed versions of [4 4- 2] cycloadditions are restricted to functionalized dieno-philes. Nonfunetionalized alkenes and alkynes cannot be activated with Lewis acids and in thermal [4 + 2] cycloadditions these suhstrates usually show low reactivity. It has been reported that intcrmolecular cycloaddition of unactivated alkynes to dienes can be accelerated with low-va-lent titanium, iron or rhodium catalysts via metal-mediated - -complex formation and subsequent reductive elimination39 44. Usually, however, low product selectivities are observed due to side reactions, such as aromatization, isomerization or oligomerization. More effective are nickel-catalyzed intramolecular [4 4- 2]-dienyne cycloadditions which were developed for the synthesis of polycycles containing 1.4-cyclohexadienes45. Thus, treatment of dienyne 1, derived from sorbic acid, with 10mol% of Ni(cod)2 and 30 mol % of tris(o-biphenyl) phosphite in tetrahydrofuran at room temperature affords bicyclic 1,4-dienes 2, via intramolecular [4 + 2] cycloaddition, with excellent yield and moderate to complete diastereocontrol by substituents attached to the substrate. The reaction is sensitive towards variation in the catalyst and the ligand. 

Nickel-catalyzed stereospecific cyclocarbonylation of cWo-cyclopentadiene dimer 3 with ( )-l -chloro-2-butene, carbon monoxide and tetracarbonylnickel in acetone/water at room temperature gives a-methyl-3-oxotetracyclo[5.5.1.02,6.08,12]tridec-9-ene-4-aceticacid (4)in 10% yield51. An X-ray investigation of the acid reveals a selective cis-exo attack at the strained double bond, while the other double bond does not react. The exo dimer of cyclopentadiene gives a different product, thus there is no isomerization, in contrast to reactions under acidic conditions. 

Formation of n-alkylnickels by addiction of Ni hydride to an alkene is important in many nickel-catalyzed reactions such as alkene dimerization and isomerization. However, stoichiometric formation of alkylnickels from alkenes and Ni hydrides is not synthetically useful because of the reactivity of alkylnickels. The regiochemistry of the addition of HNiX(PR3)2 to propene is affected by the nature of the trialkylphosphine group used in the Ni hydride complex. The observed ratios of n-propyl- to isopropyl-nickel found after the insertion step vary from 82 18 to 82 20 to 19 81 as the phosphine is changed from P(OC6Hs)3 to P(c-C6Hu) to P(t-C4H9)2(i-C3H7). ... 

Nickel-catalyzed isomerizations are thought to proceed by this mechanism. In general, in these reactions, isomerization occurs in a stepwise manner and the final isomer concentrations approach the thermodynamic equilibrium values. The ease of isomerization seems to follow the order of the complexing ability of olefins terminal > disubstituted >... 

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