Allylic compounds Nickel

In some cases it is necessary to hydrogenate allylic compounds to saturated molecules without hydrogenolysis. It was found that nickel boride is a good catalyst for this purpose.46... 

Many recent publications have described the stereospecific polymerization of dienes by ir-allyl compounds derived from Cr, Nb, Ni, etc. Of particular interest is the work of Durand, Dawans, Teyssie who have shown that ir-allyl nickel catalysts (XXI) in the presence of certain additives polymerize butadiene stereospecifically (87, 38). The active center results from reaction of acidic additives with the transition metal. 

In a sense the tr-allyl compounds of the transition metals can be regarded as the simplest of the sandwich molecules. Bis(jr-allyl)nickel, the best known of such complexes, has been shown by x-ray crystallography (104,105) to have a staggered arrangement of tr-allyl moieties and hence a C2h molecular conformation. The electronic structure of the ground state of bis(jr-allyl)nickel has been investigated by both semiempirical (47) and ab initio (274,275) methods, and a semiempirical computation has been performed on bis(7r-allyl)palladium (47). 

As the first transition series is ascended the metal—carbon bond becomes weaker and the alkyls of iron, cobalt and nickel are stable at ambient temperatures only when coordinated with strong it donors such as 1,1 bipyridyl [12], Organic groupings which form delocalized bonds with the metal are more stable and many tt complexes or 7r-allylic compounds of transition metals are known. The metal—cyclopentadienyl bond is too stable for these compounds to initiate polymerization, but some TT-allyl compounds will polymerize monomers such as butadiene where a comparable structure will be retained in the propagating chain. 

Nickel catalysts also catalyze Grignard substitution to allylic compounds including allyl alcohol [230-233] ethers [230,231,234,235 Eq. (106) 231] amines, albeit in a low product yield [231] sulfides [231,236,237], including thioacetals [238] thiols [231] selenides [239] carboxylates [240] phosphates [94,121] and halides [Eq. (107) 230], most likely via intermediate / -allyl-Ni species. Monosubstitution of bis-allyl ether was possible [Eq. (108) 235]. Most of the literatures cited in the foregoing disclosed regiochemical outcome associated with these allylic substitutions. 

TJ ecent articles of this series (18) reported investigations of the synthe-sis of cyclododecatriene from butadiene (6) using a nickel catalyst. These investigations culminated in isolating a crystalline intermediate of this cyclization reaction (19). Strong support for the proposed structure of the intermediate, I was obtained by synthesizing bis (7r-allyl) nickel II (20). Subsequently, several 7r-allyl compounds of many transition metals were synthesized (21). 

TT-Allylnickel halides are more stable, and thermal disproportionation is not observed even at higher temperatures. Recently, we found that TT-allylnickel halides can be disproportionated easily by treating them in solution with excess gaseous ammonia (2). Bis(7r-allyl)nickel and ammonia adducts of nickel dihalides are obtained in quantitative yields and can be separated easily. In fact, the disproportionation reaction represents at the moment the easiest way to synthesize bis (7r-allyl) nickel type compounds since as mentioned, all types of 7r-allylnickel halides can be prepared easily. The advantage of the new method lies in the fact that bis (TT-allyl) nickel type compounds can be prepared without prior preparation of organometallic allyl compounds, such as Grignard compounds, which are sometimes diflBcult to prepare. The disproportionation of TT-allylnickel halides has an analog in the chemistry of alkyl-mercuric halides, some of which disproportionate under the influence of ammonia (12). 

Coupling of allylic compounds. Bauld found that allyl and cinnamyl acetates react with nickel carbonyl to give biallyl and bicinnamyl in moderate yields. 

The INS spectra of the a-allyl compound allyl iodide, CH2=CHCH2l, and the r -allyl compound di-p-chloro-bis(r] -allylpalladium), [ Pd(Ti -C3H5) 2(p-Cl)2] [94] and the INS below 800 cm of nickel and palladium allyl complexes [97] have been reported. The structure of the palladium complex is shown in Fig. 7.24 [98]. The INS spectra are shown in Fig. 7.25. Peak positions and assignments are in Table 7.15. 

Perhaps the most useful of the r 3 allyl complexes (cf. 24) are the Jt-allyls of nickel.15 The simplest type 61 are rather unstable and form the bromide-bridged complex 62 on treatment with HBr. These are stable compounds officially complexes of Ni(I) but better regarded for our purpose as dimers of r 3 complexes of allyl anions and Ni(II), much as allyl Grignard reagents 2 can be regarded as o-complexes of allyl anions and Mg(II). Direct exchange of Mg(II) for Ni(II) gives the unstable complexes 61, but the stable dimer 62 can be made by oxidative insertion of Ni(0), as its cyclo-octa-1,5-diene (COD) complex, into allyl bromide 1. 

The P.E. spectra of (d8) bis-17-allyl compounds of nickel and palladium pose severe assignment problems. The spectra display a tantalizing amount of structure but as the d-orbitals ionize in the same region as the v2 M.O., they are difficult to disentangle. The literature on these compounds14 126-130) includes various M.O. calculations,... 

Coupling of allylic compounds [1, 722-723, before references] revision and extension. Bauld s observation of the coupling of allylic acetates by Ni(CO)4 cited in ref. 12 was preceded by the discovery of Webb and Borcherdt14 (du Pont) in 1951 that nickel carbonyl in methanol effects coupling of allylic chlorides in excellent yield, for example ... 

COUPLING OF ALLYLIC COMPOUNDS Dimethylcopperlithium. Iron. Nickel carbonyl. Trimethylironlithium. GRIGNARD REAGENTS Thallium(I)-bromide. 

Acetylenic carbinols, which are manufactured from aldehydes or ketones and acetylenes, can be partially hydrogenated, with the exception of propargyl alcohol itself. Hydrogenolysis is a minor side reaction unless the hydrogenation is completed to the alkane. Nickel boride (PI or P2) catalysts achieve the saturation of propargyl and allylic compounds in the presence of double bonds without hydrogenolysis, such as in 2 an intermediate in the total synthesis of the sesquiterpene sesquicarene ... 

The transition metal-catalyzed carbonylation of allylic compounds has been developed as a simple extension of carbonylations of benzylic derivatives. In this respect nickel cyanide has been used as a catalyst precursor for carbonylation of allyl... 

Nickel complex compounds Nickel(I) jt-allyl complex Ni(CO),... 

Oligomerisation.—Much detailed discussion, based to a large extent on product characterisation and distribution observations, has appeared on catalysis of olefin oligomerisation by nickel complexes. Thus for the first example 7r-2-butenylnickel chloride, in contrast to the analogous 7r-allyl compound, is not a good catalyst for polymerisation of butadiene. But in the presence of trichloroacetate the 7r-2-butenyl complex does exhibit catalytic activity. This enhancement of activity is ascribed to the intermediacy of charge-transfer complexes. The mechanism of cyclotrimer-isation of butadiene has been considerably clarified by the characterisation of an intermediate (21) and determination of the distribution of isomers... 

Some allyl complxes may be reduced to alkene and the metal. The complex dodeca-2, 6, 10-triene-l,12-diylnickel is reduced by hydrogen with the formation of nickel and dodecane. Many other metal allyl compounds react in a similar manner. 

The early synthetic processes using organonickel compounds involved the coupling of allylic halides, which react with nickel carbonyl, Ni(CO)4, to give TT-allyl complexes. These complexes react with a variety of halides to give coupling products.255... 

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