Allyl -enyl complexes

There are two main ways in which the allyl group can link to a transition element. Either it is attached through one of the terminal carbon atoms (f/ ) leaving an uncoordinated double bond, or all three carbon atoms of the allyl system are bounded 

A useful way of distinguishing between these bonding modes is by proton n.m.r. spectroscopy. The spectra of one and one complex are given in Fig. 8.1. Consider the spectrum of fy -C3H5Mn(CO)5 first. The (H ) protons are relatively strongly shielded by the adjacent metal atom. They therefore resonate at high field, 1.8 5. If and are magnetically equivalent, that is related in space by a symmetry operation on the molecule, this resonance is a doublet of relative intensity 2, 8Hz.  

The remaining protons H, are substituents on a vinyl (alkenyl) group. Such resonances appear typically at 4.5-65 in alkenes. The trans vicinal coupling constant is greater than the cis vicinal constant The 

The spectrum of rj -C K )Mn(CO) can be interpreted in terms of a symmetrical -group. There are three different proton environments (meso) attached to the central carbon, ff (syn) and (anti). The anti protons are nearer to the metal 

The spectrum of (f/ -C3H5)Mn(CO)4 conforms to the A B X pattern. Some allyl complexes show this pattern at low temperatures, but as the temperature is raised the resonances broaden, finally coalescing to an A X spectrum in which the syn and anti protons show an averaged signal. Such observations indicate that dynamic processes are proceeding at a rate comparable with the lifetime of the nuclear spin states. 

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When heated under reflux in benzene or methanol, in the absence of hydrogen, isomerisation of the terminal olefin of the pent-4-enyl phosphine n=3) is completed in less than 5 hours to yield the cis-pent-3-enyl phosphine complex. For the but-3-enyl complex ( =2) the isomerisation to the but-2-enyl phosphine complex is incomplete even after 88 hours. The mechanism involves a i-allyl hydride intermediate, whose stability, as in the case of the hydrogenation studies, is controlled by the... 

The hydrogenation of trans-l-phenyl-1,3-butadiene has been discussed49 in terms of the mechanism originally proposed,45 and 773-allyl species were invoked. NMR studies on the hydrogenation of butadiene have shown that Tj -but-2-enyl and r 3-l-methylallyl complexes are present in the reaction mixture. With isoprene, an q1 -methylbut-2-enyl complex was observed.50 By using a two-phase system and a phase-transfer catalyst, the turnover numbers attainable with conjugated dienes can be increased to about 30.51... 

Abstraction of chloride from a vinylpalladium complex by silver acetate has been reported,45 as well as halide abstraction by silver triflate from arylpalladium halides.46 More interestingly in the context of creating C-C bonds, silver perchlorate was able to promote the reaction of (r -aryl)palladium bromide with dienes. Silver-promoted bromide abstraction led to the formation of (r 1-r 2-enyl)palladium complexes, which evolved further through the regioselective formation of a C-C bond between the aryl group and the former diene. Reactions with nonconjugated dienes suggested that the reaction proceeds via carbometallation of the less crowded double bond. Isomerization and (3 elimination led to a (ri3-allyl)palladium complex (Scheme 10.26).47... 

Chemistry of coordination complexes and organometallic compounds. Allyl and enyl complexes Allyl-transition metal systems... 

Whilst reaction of acyl chloride 303 with Na[Co(CO)3(PEt3)] affords the oxocyclobut-enyl complex 306" by ring expansion and CO loss, analogous treatment with NaRe(CO)5 delivers the non-fluxional // -cyclopropenylrhenium compound 305" . In the latter case, compound 304 loses carbon monoxide with concomitant migration of the cyclopropenyl moiety from carbonyl to rhenium as an allylic rearrangement rather than a 1,2-shift. 

Another important group of PGM compounds are the o-alkyl-, o-aryl-, a-dienyl-, and a-allyl complexes and the ir-olefin, ir-alkin, rr-allyl, and ir-enyl complexes which, as well as some carbonyl and phosphin complexes, play an important role in catalytic reactions [5]. 

An interesting example of enantioselective catalytic alkylation of carbonyl groups by unqiolung of 7r-allyl palladium complexes has been disclosed recently.A number of chiral ligands have been screened in the palladium-catalyzed alkylation of cyclohex-enyl acetate. The best performing ligand was the monodentate... 

Three-electron ligands n-allyl and w-enyl complexes... 

Treatment of nickel acetylacetonate with aluminium alkyls in the presence of cyclododecatrienes or butadiene, or of bis-)p-allyl nickel with butadiene, gives cyclododecatriene nickel, a volatile blood-red crystalline compound 10.6.This 16-electron nickel complex reacts catalytically and very rapidly with butadiene at 20° liberating isomers of cyclododecatriene. The major product is the trans-trans-cis isomer small amounts of the trans-cis-cis isomer are also formed. If the reaction with butadiene is carried out at -40° the bis-w-enyl complex, 10.7, is isolated in which the ligand is the same as that found in the complex formed from butadiene and ruthenium salts (p 180). The carbonylation (Figure 53) provides evidence for the... 

The formation of 7i-enyl complexes from a mono-olefin probably proceeds via the prior formation of an unstable olefin-metal complex, followed by the loss of a hydrogen atom. The reaction may be compared to that of allyl halides which lose the halogen. Examples are... 

The reactivity of the 7r-enyl complexes varies widely. The very high reactivity of bis-7r-allyl nickel, 2.7, is shown by the displacement reactions given in Figure 11. 

The reaction of methylenesulphones with allyl halides in the presence of quaternary ammonium salts produces the 1-allyl derivatives [52], unlike the corresponding reaction in the absence of the catalyst in which the SN- product is formed (Scheme 6.5). In contrast, alkylation of resonance stabilized anions derived from allyl sulphones produces complex mixtures [51] (Scheme 6.6). Encumbered allyl sulphones (e.g. 2-methylprop-2-enyl sulphones) tend to give the normal monoalkyl-ated products. Methylene groups, which are activated by two benzenesulphonyl substituents, are readily monoalkylated hydride reduction leads to the dithioacetal and subsequent hydrolysis affords the aldehyde [61]. 

The availability of an allylic hydrogen through the formation of the 114 agostic intermediate or an enyl ([Pg.526]

The palladium-catalyzed decarboxylative coupling of allyl 2-(benzo[c(jthiazol-2-yl)acetates 118 provides a facile approach to 2-(but-3-enyl)benzo[c(jthiazoles 122 <07JA4138>. The reaction is initiated by nucleophilic attack of Pd(0) on the allyl ester to give Pd-7t-allyl complex 119, which undergoes nucleophilic attack at the less substituted allylic carbon from the benzothiazole nitrogen to produce 120. Decarboxylative dearomatization leads to intermediate 121, and a subsequent aza-Cope rearrangement driven by rearomatization affords the final product 122 and accounts for the unusual regioselectivity. This appears to be the first report of a tandem allylation/aza-Cope reaction driven by decarboxylative dearomatization/ rearomatization. 

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