Ji-allyl complex

Another interesting example of dehydrative C-C coupling involves the alkylation of benzimidazole 36 with allyl alcohol 37, which is catalysed by complex 39 [15], The reaction is believed to proceed by alkene complex formation with the allyl alcohol 37 with loss of water from the NH proton of the NHC ligand and OH of the allyl alcohol to give an intermediate Ji-allyl complex. The initially formed 2-allylbenzimidazole isomerises to a mixture of the internal alkenes 38 (Scheme 11.9). 

The reaction of but-l-en-3-yl diethyl phosphate with diethylamine produces N,N-diethylpent-3-enamide (86%), indicating that a Ji-allyl complex is involved in the carbonylation reaction. No isomerism to the a,p-unsaturated amides was observed. 

The Grignard method is generally applicable to the preparation of ji-allyl complexes, e.g. ... 

A cyclobutylmethyl-metal system provides another opportunity to observe 13-carbon elimination. The ring opening process harnesses the release of the least necessitating ring strain of a four-membered ring. Scandocene hydride 63 reacts with 3-methyl-l,4-pentadiene to afford the linear Ji-allyl complex 65 [80]. The intermediacy of cyclobutylmethyl complex 64 which undergoes P-carbon elimination accommodates the observed rearrangement. 

Among the various carbon-carbon and carbon-hetero atom bond forming reactions promoted or catalyzed by transition metals, allylic substitution via electrophilic ji-allyl-complexes is of utmost importance. Studies focused on the synthetic potential of alkyl or aryl substituted (T)3-allyl)Fe(CO)4(1+) complexes have shown that nucleophilic attack by soft carbon and hetero atom nucleophiles preferentially proceeds regioselectively at the less or syn-substituted allyl terminus.4 Additionally, polar effects on the regioselectivity of this reaction caused by electron-withdrawing functionalities (e.g., C02R, CONR2) have been examined by the... 

Many common ligands are shown there. It is seen that 361 was converted to the Ji-allyl complex with various ligands and reacted with methyl methanesulfonyl sodioacetate (359) to form a mixture of 362 and 363. A variety of carbanion nucleophiles can be reacted with jt-allylpalladium complexes.221... 

Table 8. Catalysts of anionic-coordinative polymerization based on ji-allyl complexes of transition metals [49, 68]...

Enal-derived nickel-Jt-allyl complexes are efficient partners in cross-coupling reactions with alkenyl- and arylstannanes leading to products such as 39 (Scheme 23).P 1 As described for the aUylic ether derived complexes, the mechanism of the coupling process involves transmetalation followed by reductive elimination. The reaction is catal hic in nickel, unlike couplings of enals with allstoichiometric quantities of nickel. Many other variants of nickel-catalyzed conjugate additions may involve Ji-allyl complexes, but these are treated separately in a section on alkenes (Section 1.1.4.2) because the mechanism is poorly defined in most nickel-catalyzed conjugate additions. 

Scheme 23 Addition of Organotins to Enal-Derived ji-Allyl Complexes ...

The authors suggested that the real catalyst was the neutral intermediate I79b, detected as a cluster at m/z = 611, corresponding to I79b. The oxidative addition of ailylic acetate Sib to the above intermediate afforded the formation of a Pd(IV) ji-allyl complex 184, which was not detected directly by ESI-MS. However, the authors postulate that this complex may be related to intermediates I81b,I83 and 182, which were all observed in the ESI mass spectra. Reductive elimination from 184 and interaction with S4 close the catalytic cycle. 

Relatively stable Ji-allylic complexes Cp U(allyl)3 (Cymbaluk et al. 1983) can be obtained from the precursor Cp UCl3(THF)2 reacted with (allyl)MgX (X=Br, Cl allyl = C3H5 and 2--CH3C3H4). The X-ray structure of Cp U(2-CH3C3H4)3 shows... 

An additional issue of regiocontrol arises in asymmetric induction when the ji-allyl complex possesses a primary terminus. Although steric factors favor the formation of the achiral linear product, alkylations with reactive nucleophiles often benefit from electronic effects leading to the branched product [147,148]. Of particular interest is the reaction of the crotyl system because... 

Related bimetallic systems were also prepared with M = Ni, Pd, Pt or Ag [3-5]. This unprecedented Tl2- t2-SiO bonding mode is also found in the siloxane-substituted complex 4 [6], and may assist the rapid c-Ji-isomerization observed in the allylic complex 5. 

The isomerisation of the pentenyl and butenyl phosphine [PhzP-(CH2) CH=CH2 (n = 2,3) (see above and ref 43)] complexes of rhodium has been held to go through a ji-allyl hydride intermediate. Such an intermediate would be equally applicable to the reactions of the group VI metals with ap. Fig. 43b. 

Interestingly, treating (>/4-cyclooctatetraene)Fe(CO)3 with acetyl chloride under Friedel-Crafts reaction conditions yielded unexpectedly222-223 the (>/2,>/3-8-e.x0-acetyl bicy-clo[3.2.1]octadienylium)Fe(CO)3 cation complex, presumably by rearrangement of the intermediate bicyclo[5.1.0]octadienylium isomer (Scheme 8). The structure of the rearranged cation was confirmed from the X-ray crystal structure and from the typical 1,3-cr.ji-allylic products obtained upon nucleophilic reaction with LiAlD4 and NaCN. The nucleophilic reaction of the more bulky iodide occurs, however, on the metal. 

The formation of compound 175 could be rationalized in terms of an unprecedented domino allene amidation/intramolecular Heck-type reaction. Compound 176 must be the nonisolable intermediate. A likely mechanism for 176 should involve a (ji-allyl)palladium intermediate. The allene-palladium complex 177 is formed initially and suffers a nucleophilic attack by the bromide to produce a cr-allylpalladium intermediate, which rapidly equilibrates to the corresponding (ji-allyl)palladium intermediate 178. Then, an intramolecular amidation reaction on the (ji-allyl)palladium complex must account for intermediate 176 formation. Compound 176 evolves to tricycle 175 via a Heck-type-coupling reaction. The alkenylpalladium intermediate 179, generated in the 7-exo-dig cyclization of bro-moenyne 176, was trapped by the bromide anion to yield the fused tricycle 175 (Scheme 62). Thus, the same catalytic system is able to promote two different, but sequential catalytic cycles. 

The catalytic cycle of the palladium-catalyzed diacyloxylation follows the cycle depicted in Scheme 8-6 (X = RCOO, Y = R COO ). The coordination of a quinone in the Ji-allyl)palladium intermediate was demonstrated by NMR studies including T, measurements [58]. Attack by the second nucleophile results in the formation of the 1,4-addition product and a palladium(0)-benzoquinone complex. In an independent mechanistic study, it was shown that such Pd(0)-benzoquinone complexes, which are stable at neutral conditions (pH 7), react with weak acids to give hydroquinone and the palladium(II) salt of the acid [Eq.(36)] [59],... 

Chiral allylation of imines by an allylstannane is promoted by a bis-Ji-allylpalladium complex based on the pinane skeleton, while using triallylborane to supply the allyl group a polymer-supported arenesulfonaminoisobomeol can provide a proper environment. The chirality of alkenyl sulfoxides imposes the approach of methyl lithioacetate, and therefore it enables the generation of optically active P-amino acids. ... 

For examples of P-carbon elimination in late transition metal systems, Bergman et al. identified P-methyl transfer with four-membered ruthenacycles, which is driven by the formation of Ji-allyl and Ji-oxallyl complexes. Warming the solution of oxaruthenacycle 58 to 45°C led to formation of methane and cyclic enolate complex 60 [76]. ji-Oxallyl complex 59 initially arises from P-methyl... 

Synthesis of neutral Ji-allylpalladium complexes having bisnitrogen ligands (mainly, those related to 2-(pyrazol-3-yl)pyridine, 2-(imidazol-2-yl)pyridine, and 2,2-bipyridine) and palladium-catalyzed cyclopropanation of ketene silyl acetals with allylic acetates 00YGK736. 

Nucleophilic addition to the more electron-rich cation XXIV leads as expected to a higher proportion of attack onto the olefin than for complex XXI. The cation XXVn reacts with nucleophiles such as D and [CN] on the olefin ligand as expected. Nucleophilic addition by [i-PrS]", however, occurs onto the Ji-allyl ligand ... 

Palladium-catalyzed substitutions of allylic esters 93, which proceed via intermediate ji-allylpalladium complexes such as 94, are well established [178]. In contrast, the reactions proceeding through 7r-allylpalladium intermediates, generated by addition of the palladium intermediates from haloarenes, haloalkenes, or the corresponding triflates, to allene and substituted allenes 95 (Scheme 3-26), have scarcely been tapped [79]. 

Several examples of Pd°-catalyzed carboamination reactions between allenes and aryl or alkenyl halides have been reported [50[. For example, treatment of allene 52 with iodobenzene in the presence of K2CO3 and 2mol% Pd(PPh3)4 afforded pyrrolidine 53 in 78% yield (Eq. (1.25)) [50a]. Mechanisms involving aikene amino-palladation (similar to the reactions of alkynes and alkenes noted above) have occasionally been invoked to explain these reactions. However, in many instances these transformations may involve intermediate Ji-allylpalladium complexes. Due to this mechanistic ambiguity, these transformations have been included in this section for comparison with the related reactions of alkenes and alkynes. Similar reactions involving allylic halides have also been described (Eq. (1-26)) [51]. 

Scheme 12.5 Epimerization of terminal Pd-allyl complexes via Ji-o-it-isomerization.

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