R-Allyl intermediates

Fig. 4 Branched and linear products formed from r -allyl intermediates that have one unsubstituted terminus...

The active nickel catalyst contains one bidentate phos-phinite ligand and the overall mechanism of the reaction is believed to be similar to butadiene hydrocyanation except that the final reductive elimination step is irreversible under the conditions of the reaction. jr-Allyl intermediates (7) are believed to play an important role in the exclusive formation of the branched nitrile product observed. Formation of the C-CN bond in the final reductive ehmination from the r-allyl intermediate occurs at C(2) and not C(4), because the aromaticity of the naphthalene ring is preserved only when the bond forms with C(2). A a-alkyl complex see a-Bond) with the Ni bound to C(l), which could give the linear (anti-Markovnikov) nitrile product, does not contribute because of the much greater stability of intermediate (7), accounting for the high regioselectivity observed. 

Similarly tetravalent Ti and Zr dihydride catalysts are formed from alkyl or aryl precursors. A wide range of Group 8-10 metal hydride catalysts has been isolated or formed in situ from precursor allyl complexes. The systems are generally quite active because the catalysts are necessarily ligand deficient with sites available for substrate coordination. For an 17 -allyl precursor, equation (s) initial dihydride addition to an M(r) -allyl) intermediate appears very plausible ", cf. equations (i)-(l). 

Allene also gives stable insertion products. The insertion occurs to place the alkyl group from the metal alkyl on the second carbon of the allene forming a stable 7t-allyl-metal complex as the product by way of a [Pg.74]

Hydrides and alkyls of the titanium triad metals are polymerization catalysts, so it is understandable that there are few stable r-allyl intermediates. The only example we can locate is the reaction between the zirconium hydride XIV and butadiene, which yields the homoallyl complex XV in place of the expected t-allyl complex ... 

In a related reaction, tetraene 26 underwent carbocyclization to give allylic ether 27 [Eq.(26)] [45], The reaction can be considered as an intramolecular telomerization, and leads to the 1,4-addition of a carbon and an oxygen nucleophile to one of the dienes. The reaction involves a r-allyl intermediate, which is subsequently attacked by the oxygen nucleophile. 

Interestingly, this 1,4-carbochlorination occurs syn, which constrasts with that via the vinylpalladation in Eq.(49), which occurs anti. An explanation for this difference is that the allylsilane attacks the palladium-diene complex anti, leading to a /rans-carbopalladation of the double bond. This is the first example of nucleophilic attack by an allylsilane on an olefin coordinated to a metal. Direct evidence for a /rans-carbopalladation was provided by the isolation of the proposed r-allyl intermediate of Eq.(51) as its chlorodimer 98a from reaction of 97 with Li2PdCl4 in the absence of benzoquinone [Eq.(52)] [119b]. The trans relationship between palladium and the carbon that has attacked the diene was established by the reporter ligand technique used for 41 in Section 8.3.1.1 under Intramolecular 1,4-diacyloxylation . 

In a series of )7 -l,l-dimethylallyl complexes of Pd coordinated by diphosphine chelates, attack of malonate anion took place more preferentially at the tertiary carbon as the bite angle of the diphosphine increased e.g. 8% for dppe and 61% for 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene [110c]. This was attributed to the greater distortion in the coordination mode of the allyl ligand in such a way as to elongate the bond between Pd and the tertiary carbon. Pd-catalyzed reachon of allylic acetates with nucleophiles proceeded through cationic r -allyl intermediates in which one allyl terminus closer to the Si substitutent is bound to Pd more weakly than the other terminus, leading to the nucleophilic attack at the former allyl terminus (Eq. 8.23) [llOd]. Intramolecular pyridine coordination was shown by an X-ray study to be a cause of such distortion of the allyl coordination. 

Another conceptually different approach to discriminating the nucleophilic attack at two allyl terminus of symmetrical r) -allyl intermediate was used earlier than those described above [119], As shown in Scheme 8.64, a functional group capable of interacting with nucleophile was connected by a long tether to the one side of chiral diphosphines in order to bring the nucleophile close to only one allyl terminus. For example, chiral ferroceny diphosphines shown in Scheme 8.64 provided fairly high enantioselectivity in allylic alkylation. 

Among very rare examples of catalytic transformations involving polyene and polyenyl ligands is Pd-catalyzed oxidation of diene derivatives by the use of acetic acid and quinone with unique stereochemical control being achieved by judicious choice of the reaction condition (Scheme 8.65) [121]. Thus, the oxidation carried out with high Cl concentration afforded cis diacetate product, while trans adduct was obtained in the absence of Cl ion. The initial step of the catalytic cycle would be the exo attack of OAc at the Pd-bound diene, giving rise to r -allyl intermediate with OAc and Pd positioned trans to each other. This then underwent either exo or endo attack of the second OAc in the presence or absence of Pd-bound Cl ligand, respectively. The endo attack may have proceeded in a manner similar to Scheme 8.50. The hnal step of the catalysis would be oxidation of Pd(0), formed by the OAc attack at the r -allyl terminus, with benzoquinone as an oxidant. 

Electrophilic substitution of r/ -allyl complexes, especially those of Si and Sn, has found extensive synthetic applications, but the overall transformation is stoichiometric with regard to the amount of the metal atom. A catalytically useful reaction of r/ -allyl intermediate was involved in telomerization of 1,3-dienes in the presence of Pd catalyst shown in Scheme 8.41. r/ -Allylmetal complexes were reactive with not only H+ but also electrophilic alkenes (e.g. Scheme 8.72) [62,133]. Recent development in Pd-catalyzed amphiphilic allylation of alkenes and imines (e.g. Scheme 8.73) relied on the high susceptibility of Pd-bound r -allyl ligand to the attack of unsaturated carbon electrophile [134]. 

Differentiation of two interconverting intermediates. Reaction of allylic acetates 110 and 111 proceeds via the formation of interconverting r-allyl intermediates 112 and 113 (by enantiotopie faces exchange), and affords 114 and 115 using ehiral eatalysts. In this case, eontrol of regioseleetivity to minimize the... 

In a similar manner, Grigg and co-workers have demonstrated that a combination of 102, allene and a nucleophile (YH2) affords hetero- or carbocycles in the presence of palladium(O) [92]. Presumably, insertion of aryl iodide to allene gives a r-allyl intermediate 103, intercepted by a nucleophile (YH2), the latter being trapped by addition onto the Michael acceptor to furnish hetero- or carbocycles 105 (Scheme 40). 

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