Allene complexes formation

Insertion of an allene molecule into the Pd-H bond, with regeneration of the starting allene complex 8 which then re-enters the catalytic cycle. In the absence of allene, extensive Pd black formation was observed. 

The possibility of Jt-allylpalladium complex formation through carbopalladation is excluded from the observation that no four- and/or six-membered rings are produced. The reaction apparently proceeds via an alternative pathway which involves a sequence of Jt-coordination of PhPdl to an allenic terminal double bond, oxypallada-tion and ensuing reductive elimination (Scheme 16.9). 

The very early reported allene (NMe2)2C=C=C(NMe2)2 [12, 13] can also be considered as a push-push allene and the bent structure is activated upon complex formation as shown in Fig. 34. Reaction with [ClAu(PPh3)] and CU exchange by the weakly coordinating anion SbFg results in the formation of 71 in good yields [20,21]. 

Coordination polymerisation via re complexes comprises polymerisation and copolymerisation processes with transition metal-based catalysts of unsaturated hydrocarbon monomers such as olefins [11-19], vinylaromatic monomers such as styrene [13, 20, 21], conjugated dienes [22-29], cycloolefins [30-39] and alkynes [39-45]. The coordination polymerisation of olefins concerns mostly ethylene, propylene and higher a-olefins [46], although polymerisation of cumulated diolefins (allenes) [47, 48], isomerisation 2, co-polymerisation of a-olefins [49], isomerisation 1,2-polymerisation of /i-olcfins [50, 51] and cyclopolymerisation of non-conjugated a, eo-diolefins [52, 53] are also included among coordination polymerisations involving re complex formation. 

A series of alkenyl complexes (214, 216, and 218) were prepared from the reaction of PhS with the alkyne complex 213, the / -allene complex 215, and the cationic complex 217. Complex 218, with the nucleophile linked at the a position, could be converted to an allenacyl complex (219) by the addition of CO and a catalytic amount of oxidant, such as [Cp2Fe][Bp4] or Ce VEtOH. Excess Ce VEtOH leads to the formation of an alkenyl ester, (EtO)C(0)C(Me)=C(Me)SPh (153). 

S-GsHs)2Ti(G(Me)G(Me)GH2C(=CMe2)) 174, which can also be directly prepared from ()j5-CsI Is)2TiMc2 91 with 2-butyne, suggesting that the allene complex, (7 s-CsHs)2Ti(772-Me2CCCH2), may be intermediate in the formation of the final product (Scheme 26).106... 

Reaction of nonstabilized carbanions with [Fp(olefin)] complexes generally results in either displacement of the olefin or reduction of the metal rather than formation of stable (j -alkyliron complexes. This is especially true with simple, nonstabilized organo-magnesium halide or lithio reagents. However, allylmagnesium chloride and phenylmag-nesium chloride react in modest (20-40%) yield with the ethylene, propene and butadiene (1,4 addition) iron complexes. Lithium dimethylcuprate is even more efficient, reacting in up to 70% yield with Fp complexes of styrene, butadiene (1,4 addition) isoprene (1,4 addition) and allene. Complexes of cyclopentene and allene react in low... 

Cuprous ammonium acetate extraction. Butadiene is purified by aqueous CAA extraction in a liquid-gas countercurrent process developed by Exxon (67-69). The cuprous salt forms a soluble addition complex with butadiene, which is decomposed by heat thus the process is adaptable to countercurrent multistage equipment. Typically, the C4 hydrocarbon mixture with a butadiene content of 30-40% contacts the CAA solution in a countercurrent fashion in a series of mixer-settlers. Cooling to ca -15°C is required to promote complex formation. The more saturated hydrocarbons, butanes, and butenes are first removed by distillation. Butadiene is released from the complex by further heating to 80° C. After ammonia is removed by washing with water, distillation produces butadiene that is 98-99% pure. Acetylenes and allenes are extracted with the butadiene but must... 

Although superficially similar, propargyl compounds do not form t -complex intermediates, but give t -allenic complexes. As part of a catalytic cycle, these can undergo typical reactions, such as coupling (Schemes 9.82 and 9.83), ° ° reduction by formate, alkene insertion and carbonylation (Scheme 9.84). 

Liang et al. reported a PtCla-catalyzed transformation of 3-(2-alkyl)phenyl-propynyl acetate 130 to prepare naphthalenyl acetate 131 (Scheme 50) [45]. The electrophihc Pt-allene complexes formed in situ worked as hydride acceptors. Mechanistically, the Pt(Il)-promoted [l,3]-OAc shift leads to the formation of platinum-activated allenyl ester 1, which undergoes a [l,5]-hydride shift to form 1,3,5-hexatriene II. A subsequent bir-electrocyclic ring closure affords intermediate... 

The reactions of a number of substituted allenes with [Ni(PPh3)3] have been investigated and the factors affecting the regio- and stereochemistry of iT-complex formation and coupling to form nickelacyclopentane complexes have been analysed. 

Maria, PC. and Gal, J.F. (1985) A Lewis basicity scale for nonprotogenic solvents enthalpies of complex formation with boron trifluoride in dichloromethane. J. Phys. Chem., 89, 1296-1304. Allen, F.H. and Taylor, R. (2004) Research applications of the Cambridge structural database (CSD). Chem. Soc. Rev., 33, 463-475. 

The whole synthesis [151] is based on the route previously proposed by Malakria [146]. This includes formation of the structure 2.309 containing cyclopentanone moiety (ring D in the steroid) in the initial step, which is represented in the retro-synthesis (Scheme 2.102) at a very late stage. The second propargylic moiety in 2.308 was introduced enan-tiomerically using a chiral boron-allene complex. 

The high degree of orientational specificity which controls the cycloadditions to (267) of allene [(273) (274) 30 1 ] and acetoxybutenone [rz t/-adducts (278) and (279)] is suggestive of being meaningful in mechanistic terms. Several proposals have been advanced to account for these observations, inter alia a polar ground-state complex of the reactants, (281), which undergoes photoexcitation followed by concerted bond formation to products... 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

The insertion of allenes in the Pd-C bond of cyclopalladated 3-arylisoquinoline derivatives 327 afforded compounds 328, derived from the berberinium cation (Scheme 71). This reaction takes place via the formation of an intermediate (r/ -allyOpalladium complex <2003JOM313>. This chemistry has been extended to the preparation of other cationic N-heterocycles, including naphtho[r+/ ]( uinolizinium derivatives <2004EJ01724>. 

Reaction of allenes with PdCl2(PhCN)2 in benzene leads to the formation of 2-chloro 7r-allyl complexes 69 (equation 30)61. 

Attempts to employ allenes in palladium-catalyzed oxidations have so far given dimeric products via jr al lyI complexes of type 7i62.63. The fact that only very little 1,2-addition product is formed via nucleophilic attack on jral ly I complex 69 indicates that the kinetic chloropalladation intermediate is 70. Although formation of 70 is reversible, it is trapped by the excess of allene present in the catalytic reaction to give dimeric products. The only reported example of a selective intermolecular 1,2-addition to allenes is the carbonylation given in equation 31, which is a stoichiometric oxidation64. 

Two catalytic cycles are proposed to explain the difference in selectivity. In both cases, catalytic cycle is initiated by the oxidative addition of an alkynylstannane to nickel(O) species, leading to the formation of alkynylnickel(ll) complex 77 (Scheme 24).92 Then, an allene is inserted into the nickel(ll) complex in a manner which avoids steric repulsion with the butyl group to afford the anti-ir-a y complex 80. The carbometallation of the terminal alkyne can take place at the non-substituted allylic carbon of the corresponding syn-Ti-a y complex 78. The stereoselectivity is determined by the relative rate of the two possible insertion modes which depend on the ligand used. A bidentate... 

---