T 3-Allyl complexes

Allylic C/H insertion accompanied by an allylic rearrangement has been observed for carbenoid reactions of ethyl diazoacetate with allylamines (Scheme 23)1S1). Apparently, metal-catalyzed isomerization 117 118 proceeds the C/H insertion process. Although mechanistic details have not yet been unraveled, T)3-allyl complexes... 

Although t/3-allyl complexes of platinum(II) are not rare, their occurrence is not as frequent as for -alkene complexes. This situation is reversed for palladium(II) where r 3-allyl complexes are very common, and much of modern organopalladium chemistry is becoming dominated by the reactivity of j)3-allyl complexes. 

One possible explanation is that each isoprene unit adds to the titanium (and we will drop the pretence at this point that we have any idea which other ligands are on the Ti atom) to form ant]4 diene complex. This must necessarily have the s-cis conformation. Addition of R to one end of this complex gives an T]3 allyl complex still maintaining the cis configuration. The next diene then adds to form a newt 4 diene complex, couples to the allyl complex, and so on. As the chain grows, each diene is added as an r)4 complex and an all-cis polymer results. 

The spectra of the tetra-T)3-allyl complexes of Mo, W, and Zr (Table XI) show significant differences. Whereas that of the zirconium complex has only two signals (one for the meso and one for the terminal carbon atoms), the spectra of the molybdenum and tungsten complexes consist of three... 

With soft nucleophiles, steps a and b of Scheme 12.10b are the same as those in Scheme 12.10a. The crucial difference between the two pathways originates in the next steps. Soft nucleophiles attack the T 3—allyl complex anti to the metal (step c, Scheme 12.10b), which results in another inversion of configuration. This is followed by decomplexation (step d), which occurs with retention of configuration. Overall, therefore, two inversions followed by retention result in a net retention of configuration. Equations 12.26 and 12.27 illustrate the stereochemistry attendant to the reaction of diastereomeric allylic acetates with malonate ion.57... 

Cyclopropane ring cleavage is also observed in the case of zirconocene 2-alkene and j 2-imine complexes with adjacent cyclopropane rings to give t/3-allyl, /3-azaallyl, and t/ -enamine complexes [29]. 

B. Akermark, B. Krakenberger, S. Hansson, Ligand Effects and Nucleophilic Addition to (T)3-Allyl)palladium Complexes. A Carbon-13 Nuclear Magnetic Resonance Study, Organometallics, 1987, 6, 620-628. 

As mentioned above, the calculations performed for styrene as a substrate suggests that the enantioselectivity can be directly correlated with the relative thermodynamic stabilities of the r 3-allylic complexes. Indeed, the exo stereoisomer, precursor of the enantiomeric product found in excess experimentally, becomes favoured with respect to the endo one upon t 3-coordination, and remains thermodynamically more stable until product release. However, the observed energy differences in the relative stabilities of the different allylic forms (1-2 kcal/mol) are certainly at the limit of accuracy of density functional calculations. 

Our calculations suggest that the stereoselectivity of the hydrosilylation is determined by the thermodynamic stability of the ri3-allylic complex that forms after styrene insertion. This opens up the possibility of improving the enantioselectivity by modifying the catalyst framework to alter the stability of the exo versus the endo T 3-allylic intermediate. 

Unlike CO, it is possible to polymerize isocyanides (R—N=C), isoelectronic analogs to CO. When R is a bulky group, such as tert-Bu, the polymer forms a stable helical structure. Asymmetric catalytic polymerization has been reported for t-Bu-NC using [Ni(T 3-allyl)(iV-trifluoroacetyl-proline)]2 providing (M)-helical polymer with 69% ee. The more stable helical polymer was prepared from 1,2-diisocya-nobenzene derivative initiated by a chiral Pd complex. (See Scheme 4.19.)... 

In an oxidative addition, Pd(0) complex 22 with BINAP as a ligand accepts alkenyl triflate It. The resulting Pd complex 23 is cationic, since the triflate anion is bound only loosely to the palladium and dissociates from the complex.1 Syn insertion of one of the two enantiotopic double bonds of the cyclopentadienc into the alkenyl-Pd bond of complex 23 leads firs to q -allyl-Pd complex 24. This is in rapid equilibrium with t 3-allyl-Pd complex 25. Neither 24 nor 25 contains a p-H atom in a yn relationship to palladium. Moreover, internal rotation is impossible in the con form a-tionaily fixed ring system. For this reason there is no possibility of a subsequent p-hydride elimination that would once again release the palladium catalyst. In a normal Heck reaction (see discussion) the catalytic cycle would be broken at this point. 

Cationic t 3-allyltetracarbonyliron complexes are generated by oxidative addition of allyl iodide to pentacarbonyliron followed by removal of the iodide ligand with AgBF4 under a carbon monoxide atmosphere [35]. Similarly, photolysis of vinyl epoxides or cyclic vinyl sulfites with pentacarbonyliron or nonacarbonyldiiron provides Jt-allyltricarbonyliron lactone complexes. Oxidation with CAN provides by demetallation with concomitant coupling of the iron acyl carbon to one of the termini of the coordinated allyl moiety either [3- or 8-lactones (Scheme 1.12) [36, 37]. In a related procedure, the corresponding Jt-allyltricarbonyliron lactam complexes lead to P- and 8-lactams [37]. 

The insertion of the coordinating diene monomer proceeds via an t]3-allylic species. The metal atom with the attached copolymer chain terminated by such a species gives its two coordination sites to be occupied by the -allylic group. This allows the next coordination of only one additional a-olefm. After insertion of the coordinating 7-olefin, two coordination sites at the metal atom are again available, and they are preferably complexed by another conjugated diene molecule. 

Iron complexes can also catalyze allylic amination [31,32]. Enders et al. have demonstrated the nucleophilic addition of various acyclic and cyclic amines to the optically active l-methoxycarbonyl-3-methyl-(T)3-allyl)-tetracarbonyliron cation 49 formed in high yield from reaction of 48 with iron carbonyls. Oxidative removal of the tetracarbonyliron group by reaction with CAN gives 50 with high optical purity and retention of the stereochemistry (Eq. (12)) [31]. The reaction proceeds well for the different amines, and has been used for the synthesis of a compound showing cytotoxic activity against diverse cell lines [31b]. 

Treatment of [W(CO)6] with allyl halides under UV radiation gives the 7r-allyl complexes[W2Cl3(CO)6(T)3-alIyl)], [WBr(CO)4(T 3-allyl)], and [WI (CO)4(rj3-allyl)], respectively.313 A large series of 7r-allyl complexes of the type [MX(CO)2L2(Tj3-allyl)] have been prepared by reaction of the zero-valent complexes [M(CO)4L2] with allyl halides.314... 

It is appropriate to include here two bis(T)3-allyl)nickel complexes that are formed by reacting zerovalent nickel species with 1,3-dienes and that have been shown to be involved as intermediates in the nickel-catalyzed cyclooligomerization of butadiene, viz., (T)3,i73-C12H18)Ni and (rj3-QH NiPRa. 

A typical second step after the insertion of CO into aryl or alkenyl-Pd(II) compounds is the addition to alkenes [148]. However, allenes can also be used (as shown in the following examples) where a jt-allyl-rp-Pd-complex is formed as an interme-diate which undergoes a nucleophilic substitution. Thus, Alper and coworkers [148], as well as Grigg and coworkers [149], described a Pd-catalyzed transfonnation of o-iodophenols and o-iodoanilines with allenes in the presence of CO. Reaction of 6/1-310 or 6/1-311 with 6/1-312 in the presence of Pd° under a CO atmosphere (1 atm) led to the chromanones 6/1-314 and quinolones 6/1-315, respectively, via the Jt-allyl-t 3-Pd-complex 6/1-313 (Scheme 6/1.82). The enones obtained can be transformed by a Michael addition with amines, followed by reduction to give y-amino alcohols. Quinolones and chromanones are of interest due to their pronounced biological activity as antibacterials [150], antifungals [151] and neurotrophic factors [152]. 

Extensive double bond isomerization has been found in the reactions of (T 3-allyl)Pd complexes with CO2 ref. 6 c-f. 

A wide variety of nucleophiles add to an -rf-allyl ligand. Desirable nucleophiles typically include stabilized carbanions such as CH(COOR)2 or 1° and II0 amines. Unstabilized nucleophiles such as MeMgBr or MeLi often attack the metal first and then combine with the n-allyl by reductive elimination. The Tsuji-Trost reaction, which is typified by the addition of stabilized carbanions to T 3—allyl ligands complexed to palladium followed by loss of the resulting substituted alk-ene, comprises an extremely useful method of constructing new C-C bonds, and many applications of this reaction have appeared in the literature.61 Equation 8.43 illustrates an example of a Pd-catalyzed addition of a stabilized enolate to an allyl acetate.62 The initial step in the catalytic cycle is oxidative addition of the allyl acetate to the Pd(0) complex, followed by nq1 to nq3—allyl isomerization, and then attack by the nucleophile to a terminal position of the T 3—allyl ligand. We will discuss the Tsuji-Trost reaction, especially in regard to its utility in chiral synthesis,63 more extensively in Chapter 12. 

Attack at enantiotopic termini of the irf-allyl ligand when the T 3—allyl ligand is symmetrically substituted at the two terminal positions, the two allylic positions are equivalent except in the chiral environment of the Pd complex and one position is more reactive (step d, Scheme 12.10a, or step c, Scheme 12.10b). 

The reaction starts with an allylic carbonate 291 that gives the usual T 3 allyl cation complex 294 with Pd(0) though the ligand is the less usual chelating diphos 292 (Ph2PCH2CH2PPh2). The leaving group is a carbonate anion 293. 

In chapter 19 we discussed the uses of palladium allyl cation complexes as electrophiles. We established that Pd(0) adds to the opposite face of the allylic system to the leaving group232 to form an t 3 cation complex 233 and that the nucleophile attacks from the opposite face to the Pd so that the two inversions lead to retention. We established that regioselectivity and diastereoselectivity can be well controlled. If this seems unfamiliar we suggest you read the relevant section of chapter 19 before proceeding. 

Figure 1.4. Formation of R+ species ([Pt(T 3-allyl)P(C6H5)3]+) from [Pt(r 3-allyl) XP(C6Hs)3] complexes, due to the formation of an ion pair catalyzed by the high surface charge density present in the droplet.

Similar solvent effects are observed in the stoichiometric formation of bridged (t/3-allyl)palladium trifluoroacetates40. A mixture of tetrahydrofuran and acetonitrile favors anti addition to yield the ra-adduct 19, whereas syn addition dominates in pure tetrahydrofuran. syn Attack of palladium(O) is much slower. The main effect of the polar coordinating solvent is to stabilize the transition state of the anti addition, in which charge separation is necessarily involved to a higher extent than for syn addition. The reaction conditions in the two reactions shown below represent feasible methods of preparing stable 7t-allylpalladium complexes 20. Palladium black will precipitate in the second reaction, if phenanthroline is not present. 

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... 

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