R 3-allyl complex

Figure 4. Structural data obtained for the cationic r 3-allyl complexes 26 (R = Me or CH2Ph), illustrating the differing Pd-C bond lengths which result from unfavorable steric interactions.

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. 

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. 

Allylic acetoxy groups can be substituted by amines in the presence of Pd(0) catalysts. At substituted cyclohexene derivatives the diastereoselectivity depends largely on the structure of the palladium catalyst. Polymer-bound palladium often leads to amination at the same face as the acetoxy leaving group with regioselective attack at the sterically less hindered site of the intermediate r 3-allyl complex (B.M. Trost, 1978). 

An example of a tethered arene complex of ruthenium(II) in which the auxiliary ligand is a a-bonded carbon atom is complex 80, which is formed by the action of AgBF4 and P(OMe)3 on the r 4-tetraphenylcyclobutadiene r 3-allyl complex 79 [Eq. (13)]. In the proposed mechanism, the allyl group migrates to the four-membered ring, which opens to generate an intermediate cation 81, the pendant arene of which coordinates to the metal.74... 

Bouachir F, Grenouillet P, Neibecker D, Poirier J, Tkatchenko I (1998) Cationic r 3-allyl complexes. 21. Telomerization of buta-1,3-diene with z-h compounds mediated by group 10 complexes. J Organomet Chem 569 203-215... 

The cyclohexadiene complex 29 has been further elaborated to afford either the cydo-hexenone 34 or the cyclohexene 36 in moderate yields (Scheme 1) [21]. The addition of HOTf to 29 generates the oxonium species 33, which can be hydrolyzed and treated with cerium(IV) ammonium nitrate (CAN) to release the cyclohexanone 34 in 43 % yield from 29. Alternatively, hydride reduction of 33 followed by treatment with acid eliminates methanol to generate the r 3-allyl complex 35. This species can be trapped by the conjugate base of dimethyl malonate to afford a cyclohexene complex. Oxidative decomplexation of this species using silver trifluoromethanesulfonate liberates the cyclohexene 36 in 57 % overall yield (based on 29). 

On the other hand, the other process without involving the formation of r)3-allylic complexes may operate as an alternative route in the course of the C-O bond cleavage. One is the SN2 type attack of a ligand bound to the metal such as hydride, alkyl or alkoxide on the terminal carbon of the allylic entity. The process is followed by elimination of the OX group (acetate or alkoxide) in a concerted manner as shown in Eq. 4. The other mode of cleavage is insertion-elimination type as shown in Eq. 5. The process proceeds by insertion of the olefinic moiety of the allylic entity into the M-Y bond, such as hydride, alkyl, or alkoxide ligand followed by P-elimination of the acetate or alkoxide moiety. 

Cleavage of the C-O bond in various allylic substrates by oxidative addition to M(0) species gives r 3-allylic complexes, which undergo nucleophilic attack to produce allylic compounds catalytically. A base is needed in most cases to remove HOX and to drive the catalytic cycle. There are a lot of synthetic reactions utilizing allylic oxygen bond cleavage catalyzed by palladium complexes [6, 7, 19-21]. 

Pd is very widely used in organic synthesis27 and we shall meet r 2 alkene and r 3 allyl complexes later. This chapter will end with a brief description of Pd o-complexes. Stable Pd o-complexes can be formed from ArX, MeX, and a few blocked alkyl compounds with no P-hydrogens. They react well with alkyl and acyl halides, but a major application is in carbonylation reactions.28... 

Perhaps the most useful of the r 3 allyl complexes (cf. 24) are the Jt-allyls of nickel.15 The simplest type 61 are rather unstable and form the bromide-bridged complex 62 on treatment with HBr. These are stable compounds officially complexes of Ni(I) but better regarded for our purpose as dimers of r 3 complexes of allyl anions and Ni(II), much as allyl Grignard reagents 2 can be regarded as o-complexes of allyl anions and Mg(II). Direct exchange of Mg(II) for Ni(II) gives the unstable complexes 61, but the stable dimer 62 can be made by oxidative insertion of Ni(0), as its cyclo-octa-1,5-diene (COD) complex, into allyl bromide 1. 

The generally accepted mechanism for Pd-catalyzed allylic desulfonylations is illustrated in Scheme 1. The first step is coordination of the Pd(0) catalyst to the allylic sulfone. Oxidative addition or internal SN2-type nucleophilic attack of the electron-rich palladium at the allylic position generates a neutral Pd(II) r 3-allyl complex, which leads to a more reactive cationic complex that is finally reduced. The equilibrium between the neutral and the more reactive cationic complexes depends on the nature and concentration of the palladium ligands as well as the counter anions present in solution. 

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 it-allyl-r 3-Pd-complex is formed as an intermediate which undergoes a nucleophilic substitution. Thus, Alper and coworkers [148], as well as Grigg and coworkers [149], described a Pd-catalyzed transformation 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-r 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]. 

Overall, allylic isomerization in the dodecatrienediyl-Ni11 complex is predicted to require a distinctly lower barrier than for reductive elimination (AAG > 5.5kcalmol 1, see Section 4.6). This leads to the conclusion, that isomerization should be significantly more facile than the subsequent reductive elimination, which is confirmed by NMR investigations of the stoichiometric reaction.22 Consequently, the several configurations and stereoisomers of the bis(allyl),A-/restablished equilibrium, with 7b as the prevalent form. The various bis(r 3-allyl),A-/n2H.v stereoisomers of 7b are found to be close in energy, while bis(allyl), A-cf.v forms are shown to be negligibly populated (cf. Section 4.4) and therefore play no role within the catalytic reaction course. 

The thermodynamically favorable bis(r 3),A-cis/trans configuration 7b of the [Nin(dodecatrienediyl)] complex also represents the catalytically active species for reductive elimination. The new C-C a-bond is preferably established between the terminal unsubstituted carbons on two r 3-allylic groups (Fig. 9) giving rise to the formal 16e [Ni°(CDT)] product 8b, where CDT is coordinated to nickel by its three olefinic double bonds. 

Closely related to both allyl carbenoids and the allenyl carbenoids discussed above, propargyl carbenoids 101 are readily generated in situ and insert into zirconacycles to afford species 102 (Scheme 3.27), which are closely related to species 84 derived from allenyl carbenoids [65], Protonation affords a mixture of allene and alkyne products, but the Lewis acid assisted addition of aldehydes is regioselective and affords the homopropargylic alcohol products 103 in high yield. Bicydic zirconacyclopentenes react similarly, but there is little diastereocontrol from the ring junction to the newly formed stereocenters. The r 3-propargyl complexes derived from saturated zirconacycles are inert towards aldehyde addition. 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

Efforts have been made to apply r 3-allyltitanium chemistry to the asymmetric synthesis of homoallylic alcohols and carboxylic acids. The synthesis and reactions of chiral r 3 -allyl-titanocenes with planar chirality, or containing Cp ligands with chiral substituents, have been reported [6c,15,30—32]. The enantiofacial selectivity in the allyltitanation reactions has been examined for the complexes 12 [15], 13 [30], 14 [31], 15, 16, and 17 [32] depicted in Figure 13.2. 

Reactions of aldehydes with complexes 13—17 provide optically active homoallylic alcohols. The enantioselectivities proved to be modest for 13—16 (20—45% ee). In contrast, they are very high (> 94% ee) for the (ansa-bis(indenyl))(r]3-allyl)titanium complex 17 [32], irrespective of the aldehyde structure, but only for the major anti diastereomers, the syn diastereomers exhibiting a lower level of ee (13—46% ee). Complex 17 also gives high chiral induction (> 94% ee) in the reaction with C02 [32], in contrast to complex 12 (R = Me 11 % ee R = H 19% ee) [15]. Although the aforementioned studies of enan-... 

Cyclic (diene)Mo(CO)2Cp (or In) cations have been prepared by trityl cation mediated alkoxide abstraction from cyclic ( 73-allyl)Mo(CO)2Cp (or In) complexes bearing a syn alkoxy in the a position (e.g. 55, Scheme 14)81b 86. Additionally, protonation of (r/3-allyl)Mo(CO)2In (or Cp ) complexes bearing a vinyl group (e.g. 56, Scheme 14) affords the corresponding (diene)Mo+ cations8115 87. 

The structures of jj3-allyl complexes of platinum have been fully summarized by Hartley in Chapter 39 of Comprehensive Organometallic Chemistry . This article also details the NMR methods used to investigate fluxionality of the r/3-allyl ligand. 

Fe2(CO)9 also gives small amounts (1.5 3%) of the distal ring-opened dinuclear allylic complexes (equations 336 and 337)378. 

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