7i-Allyl complex

Because of the exceptional C-F bond strength, the successful preparation of a-halocyclopropyl c-complexes is realized by substitution of 1-bromo-l-fluoro-trans-2,3-dimethylcyclopropane 179 with Fp [90], Silica gel column chromatography of the thus obtained cr-complex 180 results in ring opening to the alcohol 181 as a single stereoisomer. The allene complex 182 is produced by treatment with BF3OEt2, indicating that 181 is derived from 182 and water. The 7i-allyl complex 183 is formed by photolysis via a disrotatory process. 

Based on the discussed acylpalladium 7i-allylic complex (Scheme 5.22) and the reported X-ray structure of the (R)-MOP—Pd 7i-allylic complex [31], the acylpalladium (R)-MOP Ti-allylic complex C (Scheme 5.24) is proposed for the formation of the (R)-product. Complex D, which would give the (S)-product, suffers from steric compression between the MeO-naphthyl ring and the acyl group, while there is no such steric interaction in complex C. Thus, reductive elimination of Pd(0) from C would preferentially yield the... 

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 7i-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two lectron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

Pd-catalyzed reactions proceed via 7i-allyl complexes, which at room temperature isomerize via a ti-o-ti rearrangement. As a consequence, branched as well as Z-and B-linear starting materials yield the same products, with memory effects being minimal at room temperature [16], The isomerization processes of (allyl)Ir complexes are usually slow, and accordingly any memory effects are pronounced. The configurational stability of (allyl) Ir intermediates of the Ir-catalyzed allylic substitution was studied by an investigation of substitutions at nonracemic allylic substrates (Scheme 9.5). 

A second type of organopalladium intermediates are 7r-allyl complexes. These complexes can be obtained from Pd(II) salts and allylic acetates and other compounds with potential leaving groups in an allylic position.79 The same type of 7i-allyl complexes can be prepared from alkenes by reaction with PdCl2 or Pd(02CCF3)2.80 The reaction occurs by electrophilic attack on the n electrons followed by loss of a proton. The proton loss probably proceeds via an unstable species in which the hydrogen is bound to... 

Reaction of 3a with butadiene or isoprene results in the formation of the 7i-allyl complexes 7a and 7b, respectively, which are stable in benzene solution for days at 25 °C.18 Hydrocarbon solutions of 3a react with 2,3-dimethylbutadiene to initially form the titanacyclopent-3-ene complex 8, isolated as a purple crystalline solid, which then subsequently reacts with generated free ethylene to form a mixture of the isomers 9 and 10 (Scheme 5). In an earlier paper, Rothwell and co-workers19... 

Similar to the formation of allylmagnesium chloride (25), the oxidative addition of allyl halides to transition metal complexes generates allylmetal complexes 26. However, in the latter case, a 7i-bond is formed by the donation of 7i-electrons of the double bond, and resonance of the n-allvl and 7i-allyl bonds in 26 generates the 7i-allyl complex 27 or (/ -allyl complex. The carbon-carbon bond in the 7i-allyl complexes has the same distance as that in benzene. Allyl Grignard reagent 25 is prepared by the reaction of allyl halide with Mg metal. However, the 7i-allyl complexes of transition metals are prepared by the oxidative addition of not only allylic halides, but also esters of allylic alcohols (carboxylates, carbonates, phosphates), allyl aryl ethers and allyl nitro compounds. Typically, the 7i-allylpalladium complex 28 is formed by the oxidative addition of allyl acetate to Pd(0) complex. 

No change of the formal oxidation states of the metals occurs in most of these nucleophilic attacks. However, an exception is palladium in the 7i-allyl complex 90,... 

Both stoichiometric and catalytic reactions of allylic compounds via 7r-allyl complexes are known. Reactions of nucleophilic 71-allyl complexes with electrophiles involve oxidation of metals and hence constitutes stoichiometric reactions. 7i-Allyl complexes of Ni, Fe, Mo, Co and others are nucleophilic and undergo the stoichiometric reaction with electrophiles. However, electrophilic 71-allyl complexes react with nucleophiles, accompanying reduction of metals. For example, 71-allylnickel chloride (2) reacts with electrophiles such as aldehydes, generating Ni(II), and hence the reaction is stoichiometric. In contrast, electrophilic 7i-allylpalladium chloride (3) reacts with nucleophiles such as malonate and Pd(0) is generated. Thus repeated oxidative addition of allylic compounds to Pd(0) constitutes a catalytic reaction. 

Among 7i-allyl complexes of several transition metals, the chemistry of n-allylpalladium has been studied most extensively. From the standpoint of organic synthesis, reactions involving 7i-allylpalladium complexes are by far the most important therefore, their synthetic applications are mainly treated in this chapter. 

Vinyloxiranes are used for facile 7i-allyl complex formation [14], The -allylic ferralactone complex 41 was prepared by oxidative addition of Fe2(CO)9 to the functionalized vinyloxirane 40 and CO insertion. Treatment of the ferralactone complex 41 with optically active a-methylbenzylamine (42) in the presence of ZnCl2 gave the 7r-allylic ferralactam complex 45 via 44. In this case, as shown by 43, the amine attacks the terminal carbon of the allylic system and then the lactone carbonyl. Then, elimination of OH group generates the 7r-allylic ferralactam complex 45. Finally the /1-lactam 46 was obtained in 64% yield by oxidative decomplexation with Ce(TV) salt. The <5-lactam 47 was a minor product (24%). The precursor of the thienamycin 48 was prepared from 46 [15,16]. This mechanistic explanation is supported by the formation of both 7r-allyllactone and lactam complexes (49 and 51) from the allylic amino alcohol 50 [17]. 

Several linear cooligomers of butadiene are prepared with alkenes and alkynes. Commercially important 1,4-hexadiene (103) is prepared by the reaction of ethylene and butadiene catalysed by Ni [40], Fe [41] and Rh [42], The experiment carried out using deuterated ethylene (100) supports the mechanism that the insertion of butadiene to M—H forms the 7i-allyl complex 99. Insertion of ethylene (100) to 99 gives 101, and its -elimination affords the cooligomer 102, tetradeuterated at C-1,1,2,6 of 103. 

Ene-type products are obtained by Co- and Fe-catalysed reaction of dienynes. Cocatalysed cyclization of substrate 111 proceeds smoothly with respect to the diene, acetylene and allylic ether moiety to afford 114. In this cyclization, the 7i-allyl complex 112 is formed by insertion of the diene to Co—H, followed by domino insertions of the triple and double bonds to give 113. The final step is the elimination of the /J-alkoxidc group from 113 to form 114 [47], The six-membered ene-type products 117 and 118 are obtained from the reaction of 115 catalysed by an Fe bipyridyl complex. The reaction seems to involve oxidative cyclization to form 116. Subsequent -elimination and reductive elimination provide 117 and 118. As another possibility, insertion of the diene to Fe—H gives a 7i-allyl complex. Then double bond insertion and -elimination should give 117 and 118 [48],... 

As an example of carbometallation, the 1,4-carbosilylation product 218 is obtained by the reaction of dienes, disilanes and acid chlorides of aromatic and a,/i-unsaturatcd acids at 80 °C. The phenylpalladium 216 is formed by the oxidative addition of benzoyl chloride, followed by facile decarbonylation at 80 °C, and reacts with butadiene to generate the benzyl-7i-allylic complex 217. Then, transmetallation with the disilane and reductive elimination afford 4-silyl-2-butenylbenzene 218 [92], Regioselective carbomagnesation of isoprene with allylic magnesium bromide 219 catalysed by Cp2TiCl2 gives 220, which is useful for terpene synthesis [93,94],... 

Precisely this approach was used to prepare a variety of metallocenes and 7i-allyl complexes, for example 838 (3.292) [722] ... 

By controlling the stoichiometry of the reaction, selective conversion of metallacyclobutenes to the corresponding 7i-allyl complexes can be accomplished, illustrated for both rhenacyclobutene 25 (Equation 8) <1998JA722> and platinacyclobutene 26 (Equation 9) <19980M2953>. 

Cleavage of polymer-bound allyl esters with palladium catalysts provides general access to 7i-allyl complexes, which can react with a variety of nucleophiles. This has been used in the development of re-allyl-based linkers. Ene-yne cross metathesis and subsequent cleavage in the presence of different nucleophiles yields the corresponding functionalized dienes 93 [93] (Scheme 6.1.19). 

Another difference between the two mechanisms is that the former involves 1,2-and the latter 1,3-shifts. The isomerization of 1-butene by rhodium(l) is an example of a reaction that takes place by the metal hydride mechanism, while an example of the 7i-allyl complex mechanism is found in the Fe3(CO)i2-catalyzed isomerization of 3-ethyl-l-pentene. A palladium catalyst was used to convert alkynones RCOCSCCH2CH2R to 2,4-alkadien-l-ones, RCOCH=CHCH=CHCHR. The reaction of an en-yne with HSiCls and a palladium catalyst generated an allene with moderate enantioselectivity (see p 148 for chiral allenes). ... 

More recently, a tandem Heck/allylic substitution cascade was used as a route toward various lactams. The reaction of 631a,b with 2-isopropenyl bromide in the presence of Pd(OAc)2 and (o-tolyl)3P gave the Heck adduct 632 that reacted further with H-Pd-Br to give the 71-allylpalladium complex 633. Intramolecular attack of the amide anion on the 7i-allyl complex followed by reductive elimination of Pd(0) afforded 634a,b in 77 and 75% yield, respectively (03OL259). Other vinyl bromides were also examined and found to give substituted pyrrolidinones and piperidones in moderate to good yields (50-82%). 

Keywords Allylic substitution, AUylation, Allylic alkylation, 7i-Allyl complexes. Palladium, Molybdenum, Tungsten, Iridium... 

A 7i-allyl complex is not involved. The most favored complex is the one in which the interactions between the phosphine and the allylamine substituent are minimized [882],... 

The cationic Pd(II)-7i-allyl complexes of the formula [(Ligand)Pd(ri3-C3H5)]PF6 (Ligand = ferrocenyl imino-phosphine la-e) were isolated in the solid state by the literature procedure15,61 using [(ri3-C3H5)PdCl]2 as starting material.[7] The complexes obtained were used as catalyst precursors for the asymmetric allylic... 

Racemization by this mechanism seems to be a general problem for allyl esters and phosphates since, as the following discussion shows, successful chirality transfer in intermolecular reactions is observed only for 1,3-disubstituted 7i-allyl complexes, which cannot racemize via this pathway. 

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