7r-Allyl complexes

Typical nucleophiles known to react with coordinated alkenes are water, alcohols, carboxylic acids, ammonia, amines, enamines, and active methylene compounds 11.12]. The intramolecular version is particularly useful for syntheses of various heterocyclic compounds[l 3,14]. CO and aromatics also react with alkenes. The oxidation reactions of alkenes can be classified further based on these attacking species. Under certain conditions, especially in the presence of bases, the rr-alkene complex 4 is converted into the 7r-allylic complex 5. Various stoichiometric reactions of alkenes via 7r-allylic complex 5 are treated in Section 4. 

TT-Aliylpalladium chloride reacts with a soft carbon nucleophile such as mal-onate and acetoacetate in DMSO as a coordinating solvent, and facile carbon-carbon bond formation takes place[l2,265], This reaction constitutes the basis of both stoichiometric and catalytic 7r-allylpalladium chemistry. Depending on the way in which 7r-allylpalladium complexes are prepared, the reaction becomes stoichiometric or catalytic. Preparation of the 7r-allylpalladium complexes 298 by the oxidative addition of Pd(0) to various allylic compounds (esters, carbonates etc.), and their reactions with nucleophiles, are catalytic, because Pd(0) is regenerated after the reaction with the nucleophile, and reacts again with allylic compounds. These catalytic reactions are treated in Chapter 4, Section 2. On the other hand, the preparation of the 7r-allyl complexes 299 from alkenes requires Pd(II) salts. The subsequent reaction with the nucleophile forms Pd(0). The whole process consumes Pd(ll), and ends as a stoichiometric process, because the in situ reoxidation of Pd(0) is hardly attainable. These stoichiometric reactions are treated in this section. 

Treatment of 7r-allylpalladium chloride with CO in EtOH affords ethyl 3-butenoate (321)[284]., 3, y-Unsaturated esters, obtained by the carbonylation of TT-allylpalladium complexes, are reactive compounds for 7r-allyl complex formation and undergo further facile transformation via 7r-allylpalladium complex formation. For example, ethyl 3-butenoate (321) is easily converted into 1-carboethoxy-TT-allylpalladium chloride (322) by the treatment with Na PdCL in ethanol. Then the repeated carbonylation of the complex 322 gives ethyl 2-... 

Two monomeric and dimeric 2-substituied 7r-allylic complexes (548 and 549) are obtained by treatment of allene with PdCl2(PhCN)2. They are formed by the nucleophilic attack at the central carbon of allene[493, 494],... 

One other reaction deserves mention. From bis(cyclooctadiene)nickel and butadiene (31), and in the presence of an isocyanide (RNC, R = cyclohexyl, phenyl, tcrt-butyl) two organic oligomeric products are obtained, 1 -acylimino-11 -vinyl-3,7-cycloundecadiene and 1 -acylimino-3,7,11 -cyclo-dodecatriene. In each, one isocyanide has been incorporated. An analogous reaction with carbon monoxide had been reported earlier. The proposed mechanism of these reactions, via a bis-7r-allyl complex of nickel, is probably related to the mechanism described for allylpalladium complexes above. 

The nonacarbonyldiiron-induced transformation of oxazabicyclo[2.2.2] octenes (12) into condensed azetidinones is intriguing mechanistically but will obviously have limited synthetic application (Scheme 15).37 The /(-lactam (14a), among other products, is isolated directly from the iron carbonyl reaction but the dimethyl analog (14b) is obtained by pyrolysis of an isolable intermediate <7-7r-allyl complex (13b). 

One of the most recent developments in the field of Ni-catalyzed reactions of alkyl halides with organozinc derivatives is a study of Terao et al.411 They reported the use of three additives in the couplings 1,3-butadiene, N,N-bis(penta-2,4-dienyl)benzylamine 308a, and 2,2-bis(penta-2,4-dienyl)malonic acid dimethyl ester 308b. Addition of tetraene 308b to the reaction mixtures significantly increased the product yields (Scheme 157). The remarkable effect of these additives was explained by the formation of the bis-7r-allylic complex 309 as the key intermediate (Scheme 158). 

Cobalt hydroformylation of butadiene produced low yields (24%) of an equimolar mixture of n- and isovaleraldehyde (40). It has been established that the cobalt hydrocarbonyl adds to form a stable 7r-allyl complex (93, 94). 

This result suggests the formation of the 7r-allyl complex (20) as an... 

The carbonylation was explained by the following mechanism. Formation of dimeric 7r-allylic complex 20 from two moles of butadiene and the halide-free palladium species is followed by carbon monoxide insertion at the allylic position to give an acyl palladium complex which then collapses to give 3,8-nonadienoate by the attack of alcohol with regeneration of the zero-valent palladium phosphine complex. When halide ion is coordinated to palladium, the formation of the above dimeric 7r-allylic complex 20 is not possible, and only monomeric 7r-allylic complex 74 is formed. Carbon monoxide insertion then gives 3-pentenoate (72). 

Green demonstrated (48) that a 7r-allyl complexed to W or Mo can undergo a nucleophilic attack by H- on the central carbon of the tj3 allylic group, forming a stable metallocyclobutane. [See Eq. (20).] Cyclopropane and propylene were evolved (49) on heating the metallocyclic... 

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

Oxidative cyclizations involving 7r-allyl complexes have been reported and are thought to involve syn-CH cleavage (Equation (187)).1S1... 

The authors proposed two mechanisms. The first proceeds through a cationic intermediate and was ruled out after failed efforts to trap the cationic intermediate. The favored mechanism proceeds by allylic G-H activation, forming a 7r-allyl complex, which undergoes insertion and reductive elimination to give the cyclic product (Scheme 26). 

Other conjugated diene systems readily react with either H4Ru4-(CO)i2 or Ru3(CO)i2, to yield 7r-allyl complexes. Thus, butadiene yields the crotyl derivative HRu3(CO)9C4H5. The X-ray structure of the related adduct formed from cis-trans- or trans-trans-2,4-diene is shown in Fig. 12. The hydride is considered to bond to the Ru(l)-Ru(2) edge, as this is the longest metal-metal bond in the structure and in the... 

Some trimethylsilyl enol ethers, not able to undergo /3-H elimination, have been stoichiometrically converted into a-alkoxycarbonyl ketones via the intermediate formation of oxa-7r-allyl complexes (Scheme 23) [123,124]. 

Scheme 30 shows the proposed reaction mechanism, which involves the formation of an acylpalladium species as the key intermediate, in tautomeric equilibrium with a cyclic 7r-allyl complex (in this and in the following Schemes, unreactive ligands are omitted for clarity). The main reason for the high activity of the Pdl42 -based catalyst in this process lies in the very efficient mechanism of reoxidation of Pd(0), which involves oxidation of HI by 02 to I2, followed by oxidative addition of the latter to Pd(0). It is worth nothing that under these conditions Pd(0) reoxidation occurs readily without need for Cu(II) or organic oxidants. 

The interconversion of a- and 7r-allyl complexes has been observed by Kwiatek, Mador, and Seyler in the interesting homogeneous catalytic system of potassium pentacyanocobaltate(II), K3Co(CN)5 (46). This solution absorbs molecular hydrogen to form the active hydride species, Kj[Co (CN)jH]. Addition of butadiene to the hydride results in the formation of a [Pg.36]

A variety of 7r-allyl complexes are possible, including the transannular ones. It should be noted that, although 1,3-cyclooctadiene is thermodynamically the most stable diene, the stability of the 1,5 complex provides the driving force for the rearrangement. The 1,4 diene complex does not appear to be an intermediate, suggesting that the hydride abstraction-addition sequence is not straightforward. 

The application of tungsten-<7r-allyl complexes provided yet another elegant and efficient access to paraconic acids (Scheme 29) [68]. The propargylic al-... 

Olefin, Acetylene, and 7r-Allylic Complexes of Transition Metals R. G. Guy and B. L. Shaw... 

Subsequently to rhodium coordination with the enyne to form X, oxidative addition with the allyl chloride affords a rhodium-7r-allyl complex. Then isomerization... 

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... 

Further development of termination strategies has led to the use of allenes, which trap the organopalladium intermediate to give 7r-allylic complex. The latter undergoes facile reactions with nucleophiles. The overall process becomes a pentamolecular cascade (Scheme 17). 

Cobalt hydrocarbonyl reacts rapidly with conjugated dienes, initially forming 2-butenylcobalt tetracarbonyl derivatives. These compounds lose carbon monoxide at 0°C. or above, forming derivatives of the relatively stable l-methyl-ir-allyl-cobalt tricarbonyl. As with normal alkylcobalt tetracarbonyls, the 2-butenyl derivatives will absorb carbon monoxide, forming the acyl compounds but these acyl compounds also slowly lose carbon monoxide at 0°C. or above, forming 7r-allyl complexes. The acyl compounds can be isolated as the monotriphenylphosphine derivatives (47). 

When butadiene is treated with PdCl2 the l-chloromethyl-Tr-allylpalladium complex 336 (X = Cl) is formed by the chloropalladation. In the presence of nucleophiles, the substituted 7r-methallylpalladium complex 336 (X = nucleophile) is formed[296-299]. In this way, the nucleophile can be introduced at the terminal carbon of conjugated diene systems. For example, a methoxy group is introduced at the terminal carbon of 3,7-dimethyl-l,3,6-octatriene to give 337 as expected, whereas myrcene (338) is converted into the 7r-allyl complex 339 after the cyclization[288]. 

The intramolecular insertion of a conjugated diene into 7r-allylpalladium, initially formed in 789, generates another 7r-allyl complex 790, which is trapped with acetate anion to give a new allylic acetate 791. No further reaction of the allylic acetate with alkene takes place[489]. 

The discovery of 7r-allylic complexes is an exciting recent development in coordination chemistry. In these complexes the bonding of the allylic radical... 

A review of diene-iron carbonyl complexes has recently appeared (5) metal complexes of di- and oligoolefinic ligands have also been reviewed (6). A general review of olefin, acetylenic, and 7r-allylic complexes of transition metals is due to Guy and Shaw (7). 

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