Tr-allyl complexes

When butadiene is treated with PdCU the l-chloromethyl-7r-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-I,3,6-octatriene to give 337 as expected, whereas myrcene (338) is converted into the tr-allyl complex 339 after the cyclization[288]. 

In 1965 Kwiatek and Seyler 109) reported the existence of a cyanide-dependent equilibrium involving a- and Tr-allyl complexes, revealed by NMR spectroscopy in aqueous solution, viz.. 

Some of the evidence for such structures comes from the change in product distribution of the butenes as a function of cyanide concentration when butadiene is hydrogenated with pentaeyanocobaltate(II) catalyst or when the a butenyl complex is reduced with the hydride complex [HCo(CN)5] . Thus 1-butene is the major product in the presence of excess CN, and major product in the absence of excess cyanide. The 1-butene presumably arises from the cleavage of a tr complex, and the 2-butene via an intermediate w-allyl complex. The Tr-allyl complexes of cobalt tricarbonyl are well-characterized and can be prepared either from butadiene and HCo(CO)4 or from methallyl halide and NaCo(CO)4 [49). 

Fio. 11. The interconversion of syn and anti Tr-allyl complexes via a a-bonded allyl strueture. 

Treatment of ir-allylpalladium chloride with CO in EtOH affords ethyl 3-butenoate (321)[284]. f3, p-Unsaturated esters, obtained by the carbonylation of 7r-allylpalladium complexes, are reactive compounds for Tr-allyl complex formation and undergo further facile transformation via ir-allylpalladium complex formation. For example, ethyl 3-butenoate (321) is easily converted into 1-carboethoxy-7r-allylpalladium chloride (322) by the treatment with Na2PdCl4 in ethanol. Then the repeated carbonylation of the complex 322 gives ethyl 2-... 

XLIV). The chemical shift and spin-coupling data for these two isomers is given in Table II. Of special interest is the almost zero value of the coupling constant for the gem-protons, viz., Jhc. This phenomenon is general for Tr-allylic complexes, and has usually been explained in terms of the... 

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(I) is an example of a reaction that takes place by the metal hydride mechanism,69 while an example of the Tr-allyl complex mechanism is found in the Fe3(CO)i2-cataIyzed isomerization of 3-ethyI-l-pentene. A palladium acetate or palladium complex catalyst was used to convert alkynones RCOGsCCH2CH2R to 2,4-alkadien-l-ones RCOCH=CHCH=CHCHR. 71... 

The development of methods to effect nucleophilic addition to carbon-carbon double bonds by prior activation with metal cations has been applied, at least in a preliminary way, as a method of pyrrole ring closure. The conversion of butadienes to N-substituted pyrroles can be accomplished in two stages. In acetic add, 1,4-dienes react with PdnCl2 to give tr-allyl complexes with introduction of acetate at C-4. The ir-allyl complexes then react with amines to give a l-amino-4-acetoxy-2-butene (equation 70). When the addition of the amine is carried out in the presence of a silver salt and triphenylphosphine, a pyrrole is isolated, probably by cyclization of the amino-substituted allyl-Pd complex (equation 71) (81CC59). Although this procedure is attractive in terms of the simplicity of the... 

The treatment of the UPS of the tr-allyl complex (CsHs)2Nb(C3Hs) is similar to that of the ethylene complexes. One significant difference between the ethylene and tr-allyl complexes, however, is that the symmetry is reduced to C2 in the latter. The correlations between the appropriate orbital symmetries in the C2v and Cs point groups are indicated in Table XIV. 

Methyl ketones can be catalytically produced when an excess of TBHP is used for regenerating the initial f-butyl peroxide species from the resulting alkoxy complex in Scheme 6- To prevent the formation of a Tr-allylic complex from causing lower selectivities, a large excess of TBHP with respect to the alkene is required (equation 86).260... 

An extremely useful alternative method, employing catalytic amounts of Pd° complexes, rather than stoichiometric (or near stoichiometric) quantities of Pd11 salts has been developed. The oxa-tr-allyl complex is accessed via allyl p-ketocarboxylates360-363 or allyl alkenylcarbonates360,361,363,364 in the intramolecular cases and via enol acetates,361,363,365 enol silyl ethers361,366 or ketene silyl acetals367 with allyl carbonates in the intermolecular cases. The products of these reactions are the corresponding a,p-un-saturated ketones, aldehydes and esters (equations 138-142). 

Addition of dimethylamine to 1,3-cyclohexadiene catalyzed by Pd11 complexes proceeds via an amino-Tr-allyl complex of known configuration, to which a second equivalent of HNMe2 adds on the face opposite Pd to yield a cis- 1,4-diaminocyclohexene (equation 183).185 Similar results were obtained from an acetoxychlorocyclohexene (equation 184)195 and in the preparation of amino sugars (equation 185).208... 

This method for tr-allyl complexes is of wider application than most others and examples are found in the V, Cr, Mn, Fe, Co and Ni triads because allylic halides, esters, etc., are reactive electrophiles. Mild conditions can be used, and this is often required owing to the limited thermal stability of most organotransition-metal complexes. A second reason is that metal complexes of the above triads easily adopt low-valent,... 

Fe2(C0)fl at 50°C under pressure produces, in addition to this same 7T-allylene complex, the o-.Tr-allyl complex (CaH4)Fe2(CO)7 (66) (58,156). 

A somewhat similar reaction of 3-hexyne with acyltetracarbonyl-cobalt produces the Tr-allyl complex 3,4-alkeno-4-lactonyltricarbonyl-cobalt (282). 

Secondary or primary amines Dienes Primaiy attack resulting in a Tr-allyl complex [3]... 

Scheme 3-26 Fonnation of Tr-allyl complexes from 1,3-dienes and allenes.

PtMeL2] proceed in a Markownikov manner by electrophilic attack of Pf thus [Pt(A -2-methallyl)L2] is formed from allene and [PtMe-(acetone)L2], whereas the analogous 1,3-butadiene cation does not lead to a 7r-allylic derivative by Pt—Me insertion. The hydro cation, however, can react by either a Markownikov or an anti-Markownikov mechanism with either Pt+ or attack on the unsaturated ligand. This apparent versatility leads to the formation of Tr-allylic complexes from both allenes and 1,3-dienes with [PtHLg]. ... 

These results showed that dimethyl-1,3-77-allylic products were formed, along with pentadiene-1,3, implying that double-bond migration occurred initially, to be followed by formation of Tr-allylic complexes. This preferential formation of 7r-allyls is particularly important since the favored route via 7r-allyls in the presence of butadiene explains why it is possible to obtain high yields of hexadienes from butadiene and ethylene so long as butadiene is still present. 

Several new types of allylic complexes have recently been prepared. Atkinson and Smith prepared Tr-allylic complexes from 2,2,4-trimethy-pent-3-en-l-ol of the type [(CHg—CHMeCH)CMe2CHaOH]PdCl 2, in which the alcohol function is not coordinated to the Pd(II) (10). In contrast, the Rh(III) and Pt(II) complexes are olefin complexes, with the alcohol function complexed to the metal. 

The nature of the oxidation products is traceable to the nature of the rhodium-alkene interaction. Terminal alkenes and internal ones (e.g. cycloheptene), which form tt-complexes of rhodium(I), e.g. [RhCl(alkene)2]2, are selectively converted into methyl ketones, whereas alkenes which form Tr-allylic complexes of rhodium(IIl) e.g. cyclopen-tene) give alkenyl ethers via oxidative substitution of the alkene by the solvent alcohol. ... 

Periana RA, Bergman RG (1984) Rapid intramolecular rearrangement of a hydridocyclopro-pylrhodium complex to a rhodacyclobutane. Independent synthesis of the metallacycle by addition of hydride to the central carbon atom of a cationic rhodium Tr-allyl complex. J Am Chem Soc 106 7272-7273... 

An extraordinary effect is noted for cydizalion of the carbonate derived from dihydrocar-vone (Table 14, entries 16 and 17). When R = H, no reaction is observed with the depicted substrate (Table 14, entry 16) or with the regioisomeric allylic carbonate22. Palladium cannot coordinate to the olefin because of steric reasons. However, cyclization occurs smoothly in the presence of an alkyne group in the molecule (Table 14, entry 17), because precoordination of palladium(O) to the alkyne enables the intramolecular formation of the initial palladium(O)-alkene complex and subsequent ionization to the tr-allyl complex. The product has been further elaborated via an ene-yne cyclization and eventually transformed into the alkaloid (-)-den-drobine. Entry 18 shows the preparation of a precursor for (-)-aspochalasin B23. Cyclization to the 11-membered ring proceeds with high diastereoselectivity which has been used to advantage during further transformations. 

A number of enantiomerically pure complexes have been made, and this chemistry has been used in several natural product syntheses. Enantiopure complexes are readily available from the corresponding vinylic epoxides, and in cases where diastereoselective complexation is possible, diastereoselectivities tend to be moderate (typically 3 1 -4 1). The rationale for the origin of this diastereoselectivity has been proposed to derive from a preferential complexation of a Fe(CO)4 fragment to the alkene anti to the epoxide. Since the initial vinyl epoxide is conformationally flexible, four diastereomeric rr-complexes would be produced as a consequence of anti or syn complexation to the s-trans or 5-C/5 conformers. Isomerization of these initial rr-complexes to alkoxy-TT-allyl species would then enable interception of an iron-bound carbonyl ligand by the alkoxide to afford diastereomeric lactone complexes. Fortunately, equilibria between the two possible trans Tr-allyl complexes and their more stable cis Tr-allyl analogs simplifies the outcome significantly. Thus, for trans vinyl epoxides, the major diastereomer typically is the one designated as endo cis (the C-1 substituent points toward the iron atom) the minor diastereomer corresponds to the exo cis isomer (the C-1 substituent points away from the iron atom) (Scheme 51). For cis vinyl epoxides, this outcome is reversed - the exo cis isomer is the major product. 

The reaction is believed to proceed via a a-allyl copper complex, in which the carbon-copper bond is formed at the y-position, anti to the acetate leaving group. Reductive elimination of copper led to pure y-substitution. With cyclic aliphatic allylic acetates, the selectivity is generally lower because the o-allyl copper complex can isomeriz to the tr-allyl complex with loss of regioselectivity. 

The conducted experiments [70,71] demonstrate that PVP-Pd complexes are active, selective and stable catalysts. The composition of such catalysts represents a composite system including Pd(II) and Pd(0). The role of the polymer ligands evidently consists in the stabilization of the particular valent states of palladium which are optimum for the substrate hydrogenation. One can assume that in the given catalytic system, Pd(0) promotes the activation of hydrogen, whereas complex-bound Pd(II) promotes the formation of a tr-allyl complex with unsaturated double bonds of the substrate and thus its activation. Furthermore, pyridine rings promote substrate orientation. This assumption enables polymer-metal heterogenized catalysts to be considered as models of catalytic enzyme systems. 

In the absence of added phosphorus ligand, BD can take the place of L in intermediate 58. In that case, however, in addition to coupling Ci and Cg (or C3 and Cg), there is the possibility of coupling Ci with a terminal carbon of the coordinated butadiene. This leads to a new ftw-Tr-allyl complex 63, an intermediate in the cyclotrimerization of BD which can be isolated and is stable at low temperature in the absence of free BD or Recent C [ H] and NMR studies show that 63 exists as two isomers in solution, both with coordinated trans-oX fimc double bonds. The major one is assigned structure 64, where the double bond is parallel to the planes... 

---