Allyl complexes of palladium

Also, the trimerization reaction was observed by using jr-allyl complexes of palladium stabilized by chelating ligands, such as 28 and 29 (38). The reaction was carried out at 70°C in dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), and 1,3,6,10-dodecatetraene (27) was obtained in 60% yield at 30% conversion. 

F.Guibf and Y. Saint M Leux, The allyloxycarbonyl group for alcohol protection Quantitative removal or transformation into allyl protecting group via w-allyl complexes of palladium, Tetrahedron Lett. 22 3391 (1981). 

Polar, aprotic solvents such as DMF were necessary for the formation of (126). [Pd(diphos)2] (104) gave the best performance as catalyst. Using [Pd(DBA)2] (105) as palladium source, a number of mono- and bi-dentate nitrogen, phosphorus and arsenic ligands were investigated and Me2PCH2CH2PMe2 was found to be nearly as effective as diphos. The reaction of C02 with a bis(i73-allyl) complex of palladium was believed to be a key step in the process (equation 158). 

An approach to enantiomerically pure (R)- and (S)-2-(alkenyl)tetrahydrofurans has been devised via a nucleophilic substitution on an allylic complex of palladium(O). This substitution proceeds with chirality transfer from a preexisting stereogenic center to the newly formed center. Thus, starting from (/ )-(,E)-7-methyI-4-octene-l,6-dioI (1), an efficient transfer is realized by treatment with a catalytic amount of tetrakis(triphenylphosphane)palladium in the presence of triethylamine in acetonitrile66. After 30-45 min at 35-37 °C, (5)-tetrahydro-2-f( )-3-methyl-l-butenyl]furan (2) is obtained in 95% yield and 80% ee. The absolute stereochemistry and the optical purity [a]D —10.7 (c = 2.279, CH2C12) are conveniently determined by conversion to the known (S)-tetrahydro-2-furanmethanol. 

Hegedus and Tamura (66) postulated that reactions of acyliron, acyl-nickel, and cobalt carbonyl anions with ij -allyl complexes of palladium proceed via unstable bimetallic intermediates with bridging o-,7r-alkenyl ligands, L (RC(0))M T7 T7--CH=CHR PdL . Products isolated from the acylation of rr-allyl ligands were a,/3- and a,y-unsaturated ketones. 

Cyclopropanes can also be generated via decomplexation of cyclopropane containing allyl complexes of palladium. This reaction mode is represented by a palladium-catalyzed conversion of 1 -sulfonyloxy-1 -vinylcyclopropanes 6 or 1 -chlorovinylcyclopropanes, which, via nucleophilic substitution with allylic rearrangement, gave methylenecyclopropanes 7 with the heterofunction in the side chain.An allylic cyclopropane-containing palladium complex is suggested as the intermediate. -... 

Nucleophilic attack of the central carbon of allyl ligands represents an important access to metallacyclobutanes independent from cyclopropanes as precursors. This reaction pathway competes with the more common attack at one of the terminal allylic carbons. However, several examples of metallacyclobutane formation followed by release of cyclopropane products have been reported, especially with allyl complexes of palladium, platinum, and iridium as stable precursors or reactive intermediates in catalytic cycles. Detailed experimental (see below) and theoretical studies considering chemo-, regio- and stereoselectivity of the crucial reaction steps with respect to influence of the metal, ligands, substituents and reaction conditions are available. ... 

An example of the variation of the character of the carbon metal bonds is the 7r-allyl complexes of palladium and nickel. They are similar in structure. However, the nickel complex reacts with electrophiles such as aldehydes, ketones, and acrylonitrile 28,36,37)... 

Several recent studies have revealed the dichotomous relationship between the regioselectivity of reactions of unsymmetrical allyl complexes of palladium and that of reactions of unsjmmetrical allyl complexes of other metals. An isolated molybdenum-allyl complex containing an allyl group derived from methyl cinnamyl carbonate (PhCH=CHCH2C)COjMe) reacts with the sodium salt of dimethyl malonate in the presence of a source of CO to generate the branched substitution product (Equation 11.41). The addition of nucleophile to this allyl complex has been shown to occur by retention of configuration, perhaps by initial attack at the metal, rather than the more common external attack at Are allyl group. ... 

OLEFIN AND tt-ALLYLIC COMPLEXES OF PALLADIUM, PLATINUM AND IRIDIUM,... 

B3 OLEFIN AND ir-ALLYLIC.COMPLEXES OF PALLADIUM, PLATINUM AND IRIDIUM, M.S. Lupin, S.D. Robinson and B.L. Shaw... 

Allyl Complexes of Palladium and Platinum. A preliminary report has appeared on the structure of one of thediastereoisomersof ( -methylallyl)-(2 it,5>a-(phenylethylimino-3-penten-4 1ato)palladium(n) (92). ThePd-C... 

The various 7r-allylic-type ligands which have so far been recognized are shown in the first column of Table IV. At present more 7r-allylic complexes of palladium are known than of the other metals, but the preparation of TT-allylic complexes of other metals is more a matter of time than chemistry in that similar complexes of most of the transition metals should be obtainable. 

Allyl complexes of palladium and iridium are conveniently generated by oxidative addition between Pd(0) and Ir(I) catalysts and allylic acetates and carbonates. Chiral allyl complexes... 

Interestingly, trimethylsilylmethyl-substimted allyl complexes of palladium 11.105 and molybdenum can be obtained from the reaction of the corresponding mesylate 11.104 (a much better leaving group than acetate) in the absence of any silophilic nucleophile, while some other metals, including iridium, did give a TMM complex 11.106 (Scheme 11.35). ... 

Allylic complexes of palladium can be used as a source of the allyl moiety to synthesize other metal -allylic complexes. Redox transmetallation to a Pt(0) complex leads to a platinum 77 -allyl derivative (Equation (44)). The reaction is easier when a cationic palladium allyl is used, and can be viewed as a nucleophilic attack of the Pt(0) complex to the allylic fragment. The transmetallation takes place with inversion of configuration of the allyl group. The same reaction occurs with Pd(0) complexes, a redox exchange with inversion of configuration of the allylic fragment. However, when a neutral palladium(ll) 77 -allyl complex is employed, the reaction leads to dimeric palladium(l) derivatives, as discussed before (Equation (41)). 

The use of ir-allyl complexes of palladium as intermediates in nucleophilic displacement reactions enables both the stereochemistry and regiochemistry of the reaction to be controlled. 

In asymmetric olefins the carbon monoxide attack occurs at the most substituted carbon, although olefins higher than pentene behave differently. Carbonylation of 7r-allylic complexes of palladium also takes place at the carbon with the least number of hydrogens and upon hydrolysis gives unsaturated acids. 

Palladium.—number of papers have appeared dealing with the many facets of allyl rearrangement in A -allyl complexes of palladium. The criteria for tackling these often complicated systems have already been outlined in this section in Volume 2 of these Reports and need no further mention here. For the sake of convenience the reports will be dealt with in two categories, intrmnolecular rearrangements and rearrangements promoted by substitution. 

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