Allenylpalladium intermediates

The proposed meehanism for this eyeloearbonylation was shown to involve the insertion of Pd(0) speeies into the C—O bond of the substrate followed by rearrangement to the allenylpalladium intermediate 29 (as proposed for alkyl systems). Insertion of CO and subsequent reduetive elimination may lead to the 2,3-dienoie aeid 30, whieh undergoes eyelization, eatalyzed by traee quantities of an aeid present in the solvent, to the 2(5//)-furanone 28 (Seheme 10) (93JOC1538). 

The Pd-catalyzed coupling reaction of the propargyl acetate 53 and 4-pentynoic acid (54) in the presence of potassium bromide produced the unsaturated exo-enol lactone 55 [66], The reaction proceeded via oxypalladation of the triple bond of 54 with an allenylpalladium intermediate, which was formed from Pd(0) and 53 and the carboxylate as shown in Scheme 3.30. 

In other examples, also involving propargyl carbonates, the parent derivative 86 was first coupled with 87 - obtained by reaction of 5-octyne with the titanium diiso-propoxide - propene complex at -50 °C, providing the titanated vinylallene 88, which on hydrolysis furnished the vinylallenes 89 in good yield [29]. Carbonate 90 in the presence of a Pd° catalyst readily decarboxylated and yielded the allenylpalladium intermediate 91, which could be coupled with various vinyl derivatives to afford the vinylallenes 92. Since X represents a functional group (ester, acetyl), functionalized vinylallenes are available by this route [30]. 

Some synthetically important allenylmetallics, such as allenylzinc and allenylin-dium reagents, are prepared from allenylpalladium intermediates. These reactions are discussed in appropriate sections of this chapter. This section covers the reactions of allenylpalladium compounds without further transmetallation. Allenylpalladium complexes can be prepared from propargylic halides, acetates, carbonates, mesylates, alcohols and certain alkynes [83-87], The allenylpalladium compound prepared from 3-chloro-3-methyl-l-butyne has been isolated and characterized spectroscopically (Eq. 9.106) [83], It was found to couple with organozinc chlorides to produce homologated allenes quantitatively (Eq. 9.107). 

Scheme 9.26 Possible mechanism for heteroannulation promoted by cr-allenylpalladium intermediate.

Scheme 9.34 Oxidative transmetallation of allenylpalladium intermediates with Ini.

In situ transmetallation of allenylpalladium intermediates with Sml2 provides a route to allenylsamarium reagents [123]. These undergo protonolysis with alcohols to afford mainly allenes (Table 9.59). Additions to ketones yield either allenylcarbi-nols (A) or homopropargylic alcohols (B) depending on the allene substituents (Table 9.60). 

An alternative preparation of enantioenriched anti-homopropargylic alcohols along similar lines uses EtjZn to effect in situ transmetallation of an allenylpalladium intermediate from a propargyl mesylate and 2.5 mol % of Pd OAc)2 PPh3 in the presence of an aldehyde (Table II). The two methods are comparable. 

When the palladiozincation was performed in the absence of an aldehyde substrate, two products could be isolated. The major product was the ethyl-substituted allene resulting from reaction of the putative allenylpalladium intermediate with diethylzinc (equation 22). Interestingly, this product is formed in only trace amounts when an aldehyde substrate is present. The second adduct is a dimeric allene, which can be envisioned as arising from coupling of the allenylzinc and allenylpalladium intermediates. 

Polycyclic compounds are prepared by domino insertions of alkenes and alkynes to the allenylpalladium intermediates. As examples, 1,6-enyne 28 underwent intramo-... 

Reaction of PhZnCl with 3-acetoxy-3-methyl-1-butyne (116) gives l-phenyl-3-methyl-1,2-butadiene (119) in high yield [28-30]. The reaction can be explained by transmetallation of the allenylpalladium intermediate 117 with PhZnCl to generate the allenyl(phenyl)palladium intermediate 118, followed by reductive elimination to afford 119. 

Most of the Pd(0)-catalyzed reactions of propargylic compounds discovered so far can be understood by the formation of the allenylpalladium intermediates 8 and belong to Types I, II and III. 

Reactions via Transmetallation of Allenylpalladium Intermediates and Related Reactions (Type II)... 

Reactions via Attack by Soft Carbon and Oxo Nucleophiles on the Central Carbon of Allenylpalladium Intermediates (Type... 

Marshall and co-workers have demonstrated that allenylindium [277] and allen-ylzinc [276] reagents can be formed in situ from propargyl mesylate 389 and that these reagents react enantioselectively with aldehydes without additional Lewis acid catalysis (Scheme 11-28). The allenylzinc and allenylindium reagents 392 and 391 are derived from the allenylpalladium intermediate 390 via metathesis with Et2Zn and Ini, respectively. Allenylpalladium intermediate 390 in turn is derived from (7 )-389 via invertive displacement of the mesylate functionality of 389 with Pd(0). 

Transmetalation of an allenylpalladium intermediate with Ini proceeds stereoselec-... 

The Pd-catalyzed reaction of o-alkynylphenol 39 with tertiaryl propargyl carbonate 40 gave the 2-substituted 3-allenylbenzo[i>]furan 43 under neutral conditions. Attack of phenoxide anion to the triple bond from the opposite side of a-allenylpalladium intermediate as shown by 41 generates allenyl(benzofuryl)palladium intermediate 42, and the coupling product 43 is formed by reductive elimination [7]. 

Reactions of Nucleophiles on Central sp Carbon of Allenylpalladium Intermediates... 

The asymmetric three-component reaction of the carbonate 132 with o-methoxyphenol under CO2 atmosphere afforded the cyclic carbonate 135 in 83 % yield with 91 % ee using ( l-BINAP as a chiral ligand. In this reaction, the r-allylpalladium intermediate 133, formed by the attack of phenol to the allenylpalladium intermediate, reacts with CO2 to generate the carbonate 134. Then intramolecular attack of the carbonate anion to the r-allylpalladium terminus provides the cyclic carbonate 135 [40]. 

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