Allenylpalladium transmetallation

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

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). 

A more satisfactory sequence, which also starts with enantioenriched propargylic mesylates, generates the allenylindium intermediate by oxidative transmetallation of an allenylpalladium precursor with Ini. This catalytic process takes place in the presence of an aldehyde. The reaction employs 5-10 mol% of the palladium catalyst and a stoichiometric quantity of Ini. Based on the stereochemistry of the starting... 

Scheme 9.34 Oxidative transmetallation of allenylpalladium intermediates with Ini.

Sulfonic amides of alkynylaziridines can also be converted to allenylindium reagents by oxidative transmetallation of transient allenylpalladium complexes. When the reaction is perfomed in the presence of an aldehyde the expected adducts are produced stereoselectively (Eq. 9.150) [120],... 

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. 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-I,2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5]. The reaction can be explained by the transmetallation of the c-allenylpalladium methoxide 4 with copper acetylides to form the allenyl(alk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCN) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7],... 

Transmetallation of allenylpalladium species with indium salts has been previously reported Marshall, J. A. Grant, C. M. J. Org. Chem. 1999, 64, 8214-8219. 

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. 

Umpolung of propargyl compounds occurs in the presence of excess Et2Zn, and homopropargyl alcohols 187 are obtained by the reaction of propargyl benzoates (186) with benzaldehyde, although the exact mechanism is not known. Ethylallene, which is expected to be formed by transmetallation of the allenylpalladium with Et2Zn and reductive elimination, is not formed [41],... 

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

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

Table 2 Umpolung ofn-allyl-, 71-propargyl-, and allenylpalladium by transmetallation with organometallics...

The propargylplatinum(II) complex reacts with Pd(0) complex to form Pt(0) complex and allenylpalladium(II) complex, the latter of which undergoes spontaneous and/or Pd(0)-catalyzed rapid isomerization into the propargylpalladium(II) complex. The reaction of the produced Pt(0) complex and propargylpalladium(II) complex leads to the formation of allenylplatinum complex. Each of the transmetalation reactions in the above procedure is accompanied by allenyl-propargyl... 

The allenylpalladium 27, formed from the carbonate 26, undergoes 5-exo cycliza-tion to give 28 and its 3-exo cyclization, without f-E. elimination, generates the bicyclo[3.1.0]hexane system 29, which has no -hydrogen to be eliminated. Thus transmetallation of 29 with the tin reagent 30, followed by reductive elimination affords the azabicyclo[3.1.0]hexane 31 diastereoselectively [4], The synthesis of (—)-a-thujone was achieved by diastereoselective domino cyclization of the chiral carbonate 32, and trapping with dimethylzinc to give 33 as a key reaction [5],... 

In general, Pd-catalyzed reaction of propargyl compounds provides synthetically valuable allenyl compounds through addition, transmetallation, or oxypalladation of allenylpalla-dium intermediates. Exceptionally, soft carbon nucleophiles such as malonate and methyl acetoacetate attack the sp carbon of allenylpalladium intermediates to afford allylic compounds and furan derivatives. 

Enantiomerically pure allenylzinc and indium reagents are readily available through oxidative transmetallation of transient allenylpalladium intermediates derived from sulfonic esters of (/ )- or (5)-3-butyn-2-ol derivatives, such as 250. When these transmetaUations are conducted in the presence of aldehydes, anti homopropargylic alcohol adducts 251 are formed with high diastereoselectivity (Scheme 10.49). The preference for anti over syn adducts can be assigned to an unfavorable eclipsing interaction between the methyl group of allenyl moiety and the... 

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