Allenyl palladium intermediate

Substituted propargylic alcohols were found to undergo direct carbonylation to the corresponding butenolides in 67-98% yield (Eq. 9.120) [86]. This reaction requires a catalytic amount of Pd2(dba)3-CHC13 (4%) and l,4-bis(diphenylphosphi-no)butane (8%) in CH2C12 under an atmosphere of CO (600 psi) and H2 (200 psi) at 95 °C for 36 h. The cyclocarbonylation reaction is believed to proceed via an allenyl-palladium intermediate, which is formed by initial insertion of Pd(0) into the C-O bond of the alkynol followed by rearrangement (Scheme 9.25). 

An extension of Tsuji s methodology has recently been reported by Liang in which he is able to perform a three-component coupling for the preparation of tetrasubstituted furans [154]. A combination of methyl or ethyl acetoacetate with propargyl bromide or carbonate and an aryl iodide enables the formation of tetrasubstituted furans 184. The proposed mechanism for this transformation is similar to that of Tsuji in that an allenyl-palladium intermediate is postulated. However, a three-component reaction is possible because the initial oxidative addition in this reaction is with the aryl iodide. Interaction of this Pd(II) intermediate with the propargyl bromide or carbonate leads to the allenyl-palladium species and, eventually, the product 184. 

In 1986 Inanaga et al. found that allylic or propargylic acetates could be reduced in THF by diiodosamarium in the presence of a catalytic amount of a Pd(0) complex and 1 equiv of 2-propanol [160,161]. Some examples are indicated in Scheme 60. The mechanism of the reaction likely involves jt-allyl (or a-allenyl) palladium intermediates which are reduced first to radicals and then to carban-ions (see Scheme 60). A final protonation by the alcohol generates the products. 

Pd-catalyzed cross-coupling reactions of the in situ-generated (cr-allenyl)palladium (II) intermediates with terminal alkynes were also realized in the presence of a catalytic amount of Cul [35-38], The reactions are similar to the well-known Sonoga-... 

As shown in the previous sections, a (cr-allenyl)palladium species, which is formed from a propargyl electrophile and a Pd(0) catalyst, reacts with a hard carbon nucleophile in a manner analogous to the Pd-catalyzed cross-coupling reaction to give a substituted allene. The results indicate that the reactivity of the (cj-allenyl)palladium species is similar to that of an alkenylpalladium intermediate. Indeed, it was found that the (cr-allenyl)palladium species reacted with olefins to give vinylallenes, a reaction process that is similar to that of the Heck reaction of alkenyl halides [54]. 

The allenylindium intermediates are prepared by treatment of the aziridines with Pd(PPh3)4 in THF-HMPA containing 1 equivalent of water. In the presence of iso-butyraldehyde the expected adducts were formed with excellent diastereoselectivity (Tables 9.56 and 9.57). Interestingly, the reaction did not proceed in the absence of water. It is suggested that water is needed to protonate the sulfonamide anion of the initially formed allenyl palladium species (Eq. 9.150). 

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. 

Four cationic palladium intermediates in the Pd(0)-catalysed three-component cascade double addition-cyclization of organic halides, 2-(2,3-allenyl)malonates, and imines have been characterized by the high-resolution ESI-FTMS technology.84... 

Propargylic carbonates are valuable substrates in Pd(0)-catalyzed processes. Their chemistry differs mechanistically from that of simple alkynes in that they form allenyl palladium species. We and others have shown these versatile intermediates can serve a double function as both starter and terminating species in bis-cyclization-anion capture cascades A typical example is the process 45 46, which generates azabicyclo [3.1.0] hexanes (Scheme 5.6.12) in good yield. ... 

Palladium-catalyzed isomerization of ynones to furans has been an active area of research over the last twenty years. Huang et al. described a Pd-catalyzed rearrangement of a,P-acctylcnic ketones to furans in moderate yield [175]. For example, Pdfdbajj promotes the isomerization of alkyne 214 to a putative allenyl ketone intermediate 215, which subsequently cyclizes to the corresponding furan 216. 

The ophcally active Pd complex with a chiral allenyl ligand undergoes epimer-izahon in the presence of a catalytic amount of Pd(0) complex. This reaction does not involve the isomerization to the propargyl complex, but takes place via a dinuclear intermediate as depicted in Scheme 5.39. The -allenyl ligand in the dinuclear palladium intermediate may racemize via a vinyl-vinyidene intermediate. This type of reaction is prohahly involved in a kinetic resolution of racemic propargyl alcohols promoted hy chiral transihon metal complex [203]. The intermolecular allyl ligand transfer from Pd to Ee complexes occurs under... 

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]. 

All the key intermediates were detected by ESI-FTICRMS over time, such as those ions of m/z 707.1,891.2 and 1150.3 (Figure 4.2). After these intermediate ions were further characterized by sustained off-resonance irradiation collision-induced dissociation (SORI-CID), one mechanism was proposed as shown in Scheme 4.3. The mechanism involving the carbopalladation with 2-(2,3-allenyl)malonate yielding the 7t-allyl palladium intermediate (Scheme 4.3) was confirmed. 

The addition of a chiral allenyl metal to an aldehyde generating a 2-substituted butynyl structure is named Marshall-Tamaru MT) reaction (Scheme 5-13). An allenyl palladium species is generated via a formal Sn2 substitution of the mesylate, which in turn undergoes a transmetalation with diethylzinc yielding a nucleophilic species (Scheme 5-13). The reaction of the electrophile proceeds via a similar -ester enolate transition state as depicted in Scheme 5-13. Corresponding allenylindium reagents can also be used instead of allenylzinc intermediates. ... 

Arylative or silylative cyclizations of allenyl aldehydes or ketones have been reported (Equations (101) and (102)).459,459a The intermolecular process, that is, three-component coupling reaction of aldehydes, allenes, and arylboronic acids, is catalyzed by palladium as well (Equation (103)).46O 46Oa These reactions are proposed to proceed through nucleophilic attack of the allylpalladium intermediates to the carbonyl groups. 

Moreover, propargyl oxiranes 202 were found to react with samarium diiodide and ketones to form a,a -dihydroxyallenes 203 with moderate to high anti-diastereo-selectivities (Scheme 2.62). Aurrecoechea and co-workers [99] reported this reductive coupling to proceed smoothly in the absence of a palladium catalyst, i.e. a direct electron transfer from the samarium(II) to the substrate has to take place in order to generate an allenyl/propargyl samarium intermediate of type 184/185, which is then regioselectively trapped by the electrophile. 

The allenyl carboxylate 35 was obtained in an enantiomerically enriched form by the palladium-catalyzed reduction of the racemic phosphate 34 using a chiral proton source [53]. The two enantiomers of the (allenyl)samarium(III) intermediate are in rapid equilibrium and thus dynamic kinetic resolution was achieved for the asymmetric preparation of (i )-35 (Scheme 3.18). 

The palladium-catalyzed carbonylation of 4-amino-2-alkynyl carbonates 40 or 5-hydroxy-2-alkynyl carbonates 41 afforded a-vinylidene-/i-lactams 42 [60] or a-vinyl-idene-y-lactones 43 [61] in good yields (Scheme 3.25). The initially formed (allenyl-carbonyl)palladium(II) intermediates were trapped by the intramolecular amino- or hydroxy-nucleophiles to give 42 or 43. 

Recently, Hiroi and co-workers reported a palladium-catalyzed asymmetric transformation of chiral 2-alkynyl sulfmates 142 into allenyl sulfones 145 (Scheme 4.38) [58], Treatment of 142 with Pd(OAc)2 in the presence of a phosphine ligand afforded allenylsulfones 145 with high stereospecificities (73-89%) in good yields, probably through intermediates 143 and 144. 

The reaction of an allene with an aryl- or vinylpalladium(II) species is a widely used way of forming a Jt-allyl complex. Subsequent nucleophilic attack on this intermediate gives the product and palladium(O) (Scheme 17.1). Oxidative addition of palladium ) to an aryl or vinyl halide closes the catalytic cycle that does not involve an overall oxidation. a-Allenyl acids 27, however, react with palladium(II) instead of with palladium(O) to afford cr-vinylpalladium(II) intermediates 28 (Scheme 17.12). These cr-complexes than react with either an allenyl ketone [11] or with another alle-nyl acid [12] to form 4-(3 -furanyl)butenolides 30 or -dibutenolides 32, respectively. 

The reaction employed here is a palladium-catalyzed domino 1,6-enyne cycli/ation.12 Carbonate 10 adds oxidatively to a complex containing palladium in the zero oxidation state.n The resulting intermediate, 26a. is in equilibrium with the palladium-allenyl species 26b.14... 

Type III reactions proceed by attack of a nucleophile at the central sp carbon of the allenyl system of the complexes 5. Reactions of soft carbon nucleophiles derived from active methylene compounds, such as /i-kcto esters or malonates, and oxygen nucleophiles belong to this type. The attack of the nucleophile generates the intermediates 9, which are regarded as the palladium-carbene complexes 10. The intermediates 9 pick up a proton from the active methylene compound and n-allylpalladium complexes 11 are formed, which undergo further reaction with the nucleophile, as expected, and hence the alkenes 12 are formed by the introduction of two nucleophiles. 

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