7T-Allyl palladium

The mechanism of this reaction was considered on the basis of hydropalladation (Scheme 14). To minimize steric repulsions, the palladium hydride complex approaches the C=CH2 moiety of the allene in the anti-Markovnikov mode from the opposite side of the substituent. This addition gives a 7t—allyl palladium complex with the (Z)-configuration,18 which is converted to the (Z)-product by C-P bond formation, with regeneration of the Pd(0) catalyst. 

The reaction starts with an oxidative addition of an allylic compound to palladium(O) and a Tt-allyl-palladium complex forms. Carboxylates, allyl halides, etc. can be used. In practice one often starts with divalent palladium sources, which require in situ reduction. This reduction can take place in several ways, it may involve the alkene, the nucleophile, or the phosphine ligand added. One can start from zerovalent palladium complexes, but very stable palladium(O) complexes may also require an incubation period. Good starting materials are the 7t-allyl-palladium intermediates ... 

A mechanistic study of the reaction has highlighted the key role of ZnCl2 [90]. It is the most readily reduced species, and Zn° might be the reducing agent of the first formed 7t-allyl palladium intermediate leading to allylzinc compounds as illustrated in Scheme 6 for allylacetate. 

The first synthesis of 1,3-dioxolium-4-olates (here defined as oxamiinchnones) was reported in 1980 by Berk et al. (64) but it was work of Hamaguchi and Nagai (65,66) that demonstrated the accessibility and utility of these new mesoionic heterocycles in cycloaddition reactions. Thus, reaction of diazoacetic benzoic anhydrides 108 with a 7t-allyl palladium complex affords oxamiinchnones 109. 

Carbonylation of (n-allyl)palladium dimers. Reaction of carbon monoxide in methanol with (7t-allyl)palladium dimers requires high pressure and results in modest yields of products. However, addition of the sodium salt of a carboxylic acid permits carbonylation at 25" under low pressure of CO and results in high, yields of fi.y-... 

Aakermark B, Hansson S, Krakenberger B, Vitagliano A, Zetterberg K (1984) Alkylation of (7t-allyl)palladium systems. Mechanism and regiocontrol. Organometallics 3 679-682... 

Allylic carbonates produce the required alkoxide by decarboxylation of the carbonate anion that is displaced in the formation of the 7E-allyl palladium intermediate. Deprotonation creates the active nucleophile, which rapidly traps the 7t-allyl palladium complex to give the allylated product and regenerates the palladium(O) catalyst. 

N2CHC02Et, cat Pda2(PhCN)2 N2CHC02Et, cat 7t-allyl-palladium chloride N2CHC02Et, cat Cu... 

Reaction of 2-vinylaziridines 1464 with various heterocumulenes (i.e., 1465, 1467), catalyzed by [Pd(OAc)2] and PPh3 at room temperature, affords five-membered ring products 1466, 1468 in moderate to high yields. The reaction is regioselective and involves a tj -ri -ri interconversion of a (7t-allyl)palladium intermediates (Scheme 379) <2000JOC5887, 1998JOC17>. 

C(3j. This indicated that, after forming the 7t-allyl palladium complex, the arsenite ion OAs attacks at both positions and with the same probability and gives the deuterium-scrambled recoupling product according to equation 71. 

Ammonium formate hydrogenolysis of 7t-allyl-palladium(II) complexes formed via oxidative addition of Pd(0) to propargylic carbonate esters is a useful method for preparing disubstituted alkynes, as depicted below. This method of deoxygenation avoids the use of expensive and toxic reagents often associated with the free-radical Barton-McCombie deoxygenation. °... 

Allyl carbonates are particularly useful allylic substrates. Thus, in addition to their high reactivity in oxidative addition, allyl carbonates have the advantage that the ally-lation reaction can be conducted under neutral conditions without an added base. The (7t-allyl)palladium intermediate (17) derived from an allylic carbonate has a carbonate anion that serves as a masked base to generate carbon nucleophiles (eq (112)) [143]. 

Oxatrimethylenemethanepalladium and/or Oxa(7t-allyl)palladium Complexes and Aza Analogs Thereof as Acylcarbene Equivalents... 

Based on their results with (2-oxoalkyl) carbonates, Murai et al.371 have proposed that an oxa(7t-allyl)palladium complex 5 rather than an oxatrimethylenemethanepalladium complex such as 4 is involved in the C-C coupling reaction with the alkene. The decisive argument is provided by the observation of a successful reaction with ethyl (2-oxo-2-phenylethyl)carbonate. In this case, and in contrast to the 2-oxopropyl derivate, the intermediate oxa(a-allyl)palladium complex 5 would be unable to form an oxatrimethylenemethane complex similar to 4 by elim ination of ethanol. 

Asymmetric alkylations can be performed in the presence of catalytic amounts of chiral phase transfer catalysts or through the intermediacy of 7t-allyl-palladium complexes bearing chiral ligands. Both methods normally require relatively acidic carbon acids (pKa 5 17) such as (3-diketones, P-keto- or cyanoesters or malonic acid derivatives. 

This competition between the nucleophile and the free secondary amine for the capture of the 7t-allyl palladium complex intermediate is explained by the mechanism of deprotection (Scheme 2). 

As shown in the case of cinnamaldehyde, addition to the carbonyl functionality occurs in preference to attack at the electron-poor double bond. The extent to which stereogenic aldehydes can control the stereochemical course of these cycloadditions is discussed in Section 1.6.1.2.3.2. The success of the tin-based cocatalyst to induce reaction is explained in terms of the formation of an intermediate stannyl ether which exhibits greater reactivity towards 7t-allyl palladium cations than free alkoxide ions. The addition to ketones is less general and one of the more successful examples is given. 

The reaction can be explained by the transmetalation of the borate with PdCl2 giving phenylpalladium intermediate and the subsequent insertion of a C=C double bond of norbornadiene into the Pd-Ph bond. Cationic 7t-allyl-palladium(II) and -rhodium(I) complexes having a BPhJ counter anion induce phenyl group migration from B to the transition metals at elevated temperature to form phenyl complexes [54,55] (Eq. 5.17). 

Due to the formation of sterically encumbered 7t-allyl palladium-species, a,a- and y,y-dimethylallyl alcohols failed to react. Enolisable aldehydes were also suitable substrates, and of note, quaternaiy centres eould be accessed via disubstituted aldehydes. 

With ample supplies of 38 provided through this protocol, the Sorensen group could next attempt to attach the atoms needed to prepare 37, the projected intermediate for a second reaction based on 7T-allyl palladium complexes (a Tsuji—Trost reaction) that would hopefully lead to the 19-membered macrocycle 36. In essence, this requirement boiled down to only two key synthetic objectives generating a ketoester moiety from the Weinreb amide, and converting the allylic TES-protected alcohol function at Cl into a methyl carbonate. Neither of these tasks ultimately proved to be overly challenging to carry out, with the first accomplished by treating 38 with excess quantities of the lithium enolate of t-butyl acetate to provide 54, and the second requiring three rela-... 

Burns, B., Grigg, R., Santhkumar, V. eta/. (1992) Palladium catalysed tandem cyclisation anion capture and hydride ion capture by alkyl- and 7T-allyl-palladium species. Tetrahedron, 48, 7297-320. 

Coupling reactions between allyltins and allyl chlorides are assumed to proceed via bis(7T-allyl)palladium intermediates Yamamoto [9] has shown that in the presence of aldehydes or imines, it is possible to control the chemoselectivity of the reaction so that either the allyl halide or the aldehyde/imine is allylated by the organotin. The key element is coordination of a ligand, such as triphenylphosphine, which is suggested to form a trigonal palladium complex with an and an /jCallyl... 

In addition to benzylations, palladium-catalyzed C-H allylation reactions have also been described, but they usually involve the electrophilic substitution of an electron-rich (hetero)arene with a 7t-allyl palladium complex and therefore deviate from the scope of this chapter [41, 42]. In contrast, the palladium/copper-catalyzed allylation of polyfluoroarenes with allyl carbonates (Scheme 19.27) has been reported to occur through a different mechanism [43]. Thus, base-induced cupration of the arene would give rise to intermediate 15 that was previously characterized by X-ray crystallography [44]. Attack of the in sitw-generated JT-allyl... 

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. 

In a first stage, a 1 2 mixture of [(7t-allyl)PdCl]2 and 56 revealed a peak corresponding to the intermediate I79b, which again showed the coordination of the palladium atom with the hydrophilic ligand. This complex had an allyl moiety coordinated to the metal in an or coordination mode (Entry 6). No new peaks were observed when the allylic acetate 51b was added to the above mixture. However, when nucleophile 54 was added, two new peaks were detected after 1 h. at 50 °C (i) a 7t-allyl palladium complex (I81b) and (ii) an intermediate... 

Selective alkylation of the methyl group of geranylacetone (CCXVIII) without carbonyl protection is possible via the intermediacy of the 7t-allyl-palladium complexes CCXIX and CCXX (Scheme 12) (Trost et al., 1973). 

In the last few years, several groups have developed enantioselective domino reactions catalysed by combinations of organocatalysts with palladium complexes. As an example, Murkheqee and List have reported a domino synthesis of p-all earbon quaternary amines on the basis of a highly enantioseleetive a-allqrlation of a-branched aldehydes, involving an achiral palladium catalyst and a chiral phosphoric acid. Under the catalysis of phosphoric acid, a secondary allylamine reacted with an a-branehed aldehyde to form an enammonium phosphate salt, which upon reaction with palladium catalyst afforded a cationic 7t-allyl palladium complex (Scheme 7.2). This intermediate resulted in the formation of an a-allylated iminium ion, whieh eould be reduced to the corresponding final chiral amine in high yield and exeellent enantioselectivity of 97% ee. The synthetic utility of this transformation was also demonstrated hy a formal synthesis of (+)-cuparene. 

Six-membered Rings.—A reaction, which may be especially suited to natural product synthesis, provides a facile route to 1-azaspirocycles. Cyclization via a 7T- allyl palladium complex occurs in almost quantitative yield in some cases,... 

Exposure of prochiral cyclopentadiene 59 to catalytic Pd(OAc)2, (5)-BINAP and -BuNOAc furnished diquinane 64 in 89% yield and 80% ee. The mechanism presumably involves oxidative addition, followed by coordination of either enantiotopic double bond to yield diastereomeric intermediates 60 and 62. The energetically favoured complex 62 undergoes insertion followed by rapid o-ir isomerization to generate the 7t-allyl palladium species 63. Trapping of the intermediate with an acetate anion... 

The carbopalladaticm of allenes provides a convenient entry to 7t-allyl palladium species. In the presence of a non-conjugated ester group, such as in the 3,4-allenoates 92, the p-H elimination transforms this intermediate stereoselectively into conjugated 1,3-dienes 93 incorporating di-, tri-, or even tetra-substimted double bonds (Scheme 46) [85]. 

Allenes are also potential substrates for 7t-allyl palladium complexes, for example, if treated with aryl palladium halides (for an example, see p. 958). 

---