Palladium-catalyzed transmetalation

Yasuda S, Yorimitsu H, Oshima K (2008) Synthesis of aryliron complexes by palladium-catalyzed transmetalation between [CpFe(CO)2l] and aryl Grignard reagents and their chemistry directed toward organic synthesis. Organometallics 27 4025 027 Jonas K, Schieferstein L (1979) Simple route to Li- or Zn-metalated r -cyclopentadien-yliron-olefin complexes. Angew Chem Int Ed Engl 18 549-550... 

Palladium catalyzes allylation of carbonyl compounds with various ally lie compounds using In-InCl3 in aqueous media (Eq. 8.66).158 Various allylic compounds can be effectively applied via the formation of TT-allylpalladium(II) intermediates and their transmetalation with indium in the presence of indium trichloride in aqueous media. 

The palladium-catalyzed cross-coupling reaction featured in this procedure occurs under neutral conditions in the presence of many synthetically useful functional groups (e.g. alcohol, ester, nitro, acetal, ketone, and aldehyde). The reaction works best in N,N-dimethylformamide with bis(triphenylphosphine)palladium(ll) chloride, PdCI2(PPh3)2, as the catalyst. Lithium chloride is added to prevent decomposition of the catalyst.143 13 It is presumed that conversion of the intermediate aryl palladium triflate to an aryl palladium chloride is required for the transmetallation step to proceed.9... 

Additional publications from Sanford et al. describe the full exploration of palladium-catalyzed chelate-directed chlorination, bromination, and iodination of arenes using N-halosuccinimides as the terminal oxidant <06T11483>. Moreover, an electrophilic fluorination of dihalopyridine-4-carboxaldehydes was reported by Shin et al. <06JFC755>. This was accomplished via transmetalation of the bromo derivative, followed by treatment with A-fluorobenzenesulfinimide as the source of electrophilic fluorine. 

It is well known that alkenylzirconocenes are transmetalated to zinc and conveniently undergo further palladium-catalyzed cross-coupling reactions [7k,8a,32]. Although the transmetalation of zirconacyclopentadienes to zinc is not yet well developed, some interesting reactions that take place in the presence of ZnCl2 have already been described (Eq. 2.30) [9]. 

Synthetically useful allylstannanes are provided by palladium-catalyzed carbostan-nylation using hexamethylditin (Scheme 16.58) [63]. The reaction mechanism can be rationalized by transmetallation between ditin and a Jt-allylpalladium complex produced by reaction of an allene with an arylpalladium iodide. In this process, hexamethylditin is added to the reaction mixture slowly via a syringe pump to suppress its high reactivity towards the arylpalladium species leading to an arylstannane. 

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

Palladium-catalyzed cross-coupling reaction of organoboranes with organic halides, triflates, etc. in the presence of a base (transmetalation is reluctant to occur without the activating effect of a base). For the catalytic cycle, see Kumada coupling on page 345. 

Concerning the lithium-zinc transmetallation from intermediate 331, it has been used (THF, room temperature) to perform a palladium-catalyzed Negishi cross-coupliug reaction with aryl bromides - and a 8 2 regioselective allylation in the presence of copper cyanide. 346 and 347 were, respectively, obtained (Scheme 101). 

When the metallic additive to the intermediate 374 was zinc dihalide (or another Lewis acid, such as aluminum trichloride, iron trichloride or boron trifluoride), a conjugate addition to electrophilic olefins affords 381 . In the case of the lithium-zinc transmetallation, a palladium-catalyzed Negishi cross-coupling reaction with aryl bromides or iodides allowed the preparation of arylated componnds 384 ° in 26-77% yield. In addition, a Sn2 allylation of the mentioned zinc intermediates with reagents of type R CH=CHCH(R )X (X = chlorine, bromine) gave the corresponding compounds 385 in 52-68% yield. ... 

Palladium-catalyzed alkenylation reactions involving pyrimidines can be achieved with hydrozirconated terminal alkynes, although the reaction is carried out in the presence of zinc chloride, so transmetallation to the zinc species is presumed to occur prior to the palladium-mediated coupling <1996ACS914, B-2002MI409>. Selective reaction at the 4-position of both 2,4-dichloropyrimidine and 2,4-dichloroquinazoline can be achieved. 

The palladium catalyzed cross-coupling of organosilicon compounds and aryl halides found only limited application with azines compared to the Suzuki or Negishi coupling. In a recent paper DeShong reported the efficient coupling of bromopyridine derivatives with aryl siloxanes (7.44.) 62 The transmetalating ability of the siloxane was enhanced by the addition of tetrabutylammonium fluoride. 

The palladium-catalyzed reaction of aryl- and vinyl-tin reagents with stereochemically defined allyl chlorides proceeds with overall retention of configuration, indicating that the second step, entailing interaction of the iT-allylpalladium complex and the organotin, proceeds by transmetallation and reductive elimination (attack at Pd, retention) (equations 166 and 167).142145 Comparable results were obtained with cyclic vinyl epoxides and aryltins.143... 

Early findings by Suzuki and co-workers [109] showed that the palladium-catalyzed iminocarbonylative cross-coupling reaction between 9-alkyl-9-BBN derivatives, t-butylisocyanide, and arylhalides gives access to alkyl aryl ketones 132 after hydrolysis of the corresponding ketimine intermediates 131. Presumably, the concentration of free isocyanide is kept to a minimum by its coordination with the borane. Formation of an iminoacylpalladium(II) halide 130 by insertion of isocyanide to the newly formed arylpalladium complex followed by a transmetallation step afford the ketimine intermediates 131 (Scheme 8.52). 

The palladium-catalyzed and the copper-cocatalyzed direct arylation of imidazole 1-oxides 280 shown in Scheme 83 may involve transmetallation (2008JA3276, 2009JA3291). However, classical transmetallation like conversion of imidazolyllithium compounds to imidazolylzinc compounds has not been reported. 

The mechanism of the Sonogashira reaction has not yet been established clearly. This statement, made in a 2004 publication by Amatore, Jutand and co-workers, certainly holds much truth [10], Nonetheless, the general outline of the mechanism is known, and involves a sequence of oxidative addition, transmetalation, and reductive elimination, which are common to palladium-catalyzed cross-coupling reactions [6b]. In-depth knowledge of the mechanism, however, is not yet available and, in particular, the precise role of the copper co-catalyst and the structure of the catalytically active species remain uncertain [11, 12], The mechanism displayed in Scheme 2 includes the catalytic cycle itself, the preactivation step and the copper mediated transfer of acetylide to the Pd complex and is based on proposals already made in the early publications of Sonogashira [6b]. 

Allenylsilver compounds appear to enter the palladium coupling catalytic cycle via transmetallation. It was found that tert-butyl allenylsilver, and the corresponding argentate derivative, underwent palladium-catalyzed coupling with iodobenzene to... 

Although catalytic amounts of Ag20 have been found to promote the palladium-catalyzed coupling of aryl boronic acids and terminal alkynes, the authors in this case do not attribute the desired reactivity to the formation, and subsequent transmetalla-tion, of a silver acetylide. Rather, it is proposed that the Ag20 activates the alkynylpalladium complex to allow transmetallation from the boronic acid, and that any competing formation of the silver acetylide results in a homocoupling of the alkynes (Scheme 1.55).123... 

Palladium-catalyzed ketone synthesis B. The reaction mixture is saturated with carbon monoxide, which intervenes in step 2 by forming a palladium(II) carbonyl complex. Before the transmetalation (above referred to as step 3) takes place a rearrangement is interposed. The ligand Rmisa rji cd undergoes a [l,2]-shift from Pd(II) to the carbon atom of carbon monoxide, leading to the formation of an acylpalladium(II) complex with the structure P lllsa llra cd-(C=0)-Pd(-X) L j. With regard to the above-mentioned steps 3-4 it behaves like the acyl-Pd(II) complex of the ketone synthesis A and, after reductive elimination, i.e. in step 5, yields... 

Fig. 13.15. Palladium-catalyzed, stereoselective alkenylation of an arylboronic acid (preparation according to Figure 5.32) with a variety of iodoalkenes. The boronic acid is converted into the boronate anion A. The ion A reacts with the Pd(II) intermediate B via transmetallation subsequent reductive elimination leads to the coupling products.

The palladium-catalyzed reaction of diboron with allyl acetates or chlorides is a convenient alternative to transmetallation method for the synthesis of functionalized boronates. The reaction of diboron 119 with allyl acetates309-312 smoothly occurred without the assistance of a base, whereas the presence of AcOK was critical for the coupling with allyl chlorides313 (Equation (57)). The boron atom coupled with the less-hindered terminal carbon giving the thermally stable (if)-allyl boronates. Thus, the (E)- and (Z)-cinnamyl acetate, chloride (entries 1-4), and their secondary derivatives (entries 7 and 8), all afforded an (if)-cinnamyl boronate. The borylation of prenyl acetate was slow (entry 6), but the corresponding chloride (entry 5) and tertiary derivative (entry 9) worked well for the same purpose (Equation (58)).309,310... 

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