Palladium catalyzed reactions mechanism

As in case of other palladium-catalyzed reactions, the general mechanism of the Stille reaction is best described by a catalytic cycle—e.g. steps a) to c) ... 

Scheme5-24 Eq. (1) Proposed mechanism for Eq. (2) Stoichiometric reactions relevant to the phosphinic acid-modified palladium-catalyzed proposed mechanism hydrophosphinylation of alkynes.

Scheme 150).225 227 The pyran products predominate when the ratio of triphenylphosphine to palladium catalyst exceeds two whereas the linear oligomers are the major products when this ratio is close to unity. The suggested227 mechanism (Scheme 151) includes a step of insertion of C=0 into a C—Pd palladium-catalyzed reactions leading to the formation of pyranones (see Scheme 152)228 and piperidones (see Scheme 139 in Section V,A,2).211 It is useful to note that the 2,5-divinyltetrahydropyran derivative can be transformed catalytically by ruthenium trichloride into synthetically useful 3,4-dihydro-2//-pyran derivatives (Scheme 153).229... 

The first examples of the use of palladium as a catalyst for carbon-carbon coupling reactions were reported almost thirty years ago [14], and over recent decades a massive effort has been devoted to the extension of the scope of palladium-catalyzed reactions. Organic and organometallic chemists have received extensive input from palladium-coordination chemistry in the task of understanding the mechanisms behind these efficient synthetic procedures [14]. 

Disubstituted silole derivatives are synthesized by the palladium-catalyzed reaction of (trialkylstannyl)di-methylsilane with terminal alkynes (Equation (107)).266 The mechanism is supposed to involve a palladium silylene complex, which is generated via /3-hydride elimination from LJ3d(SiMe2H)(SnBu3) (Scheme 62). Successive incorporation of two alkyne molecules into the complex followed by reductive elimination gives rise to the silole products. 

Unsubstituted or 2,3-disubstituted stannoles can be prepared from the palladium-catalyzed reaction of an alkyne with the stable stannylenes [(Me3Si)2CH]2Sn and CH2C(SiMe3)2 2Sn . The mechanism has been traced by the isolation of intermediates,235 and by ab initio calculations (Equation (78)).236... 

The palladium catalyzed reactions of substituted vinylallenes with unactivated 1,3-butadienes proceeded with high selectivity133. A multistep mechanism, involving several palladacycles, was proposed to explain the high selectivities observed. 

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]. 

The mechanism of the palladium-catalyzed reaction is similar to that of the allyl chloride-nickel carbonyl reaction described above, but more complex, at least when phosphine ligands are present (14). The first step is believed... 

Vollmuller F, Krause J, Klein S, Magerlein W, Beller M (2000) Palladium-catalyzed reactions for the synthesis of fine chemicals, 14 - control of chemo- and regioselectivity in the palladium-catalyzed telomerization of butadiene with methanol - catalysis and mechanism. Eur J Inorg Chem 1825-1832... 

The scope of this reaction appeared to be limited to dialkylamides and electron-neutral aryl halides. For example, nitro-, acyl-, methoxy-, and dimethylamino-substituted aryl halides gave poor yields upon palladium-catalyzed reaction with tributyltin diethylamide. Further, aryl bromides were the only aryl halides to give any reaction product. Vinyl bromides gave modest yields of enamines in some cases. Only unhindered dialkyl tin amides gave substantial amounts of amination product. The mechanism did not appear to involve radicals or benzyne intermediates. 

The three component aminoallylation reaction of the activated olefins 4 was also reported by us (Scheme 7) [39]. The palladium catalyzed reaction between phthalimid 10, olefins 4 and allyl chloride 11 proceeded well in the presence of CS2CO3 to give the corresponding aminoallylation products 12 in high yields. Likewise, cyanoallylation of activated olefins was also reported (Scheme 8) [40]. In both cases only aryl substituded and f-bulyl substituded olefins are proved good substrates. The mechanism of these reactions is presumably similar to that of hydroalkoxylation reaction (see Scheme 5). 

The palladium-catalyzed reaction of 1 1 mixture of allyl chloride 11 and allyl stannane 3a with alkylidene malonitriles e.g., 4a resulted into the double-allylated product 95 (Scheme 29) [33]. The mechanism of this reaction is... 

The palladium-catalyzed reaction of allyl chloride 11 with the benzyne precursor 104 to produces phenanthrene derivatives 131 is also known [83]. A plausible mechanism for this intermolecular benzyne-benzyne-alkene insertion reaction is shown in Scheme 38. Initially n-allyl palladium chloride la is formed from Pd(0) and 11. Benzyne 106, which is generated from the reaction of CsF and 104, inserted into la to afford the aryl palladium intermediate 132. A second benzyne insertion into 132 produce 133 and subsequent carbopalladation to the alkene afford the cyclized intermediate 134. f>- Iydride elimination from 134 followed by isomerization gave 9-methylphenanthrene 131. 

The palladium-catalyzed reaction of aryl halides with cyanides to give cyanobenzenes takes place under relatively mild conditions compared to the conventional method using a stoichiometric amount of CuCN [74]. Thus, palladium catalysis has been often employed. Recently, a number of effective methods for the cyanation have been reported. The reaction of aryl iodides with NaCN under two-phase conditions [75] and those of aryl triflates [76, 77] and aryl chlorides [78] with Zn(CN)2 occur with good efficiency, while these are considered to proceed via mechanism B. 

Palladium-catalyzed carbonylation reactions have assumed increasing importance over the last 30 years, particularly for smaUer-scale laboratory and fine chemical syntheses (see Palladium Organometallic Chemistry). There is an enormous amount of work on palladium-catalyzed reactions, and it would be impossible to cite many specific examples. It is possible, however, to generalize the mechanism for producing... 

Palladium-catalyzed Reactions of Organoboron Compounds and Their Mechanism... 

It is frequently assumed that the mechanism of the hydrovinylation reaction is identical for catalysts containing the same metal, irrespective of the nature of the metal precursor. However, it is questionable whether this assumption can be extended to different metals and it should not, for example, be assumed that the nickel-catalyzed reactions have mechanisms identical to those of the palladium-catalyzed reactions. 

Jutand, A. Mechanism of palladium-catalyzed reactions Role of chloride ions. Applied Organometallic Chemistry 2004, 18, 574-582. 

The mechanism proposed (84, 95) is as follows. The formation of cis-2-butene is adequately described by steps (1), (2), and (3) proposed above for the palladium-catalyzed reaction. Since the initial cisitrana ratio in the butenes is almost independent of initial reactant pressures and temperature, and since the distribution of deuterium in these two isomers is similar, it has been concluded that produced directly from 2-butyne, and not by the subsequent isomerization of cis-2-butene. Steps (6), (7), (8), and (9) describe the simplest route which satisfies the experimental observations and involves the addition of hydrogen to a free radical form of the half-hydrogenated state, which is envisaged to be in equilibrium with the normal form. An analogous equilibrium was postulated in the mechanism for acetylene... 

The most important class of allylic substitutions are palladium-catalyzed reactions with so-called soft nucleophiles such as stabilized carbanions or amines, and with few exceptions, the enantioselective transformations discussed in this chapter belong to this category. The mechanism of these reactions has been firmly established and a detailed picture of the catalytic cycle can be drawn [1, 2,3,4,5,6,13,14,15]. The course of allylic substitutions catalyzed by metals other than palladium is less clear and information about the intermediates involved is scarce. 

This reaction is catalyzed by nickel or palladium complexes. However, the selectivity and activity of nickel catalysts are lower than those of palladium catalysts. Palladium-catalyzed reactions give linear dimers selectively and no cydization takes place. Not only the zerovalent palladium complexes but also certain bivalent ones can be used as active catalysts in combination with excess PPh3. However, the zerovalent palladium complexes are somewhat tedious to prepare and unstable in oxygen, so easily available and stable bivalent palladium compounds such as Pd(OAc)2 are generally used with PPh3. A proposed mechanism is given in Figure 1 [1 f]. 

Finally, Alper has reported the preparation of imidazolidinethiones 162 by the palladium(II)-cataly zed cyclization of aziridines 160 and sulfur diimides. Through a mechanism not yet fully elucidated, the methylene carbon of the aziridine is incorporated into both the thiocarbonyl and methylene positions of the product in this fascinating palladium catalyzed reaction [94JA1220]. 

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