Acylpalladation terminations

The scope of reaction has been further extended to allylic electrophiles. An extensive investigation of the intramolecular acylpalladation has been performed in a series of < -iodoalkenylbenzenes with both terminal and internal double bonds and 1-iodo-substituted 1,4-, 1,5-, or 1,6-dienes. ... 

The other strategy requires dienes or enynes containing allylic or propargylic ester fragments. The main pathway in this case involves the formation of allylpalladium intermediates, which perform carbapalladation of double or triple bonds with subsequent acylpalladation forming two cycles, and termination by palladium hydride elimination or other usual trapping pathways (Scheme for example, in the following examples in Scheme 22. ... 

These reactions may as well occur in the termination stages of cascade reactions, for example, involving intramolecular acylpalladation (Scheme 30) or intramolecular Heck reaction Equation (49). ... 

All of the examples of trapping of acylpalladium species with enolates discussed above as part of the Type III Ac-Pd process involve trapping with O-enolates. As discussed earlier, however, acylpalladium derivatives can also be trapped with C-enolates (Eqs. 4 and 5 in Scheme 11), and this trapping with C-enolates has since been exploited for terminating acyclic carbopalla-dation process [135] (Scheme 48). However, this process does not appear to have been used for terminating cyclic acylpalladation processes. 

In principle, carbonylative cyclization, that is, acylpalladation or Ac—Pd process, or noncarbonylative cyclization, that is, sample carbopalladation or C—Pd process, in the presence of CO and a Pd catalyst. Various possibilities with halo alkenes as representative substrates are shown in Scheme 2P Those processes that incorporate CO in the cyclization processes are discussed in Part VI including Sects. VI.4-VI.6. hi this section, those cases that do not incorporate CO during the cychzation processes but do so only after cyclization will be discussed. Such cychc carbopalladation-carbonylative termination tandem and cascade processes are represented by the Type II C—Pd process in Scheme 2, which may take place in competition with the other processes shown in Scheme 2, especially the cyclic Heck reaction (Type 1 C—Pd process) and cyclic carbopalladation involving cyclopropa-nation (Type 111 C— Pd process). 

As in the cases of termination by lactonization and lactamization, some specially structured alkenes, such as norbomene and related alkenes, can participate in intermolecular carbopalladation-carbonylative ketonization tandem processes. In the reactions shown in Scheme 22, the acylpalladium intermediates undergo intramolecular acylpalladation with arenes to provide ketones. ... 

The cyclic acylpalladation process can be terminated by carbonylative esterification, that is, Type II AcPd process, as discussed in Sect. VL4.1. In some cases, however, this process can be overshadowed by premature esterification This difficulty can be circumvented by a two-step alternative consisting of Type III AcPd process followed by methanolysis (Schemes 21 and 22). The results shown in Scheme 23 suggest that further optimization of the reaction conditions for the Type III AcPd process appears to be promising and highly desirable. 

The intennolecular acylpalladation corresponds to the addition of an acyl-palladium bond onto a rr-bond system of another molecule this elementary step can also be referred to as an insertion (Scheme I). This produces another organopalladium complex, which can in principle participate in subsequent propagation or termination reactions. This excludes processes that involve alkoxycarbonylation (R— = R O—) and hydrocarbonyla-tion (R— = H—). This section will focus on nonpolymeric intermolecular reactions of acylpalladium complexes with different 7r-bond systems (alkenes, imines, dienes, and alkynes). 

In conclusion, the intermolecular acylpalladation of alkenes is, so far, limited to specific substrates, and this is mainly due to certain strict requirements, that is, (i) acylpalladation faster than decarbonylation and (ii) a termination step faster than polymeric pathway, and yet slower than acylpalladion itself. Attempts to control this reaction by... 

There are other examples of carbonylation of terminal alkynes, which involve the in-termolecular reaction of an acylpalladium complex with an alkyne moiety, but they most probably also involve carbon-carbon bond formations via cross-coupling rather than acylpalladation (Scheme 21). 

Overall, the carbonylation of alkynes is rather complex, but it is possible to draw a general trend and to divide these processes into three classes depending on the alkyne (i) For most internal alkynes, the carbon-carbon bond-forming process can involve an acylpalladation step whether there is an isomerization or not. (ii) However, some of them may involve an electrophilic activation of the triple bond by the acylpalladium complex followed by nucleophilic attack and reductive elimination, (iii) On the other hand, terminal alkynes appear to undergo mostly cross-coupling for the first carbon-carbon bond formation. Aside from these mechanistic intricacies, it is important to point out that these processes usually involve incorporation of more than one molecule of CO and creation of two to three carbon-carbon bonds in one reaction, and they yield heterocycles in fair to good yields. Other multiple bond systems like alkenes, imines or dienes also provide nice entries to carbo- and heterocycles. The limitations are usually due to the necessary time balance between acylpalladation and the termination step to avoid polymeric or decarbonylation processes. 

CO Insertion—Acylpalladation Tandem Processes. In eqs 40 3, it was shown that, even under carbonylation conditions, CO insertion can be deferred in favor of noncarbonylative carbopaUadation that may then be terminated by CO insertion and an appropriate trapping for catalyst regeneration. In this section, those cases in which (i) CO insertion is favored and (ii) the acylpalladium species thus generated can undergo a kind of carbopaUadation termed acylpalladation (vide supra) will be discussed. Systematic investigation by Negishi and his... 

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