Tandem reactions acylpalladation

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). 

The relative rates of carbopalladation and CO insertion represent a very delicate issue, which is, to a considerable extent, dependent on the reaction conditions. The reaction of Phi with even 1 attn of CO in the presence of 1-octyne and MeOH under the influence of 5 mol % of Cl2Pd(PPh3)2 gives only methyl benzoate (73%) without the sign of either intermolecular carbopalladation or acylpalladation. One may conclude that inter-molecular C—Pd and Ac—Pd processes are, in general, decidedly slower than the CO insertion-methanolysis tandem process. 

A detailed investigation with 10 summarized in Table 2 indicates that premature esterification and cyclopropanation (Type HI C— Pd process in Scheme 2) can occur as dominant side reactions but that, under the optimized conditions (entry 7), both can be suppressed to insignificant levels (<3%). It is also important to note that, in marked contrast with the cyclic acylpalladation (Type n Ac—Pd) discussed in Sect. VI.4.1.1, monosubstituted alkenes that can readily participate in dehydropalladation (e.g., 11) cannot undergo the cyclic carbopalladation-carbonylative esterification tandem process (Type II C-Pd) since they merely undergo the cyclic Heck reaction (Type I C— Pd process in Scheme 14). The contrasting behavior mentioned above may be attributable to a chelation effect exerted by the carbonyl group in the acylpalladation (Scheme 15), which is lacking in the carbopalladation shown in Scheme 14. 

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. ... 

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