Acylpalladation process

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. 

Double or Multiple Carbopalladative Cyclization Reactions Involving One or More Cyclic Acylpalladation Processes... 

A systematic investigation of cyclic acylpalladation of haloenes, haloynes, and related electrophiles conducted since 1983 [30] has led to the development of three types of cyclic acylpalladation processes (Types I—III Ac-Pd) and Pd-catalyzed carbonylation-induced ketene [2 + 2] cycloaddition (Sect. 3.1). Collectively, these cyclic acylpalladation and related reactions have provided a number of new and attractive routes to cyclic compounds. Significantly, they nicely complement and supplement the non-carbonylative cyclic carbopalladation reactions. Thus, they have become integral and indispensable parts of the carbopalladation-based cyclization methodology. 

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. 

In all of the three cyclic acylpalladation reactions, that is. Types I-III Ac-Pd processes, polymeric acylpalladation discussed in Sect. VI.4.2 and direct noncarbonylative carbopal-ladation discussed in Sect. IV.3.3 can be not only serious side reactions but even dominant processes depending on the reaction conditions. The polymeric acylpalladation process... 

B. SINGLE-STAGE CYCLIC ACYLPALLADATION PROCESSES B.i. Type I Ac-Pd Process... 

All of the acylpalladation processes discussed above involve interaction of acylpalladium bonds with C=C bonds. The corresponding acylpalladation of C C bonds proved to be rather elusive. Thus, for example, attempts to observe cyclic acylpalladation of alkyne-containing iodobenzenes totally failed, even though the corresponding alkene derivatives undergo cyclic acylpalladationt (Scheme 23). The contrasting behavior of alkene and alkyne substrates is also schematically summarized in Scheme 23. 

Many different combinations of carbopalladation and/or acylpalladation processes are conceivable for devising cascading di- and multicyclization processes. Those consisting only of straightforward carbopalladation processes have been systematically investigated by a number of chemists since around 1988, as detailed in Sect IV.3. Aside from these carbopalladation cascades and those allylpalladation-acylpalladation cascades discussed above, only a small number of scattered examples are known, as shown below, and this area needs to be explored further. 

The [2 + 2] process for cyclobutanone synthesis is a companion path in the insertion (acylpalladation) process. The utility has been demonstrated in j8-lactam synthesis. 

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