Carbonylation acylpalladation

Cyclic acylpalladation. Another major subtopic of carbopalladation is acylpalladation. In the mid-1960s, two seemingly independent papers were published by J. Tsuji [24] and P.R. Hughes [25,26]. The former reported a perfectly alternating copolymerization of norbornadiene with CO (Scheme 8), while the latter described two related Pd-catalyzed carbonylation cyclizations shown in Scheme 9. 

Although the Pd-catalyzed alkene-CO copolymerization reaction must involve a series of acylpalladation reactions, it is outside the scope of this chapter. And, the readers are referred to recent reviews and pertinent references cited therein [27-29]. As such, the cyclic carbonylation reactions of dienes were of limited synthetic utility because of difficulties in controlling regiochemistry and other aspects of importance in fine chemicals synthesis. Whatever the reasons might have been, little had been reported further until the 1980s. 

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. 

Carbonylation of 2-iodostyrene (74) afforded indanone (75) and indenone (76) via intramolecular acylpalladation of the double bond. In the presence of BU4NCI and pyridine, protonation occurred to give indanone (75). When Et3N was used, indenone (76) was obtained. Under high pressure CO (40 atm), the keto ester 77 was the main product [35]. 

Yet another important development in the area of Pd-catalyzed carbonylation is the development of acylpalladation and related carbonyl-Pd bond addition reactions. Acylpal-ladation may be defined as a process of acyl-Pd bond addition to alkenes and alkynes. Clearly, it is a kind of carbopalladation reaction. For practical reasons, however, it is discussed in Part VI together with other carbonylation reactions mentioned above. Tsuji and Hosaka " reported in 1965 what appears to be the first example of the perfectly alternating alkene-CO copolymerization (Scheme 8). Independently, Brewis and Hughes reported also in 1965 a Pd-catalyzed cyclic carbonylation of dienes with CO and methanol (Scheme 9). Although the exact mechanism of the initiation is unclear, these reactions... 

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). 

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. ... 

Intramolecular acylpalladation-carbonylation via palladium 7t-allyl complexes O 2-Cyclopentenon-5-ylacetic acid esters... 

To complete organic synthesis via Pd-catalyzed carbonylation, the acylpalladium and related derivatives, represented by RCOPdL and YCOPdL in Scheme 2, must undergo further transformations including C—Pd bond cleavage as a mandatory step. In addition, some organopalladium interconversion processes, such as acylpalladation, may take place prior to C—Pd bond cleavage. 

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. 

Reactions of acylpalladium complexes involving acylpalladation with arenes or formally equivalent processes have been much less exploited compared with those involving acylpalladation with alkenes or alkynes discussed in the previous section, and to the best of our knowledge, their examples are limited to intramolecular reactions. A few reactions that may include the intramolecular acylmetallation with arenes are known in the case of a rhodium catalyst, but the products are limited to a narrow range of compounds such as indenones.t In contrast, the Pd-catalyzed carbonylation of 3-arylaUyl acetates or hahdes, most typically cinnamyl acetate, involving intramolecular acylpalladation with arenes results in the construction of 1-naphthol, which is a hiding member of cyclic aromatic ketones. The outline of the conversion is sketched in Scheme 1. In fact, this type of carbonylation-cyclization reaction (cyclocarbonylation) can be applied to the synthesis of a variety of fused aromatics, which is summarized in the following subsection. 

---