P-carbon elimination

Keywords Activation, C-C bond, Transition metal, Cleavage, Oxidative addition, P-carbon elimination, Directionality, Homogeneous... 

For examples of P-carbon elimination in late transition metal systems, Bergman et al. identified P-methyl transfer with four-membered ruthenacycles, which is driven by the formation of Ji-allyl and Ji-oxallyl complexes. Warming the solution of oxaruthenacycle 58 to 45°C led to formation of methane and cyclic enolate complex 60 [76]. ji-Oxallyl complex 59 initially arises from P-methyl... 

A o,jt-bonded alkyl-palladium complex undergoes P-carbon elimination on protonation of the cyclopentadiene ring [79]. 

A cyclobutylmethyl-metal system provides another opportunity to observe 13-carbon elimination. The ring opening process harnesses the release of the least necessitating ring strain of a four-membered ring. Scandocene hydride 63 reacts with 3-methyl-l,4-pentadiene to afford the linear Ji-allyl complex 65 [80]. The intermediacy of cyclobutylmethyl complex 64 which undergoes P-carbon elimination accommodates the observed rearrangement. 

Yttrium hydride reacts with methylenecyclobutane to form pentenyl chelate complex 67 [81]. P-Carbon elimination occurs to open the ring of cyclobutylmethyl intermediate 66. 

A phenylpalladium complex also causes ring-opening rearrangement of methylenecyclobutane to a ji-allylpalladium complex, which arises from p-carbon elimination of an intermediate (cyclobutylmethyl)palladium complex [82]. 

Reversible olefin insertion/p-carbon elimination occurs with a cationic pentenyl chelate platinum complex. Labeled complex 68 is reversibly converted to 70 via (cyclobutylmethyl)platinum intermediate 69 [83,84]. 

Abstract Palladium-catalyzed C-C bond cleavage via p-carbon elimination occurs in various cyclic and acyclic systems. Thus, the reaction can be utilized as one of fundamental and effective tools in organic synthesis. The recent progress in this field is summarized herein. 

Keywords C-C bond cleavage p-Carbon elimination Ring opening Palladium catalysts... 

Of the two reaction types involving -carbon elimination, the former through CPC-Pd is relatively more common. For instance, in the Heck-type reaction of vinyl bromides with MCP (Eq. 1), carbopalladation on the exo-methylene moiety takes place to give a CPC-Pd intermediate. Then, p-carbon elimination, hydrogen migration, and reaction with a carbon nucleophile successively occur to give rise to three-component coupling products [13]. 

As shown in Eqs. 2 and 3, the carbopalladation of bicyclopropylidene [14,15] and vinylcyclopropane [16] also gives the corresponding CPC-Pd intermediates, which readily undergo P-carbon elimination, hydrogen migration, and the subsequent inter- or intramolecular reaction with nucleophiles. 

The halopalladation of MCPs gives CP-Pd and CPC-Pd intermediates depending on the reaction conditions. Thus, the isomerization of alkylidene cyclopropyl ketones to 4ff-pyran derivatives takes place in the presence of a palladium chloride catalyst via chloropalladation to form a CPC-Pd and the successive P-carbon elimination (Eq. 10) [25]. In contrast, the addition of Nal changes the reaction pathway dramatically. Under the conditions, the reaction proceeds through a CP-Pd intermediate and results in the formation of furan derivatives. 

Cyclopropenyl ketones also undergo isomerization to produce furan derivatives (Eq. 11) [26]. It has been proposed that the initial chloropalladation on their unsymmetrically substituted double bond occurs regioselectively to give one of the possible CP-Pd intermediates predominantly, which undergoes p-carbon elimination and several subsequent reactions to yield the major products. 

A similar reaction can also be performed by using a Pd(0) catalyst. In this case, it has been assumed that the cyclopropoxypalladium species is formed by oxidative addition of the 0-H bond to Pd(0), which is followed by P-carbon elimination and successive p-hydrogen elimination or reductive elimination to give an enone and a saturated ketone, respectively (Eq. 13) [29]. 

On the other hand, the arylative ring opening takes place in the presence of a Pd(0) catalyst, an aryl halide, and a base (Scheme 3, reaction b) [32-35]. Oxidative addition of aryl halides toward Pd(0) gives ArPdX species, which can readily interact with the alcohols affording arylpalladium alkoxide intermediates. Then, p-carbon elimination and subsequent reductive elimination occur to give y-arylated ketones and regenarate Pd(0) species. An example is shown in Eq. 15. 

More generally, it has been reported that four-, five-, six-, eight-, and twelve-membered rings of bicyclic carbonates can be opened. An example of the six-membered ring opening is given in Eq. 21 [46]. In the reaction, oxidative addition of the allylic C-0 bond toward Pd(0) species followed by decarboxylation affords a palladacycle intermediate. The subsequent p-carbon elimination results in the formation of a dienal. 

In addition to aryl and alkyl groups, alkynyl groups in tertiary alcohols are also detachable. Thus, as shown in Eq. 33, P-carbon elimination of an sp -sp C-C bond in the reaction of propargyl alcohols with alkenes under an oxygen atmosphere gives an ene-yne product [57]. 

The decarboxylation of palladium(II) benzoates to give arylpalladium(II) species maybe regarded as a P-carbon elimination. Such a reaction seems to be involved in the Heck-type coupling of benzoic acids and alkenes (Eq. 34) [58,59]. 

The key intermediate should be a deuterated vinylpalladium species (H) generated via syn addition of D-Pd species (Scheme 9). This vinylpalladium intermediate H is similar in its structure to the Negishi intermediate in the Mizoroki-Heck-type cyclization of 2-iodo-1,6-dienes to six-membered carbo-cycles, which proceeds with olefmic geometry inversion[53-56]. Thus, the plausible mechanism follows a D-Pd syn addition, p-carbon elimination, and P H elimination mechanism effective in controlling olefin inversion via cyclopropane intermediate (E and then I). 

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