Carbopalladation of allene

A one-pot synthesis of 3,3-disubstituted indolines was achieved by taking advantage of a sequential carbopalladation of allene, nucleophile attack, intramolecular insertion of an olefm and termination with NaBPh4 (Scheme 16.6) [10]. First, a Pd(0) species reacts with iodothiophene selectively to afford ArPdl, probably because the oxidative addition step is facilitated by coordination with the adjacent sulfur atom. Second, the ArPdl adds to allene, giving a Jt-allylpalladium complex, which is captured by a 2-iodoaniline derivative to afford an isolable allylic compound. Under more severe conditions, the oxidative addition of iodide to Pd(0) followed by the insertion of an internal olefm takes place to give an alkylpalladium complex, which is transmetallated with NaBPh4 to release the product. 

Cyclizations Proceeding with Inter- and Intramolecular Carbopalladations of Allenes 327... 

Scheme 37 Two cascade cyclizations involving inter- or intramolecular carbopalladations of allenes.

Ma S (2002) Carbopalladation of Allenes. In Negishi E, de Meijere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 1491... 

The same palladium-catalyzed domino cyclization-anion capture sequence involving carbopalladation of allene (69), which was employed by Grigg et al. for the synthesis of diene 70 (Scheme 10) can yield 1,3,5-hexatriene 194, when starting from 2-bromo-l-ene-6-yne 193 instead of the aryl iodide 68 (Scheme 32) [52], Under the conditions of its formation, 194 immediately undergoes thermal 67r-electrocyclization to give the bicyclic product 195. 

Allenes undergo facile carbopalladation and are used extensively in organic synthesis. The general reaction patterns of allenes with aryl halides are as follows. In carbopalladation of allene, an Ar group attacks the central sp carbon of the allene system to generate 367 and then the 7r-allylpalladium 368 as an intermediate. Then the attack of a nucleophile mainly on a less substituted terminus yields the alkene 369. Thus 1,2-addition to one of the double bonds in the allene occurs. In the absence of nucleophiles, y3-H elimination gives 2-substituted 1,3-dienes... 

Reaction of the o-iodoaniline derivative 431 with 1,2-decadiene in the presence of the chiral bisoxazoline 433 as a ligand and Ag salt provided the indoline 432 with 80 % ee [164], Construction of indole skeletons was carried out by intramolecular carbopalladation of allenes followed by amination. The 7r-allylpalladium intermediate 435 was formed from o-iodoaniline derivative 434, and intramolecular amination afforded the indole 436 in 89 % yield [165]. 

In the Pd(0)-catalyzed reaction of allene 54 with aryl halide, carbopalladation of allene with Ar-Pd-X takes place to give the 7r-allylpalladium intermediate 55, which is attacked by a pronucleophile to give 56. For example, reaction of 1,2-hexadiene (57) with iodobenzene and pyrrolidine afforded the aUylie amine 58 [17]. The catalytic reactions of this type have been extensively studied. They are surveyed in Chapter 3.2.9.2. 

A.i. Formation of rr-Allylpalladium Species via Stoichiometric Reactions of Carbon-Palladium Species with AUenes Regiochemistry of Carbopalladations of Allenes... 

Schultz discovered the first example of a carbopalladation of allenes when PdCl2(PhCN)2 was added to 1,2-propadiene to form the rr-allylpalladium complex 3, which was confirmed by the NMR spectra (Scheme 2).f t In this case the first step is the chloropal-ladation of propadiene to form the 7r-allylpalladium complex 4, of which the Csp2—Pd bond reacted further with another molecule of propadiene to afford the final product 3. 

A.ii. Reactivities of 77-Allylpalladium Species Formed via Carbopalladation of Allenes Outline of Possible Reactions... 

B. INTERMOLECULAR CARBOPALLADATION OF ALLENES FOLLOWED BY /8-HYDRIDE ELIMINATION REACTION... 

As outlined in Sect. B and C, catalytic intermolecular carbopalladations of allenes followed by either /3-hydride elimination or intermolecular nucleophilic trapping provide 1,3-dienes or allyl derivatives bearing the nucleophile moiety, respectively, while an intermolecular carbopalladation followed by intramolecular trapping sequential reaction provides cyclic skeletons (Scheme 27). In Type I, the nucleophilic moiety is connected with the C—X bond, and in lype II it is attached to the allene moiety. 

F. INTRAMOLECULAR CARBOPALLADATION OF ALLENES FOLLOWED BY NUCLEOPHILIC TRAPPING... 

The intramolecular carbopalladations of allenes has been shown to afford either tt-... 

In conclusion, with subtle design, carbopalladation of allenes will provide efficient access to dienes, alkenes, monocycles, and oligocycles with functional groups and will show its utility in the synthesis of specific target molecules. 

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