Coupling Reaction of Allyl Halides

This reaction is another illustration of the compatibility of the organon-ickel reagent with different functional groups (an ester in the present case). 

---

Synthesis of maerolides. The intramolecular coupling reaction of allylic halides (I, 722-723 2, 290-292 3, 211) has now been shown to be applicable to the synthesis of maCrocyclic lactones. Thus addition of the dibromo ester (1) to 6 eq. of nickel carbonyl in N-methylpyrrolidone effects cyclization to the macrolide (2) as the major product. 

Tetraorganotins can be conveniently prepared from the coupling reaction of allyl halides with organotin halides under ultrasonic-assisted Barbier conditions [Eq. (37)]. 

A) Cross coupling reaction of allyl halides are efficiently run in the presence of nickel (or palladium (0)) complexes. The key intermediate is a jT-allyl nickel complex resulting from an oxidative addition of one of the two allylic bromides implied in the coupling process. 

The reaction of allylic halides with nickel tetracarbonyl to form coupled products has been known for over two decades (9), but it is only in recent years that an insight into the mechanism has been obtained. Isolation of the intermediate 7r-allylnickel complexes and the discovery that these react with activated olefins and organic halides in general have led to a considerable increase in the scope of the reaction. 

Castle and Widdowson were first to disclose alkyl-alkyl Kumada coupling reactions catalyzed by Pd(dppf)Cl2 [195]. This report was later questioned and corrected by Scott [196]. Matsubara and colleagues established formal Stille-type coupling reactions of perfluoroalkyl halides with allyl, alkynyl, or vinyl stannanes catalyzed by 10 mol% of Pd(PPh3)4, which have to be considered, however, better as radical addition/elimination reactions rather than as coupling reactions (see Sect. 3.1) [184],... 

The second approach for the nucleophilic animation reactions to be considered here will be reactions of allyl halides and allyl acetates leading to allyl amines. Allyl halides are normally very reactive in SN2 reactions, but the direct coupling of allyl halides with nitrogen nucleophiles has been performed with limited success [4], as di- and trialkylated by products often predominate. The application of the Gabriel synthesis can to a certain extent eliminate the problem with polyalkylation of amines using, e.g., the stabilized phthalimide anion 19 as the nucleophile. The allyl amine 20... 

Coupling reactions between allylic halides and alkynylcoppers have been rather heavily investigated as routes to stereochemically defined polyenyl natural products. The CuCl-catalyzed reaction of acetylenic Grignard reagents with chlorine substituted allylic chlorides has been employed in the synthesis of stereochemically pure 1,3,5-undecatrienes... 

Oxidative addition of the carbon-halogen bond is a well-documented reaction for Group 10 transition metal complexes, but it is relatively limited for ruthenium. The example given here involves the reversible oxidative addition of allyl halide to RuCp(CO)2X to produce RuCp(p -allyl)X2 [78]. Oxidative addition of allyl halide to a Ru(0) complex Ru(l,5-COD)(l,3,5-COT) is also reported, but the product yield was poor [79]. Nevertheless, a catalytic Heck-type alkenylation of bromostyrene with methyl acrylate by Ru(l,5-COD)(l,3,5-COT) proceeded smoothly [80]. A cross-coupling reaction of alkenyl halide with Grignard reagents or alkyl lithium also pro-... 

Stille cross-coupling reactions of allylic and benzylic halides with conjugated organo-... 

Homo- and cross-coupling reactions of allyhc halides and reactions of conjugated dienes with dichloroalkanes using reactive barium are reviewed in Section 5.2. The next section covers methods of generating allylic barium reagents and reactions of these carbanions with a variety of electrophiles. The last section describes examples of other carbon-carbon bond forming reactions promoted by barium compounds. 

Allylic barium reagents prepared in this way can realize highly a-selective reactions with different electrophiles, e.g. cross-coupling reactions with allylic halides or allylic phosphates, additions to carbonyl compounds or imines, and ring opening of epoxides. A selective Michael addition reaction with an a,/ -unsaturated cy-cloalkanone can also be performed by use of an allylic barium reagent. 

Metal Enolates. In parallel with additives, transition metals may be added to enolates to give transmetallated species which can undergo cross-coupling chemistry. Perhaps the earliest example of metal-catalyzed enolate reactions is the Reformatsky reaction. Transition metal-catalyzed enolate chemistry has been recently revived in the literature, particularly in the field of asymmetric catalysis. The transition metal-catalyzed coupling reactions of aryl halides, allyl epoxides, and allylic esters with alkyl enolates have been recently investigated. Generally the choice of base employed depends on the substrate and on the reaction performed. For enolate arylation, KHMDS seems to be the most... 

---