Rhodium-Catalyzed Arylzincation of Alkynes: Ligand Control of 1,4-Migration Selectivity

Abstract

The addition of arylzinc reagents ArZnCl 1 to alkynes 2 was found to be catalyzed by rhodium complexes in the presence of a catalytic amount of zinc chloride. The selectivity in giving 2-arylalkenylzinc species 3 or ortho-alkenylarylzinc species 4, the latter of which is generated through 1,4-Rh migration from alkenyl to aryl in the catalytic cycle, is controlled by the ligands on rhodium. Ligands cod and binap gave 3 and 4, respectively, with high selectivity.

1,4‐Migration of Transition Metals in Organic Synthesis

“…The reaction was monitored by D 2 O quenching. With BINAP as a ligand, the reaction gave aryl‐D product 80 with 99% selectivity; meanwhile, the formation of vinyl‐D product 79 was favored with ligand (1 Z ,5 Z )‐cycloocta‐1,5‐diene (COD) (Scheme ) . It was reasoned that the transmetallation of M59 with ZnCl 2 would give M60 , a product without undergoing 1,4‐Rh migration when COD was used as the ligand.…”

Asymmetric Synthesis of Alkylzincs by Rhodium‐Catalyzed Enantioselective Arylative Cyclization of 1,6‐Enynes with Arylzincs

“…The results on the deuterium quenching studies clearly demonstrate that alkylzinc 3 a and arylzinc 4 a were produced by the Rh-catalyzed arylzincation of 1 a. Based on the reaction pathways reported for Rh-catalyzed arylation of alkynes [6,7,12] and cyclization reactions, [9a-c] addition of an aryl-Rh intermediate to alkyne in 1 a followed by addition of the resulting alkenyl-Rh A to alkene generates the alkyl-Rh intermediate B. Transmetalation between B and ZnCl 2 [10] produces alkylzinc 3 a and a Cl-Rh species, the latter undergoing the reaction with ArZnCl regenerating aryl-Rh species. When the regiochemistry at the arylrhodation of alkyne is opposite, the alkenyl-Rh intermediate C cannot participate in the intramolecular addition to alkene.…”

“…If the ArB(OH) 2 is replaced by ArZnX and the transmetalation with ZnX 2 takes place for the alkyl-Rh intermediate, the overall reaction would be a new type of catalytic carbozincation giving alkylzinc product. During our studies on the arylzincation of carbon-carbon multiple bonds, [3,10] we found that the catalytic asymmetric carbozincation giving chiral alkylzincs shown in Scheme 1 b is realized in the presence of a chiral diene/Rh catalyst and zinc chloride.…”

“…Instead, 1,4-Rh shift takes place from alkenyl to the phenyl ring, [12,13] and transmetalation of aryl-Rh species D with ZnCl 2 produces arylzinc 4 a. [10] Although the alkylzinc product 3 a itself is not very reactive toward electrophiles other than proton, the Cumediated acylation [1,2,14] developed by Knochel proceeded well to give a high yield of the benzoylation product 7 aa which is readily analyzed for its enantiomeric purity. Thus, the reaction mixture resulting from arylzincation of 1 a, which contains 3 a, was treated first with CuCN and then with PhCOCl to give phenyl ketone 7 aa in 91 % yield with 99 % ee (reaction Scheme in Table 1).…”

“…The reaction did not take place with bulky DTBMsegphos [22] (entry 8). The presence of ZnCl 2 is necessary for the reaction to proceed efficiently, which is probably because ZnCl 2 participates in the catalytic cycle at the transmetalation step giving the final alkylzinc product [10] (see Scheme 2). While the high yield (91 %) of 7 aa was obtained under standard conditions with 0.5 equiv of ZnCl 2 (entry 1), the reaction was slower in the absence of excess ZnCl 2 (entry 9), and a large excess of ZnCl 2 retarded the reaction (entry 11).…”