Alkenylchromium reagents

Coupling of vinyl iodides with aldehydes (12, 137). Further study1 of this 1,2-addition of alkenylchromium compounds to aldehydes to form allylic alcohols indicates that the reaction is applicable to a-alkoxy and a,(i-bisalkoxy aldehydes by use of a solvent other than DMF, which can promote elimination to an enal. A wide number of other functional groups can also be accommodated. Both vinyl iodides and p-iodo enones can be used as precursors to the alkenylchromium reagent. The reaction is only modestly diastereoselective, but the stereochemistry of a disubstituted vinyl iodide is retained. 

PREPARATION AND REACTIONS OF ALKENYLCHROMIUM REAGENTS 2-HEXYL-5-PHENYL-1 -PENTEN-3-OL... 

Addition of a catalytic amount of NiCfe to CrCl2 is essential for the formation of alkenylchromium reagents.4-6 However, a substantial amount of 1,3-diene, the coupling product of the alkenyl Iodide, is produced if a higher content of NiCfe is employed.8 9... 

K. Takai, K. Sakogawa, Y. Kataoka, K. Oshima, and K. Utimoto 180 PREPARATION AND REACTIONS OF ALKENYLCHROMIUM REAGENTS 2-HEXYL-5-PHENYL-1-PENTEN-3-OL... 

Kishi and cowoikers " have also implemented alkenylchromium reagents for die synthesis of glycosides and C-methyl glycoside analogs. In the synthesis of the C-analog (217) of isomaltose (216), the C(5)—C(6) bond was established by die nickel(II) chromium(II)-mediated coupling of (Z)-iodoalkene... 

Exan les of intramolecular addition reactions of alkenylchromium reagents to aldehydes have ap-peared. In the course of synthetic studies in the brefeidin structural series, the nickel(II)/chro-miiun(II)-mediated intramolecular addition reactions of ( )-iodoalkenes (227) and (230) were studied by Schreiber and Meyers (equations 72 and 73). Treatment of (227) with CrCh and a catalytic portion of [Ni(acac)2] in DMF produced a 4 1 mixture of 4-epibrefeldin C (228) and (+)-brefeldin (229) in 60% yield. In a similar fashion, precursor iodide (230) afforded a >10 1 mixture of cyclized hydroxy lactones (231) and (232) in 70% yield. An explanation of the stereochemical preference observ has been elo-quendy offered in a discussion of loc conformational preferences found in the starting material and the product lactone, as each is relevant to a transition structure for a 13-membered ring closure. 

Takai, K., Tagashira, M., Kuroda, T., Oshima, K., Utimoto, K., Nozaki, H. Reactions of alkenylchromium reagents prepared from alkenyl trifluoromethanesulfonates (triflates) with chromium(ll) chloride under nickel catalysis. J. Am. Chem. Soc. 1986, 108, 6048-6050. 

Alkenylchromium reagents are obtained from 1,1,1-trichloroalkanes by treatment with CrCl2 - Lil in THF. These reagents add to aldehydes to form aUyhc alcohols. From 1,1,1-trichloroethanol the primary reaction products are further dehydrated to give conjugated aldehydes. a,a,a-Trichloromethylarenes afford diaryleth3mes. 

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