Preparation of chiral allenylmetal reagents from enantioenriched allenyl iodides and propargylic mesylates. A comparison of indium, bismuth, and tin derivatives

Chiral allenylmetal halides were prepared starling from enantioenriched (similar to 95% ee) 1-octyn-3-ol mesylate (1) and 3-butyn-2-ol mesylate (7), Addition of these transient metallo species to aldehydes in situ afforded mainly anti homopropargylic alcohol adducts of varying ee. Three approaches were explored. In the first, the mesylates were converted to allenylstannanes with Bu3SnLi.CuBr and the allenylmetal halide was prepared From the resulting enantioenriched allenylstannane by transmetalation with InBr3, BiBr3, and SnCl4 in the presence of cyclohexanecarboxaldehyde. The configurational stability of the transient allenylmetal halide was estimated from the ee of the derived adduct, The second approach involved metalation of the allenyl iodides prepared from mesylates 7 and I through S(N)2' displacement with LiCuI2. From these experiments, it was determined that allenylindlium bromides and allenyltin bromides are configurationally stable under the reaction conditions. The adduct obtained from the allenylbismuth bromide was nearly racemic. A superior procedure was developed in which the propargylic mesylates 7 and 1 were converted to allenylindium iodide intermediates through treatment with InI and 5 mol % of a Pd(0) dppf catalyst; in THF-HMPA, THF-DMSO, or THF-DMPU. Under these conditions, alpha-branched aldehydes were converted to anti adducts (similar to 95:5 anti/syn) of >90% ee. Additions to unbranched aldehydes were less diastereoselective, hut afforded adducts of high ee. Additions of enantiomeric allenylindium iodide reagents to (R)-beta-ODPS-alpha-methylpropanal (13) afforded diastereomeric adducts (anti,anti and anti,syn) 14, 15 and 18, 19 with excellent diastereoselectivity indicative of a high degree of reagent control. A convenient procedure for the preparation of InI from In and I-2 is also described.