COMMUNICATIONSphiles The is reaction of general of (~~3-allyl)palladium synthetic and fundamental, complexes mechanistic with nucleo-interest."] We report here on the preparation, isolation, and full characterization of a palladacyclobutane, the structure of which affords insight into the mechanism of cyclopropane formation. -d-, N-Pd ' \ -TmA*do-\ok HCI. -15'C ON-1 2 Scheme 1. R = H, aryl. alkyl: R = H. aryl. nor alkyl. LDA = lithium diisopropylamide. TMEDA = N,N,.V',N'-tetrarnethylethylenediamine.

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48 h gave the saturated silanes 21,23, and 25 in greater than 90 % yields. Exceptionally high diastereoselectivities (> 500: 1) were noted in all cases. No desilylation was observed under these conditions. Table 2. Directed hydrogenation of y-hydroxyvinylsilanes. The substrate was treated with 5 mol % of the catalyst 1 in CH,CI, for 24-48 h at 103 x lo5 Pa. Substrate Product Yield [%] [a] (syn:anti ratio) C6H11 r nR C6Hl1 / r J R ZOa. R = H, R' = PhMe,Si b, R = PhMe,Si, R = H 22a, R = Me, R = PhMe,Si 24, R = PhMe,Si, R = Me OH SnBu, uSiMe, 27 OH SnBu, uSiPhMe, 29 C6H1 LSiMe2Ph I 21 99 90 C6H1 I =R 23 a, R = Me, R = PhMe,Si 99 (> 500:l) 25, R = PhMe,Si. R = Me 95 (1:>500)[XI a) M.

COMMUNICATIONS the desired product 6a (Table 1). [4-'3C]Nicotinic acid (99% I3C) was prepared by using the method of Oberfrank et aI.l9] and biosynthetically incorporated into ~rocanase.[~I The cultivation of Pseudomorias putida nic I1 (a nicotinate auxotrophic mutant), the isolation of the labeled enzyme. and the enzymatic formation of the NAD+-imidazole propionate adduct were carried out as described by Klepp et aI.l4] 250 mg (2.03 pmol) '%labeled urocanase and 48 mg (300 pmol) [5'-'3C]imidazole propionic acid . HCI were used.