R 2 Zn in the presence of NMP or LiBr promotes the intramolecular rearrangement of 1,1-diiodoalkanes via the formation of sp 3 secondary zinc carbenoid.The intermolecular reaction between a nucleophilic organometallic and an electrophilic carbon center represents one of the widest classes of carbon-carbon bond forming reaction nowadays. Far less common are intramolecular variants in which both the nucleophilic and electrophilic partners are bound to the same metal (intramolecular rearrangement of carbenoid). When the metal bears a negative charge, such reactions are defined as 1,2-metallate rearrangements and are relatively well known for the migration of alkyl groups to sp 3 -, sp 2 -and sp-hybridized carbons. 1 Particularly attractive is the rearrangement of ahalo-triorganozincate 2 (M = Zn) as well as a-alkoxy-alkenyl lithio cuprate (M = Cu, Scheme 1). 3 Scheme 1However, an excess of electrophile is usually required to compensate the excess of alkyl groups attached to the metal. When the metal is not negatively charged, no intramolecular rearrangement is observed and only intermolecular reaction occurs between carbenoid and organometallic derivative 4 to give the alkylated and rearranged organometallics as products. 5 It was reported recently by one of us that a polar cosolvent like N-methylpyrrolidinone (NMP) permitted the 1,4-addition of R 2 Zn to enones in the absence of any copper or transition metal catalyst; named as the uncatalyzed conjugate addition reaction. 6 R 2 Zn alone did not react with any of these enones. The origin of this reaction rate increase in NMP may result from the ionization of the diorganozinc by the NMP providing a more reactive pseudozincate. 7 So, we wished to use this efficient and economic new form of activated dialkylzinc (equivalent of zincate derivatives) 8 for several different synthetic purposes, and we report here that these derivatives can be successfully used for the intramolecular rearrangement of sp 3 carbenoids. 9Indeed, treatment of 1,1-diiodoalkane 10 1 with 1.2 equivalents of Et 2 Zn in THF in the presence of NMP allows iodine-zinc exchange at -50°C to form the corresponding sp 3 secondary zinc carbenoid 2. Then, by warming the reaction mixture to room temperature, the carbenoid 2 undergoes an intramolecular nucleophilic rearrangement into the secondary organozinc iodide derivative 3 which can react with different electrophiles in good overall yields (Scheme 2 and Table). Scheme 2In order to prove the stepwise mechanism, intermediate zinc carbenoid 2 was trapped with Br 2 at -50°C to get 4.Although, the a,a'-bromo iodo derivative 4 was isolated in a promising 50% yield, minor amounts of dibromoalkanes were always present in the crude reaction mixture (probably formed by degradation of the carbenoid 2 into carbene and subsequent reaction with Br 2 ).The formation of a secondary organozinc halide 3 was checked by halogenolysis (entries 2 to 4 and entry 9), oxidation 11 (entry 6) and, finally, by allylation and 1,4-addition reactions after transmetallation of the organ...

No abstract

No abstract

Tertiary amines activated either by borane (BH 3 ) or cyanoborane (BH 2 CN) groups were α-C lithiated with sBuLi (2 equiv.) and then treated with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane to produce the α-aminoboronate com-

No abstract