Cobalt-propargyl radicals

Propargyl)Co2(CO)g radicals presumably are also involved in the Mn(III)-mediated addition of P-dicarbonyl compounds to complexed 1,3-enynes, which produces highly functionalized dihydrofiuan derivatives 122 (Scheme 4-65) [222, 223]. The chemo-, regio-, and stereoselectivity of these reactions stands in contrast to the variable selectivity associated with the corresponding reactions of free enynes [224]. The formation of ethers 123 in methanol (Scheme 4-65) suggests that the cobalt-propargyl radicals initially produced are rapidly oxidized by Mn(III) to the stabilized carbocations. 

The same radical cyclization can be performed using a more simplified vitamin B12 model, such as Z w(dimethylglyoximato)(pyridine)Cobalt chloride Co3+ Cl -Py complex (B), as shown in eq. 11.3. Treatment of propargyl P-bromoethyl ether (3) with NaBH4 and a catalytic amount of the cobalt complex (B) provides fi-exo-methylene tetrahydrofuran via 5-exo-dig manner [2-7]. 

Alternatively, 5-allyl-2(5/f)-furanones can be obtained from TMSOF and allylic acetates using lithium perchlorate in ether. 5-Propargyl-2(5//)-furanones are prepared by coupling with bex-acarbonyldicobalt complexes of propargylium cations (eq 5). Reaction of the intermediate complex with cerium(TV) ammonium nitrate removes the cobalt Fluoroalkylation is acbieved by using bis(fluoroalkanoyl) peroxides in 1,1,2-tricbloro-1,2,2-trifluoroethane (Freon 113) (eq 6). This process is believed to involve combination of a fluoroalkyl radical with the furan-derived radical cation. 

The elaborate cobalt(II)-porphyrin catalyst [Co(Pl)] further enabled intramolecular amination of C(sp )-H bonds in simple alkyl chains, adjacent to electron-withdrawing groups, or at the allylic or propargylic positions through decomposition of sulfamoyl azide (Scheme 10.21) [50]. Thus, the catalyst promotes nitrene insertion into such C(sp )-H bonds located at the y-position of the sulfamoyl group to afford six-membered cyclic sulfamides in excellent yields, which serve as versatile 1,3-diamine scaffolds. On the basis of mechanistic experiments, a mechanism that is distinct from related Rh -catalyzed C-H amination was proposed. Thus, the 15-electron metalloradical catalyst [Co(Pl)j decomposes the sulfamoyl azide to form a Co -radical nitrene species, which... 

---