Nugent reagent

To improve the utility of Nugent s and RajanBabu s conditions even further, catalytic conditions for cyclizations have been developed. They address the issue of reagent control of the cyclization and the mode of its termination. The formation of an alkyl titanocene species after reductive trapping allows two distinctive pathways for the regeneration of the catalyst. 

The (3-metaloxy radical was first exploited for synthetic purposes in C—H and C—C bond-forming reactions by Nugent and RajanBabu through the use of titanocene(III) chloride as an electron-transfer reagent [5]. They established that the (3-titaniumoxy radicals formed after electron transfer can be reduced by hydrogen atom donors, e. g. 1,4-cy-clohexadiene or tert-butyl thiol, that they add to a,(3-unsaturated carbonyl compounds, and that they can react intramolecularly with olefins in 5-exo cyclizations. 

It should not be forgotten, however, that titanocene(III) complexes are also excellent reagents for the deoxygenation of epoxides, as demonstrated independently by Schobert [13] and by Nugent and RajanBabu [5d[. An example of a reaction yielding a highly acid-sensitive product in reasonable yield is shown in Scheme 12.5. 

The general idea of this concept was first outlined by Nugent and Rajan-Babu [17-20] as shown in Scheme 3, and constitutes an analogue of the well-established opening of a cyclopropylcarbinyl radical [21,22]. Titanocenes have emerged as the most powerful reagents in these transformations. However, it is clearly attractive to find other metal complexes in order to develop novel reactivity patterns. 

Nugent has isolated and structurally characterised the zirconocyclopentanes derived from 1,6-heptadiene and either Cp2ZrCl2/2BuLi or Cp ZrCl3/Na(Hg). The former 25 is monomeric while the latter 25a involved a Zr2(p-Cl)2 structure. However, of particular note is that while the Negishi reagent provides primarily (97 3)... 

Reduction of Cp2TiCl2 by zinc produces an active species for the radical cyclization reactions of substituted a-(prop-2-ynyloxy) epoxides.1162 Cp2TiCl2 is reduced with powdered zinc in THF to produce Cp2TiCl in situ, Nugent s reagent, an efficient catalytic agent for organic reactions.1163,1164... 

Nugent WA, Thom DL, Harlow RL (1987) Cyclization of diacetylenes to E, E exocychc dienes. Complementary procedures based on titanium and zirconium reagents. J Am Chem Soc 109 2788-2796... 

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