Reactions with samarium diiodide

Epoxy-l-haloalkenes undergo novel reductive cleavage reactions with samarium diiodide to give cyclopropanols or allylic alcohols depending upon the substrates and reaction conditions. Formation of the cyclopropanol is believed to proceed through a diradical intermediate <1998SL1073>. 

Per-benzoylated aldonolactones undergo a reductive elimination reaction with samarium diiodide leading to unsaturated derivatives (Scheme 3). Deoxygenation at the 2-position can also take place. The difference in the reductive elimination patterns between the D-g/uco and D-galacto isomers may be due to the axial disposition of the OBz in the galacto isomer. 

In Scheme 4.16 is depicted another reduction reaction, involving carbohydrate lactones, which are converted into the corresponding 2-deoxylactones by reaction with samarium diiodide in a water-THF mixture (Hanessian and Girard, 1994). The best amount of water for obtaining the hydroxyketone was 1-2% of total volume. More water led to significant amounts of benzaldehyde (Hasegawa et al, 1996). 

Burk et al. showed the enantioselective hydrogenation of a broad range of N-acylhydrazones 146 to occur readily with [Et-DuPhos Rh(COD)]OTf [14]. The reaction was found to be extremely chemoselective, with little or no reduction of alkenes, alkynes, ketones, aldehydes, esters, nitriles, imines, carbon-halogen, or nitro groups occurring. Excellent enantioselectivities were achieved (88-97% ee) at reasonable rates (TOF up to 500 h ) under very mild conditions (4 bar H2, 20°C). The products from these reactions could be easily converted into chiral amines or a-amino acids by cleavage of the N-N bond with samarium diiodide. 

Two convenient methods have been developed for the preparation of trifluoro-methyl-substituted alkoxyallenes. Reductive elimination of allylic acetates 30 with samarium diiodide leads to 31 (Scheme 8.11) [38], whereas reaction of Wittig cumu-lene 32 with phenyl trifluoromethyl ketone (33) and thermolysis of the intermediate 34 provides 35 (Scheme 8.12) [39]. 

Transannular cyclization of ketoalkenes was first reported in 1965. Treatment of the conformationally restricted ketone (104) with sodium in moist ether gave the alcohol (105 equation 127). Similarly the ketoalkene (106) transannulated in 73% yield by exposure to sodium in refluxing propanol (equation 128). Conformational restriction is not a prerequisite for transannular reaction thus the caryophyllene (107) undergoes cyclization to the alcohol (108) with lithium in liquid ammonia (equation 129). Transannulation across a nine-membered ring has also been observed upon treatment of ketone (109 equation 130) with samarium diiodide, via cyclization of the ketoalkene (110). Of more practical importance, the electrochemical transannulation of the cyclooct-4-en-l-one gives the bicyclo[3.3.0]octanol (111 equation 131). ... 

Reductive deamination of pyrrolidine (3) with samarium diiodide in THF-HMPA in the presence of methanol as the proton source yielded the desired piperidone (4) stereoselectively in 90% yield (Eq. (2)), where a carbon-nitrogen bond cleavage reaction and subsequent recyclization took place simultaneously (05TL5161). 

Despite the tremendous amount of work on samarium diiodide mediated reactions, few examples of cyclic ketone reductions have been described. One example is a step in the synthesis of racemic atractyligenin49 where an annulated cyclohexanone was reduced efficiently with samarium diiodide in tetrahydrofuran/ water at 20 C to give the 2/J-alcohol equatorioselectively. 

The well-known Sm ry -arene complexes are generally prepared by the reaction of arenes with rare-earth trihalides in the presence of aluminium halides. This reaction was also effective with samarium diiodide and was extended to thulium diiodide however, an attempt to react Tml2 with naphthalene in toluene in the presence of aluminium trichloride was not successful but resulted in the isolation of an ry -toluene complex of Tm [77 -(CH3C6H5)Tm(AlCl4)3] (Figure 6) (Fagin et al., 2005). 

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