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Monday, October 23, 2006
Douglass Taber
University of Delaware

Functional Group Transformation

Two particularly useful conversions of alcohols were recently reported. Abdol Reza Hajipour of the University of Wisconsin has found (Tetrahedron Lett. 2006, 47, 4191. ) that grinding an alcohol such 1 with triphenylphosphine and I2, then microwaving the resulting solid mixture for a minute or less gives the corresponding iodide 2. The work up is particularly convenient. Phosphonium salts such as 4 have usually been prepared from the corresponding alkyl halides. Roman Mazurkiewicz of the Silesian University of Technology has now shown (Tetrahedron Lett. 2006, 47, 4219. ) that Mitsunobu coupling of the alcohol 3 with Ph3P.HBF4 gives the phosphonium salt 4 directly.

For small-scale oxidation of alcohols to aldehydes and ketones, it is often desirable to be able to simply filter away the spent (and any excess) reagent. Viktor Z. Zhdankin of the University of Minnesota, Duluth has developed (Org. Lett. 2006, 8, 167. ) a hypervalent iodine-based oxidizing resin (0.7 – 0.8 mmol/gram) that looks attractive for this application. For larger scale work, Tomoya Kitazume of the Tokyo Institute of Technology has reported (Tetrahedron Lett. 2006, 47, 2797. ) an ionic liquid based oxidation using catalytic N-hydroxyphthalimide, catalytic Co complex and O2. Remarkably, replacement of the Co and O2 with HNO3 gives a mixture that efficiently converted 9 to the nitroalkane 10.

Several useful methods for reduction have also been introduced. G. K. Surya Prakash and George A. Olah of the University of Southern California have found (J. Org. Chem. 2006, 71, 3952. ) that in the presence of the superacid formed from trifluoroethanol and BF3, triethylsilane will reduce a ketone such as 11 to the corresponding hydrocarbon 12. Tanmaya Pathak of the Indian Institute of Technology, Kharagpur has developed (Org. Lett. 2006, 8, 1303. ) a protocol for reductive desulfurization, e.g. of 13, using catalytic NiBr2 and stoichiometric Mg in methanol. Benzyl ethers are stable to the conditions. Although tosylates are unreactive, Hironao Sajiki of Gifu Pharmaceutical University has found (Org. Lett. 2006, 8, 987. ) that mesylates such as 15 are inexpensive replacements for triflates for reductive deoxygenation of phenols. And, David Milstein of the Weizmann Institute, revisiting a traditional transformation, has designed (Angew. Chem. Int. Ed. 2006, 45, 1113. ) a Ru catalyst that effects hydrogenation of esters under Parr shaker conditions at 115 °C.

Two important new transformations of terminal alkynes have recently been reported. Bernhard Breit of the Universität Freiburg has prepared (Angew. Chem. Int. Ed. 2006, 45, 1599. ) a Ru catalyst that converts terminal alkynes such as 19 into the corresponding aldehydes. Valery P. Fokin of Scripps La Jolla has found (Angew. Chem. Int. Ed. 2006, 45, 3154. ) that under Cu catalysis, a terminal alkene such as 21 reacts with an arenesulfonyl azide to deliver the corresponding amide 22.

D. F. Taber, Org. Chem. Highlights 2006, October 23.