Functional Group Manipulation Usin Organoselenium Reagents

22 Reich Accounts of Chemical Research (25), bound covalently or by physical forces to the possibilities. One is that cosynthetase alters the con- enzyme system, is then converted into uro’gen-I11 (1) formation of the deaminase-bilane complex to direct by an intramolecular rearrangement which directly cyclization of 25 at C-16. There are indication^",^^ that affects only ring D and the two carbons which become deaminase associates with cosynthetase, and it has been C-15 and C-20. The nature of the intermediate between ~uggested”~,~~that cosynthetase acts as a “specifier the bilane and uro’gen-111 remains to be established, protein” in the way lactoalbumiri works during the and this leads to the concluding section. biosynthesis of lactose. The other possibility is that the Prospect. In the presence of deaminase alone, and bilane (25) is the product from deaminase but is then also nonenzymically, the cyclization of bilane (25) occurs the substrate for cosynthetase which brings about ring at C-19 to produce 26, leading to uro’gen-I (11). We closure with rearrangement to produce uro’gen-I11 suggest that in the presence of cosynthetase cyclization specifically. occurs at C-16 rather than at (2-19 (Scheme XIII). The Work is in hand on these aspects and on the problem postulated attack at C-16 would produce the spiro of the structure of the intermediate5’ between the bilane intermediate 27; the labeling arising from 25b and 25c and uro‘gen-111. It will be good to have the answers to is shown. Fragmentation and cyclization again as il- the few remaining questions. lustrated would generate ~ro’gen-111.~~The spiro Weare glad to have this opportunity to record our debt and system is related to that proposed by Mathewson and thanks to our young colleagues whose courage in this demanding Cor~in~~in 1961, and its intermediacy is consistent with field and experimental skills made the work posible. Their names all the work summarized above. Notice that both 26 are given in the literature references. Weare also grateful for and 27 contain a new sp3 center, and they differ less in financial support from the Nuffield Foundation, Science Re- shape than may appear from a planar diagram. search Council, and Roche Products. Wow does cosynthetase divert the unrearranged bi- (48) R. B. Frydman and G.Feinstein, Biochim. Riophys. Acta, 380, lane to exclusive type-111 formation when in its absence 358 (1974). type I is exclusively formed? We consider here two (49) M. Higuchi and L. Bogorad, Ann. N.Y. Acad. Sci., 24.1.401 (1975). (50) B. Middleton, Cambridge, quoted by A. R. Battersby and E. (46) The alternative, and equivalent. fragmentation from ring .4 would McDonald in ref 2, p 96. simply regenerate an equivalent of the unrearranged bilane ready for (51) The only alternative to the spiro intermediate involves fission of re-formation of the spiro system 27. the C-15/C-16 bond of bilane (25) while it is bound to the enzyme, followed (47) J. H. Mathewson and A. H. Corwin, J. Am. Chem. Soc., 83, 136 by inversion of ring D with strictly no exchange with the medium of the (1961). now separated pyrrole fragment. We find this possibility less attractive. Functional Group Manipulation Usin Organoselenium Reagents HANSJ. REICH Department of Chemisti-3, L’niuersit> of Wisconsin, Madison, Wisconsin 53706 ReceiLed March 13, 1978 The use of second, third, and fourth row elements in The utility of selenium and sulfur is derived from the organic synthesis received its major modern impetus great ease with which a wide variety of organoselenium from the discovery of the Wittig olefin synthesis. Since and organosulfur compounds can be prepared, as well then many applications of silicon, tin, phosphorus, as the facility of transformations of t,hese compounds sulfur, selenium, and other metalloids in organic syn- to effect useful functional group interconversion. Both thesis have been proposed. and a significant number of nucleophilic and electrophilic reagents are readily these procedures are finding their place in routine available for the introduction of selenium. The large organic synthesis. stabilization of carbanions provided b57 sulfur and In this Account some recent developments in the selenium substituents (at least 10-15 pK, units for PhS chemistry of organoselenium compounds which have and PhSe, more for sulfoxides, selenoxides, and synthetic potential are described, with particular em- sulfones’) has resulted in their use as activating groups phasis on the exploitation of chemical properties of for a variety of carbon-carbon bond-forming procedures selenium which differ from those of sulfur. In addition involving organometallic reagents. to work at Wisconsin, research groups headed by The chemistry of sulfur and selenium is very closely Sharpless, Seebach, Grieco, Krief, Nicolaou, and others related. The greater expense of selenium and the have been active in this area.l hazard resulting from its toxicity3 are justifiable only Hans J. Reich was born in 1943 in Danzig, Germany. After a B.Sc. degree (1) For a comprehensive account of synthetic organoselenium chemistry, at the University of Alberta and Ph.D. at UCLA with D. J. Cram, he spent 2 years see H. J. Reich in “Oxidation in Organic Chemistry, Part C”, W. Tra- doing postdoctoral work, first at Cal Tech with J. D. Roberts and then at Harvard hanovsky, Ed., Academic Press, New York, 1978, p 1. with R. 8. Woodward. In 1970, he joined the faculty at the University of (2) F. G. Bordwell et ai., J. Org. Chem., 42, 326 (1977). Wisconsin-Madison, where he is now Associate Professor. He is an Alfred (3) See K. Schwartz and K. D. Pathak, Chem. Scr., SA, 85 (1976), and P. Sloan Fellow and has research interests in the chemistry of the organometalloids D. V. Frost and D. Ingvoldstad, ibid.. 8A, 96 (1975), for reviews on the and their application to organic synthesis. beneficial and harmful effects of dietary selenium. 0001-4842/79/0112-0022$01.00/0 IC 1979 American Chemical Society Vol. 12, 1979 Organoselenium Reagents 23 if there is some qualitative or quantitative chemical Organoselenium Reagents. Most synthetic difference which allows introduction, modification, or transformations using organoselenium reagents have removal of the organoselenium group under conditions involved use of the arylseleno (usually phenylseleno) not possible with sulfur: group, for which the primary source is a diary1 di- (1) Selenium forms weaker c bonds than sulfur; selenide or an aryl selenocyanate. Diphenyl diselenide hence, many reactions which involve cleavage of C-Se, 0-Se, and N-Se bonds are more rapid than cleavage PhMgBr PhSeMgBr 8rz PhSeSePh of analogous bonds to sulfur. In this regard, alkyl selenoxides undergo syn elimination about 1000 times Ag02CCF3 PhSeOzCCF3 as fast as do sulfoxide^,^ [1,3] and [2,3] sigmatropic Ph-Se-Se-Ph -Clz PhSeCl 4 PhSe02CCH3 rearrangements proceed at markedly lower tempera- ~r~ PhSeBr PhSeNR2 ture~,~~~and episelenides spontaneously lose selenium > at room temperature.7-9 is readily available by a Grignard synthesi~.~~,~~Aryl (2) Selenides and selenolate anions are less basiclo and selenocyanates are usually prepared by the reaction of more nucleophilic"J2 than corresponding sulfur com- diazonium salts with KSeCN.1gds20b pounds. Selenoxides are more polar and more basic Powerful nucleophiles attack Ph2Sezat selenium, but than are ~u1foxides.l~ more efficient sources of electrophilic selenium are the (3) Selenium is oxidized somewhat more easily to selenenyl halides (PhSeC1, PhSeBr). Both are crys- Se(IV), but with more difficulty to Se(V1) than the talline, shelf-stable materials which can in turn be corresponding sulfur oxidations. Thus most oxidizing converted to other useful reagents: mixed selenenic agents convert selenides to selenoxides, selenols to anhydrides (PhSe02CCF3,21PhSe02CCH322) or selen- seleninic acids (RSe02H), and hydrogen selenide to enamides (PhSeNR223). selenium dioxide, but higher oxidation states (selenone, Benzeneseleninyl chloride and benzeneseleninic selenonic acid, selenium trioxide) are difficult to anhydride are used for the electrophilic introduction achieve. of the seleninyl Utilization of a nucleophilic (4)Organoselenium compounds are more subject to nucleophilic attack at selenium. Thus a-phenylseleno 0 0 I/ ketones are easily deselenated in the presence of base,14 PhSeCl 2 Ph-Se-C1 and most organoselenium compounds are attacked at 00 selenium by organolithium reagents.15J6 This property 0, II /I Ph-Sese-Ph -+ Se Se permits the convenient synthesis of a-lithio selenides \ 1 \ by cleavage of selenoacetals and -ketals.16a-d 0 Ph (5) Tetracovalent selenium compounds (selenuranes) seleninyl reagent (ArSeO-, benzeneselenenate anion) are usually more stable and more easily prepared than has been reported. Decomposition of the selenoxide 1 corresponding sulfur systems.17 (6) The stereochemical lability of selenoxides is much greater than that of sulfoxides. Selenoxides are readily racemized in neutral or weakly acidic media.ls It is this property of facile acid catalyzed nucleophilic exchange at Se(IV),coupled with resistance to further oxidation 1 3 2 (3, above) that permits benzeneseleninic acids to serve as catalysts for epoxidation of olefins with hydrogen in the presence of sodium hydride and benzyl bromide peroxide (ArSe03H is thought to be the active oxi- leads to the selenoxide 2, presumably by Se-alkylation dant).I9 of the intermediate sodium selenenate 3.25 (4) (a) D. N. Jones, D. Mundy, and R. D. Whitehouse, Chem. Commun., Diphenyl diselenide is reduced to benzeneselenol 86 (1970); (b) H. J. Reich, S. Wollowitz, J. E. Trend, F. Chow, and D. (PhSeH or PhSe-) by a variety of reducing agents of F. Wendleborn, J. Org. Chem., 43, 1697 (1978). which NaBH4 in ethanol is the most convenient. Less (5) (a) K. B. Sharpless and R. F. Lauer, J. Org. Chem., 37,3973 (1972); (b) K. B. Sharpless and R. F. Lauer, J.

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