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Selenium Stabilized Carbanions. Preparation of A-Lithio Selenides and Applications to the Synthesis of Olefins by Reductive Elim

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Selenium Stabilized Carbanions. Preparation of A-Lithio Selenides and Applications to the Synthesis of Olefins by Reductive Elim 6638 Journal of the American Chemical Society / 101:22 / October 24, I979 Selenium Stabilized Carbanions. Preparation of a-Lithio Selenides and Applications to the Synthesis of Olefins by Reductive Elimination of ,&Hydroxy Selenides and Selenoxide Syn Elimination Hans J. Reich,** Flora Chow, and Shrenik K. Shah Contribution from the Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706. Received May 10, 1979 Abstract: The deprotonation of several alkyl aryl selenides with lithium amide bases has been studied. The kinetic acidity of methyl m-trifluoromethylphenyl selenide was found to be 1/33 that of the sulfur analogue. The introduction of a m-trifluoro- methyl substituent into methyl phenyl sulfide increased the kinetic acidity by a factor of 22.4. A variety of @-hydroxy selenides have been prepared by the reduction of a-phenylseleno ketones and the addition of a-lithio selenides (prepared by deprotona- tion of benzyl phenyl selenide, bis(phenylseleno)methane, methoxymethyl m-trifluoromethylphenyl selenide, and phenylsele- noacetic acid, and by n-butyllithium cleavage of bis(pheny1seleno)methane) to carbonyl compounds. These @-hydroxyselen- ides are converted to olefins on treatment with rnethanesulfonyl chloride and triethylamine. This reductive elimination pro- ceeds exclusively or predominantly with anti stereochemistry. Simple olefins, styrenes, cinnamic acids, vinyl ethers, and vinyl selenides have been prepared using this technique. Attempts to carry out the syn reductive elimination of @-hydroxyselenox- ides or related compounds have not been successful. a-Lithio selenides can also be alkylated, and the derived selenides convert- ed to olefins by selenoxide syn elimination. The unique chemical properties of organoselenium com- monoactivated cyclopropanes,ll quaternary ammonium pounds have been successfully exploited for a variety of ole- salts,I2 and amines.13 Furthermore, selenides are available by fin-forming proce~ses.~The synthetic utility of these reactions routes not involving sN2 displacements at carbon, such as the is amplified by the capacity of the selenium function to serve reaction of organometallic reagents with benzeneselenenyl as an activating group for carbon-carbon bond formation, in chloride, or the acid-catalyzed or free-radical addition of addition to its role in the introduction of the double bond. The benzeneselenol to certain olefins. preparation, properties, and reactions of a-lithio selenides will Prior to our research in this area, only a few selenium sta- be the subject of this paper. The following paper4 describes our bilized carbanions had been prepared by deprotonation: the work on the chemistry of a-lithio selenoxides. nietalation of tert-butoxyselenophene (to give 4) and benzo- All of the standard procedures for the preparation of alk- selenophene (to give 5) with ~-BuL~~~and the deprotonation yllithium reagents are, in principle, applicable to the prepa- ration of a-lithio selenides. In practice, only two methods have been used widely: the n-butyllithium cleavage of selenoacetals and -ketals5s6 and the deprotonation of selenides using strong 4 5 base~.l~%~.~The former procedure developed by Seebach and of (PhSe)2CH2 and (PhSe)3CH with lithium diisobutylamide.' co-w~rkers~.~~,~is generally applicable to systems 1 where Rl Although alkyllithium reagents are frequently and routinely and Rl can be hydrogen or an alkyl or aryl group. The resulting used for the deprotonation of sulfides, their application to the lithium reagents 2 have high nucleophilicity; they react with deprotonation of selenides is rarely successful, an observation made in early work by GilmanI4 and Seeba~h,~and confirmed repeatedly in subsequent work in our laboratory and elsewhere. In fact, the deprotonation of phenylselenotrimethylsilyl- 1 2 3 methane with ~ec-butyllithium~~is the only successful reaction a variety of electrophiles including alkyl halides, epoxides, of this type in addition to the selenophene derivatives men- ketones, aldehydes, esters, amides, silyl halides, and disulfides. tioned above. The major process is usually attack of the lithium The products (3) themselves are useful starting materials for reagent at selenium, presumably giving an ate complex 6 fol- the preparation of a variety of selenium-free compounds. Two major disadvantages of the selenoketal cleavage are apparent: ( 1 ) The precursor bis(phenylselen0) compounds 1 (except where RI = R2 = H) are usually prepared by the re- 6 action of ketones and aldehydes with malodorous and air- lowed by Se-C bond cleavage.6b,dAs a result it becomes nec- sensitive benzeneselenol under strongly acidic conditiom8 This essary to use lithium dialkylamides or similar bases for de- requires that any functional groups in R,and R2 be stable in protonation of selenium compounds. both strongly acidic and strongly basic media. (2) The process Results and Discussion uses up an expensive phenylseleno group, which then con- taminates the product as butyl phenyl selenide. It would clearly Preparation of Selenides. The selenides used in this work be more advantageous to deprotonate the appropriate selenides, were prepared by standard procedures involving nucleophilic a procedure which avoids the necessity of handling ben- displacements by PhSeNa in ethanol. For small-scale reactions zeneselenol, the harsh conditions for selenoketal formation, the reduction of diphenyl diselenide using sodium borohydride and contamination by BuSePh. The great nucleophilicity of presents a convenient source of ethanolic PhSeNa.I5 We have PhSe- assures that alkyl selenides can be prepared not only frequently used sodium hydroxymethyl sulfoxylate (Rongalite, from traditional substrates for sN2 displacements such as Na+-OSOCH20H)I6 in alkaline ethanol as a reducing agent. halides and sulfonates, but also from lactone^,^ esters,I0 This procedure has several features which make it particularly 0002-7863/79/1501-6638$01 .OO/O 0 1979 American Chemical Society Reich, Chow, Shah / Preparation of a-Lithio Selenides 6639 lable I. Substituted Arylselenornethyllithium Reagents Prepared lable 11. Substituent Effects for Reactions Involving Carbanion by Deprotonation Intermediates Ar.SavX -t A,,SayX quantity km-c~31 Li comDd measured base/solvent kH p ref 7a Ar= Ph 8 ArSCH3 ka LiTMP/THF- 22.4 3.14h c b Ar = m-CF,-C,H, hexane X basea conditions ref ArCZCC H3 k " 1.3 25 ArPh2CH k" PhLi 2.2 26 Ar = Phenyl ArCH2T kd LiNH-c-Hex/ 68 4.0 27 PhSe- LiNR2" -78 OC, 60 min 5 H2N-c- Hex LDA~ -78 OC, <I5 min c ArCH3 pK,, N aCH 2SOC H3 1 12' 28 Si Me 3 sec - Bu Li / T M EDA 25 OC, 90 min la MezSO LDAITMEDA -78 OC, 45 min 18 Ph LDA -78 OC, 5 min c, 7c Rate of deprotonation. Two points only. This work. Isotopic vinyl LDA, LiTMP f 7e cxchange rate. Based on strongly electron-withdrawing substituents cthynql LDA f If only. COzLi LDA -78 OC, <IO min c COIEt LiNR2g -78 "C <5 min 7b Vinyl" and allenyl phenyl selenides21acan also be depro- COPh LDA -78 OC, <5 min 7h tonated with LDA. C H 3S( 0)CH 2Na 20a Kinetic Acidity of Simple Selenides and Sulfides. Our ob- Ar = m-Trifluoromethylphenyl servation that m-trifluoromethylphenyl (henceforth, Ar) H LiTMP -55 "C, 120 min c selenides are substantially more acidic than the phenyl ana- OCH, LiTMP -78 OC, 5 min c logues prompted a brief study of some relative kinetic acidities, SiMe3 LiTMP -40 OC, 40 min 7d measured under typical synthetic conditions. Competitive (' In THF unless otherwise noted. Lithium diisobutylamide. deprotonations of two-component mixtures were carried out. Each sample was treated with an amount of LiTMP in hex- L Present work. Ether can also be used. In hexane. ./Variable, depending on substitution. fi Lithium N-isopropylcyclohexylamide. ane/THF at -56 OC such that only partial deprotonation of In Me2SO. the most acidic compound occurred. This mixture of anions was either methylated (CH31) or silylated ((CH3)3SiCl), and the product mixture analyzed by GLC. Control experiments attractive for large-scale reactions: it is inexpensive and, unlike showed that proton transfer between the lithiated and pro- the borohydride reduction, there is no hydrogen evolution. tonated compounds was not occurring. The relative rates ob- PhSeH and PhSeNa have also been prepared by reduction of tained are summarized below. diphenyl diselenide with hypophosphorous acid.]' PhSeNa in Ph".CH, ArSS'CH3 Ar"'. CH3 Ar""CH,SiMe, DMF'.' and HMPAlo-9band PhSeLi with 12-crown-4 in bel, 1.0 22.4 59 2.1 benzene' I show superior reactivity over that observed in protic solvents. These results show that ArSMe is kinetically about four Deprotonation of Selenides. We have routinely used lithium times as acidic as ArSeMe. This is compatible with published diisopropylamide (LDA) and lithium 2,2,6,6-tetramethylpi- results: (1) The kinetic acidity of PhSMe exceeds that of peridide (LiTMP) for the deprotonation of selenides. The latter PhSeMe by a factor of IO using KNHz/NH3.22 (2) The acidity appears to be at least a power of ten more reactive, although of (PhS)2CH2 exceeds that of (PhSe)*CH2 by about two or- steric effects are important. Our survey of a number of selen- ders of magnit~de.~(3) The pK,s (in Me2SO) of phenylthio- ides had shown that any phenylselenomethane with one addi- and phenylselenoacetophenones are 17.1 and 18.6, respec- tional anion stabilizing group (7) can be deprotonated with tively.20" The results are not, however, compatible with a LDA (Table I). However, even LiTMP does not give complete theoretical prediction of greater stability for a-seleno carb- :inion formation for the methoxy-substituted compound anions than a-thio carbanions.23 PhSeCHzOCH3. It is, therefore, necessary to use the methoxy The magnitude of the m-trifluoromethyl substitution effect selenide 7b with the m-trifluoromethyl group, a technique was not measured in the selenium compounds because of the which enhances the kinetic acidity of the methylene group by slow rate of deprotonation of PhSeCH3. It was found to be 22.4 approximately a factor of 20 (see below).
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