6638
Journal of the American Chemical Society
/
101:22
/ October 24, I 9 7 9
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 / 3 3 that of the sulfur analogue. The introduction of a m-trifluoromethyl 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 deprotonation of benzyl phenyl selenide, bis(phenylseleno)methane, methoxymethyl m-trifluoromethylphenyl selenide, and phenylselenoacetic acid, and by n-butyllithium cleavage of bis(pheny1seleno)methane) to carbonyl compounds. These @-hydroxyselenides are converted to olefins on treatment with rnethanesulfonyl chloride and triethylamine. This reductive elimination proceeds 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 @-hydroxyselenoxides or related compounds have not been successful. a-Lithio selenides can also be alkylated, and the derived selenides converted to olefins by selenoxide syn elimination.
The unique chemical properties of organoselenium compounds have been successfully exploited for a variety of olefin-forming proce~ses.~ The synthetic utility of these reactions is amplified by the capacity of the selenium function to serve as an activating group for carbon-carbon bond formation, in addition to its role in the introduction of the double bond. The preparation, properties, and reactions of a-lithio selenides will be the subject of this paper. The following paper4 describes our work on the chemistry of a-lithio selenoxides. All of the standard procedures for the preparation of alkyllithium reagents are, in principle, applicable to the preparation 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 b a s e ~ . l ~The % ~former . ~ procedure developed by Seebach and c o - w ~ r k e r s ~is. generally ~ ~ , ~ applicable to systems 1 where Rl and Rl can be hydrogen or an alkyl or aryl group. The resulting lithium reagents 2 have high nucleophilicity; they react with
1
2
3
a variety of electrophiles including alkyl halides, epoxides, ketones, aldehydes, esters, amides, silyl halides, and disulfides. The products (3) themselves are useful starting materials for 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 R I = R2 = H) are usually prepared by the reaction of ketones and aldehydes with malodorous and airsensitive benzeneselenol under strongly acidic conditiom8 This requires that any functional groups in R,and R2 be stable in both strongly acidic and strongly basic media. (2) The process uses up an expensive phenylseleno group, which then contaminates the product as butyl phenyl selenide. It would clearly be more advantageous to deprotonate the appropriate selenides, a procedure which avoids the necessity of handling benzeneselenol, the harsh conditions for selenoketal formation, and contamination by BuSePh. The great nucleophilicity of PhSe- assures that alkyl selenides can be prepared not only from traditional substrates for s N 2 displacements such as halides and sulfonates, but also from lactone^,^ esters,I0 0002-7863/79/1501-6638$01 .OO/O
monoactivated cyclopropanes,ll quaternary ammonium salts,I2 and amines.13 Furthermore, selenides are available by routes not involving s N 2 displacements a t carbon, such as the reaction of organometallic reagents with benzeneselenenyl chloride, or the acid-catalyzed or free-radical addition of benzeneselenol to certain olefins. Prior to our research in this area, only a few selenium stabilized carbanions had been prepared by deprotonation: the nietalation of tert-butoxyselenophene (to give 4) and benzoselenophene (to give 5) with ~ - B u and L ~the ~ deprotonation ~
5
4
of (PhSe)2CH2 and (PhSe)3CH with lithium diisobutylamide.' Although alkyllithium reagents are frequently and routinely used for the deprotonation of sulfides, their application to the deprotonation of selenides is rarely successful, an observation made in early work by GilmanI4 and S e e b a ~ hand , ~ confirmed repeatedly in subsequent work in our laboratory and elsewhere. I n fact, the deprotonation of phenylselenotrimethylsilylmethane with ~ec-butyllithium~~ is the only successful reaction of this type in addition to the selenophene derivatives mentioned above. The major process is usually attack of the lithium reagent a t selenium, presumably giving an ate complex 6 fol-
6
lowed by Se-C bond cleavage.6b,dAs a result it becomes necessary to use lithium dialkylamides or similar bases for deprotonation of selenium compounds.
Results and Discussion Preparation of Selenides. The selenides used in this work were prepared by standard procedures involving nucleophilic displacements by PhSeNa in ethanol. For small-scale reactions the reduction of diphenyl diselenide using sodium borohydride presents a convenient source of ethanolic PhSeNa.I5 We have frequently used sodium hydroxymethyl sulfoxylate (Rongalite, Na+-OSOCH20H)I6 in alkaline ethanol as a reducing agent. This procedure has several features which make it particularly 0 1979 American Chemical Society
Reich, Chow, Shah
/ Preparation of a-Lithio Selenides
l a b l e I . Substituted Arylselenornethyllithium Reagents Prepared by Deprotonation Ar.SavX
-t
7 a A r = Ph b A r = m-CF,-C,H,
8
basea
PhSeSi Me 3 Ph vinyl cthynql COzLi COIEt COPh
H OCH, SiMe3
conditions
Ar = Phenyl LiNR2" -78 LDA~ -78 sec - Bu Li / T M ED A 25 LDAITMEDA -78 LDA -78 LDA, LiTMP f LDA f -78 LDA LiNR2g -78 -78 LDA C H 3S( 0 ) C H 2Na
ref
OC, 60 min OC, < I 5 min O C , 90 min OC, 45 min OC, 5 min
5 c la 18 c, 7c 7e
If OC, < I O min "C 99% trans-2-decene was converted
18b
ination, although no independent proof of the stereochemistry of the hydroxy selenides 17 was obtained. The geometry of the vinyl ethers 18a and 18b was assigned on the basis of their N M R Eu(fod)3 shifts. The stereochemical evidence and other mechanistic considerations suggest a pathway involving an episelenonium ion. Since the crucial olefin-forming step is reversible, some method for disposal of the active electrophile 20 must be available. We yh 0 sa+ -0 S CH
PhSe 3
=eo A
qoso,cH3
19
X 0 PhSa-OSCH, 0
20
suggest that for the MsCI/NEt3 reagent sulfene may play this role; the reaction appears to be clean and complete only when an excess of the reagent was used. This mechanism is supported by other observations: only about 30% of the selenium appears ;is diphenyl diselenide after the reaction, the remainder being lost as water-soluble material, presumably 21, X = O H ; p -
X-
20
0
Ph SWCH~S-X 0 21
toluenesulfonyl chloride is not a satisfactory replacement for methanesulfonyl chloride. The involvement of 19 as an interR = C,HS5 14 mediate is also supported by the observation2Ibthat when phenyl is replaced by m-trifluoromethylphenyl more vigorous via epoxide and 14 to >99% trans-2-decene, and 97% cis-2reaction conditions are sometimes needed to achieve “reductive decene was similarly converted to 95% cis-2-decene. Since the elimination”. Additional evidence pertaining to the mechanism epoxidation and nucleophilic opening proceed with defined has been presented by Rimion and Krief.3’” stereochemistry, the reductive elimination must proceed anti. Olefins from a-Lithio Selenides and Carbonyl Compounds. Rtmion and K ~ - i e fhave ~ ~ ” also shown that other conditions for Table I l l summarizes the olefins prepared by addition of elimination of PhSeOH proceed with anti stereochemistry. ci-lithio selenides to aldehydes and ketones, followed by ‘‘reSimilar conclusions can be drawn from the experiments ductive elimination”. In addition to simple olefins, a vinyl outlined in Scheme I . Addition of phenylselenoacetic acid diselenide and several vinyl ethers have been prepared. Two atanion to benzaldehyde gave stereoisomeric adducts, whose tempts at preparing dienes by addition of phenylselenomethesters (15) could be chromatographically separated. Isomer yllithium to enones failed at the reductive elimination stage. 15b was cleanly converted to methyl trans-cinnamate, whereas Dienes with other substitution patterns have, however, been 15a was converted to methyl cis-cinnamate and the dihydration prepared successfully by this t e ~ h n i q u e . ~ . ~ ~ ” product 16. Addition of PhSe02CCF3 to methyl trans-cinAs illustrated in Schemes I and I 1 and Table IV, the addition naniate gave only isomer 15b. This agrees with published results which show that addition of ~ e l e n e n i c ~and ’ . ~~ ~u l f e n i c ~ ~of chiral lithium reagents (those bearing three different substituents at the anionic carbon) leads to diastereomeric hydroxy acid derivatives to olefins invariably proceeds with anti steselenides upon reaction with an aldehyde or unsymmetrical reochemistry. ketone. These diastereomers in turn give isomeric E or Z olefin We have also shown that the diastereomeric selenides 17 in a largely stereospecific reaction. Some of the cases we have (Scheme I I ) formed as shown give -90% stereospecific elim-
Reich, Chow, S h a h
/
664 1
Preparation of a-Lithio Selenides
Hydroxy Selenides and Olefins Prepared from a-Lithio Selenides and Carbonyl Compounds
‘fable Ill.
Table IV. Diastereomer Ratios of &Hydroxy Selenides from Addition of a-Lithio Selenides and Selenoxides“ to Ketones and A ldchydes LITHIUM REAGENT
RUN NO
CARBONYL COMPOUND
R B b RR-SS
0
66
2
e
ph,saYcozLI LI
39/61
PhKH
O , Y2O4, Tl(NO3)3, ~ - B U O O H only ) , ~ two, ~ ozone and m-chloroperbenzoic acid, met these requirements. Ozone has a long history as a convenient oxidant for the preparation of selenoxides.1° Further oxidation to selenone can
0 1979 American Chemical Society
