J. Braz. Chem. Soc., Vol. 9, No. 4, 327-340, 1998. © 1998 Soc. Bras. Química Printed in Brazil. 0103 -- 5053 $6.00 + 0.00 Review Selenium Stabilized Carbenium Ions on Organic Synthesis Claudio C. Silveira, and Enrique L. Larghi Departamento de Química, Universidade Federal de Santa Maria, UFSM, C.P. 5001, 97105-900 Santa Maria - RS, Brazil; e-mail: [email protected] Received: August 10, 1998 Este artigo resume as aplicações sintéticas de íons de carbenio estabilizados por grupos organoselênio. São descritas reações de diferentes tipos de íons de carbenio estabilizados por selênio com compostos aromáticos, éteres enólicos de silício, alilsilanos/alilestananas e alcenos. This review summarizes the synthetic application of selenium stabilized carbenium ions. Are presented reactions of different types of selenium stabilized carbenium ions with aromatic com- pounds, silyl enol ethers, allylsilanes/allylstannanes and alkenes. Keywords: selenium, carbenium ions, organic synthesis Introduction an adjacent carbenium ion. These carbenium ions were generated from different selenium species and reacted with Organoselenium compounds have been known for a several nucleophiles, like silyl enol ethers, alkenes, allylsi- long time as versatile reagents in organic chemistry. In the lanes/allylstannanes and aromatic compounds. In the fol- last years there has been a considerable development of lowing, we will be presenting these reactions in detail. selenium-based methods for organic synthesis1. Many of these methods are currrently been used as standard proce- Reaction of selenium stabilized carbenium ions with dures to introduce new functional groups under mild con- aromatic compounds ditions. From the several areas of interest in The first aplication of this chemistry in order to produce organoselenium chemistry, selenium stabilized carboca- a new carbon-carbon bond, was made by the use of se- tions has emerged as a useful class for interesting transfor- lenoallyl cations in reaction with N-methyl-pyrrole (1) and mations and new methods for carbon-carbon bond furan (2)3. These carbocations, stabilized by both selenium formation. and allylic resonance, were generated in situ from a mixture Aspects related to heterosubstituted carbenium ions, of E and Z 1,3-bis(methylseleno)-propene (3) and 1,3- like their formation, structure and stability, has been very 2 bis(phenylseleno)-propene (4) mediated by silver perchlo- well studied and revised by Hevesi in the last years , and rate and sodium carbonate in nitrometane to give adducts will not be the subject of this article. In the following like 5, as depicted in Fig. 1. chapter we will detail synthetic aspects of selenium stabi- The reaction proceed smothly, with relatively good lized carbocations as electrophiles. yields and stereospecificity. In the case of N-methyl-pyr- Synthetic Applications role variable ratios of regioisomers were obtained. After reaction are recovered important quantities of the starting Several new reactions were developed in the last years material but one the isomers when are used as Z and E using the capability of an organoselenium group to stabilize mixture.There have not been detected products derived of X X RSe SeR AgClO4/CaCO3 SeR + SeR CH3NO2/-5 °C R=CH3 (3) X=CH3-N (1) R=Ph (4) X=O (2) 5 Figure 1. 328 Silveira & Larghi J. Braz. Chem. Soc. a [4 + 3 ® 7] cycloaddition. This latter reaction occurs with thiophene the products are obtained as a mixture of substi- allyl cations substituted at position 2, especially with 2- tuion at 2- and 3-position. oxyallyl systems3. Although 1-selenoallyl cations 7 also undergo Friedel- The same type of reaction have been exploited using Crafts reactions with electron rich heterocycles, the nature other sources of allylic cationic species, like b-bromovinyl of the final product is highly dependent on the susbstitution selenides 6a-f and electron-rich aromatic compounds as pattern of 8 and 9 as well as on the reaction solvent. This is illustred in Fig. 3 for the case of N-methyl pyrrole5. furan, N-methyl-pyrrole, thiophene and 1,3,5-trimethoxy- benzene4. Unsubstituted 8 and 9 (R1=R2=H) suffers nucleophilic Reagents like 6a and 6b have been prepared by addition attack by N-methyl pyrrole to give 10 (path a), whereas disubstituted 8 and 9 (R1=R2=alkyl, aryl) follows path b. SePh Compound 11 cannot be isolated most likely because of the R1 Br high stability of cation 12 which in turn reacts with a second molecule of N-methyl pyrrole to give 13 or 14, depending R2 R3 on the solvent used. Paths a and b also shows some sensi- 6 a: R1= H, R2= Cl, R3= H; b: R1= H, R2= Br, R3= H; tivity to solvent, but the most spectacular effect is seen at c: R1=R2= R3= H; d: R1= H, R2=R3= CH3 (Z + E); the last stage of the reaction, i.e. attack of 12 by N-methyl e: R1= C2H5, R2=R3= H (Z + E); f: R1=R2= CH3, R3= H pyrrole. 1,3-dipyrryl propenes 13 are almost exclusively formed in nitromethane, while regioisomers 14 are largely of benzeneselenenyl chloride or bromide to propargyl bro- predominant when the reaction is carried out in DMF. The mide in dichloromethane solution at room temperature in amounts of 15 and/or 14 can in each case be minimized by 91% and 69%, respectively. In an analogous fashion 6c-f the use of an excess of N-methyl pyrrole. have been prepared by the addition of benzeneselenenyl One of the most interesting examples is the synthesis of bromide to the corresponding allenes in quantitative 4 porphyrins by this type of reagents. The high selective yields . access to 1,1-dipyrryl propenes 14, suggest a straightfor- In Fig. 2 below, are presented some of the results ward synthesis of phorphirins bearing substitited vinyl obtained on the reaction of selenium stabilized carbenium groups at their meso-positions. Indeed, with the vinylse- ions with N-methyl pyrrole, thiphene, furan and 1,3,5- lenoacetal 16 (a-g) like source of electrophile, a series of trimethoxybenzene. In the case of N-methyl pyrrole or porphyrins 17 (a-g) have been prepared under the mild N N SePh AgClO4/MeNO2 N Na2CO3, -15 ºC + SePh SePh S S SePh S Br + AgClO4/MeNO2 CaCO3, -15 ºC + 6 c SePh O O SePh AgClO4/MeNO2 CaCO3, -15 ºC SePh OMe OMe Cl Br + AgClO4/MeNO2 CaCO3, 0 ºC SePh Cl MeO OMe MeO OMe 6 a Figure 2. Vol. 9, No. 4, 1998 Selenium Stabilized Carbenium Ions 329 N a R1 SeR SeR R2 N X R1 R2 R1 SeR 8 L.A. + 10 R 2 SeR R R1 X 1 b L.A. 7 N R R2 SeR 2 9 11 R1 R2 R1 R2 R1 R2 c N N N N N N R1 N 13 15 + R2 R d N 1 12 R2 N 14 Figure 3. conditions6 using the one pot reaction sequence outlined in ing an acyl group with an a-substituent, under these condi- Fig. 4. tions, into an aromatic ring is expected to be difficult, Another synthesis of an interesting class of meso-por- owing to the desactivation of the intermediary electrophilic phyrins have been developed in the same fashion7. In this complex by the electronegative acyl group. The presence case, meso-tetraalkynyl porphyrins have been obtained of an organoselenium moiety at the a-position of the acyl from selenoacetals 18a-b derived from 2-butynal and 3- group could circunvent this problem. phenylpropynal, with pyrrole and silver perchlorate in The synthetic procedure involves treatment of aromatic DMF. A further treatment with DDQ furnish the porphyrin hydrocarbons with a mixture of ethyl a-bromo-a-phenyl- 19a and 19b albeit in low yield (Fig. 5). seleno acetate 20a and ethyl a-a-bis-(phenylseleno) ace- The use of selenocarbenium ions containing an acyl tate 20b and TiCl4 at room temperature, affording the moiety and generated under Friedel-Crafts conditions have related benzylic selenides 22a-h as shown in Fig. 6. The proved to be an efficient route to aromatic derivatives of mixture of ethylacetate derivative 20a and 20b is easily phenyl acetic acids8. The interest in this synthetic route is avaiable from reaction of ethyldiazoacetate with phenylse- due to that direct introduction of a two carbon unit contain- lenenyl bromide in THF at 0 °C (Fig. 6). We observed lately R1 R1 R2 Yied (%) R2 R1 Me Me 10 N Et Et 20 R1 SeMe N R2 + 1)AgClO4/DMF i-Pr i-Pr 25 N N R SeMe 2)DDQ 2 Ph Ph 28 R2 N Me Ph 28 R1 R2 o-NO2Ph H 4 R1 16 a-g 17 a-g Ph H 0 Figure 4. 330 Silveira & Larghi J. Braz. Chem. Soc. R R N SeM e -3 N 1)AgClO , DMF 10 M R + 4 N N SeMe 2)DDQ 30 min . N 18 R R a-b 19 a-b a- M e a- 0.4% b- Ph b- 2% Figure 5. that pure ethyl a-bromo-a-phenylseleno acetate can be Table 1. obtained exclusively if the reaction is made by adding ethyl diazoacetate to a reflux benzene solution of PhSeBr. The 21 Aromatic Yield (%) Reaction time chloro derivative may also be prepared by the reaction of hydrocarbon [isomer ratios] (min) PhSeCl and ethyldiazo acetate, as the sole product9. Both a benzene 87 50 these reagents gives the Friedel-Crafts reaction under the b toluene 68 [2:1] 75 same conditions described. Also, has been developed an c ethylbenzene 75 [1.5:1] 30 efficient way to remove the phenylseleno group from the alkylation products. By treatment of benzylic selenide 22 d naphtalene 94 50 with catalytic amounts of thienylditelluride in ethanol and e phenantrene 76 40 equimolar amounts of sodium borohydride under basic f p-cymene 64 [1:2] 60 conditions10 the corresponding deselenate derivative 23 is g mesitylene 82 60 formed, as depicted in Fig.