[2+2]cycloaddition of substituted thiophenes to acetylenes

TetrahedronLetter6Uo. 52, pP 5269 - 5272, 1972.

PergamonPress.

Printed in Great Britain.

[2+2]CYCLCADDITION OF SGESTITGTED'EiIOPIiENES ti ACETYLENES D.N. Reinhoudt,H.C. Volger and Mrs. C.G. Kouwenhoven, Koninklijke/Shell-Laboratorium, Amsterdam (ShellResearchN.V.) E!adhuisweg 3. Amsterdam,The Netherlands, and H. Wynberg and R. Helder, Departmentof Organic Chemistry, 'IheUniversity,Zernikelaan,Gronlngen.The Netherlands. (Rwei.vedin nK 6 yovemb& 1972; accepted for publication23

AOVO&O~

1972)

Recentlyboth groups of authorsreported independentlythe first synthesisof a monocyclic thiepine"*. Two modes of preparationwere recorded.One comprisedthe ogcloaddltionof 3-methyl-4-pyrrolidinothlophene to dlmethylacetylenedicarboxylate at -30 '. Subsequentiscueerizatlon,at this temperature,of the Initiallyformed 5-pyrrolldino-2-thlabioyolo~.2.O~epta3,6-diene (I) yielded a monocyclicthlepine (II)' (Scheme1). The structureof this thiepine was

confirmedby PMR end IR spectroscopy.At-X

' the sulphur was slowly extrudedfrom the

thieplne to give the correspondingbenzene derivative.

I

R,,Q=H,

CH3 SCHEME1

The other synthesiswas describedas a faoile reaction at room temperaturebetween tetramethylthiophene and dicyanoacetylenein the presenceof AlC On the basis of spectro132* scoplcal data (IR, W, FNR and MS) of the reactionproduct as well as of its sulphone,a thieplnestructure (V) was assigned.The product,m.p. 93-9 ', could be pyrolysedat 300 o to tetramethylphthalonitrlle; heating at 140 Oyieldedan isomer,m.p. 97.5-B O, to which structureIV was assigned (Scheme2).

5269

NO. 52

5270

R= CH,

300 “C,

R

CN

R R IV

CN R

SCHEME 2

Tne differencein thermal stabilitybetween II and V was strikingand difficultto lnterpret3.The theoreticallypredictedinstabilityof tbieplnes4cast some doubt on the structuralassignmentof the reaction product of tetramethylthiophene and dioyanoacetylene. -1 In addition,the medium strong absorptionat 1610 cm in the Ih spectrumof this product 6 could equally well be assignedto a bicyclobutenestructure as to a thiepine. In order to arrive at an unambiguousassignment13C NMR spectrawere recorded for the product of this reactionend its isomer (IV). They showed that the two compoundshad the same structuralskeleton:4

C atoms, 2 nitrlle C atoms, 2 bridgeheadC atoms and 4 methyl

C atoms (the latter showed up as quartetson partial decoupling,Table 18). These data leave no doubt that the reactionproduct of tetramethylthiophene and dicyanoacetylene is not a (III) formed by a thlepine (V), as proposedearlier2,but a 2-thiabicyclo[3.2.0]heptadiene [2+2]cycloaddition.

5271

No. 52

Table I

l3C NMR chemicalshifts of 2-thiabicyolo[J.2.O]hepta-j,6-dienesa

No. of compound

b

d '3s4s6.7

&dI

4,

1.5

III

131.3 128.7 126.8 124.4

110.7 110.1

72.8 63.7

19.4 15.2 14.9 12.7

Iv

162.3 145.9 140.0 114.9

113.7 100.5

66.4 62.0

22.2 18.2 10.8 9.2

a. In CC14 solutionwith CS2 as internalstandard. b. Chemical shifts are given In ppm relativeto 'DE3= 0. c. Quartetson partialC-H decoupling. The differencein thermal stabilitybetween the 2-thiabicyclo[J.2.O]hepta-3,6-dienes I and III can be explainedas follows.The cyclobutenering in III is thermallystable because the synrmetry-allowed conrotatoryring opening would give rise to a e,

%,

e-thiacyclo-

heptatriene.In I the opening of the cyclobutenering is aoceleratedby the presenceof the donatingbridgeheadpyrrolldinosubstituentand proceedsvia a two-step prooess (compare ref. 9).

. The isomerlzationof III Into IV and of VI into VII are the first examplesof a

[3,3] antara,antaraCope rearrangementof 2-heterobicyclo[3.2.O]hepta-3,6-dlenes.

m ; X=S, R=CH3 IKa; X=S, R-C&D WI ; X =SD2,R=CHJ

IV ;X=S , R=CH3 IVa;X=S , R=CH# VII ;X= Q., R= CH3

Mukai et al." and Ealdwin et al." recentlyreported a similar rearrangementin the carbocycllcseries (X = CO snd C(C ),). Other postulatedpathways,a twofold antara,supra If [1,3] sigmatropicreaction and the synunetry-allowed conrotatoryopening of the cyclobutenering, 10,ll are unlikely . The first would give rise to a thianorcaradiene as an intermediate- a

5272

stioture

No.

whloh

will eliminatesulphurrapidly - and the second to a highly strainede,

system. -ois-thlaoyoloheptatrlene

Desulphurieation

52

trsns,-

of III and IV at 3C~0* may well proceed via

thleplneV (oompareref. 10). Aoknowledgement We thank Dr. A.D.M. Clague and Mr. L.L. Blaauw of the KoninhllJke/S~ll-Lratorium for reoording

and Interpretingthe 13C NMR spectra.

FtEmRENcEs ANDmmm 1.

D.N. Reinhoudtand

C.G.

Kouwenhoven,Chem. Comn., in press.

2. H. Wynberg and R. Helder, TetrahedronLetters,E,

3647.

3. The observedhigh thermal stabilityof an ennulatadthleplnereportedby Hoffman and Sohleeslnger5is misleadingbeoause in this oompoundthe two bullsygroups at C2 and C7 render desulphurization difficult. 4. M.J.S. Dewex and N. TrinaJstio,J. Am. Chem. SQC.~ 2, -

1453, (1970).

5. J.M. Hoffman and R.H. Sohlesslnger,J. Am. Chem. Soo., g,

5263,

-

(1970).

6. In oomparable2-thlabioyclo[3.2.0]heptadienes the oyclobutenemoiety absorbs in the -1 1640-1600cm region1,7. 7a. D.N. Relnhoidtand C.G. Lellveld,TetrahedronLetter=&E,

3119.

7b. D.N. Relnhoudt and C.G. Kouwenhoven,Chem. Comn., In press. 8. 13C NMR spectrawere recordedin CC14 solutionwith CS2 as internalstandard.Cl%lllCti shifts were comparedwlth those of known compounds. $& 9. R. Criegee,D. Seebaoh,R.E. Winter, B. l%rretzensnd H.A. Brune, Chem. Ber., 10a. T. Mlyashi,M. Nitta end T. Mukal. TetrahedronLetters,m, lob. T. Mlyashi,M. Nitta and T. Mukal, J. Am. Chem. Soo., 3, -

2339 (1965).

3433.

3441

(1971).

Verlag Chemie, Weinheim/ 100. R.B. Woodward and R. Hoffmann, "Me Erhaltungder Orbltalsynmetrie", Bergstr.m,

126.

11s. J.E. Baldwin and M.S.

Kaplan,

llb. Ibid.,$14,668 (1972). -

J. Am. Chem. Sot., 42, m9 -

(1971).