An AM1 calculational study of the Diels–Alder addition of maleic anhydride to C5-substituted pentamethylcyclopentadienes and 2,5-dimethylthiophene oxide. An attempt to ascertain the factors controlling the π-facial selectivities and relative reactivities

Abstract

NICK HENRY WERSTIUK and JIANGONG MA. Can. J. Chem. 72,2493 (1 994).The Diels-Alder reactions of maleic anhydride (1) with a group of C5-substituted cyclopentadienes and 2,5-dimethylthiophene oxide (5) have been studied with the semi-empirical method AM1. We find that the facial selectivities and relative reactivities found experimentally for six dienes (2n-2f) are accurately reproduced at this level of theory. The results of the calculational study provide no concrete support for the proposal that a-a* hyperconjugation in the transition state -an interaction between a a bond at C5 of the cyclopentadiene and the incipient a bonds anti to it (the Cieplak effect) -is the major source of the syn facial selectivity observed for 2a-2d. In addition, the relative reactivities and facial selectivities are predicted for a number of cyclopentadienes which will be synthesized and studied experimentally.NICK HENRY WERSTIUK et JIANGONG MA. Can. J. Chem. 72,2493Chem. 72, (1994. Faisant appel 2 la methode semi-empirique AM I, on a CtudiC les rkactions de Diels-Alder de l'anhydride maleique (I) avec un certain nombre de cyclopentadiknes substituCs en position 5 et avec l'oxyde du 2,5-dimCthylthiophkne (5). On a trouvC que, h ce niveau de la thCorie, il est possible de reproduire fidklement les sClectivitCs faciales et les rCactivitCs relatives observCes expCrimentalement pour les six diknes (2n-2f). Les rCsultats de I'Ctude par calculs ne fournit aucun support concret pour la proposition qui a CtC faite selon laquelle de l'hyperconjugaison a** dans I'Ctat de transition -une interaction entre une liaison a au niveau C5 du cyclopentadibne et les liaisons a qui se forment anti par rapport h elles (effet Cieplak) -serait la cause principale de la sClectivitC faciale syn observCe avec les composCs 2n-2d. De plus, on a prCdit les rCactivitCs relatives et les sClectivitCs faciales d'un certain nombre de cyclopentadiknes qui seront synthCtisCs et CtudiCs expCrimentalement.[Traduit par la redaction] Introduction The synthetic utility of the Diels-Alder reaction is well established although the ability to control the facial selectivity which would render the reaction enantioselective with asvmmetric planar 1,3-dienes and potentially generate five or more chiral centres in one operation, is limited in the absence of chiral auxiliaries. In the reaction of C5-substituted cyclopentadienes where n-facial diastereoselectivity has been observed in the absence of chiral auxiliaires, the origin of the selectivity is still imperfectly understood even though a wide range of substrates have been studied experimentally (1-16) and computationally (17-25). These investigations revealed that a number of factors are important and various suggestions have been advanced as possible sources of the pattern observed. These include steric effects, complexation between diene and dienophile, secondary orbital interactions, polarizability, electrostatic interactions, orbital mixing, and transition state hyperconjugation. This latter interpretation proposed by Ci...

Diels-Alder Additions as Mechanistic Probes–Interception of Silyl-Isoindenes: Organometallic Derivatives of Polyphenylated Cycloheptatrienes and Related Seven-Membered Rings

“…Fortunately, X-ray crystallography resolved these issues when the product was revealed as the imino lactone, 62 , whereby the hydroxyl moiety was added across an exo-nitrile linkage, thus breaking the mirror symmetry and rendering the other three nitriles nonequivalent ( Scheme 22 ). Evidently, the reaction of TCNE with 59 proceeded via cycloaddition syn to the hydroxyl substituent, in accord with theoretical calculations [ 66 ].…”

π-Facial selectivity in Diels–Alder reactions of cyclopentadienes having π-systems at 5-positions and the solvent effect †

“…Macaulay and Fallis 3b claimed that the selectivity could be explained in terms of "Cieplak effects". 7d, 12 Coxon 13 and McEvoy 14 agreed with this proposal, although semiempirical calculations reported by Werstiuk and Ma 15 and us 16 provided no concrete support for the proposal. Recently, Poirier and Burnell reported a † Calculated Z-matrices and heats of formation of the stationary structures of compounds 1d-f as global minima and other local minima of the cyclopentadienes 1d-f and their relative total energies (Fig.…”

Thiophene S-oxides

“…28 While the formal cycloadducts of thiophene S-oxides with alkynes, the 7-thiabicyclo[2.2.1]heptadiene S-oxides, extrude the sulfoxy bridge spontaneously and aromatise (22), the reaction of thiophene S-oxides with alkenes leads to 7-thiabicyclo[2.2.1]heptene S-oxides (20). 31b,33 The activation barriers of both syn-and anti-addition have been calculated by semi-empirical methods 34,35 as well as at the RHF/6-31G* level. The cycloaddition shows high to exclusive endo-selectivity, the lone pair of the sulfur of the sulfoxy-bridge coming to lie on the side of the newly formed double bond of the bicyclic product (20).…”

“…31b,32a The syn-π-facial selectivity of this reaction can best be explained by the Cieplak effect. 31b,33 The activation barriers of both syn-and anti-addition have been calculated by semi-empirical methods 34,35 as well as at the RHF/6-31G* level. 32a The 7-thiabicyclo[2.2.1]heptene S-oxides (20) have been found to be viable starting materials for diaryl disulfides (21), arenes (22), 31c,e,f and cyclohexadienes (24).…”

“…V Complexes with thiophene S-oxide as ligand Rauchfuss and co-workers 43 have shown that Cp*Rh(η 4tetramethylthiophene) (33) oxidises in a dry oxygen atmosphere to Cp*Rh(η 4 -tetrathiophene S-oxide) (34). The reaction of a suspension of Cp*Rh(η 4 -tetramethylthiophene) dication 32 -which features a Rh(III) species -with potassium trimethylsiloxate (KOSiMe 3 ) also gives 34.…”