Tandem rearrangements of a cyclic bis-allene (original) (raw)
Journal of the American Chemical Society, 1976
The mechanisms of the dimerization of butadiene and piperylene and the thermal rearrangements of the corresponding dimers are investigated by kinetic and stereochemical techniques. Particular attention is given to the question whether, in the Diels-Alder dimerization of the dienes la or Ib, intermediates are involved that are common to the 1,3-sigmatropic rearrangements of the corresponding [2 + 21 dimers 3a and 3b. Substituents on the terminal vinyl position of cis-1,2-divinylcyclobutane (Sa) retard the normal stereospecific boat-like Cope rearrangement to 3,4-dimethyl-cis,cis-cycloocta-1,5-diene and permit the detection of a new "nonboat" process, whicjl leads to a stereoisomeric prbduct. The boat-like rate constant declines with increasing terminal cis-methyl sustitution in the series Sa > cTT-8 > cCT-8 > cCC-8. The total range of the effect amounts to a factor of 1.81 X lo5. The trans-1,2-dipropenylcyclobutanes also give Cope rearrangement products, but this reaction occurs exclusively by an indirect mechanism: prior epimerization to the cis isomer followed by Cope rearrangement of the latter. The rearrangement of trans-3,4-dimethyl-cis,trans-cycloocta-1,5-diene (16) to cis-3,4-dimethyl cis,cis-cycloocta-1,5-diene (lo), involving overall epimerization at one asymmetric center and geometric isomerization at one olefinic site, proceeds by a two-step mechanism in which cis.-1,2-trans,trans-dipropenylcyclobutane (cTT-8) is an intermediate. The 1,3-sigmatropic rearrangement of (lR,2R)-(+)-trans-1,2-divinylcyclobutane (3a) gives (R)-(+)-4-vinylcyclohexene (2a) with 7.7% preservation of enantiomeric purity (corrected for competing racemization of 3a). This corresponds to 54% inversion and 46% retention of configuration of the migrant carbon. By attaching stereochemical labels to the terminal vinyl positions as in optically active tTT-9 and tCT-9, the stereochemistry of the 1,3-sigmatropic rearrangement can be subdivided into the four possible pathways (Schemes IX and X), suprafacial inversion, antarafacial retention, suprafacial retention, and antarafacial inversion. In this way, it can be shown that relative rates through these four pathways are, respectively, 50.2,6.0,41.1, and 2.7 from tTT-9, and 49.5, 2.8.46.8, and 0.9 from tCT-9. These results can be fitted by a biradical mechanism, but are more fruitfully interpreted as mainly the outcome of two competing concerted reactions, one allowed (supra facial inversion) and one forbidden (suprafacial retention). The absence of any substantial antara contribution in the dipropenyl systems rules out a stereorandom biradical intermediate in the tTT-9 and tCT-9 rearrangements and makes it unlikely in the divinyl system 3a. The Diels-Alder dimerization of trans-penta-1,3-diene-t~a~s-l-d (45, Scheme XIV) in both the exo and endo orientations gives exclusively the product of reaction cis-on-the-diene,&-on-the-dienophile. This is consistent with a concerted [4s + 2s]cycloaddition and rules out common intermediates in the formation of product tT-13 and cT-12 from the two alternative pathways of Diels-Alder dimerization of piperylene and 1,3-sigmatropic rearrangement of tTT-9. 3b-2b, 3b-. 4b, and 5b-. 4b, in the piperylene dimer series. A related study concerns the possibility that cycloocta-1,5dienes with a trans double bond may play a role in these rearrangements. Cope Rearrangements of cisand trans-1,2-Dipropenylcyclobutanes. The chair-like geometry normally favored in the acyclic Cope rearrangement'8,t9 should be difficult to achieve from cis-1,2-divinylcyclobutane because the small ring wopld resist the internal rotation needed to generate the true chair, and because, even if a quasi-chair conformation could be attained, the product, &,trans-cycloocta-1 $diene, would be severely strained.21 Although the transient intermediacy of the latter substance cannot be excluded on purely energetic ground^,^ the rearrangement of cis-1,2-divinylcyclobutane (5a) to cis,cis-cycloocta-1,5-diene (4a) usually is formulat-edI4,l5JS with a boat-like transition state, the free energy of which in the acyclic system normally lies about 6 kcal/mol above that of the chair.1s.20 Our studies support this formulation and, moreover, they show how cis-1,2-dialkenylcyclobutanes can be subjected to incremental steric effects that gradually deny access even to the "second-best" boat-like reaction. cis-Cyclobutane-1,2-dicarboxylic acid anhydride (6) serves as the starting material for the syntheses of the three cis-1,2-dipropenylcyclobutanes. Dimethyl cis-cyclobutane-1,2-6
Tetrahedron Letters, 1987
onitrile rearranges selectively under direct irradiation in acetonitrile to tricyclo[3.2.2.0" $ Inon-6-ene-8,8,9,9_tetracarbonitrile, and upon acetone sensitization to bicyclo[3.2.2]nona-2 ,6-diene-8,8,9,9-tetracarbonitrile. Both products are derived from sigma bond cleavage , in contrast to the parent unsubstituted diene where only the n-bonds react.
The rearrangement of cyclopropylcarbenes
Chemical Communications (London)
When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name Chapter 4-Introduction, CNDO Calculations Chapter 5. Calculation of Coordinates Chapter 6.
Competitive Pseudopericyclic [3,3]- and [3,5]-Sigmatropic Rearrangements of Trichloroacetimidates
The Journal of Organic Chemistry, 2015
The Woodward−Hoffmann rules predict whether concerted pericyclic reactions are allowed or forbidden based on the number of electrons involved and whether the cyclic orbital overlap involves suprafacial or antarafacial orbital overlap. Pseudopericyclic reactions constitute a third class of reactions in which orthogonal orbitals make them orbital symmetry allowed, regardless of the number of electrons involved in the reaction. Based on the recent report of eight-centered ester rearrangements, it is predicted that the isoelectronic eight-centered rearrangements of imidates would also be allowed. We now report that these rearrangements occur, and indeed, an eight-centered rearrangement is slightly favored in at least one case over the well-known six-centered Overman rearrangements, in a trichloroacetimidoylcyclohexadienone, a molecular system where both rearrangements are possible.
Angewandte Chemie International Edition, 2007
The unknown C 3v-symmetric all-cis triscyclobutenocyclohexane 1 [1] is of fundamental interest as a key structure on which to probe the mechanism of [2+2+2]cycloreversions of cyclo-propa-and cyclobuta-fused cyclohexanes ("tris-s-homobenzenes"). [1a, 2] Thus, various examples of the cyclopropa-fused derivatives undergo facile [s 2 s +s 2 s +s 2 s ] retrocyclization through aromatic transition states, whereas all-cis triscyclobutacyclohexane is deflected to a radical decomposition pathway, [3] which is the result of unfavorable through-bond interactions. [2c,d] In contrast, the double bonds in 1 should not only activate by introducing further strain, [4] but also enable additional orbital symmetry control in the form of stepwise conrotatory cyclobutene ring openings, conflicting stereochemically with a concerted all-disrotatory unraveling of the cyclohexane ring. While conrotatory motion is seemingly constrained in bicyclo[n.2.0]alkenes, leading to much speculation regarding the feasibility of forbidden disroratory pathways in such systems, [5] cis-bicyclo[4.2.0]oct-7-ene, the pertinent subunit of 1 (highlighted), chooses this option. [6] More generally, such cyclohexane cycloreversions constitute one strategy for gaining access to the interior of fullerenes. [7] In addition, triene 1 (and its derivatives) are of importance structurally, because of their potential central-ring planarity, [8] and synthetically, as new precursors to novel [12]annulene isomers [9] and as new 6p-electron ligands for catalysis. [10] We report the synthesis, structure, and thermal rearrangement of the first example of structural motif 1, namely the triply annelated hydrocarbon 2 and its congeners 3 and 4, and compare their behavior to that of their common precursor, the trisbenzo analogue 5, [11] for which all-disrotatory retro-[2+2+2]cycloaddition has been proposed. [11b] To assist in the interpretation of the results, DFT calculations were employed. These calculations suggested an unexpected switch-over in the mechanism of cyclohexane ring fissure along the series. Birch reduction [12] of 5 [11b] could be controlled to give 6 a, either pure (93 %), or admixed with varying amounts (depending on the quantity of lithium used) of the mono-(6 b) and bisbenzo relatives (6 c), which were separated by column chromatography. [13] Selective dihydrogenation with Wilkinsons catalyst then furnished the targets 2-4, respectively, in good yields. Inspection of the NMR spectroscopic data of the series 2-5 reveals the trends expected for progressive, peripheral aromatization. Noticeable are the fairly invariant chemical shifts of the central cyclohexane carbon atoms, namely d = 43.3, 42.3 (average), 41.5 (average), and 40.6 ppm, respectively, and the small shielding trend, reflecting that observed for the corresponding carbon atoms in cyclobutene (d = 31.4 ppm) and benzocyclobutene (d = 29.5 ppm). Because of their novelty, X-ray structural analyses
J Am Chem Soc, 1990
Curiously, this coupling reaction is only observed for the amido deri~ative.'~ There is a hint, here, that ruthenium carbonyl cluster complexes activated by nitrogen bases may be potential synthons for promoting carbon-carbon bond formation between different kinds of olefins. The next challenge will be to render the codimerization process catalytic. Acknowledgment. This work was supported by the CNRS. We are grateful to Johnson-Matthey for generous loans of ruthenium trichloride, to Dr. Rent Mathieu for helpful discussions, and to Ms. Sylvie Segura for her contribution. Supplementary Material Available: Details for the preparation and characterization of the complexes [PPN] [la$], [PPN][2a-e], 3a,b. 4a.b. Sa,b, and 6, details of hydrogenation and dimerization reactions, prcliminary crystallographic data for [PPN] [ lc] and [PPN][2a], and a listing of full crystallographic data for 5a including atomic coordinates, thermal parameters, and selected interatomic distances and bond angles (17 pages); listing of observed and calculated structure factor amplitudes for 5a (27 pages). Ordering information is given on any current masthead pagc.
Journal of The American Chemical Society, 1982
A mixture of syn-and anti-7-(1,2-butadienyl)bicyclo[2.2.l]hept-2-ene [la (30%) and lb (70%)] has been synthesized through a Grignard reaction employing syn-7-bromonorbornene and 3-bromo-1-butyne. Gas-phase pyrolysis of the mixture at 275 O C for 24 h gives 1-ethylindan (7) as the single isolable product in 28% yield. This result is interpreted in terms of an initial concerted [,2, + .2, + (,28 + ,2a)] rearrangement of la to l-ethylidene-3a,4,5,7a-tetrahydroindene (5), which through a series of not less than five allowed [1,5] hydrogen shifts may give 1-ethylindan.
Heterocyclic Rearrangements: New Cumulenes and Acetylenes
Bulletin des Sociétés Chimiques Belges, 1982
The present paper describes several applications of flash vacuum pyrolysis in the preparation of unusual molecules. Aromatic carbene-carbene and carbene-nitrene rearrangements have been shown to proceed via seven-membered ring intermediates. ' The exact nature of thcse intermediates has, however, been a subject of much discussion until recently. As an example, the nitrogen scrambling observed on pyrolysis of tetrazolo-[1,5-a]pyridine was interpreted in terms of nitrene-nitrene interconversion via a diazacycloheptatrienylidene (Scheme 1). 2 2 % 85% L % Scheme 1 Recent experiments have shown that it is possible to isolate these intermediates at low temperatures, and that they are carbodiimides rather than carbenes (Scheme 2) .3