Tandem rearrangements of a cyclic bis-allene (original) (raw)
Related papers
Tetrahedron Letters, 1992
are proposed. Furthermore, some reactions of cyclobutenes with dihalocarbenes are described. Additions to carbon-carbon double bonds a~. the archetypical reactions of carbenes. While thousands of dihalocarbene reactions with open chain and cyclic olefins larger than four-membered rings can be found in the literature,' only a few studies with small-ring olefins have been reported.2 In the dichloro-and dibromocarbene reactions, however, in no case could the likely intermediate adducts 2 and 5 either be isolated or spectroscopically proven without doubt, and only 3 and 6 were found, probably deriving from cationic cyclopropyl ally1 (CCA) rearrangements. 3 While 6,6-dibromobicyclo[3.l.O]hexane rearranges to 2,3dibromocyclohex-2-ene above 155T ,4 Vogel found that the reaction of dichlorocarbene with cyclobutene afforded 6 (X=Cl) at 0% 2e Theoretical analyses show that fluorine substituents at C5 of bicyclot2.1.Olpentane 5 (X=F) reduce the barrier for cleavage of the central bond.5 Mahle&, however, reported that perfluoro-1,3-dimethylbicyclo[ 1.1 .O]butane, formed from the gas-phase pyrolysis of (CF&PF2 in the presence of peffluorobut-2-yne, at 3OOT only slowly rearranged to perfluoro-2,3_dimethylbutadiene in low conversion.
Journal of Organometallic Chemistry, 1987
The reaction of tricarbonyl(tropone)iron with tetracyanoethylene (TCNE) was reinvestigated. Two primary cycloadducts, the 3 + 2 and 4 + 2 isomers are formed in a 96/4 ratio. The second order rate constant for the reaction is k 4.4 x 10e2 M-" s-l (at 24"C, in acetone) with a free activation energy of AG# 19.2 kcal mol-'. The 3 + 2 cycloadduct undergoes a facile sigmahaptotropic rearrangement to the formal 5 + 2 adduct with rate constants of k 3.8 X 10e4 s-' (24"C, acetone) and k 5.1 X 10m4 s-l (24°C methanol), and with activation parameters AH* 20 kcal mol-' and AS* -7 e.u. (acetone). The kinetic results suggest that both the cycloaddition and the rearrangment are concerted, nonsynchronous, one-step reactions, involving a slightly polar transition state. Frontier molecular orbital considerations imply that the 3 + 2 and 4 + 2 cycloadditions are symmetry allowed reactions. The structural reorganization which takes place in the [3,3]-sigmahaptotropic rearrangement suggests that the metal migration proceeds by way of a Berry pseudorotation, and the simultaneous 1,3-sigmatropic shift occurs with configuration retention at the migrating carbon. A detailed molecular orbital analysis of the rearrangement is given. * For previous paper in this series see ref. 1. 0022-328X/85/$03.30 0 1985 Elsevier Sequoia S.A.
Tandem RCM-Claisen Rearrangement-[2+2] cycloaddition of O,O'-(But-2-en-1,4-diyl)-bridged binaphthols
2012
Attempted RCM of 2,2'-bis(allyloxy)-1,1'-binaphthyl resulted in a Claisen-type rearrangement of a postulated labile dioxacyclodecine proceeding at room temperature and followed by [2+2] cycloaddition. Structures of products were confirmed by X-ray crystallography. A mechanistic rationalisation based on relative stabilities of proposed intermediates and transition states is provided. Scheme 2. Synthesis of (R,R)-3a through macrocyclisation. Reagents and Conditions: a. Methyl 4-bromocrotonate, K 2 CO 3 , 24 h, r.t.; b. DIBAL, DCM, −78 °C −20 °C, 12 h; c. PBr 3 , THF, −40 °C r.t. overnight; d. (R)-1a, KOH, THF, 3 days, reflux.
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
Acid catalyzed rearrangements in bicyclo[3.3.1]nonanes
Tetrahedron, 1985
A&w%-Tkaiment of end&? or ex~~~~ro~y-l~~~tu~~ ketds In-4 with ptoiumc&fonic acid in dry ~rc~ultsmarrvcnibkC,bsidgcdcavagcandaflord3~quitibriumrmxtuns wkre2-substituted-wt,3dioxolan-t-ylbcyclooctanona 6-d art present as main products. Y&ids in 64 inmare as the stcric hindrancz of the substitucnu at C, in she substrate increase as we% &uterium exchange cxpcnmcnts arc in favour of an intramokcular 1,3-hydride shift from Cl to C,. Reactions alfording cyclooctanc derivatives are interesting because g-membered rings arc present in some ctasscs of naturally occurring compounds.' Direct cyclization of linear precursors results in poor yictds because of unfavorable entropic and, mainly, enthalpic factors.' Better results are obtained by oneor two-carbon ring expansions3 as well BS by fra~~ntation reactions of bicyclic precursor~,~ i.e. bicycio[3.3.l]nonane dcrivativa.'~' As far as b~yc~ononaoe fra~~ntatjo~ art concerned, one of us previously reported that the hydroxy ketaf en&la atfords the cyclooctanonc 6a (Schcmc 1) in high yield when treated with p tolucncsulfonic acid (TsQH).~ Since compound 6a andjor refated products are suitable for further synthetically useful modifications, and in view of the quite easy preparation of bi~cfo[3*3.I~nonan~
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.