5- Chloro-l,3-cyclopentadiene

Inagaki, Fujimoto and Fukui demonstrated that ir-facial selectivity in the Diels-Alder reaction of 5-acetoxy- and 5-chloro-l,3-cyclopentadienes, 1 and 2, can be explained in terms of deformation of a frontier molecular orbital FMO [2], The orbital mixing rule was proposed to predict the nonequivalent orbital deformation due to asymmetric perturbation of the substituent orbital (Chapter Orbital Mixing Rules by Inagaki in this volume). 

The orbital mixing rule demonstrates that the direction of the FMO extension is controlled by the relative energies of the Jt-HOMO (ej and the n-orbital of X (8 ). In the case of 5-acetoxy- and 5-chloro-l,3-cyclopentadienes, the jt-HOMO lies higher than n (e > ej. In this case, the ji-HOMO mainly contributes to the HOMO of the whole molecule by an out-of-phase combination with the low-lying n. The mixing of a-orbital takes place so as to be out-of-phase with the mediated orbital n. The HOMO at Cl and C4 extends more and rotates inwardly at the syn face with... 

This chemical behavior is to be contrasted with that of 5-chloro-l,3-cyclopentadiene, which cannot be made to form a stable salt. 

Chloro-l,3-cyclopentadiene (below) undergoes SnI solvolysis in the presence of silver ion extremely slowly even though the chlorine is doubly allylic and allylic halides normally ionize readily (Section 15.15). Provide an explanation for this behavior. 

Ionization of 5-chloro-l,3-cyclopentadiene would produce a cyclopentadienyl cation, and the cyclopentadienyl cation (see Problem 14.8) would be highly unstable because it would be antiaromatic. 

Diels-Alder reaction. The first step in a new stereo-controlled synthesis of prostaglandins F2(t and E2 involved a Diels-Alder reaction between 2-chloro-acrylonitrile (2) and 5-methoxymethyl-l, 3-cyclopentadiene (1 ). This diene readily undergoes a 1,5-hydrogen shift to give l-methoxymethyl-l,3-cyclopentadiene. In... 

The basic permanganate oxidation of conjugated 1,3-dienes is different from those reported for the 1,4-dienes above. Therefore, in the oxidation of cyclopentadiene and 1,3-cyclohexadiene (13) a mixture of five products 14-18 is obtained (equation 4) . In this reaction 14 and 16 are derived from the normal cis hydroxy la tions whereas 17 and 18 were said to be formed from 15 by hydration in what the authors term an epoxidic pathway. Formation of epoxy-diols under similar permanganate oxidations has been found for occidentalol (19), levopimaric acid (20) and a 1-chloro- or 1-bromo-l,3-diene (21) as an intermediate step in the synthesis of (+)-D-c/tfrc-3-inosose and (+)-D-c/r/ro-3-inositol (equation 5). The explanation for the epoxide formation in the oxidation of 1,3-dienes (13, 19-21) is not clear but may be connected to the fact that other additions of permanganate to dienes are possible beyond those commonly observed... 

Cyclobutanes from the addition of 1-chloro-l-fluoroethylene to chlorotrifluoro- or tetrafluoro-ethylene, of 2-fluoropropene to tetrafluoroethylene, and of 1-chloro-2,2-difluoroethylene to vinyl fluoride or propene have been claimed to be anaesthetics. Diels-Alder addition of cyclopentadiene to acids of the type, trans-RpCHiCH COaH (Rf = CHaF, CHFa, CFs, C2F5, or n-CsFv) and a number of related esters and other derivatives, at 25 °C, results in predominant formation of adduct witii an endo fluoroalkyl group. The addition of cyclopentadiene to the 1,1-difluoroethylenes, CFalCFa, CFa CF CFs, CFa CCla, CFa C(CF3)2, CFaiCCl-CFaCl, and CFa.CFH has been recorded and the fiee-radical bromina-tion of the resulting norbomenes studied. Buta-1,3-diene and trifluorovinyl-sulphur pentafluoride yield essentially a mbcture of cis- and rmns-cyclobutanes (116), with little or no cyclohexene formation the olefin resembles perfluoropropene which yields ca. 5% of cyclohexene, in this respect. Perfluoroindene, which adds... 

Cl2Rh2CgH,g, Rhodium(I), di-/jtetrakis(ethene)di-, 28 86 Cl2Rh2CigH24, Rhodium(I), di-/i-chloro-bis-( ( -l,5-cyclooctadiene)di-, 28 88 CI2RUC7H8, Ruthenium(II), (f/ -bicydo-[2.2.1]hepta-2,S-diene)dichloro-, 26 250 Cl2RuCgH]2> Ruthenium(II), dichloro(t -cycloocta-l,S-diene)-, 26 253 —, Ruthenium(II), di- j-chloro-(j -l,5-cyclooctadiene)-, polymer, 26 69 Cl2Sc2Si4C44Hg4, Scandium, tetrakis[ -l,3-bis(trimethylsily )cyclopentadien-yl]di-/i-chloro-di-, 27 171 Cl2Si2ThC22H42, Thorium(IV), bis(i -l,3-bis(trimethylsilyl)cyclopentadien-yl]dichloro-, 27 173 Cl2Si2UC22H42, Uranium(IV), bis(i -1,3-bis(trimethylsilyl)-cyclopentadieny 1 ]dichloro-,... 

The ozonolysis reaction, followed by reductive workup with sulfur dioxide, as described in Part A of the present procedure, illustrates a general method which has been developed for the preparation of acetals. Application of the procedure is illustrated by conversion of the following olefins in alcoholic solution to the corresponding acetals (1) l-chloro-4-(o-nitrophenyl)-2-butene to o-nitrophenylacetaldehyde dimethyl acetal in 84% yield (2) l,4-dibromo-2-butene tobromoacetaldehyde dimethyl acetal in 67% yield (3) 3-butenoic acid to malonaldehydic acid diethyl acetal ethyl ester in 61% yield (4) cyclopentadiene to malonaldehyde bis(diethyl acetal) in 48% yield and (5)... 

Nickel-catalyzed stereospecific cyclocarbonylation of cWo-cyclopentadiene dimer 3 with ( )-l -chloro-2-butene, carbon monoxide and tetracarbonylnickel in acetone/water at room temperature gives a-methyl-3-oxotetracyclo[5.5.1.02,6.08,12]tridec-9-ene-4-aceticacid (4)in 10% yield51. An X-ray investigation of the acid reveals a selective cis-exo attack at the strained double bond, while the other double bond does not react. The exo dimer of cyclopentadiene gives a different product, thus there is no isomerization, in contrast to reactions under acidic conditions. 

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