Cyclopentadiene structure

Many heterocyclic bases having a cyclopentadiene structure are sensitive to oxydation they form dimers110,113 or cyclopentadienones51,1 25,132 in the presence of atmospheric oxygen. Therefore, the quaternization should be performed under an inert atmosphere. Further difficulties arise from the tautomerism of some heterocyclic bases (Scheme 5). This can lead to the... 

Many heterocyclic bases having a cyclopentadiene structure are sensitive... 

The cyclopentadiene structure makes the polymer hydrophobic. The manufacturer s specified contact angle for the polyolefin we used was 94°. The observed contact angle for the untreated Zeonor 1020R in our experiment was 91 2°.21 We found that surface oxidation of cyclo-olefin surface using an... 

Chapter 5, may provide a rationale. Conclusions derived from a number of H-MVIR measurements indicate that cyclopentadiene has a high affinity for the interior of the micelles that were investigated, whereas the dienophile prefers the outer regions. In view of the structures of most dienes and dienophiles such a spatial separation can be expected for the majority of Diels-Alder reactions. This arrangement accounts for the unexpectedly small influence of micelles on the rates of Diels-Alder reactions as reported in the literature. 

Reduction of indolenines with sodium and ethanol gives indolines. The pentachloropyr-role, obtained by chlorination of pyrrole with sulfuryl chloride at room temperature in anhydrous ether, was shown by spectroscopic methods to have an a-pyrrolenine (2H-pyrrole) structure (222). It is necessary, however, to postulate that it is in equilibrium with small but finite amounts of the isomeric /3-pyrrolenine form (3//-pyrrole 223), since pentachloropyrrole functions as a 2-aza- rather than as a 1-aza-butadiene in forming a cycloadduct (224) with styrene (80JOC435). Pentachloropyrrole acts as a dienophile in its reaction with cyclopentadiene via its ene moiety (81JOC3036). 

Mention has already been made in this chapter of metallocene-catalysed polyethylene (see also Chapter 2). Such metallocene catalysts are transition metal compounds, usually zirconium or titanium. Incorporated into a cyclopentadiene-based structure. During the late 1990s several systems were developed where the new catalysts could be employed in existing polymerisation processes for producing LLDPE-type polymers. These include high pressure autoclave and... 

The monomer, norbomene (or bicyclo[2.2.l]hept-2-ene), is produced by the Diels-Alder addition of ethylene to cyclopentadiene. The monomer is polymerised by a ring-opening mechanism to give a linear polymer with a repeat unit containing both an in-chain five-membered ring and a double bond. Both cis-and trans- structures are obtainable according to the choice of catalyst used ... 

One interesting phenomenon was the effect of the boron substituent on enantioselectivity. The stereochemistry of the reaction of a-substituted a,/ -unsatu-rated aldehydes was completely independent of the steric features of the boron substituents, probably because of a preference for the s-trans conformation in the transition state in all cases. On the other hand, the stereochemistry of the reaction of cyclopentadiene with a-unsubstituted a,/ -unsaturated aldehydes was dramatically reversed on altering the structure of the boron substituents, because the stable conformation changed from s-cis to s-trans, resulting in production of the opposite enantiomer. It should be noted that selective cycloadditions of a-unsubsti-tuted a,/ -unsaturated aldehydes are rarer than those of a-substituted a,/ -unsatu-... 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

Absolute configurations of the isoxazolidines obtained in the nitrone cydoaddition reactions described in Schemes 7.21 and 7.22 were determined to be 3S,41 ,5S structure by comparison of the optical rotations as well as retention times in a chiral HPLC analysis with those of the authentic samples. Selection of the si face at C/ position of 3-crotonoyl-2-oxazolidinone in nitrone cydoadditions was the same as that observed in the Diels-Alder reactions of cyclopentadiene with 3-croto-noyl-2-oxazolidinone in the presence of the J ,J -DBF0X/Ph-Ni(C104)2-3H20 complex (Scheme 7.7), and this indicates that the s-cis conformation of the dipolaro-phile has participated in the reaction. 

The mechanism for the catalytic enantioselective carbo-Diels-Alder reaction of N-alkenoyl-l,3-oxazolidin-2-one 4 with, e.g., cyclopentadiene 2 catalyzed by chiral TADDOL-Ti(IV) complexes 6 has been the subject for several investigations and especially, the structure of the intermediate for the reaction has been subject to controversy. The coordination of 4 to 6 can give five diastereomeric complexes A, B], B2, C], and C2, as outlined in Fig. 8.8. 

Aldrin, a chlorinated insecticide now banned for use in the United States, can be made by Diels-Alder reaction of hexachloro-l,3-cyclopentadiene with norbomadicne. What is the structure of aldrin ... 

Cyclopentacliene undergoes thermal polymerization to yield a polymer that has no double bonds in the chain. On strong heating, the polymer breaks down to regenerate cyclopentadiene. Propose a structure for the polymer. 

The Use of the Interatomic Distance Function in Discussing the Electronic Structure of Molecules.—The observed interatomic distances listed in Table II are interpreted with the aid of the curve of Fig. 1 to lead to the assignment of fractional double bond character as given in the last column. It is seen that a single bond between conjugated double bonds (cyclopentadiene) or... 

The Electron Diffraction Investigation of the Structure of Benzene, Pyridine, Pyrazine, Butadiene-1,3, Cyclopentadiene, Furan, Pyrrole, and Thiophene... 

To conclude, we shall mention some metal-atom reactions with boranes (172) and carboranes (173). When cobalt atoms reacted with pentaboraneO) and cyclopentadiene, a number of new metalloborane clusters were formed (172), two of which were 65115003(17-05115)3 and cyclopentyl-B5H40o2(i7-05H5)3. Possible structures for the former are shown in Fig. 42. The reaction of cyclopentadiene, pentaboraneO), and 2-butyne with cobalt atoms yielded the metallocarborane species illustrated in Fig. 43 (173). 

Bis(oxazohnes) figands have been so widely used for the Diels-Alder reaction between N-2-alkenoyl-l,3-oxazolidine-2-one and cyclopentadiene that Lipkowitz and Pradhan developed a QSAR (quantitative structure-activity relationship) using Comparative Molecular Field Analysis (CoMFA) for a set of 23 copper-catalysts containing mainly bis(oxazoline) figands. The generated... 

Assess the Diels-Alder reaction between cyclopentadiene and 1,3-butadiene, drawing structures for all likely products from the reaction. Suggest ways in which the selectivity of the reaction may be improved. 

Perhaps chemists will be able to mimic nature without duplicating the iron-sulfur-molybdenum structure. For example, a zirconium complex with tetramethyl cyclopentadiene can bind dinitrogen in a manner that breaks the NON bond, as shown below. Treatment with hydrogen gas results in formation of small amounts of ammonia. Although the yields are too low to make this a viable commercial process, researchers hope to make the process more efficient through chemical modifications and changes in conditions. 

---