Cyclopentadienes site-selective

Marchand and coworkers102 reported a difference in site selectivity between the thermodynamically and kinetically controlled Diels-Alder reactions of cyclopentadiene with 2,3-dicyano-p-benzoquinone (126) (equation 37). Under kinetic conditions, the more reactive double bond of 126 reacted with cyclopentadiene affording 127, whereas the less substituted double bond reacted under thermodynamic conditions affording 128. Both reactions proceeded with complete endo selectivity. These findings were in agreement with ab initio HF/3-21G calculations. 

There is a special kind of site-selectivity which has been called periselectivity. When a conjugated system enters into a reaction, a cycloaddition for example, the whole of the conjugated array of electrons may be mobilized, or a large part of them, or only a small part of them. The Woodward-Hoffmann rules limit the total number of electrons (to 6, 10, 14 etc. in all-suprafacial reactions, for example), but they do not tell us which of 6 or 10 electrons would be preferred if both were feasible. Thus in the reaction of cyclopentadiene (355) and tropone (356), mentioned at the beginning of this book, there is a possibility of a Diels-Alder reaction, leading to 354, but, in fact, an equally allowed, ten-electron reaction is actually observed,121 namely the one leading to the adduct (357). The product is probably not thermodynamically much preferred to the... 

Marchand and coworkers reported a difference in site selectivity between the thermodynamically and kinetically controlled Diels-Alder reactions of cyclopentadiene with... 

In order to develop catalytic effects of cyclodextrins for bimolecular reactions, it needs to include two guest molecules simultaneously in a cyclodextrin (CD) cavity. Several examples of cyclodextrin-catalyzed bimolecular reactions have been reported. Rideout and Breslow have found that Diels-Alder reactions of cyclo-pentadiene with butenone, cyclopentadiene with acrylonitrile, and anthracene-9-carbinol with N-ethylmaleimide in water are markedly accelerated by 3-cyclodextrin (3-CD) (1). Komiyama and Hirai have reported site-selective Reimer-Tiemann reactions of phenols in cyclodextrin solutions (2). In most of these reactions, however, each substrate molecule is relatively small so that a 3-CD cavity may include simultaneously an additional reactant molecules. We found previously that the fluorescence quenching of pyrene and naphthalene by trimethylamine (TMA) or dimethylamine (DMA) in water is catalyzed by g-CD (3) Since the pyrene molecule is too large to be incorporated completely in the 3-CD cavity, it has been assumed that pyrene binds to a rim of the CD cavity to form a pyrene-capped CD complex and a remain-... 

Hayakawa, K., Aso, M., and Kanematsu, K., Site-selective Diels-Alder reaction of 1,4,5,8-naph-thodiquinones with anthracenes and successively with cyclopentadiene electronic effects vs. steric... 

The frontier orbital interactions at other than reaction sites can determine the selectivity [14]. The interaction between the HOMO of cyclopentadiene and the LUMO of maleic anhydride is illustrated in Scheme 26. The HOMO of cyclopentadiene has the same phase property as butadiene (Scheme 18). The LUMO of maleic anhydride is an in-phase combined orbital of and transition state for the... 

Such higher order prerequisites could be fulfilled by ensemble operation of several sites. For example, a dimeric cluster of cuprous ions on silica gel is very active for the oxidation of CO with NzO at room temperature, but isolated cuprous ions are entirely inactive for this reaction 60). More interesting selectivity may be found in the reaction of olefins with methylene complexes the reaction of olefins with mononuclear methylene undergoes an olefin metathesis reaction, but the reaction of ethylene with bridging methylene in /i-CH2Co2(CO)2(Cp)2 61), /<-CH2Fe2(CO)8 (62), and /<-CH2-/i-ClTi(Cp)2Al(Me)2 (65) (Cp = cyclopentadiene) leads to propene formation (homologation reaction). 

The complexes are isolated, characterized and used as chiral Lewis acids. Dissociation of the labile ligand liberates a single coordination site at the metal center. These Lewis acids catalyze enantioselective Diels-Alder reactions. For instance, reaction of methacrolein with cyclopentadiene in the presence of the cationic iron complex (L = acrolein) occurs with exo selectivity and an enantiomeric excess of the same order of magnitude as those obtained with the successful boron and copper catalysts (eq 3). ... 

These exciting selectivities are believed to be strongly dependent on the specific coordination number of Yb(III) [69] (Scheme 11). Two binding sites for the ligands are now postulated in the Yb catalysts. 29 or 30 coordinates in site A under equilibrium conditions to stabilize the original catalyst system. When 29 coordinates Yb(III), cyclopentadiene attacks from the si face of 29 (site A favors si face attack). On the other hand, in catalyst B (the original catalyst system and PAA), site A is occupied by PAA [75]. Since another coordination site still remains in the Yb(III) catalyst owing to the specific coordination numbers, 29 coordinates at site B and cyclopentadiene attacks from the re face (site B favors re face attack). 

Cu(ll), Fe(lll) and Ce(IV) calcined clays behave differently. When Fe(lll) or Ce(IV) clays are used as catalysts, both selectivities decrease with increasing conversions, which is particularly noticeable with Ce(IV) clay. With Cu(ll) clay the reaction stops at low conversions. This behaviour can again be attributed to the polymerization of cyclopentadiene. Given that calcination eliminates most of Brensted acid sites, a cation radical mechanism must be invoked for the extensive diene polymerization. Ce(IV), Fe(lll) and Cu(ll) are the most easily reducible cations of those used and their EPR spectra in the presence of cyclopentadiene show the above-mentioned signal of organic radicals. 

Calcined clays are better catalysts than dried clays, which may be due to the elimination of Bronsted acid sites upon calcination. When the exchanged cation is easily reducible low selectivities, due to the deactivation of the catalyst by cyclopentadiene polymers, are obtained. Ti(IV) calcined clay is the most efficient catalyst, but the best asymmetric inductions are obtained with Cr(lli) and Ca(ll) calcined clays. 

Cyclopentadiene has received remarkably little attention " this is strange, because the molecule is planar and rigid, and should be invaluable for mapping the geometry of adsorption sites. On various copper catalysts, it is hydrogenated with 100% selectivity to cyclopentene with an activation energy of 54 kJ mol chemisorption is however weak, because the rate equation is... 

A score of literature data can be rationalized with the following hardness sequence in mind ils < t)8 < Ho < Oh — Hn- The formal substitution of soft by hard elements will soften the neighbouring atoms. This explains the decreased a-sclcctivity for electrophilic substitution in 5-membered hetcrocyclcs [41,61,62] upon going from pyrrole over furan to cyclopentadiene (Table 4a), assuming that the selectivity is determined by the Scjscp softness ratio, i.e. the softest site being the most reactive (orbital control). 

The Diels-Alder reaction was performed with cyclopentadiene and methacroleine (Scheme 56) in presence of 15 mol% of the oxazaborolidine and oxazaborolidinone catalysts derived respectively from supported aminoalcohols and from V-sulfonylamino acid polymers. The oxazaborolidine and oxazaborolidinone catalysts were formed in situ by action of BH3, BH2Br, BHBr2 or BBrs. The diastereoselectivity was excellent in favour of the exo adduct and yields from 65 to 99%. It is noteworthy that higher loading of chiral catalyst site in the polymer, lower exo selectivity and enantioselectivity. The diastereoselection depended not only on the nature of the supported ligand, the crosslinker but also on the borane and the solvent. Results are summarized in Table 6. 

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