Cyclopentadienes Subject

The synthesis of natural products containing the quinonoid stmcture has led to intensive and extensive study of the classic diene synthesis (77). The Diels-Alder cycloaddition of quinonoid dienophiles has been reported for a wide range of dienes (78—80). Reaction of (2) with cyclopentadiene yields (79) [1200-89-1] and (80) [5439-22-5]. The analogous 1,3-cyclohexadiene adducts have been the subject of C-nmr and x-ray studies, which indicate the endo—anti—endo stereostmcture (81). 

The monomer, CPD, obtained via cracking of the dimer, DCPD, and the dimer both have extensive uses. Cyclopentadiene is probably the most widely studied conjugated, cycHc diolefin system. Eleven review articles dealing with the chemistry of cyclopentadiene have been pubHshed (1—11). An article dealing specifically with European uses of DCPD has also been pubHshed (12). The discovery ia 1951 of stable metal derivatives has given additional impetus to the study of the chemistry of cyclopentadiene. Eive review articles have been pubHshed on this subject (13—17). 

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. 

Propene at 955 bar and 327°C was being subjected to further rapid compression. At 4.86 kbar explosive decomposition occurred, causing a pressure surge to 10 kbar or above. Decomposition to carbon, hydrogen and methane must have occurred to account for this pressure. Ethylene behaves similarly at much lower pressure, and cyclopentadiene, cyclohexadiene, acetylene and a few aromatic hydrocarbons have been decomposed explosively [1], It is mildly endothermic (AH°f (g) +20.4 kJ/mol, 0.49 kJ/g) and a minor constituent of MAPP gas [2],... 

The parent fulvene, 5-methylene-1,3-cyclopentadiene, was the subject of numerous calculations and conformational considerations. Both structures derived from microwave... 

The father of the subject is F. Williams, because he was the first to devise how to obtain kp+ values from rate measurements on the polymerisation of an alkene (cyclopentadiene) by ionising radiation (Bonin et al., 1965), and he wrote what is still the most comprehensive and critical account of a difficult subject (Williams, 1968). He refers characteristically to .. . an alarming number of misinterpretations and errors. .. . Other, shorter, expositions have been given by Metz (1968), Hayashi (1971,1980) and Stannett and Silverman (1983). 

The flash vacuum pyrolysis of alkynes, arynes, and aryl radicals has been reviewed. A discussion of secondary reactions and rearrangements is included. The pyrolysis of cyclopentadienes has also been examined. The rates for the initial C—H bond fission and the decomposition of C-C5H5 have been calculated. A single-pulse shock study on the thermal decomposition of 1-pentyl radicals found alkene products that are formed by radical isomerization through 1,4- and 1,3-hydrogen migration to form 2- and 3-pentyl radicals. The pyrrolysis of f-butylbenzene in supercritical water was the subject of a report. ... 

Since the cyclopentenones obtained by PKR can be subjected to further chemical transformations, there are plenty of chances to prepare various kinds of Gp-type ligands. The efforts are summarized thoroughly by Ghung. Only a couple of examples are mentioned here. A nice demonstration is the synthesis of 66, which clearly shows that a combination of various reaction sequences opens an access to cyclopentadienes (Equation (33)). ... 

Tropolone has been made from 1,2-cycloheptanedione by bromination and reduction, and by reaction with N-bromosuccinimide from cyclo-heptanone by bromination, hydrolysis, and reduction from diethyl pimelate by acyloin condensation and bromination from cyclo-heptatriene by permanganate oxidation from 3,5-dihydroxybenzoic acid by a multistep synthesis from 2,3-dimethoxybenzoic acid by a multistep synthesis from tropone by chlorination and hydrolysis, by amination with hydrazine and hydrolysis, or by photooxidation followed by reduction with thiourea from cyclopentadiene and tetra-fluoroethylene and from cyclopentadiene and dichloroketene. - The present procedure, based on the last method, is relatively simple and uses inexpensive starting materials. Step A exemplifies the 2 + 2 cycloaddition of dichloroketene to an olefin, " and the specific oycloadduot obtained has proved to be a useful intermediate in other syntheses. " Step B has been the subject of several mechanistic studies, " and its yield has been greatly improved by the isolation technique described above. This synthesis has also been extended to the preparation of various tropolone derivatives. - " ... 

The reaction of nitrosyl chloride with cyclopentene, cyclohexene, and cyclo-heptene in carbon tetrachloride solution at 5°C afforded the corresponding adducts in good yield, although during the cyclohexene reaction considerable hydrolysis to 2-chlorocyclohexanone took place and, in the case of cyclohep-tene, the crude product was not identified but rather was immediately subjected to levulinic acid hydrolysis to 2-chlorocycloheptene. From the products isolated, it may be inferred that cyclopentadiene adds nitrosyl chloride in a 1,4-manner in moderate yields [55]. 

The phenomenon involving failure of a material subject to repeated loading is called fatigue. Failure occurs at stress levels below those observed in the "static" tests described above. Lee et al (22) examined the characteristics of some sulphur concretes subject to fatigue. Fatigue lives (the number of cycles to failure) considerably in excess of those for portland cement concretes were observed. Polymerization of the sulphur with di-cyclopentadiene was observed to reduce fatigue life. 

Compounds with a narrow HOMO-LUMO gap (Figure 5.5d) are kinetically reactive and subject to dimerization (e.g., cyclopentadiene) or reaction with Lewis acids or bases. Polyenes are the dominant organic examples of this group. The difficulty in isolation of cyclobutadiene lies not with any intrinsic instability of the molecule but with the self-reactivity which arises from an extremely narrow HOMO-LUMO gap. A second class of compounds also falls in this category, coordinatively unsaturated transition metal complexes. In transition metals, the atomic n d orbital set may be partially occupied and/or nearly degenerate with the partially occupied n + 1 spn set. Such a configuration permits exceptional reactivity, even toward C—H and C—C bonds. These systems are treated separately in Chapter 13. 

The importance of the rigid aminoindanol backbone in asymmetric catalytic Diels-Alder reactions is a subject of continued interest.16 50 One example immobilized the copper-inda-box complex onto mesoporous silica in the context of continuous large-scale production of chiral compounds.16 Using 10 mol% of this catalyst (Figure 17.5), the Diels-Alder reaction between /V-acryoyloxazo-lidinone and cyclopentadiene proceeded in 99% yield, 17 1 endo. exo selectivity, and 78% ee of endo cycloadduct. The catalyst could easily be recovered and reused several times without significant loss of diastereoselectivity (15 1 endo. exo selectivity after the fifth reuse) or enantioselectivity (72% ee after the fifth reuse).16 The same remarkable reactivity was observed with a number of diene-dienophile partners. 

The polymerization of cyclopentadiene represents only one type of monomer which can be employed in a Diels-Alder polymerization. In theory, a polymerization reaction should be successful if a monomer which contained both a diene portion and a dienophilic portion were subjected to polymerization conditions. The polymerization of cyclopentadiene represents a special case of this reaction type. Another type... 

Caution. Tetrachloro-5-diazo-l,3-cyclopentadiene, Cs CUN2, is said to be very stable8 and we exerienced no difficulty. However, it is advisable to use care when subjecting the compound to extreme reaction conditions. 

A recent synthesis of the tricyclic secoiridoid ( )-sarracenin (98) relied on the Patemo-Biichi addition of acetaldehyde and cyclopentadiene as the initial step. Irradiation of cyclopentadiene and acetaldehyde provided a 5 1 mixture of bicyclic oxetanes (97) and (96) in 5-10% yield. Treatment of the crude photolysate with CSA and methanol followed by tosylation of the cmde product gave (99), which represents the toluenesulfonate ester derived from the major oxetane (97). The tosylate was displaced by the anion prepared from dimethyl 3-styrenylmalonate to afford the substituted malonate (100) in 84% yield (Scheme 10). Attempts to effect ring opening of the oxetane mixture were unsuccessful. Decarboxylation and demethylation gave the alcohol (102) which was subjected to ozonolysis and reductive work-up to afford ( )-sarracenin in 60% yield. The oxetane-based synthesis is noteworthy due to its brevity and use of a biosynthetically postulated trialdehyde equivalent. 

Several reports of the participation of azirines as hetero dienophiles have appeared, and this subject has been reviewed.Simple alkyl and aryl azirines react with cyclopentadienones to afford azepines as the ultimate products (equation 23). This type of azirine does not react with cyclopentadiene, but the benzoyl azirine shown in equation (24) does to afford an isolable Diels-Alder adduct. Azirines have also been shown to react stereoselectively in [4 + 2] fashion with tetrazines such as (64) and isobenzo-furan. 

As the focus of this chapter is on the synthetic utility of the rDA reaction, an overview of mechanism is beyond the scope of this review however, the subject has beoi reviewed previously. Structural and medium effects on the rate of the rDA reaction are of prime importance to their synthetic utility, and therefore warrant discussion here. A study of steric effects cm the rate of cycloreversicHi was the focus of early work by Bachmann and later by Vaughan. The effect of both diene and dioiophile substituticHi on Ae rate of the rDA reaction in anthracene cycloadducts has been reported in a study employing 45 different adducts. If both cycloaddition and cycloreversion processes are fast on the time scde of a given experiment, reversibility in the DA reaction is observed. Reversible cycloaddition reactions involving anthracenes, furans, fulvenes and cyclopentadienes are known. Herndon has shown that the well-known exception to the endo rule in tiie DA reaction of furan with maleic anhydride (equation 2) occurs not because exo addition is faster than endo addition (it is not), but because cycloreversion of the endo adduct is about 10 000 times faster than that of the exo adduct. ... 

The effects of heavy atoms and a consideration of relativistic effects are also the subject of an investigation of the cyclopentadiene homologous series, C4H4XH2(X = C, Si, Ge, Sn) by Alparone et al.106,107 The a and y responses are found to increase monotonically with the atomic number of the heavy atom. Vibrational contributions are also calculated and compared with calculations on furan. 

Oxidation of 4-methoxycarbonyl-2-methoxyphenol (421) with PhI(OAc)2 in MeOH was carried out in the presence of cyclopentadiene (CPD) to afford in 87% yield a Diels-Alder adduct 434, which was subjected to photochemical reaction followed by Ac20-BF3-Et20 treatment to give a linear triquinane 435 (47.6% in 2 steps). ... 

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