2- phenyl-1,3-cyclopentadiene

The first phase of our efforts was the unambiguous synthesis of each model substrate. PN and PX were already well characterized materials (1) While direct synthesis of the phenyl and carbomethoxy compounds from PN and/or PX was attempted, this approach was unsuccessful due to the sluggish reactivity of the norbornenyl double bonds in these molecules (2). A successful approach to CBN and (fiBN based on N-phenyl maleimide (NPMI) trapping of the respective thermodynamically favored 1-substituted cyclopentadienes is shown in Equation 1. Similarly, kinetic trapping of 2-phenyl cyclopentadiene, from the in situ dehydration of 3-hydroxy, 3-phenyl cyclopentene, gives a clean yield of (f)VN (Equation 2). The remaining phenyl isomer (VX) and the three other carbomethoxy isomers (CBX, CVN, CVX) were all obtained by the thermal isomerization chemistry described in the next section of this paper. They were each isolated in pure form by liquid chromatography We were unable to obtain any (f)BX or any of the 7-substituted isomers by any means. 

In each case the fused ring products probably arise from rearrangement of an initially formed spirene [98]. Insertion products, i.e. phenylated cyclopentadienes, are also formed as by-products [97,100]. 

Preparations of macro-initiators or telechelic polymers by cationic methods have been executed primarily by polymerizing isobutylene in the presence of a co-initiator that also functions as a chain transfer agent. A typical reaction sequence is shown in Scheme 1, outlining the synthesis of difunctional polyisobutylene (PIB), which is then used to initiate the polymerization of a-methyl styrene (ffi-MS) to produce an A-B-A type block copolymer. By similar methods, polyisobutylenes with phenol, phenyl, cyclopentadiene, and olefin termini have been synthesized. 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

Finally, in Chapter 5, micellar catalysis of Diels-Alder reactions is discussed. In view of the nonpolar nature of most Diels-Alder reactants, efficient micellar catalysis of this reaction was anticipated However, this has not been observed. The results for the Diels-Alder reaction between cyclopentadiene and substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-one dienophiles, discussed in... 

In summary, the work in this thesis provides an overview of what can be achieved with Lewis-acid and micellar catalysis for Diels-Alder reactions in water as exemplified by the reaction of3-phenyl-l-(2-pyridyl)-2-propene-l-ones with cyclopentadiene. Extension of the observed beneficial effect of water on rates and particularly enantioselectivities to other systems is envisaged. 

There has been new information on the products of photolysis of derivatives of compound 1. Low temperature irradiation of the ester 254 gives a ketene (93JACS8621) the isolation of an isomeric ketene from a 3-pyridyldiazo ester suggests the involvement of the open chain form 255. Photolysis of the 3-phenyl derivative 256 in the presence of cyclopentadiene gives exo and endo cyclopropanes and a dipyridylstilhene, suggesting the intermediacy of the carhene 257 (99JOC6635). 

Evans s bis(oxazolinyl)pyridine (pybox) complex 17, which is effective for the Diels-Alder reaction of a-bromoacrolein and methacrolein (Section 2.1), is also a suitable catalyst for the Diels-Alder reaction of acrylate dienophiles [23] (Scheme 1.33). In the presence of 5 mol% of the Cu((l )-pybox)(SbF5)2 catalyst with a benzyl substituent, tert-butyl acrylate reacts with cyclopentadiene to give the adduct in good optical purity (92% ee). Methyl acrylate and phenyl acrylate underwent cycloadditions with lower selectivities. 

Cyclohexene-l,4-dione, 2,3,5-tiichloro-3, 6-bis(l,l-dimethylethyl)- [5-Cyclo-hexcne-1,4-dione, 2,3,5-tnchloio-3,6-dwert-butyl-], 55, 33 Cyclopentadiene, 55, 15,16 Cyclopentane acetyl-,55,25 Cyclopentane 1-cyano-l-phenyl-, 55,94 Cyclopentane methyl-, 55, 62 Cyclopropane, 1-acetyl-l-phenyl-, 55 94... 

As a continuation of these studies, Bauld recently reported evidence of a stepwise mechanism in the cation-radical Diels-Alder reaction of phenyl vinyl sulfide with cyclopentadiene [34, 35] (Scheme 1.6). 

By using unactivated K-10 montmorillonite in the absence of solvent, the endo-exo selectivity of the cycloadditions of acrolein and methyl vinyl ketone with cyclopentadiene and cyclohexadiene is low [8] (Table 4.2, entry 3), while highly reactive dienophiles such as 1,4-benzoquinone and N-phenyl... 

Table 4.16 Micellar catalysis of Diels-Alder reactions of cyclopentadiene (1) with 3-(p-substituted phenyl)- -(2-propen-1-one (113) in water at 25 °C. Relative rate constants ( rei) to the reactions performed in sole water...

Recently we pointed ont that the cyclopentadiene having phenyl moiety at the 5-position is the diene of omo- phenyl moiety can mediate orbital... 

The values of X = NH, OH, F, Cl, and CH3 are smaller than that of X = H, in accordance with the observed selectivity. Excellent correlation was found for all other cyclopentadienes described above. Syn rr-facial selectivity in the reactions between 4-phenyl-l,2,4-triazoline-3,5-dione and cyclopentadiene having simple alkyl group at 5 positions are reported by Burnell and coworkers [46] (Scheme 37). 

Copper-complexes prepared with other type of N-chelating ligands have been also prepared and evaluated as catalysts for the Diels-Alder reaction. Eng-berts et al. [103] studied enantioselective Diels-Alder reaction of 3-phenyl-l-(2-pyridyl)-2-propen-l-one with cyclopentadiene in water (Scheme 39). By using coordinating chiral, commercially available a-amino-adds and their derivatives with copper salts as catalysts, they obtained the desired product with yields generally exceeding 90%. With L-abrine (72 in Scheme 39) as chiral moiety, an enantiomeric excess of 74% could be achieved. Moreover, the catalyst solution was reused with no loss of enantioselectivity. 

Fujisawa et al. [Ill] have reported that the magnesiiun complex prepared from chiral 2-[2-[(tolylsulfonyl)amino]phenyl]-4-phenyl-l,3-oxazoline 81 and methyl-magnesium iodide was efficient, in a stoechiometric amount, for promoting the enantioselective Diels-Alder reaction of 3-alkenoyl-l,3-oxazohdin-2-one with cyclopentadiene (Scheme 45) leading exclusively to the endo adducts in up to 92% ee. The use of 10 mol% of the complex led to an important decrease in enantioselectivity of the product (51% ee). 

It has been found that the combination of Lewis acids and surfactants is particularly effective for catalyzing Diels-Alder reactions in water. The effect of micelles of SDS, CTAB, dodecyl heptaoxyethy-lene ether (Q2E7), and copper and zinc didodecyl sulfate [M(DSb] on the Diels-Alder reaction of 3-(p-substituted phenyl)- l-(2-pyridyl)-2-propen-l-ones (Figure 12.1) with cyclopentadiene was studied. 

Reactions of methoxycarbonylformonitrile, furonitrile and substituted benzoni-trile oxides (4-Me, 4-OMe, 3-OMe, 4-C1, 3-C1, 2,4-di-Cl, 4-F as substituents) with dimethyl 7-(diphenylmethylene)bicyclo[2.2. l]hept-2-ene-5,6-dicarboxylate led exclusively to exo cycloadducts 82 (R = C02Me, 2-furyl, substituted phenyl), which, on irradiation with a low-pressure mercury lamp, afforded 3-azabicyclo [4.3.0]nonadiene-7,8-dicarboxylates 83 as the only products. The 1,3-dipolar cycloaddition, followed by a photorearrangement, provides a new method for obtaining tetrahydro-27/ -pyridine derivatives from cyclopentadiene (245). 

On converting 1-benzothiophene into 1-phenyl-1-benzothiophenium triflate (95), this salt becomes a dienophile and reacts readily with cyclopentadiene or 1,3-diphenylisobenzofuran to give the adduct 96 (Scheme 45).143 This example of the dienophilic nature of the double bond in the benzothiophene ring arises from reduced aromaticity. 

The treatment of 23 with methyllithium in the presence of furan gave rise to the tetracyclic product 26, which is obviously a [4 + 2]-cycloadduct of furan to the 1,2-cyclopentadiene derivative 25 [27]. The feature that the oxanorbornene system of 26 carries its saturated substituent in the endo-position is analogous to the [4 + 2]-cycloadducts of furan to all six-membered cyclic allenes (see Section 6.3). Balci et al. [36] also provided evidence for the generation of l-phenyl-l,2-cyclopentadiene. They postulated this species to be an intermediate in the reaction of l-phenyl-2-iodocydo-pentene with potassium tert-butoxide in benzene at 240 °C, which resulted in the formation of 1-phenyl- and 1,2-diphenylcyclopentene. Both products were considered as evidence in favor of the diradical nature rather than the allene structure of 1-phe-nyl-1,2 -cyclopentadiene. 

Table 12.8 Reaction of allenyl phenyl sulfones with cyclopentadiene.

Corey and co workers2056, 24X reported the reactive cationic oxazaborinane catalyst and afforded 398a which promoted cycloadditions between cyclopentadiene and several a,/J-cnals good enantioselectivities. Excellent results were obtained in cycloadditions of several modestly reactive dienes to a-bromoacrolein in the presence of catalyst 398b having tetra[3,5-bis(trifluoromethyl)phenyl]borate as the counterion (Table 23). 

If, instead of electrocyclization, electrophilic attack of the closer upper edge of the phenyl group by the carbene carbon atom occurs, a zwitterionic intermediate might result, which upon 1,4-elimination of (COljW would yield a 1-methoxy-1,3-cyclopentadiene. Suprafacial hydrogen migration would finally lead to the formation of the observed diastereomer. 

The Diels-Alder reaction between a 2-fluoroacrylic acid derivative of 8-phenyl-menthol (83) and cyclopentadiene shows high exo- and jr-diastereofacial selectivity (Scheme 30). The C(2) of endocyclic cross-conjugated 2-(acylamino)-l,3-dienes exerts excellent diastereofacial control on the Diels-Alder addition with electron-deficient dienophiles to produce octahydroquinolines. ... 

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