Cyclo pentadiene

The l,5-hexadien-3-ol derivatives 792 and 794 are cycli2ed to form the cyclo-pentadiene derivatives 793 and 795 by insertion of an alkene into -allylpalla-dium formed from allylic alcohols in the presence of trifluoroacetic acid (lO mol%) in AcOH[490],... 

The same anion is formed by loss of the most acidic proton from 1 methyl 1 3 cyclo pentadiene as from 5 methyl 1 3 cyclopentadiene Explain... 

Another application of catalyst 8 is to the reaction of acetylenic aldehydes [10c] (Scheme 1.18, Table 1.6). Two acetylenic dienophiles have been reacted with cyclo-pentadiene or cyclohexadiene to give bicyclo[2.2.1]heptadiene or bicyclo[2.2.2]octa-diene derivatives in high optical purity. A theoretical study suggests that this reaction proceeds via an exo transition state. 

Yamamoto et al. have reported a chiral helical titanium catalyst, 10, prepared from a binaphthol-derived chiral tetraol and titanium tetraisopropoxide with azeotropic removal of 2-propanol [16] (Scheme 1.22, 1.23, Table 1.9). This is one of the few catalysts which promote the Diels-Alder reaction of a-unsubstituted aldehydes such as acrolein with high enantioselectivity. Acrolein reacts not only with cyclo-pentadiene but also 1,3-cyclohexadiene and l-methoxy-l,3-cyclohexadiene to afford cycloadducts in 96, 81, and 98% ee, respectively. Another noteworthy feature of the titanium catalyst 10 is that the enantioselectivity is not greatly influenced by reaction temperature (96% ee at... 

Diels-Alder reaction of dienophiles, N-allylic enamides and a,/l-unsaturated lactam derivatives with open chain and inner ring dienes is promoted by iodine [98]. Thus the cycloaddition of N-benzyl-N-methallyl acrylamide 147 with cyclo-pentadiene (1) proceeds smoothly in DMF at —78 °C in the presence of I2 (2 eq.) to give a prevalence of endo adduct l Vd) in 88% yield (Equation 4.17). 

Thus, a series of bis-cyclopentadienetitanium(III) dithiocarbamate and xanthate complexes have been prepared by Coutts et al. (49-51) by reaction of the sodium salts of the ligands with [Cp TiJCl (Cp =7r-cyclo-pentadiene) in air-free water under an inert atmosphere the dithiocarbamate complexes are bright-green, and the xanthates are blue. 

In the same area, a (5)-tryptophan-derived oxazaborolidine including a p-tolylsulfonylamide function has been used by Corey et al. to catalyse the enantioselective Diels-Alder reaction between 2-bromoacrolein and cyclo-pentadiene to form the corresponding chiral product with an unprecedented high (> 99% ee) enantioselectivity (Scheme 5.27)." This highly efficient methodology was extended to various 2-substituted acroleins and dienes such as isoprene and furan. In addition, it was applied to develop a highly efficient total synthesis of the potent antiulcer substance, cassiol, as depicted in Scheme 5.21... 

Significantly, there is no variation in the relative product distribution in this case as a function of the sensitizer triplet energy. 9-Anthraldehyde (Et = 42 kcal/mole) was ineffective as sensitizer for this reaction, presumably because its lowest triplet is lower in energy than that of cyclo-pentadiene.<23>... 

As expected, the azaquinone C=N double bond is strongly dienophilic. This was demonstrated by reactions with several 1,3-butadienes, cyclo-pentadiene, and cyclohexadiene (Scheme 7). These reactions, carried out... 

In addition to phosphine ligands, a variety of other monodentate and chelating ligands have been introduced to functionalized polymers [1-5]. For example, cyclo-pentadiene was immobilized to Merrifield resins to obtain titanocene complexes (Fig. 42.13) [102]. The immobilization of anionic cyclopentadiene ligands represents a transition between chemisorption and the presently discussed coordinative attachment of ligands. The depicted immobilization method can also be adopted for other metallocenes. The titanocene derivatives are mostly known for their high hydrogenation and isomerization activity (see also Section 42.3.6.1) [103]. 

Stable aryl boronates derived from tartaric acid catalyze the reaction of cyclo-pentadiene with vinyl aldehyde with high selectivity. Chiral acyloxy borane (CAB), derived from tartaric acid, has proved to be a very powerful catalyst for the enantioselective Diels-Alder reaction and hetero Diels-Alder reaction. Scheme 5 23 presents an example of a CAB 73 (R = H) catalyzed Diels-Alder reaction of a-bromo-a,/i-cnal 74 with cyclopentadiene. The reaction product is another important intermediate for prostaglandin synthesis. In the presence of... 

The biaryl compound (50) forms a complex with diethylaluminium chloride to provide a catalyst able to promote enantioselective reaction between cyclo-pentadiene and methacrolein or acrylates (Scheme 45). The addition of A -tert butyl 2,2-dimethylmalonate to the reaction mixture was found to enhance the enantiomeric excess of the product11311. 

Schafer and coworkers devised a useful synthetic procedure based on this reaction [26]. Thus, the Diels-Alder reaction of /i-silylacrylic acid with cyclo-pentadiene gave the adduct which was oxidized anodically with the elimination of the carboxyl and the silyl groups. Successful formation of norbonadiene indicates that /J-silylacrylic acid can be used an a synthon of acetylene in Diels-Alder reactions with dienes (Scheme 33). 

Ever since 1962, when Williams, Okamura, and their associates started to publish propagation rate-constants k+p for the cationic bulk polymerization of cyclo-pentadiene, isobutene, styrene, a-methylstyrene and isopropylvinyl ether by ionizing radiations, these constants have been accepted as the best, most likely, values for the k+p of unpaired cations in a medium of low-polarity, and those obtained subsequently by Stannett and his collaborators, using similar methods, enjoyed the same status, (The loci classici are Bates et al. (1962), Bonin et al. (1964), Taylor Williams (1969) and the three papers by Ueno et al. (1967), Hayashi et al. (1967) and Williams et al. (1967).)... 

The addition of methanol or hydrazoic acid to ethenylidenecydopentadiene 3 demonstrates that 3 behaves like an acceptor-substituted allene (Scheme 7.27) [226, 227]. More examples of nudeophilic additions to alkyl-substituted derivatives of 3 were reported by Hafner [228]. Photoelectron spectroscopy of the spirocyclic compound 165b, easily accessible from azide 164b, shows that the lone-pair orbital n(N) of the 2H-azirine nitrogen atom interacts strongly with the Jt1-orbital of the cyclo-pentadiene ring [227]. 

Monochlorotitanium complex 418, prepared from (l/J,25 )-Af-(2,4,6-trimethylbenze-nesulfonyl)-2-amino-l-indanol and titanium tetraisopropoxide followed by treatment with titanium tetrachloride effectively catalyzed the cycloaddition of a-bromoacrolein to cyclo-pentadiene, affording 366 with 93% ee (equation 125)259. Catalyst 418 induced an ee of 90% in the reaction of isoprene with a-bromoacrolein. 

In a similar study, Park has shown125 that cyclic dienes other than cyclo-pentadiene will also undergo a [4 + 2] cycloaddition reaction with the (trimethylsilyl)alkynylcarbene complex (184.b). When 184.b was reacted with 1,3-cyclohexadiene in THF, the diene complex 191 was isolated along with the vinylketene complex 192, which was prone to hydrolysis during column chromatography as expected.123 When 192 was stirred with silica gel and water in hexane, an almost quantitative conversion to the aldehyde complex 193 was observed. 

Figure 19.7. Cyclic batch elution chromatography obtaining high product purity and high throughput by using incomplete resolution (overlapping bands) and recycling the mixed fraction (mf) to the feedstock (a) Control of band separation and cut points determines fractional impurities t mij mr and Ami /mr2-<4l)> (b) Chromatogram for separation of pure ds- and trans- 1,3-pentadiene. Components 1, isoprene 2, trans- 1,3-pentadiene 3, cis-l,3-pentadiene 4, cyclo-pentadiene. Component 1 is eluted at almost the same time as component 4 of the...

Depending on the types of substituents and the precise reaction conditions (l,3-butadien-l-yl)carbene complexes can undergo direct cyclization to yield cyclo-pentadienes [337,350]. As mentioned in Section 2.2.5.1, cyclopentadiene formation occurs particularly easily with aminocarbene complexes [351]. Alternatively, in particular at higher reaction temperatures, CO-insertion can lead to the formation of a vinylketene complex, which, again depending on the electronic properties of the substituents and the reaction conditions, can cyclize to yield cyclobutenones, furans [91,352], cyclopentenones, furanones [91], or phenols (Dotz benzannulation) [207,251,353]. 

Diels-Alder reactions with alkynes as dienophiles have been known for a long time. Iminoboranes, however, will more readily cyclodimerize than react with dienes, and even the cyclodimers are generally superior to dienes in the competition for excess iminoborane. Among many attempts, therefore, only two reactions with iminoboranes as dienophiles have been successful, and in both of them the diene is cyclo-pentadiene [Eq. (55)] (9, 14). 

Upon reaction of Cjq with Meli a bis-epoxide bridged derivative of this cyclo-pentadiene adduct type has been isolated and characterized by single-crystal X-ray analysis [55]. Both epoxide groups bridge [5,6]-bonds in the same pentagon. 

MET MY I.ATION /u-Chlorohis(ij-2,4-cyclo-pentadien-l-yl)(dimethylaluminum)-(i-inethylcnelitanium. Triisopropoxy-methyllitanium. 

Thus, the niobium hydrides in fact do exhibit the desired hydridic character they can reduce acetone but cannot react with CO reduction products. From the color, the absence of any NMR, and the formation of 1 mol of free cyclo-pentadiene, the niobium-containing product appears to be a monocyclopenta-dienyl Nb(IV) complex. The rather facile removal of one of the Cp groups appears somewhat surprising but has been observed in related systems. For example, alcoholysis of Cp2TiCl2 gives CpTiC OEt) plus cyclopentadiene, even at room temperature (26). 

---