Pentadiene, with

Attempted reaction of 1,3-pentadiene with the optically active diboron derived from dialkyl tartrate in the presence of a phosphine-free platinum catalyst gave poor diastereoselectivity (20% de).63 Better selectivity has been attained with a modified platinum catalyst bearing a PCy3 ligand (Scheme 6).64 The reaction of allylborane thus obtained with an aldehyde followed by oxidation with basic hydrogen peroxide affords the corresponding diol derivative with moderate ee. 

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... 

Bicyclo[2.2.1]hepten-7-one is a useful intermediate in the synthesis of a variety of norbornane derivatives. The present procedure involves a four-step synthesis from hexachlorocyclo-pentadiene with a 39% overall yield. The next best method3 involves a four-step synthesis from norbornadiene with a 15% overall yield. 

The reaction of 2,3-pentadiene with I2 in MeOH afforded a 9 1 Z E ratio of 3-iodo-4-methoxy-2-pentene [17]. 

However, treatment of cis-l,3-pentadiene with Fe(CO)5 leads to formation of the hydride intermediate as shown for the conversion of 1,4-pentadiene to the borohydride treatment of 7r-pentadienyliron tricarbonyl cation (see below). 

Problem 8.40 Give possible products, with lUPAC names, of the following addition reactions (a) 1,3-butadiene and 1,4-pentadiene, each with 1 and 2mol of HBr (b) 2-methyl-l,3-butadiene and 1,3-pentadiene with 1 mol of HI. 

This mechanism is supported by the transformation of preformed metallacyclo-pentadienes with alkynes,73,76-78 and labeling experiments79 that excluded the involvement of cyclobutadiene intermediates. It also accounts for the observation that terminal alkynes yield 1,2,4- (and 1,3,5-) trisubstituted benzene derivatives as the main product but not 1,2,3 derivatives. In contrast with this picture in cyclotrimerization with PdCl2-based catalysts, stepwise linear insertion of alkynes takes place without the involvement of palladacyclopentadiene.80... 

Cyclohexadiene and singlet oxygen form (103). Most of the important synthetic approaches for 1,2-thiazines use a fragment in which the S-N bond already exists. Syntheses via [4 + 2] cycloadditions constitute a major part of the synthetic repertoire for this class of compounds (CHEC-II(6)370). Thus, pentadiene with TsNSO gives (104) and perfluoro-l,2-thiazine (105) is prepared by the cycloaddition of thiazyl fluoride (FSN) and perfluoro-1,3-butadiene CF2 = CFCF = CF2 (79CC35). 

Reductive cleavage of the potassium anions (3) of (methylthio)pentadienes with Li in NH, followed by methylation gives 4 with retention of configuration of the Ci — ( bond, but the C3—C4 bond is always (Z) regardless of the configuration in 3. 

The two Diels-Alder adducts formed in the reaction of 1,3-pentadiene with acrolein arise by the two alignments shown ... 

The validity of the proposed mechanism is borne out by the results of the polymerisation of (E,E)-1 -(2H)-1,3-pentadiene with Nd(OCOR)3—AlEt2 Cl A1 (z-Bu)3 and Co(Acac)2 AIEt2Cl—H20 catalysts [20-22,195], The Nd-based catalyst gave a czj-1,4-eryt/zro-diisotactic polymer, while the Co-based catalyst yielded a cA-l,4-t/zra>-disyndiotactic polymer. The formation of such polymers is shown by the schemes in Figure 5.5 [7,41],... 

Random copolymers of butadiene, isoprene and/or pentadiene with ethylene and/or propylene have been obtained in the presence of various catalysts, mainly based on Ti or V compounds [206,207]. Statistical butadiene/ethylene copolymers can also be formed with zirconocene catalysts [162]. 

Early studies on the homopolymerization of E-l,3-pentadiene yielded polymers with a high cis-1,4-content and an isotactic structure, whereas E-2-methyl-l,3-pentadiene resulted in a polymer with a mixed czs-1,4/transit-structure [487-492]. Investigations on the polymerization of E-1,3-pentadiene with the system NdN/TIBA/DEAC partially support these findings as a poly(l,3-pentadiene) with a cis- 1,4-threo-disyndiotactic structure was obtained [492]. A somewhat lower cis- 1,4-content of 70% was obtained when the polymerization of E-l,3-pentadiene was catalyzed by (CF3COO)2NdCl/TEA [493,494]. When 2,3-Dimethyl-1,3-butadiene is polymerized with the catalyst NdN/TIBA/EtAlC the resulting poly(2,3-dimethyl-butadiene) predominantly contains cis-1,4-units [495,496]. 

In later studies on the homopolymerization of E-l,3-pentadiene with NdO/ TIBA/DEAC crystalline polymers with cis- 1,4-contents in the range 84-99% and a high isotacticity were obtained. It was found that the cis- 1,4-content increases when the polymerization temperature is decreased from room temperature to -30°C. The polymerization of E-2-methyl-l,3-pentadiene resulted in polymers which almost exclusively comprised cis- 1,4-units and no dependence of the cis- 1,4-content on polymerization temperature was observed. The obtained poly(2-methyl-l,3-pentadiene) was composed of various polymer fractions with different stereo regularities [165,166]. 

Wiberg and coworkers published relative rate constants and the products of reaction of silene 6 with a number of alkenes and dienes in ether solution at 100 °C6 106-108. These data are listed in Table 2 along with an indication of the type of product formed in each case. As is the norm in Diels-Alder additions by more conventional dienophiles, the rate of [2 + 4]-cycloaddition of 6 to dienes increases with sequential methyl substitution in the 2- and 3-positions of the diene, as is illustrated by the data for 2,3-dimethyl-1,3-butadiene (DMB), isoprene and 1,3-butadiene. The well-known effects of methyl substitution at the 1- and 4-positions of the diene in conventional Diels-Alder chemistry are also reflected with 6 as the dienophile. For example, lruns-1,3-pen tadiene reacts significantly faster than the f/.v-isorrier, an effect that has been attributed to steric destabilization of the transition state for [2 + 4]-cycloaddition. In fact, the reaction of c/s-l,3-pentadiene with 6 yields silacyclobutane adducts, while the trans-diene reacts by [2 + 4]-cycloaddition108. No detectable reaction occurs with 2,5-dimethyl-2,4-hexadiene. The reaction of 6 with isoprene occurs regioselectively to yield adducts 65a and 65b in the ratio 65a 65b = 8.5 (equation 50)106,107. 

The usual rule of endo-selectivity is not observed in the cycloaddition of cyclo-pentadiene with methacrolein (equation II). In this reaction both DAD and MAD enhance exo-selectivity because of steric repulsion between the bulky catalyst and... 

The intermediate 1,3-diphosphapropenes can be synthesized in a separate reaction of bis(trimethylsilyl)phosphanes with phosgene or isocyanide dichlorides (29, 99), respectively, and can be converted to the unsymmetrically substituted pentadienes with phosphaketene [Eq. (47)]. 

The 1,5 cyclooctadiene complex [Cp Ru(jj rf--C Yiu) (CO)]OTf was isolated upon treatment of Cp Ru(j)" -butadiene)X (X = Cl, Br) with butadiene, AgOTf, and CO. A similar [4-1-4] cycloaddition (a thermally forbidden reaction see Woodward-Hoffmann Rules)) is observed when Cp Ru(isoprene)Cl is treated with iso-prene, AgOTf, and CO. Likewise, the reaction of 1,3-pentadiene with Cp Ru( ) -l,3-pentadiene)Cl results in linear dimerization to form [Cp Ru(4-methyl-(l,3-jj 6-8-j) )-nonadienediyl)]OTf. These types of dimerization occur with both stoichiometric and catalytic amounts of the ruthenium complex. ... 

The protic acid and Lewis acid-catalyzed [4 + 2] cycloaddition reactions of electron-rich alkenes with imines derived from anilines and aryl aldehydes constitute an extensively explored class of 2-azadienes capable of providing the products of a formaJ Diels-Alder reaction (equation 9).i5.27.i77 a useful extension of these studies and in efforts to increase the rate of the Att participation of simple 2-aza-1,3-buta-dienes in [4 + 2] cycloaddition reactions, Mariano and coworkers have examined the Lewis acid-catalyzed intermolecular reactions of (l ,3 )-l-phenyl-2-aza-l,3-pentadiene with electron-rich dienophiles, including enol ethers. Reductive work-up of the cycloaddition reactions provided the pro-... 

The intermediacy of bis(jt-allyl)ruthenium complexes has been strongly suggested by the fact that a similar reaction of (C5Me5)RuCl(>/ -l,3-pentadiene) with 1,3-penta-diene in the presence of AgOTf affords [(C5Me5)Ru 4-methyl-(l-3->/ 6-8-> )-nona-dienediyl ]OTf via a regioselective tail-to-head dimerization reaction (Eq. 5.43). 

The preparation of (/ra ,< -l,3-pentadieiie)Fe(CO).3 by reaction of 1,4-pentadiene with Fe3(CO),2 has been reviewed (47, reference 160). Although earlier attempts to prepare the cis analog by direct interaction of cw-l,3-pentadiene with Fe(CO)5 lead only to formation of the trans complex (188, 387), it has been successfully jwepared wdth Fe2(CO)B (363). Although, in general, the cis-substituted dienes undergo isomerization upon interaction with Fe(CO)5, cis,/mw -2,4-hexadiene has been observed to retain its configuration (188). There may be a relationship between the effect of steric hindrance on complex stability and the tendency for isomerization to occui (188). 

Rearrangement processes of alkyltitanocene dichlorides that occur under electron impact have been investigated using deuterium labelling. A novel type of zirconium-mediated coupling reaction of alkynes with vinyl bromide to afford 2,3-disubstituted dienes has been reported (see Scheme 105), and an inter-intramolecular reaction sequence has been proposed for the observed formation of vinylcyclohexadienes and/or methylenecycloheptadienes from the copper-catalysed reaction of zirconacyclo-pentadienes with allylic dichlorides. The essential step in these processes appears to be transmetallation of the zirconium-carbon bond of the zirconacyclopentadiene to produce a more reactive copper-carbon bond. New phosphorus heterocycles, e.g. (417), have been constructed by the thermal rearrangement of a [l,4-bis(trimethylsilyl)->/ -cyclooctatetraene]- ,3,5-triphospha-7-hafhanorbomadiene complex (416). 

In an attempt to prepare polycyclopentadiene which would be stable in toluene solution, the polymer was hydrogenated over a platinum oxide catalyst in a Parr bomb immediately after the completion of the polymerization reaction. Infrared analysis indicated the presence of residual unsaturation and the polymer became insolubilized on standing. An attempted copolymerization of cyclo-pentadiene with propylene gave a product whose infrared spectrum indicated the presence of C-methyl groups but which was still insoluble in toluene. No attempt was made to determine whether copolymerization had occurred. 

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