Hydrocarbons cyclopentadiene

A very remarkable series of organometallic compounds, derived principally from the hydrocarbon cyclopentadiene, (III), and having sandwichlike structures, was first synthesized in the 1950 s, chiefly by G. Wilkinson. The first member of the series, ferrocene (biscyeiopentadienyl-iron), abbreviated Fe(cpd)2, may be made by heating cyclopentadiene... 

Pyrrole is much more acidic than comparable saturated amines. The pKa of pyrrolidine is about 35, but pyrrole has a pKd of 16.5 making it some 1023 times more acidic Pyrrole is about as acidic as a typical alcohol so bases stronger than alkoxides will convert it to its anion. We should not be too surprised at this as the corresponding hydrocarbon, cyclopentadiene, is also extremely acidic with a p- Ca of 15. The reason is that the anions are aromatic with six delocalized Tt electrons. The effect is much greater for cyclopentadiene because the hydrocarbon "is not aromatic and much less for pyrrole because it is already aromatic and has less to gain. 

The cyclopentadienyl group is probably the most versatile of all the ligands in organotransition metal chemistry. Cyclopentadienyl-metal chemistry has its origin in 1901 when Thiele 232) discovered that the unsaturated hydrocarbon cyclopentadiene, when treated with potassium metal in benzene, liberated hydrogen and was converted to a potassium salt fXVIII). 

For a hydrocarbon, cyclopentadiene is an unusually strong acid Ka = 10 i5) indicating that loss of a hydrogen ion gives a particularly stable anion. (It is, for example, a much stronger acid than cycloheptatriene. Kg = despite the... 

W.J.Evans (33,34) prepared some divalent organolanthanides by co-condensation at low temperature of lanthanide metal vapours with unsaturated hydrocarbons (cyclopentadienes, alkynes) containing acidic hydrogen. Some organolanthanides showed catalytic activity. Thus, Sm(C Me ) (THF) catalyzes hydrogenation of 3-hexyne into cis-hexene (cis trans > 99 1 under mild conditions 25°C, 1 atm of hydrogen. The reaction is believed to involve the addition of a hydride Ln-H to the triple bond followed by hydrogenolysis with H (35). The same complex polymerizes ethylene (35). /... 

Hydrocarbon cyclopentadiene fluorene acetylene toluene benzene... 

The generic term azulene was first applied to the blue oils obtained by distillation, oxidation, or acid-treatment of many essential oils. These blue colours are usually due to the presence of either guaiazulene or velivazulene. The parent hydrocarbon is synthesized by dehydrogenation of a cyclopentanocycloheptanol or the condensation of cyclopentadiene with glutacondialdehyde anil. 

Gyclopentadiene/Dicyclopentadiene-Based Petroleum Resins. 1,3-Cyclopentadiene (CPD) is just one of the numerous compounds produced by the steam cracking of petroleum distillates. Due to the fact that DCPD is polymerized relatively easily under thermal conditions without added catalyst, resins produced from cycloaHphatic dienes have become a significant focus of the hydrocarbon resin industry. 

Cyclodienes. These are polychlorinated cycHc hydrocarbons with endomethylene-bridged stmctures, prepared by the Diels-Alder diene reaction. The development of these insecticides resulted from the discovery in 1945 of chlordane, the chlorinated adduct of hexachlorocyclopentadiene and cyclopentadiene (qv). The addition of two Cl atoms across the double bond of the ftve-membered ring forms the two isomers of chlordane [12789-03-6] or l,2,4,5,6,7,8,8-octachloro-2,3,3t ,4,7,7t -hexahydro-4,7-methano-lJT-indene, QL-trans (mp 106.5°C) and pt-tis (32) (mp 104.5°C). The p-isomerhas signiftcantiy greater insecticidal activity. Technical chlordane is an amber Hquid (bp 175°C/267 Pa, vp 1.3 mPa at 25°C) which is soluble in water to about 9 fig/L. It has rat LD qS of 335, 430 (oral) and 840, 690 (dermal) mg/kg. Technical chlordane contains about 60% of the isomers and 10—20% of heptachlor. It has been used extensively as a soil insecticide for termite control and as a household insecticide. 

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

In the United States, Europe, and Japan, DCPD streams of 70—95 wt% purity are available. Estimates of recoverable DCPD production capacity in the United States for 1990 for all grades of DCPD is >127, 000 metric tons (39) and in Europe is 48,000 metric tons (40). The vast majority of this production is from hydrocarbon steam-cracking operations. Based on the total operations, more CPD is produced than indicated above, but because of the relatively small quantities available at a single location, much of the cyclopentadiene caimot be recovered profitably. Important producers in the U.S. are Dow, Exxon, LyondeU, SheU, and Texm ark in Europe, Dow and SheU and in Japan, Nippon Zeon (40). 

Cyclopentadiene, b.p. 40°, is obtained by heating commercial 85% dicyclopentadiene (e.g., from Matheson, Coleman and Bell Company, Norwood, Ohio) under a short column (M in. diameter X 8-12 in. length) filled with glass helices. The distilled cyclopentadiene is collected in a receiver which is maintained at Dry Ice temperature until the cyclopentadiene is used. Methylcyclopentadiene and other substituted cyclopentadienes such as indene may also be employed for the synthesis of the correspondingly substituted ferrocenes. In these cases, the reaction of the hydrocarbon with sodium is much slower than with cyclopentadiene, and refluxing for several hours is required to complete the reaction. 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

With a p fa of 16, cyclopentadiene is only a slightly weaker acid than water (pA a = 15.7). It is much more acidic than other hydrocarbons—its for ionization is 10 ° times greater than acetylene, for example—because its conjugate base is aromatic and stabilized by electron delocalization. 

The pKa of 1,3-cyclopentadiene is 15, making it more acidic than water, as well as more acidic than almost any other hydrocarbon. This unusual acidity is presumably due to resonance stabilization of the conjugate base, which can be drawn as a hybrid of five resonance contributors. 

In practice, both the cyciopentadienyl cation and the radical are highly reactive and difficult to prepare. Neither shows any sign of the stability expected for an aromatic system. The six-77-electron cyciopentadienyl anion, by contrast, is easily prepared and remarkably stable. In fact, cyclopentadiene is one of the most acidic hydrocarbons known, with p/C, = 16, a value comparable to that of water Cyclopentadiene is acidic because the anion formed by loss of H+ is so stable (Figure 15.5). 

Butadiene-1,3 and Cyclopentadiene.—The value 1.46 A. for the single bond between conjugated double bonds in butadiene-1,3 and cyclopentadiene has been discussed already in connection with the values found in other hydrocarbons containing conjugated systems.16 Penney s10 predicted value for the conjugated bond in butadiene, 1.43 A., appears to be a little too low. 

Rideout and Breslow first reported [2a] the kinetic data for the accelerating effect of water, for the Diels Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile and the cycloaddition of anthracene-9-carbinol with N-ethylmaleimide, giving impetus to research in this area (Table 6.1). The reaction in water is 28 to 740 times faster than in the apolar hydrocarbon isooctane. By adding lithium chloride (salting-out agent) the reaction rate increases 2.5 times further, while the presence of guanidinium chloride decreases it. The authors suggested that this exceptional effect of water is the result of a combination of two factors the polarity of the medium and the... 

The diastereoselectivity of the cycloaddition of cyclopentadiene with methyl acrylate in SC-CO2 at 40 °C and subcritical liquid CO2 at 22 °C is practically the same endojexo = 75 25 and 76 24 respectively) and is comparable to that found in hydrocarbon solvents (73 27 and 75 25 in heptane and cyclohexane, respectively). This shows that CO2, in these states, behaves like an apolar solvent with very low polarizability [82]. 

Kuhn s carbanion, the all-hydrocarbon anion tris(7//-dibenzo[c,g]-fluorenylidenemethyl)methanide ion [2 ] (Kuhn and Rewicki, 1967a,b), is a stabilized system with tt electrons widely spread over the sp hybridized carbon framework, and was isolated as the potassium salt. It also appears in DMSO solution by dissolving the parent hydrocarbon, resulting in a deep green colour. The pKg value of the precursor hydrocarbon [2]-H is 5.9 in aqueous HCl-DMSO (Kuhn and Rewicki, 1967a,b), and its enormous stability, as compared with cyclopentadiene [9]-H, pKa 18 in DMSO... 

The Diels-Alder reaction is one of the most important methods used to form cyclic structures and is one of the earliest examples of carbon-carbon bond formation reactions in aqueous media.21 Diels-Alder reactions in aqueous media were in fact first carried out in the 1930s, when the reaction was discovered,22 but no particular attention was paid to this fact until 1980, when Breslow23 made the dramatic observation that the reaction of cyclopentadiene with butenone in water (Eq. 12.1) was more than 700 times faster than the same reaction in isooctane, whereas the reaction rate in methanol is comparable to that in a hydrocarbon solvent. Such an unusual acceleration of the Diels-Alder reaction by water was attributed to the hydrophobic effect, 24 in which the hydrophobic interactions brought together the two nonpolar groups in the transition state. 

---