Bicyclo pentadiene

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. 

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. 

Scheme 6.30 Structure ofthe trapping products of bicyclo[3.2.1]-octa-2,3-diene (123) with 1,3-cyclopentadiene, (Z)-l, 3-pentadiene and 2,3-dimethyl-l, 3-butadiene and ofthe dimers of123.

However, the fact that dichlorocarbene reacts with methyl bicyclo[l. 1.0]butane-l-carboxylate (4) to generate exclusively pentadienes 5 and 6 in a total yield of 41 % renders this type of reaction impracticable for the synthesis of bicyclo[l.l.l]pentanes.3 Numerous examples have indeed shown that the reaction between bicyclo[1.1.0]butanes and carbenes are synthetically practical routes for pentadiene preparation4 9 and interesting evidence has been obtained to indicate that a concerted mechanism occurs.10-11... 

Exercise 21-22 Unlike the conversion of bicyclo[2.1,0]-2-pentene to 1,3-cyclo-pentadiene, bicyclo[4.1.0]-2,4-heptadiene is transformed to 1,3,5-cycloheptatriene very rapidly at low temperatures by what appears to be a wholly concerted mechanism. Account for this difference. 

In the presence of (+)-ephedrine, ( )-l-acetylcyclo-octene 59dE photoisomerizes to enantiomeric (Z)-isomer, which is trapped by cyclo-pentadiene to give the Diels-Alder adduct 62d in 22% ee (Sch. 21) [118]. Direct irradiation at >280 nm of doubly bridged (Z)-l(10)-bicyclo[8.8.0] octadecen-2-one 87Z in diethyl (+)-tartrate as a solvent affords the (E)-isomer 87E in favor of the levorotatory enantiomer where the EjZ ratio is 7 (Sch. 31) [162]. 

CsHjo Cyclohexene, methylcyclopentene, methylene cyclopentane, bicyclo--[3.1.0]-hexane, 2-methyl-1, 4-pentadiene 16.0-18.1 g, m, p 927... 

Pentadien 2-Ethylthio- V/lc, 356 4-Methyl-3-methylthio- V/lc, 356 Propin 1-Butylthio- V/2a, 200, 202 7-Thia-bicyclo 4.1.0 heptan 2-... 

Bicyclo[2.2.1]heptene 5-Chloro-5-cyan-7-fluoro- E10 (5-Ti —cyclo-pentadien + En/N—F-Comp.)... 

Metbyl 13 pentadiene Cycloh ene Bicyclo[3.1.0jhexane 4 Methyi 2 pentyiie (two double bonds) (one ring, one (two rings) (one triple bond)... 

Bicyclo 4.1.0)hepta-2,4-dien-<7-spiro-5>-cyclo-pentadiene 1198... 

AuBer Butadien und Isopren wurden aueh cis- und fra ts-Pentadien-(l,3) und Cyelo-pentadien an Benzol photoaddiert, doch konnte die Struktur der entstehenden Addukte bisher nicht aufgeklart werden3. Mit 2,3-Dimethyl-butadien-(l, 3) entsteht endo-6-Methyl- sowie exo-6-Methyl-6-isopropenyl-tricydo 3.3.0.0z 8]octeM-(3) und 3,4-Dimethyl-bicyclo[4.2,2]decatrien-(3,7,9). ... 

The isomerization of hydrocarbons by Lewis acids found a surprising use in the hands of Schleyer and Donaldson 156 The action of aluminum halides on bicyclo[5.2.1.02,6]decane (the hydrogenation product of the dimer of cyclo-pentadiene) afforded adamantane by a series of isomerizations ... 

The 8.4 eV photodecomposition of cyclopentene was also studied in the same laboratory. Ethylene, acetylene, and cyclopentadiene are the major products, although a high yield of hydrogen atoms was also observed (94). On the other hand, methylenecyclobutane (d> = 0.04) and bicyclo[2. l.Ojpentane (d> = 0.03) were the only primary products observed in the 184.9 nm photochemistry of n-heptane solution of cyclopentene (95). The formation of 1,4-pentadiene (d> = 0.01) was ascribed to secondary processes. Very recently we have undertaken a systematic study of the 184.9 nm gas phase photochemistry of cyclopentene at pressures from 1 Torr to 6 atm of added propane. Again at low pressure, ethylene (d> = 0.12), acetylene (d> = 0.03), allene (d> = 0.06), and cyclopentadiene (d> = 0.22) are the main products (96). About 80% of the formation of cyclopentadiene involves the elimination of a hydrogen molecule in agreement with the Woodward-Hoffmann allowed 1,4 concerted molecular elimination process (97). Moreover, several isomers are also formed provided... 

Most cyclic and acyclic 1,3-dienes, such as cyclopentadiene, undergo photochemical ring-closure to cyclobutenes. C> clopentadiene-5-d, cyclopentadiene-d , 2-methyl[Pg.93]

The synthesis of cyclopropanes by simple cyclization of 2-halo-1,4-pentadienes has not been achieved, presumably because the product 1,2-dimethylenecyclopropane would not be stable under the reaction conditions necessary for ring closure. However, the formation of bicyclo[n.l.O]alkanes from certain 2-halo-l,3-pentadienes (Sect. IV.2.2.1.C.iii.a) or by a cascade reaction (Sect. IV.3.1) has been achieved. 

These and many other syntheses of the parent and derivatives have been reviewed [10]. The best route to 1 is perhaps reduction of the readily-available[ 1.1.1] propel-lane 2 (below) [11]. Bicyclo[l.l.l]pentane, a liquid with an estimated boiling point of 36°C, is very stable thermally heating it at 305° C for 9 hours gave a 1 1 mixture of the starting compound and 1,4-pentadiene [5, 6],... 

One of the more interesting rearrangements which has received little attention is the retro 2 + 2 reaction of bicyclo[l.l.l]pentane to 1,4-pentadiene. The reaction has log = 15.24 - 49 000/2.47 r (Scheme 6.25). Pyrolysis of the 1,3-dimethyl derivative gives 2,4-dimethyl-1,4-pentadiene with log k = 16.2 — 53000/23RT. 

An alternative pathway for reaction of 1,4-dienes is intramolecular cycloaddition giving bicyclo[2.1.0]pentanes. This pathway is usually not observed, but photolysis of l,5-diphenyl-l,4-pentadiene is an example of a compound that takes this... 

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