Cyclopentadiene formation

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]. 

The kinetic curves in Figure 4.4 show that the cyclopentene consumption rate decreases insignificantly after reaching its maximal yield. This testifies to an insufficient concentration of H202 at the stage of cyclopentadiene formation. The kinetic curves of cyclopentadiene accumulation are S-shaped, which indicates autocatalytic type of the process with an autoacceleration period from the beginning of these curves to inflection points. 

Table 7. Vinylallene vs. Cyclopentadiene Formation in the Doering-Moore-Skattebol Reaction Br Br...

Carboxylate anions are better nucleophiles for allylation. The monoepoxide of cyclopentadiene 343 is attacked by AcOH regio- and stereoselectively via tt-aliylpalladium complex formation to give the m-3,5-disubstituted cyclopen-tene 344[212]. The attacks of both the Pd and the acetoxy anion proceed by inversion (overall retention) to give the cis product. 

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

Diels-Alder reaction of 2-bromoacrolein and 5-[(ben2yloxy)meth5i]cyclopentadiene in the presence of 5 mol % of the catalyst (35) afforded the adduct (36) in 83—85% yield, 95 5 exo/endo ratio, and greater than 96 4 enantioselectivity. Treatment of the aldehyde (36) with aqueous hydroxylamine, led to oxime formation and bromide solvolysis. Tosylation and elimination to the cyanohydrin followed by basic hydrolysis gave (24). 

If pure monomer is to be used ia a reactioa, it must be used iaimediately or stored at < — 20° C to preveat dimerization to any appreciable extent. Chemical inhibition does not prevent dimerization low temperature is preferred. If the monomer has to be stored for more than a few hours, it must be protected against oxygen to prevent peroxidation and polymer formation. Cyclopentadiene monomer reacts spontaneously with oxygen of the air to form brown, gummy peroxide-containing products. 

The Diels-Alder reaction of a diene with a substituted olefinic dienophile, e.g. 2, 4, 8, or 12, can go through two geometrically different transition states. With a diene that bears a substituent as a stereochemical marker (any substituent other than hydrogen deuterium will suffice ) at C-1 (e.g. 11a) or substituents at C-1 and C-4 (e.g. 5, 6, 7), the two different transition states lead to diastereomeric products, which differ in the relative configuration at the stereogenic centers connected by the newly formed cr-bonds. The respective transition state as well as the resulting product is termed with the prefix endo or exo. For example, when cyclopentadiene 5 is treated with acrylic acid 15, the cw fo-product 16 and the exo-product 17 can be formed. Formation of the cw fo-product 16 is kinetically favored by secondary orbital interactions (endo rule or Alder rule) Under kinetically controlled conditions it is the major product, and the thermodynamically more stable cxo-product 17 is formed in minor amounts only. 

Cyclooctanone, condensation with diethyl carbonate, 47, 20 Cyclopentadiene, adduct formation with 1,2,3-benzothiadiazole 1,1-diox-ide, 47, 8... 

The preparation of cyclopentadienes with up to four trimethylsilyl groups can be performed easily on a large scale starting with monomeric cyclopentadiene by repeated metalation with n-butyllithium and treating the resulting anion with chlorotrimethylsilane [84], Any complication caused by formation of regioisomers does not occur, since all trimethylsilyl-substituted cyclopentadienes are fluxional by virtue of proto- and silatropic shifts [85], Upon deprotonation with n-butyllithium the thermodynamically most favorable anion is formed selectively (Eqs. 20, 21). Thus, metalation of bis(trimethylsilyl)cyclopentadiene 74, which exists preferentially as the 5,5-isomer, selectively affords the 1,3-substituted anion 75. Similarly, tris(trimethylsilyl)cyclopentadiene 76, which is found to be mainly as the 2,5,5-isomer, affords the 1,2,4-substituted anion 77. 

Ahern and Gokel (1979) briefly mention that (jE -arenediazocyanides also react with a variety of dienes (cyclopentadiene, cyclohexadiene, butadiene, ( )-piperylene, etc.) in a [4+ 2]-cycloaddition reaction with formation of tetrahydropyridazines (Scheme 6-31). Here the two azo nitrogen atoms of the diazocyanide react as a dieneophile in a bis-aza Diels-Alder reaction. 

Ethylene carbonate, reaction with potassium thiocyanate, 42, 59 Ethylene sulfide, 42, S9 Ethyl formate, reaction with cyclo-hexylamine, 41, 14 Ethyl- -hexylamine, 43, 47 5-(2-Ethylhexyl)-l,2,3,4,5-pentachloro-cyclopentadiene, 43, 93 Ethylhydrazinium hydrogen oxalate,... 

Scheme 13 Possible mode of formation of the cyclopentadiene 61 isomeric with 60a by 1,2-migration of the dimethylamino group via a bridged zwitterionic intermediate 62 [44]...

The simple cyclopropylmethoxycarbenechromium complex 142 reacts with alkynes to afford cyclopentenones 143 and 144 via the cyclopentadiene intermediate 145, which is hydrogenated with the aid of the chromium(O) residue and water (Scheme 31) [100-103]. Formation of 145 can be regarded as... 

Scheme 38 Formation of five-, six-, and seven-membered cycloalkane-annelated ethoxy-cyclopentadienes 175 from the phenylethynylcarbene complex 173 and cyclic enamines 174 [119,120]...

Titanoxy alkenylcarbene complexes have been used as dienophiles in their reaction with cyclopentadiene to give predominantly the exo cycloadduct in high yield. The unexpected formation of the exo isomer is attributed to the... 

The cyclopentadiene having substituents of C CR and C N are typical examples of the former case ,i homo)- Theoretical calculation showed that the diene 13 is expected to react with highly syn tr-facial selectivity [15], Experimental smdy of the selectivity in the reactions of the pentamethylcyclopentadiene derivatives 18-21 is particularly of interest (Scheme 11). Exclusive formation of syn attack product in the reaction of the diene 18 is well consistent with the prediction [16]. In the case of the dienes 19-21, the efficiency of the orbital mixing mediated by or orbital was dependent on the conformation of the substituents. However, the selectivity observed was well consistent with the theory. In the reactions of the dienes 19 and 20 where considerable formation of syn... 

Secondary orbital interaction had been proposed to explain predominant formation of endo attack prodncts in Diels Alder reaction of cyclopentadiene and dienophiles by Hoffmann and Woodward [22]. According to this rnle, the major stereoisomer in Diels-Alder reactions is that it is formed through a maximum accumulation of double bonds. In the Diels-Alder reactions, secondary orbital interaction consists of a stabilizing two-electron interaction between the atoms not involved in the formation or cleavage of o bonds (Scheme 19). 

Another case in which bonds were made where none existed previously is that of irradiation of AsClj in benzene solution (76) which led to formation of PhAsCl2 and PhjAsCl, and even PhjAs. Yet another example 51) is the formation of FeCpj by irradiation of Fe(CO)j in cyclopentadiene (monomer) solution. While these examples may not tell us when the reaction does occur, they do show that initial bonding is not indispensable. 

Ruthenium complexes have also been reported as active species for enan-tioselective Diels-Alder reactions. Faller et al. prepared a catalyst by treatment of (-)-[( ] -cymene)RuCl(L)]SbF6 with AgSbFe resulting in the formation of a dication by chloride abstraction [95]. The ligand was (-l-)-IndaBOx 69 (Scheme 36) and the corresponding complex allowed the condensation of methacrolein with cyclopentadiene in 95% conversion and 91% ee. As another example, Davies [96] prepared the complex [Ru(Fl20)L ( i -mes)] [SbFe]2 (with 70 as L in Scheme 36), and tested its activity in the same reaction leading to the expected product with similar activity and lower enan-tioselectivity (70%). 

---