1,3-Cyclopentadiene, cycloaddition with dimerization

Thiophene 1,1-dioxide did not undergo cycloaddition with electron-deficient dienophiles. In most of the cases the dihydrobenzothiophene derivative 109 was obtained as the major product. This shows that self-dimerization is faster than cycloaddition with a different molecule. In the case of dimethyl acetylenedicarboxylate (DMAD) and 4-phenyl-3//-l,2,4-triazole-3,5(4//)-dione (PTAD), the Diels-Alder adducts 111 and 112 of 109 were obtained <1997JA9077>. However, cyclopentadiene gave the Diels-Alder adduct 113 with thiophene 1,1-dioxide. The DMAD adduct 111 on thermolysis undergoes a retro-Diels-Alder reaction to give dimethyl phthalate and thiophene 1,1-dioxide. Azulene was isolated in the thermolysis of 108 in the presence of 6-(dimethylamino)-fulvene this was the result of a [4-1-6] cycloaddition of the thiophene 1,1-dioxide formed in the reaction followed by elimination of SO2 and dimethylamine (Scheme 28) <1999BCJ1919>. 

The data listed in Table 10.17 give clear evidence of the enormous rate acceleration in aqueous solution, in accordance with all previous kinetic reports [51, 61, 72), particularly for the corresponding cyclopentadiene cycloadditions [73]. The highest values for the kinetic ratio ks/fcref (> 1000) are observed for reactions involving the least hydrophilic molecules (HCCP -1- styrene, isoprene I-methyl acrylate, dimerization of isoprene). Intermediate values are found in reactions involving ketones. 

Cyclopentadiene is a very reactive diene and exists as its dimer that needs to be cracked (retro-Diels-Alder reaction) to prepare the diene. Cycloaddition with dienophiles forms bridged compounds of the bicyclo[2.2.1]heptane series. The reaction of cyclopentadiene with mono-and cw-disubstituted alkenes could give rise to two stereochemically distinct products, the endo- and ejco-bicyclo[2.2.1]heptene derivatives. It is found in practice, however, that the endo isomer predominates. 

Low-temperature trapping experiments of the photolysis products of disilacycloheptene (75), which was prepared by insertion of ethyne into the Si—Si bond of l,l,2,2-tetramethyl-l,2-disilacyclopentane <75JA931>, provided evidence for the trans isomer of (75) <90JA6601>. GC-MS analyses indicated that, as the decay of trans-(75) progressed, a mixture of six dimers of mjz 368 was formed. Only one of the dimeric products was successfully isolated in pure form by preparative GC this dimer was assigned the unsymmetrical c/s,fra 5-fused cyclobutene structure (76) on the basis of NMR data. Trapping experiments of metastable trans-(75) by Diels-Alder cycloadditions with cyclopentadiene and 9,10-dihydro-l l,12-dimethylene-9,10-ethanoanthracene afforded (77) and (78), respectively. The crystal and molecular structures of (78) were determined. 

Pentalenes.—The X-ray structure of the tri-t-butylpentalene (60) has been determined the compound is planar and the double bonds are localised. Pentalenes (62), together with dimers, are produced by the action of acetylenes RC=CH (R = C02Me, CHO, or CN) on l,3-di-t-butyl-6-dimethylamino-fulvene (61). The aldehyde (62 R = CHO) dimerized to yield mainly compound (63), whereas the head-to-tail dimer (64) was isolated from the cyano-derivative (62 R = CN). The latter adds water under acidic conditions to give the alcohol (65), while dimethylamine affords the rearranged adduct (66). Cycloaddition reactions of the pentalene di-ester (67) proceed differently with dicyanoacetylene and cyclopentadiene the former yields the cyclobutene derivative (68), the latter the Diels-Alder product (69). ... 

For a discussion of the mechanistic course of the reaction, many aspects have to be taken into account. The cisoid conformation of the diene 1, which is in equilibrium with the thermodynamically more favored transoid conformation, is a prerequisite for the cycloaddition step. Favored by a fixed cisoid geometry are those substrates where the diene is fitted into a ring, e.g. cyclopentadiene 5. This particular compound is so reactive that it dimerizes easily at room temperature by undergoing a Diels-Alder reaction ... 

In contrast to those unreactive dienes that can t achieve an s-cis conformation, other dienes are fixed only in the correct s-cis geometry and are therefore highly reactive in the Diels-Alder cycloaddition reaction. 1,3-Cyclopentadiene, for example, is so reactive that it reacts with itself. At room temperature, 1,3-cycIopentadiene dimerizes. One molecule acts as diene and a second molecule acts as dienophile in a self Diels-Alder reaction. 

Cyclic enones with ring sizes of six-to-eight carbons can be photochemically induced to undergo [4+2] cycloadditions via isomerization to a strained trans isomer (Schs. 22-24). Irradiation of 2-cycloheptenone 99 leads to [2+2] dimerization of an intermediate r -2-cycloheptenone 100, but if this irradiation is conducted with an excess of cyclopentadiene 32 at —50 °C, a single [4+2] adduct 101 is isolated in very high yield [65,66]. The somewhat less strained nms-2-cyclo-octenone can be generated and trapped by subsequent addition of a cyclopentadiene [67,68]. Extension of this reaction to intramolecular examples has recently been reported [69]. 

The reaction of adamantylidene cyclopentadiene with 4-phcnyl-(37/)-l,2,4-triazole-3,5(47/)-dione gives different products depending on the reaction conditions14. At — 78 °C the [4 4- 2] cycloadduct is initially formed (> 98 % yield) ( H NMR), but is then converted at higher temperature to the [6 + 2] cycloaddition product, which in turn undergoes dimerization and/or the [4 -I- 2] addition of a second molecule of diazene. Either the dimer (X-ray) or the 2 1 cycloadduct ( h NMR) are obtained cleanly by using the proper ratio of the reagents. 

With methyl 2-alkylcycloprop-2-enecarboxylates, on the other hand, only [2-1-1] cycloaddition to the tricyclic dimers of cyclopentadiene (18) occurs. Only the strained double bond in the norbornene substructure is cyclopropanated, and the exo-adducts are formed. The Z/E isomeric ratios (6 1 and 2.5 1) are comparable to the ratios obtained in the reactions with norbornadiene vide supra). A mechanism proposing ring opening of the cyclopropene and attack on the metal-coordinated alkene in a concerted way has been suggested in order to explain the stereoselectivity of the reaction. ... 

Yet another rather general approach towards highly substituted cyclopenta-dienones has been developed via /1-aminosubstituted a,/ -unsaturated chromi-umcarbene complexes such as 32. Their cycloaddition to alkynes proceeds without carbonyl insertion to yield 1,2,3-trisubstituted 5-dimethylamino-3-ethoxy-cyclopentadienes which are readily hydrolyzed to the correspondingly substituted cyclopentenones [26]. After quaternization with methyl iodide the ammonium salt 33 is obtained. Treatment of the latter with a base such as NaOMe or NaOH results in deprotonation and elimination of trimethylamine to yield a trisubstituted cyclopentadienone which immediately dimerizes by [2 + 2] cycloaddition probably via a 1,4-diradical intermediate like 35 to yield the bis(cyclo-pentenone)-annelated cyclobutane derivative 34 (Scheme 8) [27]. 

Reports of studies on the reactions of diphosphenes have begun to appear. Like (137), (140) is stable towards methanol but is cleaved by hydrogen chloride. The less hindered (143) dimerizes at room temperature in a few hours, and gives cycloaddition products with Sg and cyclopentadiene (Scheme 11). Oxidation... 

Bicyclic 1,3-dioxepanes (8) and (10) were prepared in a stereoselective manner by direct acetalization of aldehydes and ketones with diols (7) and (9 X = H), or chlorendic diol (9 X = Cl) (Scheme 2) <75BSF1763,76USP3984438>. For another approach to dioxepane (10) by [4 4-2] cycloaddition of 2-ethyl- or 2-isobutyryl-4,7-dihydro-1,3-dioxepin with cyclopentadiene dimers see Section 9.11.1.2.2. Acetalization of hexafluoroacetone with 1,4-butanediol to 1,3-dioxepane (11) has been effected with dicyclohexyl carbodiimide as condensating agent <87MI 9ll-0l>. 

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