Cyclopropenes 1,3,5-cycloheptatriene

Diazomalonic esters serve as intermediates for the synthesis of a wide variety of compounds including cyclopropanes, cyclopropenes, cycloheptatrienes, sulfur ylides, lactones, and substituted malonates... 

An interesting cycloheptatriene (182) synthesis has been described using thiophene 1, 1-dioxides (180) and cyclopropenes 181 (equation 121)ns. Concerted [4 + 2]cycloaddition and subsequent cheletropic extrusion of sulfur dioxide are suggested by the second-order kinetics (first in each reactant), and by the large negative activation entropy. 

Cyclopropene also adds to less reactive, acyclic, dienes218), though it is worth noting that the reaction with cyclohexadiene only proceeds in 10% yield 219). Addition of a range of alkylcyclopropenes to thiophene dioxides leads to cycloheptatrienes, presumably by cheletropic elimination of sulphur dioxide from the intermediate adduct 220) ... 

In the presence of a bis(ty -cycloocta-l,5-diene)nickel(0)/triaryl phosphite catalyst two molecules of alkyne couple to one cyclopropene moiety yielding substituted norcaradienes and cycloheptatrienes. 

A wide range of 3,3-disubstituted cyclopropenes, e.g. the dimethyl and diphenyl derivatives, as well as spiro[2.4]hept-l-ene or 6,6-dimethyl-4,8-dioxaspiro[2.5]oct-l-ene, can thus be reacted with a variety of mono- and disubstituted alkynes. Usually, the chemoselectivity of this cy-clocotrimerization reaction is remarkably high. The norcaradiene derivatives initially obtained are in equilibrium with the valence tautomeric cycloheptatrienes. The equilibrium ratio of the valence tautomers is strongly dependent on the position and kind of substituents, especially those in the 7-position of the newly formed norcaradienes and cycloheptatrienes. When one or two phenyl groups are present in this position, only norcaradiene products can be detected by and NMR spectroscopy at room temperature, whereas in the case of methyl substituents, both valence tautomers are formed in almost equal amounts. When cyclic alkynes, such as cyclooctyne, are employed in the reaction, only norcaradienes are formed regardless of the substituents present in the cyclopropene cosubstrate. ... 

With monosubstituted alkynes, mixtures of regioisomeric cyclocotrimers are obtained, with the unsymmetrically substituted isomers being formed preferentially. Due to the influence of the cyclopropene substituents (vide supra), in the reaction of cyclopropenes 9 with monosubstituted alkynes 10 the equilibrium of the valence tautomers lies predominantly on the side of the cycloheptatrienes. ... 

Related carbenes, i.e. 3//-benzocycloheptatrien-3-ylidene and 5/7-dibenzo[a,c]cycloheptatrien-5-ylidene generated from the corresponding tosylhydrazones, thermally or photolytically, isomerize to benzo-12 or dibenzobicyclo[4.1.0]hepta-2,4,6-trienes, respectively. In contrast to cycloheptatrienylidene, 5/7-dibenzo[a,c]cycloheptatrien-5-ylidene underwent ring contraction to form a cyclopropene derivative 13 before reaction with alkenes. 

Co-cyclotrimerization of cyclopropenes with alkynes is achieved with phosphane-free cobalt catalysts, e.g. (t/ -cycloocta-l, 5-diene- -cyclooctenyl)cobalt(I), to give tricyclo[5.1.0.0 ]oct-2-ene systems 22 as the predominant products. Norcaradienes and cycloheptatrienes were observed as side products. 

Similar to the addition of halogens, cycloproparenes react under hydrogenation conditions either by initial attack at the cyclopropene bond or via cleavage of one of the lateral cyclopropane bonds. In the former case, the intermediate norcaradiene opens to a cycloheptatriene which is further reduced to cycloheptane. In the latter, the reduction leads to toluene or methyl-cyclohexane. For example, benzocyclopropene, on reaction with platinum oxide in acetic acid, was hydrogenated to cycloheptane (49%) and methylcyclohexane (51%). 1,1-Difluoroben-zocyclopropene, under the same conditions, gave cycloheptane exclusively (85%). Hydrogenation of benzocyclopropene with Raney nickel gave toluene in 90% yield. 

A variety of diverse synthetic methods have been empioyed for the preparation of cyclopropane (1 j. Schlatter and Demjanov and Dojarenko pyrolyzed cyclopropyltrimethylammonium hydroxide at 320°C using platinized asbestos as the catalyst. About equal amounts of cyclopropene (1) and cyclopropyidimethylamine are formed, contaminated with some dimethyl ether and ethylene. Treatment with dilute hydrochloric acid removed the amine from the gas stream and 1 was separated from the other products by gas chromatography. Alder-Rickert cleavage of the Diels-Alder adduct formed from cycloheptatriene and dimethyl acetylenedicarboxylate resulted only in the formation of a polymer and trace amounts of 1. A simple approach by Closs and Krantz based on the synthesis of 1-methylcyclopropene involved the addition of allyl chioride to a suspension of sodium amide in mineral oil at 80°C. Under the conditions employed, 1 could readily escape from the reaction mixture. Though a number of variations were tried, the yield of 1 never exceeded 10%. 

In 1945 Michael J. S. Dewar suggested that the tropylium ion (the cation derived from cycloheptatriene) should also be aromatic (Figure 9). This was confirmed in 1954 since then, the dianion of butadiene and the dication of cyclooctatetraene have also been shown to be aromatic. Like benzene, all four of these ions are planar rings with six tt electrons. According to Hiickel s rule the cyclopropene cation should also exhibit aromaticity, and it does. (In this case n = 0, and 4n + 2 = 2.) The planar anion of cyclononatetraene and the dianion of cyclooctatetraene should also be aromatic (n = 2, and 4n + 2 = 10), and both of them are. 

A stepwise 1,7-vinyl shift was proposed to account for the reaction after carbene addition to make the cyclopropene. Deuterium labeling studies were consistent with this pathway. Interestingly, upon heating to 145°C the phenyl-substituted 5.2.0 tetraene isomerized to 2-phenylindene in what appears to be a cyclization of the cycloheptatriene moiety followed by opening of the bicyclo[2.1.0]pentene and a 1,5-hydrogen shift (Scheme 10.9). 

Cycloheptatrienes were isolated from reactions of 1,2,3-triphenylcyclopropane with cyclopentadienes. Thermolysis of the Diels-Alder adduct of cyclopropene... 

Any neutral monocyclic unsaturated hydrocarbon with an odd number of carbons in the ring must of necessity have at least one CH2 group in the ring and therefore cannot be aromatic. Examples of such hydrocarbons are cyclopropene, cyclopentadiene, and cycloheptatriene. 

In Chapter 2 we showed that aromaticity strongly stabilizes organic structures. Accordingly, large effects are prevalent in acid-base chemistry. Tables 5.6E and I highlight some examples. The most well known is the acidity of cyclopentadiene (pK 16.0), which is similar to that of water. The resulting anion is aromatic. However, when the resulting anion is antiaromatic the compounds are dramatically less acidic, as expected the cyclopropene pKj, is 61 and the pK of cycloheptatriene is 38.8. 

The purported exo-addition of 3,3-difIuoro-l,2-di(trifluoromethyl)cyclopropene to cyclopentadiene has been re-examined, and by using the n.m.r. data of (103)—(105) the major product has been reassigned as the corresponding e/uio-tricyclo-octene. A further claim to [2 + 4] exo-addition of cyclopropene has been made from the reaction between 1,3,3-trimethylcyclopropene and isobenzofuran. Further details on the reaction of cyclopropenes with thiophen 1,1-dioxides to give isomerically pure cycloheptatrienes have appeared. ... 

Cycloheptatriene.—Synthesis. Details have been published on the preparation of specifically substituted cycloheptatrienes by the cycloadditon of cyclopropenes to thiophene-1,1-dioxides followed by expulsion of S02. Substituted tropones have been obtained from the furan adducts of bromo-oxyallyl cations e.g. treatment of l,l,3,3-tetrabromo-4-methylpentan-2-one with di-iron nonacarbonyl in the presence of furan gave adduct (141) which was converted into a-thujaplicin (142). Acid hydrolysis of the norbornyl ketal (143) followed by warming to 70 "C gave 7,7-dimethyl-cycloheptatriene. ... 

Scheme 95 Preparation of methylated cycloheptatrienes from methylated cyclopropenes and thiophene 1,1-dioxide [118]...

Methylated cycloheptatrienes were prepared by cycloaddition of methyl-cyclopropenes and thiophene 1,1-dioxides after expulsion of SO2 the resulting cycloheptatrienes could be used to synthesize tropylium ions (Scheme 95) [118], Benzocyclobutene reacted with thiophene 1,1-dioxides at a relatively mild temperature to give the corresponding benzocyclo-octenes 101 in acceptable yields. For example 2,5-dimethyl-, 3,4-dimethyl-, 2,3,4,5-tetramethyl-, and 2,3,4,5-tetraphenylthiophene 1,1-dioxides gave the corresponding products in 38-78% yields. Elimination of sulfur dioxide took place under the reaction conditions, but the initial adducts 100 were not observed (Scheme 96) [119],... 

Among [4 + 2] cycloadditions of acetylenes is addition of (108) to the diazanorcaradienes (109) to yield cycloheptatriene derivatives (110) diaza-norcaradienes are available by addition of cyclopropenes to 1,2,4,5-tetra-zines. ... 

Carbene chemistry constitutes a particular but challenging field in organic synthesis. Carbenes offer a straightforward access to small rings (cyclopropanes, cyclopropenes) as well as to cycloheptatriene derivatives (the Buchner reaction) from cheap raw material (olefins, acetylenes, benzenic compounds, etc.). 

Subsequently, the electrochemical reduction of a cyclopropenium ion using second harmonic AC voltammetry was studied. This led to an estimated p/C of 61.3 0.5 for cyclopropene as compared to 38.8 0.4 for cycloheptatriene, which corresponds to a difference in acidity of 31 kcal/mol. The calculated gas-phase value (Table 3) is somewhat higher (43 kcal/mol), but the difference will probably be smaller in solution because the small cyclopropenyl anion should be better stabilized by a solvent than is the larger cycloheptatrienyl anion. 

The 4/7 cycloheptatrienide anion was obtained by the treatment of 7-methoxycycloheptatriene with K-Na alloy in THE at —20 °C and has high reactivity. NMR spectroscopy of some monosubstituted derivatives showed that they have nonplanar structures, in contrast to the cation. The AMcid of cycloheptatriene is 375 3 kcal/mol, making it 34 3 kcal/mol more acidic than ethylene. The calculated values in Table 3 are in good agreement with this difference. This is in sharp contrast with cyclopropene, which is 11 kcal/mol less acidic than ethylene. A number of calculations have been reported for cycloheptatrienyl anion, as well as studies of its formation and reactions in the gas phase. ... 

---