Cyclopropenyl cation reactions

Alkylation of enamines with epoxides or acetoxybromoalkanes provided intermediates for cyclic enol ethers (668) and branched chain sugars were obtained by enamine alkylation (669). Sodium enolates of vinylogous amides underwent carbon and nitrogen methylation (570), while vicinal endiamines formed bis-quaternary amonium salts (647). Reactions of enamines with a cyclopropenyl cation gave alkylated imonium products (57/), and 2-benzylidene-3-methylbenzothiazoline was shown to undergo enamine alkylation and acylation (572). A cyclic enamine was alkylated with methylbromoacetate and the product reduced with sodium borohydride to the key intermediate in a synthesis of the quebrachamine skeleton (57i). 

Tapia, O., Andres, J. and Cardenas, R. Transition structure for the hydride transfer reaction from formate anion to cyclopropenyl cation a simple theoretical model for the reaction catalyzed by formate dehydrogenase, Chem. Phys. Lett, 189 (1992), 395-400... 

The tropylium cation (274) first observed 1891 and rediscovered in 1957 is perfectly stable and isolable. Cyclopropenyl cations have been observed in solution a long time ago, but 273 remained elusive until very recently. Benzocyclo-propene (1) reacts with triphenylfluoroborate via hydride transfer some 5 times less rapidly than cycloheptatriene. The reaction of deuterated 1 exhibits a kinetic isotope effect of 7.0. However, only a low yield of benzaldehyde (277), the expected hydrolysis product of 273, could be isolated from the reaction mixture. ... 

The course of these additions of lithium hydride resembles that found for the addition of borane (Nagase et al., 1980 Graham et al., 1981). With ethylene, the initial step is exothermic formation of a Jt-complex without barrier, then rate-determining transformation to the borane via a four-centre transition structure. In both the borane and lithium hydride additions, there is relatively little development of the new C—H bond with distances of 1.692 and 1.736 A respectively in the transition structures. When a carbanionic product is not formed, for example in the reaction of lithium hydride with cyclopropenyl cation yielding cyclopropene and lithium cation (Tapia et al., 1985), reaction again occurs via a hydride-bridged complex, but the C- H- -Li array remains nearly linear throughout the reaction. 

The product of the reaction of 3-chlorocyclopropene with AgBF4 is the cyclopropenyl cation C3H3+. The resonance structures of the cation indicate that all hydrogen atoms are equivalent, and the NMR spectrum, which shows only one type of hydrogen atom, confirms this equivalence. The cyclopropenyl cation contains two n electrons and is aromatic according to Hlickel s rule. (Here, n = 0.)... 

According to this procedure, with proper choice of reaction conditions, a sizeable amount of diphenylacetylene can be converted quantitatively to triphenyteyclopropenium bromide in a few hours. The reaction is of wide generality and can be applied to p-anisylphenylacetylene and to di-p-anisylacetylene, or with p-anisal chloride instead of a,a-dichlorotoluene. to prepare p-methoxy derivatives of the title compound.2 Since the initial preparation of this derivative of the cyclopropenyl cation by a less efficient procedure,3 many aryl-, alkyl- and heteroatom-substituted derivatives of this simplest cyclic aromatic system have been synthesized,4 including... 

The reaction proceeds via a nucleophilic attack of the ML4 moiety on the carbon atom bearing the cyano groups. An alternative mechanism would involve a possible dissociation of tetracyanocyclopropane (171) to give a biradicaP . Reactions of cyclopropenyl cations... 

Cyclopropene and its deuterium-labelled derivatives can be obtained by the photo-decarbonylation of the corresponding furan at 254 nm but the method is of strictly limited value because of the photolability of many cyclopropenes (Section IV.B.2). West and his coworkers have shown that aryltrihalo- and diaryldihalocyclopropenes are available from classical Friedel-Crafts aromatic substitution reactions employing the cyclopropenyl cation as electrophile. Thus tetrachlorocyclopropene is converted to its derived cation which is then allowed to react with an aromatic compound. The exothermic reaction provides monoarylcyclopropene at low ( 0°C) temperature and the diaryl derivative at higher ( > 50° C) temperature (equation 26). In this way 2-phenyl-1,3,3-trichlorocyclopropene can be obtained in 58 % yield and the p-fluorophenyl analogue in... 

Simple calicenes " are available from the condensation of a cyclopropenone with a cyclopentadiene or an indene (equation 39) or by reaction of a cyclopropenyl cation with a... 

The majority of simple substitutions at C(3) of a cyclopropene involve initial ionization and subsequent nucleophilic capture of cyclopropenyl cation. For example, tetrachlorocyclopropene is converted into its trichloro-3-fluoro analogue by reaction with silver fluoride. In this way the 3-chloro substituent has been replaced by azide , triphenylphosphine and p-dimethylaminopyridine as well as by sulphur and some carbon nucleophiles (equation 44) although the reaction does not necessarily... 

Cation-with-anion reactions are rare in organic chemistry. One known example is a cyclopropenyl cation reacting with a cyclopropenyl anion. The reaction is not even direct C—C bond formation, but first an electron transfer from the anion to the cation, followed by combination of the two radicals, see R. W. Johnson, T. Widlanski and R. Breslow, Tetrahedron Lett., 1976, 4685. 

A radical-based homodesmotic reaction gives a value of 30.4 kcal/mol, which compares with 29.1 kcal/mol for benzene by the same approach. " The gas phase heterolytic bond dissociation energy to form cyclopropenium ion from cyclopropene is 225 kcal/mol. This compares with 256 kcal/mol for formation of the allyl cation from propene and 268 kcal/mol for the 1-propyl cation from propane. It is clear that the cyclopropenyl cation is highly stabilized. 

An ion cyclotron resonance study of ion-molecule reactions in benzene has shown that the cyclopropenyl cation arises from a a-type complex between protonated benzene and benzene. The mass spectral fragmentation of cyclopropanol monoethers has been examined and subjected to theoretical study and the fragmentation of a series of underivatized aminocyclitol-aminoglycoside antibiotics is believed to involve the ion (387). The chemical ionization mass spectrum of oct-6-en-3-one has been interpreted as involving the cyclobutyl-cyclopropylcarbinyl cation rearrangement (388)- (389). ... 

The G2 calculated reaction enthalpy derived by eq 4 for the cyclopropenyl radical is only 8.9 kcal/mol compared to values of 59.1 and —4.3 kcal/mol for the comparable reactions of the cyclopropenyl cation and anion (eq 5), and the much lower stabilization of the radical compared to the cation was interpreted as indicating that the cyclopropenyl radical is not aromatic. The large differences between the three... 

The reactions of cyclopropenone with a wide variety of reagents have been reported by Breslow, Oda, and Pecoraro. " Bromine gives an acyclic product after initial electrophilic attack at the carbonyl oxygen rather than at the double bond. As expected, triethyloxonium tetrafluoroborate gives ethoxycyclo-propenium tetrafluoroborate, and trifluoroacetic acid opens the C-1—C-2 bond, but this could conceivably occur after initial addition to the C-2—C-3 double bond. A cyclopropenyl cation is also a proposed intermediate in the... 

The stable triphenylcyclopropenium cation (81) undergoes an electron-transfer reaction when photolyzed in acidic medium (van Tamelen et al., 1968, 1971). Irradiation of 81 for 4 hours in 10% aqueous sulfuric acid resulted in a 49% yield of hexaphenylbenzene (82). The reaction is presumed to proceed by initial charge transfer to produce the cyclopropenyl radical 83, which then couples to give 84. This compound in... 

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