Cyclopropenyl anions

Draw an energy diagram for the three molecular orbitals of the cyclopropenyl system (C l I3). How ate these three molecular orbitals occupied in the cyclopropenyl anion, cation, and radical Which of the three substances is aromatic according to Hiickel s rule ... 

Two other systems that have been studied as possible aromatic or antiaromatic four-electron systems are the cyclopropenyl anion (59) and the cyclopentadienyl cation (60). In these cases also the evidence supports, antiaromaticity, not aromaticity. With respect to 59, HMO theory predicts that an unconjugated 61 (i.e., a single canonical form) is more stable than a conjugated 59, so that 61 would... 

Very high level ab initio [CCSD(T)//MCSF] calculations have been applied to singlet and triplet cyclopropenyl anion and cyclopropenyl radical. The anion ground state, a singlet with Cg symmetry, is destabilized relative to cyclopropyl anion as expected for an antiaromatic structure it is stabilized, with respect to its conjugate acid and the corresponding radical, by electron-withdrawing substituents such that 1,2,3-tricyanopropene has a predicted pK of 10-15. ... 

The CH2 group of cycloproparenes is relatively acidic. Extended HUckel calculations predict that the benzocyclopropenyl anion 294 should be a resonance-stabilized species, contrary to the cyclopropenyl anion 295 or cyclohepta-trienide (296), which are at least potentially antiaromatic. This prediction has been experimentally verified benzocyclopropene (1) may be deprotonated with BuLi, and the intermediate anion 294 has been trapped with trimethylsilane to afford 236. From the rate of hydrolysis of 236, the pX of 1 has been estimated to 36, i.e., some 5 units below that of toluene (pXj = 41). Theoretical calculations (STO-3G) give a pK of 33 for 1. Metallation at the CHj group of cycloproparenes is the key step for the synthesis of alkylidenecycloproparenes (see above) however, it should be noted that so far, no benzocyclopropenyl anions have... 

A concrete example will help clarify these concepts. In C3V symmetry, the n orbitals of the cyclopropenyl anion transform according to aj and e symmetries... 

Problem 10.30 Design a table showing the structure, number of tt electrons, energy levels of tt MO s and electron distribution, and state of aromaticity of (a) cyclopropienyl cation, b) cyclopropenyl anion, (c) cyclobutadiene, (d) cyclobutadienyl dication, (c) cyclopentadienyl anion, (/) cyclopentadienyl cation, (g) benzene, (h) cycloheptatrienyl anion, (/) cyclooctatetraene, (/ ) cyclooctatetraenyl dianion. ... 

R = COPh).160 Where R = CN, the ratio is about 10,000.161 This indicates that 55 are much more reluctant to form carbanions (which would have to be cyclopropenyl carbanions) than 56, which form ordinary carbanions. Thus the carbanions of 55 are less stable than corresponding ordinary carbanions. Although derivatives of cyclopropenyl anion have been prepared as fleeting intermediates (as in the exchange reactions mentioned above), all attempts to prepare the ion or any of its derivatives as relatively stable species have so far met with failure.162... 

The chemistry of oxiranyl anions (117) and aziridinyl anions (118) has been reviewed186 and tris-1,2,3-/ -nitrophenylcyclopropcnc has been found to resist conversion to the corresponding anti-aromatic cyclopropenyl anion by deprotonation even though it has been estimated to have a pAa of 32.187... 

The geometries of the ground states and of the first excited singlet and triplet states of 150 with X = SiH-, CH-, NH, O, PH and S were calculated by the Cl version of the semi-empirical SINDOl method167. We will discuss here only the silirenyl anion (150, X = SiH-) and compare the properties of this anion to those of the analogous cyclopropenyl anion (150, X = CH-). The calculated optimized geometries and bond orders for 150, X = SiH- and CH- are given in Table 15. 

TABLE 15. Calculated geometries and bond orders of the silirenyl anion (150, X = SiH ) and the cyclopropenyl anion (150, X = ( II ) in their ground and first excited singlet and triplet states"... 

Another group of unstable carbanions are those with antiaromatic character (Scheme 5.71). Thus, cyclopropenyl anions or oxycyclobutadienes, generated by deprotonation of cyclopropenes or cyclobutenones, respectively, will be highly reactive and will tend to undergo unexpected side reactions. Similarly, cyclopentenediones are difficult to deprotonate and alkylate, because the intermediate enolates are electronically related to cyclopentadienone and thus to the antiaromatic cyclopenta-dienyl cation. 

An ab initio study of the effect of a-substituents on the acidity of cyclopropaben-zene has shown that a-substituents stabilize the cyclopropabenzenyl anion (5) less efficiently than the cyclopropenyl anion (6).2 The attachment of induedvely/field acting substituents attached to the carbanionic site predominantly stabilize the cyclopropenyl anion by increasing the, v character of the lone pair, diminishing the antiaromatic character of the three-membered ring at the same time. 

An overview on cycloproparenyl anions has also been reported.3 According to theoretical calculation, cyclopropabenzenyl anion is by ca 145 kJmol-1 more stable than the parent cyclopropenyl anion. It has been shown that the stability of the cyclopropabenzenyl anion could be considerably enhanced by substitution of the aromatic ring with fluorine and cyano groups, and also by a linear extension of the aromatic backbone. 

Compare the piQ of cyclopentadiene with that of cycloheptatriene. Whilst the anion of the former has 6 7t electrons (which makes it isoelectronic with benzene), the anion of the latter has 8 ti electrons. Remember that on p. 176 we saw how 4n n electrons made a compound anti-aromatic The cycloheptatrienyl anion does have 4 7t electrons but it is not anti-aromatic because it isn t planar. However, it certainly isn t aromatic either and its pKa of around 36 is about the same as that of propene. This contrasts with the cyclopropenyl anion, which must be planar since any three points define a plane. Now the compound is anli-aromatic and this is reflected in the very high pKit (about 62). Other compounds may become aromatic on losing a proton. We looked at fluorene a few pages back now you will see that fluorene is acidic because its anion is aromatic (14 n electrons). 

In this and similar compounds the acetylene bond is supposed to donate only two jt-electrons to the conjugated system while the other jt-bond is located in the plane of the molecule and does not participate in the conjugation. Consequently, this compound satisfies the Hiickel rule for = 4. It indeed possesses aromatic properties. Anti-aromaticism. When a cyclic polyene system is studied it is important to know whether this system is nonaromatic, i.e.not stabilized by conjugation and sufficiently reactive due to the internal tension and other causes, or destabilized by conjugation, i.e. the cyclic delocalization increases the total energy of the system. In the latter case the molecule is called anti-aromatic. Here are typical examples of anti-aromatic systems cyclobutadiene, a cyclopropenyl anion, a cyclopentadi-enyl cation, and others. 

The cyclization in Step B is an improvement of Butler s procedure for the synthesis of which employs less convenient reagents, KNH and l-bromo-3-chloroacetone acetal. Beside the acetals derived from neopentyl glycol, those derived from ethanol, 1,3-propanediol and 2,4-pentanediol have been synthesized by the present method. The second part of Step B involves the formation and the electrophilic trapping of cyclopropenyl anion 2, which is the key element of the present preparations. Step B provides a simple route to substituted cyclopropenones, but the reaction is limited to alkylation with alkyl halides. The use of lithiated and zincated cyclopropenone acetal, on the other hand, is more general and permits the reaction with a variety of electrophiles alkyl, aryl and vinyl halides, Me3SiCl, Bu3SnCl, aldehydes, ketones, and epoxides. Repetition of the lithiation/alkylation sequence provides disubstituted cyclopropenone acetals. 

The effect of the cyclopropene double bond on acidity of the allylic (C3-H) protons is striking in comparison to the situation for the vinyl (Cj-H) protons. As the data in Table 1 reveal, cyclopropene is at least 10 pX units less acidic than cyclopropane. On classical grounds, resonance stabilization of the cyclopropenyl anion (D31, structure. Scheme 1) should provide an acid-strengthening effect however, increased ring strain associated with planarization of the final ring carbon could offset this stabilization. If 7c-conjugative effects are considered unimportant, then by analogy to the cyclopropyl anion the nonplanar Q... 

Efforts directed towards the synthesis of stable, perhaps isolable, cyclopropenyl anions having three identical anion stabilizing groups, such as cyano or benzoyl, have been attempted without much success. Tricyanocyclopropene (22a) could not be prepared except as a transient intermediate which could be trapped with diphenylisobenzofuran The corresponding triketone (22b) was obtainable, but attempted generation of the corresponding anion was thwarted by conjugate (Michael) addition of the base to the double bond. The cyclopropenyl anion remains an illusive species ... 

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