Cyclopropenyl cation structure

Quasi-aromatic structures are also known in which the stabilised cyclic species is an ion, e.g. the cycloheptatrienyl (tropylium) cation (15, cf. p. 106), the cyclopentadienyl anion (16, cf. p. 275), both of which have 67te (n = 1), and even more surprisingly the cyclopropenyl cation (17, cf. p. 106) which has 2ne (n = 0) ... 

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... 

Structures that are also aromatic are the cyclopropenyl cation (2 jt electrons n = 0) and the cyclopentadienyl anion (6 n electrons n = 1). Although we do not wish to pursue these examples further, they are representative of systems where the number of jr electrons is not the same as the number of carbon atoms in the ring. 

In 1997, Jemmis, Schleyer and coworkers studied the structure and energetics of lithi-ated cyclopropenyl cation and their acyclic isomers using ab initio MO (HF/6-31G ) and density functional theory (DFT, B3LYP/6-31G ) methods . Successive lithiation results in... 

The structurally characterized derivative of the diphosphirenium cation 12 is a known species [112-116], The gas-phase synthesis and pentaquadrupole MS characterization, and CID (with argon) of the first azaphosphirenium cation 13 (the N,P analogue of the cyclopropenyl cation) were also reported recently [117] (Scheme 14). Although this compound has not been characterized structurally, the similarity of the B3LYP/6-311++G calculated PC and CN(exocyclic) bond lengths of 13 to... 

The stability of these cations is enhanced by the amino substituent, which is also known to increase considerably the stability of the cyclopropenyl cation [118]. The 1.69 and 1.74 A PC bond length of 13 (calculated) and 12 (measured), respectively [118], are clearly longer than the 1.63 A value of the neutral 27/-phos-phirene [119], whereas the CN bond length is clearly in the CN double-bond range. Thus, for description of the bonding in 12 and 13 the b-type mesomeric structure was preferred [118],... 

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

Figure 5.50 shows three related molecules, the 7-methyl substituted (the visual orbital progression explained here is not quite as smooth for the unsubstituted molecules) derivatives of the 7-norbomyl cation (a), the neutral alkene norbomene (b), and the 7-norbomenyl cation (c). For each species an orbital is shown as a 3D region of space, rather than mapping it onto a surface as was done in Fig. 5.49. In (a) we see the LUMO, which is as expected essentially an empty p atomic orbital on C7, and in (b) the HOMO, which is, as expected, largely the n molecular orbital of the double bond. The interesting conclusion from (c) is that in this ion the HOMO of the double bond has donated electron density into the vacant orbital on C7 forming a three-center, two-electron bond. Two n electrons may be cyclically delocalized, making the cation a bishomo (meaning expansion by two carbons) analogue of the aromatic cyclopropenyl cation [326], This delocalized bishomocyclopropenyl structure for 7-norbomenyl cations has been controversial, but is supported by NMR studies [327].

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. 

Problem 10.26 Design a table showing the structure, number of n electrons, energy levels of n MO s and electron distribution, and state of aromaticity of (a) cyclopropenyl cation, (b) cyclopropenyl anion, (c) cyclobutadiene, (d) cyclobutadienyl dication, (e) cyclopentadienyl anion, (/) cyclopentadienyl cation, (g) benzene, (h) cycloheptatrienyl anion, (/ ) cyclooctatetraene, (j) cyclooctatetraenyl dianion. M... 

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.)... 

The cyclopropenyl cation is the simplest aromatic system, and thus of some theoretical interest. In addition, the chemistry of cyclopropane derivatives is full of Interesting rearrangements to other novel structures,9 reflecting the great strain energy Of the cyclopropene ring. 

Thus there is only one possibility for such a CjH, ion, the cyclopropenyl cation [340]. For the C,H group of ions there are three regular structures the cyclopentadienyl cation [342], the bicyclo[2.1.0]pentenyl cation [343] and the bicyclic homotetrahedryl cation [344]. Only a few of the many possible C,H, C,H and cations will be considered. 

This carbocation has to have a monocyclic structure and is the simplest potentially degenerate (CH) carbocation. It can either be a degenerate cyclopropenyl cation [339] rearranging as in (221) or a static aromatic 2 t-electron structure [340], i.e. the simplest aromatic system (222). The ion... 

Aromatic systems that obey Hiickel s 4 + 2 rule where = 0 and so possess two 7i-electrons do exist and are indeed stable. The smallest possible ring is three membered and the derived unsaturated structure is cyclopropene. The theoretical loss of a hydride ion from this molecule leads to the cyclopropenyl cation, which contains two 7t-electrons distributed over the three carbon atoms of the planar cyclic system (Figure 1.8). 

As has been discussed briefly in Section III above, the ring strain of the cyclopropenes is accommodated, in part, by somewhat unusual bonding to the substituents. Structural and spectroscopic studies suggest that the vinylic C-H bonds have ca. 44% s character and that this is approximately constant in bonding to other substituents. Consequently aspects of the chemistry of the cyclopropenes are expected to reflect this with enhanced acidity of the vinylic proton. Cleavage of an exocyclic bond at C(3) will result in the cyclopropenyl cation, radical or anion with consequent interaction with the n bond and perturbation of the energy of the system. By comparison addition to the n-system to yield a cyclopropane relieves ring strain by ca. llOkJmol" ... 

Cyclopropenone (27) and its derivatives are expected to share some of the aromatic stability of the cyclopropenyl cations, to which they are related by structure 27a. It is not... 

IUPAC nomenclature is generally followed . The cyclic structures are called heterometallacycles and dimetallacycles. When the ring is composed of a metal, a nonmetal, and a carbon, the rings are numbered in that order. When two metals are present, the higher atomic number metal takes precedence. Metallacyclic three-rings which contain a double bond may possess cyclopropenyl cation-like aromaticity and such structures have been proposed <84AG(E)89>. 

Another consequence of the donor/acceptor properties of the lithium atom is the dramatic stabilization by 37 kcal/mol (155 kJ/mol) of the cu-planar dilithiomethane 3b (cis) compared with the rra s-planar structure 3b (trans). In contrast to 3b (trans) the lithium atoms in 3b (cis) c n interact electronically to form together with the p orbital at the carbon atom a (4n + 2)-Huckel system (n = 0) isoelectronically to the cyclopropenyl cation 6 ... 

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