Nonclassical Structure

Many other cations besides the norbomyl cation have nonclassical structures. Scheme 5.5 shows some examples which have been characterized by structural studies or by evidence derived from solvolysis reactions. To assist in interpretation of the nonclassical stmctures, the bond representing the bridging electron pair is darkened in a corresponding classical stmcture. Not surprisingly, the borderline between classical stmctures and nonclassical stmctures is blurred. There are two fundamental factors... 

The structure of nonclassical carbocations, such as norbomenyl 3, has been the subject of debate since the 1950s when Saul Winstein published his milestone studies on the solvolysis of tosylated norbomenyl compounds. It was proposed that the norbomenyl cation should be represented as the nonclassical structure 4+, with a 3-center, 2-electron cyclic system (3c-2e), rather than as the classical equilibrium... 

Selenolo[3,2-h]selenophene (12) and selenolo[2,3-b]selenophene (13) have been synthesized from lithium derivatives of 2-(3-bromo-2-selienienyl) 1,3-dioxane and 2-(3-selenienyl) 1,3-dioxane, respectively, by reaction with selenium and methyl chloroacetate followed by Dieckmann cyclization.46 Even the third classical selenophthene (11) has been synthesized by two different routes, using 2,3-bischloromethyl-5-carbomethoxyselenophene (14) or preferably 4-methylseleno-3-selenophene aldehyde (IS).46 The fourth selenophthene isomer (16), which has a nonclassical structure, has not yet... 

Bicyclic systems such as trithiapentalene 61 or thia-aza-pentalenes 62 (Scheme 30) show significant charge separation. Alternative representations for the above systems include nonclassical structures 61b,c and 62c with S(IV). Density functional calculations showed that the stability and aromaticity of bis-... 

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

The combination of steps 1 and 2 corresponds to a 1,2 hydride shift (R = H) or a Wagner-Meerwein rearrangement (R = alkyl). The intermediate bridged nonclassical structure... 

The C—H—C bond is not linear, the angle being about 170° according to high-level MO calculations. Several bridged cycloalkyl carbocations of the type 2 have been prepared [236]. Complexes between a number of alkyl cations and alkanes have been detected in mass spectrometric experiments [235]. The nonclassical structure of the ethyl cation, 3, may be cited as another example of hydride bridging (for a discussion, see ref. 55). 

Olah and co-workers represented the nonclassical structure as a corner-protonated nor-tricyclane (52) the symmetry is better seen when the ion is drawn as in 53. Almost all the... 

On the basis of Hiickel s An +2) n- electron rule, all of these systems can be expected to be aromatic in nature. They do indeed exhibit varying degrees of aromatic stabilization depending on the nature and position of the heteroatom. They cannot all be represented by conventional classical structures. Structures (la)-(lc) can be represented by classical covalent bonded structures whereas those of the (Id) type form the nonclassical structures in the sense that they can be drawn only as charge-separated systems or biradicals in systems wherein X/Y are sulfur or selenium atoms, d- orbital participation in bonding is conceivable, leading to tetravalent sulfur or selenium. 

The 1-methylcyclobutyl cation 8, which can also be considered as equilibrating 1-methylcyclopropylcarbinyl cations, was shown to have a nonclassical structure by 13C NMR spectroscopic studies in conjunction with Saunders isotopic perturbation of equilibria techniques. 

Extensive theoretical studies have been carried out in order to rationalize the rearrangement pathways in the cationic system92. The characterization of the potential surface of C9H9+ cations93 at MP2-, MP3- and MP4(SDQ)/6-31G levels showed that the open 9-barbaralyl cation 83a is more stable than the completely charge delocalized (nonclassical) structure of Z)1h symmetry, and the bicyclo[3.2.2]nona-3,6,8-trien-2-yl cation 83b by 6.9... 

Heilbroner and co-workers found the UV spectra of A-protonated 1,2-diazocine 8 and M-protonated (Z)-azobenzene were very similar. This and other spectroscopic and chemical evidence indicated that upon protonation (Z)- and ( )-azobenzene retain their configuration and are classical rather than bridged or nonclassical cations (60HCA1890). Haselbach and co-workers studied protonated 8 (and other azo compounds) using electron spectroscopy for chemical analysis (ESCA) and concluded that the cation has classical structure 69 as opposed to a nonclassical structure (70) (72HCA705). 

Preparation of Spirocyclic-P-Lactams with Nonclassical Structure. 4... 

Preparation of Fused 0-Lactams with Nonclassical Structure 3.1 Using Cycloaddition Reactions... 

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