Bisected geometry

Only the bisected conformation aligns the cyclopropyl C—C orbitals for effective overlap. Crystal structure determinations on two cyclopropylmethyl cabons with additional stabilizing substituents, C and D, have confirmed file preference for the bisected geometry. The crystal structures of C and D are shown in Fig. 5.8. 

The a-spirocyclopropylcycloalkyl cations have the ideal bisected geometry of the cyclopropyl group with respect to the cation center. Hence solvolysis of such substrates reveals enormous rate accelerations. Thus the solvolytic rate acceleration of 87, as compared with the analogous a,a-dimethyl system 88, is in the range of 10596. Even the tertiary substrate 89 undergoes enhanced solvolytic rates.as compared with its a,a-dimethyl analogue 90. 

The stabdities of protonated cyclopropylcarbinyl ketones are a long-standing puzzle. Richie provided evidence that the bisected cyclopropylcarbinyl carbenium ion (66a) was the more stable conformation, rather than the perpendicular geometry (66b). Of the protonated, rigid ketones, (67), (68), and (69), spiro compoimd (67) is most stable, but the bicyclo compound (68) proved more stable than the nortricyclic system (69), although the latter has a bisected geometry, while (68) is imable to achieve this. The anomaly appears to have been resolved by semiempirical calculations of heats of formation of the ketones and ions, and an analysis of the effects of syn- and... 

This section is confined to cyclopropenium ions. Cyclopropylcarbinyl cations are not considered although these ions are of interest on two counts (1) the relationship with the bicyclobutonium ion, (2) their bisected geometry in certain cases (which has relevance to the conformation of nitrocyclopropane ) as they have recently been reviewed ... 

Molecular structures of the cyclopropylcarbinyl cation salts lOa-c have been determined by X-ray. Both 10a and 10b show a bisected geometry that optimizes cyclopropyl conjugation with the cationic center, whereas in 10c such conjugation could lead to antiaromatic destabilization, and it was suggested that the structure reflected geometrical distortion to avoid interaction of the cyclopropyl group with the allyl cation portion of 10c. 

The lower IP for the conformationally fixed derivative 19 compared to c-PrCMe=CH2 is evidence that bisected geometries have the greatest interactions in the ionized states. Similarly, the lower IP for 21, in which one has bisected cyclopropyl, compared to 20, in which both groups are perpendicular, provides further evidence for this effect ... 

The X-ray structure of 1,2,3-triphenylcyclopropene showed a preference for the bisected geometry 51a of the 3-phenyl and a coplanar arrangement for the 1- and 2-phenyls ... 

The molecular structures of compounds of the type c-PrCOX all show a preference for bisected geometries, with both the cis (54) and trans (55) geometries being observed ... 

Calculations also favor a bisected geometry when X =... 

The rate effects of substituents on the cyclopropyl ring are discussed in earlier reviews and in Section II. It was particularly notable that these substituent effects appeared to be additive and were interpreted in terms of symmetrical 7r-donation through a bisected geometry (see Figure 1). 

In their study of the solvolysis of tosylate (87) Ree and Martin addressed the question of why there was an actual rate deceleration relative to 95 (k (95)/k(87) = 360) as compared to the rate acceleration of approximately 10 for a bisected geometry. 

Phenonium ions, of which 105 is the parent species, have long attracted attention, initially concentrated on kinetic and stereochemical evidence for their, generation in solvolysis reactions (equation 10), and more recently with direct observation of the ions in solution " . It has been persuasively argued that these ions are best represented as cyclopropyl-substituted carbocationic systems, with the bisected geometry best suited for TT-donation from the cyclopropyl ring. 

A simple HMO calculation for the cyclization of the allylcarbinyl to the cyclopropylcarbinyl anion (161) has appeared (equation 33) and, based on proton affinities, 161 was calculated to be 10 kcal mol" more stable than ethyl anion (SCF double-zeta basis) and 14.3 kcal mol" more stable than i-PrCH (STO-3G). The bisected geometry was predicted to be most stable, with electron donation from the filled p orbital at C(4) to the unoccupied Walsh 4ai MO (162) at C(l). Inductive electron withdrawal by cyclopropyl would also stabilize this anion. 

With simple secondary spirocyclopropylcarbinyl systems the cyclopropyl group, owing to its favorable bisected geometry for orbital overlap, generally produces a rate acceleration of about 10 over corresponding 2,2-dimethylcycloalkyl systems The relative rates for the secondary spiro systems shown below " " are for typical... 

All cyclopropylcarbinyl cations show some charge delocalization into the three-membered ring, but only the primary system (29) is unique in its chemical shifts which suggest a delocalized structure61. The C—1 —H coupling constants, however, do not support this conclusion. The non-equivalence of the methyl groups establishes the bisected geometry of (31), with slow rotation about the exocyclic C—C bond. The... 

The first cyclopropylmethyl cation to be directly observed was the tricyclo-propylmethyl cation (100) and the subsequent study of a variety of cyclopropylmethyl cations " led to the conclusion that the tertiary cations are static and, in the absence of constraining skeletal rigidity, adopt a bisected geometry rather than an eclipsed one (making the ex substituents on the carbenium ion... 

Additionally, this cyclopropane is set in conjugation with the C5-C6 insaturation in such a way that the bisected geometry derived from its spyrocyclic disposition maximizes the overlap between the n and cyclopropane 3d molecular orbitals. The setting is ideal for the constitution of a non-classical carbenium ion (8) if a proton is trapped by the C5-C6 double bond, Fig (5) route A. The nucleophilic attack of water from the medium would yield the rupture of the cyclopropane unit, and water elimination from the tertiary alcohol in the resulting diol (9) would furnish a conjugated dienone (10). Aromatization to the observed product, pterosin B (11), would occur by acid-induced glycolysis. Conversely,... 

Solvolysis rate studies also indicate that there is greater stabilization by a cyclopropyl group in a bisected geometry. Tosylate 1, in which the cyclopropane ring is locked into an orientation that affords a perpendicular arrangement, reacts 300 times more slowly than the model compound 2. Tosylate 3, which corresponds to the bisected geometry, undergoes acetolysis at least 10 times faster than the model 2-adamantyl tosylate, 4. ... 

---