1,2-Cyclopropyl shifts

Timberlake and coworkers have studied the degenerate rearrangement of pentacyclo-propylethyl cation 56 (involving 1,2-cyclopropyl shifts) under long-lived stable ion conditions81 82 (equation 39a). The rearrangement could not be frozen even at -80 °C. However, additivity of 3C NMR chemical shift analysis7 indicates the classical trivalent nature of the carbocation. 

Intermolecular trapping of carbene complexes (39) has also been reported. In this case, a second carbene (41) arising from a cyclopropyl shift from (39) can also be trapped to yield (42).39... 

The NOESY spectrum of buxatenone shows four cross-peaks, A-D. Cross-peak B represents the dipolar coupling between the most upfield C-19 cyclopropyl proton (8 0.68) with the most downfield olefinic proton (8 6.72). This could be possible only when the double bond is located either between C-1 and C-2 or between C-11 and C-12. The possibility of placing a double bond between C-11 and C-12 can be excluded on the basis of chemical shift considerations, since conjuga-... 

Spectra obtained by 1H NMR allowed the progress of the hydrogenolysis reaction to be followed quantitatively. The cyclopropyl protons in the polymer have an 1H NMR peak at 61.0 ppm, while the protons on the ring-opened, gem-dimethyl repeat unit (Equation A) have an XH NMR peak at 61.35 ppm. The gem-dimethyl peak assignment was based on chemical shift calculations (17). Also the dimethyl acetal of acetone has protons in a similar chemical environment to those on the gem-dimethyl ketal repeat unit and they have a chemical shift between 61.3 ppm and 61.A ppm (18). A proton NMR spectrum of partially hydrogenated polycyclopropanone is shown in Figure 7. 

As mentioned earlier (cf. chapter 2.1.1), p,y-enones undergo an 1,3 acyl shift from the Si-state. In sensitized reactions they rearrange to cyclopropyl ketones in the so termed oxadi-pi-methane rearrangement, as illustrated for... 

The analogous 2,6-cycloheptadien-l-ones display zwitterion-derived reactivity as well52,53. For example, the parent compound 113, upon irradiation in either acetic acid or t-butanol, gave diastereomeric solvent adducts 114 and 115 (Scheme 28)52a. On the other hand, tetramethoxy derivatives 116 furnished rearranged products 117, which are equivalent to the cyclopropyl ketone 1,4-shift products seen with cyclohexadienones52b. 

In contrast to the cyclopropyl- and cyclobutylhalo- (and acetoxy)carbenes, the cyclopentylhalo- and acetoxycarbenes (91), and their corresponding benzo derivatives, the indanylcarbenes (94), prefer 1,2-H shifts over 1,2-C shifts Eqs. 35 and 36.115 In the absence of strain relief to drive ring expansion, as with the cyclobutylcarbenes, 17, where kc exceeds ku, the 1,2-H shift is preferred.115... 

The comparative study of the experimental NMR spectra of Zs-1-cyclopropyl -2-(triisopropylsilyl)ethyl cation (17) and the computational model structure E-1-cyclopropyl-2-(trimethylsilyl)ethyl cation (18) demonstrates another application of calculations of 3H and 13C NMR chemical shifts and nuclear spin-spin coupling constants. In particular vicinal3./(11,11) spin-spin coupling constants are useful for... 

However, contrary to the parent cation, the corresponding isomeric cyclopropyl-methyl cation structures 58 and 59 are no minimum structures (MP2/6-31G(d)) and do not contribute to the averaged chemical shifts. 

The opening of cyclopropylcarbinols to homoallylic bromides constituted the first use of cyclopropyl compounds for the stereocontrolled synthesis of natural products. The cyclopropyl conjunctive reagents enhance the richness of this notion. The stereocontrolled opening of vinylcyclopropanes by a homopentadienyl proton shift provides an approach to trisubstituted olefins and thereby a new strategy. The fungal prohormone methyl trisporate B (224) as summarized in Scheme 15 illustrates this conceptual development97). 

One such typical transformation is the thermal isomerization of the spiropentane derivative 76 into triene 80 which is assumed to occur via the diene intermediate 78 with the intermediate participation of the cyclopropyl-trimethylenemethane (TMM) 77 and the vinyl-TMM 79 diradicals (equation 29)44. It was shown by using deuterium labels that the diradical 79 forms the triene 80 by 1,6-hydrogen shift. The pathway 76 — 80 which occurs via tetramethylene-ethane diradical was recognized as a less probable route. 

Both acridone and dibenzo[6,/]azepine produce unexpected products (Scheme 7.39) when reacted with dimethylvinylidene carbene (7.1.18.A). Acridone reacts initially at the nitrogen atom to produce the 10-(3,3-dimethylallenyl) derivative (13%) and a pyrroloacridone (10%) which, if the structure is correct, could be derived from the allene by sigmatropic shifts [16]. The dibenzoazepine reacts as expected to produce a cyclopropyl derivative but, under the reaction conditions, the adduct rearranges spontaneously to yield a 1,6-methanodibenzo[b,/]cyclo-prop [J]azepine, the structure of which was confirmed by X-ray crystallography [17]. 

Comparison of Chemical Shifts of Methyl, Cyclopropyl, and Phenyl Carbonium Ions"... 

Still another barrier to employing the shifts to resolve the norbomyl problem stems from recent studies of H. C. Brown and Peters (1973). These authors have measured the relative rates of solvolysis of 2-methyl-, 2-t-butyl-, 2-phenyl- and 2-cyclopropyl-2-propyl p-nitrobenzoate in 80% aqueous acetone to gather information about the electron-releasing ability of these groups. Upon comparing their results with the C shifts of the carbonium carbon of the same cation, they found no correlation. Although it might be... 

On the other hand, if the non-classical ion is a stable intermediate, the transition state for the 3,2 hydride shift requires a subst mtial reorganization, including the cleavage of the cyclopropyl ring, and, by analogy with unimolecular gas phase processes, a much higher pre-exponential factor might be expected. [In the cyclopropane-propylene reaction log A is 1ST 7 (Chambers and Kistiakowsky, 1954).] Contrary to expectation, the observed pre-exponential for the 3,2-shift is actually a little lower than for the 1,2,6-equilibration process. 

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