Cyclopropanation structure

The energy surface for C3H7 has been calculated at the 6-311G /MP4 level. The 1-and 2-propyl cations and comer- and edge-protonated cyclopropane structures were... 

The acyl selenide 19 affords the decarbonylated )S-lactam in good yield. A N-hydroxypyridine-2-thione ester 20 is used in the key step to construct the chiral cis-cyclopropane structure in compounds designed as antidopaminergic agents. The observed high cis selectivity is due to the hydrogen abstraction from the sterically demanding (TMSlsSiH, which occurs from the less-hindered side of the intermediate cyclopropyl radical. 

The main building block of PEDC (1 -phenyl-2-[(S)-l-aminoethyl] -N,N -di-ethylcyclopropanecarboxamide), a potent NDMA (N -methyl-D-aspartic acid) receptor antagonist of a cyclopropane structure, N -benzyl-C-cyclopropyl nitrone... 

Furthermore, cyclopropane structures have often served as intermediates in organic synthesis. For these reasons, olefin cyclopropanation has proved to be a useful tool for synthetic organic chemists. This has led to the development of several methods for cyclopropanation reactions,91 including the metal-catalyzed reactions of diazo compounds with olefins, as well as the Simmons-Smith reaction. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

The visual and conceptual impact of seeing the timed sequence of structures, a full representation of atomic-scale events as a complex chemical reaction took place, was powerful. This achievement, the product of state-of-the-art calculations applied to an ambitious objective as well as excellent presentation graphics, was not diminished through a repressed awareness that it aU depended on theory. Nothing experimentally based provided an anchor for the visually compelhng rendition of the reacting system as a cyclopropane cleaved a C C bond, formed a trimethylene diradical intermediate, and executed a net one-center epimerization before reverting to the cyclopropane structure. 

Considering that cyclopropane possesses Z)3h symmetry, structures 4, 5, 6 and 7 can be excluded. Considering further that all C atoms in cyclopropane are tetravalent, only 1 and 2 remain as possible cyclopropane structures. Finally, structure 1 is left as the only possibility if one considers that each CH2 group in the molecule has to be perpendicular to the ring plane. 

Since both structures 6 and 7 are highly unstable, we can conclude that the cyclopropane structure 1 is by far the most stable cyclic C3H6 system possible. It complies with the rules of classical carbon chemistry [tetravalent C, (distorted) tetragonal geometries] and, therefore, it is relatively stable. 

The schematic representation of Figure 5 illustrates the paths relating one EE intermediate with four cyclopropane structures. An alternative schematic, Figure 6, shows the paths relating one cyclopropane with four EE intermediates by way of six transition structures. In addition to the paths given explicitly in Figures 5 and 6, there are 4 direct paths by way of EF transition structures relating each cyclopropane with one-center epimeriza-tion products. 

Trans-selective alkylation of dibromocyclopropanes via a bromochlorocyclopropane synthesis of trans- 1-butyl-2-[(phenylmethoxy)methyl]cyclopropane (Structure 22)8c... 

Reactions of trimethylsilyl enol ethers with diazo ketones give cyclopropanes contaminated by ring opened compounds 60,61). Use of the more stable tert-BuMe2Si-derivatives or of Rh2 (0Ac)4 as a catalyst might eventually improve the situation. O-Silylated ketene acetals and O.S-ketene acetals, respectively, did not provide products with cyclopropane structure 61 ... 

The catalyst causes a classical carbenium ion to be formed by acid catalyzed activation reactions. The classical carbenium ion is transformed into the key intermediate which can be described as a protonated cyclopropane structure. After some rearrangements cracking occurs. The formation of branched paraffins is very fortunate since branched paraffins have high octane numbers and the isobutane produced can be used in alkylation. The preferred products are those of which the formation proceeds via tertiary carbenium ions. Carbenium ions can also be generated by intermolecular hydride transfer reactions between alkane and carbenium ions that are not able to form tertiary carbenium ions (see Chapter 4, Section 4.4). Under more severe conditions lower paraffins can also be cracked. 

The well known ease with which a terlary bromide loses hydrobromic.,acid to give an unsaturated compound has led Ostling to question the structure of 1,1,2 trimethylcyclopropane. Zelinsky who prepaBed it states that it slowly decolorizes potassium permanganate. Certain products of the action of nitric acid, however,support tha cyclopropane structure very strongly and the compound can best be considered to have a cyclic structure. 

These observations, when compared with spectra of the 7-norbornenyl and 7-norbornadienyl cations, confirmed the protonated cyclopropane structure [4] for the norbornyl cation. Average chemical shifts and coupling constants estimated for C(l), C(2) using equilibrating classical models were not in accord with the experimental i C-nmr data (Olah, 1976). 

The NMR spectrum is consistent with structure 46. The single vinyl proton appears as a triplet at 4.74, 4.61, and 4.48 r and the corresponding splitting is found in the doublet at 6.88 and 6.75 t assigned to the methylene group a to the carboxyl group. The methene protons and those of the g m-dimethyl group appear at 5.33 and 8.90 t, respectively. No resonance characteristic of cyclopropane structures is observed. 

Although absorption bands assigned to cyclopropane ring vibrations are sometimes said to be of questionable significance, to be occluded or apparently absent, they have nevertheless been cited consistently in support of cyclopropane structures. A limited number of examples will be given. 

An ESR spectrum consistent with the cyclopropane structure 130 was obtained on electrolytic reduction of 130. Undoubtedly 130 , if it has the proposed structure, will owe its stability to the many and excellent electrophoric substituents as this is the case with 127 , 128 and 129 . The facile ET to 130 from the nucleophiles mentioned is remarkable. 

Electrophilic addition reactions of carbenes to cyclopropane structures are rare and not synthetically important. Reaction of carbenes with bicyclo[2.1.0]pentane generally gave products arising from C —H insertion at one of the methylene groups of the four-membered ring. ° An exception was difiuorocarbene which gave 1,1-difluorohexa-l,5-diene (2) in very low yield by cleavage of two C —C bonds. - ... 

The susceptibility of nucleophiles to ring-opening reactions is often increased after the activated cyclopropane structure has been incorporated into a polycyclic system due to the additional strain. 3-enaqueous tetrahydrofuran. When 3-cnt/o-methoxytricyclo[3.2.0.0 ]heptan-6-one was dissolved in methanol, the corresponding ring-opened dimethoxy derivative 22 (R = Me) was isolated in high yield. 

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