In cyclopropanation

FIGURE 3 10 Bent bonds in cyclopropane (a) The orbitals involved in carbon-carbon bond formation overlap in a region that is displaced from the internuclear axis (b) The three areas of greatest negative electrostatic potential (red) correspond to those predicted by the bent bond description... 

For a review of various descriptions of the bonding in cyclopropane, see A. de Meijer, Angew. Chem. Int. Ed. Engl. 18 809 (1979) K. B. Wiberg, in The Chemistry of the Cyclopropyl Group, Z. Rappoport, ed., John Wiley Sons, New York, Chapter 1, 1987 B. Rozsondai, in The Chemistry of the Cyclopropyl Group, Vol. 2, Z. Rappoport (ed.), John Wiley Sons, New York, Chapter 3, 1995. 

In solutions of n-propyllithium in cyclopropane at 0°C, the hexamer is the main species, but higher aggregates are present at lower temperatures. The reactivity of the organo-... 

The addition of nucleophiles to cyclic fluoroolefins has been reviewed by Park et al. [2 ]. The reaction with alcohols proceeds by addition-elimination to yield the cyclic vinylic ether, as illustrated by tlie reaction of l,2-dichloro-3,3-di-fluorocyclopropene Further reaction results in cyclopropane ring opening at the bond opposite the difluoromethylene carbon to give preferentially the methyl and ortho esters of (Z)-3-chloro-2-fluoroacrylic acid and a small amount of dimethyl malonate [29] (equation 8). 

Boche, G, Walborsky, H M In Cyclopropane Derived Reactive Intermediates, Fatal, S, Rappoport, Z, Eds, John Wiley Chichester, 1990, Chapter 1... 

C-C bond Fluonnation increases the bond strengths in cycloalkanes, including cyclobutanes [75, 94], but by contrast, it decreases C-C bond strengths and increases nng strain in cyclopropanes and other three-membered nng compounds [75 94. 9S]... 

In keeping with the "bent-bond" description of Figure 3.10, the carbon-carbon bond distance in cyclopropane (151 pm) is slightly shorter than that of ethane (153 pm) and cyclohexane (154 pm). The calculated values from molecular models (see Learning By Modeling) reproduce these experimental values. 

Conformational analysis is far- simpler in cyclopropane than in any other cycloalkane. Cyclopropane s three carbon atoms are, of geometric necessity, coplanar-, and rotation about its carbon-carbon bonds is impossible. You saw in Section 3.4 how angle strain in cyclopropane leads to an abnormally large heat of combustion. Let s now look at cyclopropane in more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry. 

Catalytic, enantioselective cyclopropanation enjoys the unique distinction of being the first example of asymmetric catalysis with a transition metal complex. The landmark 1966 report by Nozaki et al. [1] of decomposition of ethyl diazoacetate 3 with a chiral copper (II) salicylamine complex 1 (Scheme 3.1) in the presence of styrene gave birth to a field of endeavor which still today represents one of the major enterprises in chemistry. In view of the enormous growth in the field of asymmetric catalysis over the past four decades, it is somewhat ironic that significant advances in cyclopropanation have only emerged in the past ten years. 

The most logical starting point is a discussion of the structure of the zinc carb-enoid, followed by a somewhat chronological presentation of major advances in the use of zinc carbenoids in cyclopropanation. After a brief historical recounting of Simmons and Smith s original studies, the crucial implications of diastereose-... 

In this model, the intermediacy of a monomeric zinc species is postulated. To support this assumption, an examination of the effect of stoichiometry and solvent in cyclopropanation involving the 2,4-pentanediol auxiliary was preformed [59]. In the initial reaction protocol, a large excess of both diethylzinc and diiodo-methane is employed. Such excessive conditions are justified on account of the instability of the zinc carbenoid under the reaction conditions. To minimize the un-... 

Carbon-carbon bonds are not easily cleaved under mild conditions unless weakened by strain 3,86,182) or activation. The most common examples of carbon-carbon bond cleavage occur in cyclopropanes. 

Figure 4.1 (a) Rotation occurs around the carbon-carbon bond in ethane, but (b) no rotation is possible around the carbon-carbon bonds in cyclopropane without breaking open the ring. 

Each H <— H eclipsing interaction in ethane costs about 4.0 kj/mol. Wow many such interactions are present in cyclopropane What fraction of the overall 115 kj/mol (27.5 kcal/mol) strain energy of cyclopropane is due to torsional strain ... 

The present method gives better yields and is a simpler procedure than those described previously The ease of preparation of a-chloro ethers enables one to effectively remove a protecting methyl group from a phenol2 a-Chloroanisole is used to produce phenoxycarbene, a reactive intermediate in cyclopropane synthesis8... 

CC and CH bond orbitals but also for the CTL, ami CH3 group orbitals. If the local symmetry elements are preserved in the full molecule, the 7r (or a) local orbitals can combine to give v (or o) molecular orbitals. The reader should, therefore, not be surprised to find, for instance, tt type molecular orbitals in cyclopropane which are delocalized over the CH2 groups. 

In addition, Suehiro et al. (1987) were able to accurately determine the rates of dediazoniation of twenty substituted aryldiazenyl radicals formed from aryl(aryl-thio)diazenes (Scheme 8-36), in cyclopropane and in alkenic solvents at -48 to — 117°C, using a time-resolved ESR method. 

Kinetic ESR spectroscopy has been used to determine the rate constants for self-termination of MeS02, EtS02, PhSO 2 and 2,5-Cl2C6H3S02 radicals18. At 233 K and in cyclopropane as a solvent the values found for 2k, were in the range (4.5 1.5) x 109 m" 1 s 1 for all RSO 2 radicals. 

Small amounts of cyclopentene derivatives are detected in cyclopropanation reactions of electron-deficient dienes, but they may result from thermal rearrangement of the corresponding vinyl cyclopropanes and not from a direct [4+1] cycloaddition... 

Cyclopropane, C.H, is a hydrocarbon composed of a three-membered ring of carbon atoms, (a) Determine the hybridization of the carbon atoms, (b) Predict the CCC and HCH bond angles at each carbon atom on the basis of your answer to part (a), (c) What must the real CCC bond angles in cyclopropane be (d) What is the defining characteristic of a cr-bond compared with a ir-bond, for example (e) How do the C—C cr-bonds in cyclopropane extend the definition of conventional o-bonds (f) Draw a picture depicting the molecular orbitals to illustrate your answer. 

The points are plotted in Fig. 13.11. The graph is a straight line, confirming that the reaction is first order in cyclopropane. When we use points A and B on the plot, the slope of the line is... 

In a note added at the end of his last paper6 Wierl stated that he had found for the carbon-carbon distance in cyclopropane the value 1.6 0.1 A. 

It might be expected that the strain in cyclopropane, in which the C-C-C bond angles are distorted from 109°28 to 60°, would weaken the bonds and thus lead to an increase in the bond distance. This effect is not observed, however, the carbon-carbon distance in cyclopropane being the same as in the other hydrocarbons to within the accuracy of the investigation. There is even some small indication that the C-C distance in cyclic aliphatic hydrocarbons is slightly smaller (by about 0.01 A.) than the normal distance, the three values reported being 1.53, 1.52, and 1.53 A. 

FIGURE 4.6 Orbital overlap in cyclopropane. The arrows point toward the center of electron density. 

Data are extant in the literature for four tra s-cyclopropylene sets. Of these, two are disubstituted (sets 39-9 and 39-11). Positions trans-2 and tranS 3 are completely equivalent in cyclopropanes bearing the reaction site at position 1. These sets have been correlated with eq. (30). The other two sets have been correlated with eq. (2). Three of the four sets studied gave significant correlations. The fourth set had only four points. The results obtained clearly show a significant resonance effect. They clearly demonstrate that the frans-cyclopropylene system does involve a resonance interaction between the substituent and the cyclopropane group. The rra s-cyclopropylene system again... 

Ruthenium porphyrin complexes are also active in cyclopropanation reactions, with both stoichiometric and catalytic carbene transfer reactions observed for Ru(TPP)(=C(C02Et)2> with styrene. Ru(Por)(CO)orRu(TMP)(=0)2 catalyzed the cyclopropanation of styrene with ethyidiazoacetate, with aiiti.syn ratios of 13 1... 

A. N. Tarnovsky, V. Sundstrom, E. Akesson, and T. Pascher, Photochemistry of diiodomethane in solution studied by femtosecond and nanosecond laser photolysis. Formation and dark reactions of the CH2I-I isomer photoproduct and its role in cyclopropanation of olefins. J. Phys. Chem. A 108(2), 237-249 (2004). 

D. L. Phillips, W.H. Fang, and X. Zheng, Isodiiodomethane is the methylene transfer agent in cyclopropanation reactions with olefins using ultraviolet photolysis of diiodomethane in solutions a density functional theory investigation of the reactions of isodiiodomethane, iodomethyl radical, and iodomethyl cation with ethylene. J. Am. Chem. Soc. 123(18), 4197-4203 (2001). 

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