C.H. Cyclopropane

The cyclopropane ring is necessarily planar, and the question of conformation does not arise. The C—C bond lengths are slightly shorter than normal at 1.5 A, and the H—C—H angle of 115° is opened somewhat from the tetrahedral angle. These structural... 

In addition to angle strain, cyclopropane is destabilized by torsional strain. Each C—H bond of cyclopropane is eclipsed with two others. 

It is easily seen that for given m there exists only one ring-shaped C-graph with m vertices. The construction described in Sec. 37 defines a topologically unique C-H graph For a given m there exists with respect to structure exactly one purely ring-shaped cycloparaffin. It is, in the simplest case w 3, the above (Secs. 56 - 58) discussed cyclopropane. 

Cyclopropane is the most strained of all rings, primarily because of the angle strain caused by its 60° (,-C-C bond angles. In addition, cyclopropane also has considerable torsional strain because the C-H bonds on neighboring carbon atoms are eclipsed (Figure 4.4). 

Figure 4.4 The structure of cyclopropane, showing the eclipsing of neighboring C-H bonds that gives rise to torsional strain. Part (b) is a Newman projection along a C-C bond.

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

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. 

Cyclopropane gave photographs showing four rings, with qualitative appearance corresponding closely to the calculated curve of Fig. 4. The s0 and I values given in Table V lead to the radial distribution curve shown in Fig. 3 (curve A), with peaks at 1.12 A. (C-H bond distance), 1.52 A. (C-C bond distance) and 2.2-2.5 A. (larger C-H... 

The allyl radical [115] trapped in an argon matrix can be photolytically (A = 410 nm) converted into the cyclopropyl radical [116] (Holtzhauer er a/., 1990). Dicyclopropane and cyclopropane were formed when the photolysed matrix was warmed from 18 to 35 K. The intermediate [116] was shown to be a cr-type (Cs symmetry) and not a rr-type symmetry) radical. Normal coordinate analysis of the radical [116] has been carried out and the IR band at 3118 cm has been assigned to the stretching vibration of the C—H bond at the radical centre. 

Esters of a-diazoalkylphosphonic acids (95) show considerable thermal stability but react with acids, dienophiles, and triphenylphosphine to give the expected products. With olefinic compounds in the presence of copper they give cyclopropane derivatives (96), but with no such compounds present vinylphosphonic esters are formed by 1,2-hydrogen shift, or, when this route is not available, products such as (97) or (98) are formed, resulting from insertion of a carbenoid intermediate into C—C or C—H bonds. The related phosphonyl (and phosphoryl) azides (99) add to electron-rich alkynes to give 1,2,3-triazoles, from which the phosphoryl group is readily removed by hydrolysis. 

Carbenes and carbenoids can add to double bonds to form cyclopropanes or insert into C-H bonds. 

Fig. 16 The experimental geometries of methylenecyclopropane- HC1 and methylene-cyclopropane- -CIF, drawn to scale. The n-electron model for the Lewis base is also shown. The angles C- - H and C- Cl, respectively, where is the centre of the C - C double bond, are both close to 90°, as required by rule 2. The halogen bond again exhibits a smaller non-linearity 6 than the hydrogen bond. See Fig. 1 for key to the colour coding of atoms... 

Fig. 14. Potential energy barrier heights as calculated using ab initio theory for insertion of transition metal atoms into a C-H or C-C bond of (a) ethane and (b) cyclopropane. Values taken from Ref. 22.

Like other metal reactions studied previously in our laboratory, H2 elimination is initiated by insertion into one of the C-H bonds forming HMC3H5. The reaction rate constant for Y + cyclopropane was found to be very small at room temperature, 0.7 x 10 12 cm3 s 1, and it was suggested that the reaction most likely involved termolecular stabilization of C-H or C-C insertion complexes, rather than molecular elimination.22 By analogy with other systems studied, the dynamically most favorable route to H2 loss in this case is likely via H atom migration to the Y-H moiety, with concerted... 

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