Coplanarity of Reacting Centers

This so-called stereoelectronic factor operates to maximize or minimize orbital overlap, as the case requires, to obtain the most favorable energy. This was evident from the three- and four-center systems we have discussed by the VB and HMO methods. It was also implicit in favored anti-1,2-additions, 1,3-cyclizations (Fig. 23), fragmentations (e.g. (174)), etc. Here we have selected several reaction types to illustrate the principle. In this and other sections, we show that the tendency for reaction centers to be collinear or coplanar stems largely from orbital symmetry (bonding), but may also derive from steric and electrostatic effects, as well as PLM. 

The syn-periplanar eliminations by pyrolysis of esters, xanthates, sulfoxides and amine oxides are symmetry-allowed. With respect to the alkene portion of the transition state, the centers presumably are or 

It is well known that in many brominations and protonations of cyclohexenols (91) axial entry is favored (Eliel et al., 1965). This is attributed to the parallel alignment of the v orbitals on the three centers. The overlap preference is well illustrated in the oxidation of allyl vs. saturated alcohols. Normally, axial alcohols are oxidized more rapidly by chromic acid than equatorial alcohols. In the absence of large strain factors, equatorial allyl alcohols are oxidized faster than axial alcohols by chromic acid hydrogen is abstracted in the rate-determining step. The contribution of a-j8 ketonic resonance lowers the activation energy, 

Czech workers have also found several competitive anti- and syn-eliminations (Pankova et al., 1967 Zavada et al., 1967). 

The erythro compound shows little or no kinetic isotope effect, but the threo compound has a moderate one, H/fcD 2-3-3-3, for both syn and anti processes. This suggests that an E2 process is involved. Eliminations from the cyclic bromides may produce trans alkenes by syn eliminations or by anti eliminations, if n 8 in(189). 

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A classic diagnostic use of such stereochemical requirements, due to Ruzicka, is the ring contraction induced in natural products containing the 4,4-dimethyl-5a-3 -ol system (94). The epimeric, axial 3a-alcohols (95) dehydrate without ring contraction. Barton suggested that it is necessary for the four reacting centers (hydroxyl, C-3, C-4, C-5) to be coplanar for ring contraction to occur, and this is only the case with the 3)5-alcohol. 

Experimental evidence and computational analysis point to a mechanism in which the alkene (or alkyne) carbons and the M-H bond must be nearly coplanar to react. Once the metal alkene complex has achieved such geometry, 1,2-insertion can occur. During insertion, the reactant proceeds through a four-center transition state. 14The reaction involves simultaneous breakage of the M-H and C-C n bonds, as well as the formation of an M-C a bond and a C-H bond at the 2-position of the alkene (or alkyne). The result is a linear compound, L M(CH2CH3), in the case of ethene insertion. The reverse reaction, (3-elimination, follows the same pathway starting from a metal-alkyl complex with an open coordination site. 

Three-center SN2 displacement and anti-eliminations from unsaturates are obvious examples of the coplanarity principle. DePuy et al. (1965) noted that when anti eliminations cannot have coplanar reacting centers the syn coplanar-transition state may become more favorable. Syn bimolecular eliminations had been noted in various systems previously, e.g. haloethenes (Miller, 1961), but these were generally slower than anti eliminations. There were, however, syn bimolecular eliminations whose rates approached that of the anti form or exceeded it. The relative rates of elimination of 2-phenylcyclopentyl and cyclohexyl tosylates with t-butoxide in t-butyl alcohol at 50° are as follows syn-cyclopentyl, 3 awii-cyclopentyl, 26 syw-cyclohexyl, 0 and anii -cyclohexyl, 2. In the cyclopentyl system in which the torsional angle (r) between the leaving groups approaches zero the syn rate is close to the anti rate. In the cyclohexyl system in which t of the stable form is ca. 60° the syn rate is 0. LeBel et al. (1964), report that in the reactions of t-butoxide with the 2,3-dihalobornanes, 92-95,... 

A further aspect of the rearrangement that we would like to explore is the geometrical requirement of the transition state. If a concerted mechanism is accepted, a first notion of the transition state might be one in which the reacting centers are located in a coplanar, approximately rectangular array, essentially as drawn in Fig. l,d. 

Ti -Cyclopentadienyl(triphenylphosphine)cobalt reacts with phosphites and forms complexes of 1-alkoxyphosphole oxides 251 (R = Me, Et, Ph) through a step involving (ri -cyclopentadienyl)(phosphite)cobalt (80JA4363). (ri -Cp)Co(PF3)2 reacts with hexafluorobut-2-yne and 252 is formed, which hydrolyzes into 253 (X = OH) [73JCS(CC)583 75JCS(D)197]. The five-member ring has the envelope conformation, in which the carbon atoms are coplanar, and the phosphorus atom deviates from this plane in the direction opposite to the cobalt atom. The heterocycle is a four-electron donor relative to the metal center. 

Two equiv. of 6,6-di(cyclopropyl)fulvene react at 60 °C over a period of a week with Ca[N(SiMe3)2]2-(THF)2 bis in THF to yield the metallocene 170. The heteroleptic amido complex 171 is detected as an intermediate with 111 and 13C 1H NMR spectroscopy. A 1 1 reaction of the calcium amide and 170 also produces 171 in solution, an equilibrium involving these three derivatives exists (Equation (30)). The calcocene 170 crystallizes at — 20 °C from THF as colorless cuboids. The metal center is surrounded by the four ligands in a distorted tetrahedral manner, and the cyclopentadienyl group and the propylidene fragment are coplanar with each other.393... 

A pentopyranosyl radical is much more flexible than a hexopyranosyl radical. Because the alkyl-anchor at C-5 is absent, the radical is now so flexible that several species of similar energy can coexist. According to ESR spectroscopic data, the arabinopyranosyl radical 9 exists as an equilibrium between the 4C19a and the B03 9b conformation, which both realize a coplanar arrangement of the C-O bond and the SOMO [9] (Scheme 6). Reactions with alkenes are unselective. However, the arabinofuranosyl radical 10 reacts with high stereoselectivity [9]. This is due to its 2E conformation in which the si-side of the radical center is sterically hindered by the large benzoyl group. 

However, the luminescence measurements show quenching of fluorescence in the trimer, which is attributed to a photo-induced electron transfer from the axial ruthenium(II) porphyrin to the excited state of the basal tin porphyrin. Not only ruthenium(II) porphyrins, but also rhodium(III) porphyrins can easily be incorporated into the arrays with the same strategy [33]. Again, the isonicotinic acid is first reacted with the bis-hydroxy tin porphyrin to give the bis-isonicotinic acid complex. Addition of two equivalents of rhodium(III) porphyrin readily yields the trimeric array of the composition Rh-Sn-Rh. The X-ray structure of this complex, which is shown in Fig. 34c, shows that the ligands on the tin center (carboxylates) are in an off-direction which is close to orthogonal to the porphyrin plane, and the three porphyrins adopt a near coplanar arrangement. The tin porphyrin is tilted by about 8.6° with respect to the rhodium(III) porphyrins. 

The criteria of allowedness discussed in the preceding two sections do not require the explicit consideration of orbital symmetry, in the sense that the symmetry elements retained along the reaction path do not enter directly into the analysis consequently, they were not drawn in the figures. However, it is easy to ascertain from Fig. 1.1, for example, that two ethylene molecules in either the coplanar or [s -f s] orientation have three perpendicular mirror planes one common to the four carbon atoms, another reflecting one molecule into the other, and a third bisecting both of them three twofold axes of rotation (one at the intersection of each pair of mirror planes) and a center of inversion at the point where the three rotational axes intersect. After both molecules have been twisted so as to react in the [a + a] mode (Fig. 1.1c), only the rotational axes remain, whereas the off-orthogonal orientation of Fig. 1.4b retains a single twofold rotational axis and no other element of symmetry. 

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