Nonplanar Intermediates

By balancing calculated values of angle strain against MO delocalization energy as a function of configuration, it has been possible to predict the relative stability and geometry of some interesting postulated reaction intermediates. 

Nearly the same approach has been applied with consid- 

Winstein verbiage almost completely conceals the use of the interesting idea that a. p orbital on a. sp. -hybrid zed carbon connected to another carbon that has a less—than—normal bond angle will not be perpendicular to the C—C internuclear line. This idea is best illustrated by a diagram  

The rationale is that the internuclear line should not lie in the plane determined by the sp —hybrid orbitals when the bond angle is less than normal because the best overlap will be obtained with a bent bond as indicated above by the dotted line. One way of correcting for this effect is as follows  

The bent bond can be taken as following a circle passing through the nuclei and tangent to the normal 109. 5° direction of the sp orbital. The orbital of interest can be drawn 

---

From what has been so far reported, it would seem that the intermediacy of the cis-cisoid ring-opened nonplanar intermediate X is much better documented for nitrospiropyrans than for spiroazines, for which the matter is still controversial. No studies have so far been reported on the importance of the X isomer in the photodegradation mechanistic schemes. 

The rate-determining step in the formation of the x-lithio ethers is the formation of a carbon radical as a precursor to the anion. The intermediate radical in the tetrahydropyranyl system is expected to be nonplanar, to be capable of rapid equilibration between the quasiequatorial and quasiaxial epimers, and to exist largely or entirely in the axial configuration at — 78 °C. However, treatment of the a-phenylthio ether 4 with LDMAN at higher temperature in the presence of A, A, lV, ./V -tetramethylethylenediamine leads to the more stable equatorial epimer of the lithio ether 5 and, after addition to benzaldehyde, the axial- and equatorial-substituted products were obtained in a ratio of 13 87. 

This rearrangement, which accounts for the scrambling, is completely stereospecific.The rearrangements probably take place through a nonplanar cyclobutyl cation intermediate or transition state. The formation of cyclobutyl and homoallylic products from a cyclopropyl-methyl cation is also completely stereospecific. These products may arise by direct attack of the nucleophile on 58 or on the cyclobutyl cation intermediate. A planar cyclobutyl cation is ruled out in both cases because it would be symmetrical and the stereospecificity would be lost. 

In the examples given, there is good evidence for the formation of an unstable epoxide intermediate in the production of monohydroxymetabolites. However, there is an ongoing debate about the possible operation of other mechanisms of primary oxidative attack that do not involve epoxide formation, for example, in the production of 2 OH 3,3, 4,4 -TCB (Figure 6.3). As mentioned earlier, P450s of gene family 1 (CYP 1) tend to be specific for planar substrates, including coplanar PCBs they do not appear to be involved in the metabolism of nonplanar PCBs. On the other hand. 

In the latter example, the most slowly eliminated compounds were nonplanar, and lacked vicinal carbons in either the ortho-meta or the meta-para positions that were without any chlorine snbstitntion (i.e., there were no vicinal ortho-meta or meta-para positions that were snbstitnted solely with hydrogen). The more rapidly eliminated compounds all possessed vicinal ortho-meta positions that were without chlorine sub-stitntion. In the former example, the most persistent compound was nonplanar, and lacked carbons unsubstitnted by chlorine in the meta-para positions. Interestingly, both of the coplanar componnds were eliminated rapidly, even though one of them (3,3, 5,5 -TCB) lacked nnsnbstitnted vicinal carbons in either position. This suggests that P4501A1/1A2 was able to hydroxylate the molecule reasonably rapidly without any vicinal unsubstitnted carbons, presumably without the formation of an epoxide intermediate. 

At present, the problems in thiepin chemistry awaiting solution are (i) how to construct the thiepin skeleton under mild reaction conditions, (ii) what are the structural effects on thermal stability of thiepin, (iii) whether the thianorcaradiene is an intermediate of sulfur extrusion reaction or not, (iv) what is the molecular structure of the thiepin (planar or nonplanar), (v) what is the antiaromaticity of the thiepin ring. 

Several aromaticity indices (bond lengths, bond orders, Jug and Francois s aromaticity index) indicate that, despite the nonplanarity of the five-membered ring in 2,5-diphenylthiophene-l-oxide (108), this compound is intermediate in aromaticity between the corresponding thiophene 107 and the nonplanar 1,1-dioxide 109 (Scheme 48).149 The theoretical calculations were supported by experimental electrochemical data.150... 

The transition state for erythro epoxidation, 5yn-(2/ ,3/ ,45 )-TS, is only 0.9 kcalmoG higher in energy and possesses a nonplanar peracid approaching the C=C bond in a manner intermediate between spiro and planar. The relative energy and nonplanarity of this syn transition strucmre is highly sensitive to the basis set applied. It was shown that... 

Disubstitution (A and B H), on the other hand, destabilized intermediate conformations 460 and 462, causing then to be nonplanar and so facilitating their closure to a heterocycle (459). 165.167.174.2°8.395. .402 Cyclic forms 459 seem to exist only in cases where their valence isomeric cis dienones possess no stable planar conformation type of 460 or 462,164 but... 

The formation of the tra/w-fused 4/ ,5a-ethylene adduct (262) has analogies in the addition of olefins to simple a,/ -unsaturated ketones. The possibility is that this compound derives in a concerted antara-antarcfi cial process from an excited state of the dienone (260) in which the a,j8-bond is nonplanar. Alternatively, it has been argued that upon a stepwise addition of the olefin to the excited (di)enone system a diradical is formed (e.g., through the formation of a single bond to the /2-carbon in the first step), and that part of the excess energy of this intermediate is absorbed in the formation of the strained trans-fused system. A two-step reaction is consistent with the result that the addition of either cis- or traws-dichloroethylene to enone (267) affords the same 1 1 mixture of 16a,17a-adducts [(269) and (270)]. 

Porphycenes 51 bearing substituents at ethylenic carbons have been prepared from 5,5 -acyl bipyrroles 53 (05OL1887) (Scheme 26). However, oxidation of the intermediate annulene 54 is difficult, owing to the presence of the alkyl chains on the alkene bridges. These reduce conformational flexibility, resulting in a near-nonplanar geometry, which resists oxidation and/or aromatization (08CSR215). 

---