Forbidden cyclizations

Scheme9.3. Examples of allowed and forbidden cyclizations according to Baldwin s rules [2, 5],...

Figure 2 Occupation numbers of natural orbitals in thermally forbidden cyclization of butadiene to cyclobutene in dependence on the value of the generalized reaction coordinate cp.

As can be seen from this comparison, the resulting values are affected by the choice of the critical structure and on going from X(n/4) to X(-7t/4), the systematic shift of the dominant similarity from the zwitterionic state Z + Z2 to the state Zj -Z2 is observed. We can thus see that the predictions for both types of critical structures differ and the problem thus appears which of the above two critical structures should be regarded as a true model of the transition state in forbidden reactions. Similarly as in the case of allowed reactions such a decision does not arise from the approach itself, but some external additional information is generally required. This usually represents no problem since the desired information can be obtained, as in the case of allowed reactions, from the simple qualitative considerations based on the least motion principle [80,81], or from the direct quantum chemical calculations.This is also the case with us here, where the desired information is provided by the quantum chemical study [63] of the thermally forbidden cyclization of the butadiene to cyclobutene. From this shufy it follows that the ground state of the transition state should correspond to the ground state of the cyclobutadiene which is the Zj - Z2 state. 

Similarly, thermal + ji4J and [ 2a + 2a] cyclizations are forbidden, while thermal [,t2a +, i4a] and [ 2s + n2a] cyclizations are allowed, and these consi-... 

The [4+ 4]-homolog of the [4 + 2]-Alder-ene reaction (Equation (48)) is thermally forbidden. However, in the presence of iron(m) 2,4-pentanedioate (Fe(acac)3) and 2,2 -bipyridine (bipy) ligand, Takacs57 found that triene 77 cyclizes to form cyclopentane 78 (Equation (49)), constituting an unprecedented formal [4 + 4]-ene cycloisomerization. The proposed mechanism for this transformation involves oxidative cyclization followed by /3-hydride elimination and reductive elimination to yield the cyclized product (Scheme 18). 

Recently, we analyzed the role of electron repulsion relative to bond breaking and antiaromaticity effects on a quantitative basis using Natural Bond Orbital (NBO) analysis.24 Two other destabilizing factors were considered at the initial stage of the cyclization in addition to four-electron repulsion between the filled in-plane acetylenic re-orbitals - distortion/breaking of the acetylenic bonds as a result of their bending, and the fact that, at a distance of ca. 3 A, the in-plane re-orbitals become parallel and reach a geometry that resembles the antiaromatic TS of the symmetry forbidden [2S + 2S] cycloaddition (vide infra). 

Nitrosoimines can undergo thermal reaction, a unimolecular, two-step mechanism has been proposed, as shown in Scheme 3.22 [193]. In this mechanism, a concerted electrocyclization is envisioned to form the strained four-membered ring in 41, followed by a presumably forbidden, but highly exothermic, deazetization to give 41. The electrocyclic ring closure is, at first glance, a 4-electron process, analogous to the cyclization of butadiene [194] or acrolein [194, 195]. This would be expected to involve rotation around the C=N bond coupled with C-O bond formation. 

We ascribe the increased stability of 162 to its bis-homoaromatic nature and to the fact that the increase in strain relative to 161 is minimized, as the two pivotal cyclopropane bonds are only partially formed. The cyclization of 161 must be fast even at - 50 °C the barrier is estimated to lie near 5 kcal/mol. The case of cycloaddition is surprising since it is formally of the [3 + 2]-type and, therefore, symmetry forbidden (vide supra). The driving force for the conversion 160 - 161 lies in the bis-homoaromatic stabilization of 161, and the conversion must have an early transition state [425]. 

If LVMO and HOMO are not compatible, a forbidden or high-energy process would normally be necessary to effect cyclization otherwise, anti cycloaddition (Fig. 10c) might be possible, as will be discussed shortly. It may be even simpler to imagine the reactants as they would appear in the transition state and inquire whether one has an incipient aromatic system, i.e. Htickel (4% + 2)77 cycle (Dewar, 1966 Fukui, 1965, 1966). If so, the cycloaddition is thermally allowed otherwise, forbidden. The nature of these predicted closures would normally be reversed for reactions of the first excited states. 

The dimerization of acyclic polyenes in which all n bonds are lost would lead to the open structures of (54) and (55). A schematic orbital correlation diagram (Fig. 15) for process (54) shows that allyl dimerization is improbable. The cyclization of higher acyclic polyenes, e.g. to cis-or trans-7 in (55), is subject toa similar prohibition, but the formation of 8 is allowed. In general, processes in which the products retain elements of symmetry inherent in the reactants are symmetry-forbidden the argument used to demonstrate this is analogous to that used for ethylene. One dimerization of 1,3-butadiene, namely to 9, is unique this... 

Hoffmann and Woodward (1965b) consider possible 2 + 2, 2 + 4, and 4 + 4 reactions of cyclobutadiene with itself. The fact is that the 2 + 2 and the 2 + 4 processes are not distinguishable, but the result is an endo favored cyclization (64). We have already discussed the forbidden 4 + 4... 

The classes of cyclization reactions are important, not because we have a compulsive Victorian desire to classify everything, but because which class a reaction falls into determines whether or not it is likely to work. Not all cyclizations are successful, even though they may look fine on paper The guidelines that describe which reactions will work are known as Baldwin s rules they are not really rules in the Woodward-Hoffmann sense of the term, but more empirical observations backed up by some sound stereoelectronic reasoning. To emphasize this, the rules are couched in terms of favoured and disfavoured , rather than allowed and forbidden . We will deal with the rules step by step and then summarize them in a table at the end. 

Metal complexes of heterocyclic compounds display reactivities changed greatly from those of the uncomplexed parent systems. All of the -electron system(s) of the parent heterocycle can be tied up in the complex formation, or part can be left to take part in alkenic reactions. The system may be greatly stabilized in the complex, so that reactions, on a heteroatom, for example, can be performed which the parent compound itself would not survive. Orbital energy levels may be split and symmetries changed, allowing hitherto forbidden reactions to occur. In short, a multitude of new reaction modes can be made possible by using complexes dimerization of azirines with a palladium catalyst serves as a typical example (Scheme 81). A variety of other insertion reactions, dimerizations, intramolecular cyclizations, and intermolecular addition reactions of azirines are promoted by transition metals. 

An objection that may be raised to a concerted four-center mechanism is the possibility of violation of orbital symmetry rules (68, pp. 65-78). Formally, the cyclization and cleavage may be viewed as forbidden + J2,] and [Jl, + J2,] cycloadditions (see transition state structure 41). The polarity of the bonds involved may alleviate the situation somewhat (21, 41). However, if the magnesium atom utilizes an additional orbital as in 42 (formally occupied partially by binding a solvent... 

Fig. 14.21. Cyclization of the cis-butadiene. (a) Start from the top coniotation (leftward) and disrotation (rightward) lead in general to different products, but only one of these transformation is symmetry allowed, (b) The doubly occupied n orbitals of the butadiene (p (HOMO-1) and (HOMO), (c) The transformation of >P2 under the conrotation and disrotation. The conrotation is symmetry-allowed, and the disrotation is symmetry-forbidden.

Alkylpalladium(ll) species 97, generated via initial palladium cyclization of an aryl iodide onto a proximate aUcene, are highly reactive and will attack proximate aromatic or heteroaromatic rings, both electron rich and electron poor, leading to spirocycles 98 (Scheme 40). The second cyclization step onto the aromatic ring is expected to occur with cA-stereo-chemistry. The (3-hydride elimination step normally occurs with cw-stereochemistry. This process is not possible in the present case. Similarly, formally forbidden eliminations are not uncommon especially when the Pd(ll) species is located at a benzylic position and may involve prior stereomutation of the Pd(ll) moiety or a slower irani-elimination. ... 

The formation of the permethyl Dewar benzene (and presumably the parent compound) from the prismane appears to be a symmetry forbidden retro 2 + 2 cyclization perhaps proceeding via a biradical which can also give the benzvalene via a 1,2-shift of the cyclopropane (Scheme 7.9). 

The nature of the transition state in the 3,3-shift was examined by Berson who found that (5 )-4-methyl-2-hepten-6-yne gave recovered starting material and both RZ)- and (5 )-4-methyl-l,2,5-heptatriene and the latter two gave (/ Z)-4-methyl-2-hepten-6-yne all reversibly with high stereospecificity prior to formation of cyclized and dimeric products (Scheme 7.51). These results are consistent with an allowed stereopathway via a chair-like transition state as opposed to a forbidden one or the one in which an effectively planar cyclohexene-1,4-diyl is involved. 

In their now classic monograph [1], Wooodward and Hoffmann concentrate on three basic types of no mechanism reaction Electrocyclic reactions -notably polyene cyclizations, cycloadditions, and sigmatropic rearrangements. These three reaction types will be taken up in this and the next two chapters from the viewpoint of Orbital Correspondence Analysis in Maximum Symmetry (OCAMS) [2, 3, 4], the formalism of which follows naturally from that developed in Chapter 4. The similarities to the original WH-LHA approach [5, 6], and the points at which OCAMS departs from it, will be illustrated. In addition, a few related concepts, such as allowedness and forbiddenness , global vs. local symmetry, and concertedness and synchronicity , will be taken up where appropriate. 

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