Cyclopropyl cation, disrotatory

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

The disrotatory mode, in which the methyl groups move away from each other, would be more favorable for steric reasons. If the ring opening occurs through a discrete cyclopropyl cation, the W-shaped allylic cation should be formed in preference to the sterically less favorable U-shaped cation. This point was investigated by comparing the rates of solvolysis of the cyclopropyl tosylates 6-8 ... 

This interpretation is supported by results on the acetolysis of the bicyclic tosylates 9 and 10. With 9, after three months in acetic acid at 150°C, 90% of the starting material was recovered. This means that both ionization to a cyclopropyl cation and a concerted ring opening must be extremely slow. The preferred disrotatory ring-opening process would lead to an impossibly strained structure, the /ran -cyclohexenyl cation. In contrast, the stereoisomer 10 reacts at least 2x10 more rapidly because it can proceed to a stable cis-cyclohexenyl cation ... 

For example in cyclopropyl cations, it can be seen that groups situated on the same side of the ring as the leaving group will move inwardly. The same process will impose an ontoward disrotatory motion to the groups situated in anti as follows. 

Note that we predict ring opening of the cyclopropyl cation to require activation this at first sight seems to be at variance with evidence that rearrangement occurs as a concerted process in the solvolysis of cyclopropyl esters and indeed acts as a driving force 44). Moreover the evidence shows very clearly that this is so only for one of the possible disrotatory processes, i.e. that indicated in 25 ... 

The electrocyclic reactions of 3-membered rings, cyclopropyl cation and cyclopropyl anion, may be treated as special cases of the general reaction. Thus the cyclopropyl cation opens to the allyl cation in a disrotatory manner (i.e., allyl cation, n = 0), and the cyclopropyl anion opens thermally to the allyl anion in a conrotatory manner (i.e., allyl anion, m = 1). Heterocyclic systems isoelectronic to cyclopropyl anion, namely oxiranes, thiiranes, and aziridines, have also been shown experimentally and theoretically to open in a conrotatory manner [300]. 

Figure B14.2. Orbital correlation diagram for electrocyclic opening of cyclopropyl cation S, A and solid lines indicate disrotatory opening S, A and dotted lines indicate conrotatory opening.

In their first paper, Woodward and Hoffmann (1965a) make a prediction based on orbital properties favoring one of two disrotatory paths in (27). The process of ionization to produce a cyclopropyl cation also produces a negative ion, X-. (We can also think of the reversal, namely, the attack of X- on a three-atom cyclic cation.) Here, tpi is a and i/r2 is b (Fig. 1). From either point of view the two a orbitals involved in the... 

When the latter reaction is carried out in MeOD, the distribution of the label in the products is consistent with attack of silver ion at the 7i-bond and disrotatory opening of an incipient cyclopropyl cation towards the centre of developing positive charge 284) ... 

Although the initial product of the ring opening is a cation, and therefore a hard-to-observe reactive intermediate, some nice experiments in superacid media (Chapters 17 and 22) have proven that cyclopropyl cation ring openings are indeed disrotatory. 

Stable than cation 35,2 kcalmoT less stable than corner protonated cyclopropene (39) and 1 kcal mol less stable than the perpendicular allyl cation (36). It has been speculated that 34 is the transition state for the stereomutation of the planar allyl cation This is confirmed by MP2/6-31G(d,p) calculations ", which show that the cyclopropyl cation is located at a first-order transition state possessing one imaginary frequency of symmetry that describes a disrotatory movement of the CH2 groups in line with a stereomutation process of the allyl cation. Alternatively, stereomutation can also follow a stepwise route via perpendicular allyl cations 36 that are somewhat more stable than 34 (Table 26) and therefore lead to lower stereomutation barriers. ... 

The selection rules can be applied to charged species as well as to neutral molecules. The only requirement is that the reaction be a concerted process involving electrons in overlapping p orbitals. For example, the conversion of a cyclopropyl cation to the allyl cation can be considered as a tt -electrocyclic process. For this process, the selection rules predict a disrotatory process. 

The cationic cyclopropyl to allyl rearrangement is always disrotatory. If a cyclopropyl cation 9 or 14 is involved, both disrotatory modes are electronically equivalent, and their ratio is determined by steric interactions in the transition state. If, however, cyclopropane 16 rearranges to an allyl cation 17 in a synchronous reaction, bypassing the cyclopropyl cation 4, only the disrotatory mode, that moves the electron density of the breaking bond to the rear side of the leaving group X, occurs. This anchimeric assistance causes a kinetic acceleration of the rearrangement by a factor of at least 10 to 10 . °... 

However, the formation of a-fluoro-a,j8-unsaturated carbonyl compounds from difluorocarbene adducts may proceed in some cases via a classical cationic disrotatory cyclopropyl to allyl rearrangement, e.g. 12- 13->14. °... 

All these facts are perfectly consistent with solution chemistry results showing the essentially barrierless electrocyclic disrotatory ring opening of cyclopropyl cations... 

As with cyclobutenes, there are two possible directions for the allowed disro-tation in substituted cyclopropyl cations. For a a>2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to structurally distinct allyl cations. 

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