Formation of a Three-Membered Ring

The ready formation of the episulphonium compound is reasonably explained in terms of the directionality and polarizability of the non-bonded electron-pairs of sulphur, a soft nucleophile which naturally forms bonds at angles close to 90°. We may presume that the directional requirements of the nucleophilic orbitals of first-row elements (N, O) are stricter, and that this factor shows up most clearly in a situation (the formation of a three-membered ring) which makes extreme demands on orbital flexibility. 

Carbonium Ions as Reaction Intermediates.—The properties of electron-deficient substances may be expected to be of great importance in the theory of chemical reactions. For example, a positively charged (and hence electron-deficient) carbon atom in a complex carbonium ion would be expected to cause adjacent atoms to increase their ligancies, as by the formation of a three-membered ring and by the use of bridging hydrogen atoms. The analysis of the mechanisms of chemical reactions may in the course of time permit much more precise principles to be formulated than are now at hand. 

The 31P-NMR spectrum of 59 is of the AX type and contains signals of three- and pentacoordinated phosphorus at 11.9 and —64.9ppm (Vp p = 168.1 Hz), respectively. The established values of Jpc and Jpn allowed the conclusion that the corresponding chloromethyl groups are relatively stable and do not participate in the formation of a three-membered ring with the adjacent phosphorus atom <1997ZFA1325>. 

An oxygen atom is analogous to a carbene in that it has only six electrons in its valence shell. Like a carbene, it might be expected to act as both an electrophile and a nucleophile in its reaction with an alkene. resulting in the formation of a three-membered ring containing an oxygen—an epoxide. 

To explain this reaction, consider the fact that, due to the overlapping p orbitals, double bonds are electron rich. This property allows olefins, under certain conditions, to act as nucleophiles. In the case of a double bond reacting with molecular bromine, the result is formation of a three-membered ring containing a positively charged bromine atom. This three-membered ring is known as a bridged bromonium ion. Concurrent to formation... 

The sulfenylation, on the other hand, has a moderate negative p value. No cation is formed that is delocalized round the ring, but electrons flow out of the ring and we suspect some loss of conjugation. All this fits well with the formation of a three-membered ring intermediate. From experiments like this we learn that PhSCl is much more likely than bromine to react stereo specifically with alkenes through cyclic cation intermediates. 

Three-membered cyclic sulfonium ions, representing [3 carbocations, are often encountered in participation reactions. We have seen this already in the way mustard gas works, but almost any arrangement of a sulfide with a leaving group on the (3 carbon atom leads to participation and the formation of a three-membered ring. The product is formed by migration of the PhS group from one carbon atom to another (Chapter 37). 

The oxenoid mechanism implicates the insertion of an oxygen atom (oxenoid) across the C-H bond of the hydrocarbon (Hamilton et al., 1973). This mechanism is evidently energetically preferable, since it is accompanied by the formation of three bonds, two of which, O-H and C-O, are extremely strong and compensate the rupture of the relatively weak C-H and 0-0 bonds. The transition state, however, involves the formation of a three-membered ring with oxygen, whose formation is accompanied by a strain with an energy of about 30 kcal/mole. More over, the insertion of O to C-H or H-H bonds is a symmetrically forbidden process. 

The reaction of alkenes with Fischer carbene complexes most typically leads to cyclopropane products however, the formation of a three-membered ring product from a reaction with an alkyne has been observed on only one occasion. The reaction of the cationic iron-carbene complex (199) with 2-butyne presumably leads to the formation of the cyclopropene (200), which was unstable with respect to hydride abstraction by the starting carbene complex and the ultimate product isolated from this reaction was the cyclopropenium salt (201) and the benzyl-iron complex (202). Cyclopropene products have never been observed from Group 6 carbene complexes despite the extensive investigations of these complexes with alkynes that have been carried out since the mid 1970s. 

Under some experimental conditions, electrophilic addition of either CU or a proton may form stable three-membered-ring intermediates. Thus, when a double bond undergoes stereospecific anti addition, formation of a three-membered-ring intermediate analogous to the bromonium ion is often part of the mechanism. 

The pyridine adduct of the early transition metal titanium undergoes a unique cyclometallation with formation of a three-membered ring adduct ... 

Let us now look at the formation of a three-membered ring. Draw a conformation in which the terminal carbons of a propane molecule are close together. 

The examples given so far have involved the formation of a three-membered ring. However, this effect is not limited to cases in which a three-membered ring may be formed. Suggest which other ring sizes may be observed. 

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