Other Small-ring Intermediates

Other Small Ring Intermediates. Arguments and evidence for phenonium ions in a variety of cation-forming reactions of 2-phenylethyl derivatives have been presented. For example, acetolysis of [l- C]-2-phenylethyl triflate proceeds with ca. 32 % rearrangement of the label to the 2-position, and recovered starting material is also isotopically scrambled.  

4-(l-Furyl)-l-diazobutan-2-one undergoes an interesting rearrangement to a cyclo-pentenone on decomposition with copper sulphate, believed to occur via the intramolecular carbene addition product (836).  

A related reaction is the vinylogous Wolff rearrangement of Py-unsaturated ketones (837 n = 1 or 2 R = H or Me) to y5-unsaturated esters (839). Labelling at the 2-position (799 R = D or Me) implicates the intermediacy of a bicyclopentanone (838) in this rearrangement.  

The thermal decomposition of the sulphone (840) has been suggested to occur via 

Further evidence for the intermediacy of cyclopropenes in the cycloheptatrienyli-dene-arylcarbene interconversion comes from the trapping of the cyclopropene by Diels-Alder reactions with cyclopentadiene, furan, butadiene, and tetracyclone (see also p. 97). 

Attempts at making epoxycyclopropanes (595) from a -unsaturated ketones and cyclopropyl ketones with ylide methylene-transfer reagents have been reported. However, compounds were isolated which are believed to be products of rearrangement of the unisolable epoxycyclopropanes. 

It is known that phenylcarbene interconverts in the gas phase with cyclo-heptatrienylidene and that it undergoes ring contraction to give fulvenallene and 1-ethynylcyclopentadiene. It has now been reported that phenylcarbene 

Miscellaneous.—a-Elimination. The reaction of 1,1-dibromocyclopropanes with alkyl-lithiums continues to be used for routine generation of allenes.  

As part of an extended theoretical study of carbenes by the MINDO/2 method, Bodor, Dewar, and Wasson have considered the electronic ground states of cyclopropylidene and cyclopropenylidene. The former was calculated to have a triplet ground state, like simple, acyclic carbenes, but with a smaller triplet-singlet energy difference. The calculations suggest that cyclopropenylidene has a singlet electronic ground state. 

As reported last year, 6,6-dibromobicyclo [3,1,0] hexane (603) with methyl-lithium gives cyclohexa-1,2-diene, which has but a transient existence before it either dimerizes or is trapped intermolecularly. The dimers, prepared from mono- and di-deuterio-(603) have been made and investigated by Fourier-transform n.m.r. spectroscopy. The vinyl lallyl deuterium ratio in the dimer from [ H]-(603) was found to be 1.04 and this corresponds to for the formation of the doubly allylic C—C bond in (604). Although the communication illustrates an interesting technique, it does not lead to a firm mechanistic conclusion about the nature of the allene dimerization process. 

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This chapter follows the organization used in the past. A summary of the electronic properties leads into reports of electrocyclic chemistry. Recent reports of studies of HDS processes and catalysts are then summarized. Thiophene ring substitution reactions, ring-forming reactions, the formation of ring-annelated derivatives, and the use of thiophene molecules as intermediates are then reported. Applications of thiophene and its derivatives in polymers and in other small molecules of interest are highlighted. Finally, the few examples of selenophenes and tellurophenes reported in the past year are noted. 

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 71-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two-electron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

As mentioned above, the higher reactivity of compared to D4 is due to the higher nucleophilicity of the former, which results in higher concentration of the silyloxonium intermediate. However, some enhancement of the reactivity of D2 because of a small ring strain cannot be excluded. On the other hand, relatively fast cyclics interconversion in the polymerization of E>2 results finm the high basicity of ojg gen atoms in the polyoxydisilylene chain (Table 1). 

Other biradicals are formed as intermediates in the pyrolysis of small ring compounds. If we consider olefins as two-membered rings, the thermal cis-trans isomerization of olefins can be considered as proceeding through a transition state which is a twisted (90° twist) 1,2-biradical ... 

The structural complexity of organic compounds arises from carbon s small size, intermediate EN, four valence electrons, ability to form multiple bonds, and absence of d orbitals in the valence level. These factors lead to chains, branches, and rings of C atoms joined by strong, chemically resistant bonds that point in as many as four directions from each C. The chemical diversity of organic compounds arises from carbon s ability to bond to many other elements, including O and N, which creates polar bonds and greater reactivity. These factors lead to compounds that contain functional groups, specific portions of molecules that react in characteristic ways. 

Azides are important for the synthesis of two classes of small ring nitrogen heterocycles 2H-azirines and aziridines. The synthetic methods fall into two broad categories (1) those in which organic azides are the direct precursors and (2) those in which azides are used to provide intermediates that are then converted into heterocycles. We aimed to provide examples of both types. Routes that lead to 2//-azirines and those that lead to aziridines are described in Sections 6.2 and 6.3. Azides have been used less frequently for the preparation of other small nitrogen heterocycles but some examples are given in Sections 6.4 and 6.5. 

Although reasonably stable at room temperature under neutral conditions, tri- and tetrametaphosphate ions readily hydrolyze in strongly acidic or basic solution via polyphosphate intermediates. The hydrolysis is first-order under constant pH. Small cycHc phosphates, in particular trimetaphosphate, undergo hydrolysis via nucleophilic attack by hydroxide ion to yield tripolyphosphate. The ring strain also makes these stmctures susceptible to nucleophilic ring opening by other nucleophiles. 

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