Ring expansion heteroatoms

Cyclopropylcarbinyl-cyclobutyl ring expansions (Eq. 5) are facilitated by the presence of the heteroatom substituent in the order O > S > Se. In this case, the heteroatom stabilized cyclobutyl cation (see Eq. 39) can suffer hydrolysis to give the... 

Heteroatoms can also participate in the ring expansion reaction mentioned above. For example, a hydroxyl methoxyallenylphthalan gave rise to a 4-isochromanone derivative. A hydroxymethoxyallenylisoindolinone afforded an isoquinolinedione derivative efficiently (Scheme 16.23) [27, 28]. 

Some of the ring expansion reactions discussed in Section 2.03.3.3.1 can be extended to five-membered heterocycles containing two or more heteroatoms. Reaction of imidazoles and pyrazoles with dichlorocarbene, for example, gives chloropyrimidines together with small amounts of chloro-pyrazines or -pyridazines, and oxidative ring expansion of 1-aminopyrazole with nickel peroxide gives 1,2,3-triazine (this, in fact, constitutes the only known synthesis of the unsubstituted triazine). There are, however, a number of interesting and useful transformations which are unique to five-membered polyheteroatom systems. 

Furthermore, if we consider the carbometalative ring expansion to produce the corresponding five-membered ring zirconacycle 86, the carbon-heteroatom bond of the sp3 metalated center Cx should isomerize to produce the most stable intermediate. Such isomerization could be due to an interaction between the heteroatom moiety XR and the zirconium atom [65], which would produce a weakness of the Cj-Zr bond and would facilitate the isomerization. Thus, whatever the stereochemistry of the starting material, a conformation is always possible in which Cj-SR is antiperiplanar to C2-C3 in 86 with a trans relationship between R and the ZrCp2 fragment. The elimination reaction, or decarbo-zirconation, occurs in a concerted way to give the E-vinyl zirconium 83. Unfortunately, neither the zirconacyclopentane nor the zirconacyclopropane have been trapped as intermediates. 

Several ring expansion reactions of azoniaboratacyclopropanes have appeared (Scheme 2) <1996AOM147>. All involve, in essence, insertion of substrate into the B-N bond, with attachment of the substrate heteroatom to boron,... 

The temperatures required for these expansions are typically below 1(X) °C if one or two electron-donating heteroatom substituents is attached to the bridgehead positions. Considerably higher temperatures are required in the absence of such substituents. For example, thermolysis of (97) requires 180-2(X) C, the amide (98) must be heated to over 450 C in order to achieve ring expansion, and the cyclobutene (99) is reported to be thermally stable. ... 

Cyclopropanes adjacent to an electron-deficient centre X (carbon or heteroatom) undergo C3 - C4 ring enlargement into four-membered ring derivatives via routes involving cations, radicals or car bene intermediates. Furthermore, when the cyclopropanes also bear on the same carbon an electron-donor substituent Y they undergo a specific C3 - C4 ring expansion, important from the synthetic point of view, into cyclobutanone (or related) derivatives (equation 67). 

In principle, this ring-expansion reaction should proceed for any metalloid heterocycle with a chloromethyl substituent. However, no system other than one with a silicon heteroatom has been studied. 

Ring expansions of appropriately a-substituted ketones via their vinyl carbinol derivatives has also been carried out under cationic conditions. The basic sequence, outlined in Scheme 66, begins with the addition of a vinyl organometallic to an a-substituted ketone, where X is carbon or a heteroatom. Departure of a leaving group Y then produces a cationic species, which may react further to form products. Two main pathways appear to be operative one is a cationic alkene cyclization, followed by a pinacol-like rearrangement while the other is a 3,3 or 3,3-like rearrangement, followed by an intramolecular alkylation of the intermediate enol. 

Another type of reactivity typical of zirconium and hafnium alkyne complexes is ring expansion to yield five-membered and larger metallacycles. A variety of unsaturated organic reagents can be added including alkenes, alkynes, and ketones. This chemistry has been reviewed in detail. Heteroatom-stabilized zirconocene alkyne... 

The introduction of a carbonyl moiety into an organic molecule using carbon monoxide requires the presence of a transition metal complex either as a catalyst or as a stoichiometric reactant. The insertion of carbon monoxide into a carbon-heteroatom bond of the heterocyclic compound comprises a simultaneous ring expansion and functionalization of a heterocyclic substrate. The carbonylation reaction provides a very convenient and effective one-step procedure for ring homologation (95ACR414). 

If one of the heteroatoms is present as an OH group then only one nucleophile is involved and molecules such as cyanohydrins (7) are obviously made from carbonyl compounds and HCN. Hence hydroxy amine (8), needed for a ring expansion (see Chapter 30), can be made by reduction of (9) (see Chapter 8) and hence from cyclohexanone. 

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