Ring expansion catalysis

Treatment of aziridinecarboxylic esters having an electron-withdrawing substituent at nitrogen with acetonitrile under BF3 catalysis leads to a smooth ring expansion reaction as depicted in Scheme 28 [31]. 

In 1996, Brookhart and co-workers developed a remarkable class of Pd complexes with sterically encumbered diimine ligands (Scheme 4, S4-1, S4-2, S4-4, and S4-5). These examples are capable of mediating the co-polymerization of ethylene with methyl acrylate (MA) to furnish highly branched PE with ester groups on the polymer chain ends by a chain-walking mechanism (Scheme 10). " This represents the first example of transition metal-catalyzed ethylene/MA co-polymerization via an insertion mechanism. The mechanism for co-polymerization is by 2,1-insertion of MA and subsequent chelate-ring expansion, followed by the insertion of ethylene units. The discovery of these diimine Pd catalysts has stimulated a resurgence of activity in the area of late transition metal-based molecular catalysis. Recently, the random incorporation of MA into linear PE by Pd-catalyzed insertion polymeriza-... 

An asymmetric [1,2]-Wagner-Meerwein shift has been achieved under Pd catalysis, allowing ring expansion of l-(alkyloxyallenyl)cyclobutanol and simple derivatives to ... 

Silacyclobutanes as well as silacyclopropanes undergo aldehyde insertion under catalysis by /-BuOK (Equation (77)).292 The reaction of silacyclobutanes with lithium carbenoids such as dihalomethyllithium and oxiranyllithium gives 2-substituted silacyclopentanes (Equation (78)). Treatment of l-(l-iodoalkyl)- and 1-oxiranyl-silacyclobutanes with a stoichiometric amount of an alkali alkoxide leads to silacyclopentanes by anionic 1,2-shift of the ring carbon adjacent to silicon. These ring-expansion reactions proceed probably via a pentacoordinate silane intermediate. 

A diazo ester or alkane will react with a ketone, under Lewis acid catalysis, to give the homologated product. This reaction is most often used for one-carhon ring expansion, as illustrated hy the conversion of (166) to (167 equation 69). The regioselectivity of the (166) to (167) transformation, 98 2 in the example illustrated, was found to depend on both the diazoalkane and the Lewis acid used. With diazomethane and boron trifluoride, a 1 1 mixture of regioisomeric products was observed. 

The homologation of ketones by the addition of diazoalkanes complements the Tiffeneau-Demjanov rearrangement. Epoxide formation is a side reaction which can be minimized if polar aprotic solvents are avoided (Scheme 7). Rearrangement i.e. homologation) is maximized in ether solvents or by Lewis acid catalysis. The reaction is most effective in the ring expansion of cyclic ketones. 

MORIN Penicillin Rearrangement Ring expansion of penams to cephems under acidic catalysis. 

The benzofuro-annelated azabicyclo[3.2.0]heptenone (34) undergoes both thermal and acid-catalysed ring-expansion. The former gives (36) directly while acid catalysis gives firstly (35), which is isomerized to (36) by treatment with base. ... 

Removal of a proton from 1,2-disubstituted diazetidinones (91) results in an instantaneous ring-expansion to imidazolidinones (92) in a similar reaction to Stevens rearrangement <86JOCl537>. Another interesting rearrangement to pyrroloimidazoles (94) has been described from trisubstituted aziridine (93) using base catalysis <85JOC2220>. Both reactions are shown in Scheme 16. 

Under the catalysis of Pd(PPh3)4, l-propadienyl-2-(Z-o)-iodo-(allylpalladium intermediate 146 after oxidative addition and intramolecular carbopalladation. Nemoto and co-workers observed that the ring strain of the four-membered ring in 146 was released by ring expansion to afford the bicy-clo[6.3.0]undecane derivatives 147 and/or 148 with a cyclopentanone moiety (Scheme 47)J ... 

Cyclopentanones can be obtained from the ring expansion of methyl-enecyclobutanone and its 3-substituted derivatives with Pd catalysis, and Binger and Schuchardt have shown that cycloaddition reactions between methyl-enecyclopropane and alkenes lead to methylenecyclopentanes (52) when the reactions are catalysed by Pd°. ... 

The proposed reaction mechanism is shown in Scheme 6.75. The nitroalkene moiety of bifunctional ortAo-alkyne-substituted nitrostyrenes 159 is activated through hydrogen bonding with catalyst 160 to incorporate the stereoehemieal information in the first AFC reaction. Then the alkyne is activated under gold catalysis to affect the seeond AFC/ring expansion cascade. 

Figure 4.37 Proposed mechanisms of DAOCS/DACS catalysis of the ring expansion of penicillin N to DAOC.

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