Cycloadditions and Ring Expansion Reactions

Iron-catalyzed Cycloadditions and Ring Expansion Reactions 

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Since aromatic substitutions, aliphatic substitutions, additions and conjugate additions to carbonyl compounds, cycloadditions, and ring expansion reactions catalyzed by Fe salts have recently been summarized [17], this section will focus on reactions in which iron salts produce a critical activation on unsaturated functional groups provided by the Lewis-acid character of these salts. 

This chapter will focus on preparative applications of iron catalysts in the synthesis of cyclic ring systems by means of intra- and intermolecular cycloadditions, Alder-ene reactions and ring expansion reactions. Previous reviews concerning iron-catalyzed chemistry have either focused their attention on different aspects involving iron-containing compounds as active catalysts [1] or have concentrated on certain reactions [2] and on the synthesis of specific substance classes [3]. A more recent general review concerning all aspects of modern applications in iron-catalyzed reactions has been summarized by Bolm et al. [4]. 

On the other hand, small-ring alkenes such as diphenylcyclopropene derivatives (125) may serve as substrates for the preparation of six-membered heterocycles by way of the cycloaddition-extrusion-ring-expansion reaction. Thus, thd reaction of (9) with (125a) leads to formation of (127a) via the primary adduct (126), whereas the reaction of (9) with (125b) yields (127b) and the tricyclic derivative (128) (75JCS(P1)632). 

Cycloaddition-extrusion-ring expansion reactions of five-membered mesoionic oxazolones and thiazolones with 2,3-diphenylcyclopropenethione produced pyridine-4(l//)-thiones... 

Further examples including extrusion and ring expansion reactions <75JCS(Pi)632> as well as intramolecular [3 -I- 2] cycloadditions have been reported <81LA347>. In addition, the mesoionic 1,3-dithiolones (124) react with formaldehyde regiospecifically in a [3 -i- 2] cycloaddition reaction with formation of the 2-oxa-6,7-dithiabicyclo[2.2.1]heptanone derivative (125) (Equation (17)) <78CB3l7l>. Furthermore, upon reaction with the N=N bond of dimethyl azodicarboxylate, (126) is obtained (Equation (18)) <78CB3171>. 

The highly strained and reactive 2iT-azirines have been extensively studied for various synthetic purposes, such as ring expansion reactions, cycloaddition reactions, preparation of functionalized amines and substituted aziridines. The older literature on azirines in synthesis has extensively been reviewed [69]. Concerning azirines with defined chirality only scarce information is available. Practically all reactions of azirines take place at the activated imine bond. Reduction with sodium borohydride leads to cz5-substituted aziridines as is shown in Scheme 48 [26,28]. 

The ring expansion reaction of diaryl cyclopropenones and cyclopropene thiones occurring with pyridinium, sulfonium, and phosphonium enolate betaine 427268-270) is related to 1,3-dipolar cycloadditions. This process results in formation of 2-pyrones 428 by loss of pyridine (or sulfide or phosphine) and insertion of the remaining fragment C=C-0 to the C1(2)/C3 bond of the cyclopropenone ... 

The reaction of methylenecyclopropanes with transition metal complexes is well known to promote a catalytic a-ir cycloaddition reaction with unsaturated compounds, in which a trimethylenemethane complex might exist71-76. Recently, much interest has been focused on the interaction of strained silicon-carbon bonds with transition metal complexes. In particular, the reaction of siliranes with acetylene in the presence of transition metal catalysts was extensively investigated by Seyferth s and Ishikawa s groups77-79. In the course of our studies on alkylidenesilirane, we found that palladium catalyzed reaction of Z-79 and E-79 with unsaturated compounds displayed ring expansion reaction modes that depend on the (Z) and (E) regiochemistry of 79 as well as the... 

Analogous to the cycloadditions described above, the first step of these ring expansion reactions is believed to involve the initial oxidative coupling between the carbonyl and the alkyne to afford a nickelapentenacycle (12, Scheme 2) [37,38]. Subsequent (3-carbon elimination relieves ring strain and affords a seven-membered nickelacycle 13a that reductively eliminates the... 

The reaction of carbenes with dienamines is complex and results in cycloaddition to either or both double bonds (Scheme 15) and ring expansion (see Section VII.D). Ethoxycarbonylcarbene gives only the ft- and <5-ethoxycarbonylmethyl derivatives, presumably by ring opening of the corresponding cycloadducts24. 

Photochemical cyclobutane annealings are much more promising and find their way into synthesis to a greater extent. Photochemical cycloaddition reactions to enolized 1,3-diketones and consecutive ring openings via retroaldol reactions have been applied in ring expansion reactions numerous times. 

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