Epoxides ring-expansion carbonylation

Abstract Development in the field of transition metal-catalyzed carbonylation of epoxides is reviewed. The reaction is an efficient method to synthesize a wide range of / -hydroxy carbonyl compounds such as small synthetic synthons and polymeric materials. The reaction modes featured in this chapter are ring-expansion carbonylation, alternating copolymerization, formylation, alkoxycarbonylation, and aminocarbonylation. 

The ring-expansion carbonylation of epoxides is the most widely studied field in the epoxide carbonylation chemistry since the product lactones are highly attractive targets particularly, /1-lactones are useful compounds due to their versatility in organic synthesis [ 14,15] as well as their utilization as monomers to produce poly(3-hydroxyalkanoate)s, naturally occurring biodegradable polyesters [16-19]. 

Altihough many of the early examples of the carbonylation of heterocycles included reactions of tetrahydrofurans, oxetanes, and azetidines, the majority of recent work has focused on the reactions of epoxides and aziridines. At this point, the ring-expansive reactions of epoxides are more general than the reactions of aziridines and occur imder milder conditions. Prior to 1994, ring-expansive carbonylation of epoxides was restricted to a few substrates. The patent by Droit and Kragtwijk s in 1994 inspired further work on these t)rpes of carbonylations, and tiiis work led to dramatic improvements in reaction scope. 

Disubstituted epoxides undergoing ring-expansion carbonylation by catalysts 4 and 5 of Scheme 17.24. 

Scheme 9.45 Carbonylative iron-catalyzed ring expansion of epoxides.

The transition-metal-catalyzed carbonylation reaction has been extensively investigated, and especially the carbonylative ring expansion reaction of strained heterocycles has been shown to be a useful and efficient procedure to synthesize lactams, lactones, and thiolactones.203 The carbonylation of epoxides and aziridines 450 is a powerful tool to construct the /Mactone and /Mactam skeletons 451 (Scheme 142).204 This type of reactions can be regarded as a hetero-[3 + 1]-cycloaddition. 

Treatment of the epoxide (429) with dimethylthioformamide in the presence of TFA produces a mixture of the thiirane (430), furan (431), and thiophene (432) (Scheme 86) <88S467>. The thiirane (430) is converted to (432) in good yield by treatment with TFA. Protonation of the carbonyl group of (430) followed by ring expansion and dehydration would explain the formation of (432) (Scheme... 

With the same concept, but using the more reactive Ti(III) cationic radical [Cp2TiCl(THF)2] or a cationic salphen aluminum complex in combination with the cobalt anion [Co(CO)4] , Coates et al. succeeded to make the epoxide or aziridine carbonylative ring expansion reaction catalytic (Scheme 60) [149]. For both substrates, it is proposed a nucleophilic attack of the cobalt anion at the least-substituted carbon atom of the three-membered ring, the latter being activated by the Lewis acidic part of the catalyst. Of note, catalysts 106 and 107 used in this reaction are described as ion pairs rather than M-Co bond containing complexes. 

The carbonylation of epoxides and aziridines has been studied for several decades, and two forms of this process are now well established. Highly active catalysts for the ring expansion of epoxides and aziridines to p-lactones and 3-lactams are now known. In addition, conditions have been developed for the hydroformylation of epoxides to a-hydroxy aldehydes (including protected a-hydroxyacetals), and similar conditions have been developed for the tandem hydroformylation and hydrogenation of epoxides to generate... 

The accepted mechanism for the carbonylation of epoxides is shown in Scheme 17.26, and the basic steps of this cycle are also thought to occur during the carbonylation of aziridines. Alper first proposed a catalytic cycle for the expansion carbonylation of aziridines by [Co(CO)J, and Coates has proposed a similar cycle for epoxide carbonylation catalyzed by complexes containing both Lewis acids and cobalt-carbonyl anions (Scheme 17.26). This mechanism consists of four steps (1) the activation of substrate by coordination to a Lewis acid (2) the S 2 attack on the substrate by [Co(CO)J (3) the insertion of CO into the new cobalt-carbon bond, and the subsequent uptake of CO and (4) ring closing with extrusion of product and regeneration of the catalytic species. 

Transition metal catalyzed ring expansions of cyclic ethers to lactones under pressures of CO [51, 52] have been reported for tetrahydrofuran [53], oxetanes, and epoxides [54—56]. Carbonylation of epoxides is particularly important since P-lactone products are challenging synthetic targets (see Section 2.2.5). Using Co(CO)4 in combination with a Lewis acidic Al-salen counterion, the reaction of (R)-propylene oxide and CO occurs with stereochemical retention (Scheme 2.23) [57]. The mechanism is believed to involve Lewis acid activation of the epoxide followed by nucleophilic ring opening with Co(CO)4 [58]. 

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