Epoxide carbonylation

The mechanism of catalytic epoxide-expansion carbonylation has been the subject of several investigations. Because the catalytic formation of 3-lactones from epoxides generates a new carbonyl compound, it is easily monitored by in situ IR spectroscopy. In addition to kinetic and reactivity studies, computational studies - have been conducted to probe the mechanism of epoxide-expansion carbonylation. The mechanistic aspects of the carbonylation chemistry based on these results are presented in this section. 

Catalytic cycle for the carbonylation of epoxides catalyzed by the aluminum/cobalt catalyst. L = Lewis base (solvent, epoxide, lactone). 

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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]. 

E+ = alkyl halides, epoxides, carbonyl compounds, 2,/1-unsaturated carbonyl compounds... 

Enals vinyl silyl ketones.1 The anion of I reacts smoothly with va rious electrophiles (alkyl halides, epoxides, carbonyl compounds). The products are converted to (E)-enals by oxidation with 30% H202. 

Vinyl selenides have been lithiated at the a-position by LDA983,984 at —78 °C in THF to give a-(arylselanyl)vinyllithiums 680, a-(methylselanyl)vinyllithiums 681 being obtained by selenium-lithium transmetallation from l,l-bis(methylselanyl)alkenes with n-BuLi in THF or t-BuLi in ether at —78 °C985 986. These intermediates reacted with alkyl halides, epoxides, carbonyl compounds and DMF985, the final deprotection being performed by mercury(II) salts986. 

Metallated 1-ethoxy-1,3-dienes 697 and 712, obtained from the corresponding acetals by means of the LICKOR base, have been treated with alkyl halides, epoxides, carbonyl compounds, carbon dioxide and carboxylic esters affording ( )-l-substituted 1-ethoxy-1,3-dienes and, after hydrolysis, a,P-unsaturated carbonyl compounds1007-1010 (Scheme 186). Intermediates 697 and 712 have been transformed into the corresponding vinyl stan-nanes, which were submitted to Stille couplings with iodobenzene and benzoyl chloride823. 

Instead of epoxides carbonyl compounds may be used as electrophiles to generate the y-hydroxy carboxylic acid intermediate. In the classical Stobbe reaction (equation 79) succinic esters are deprotonated and treated with aldehydes or ketones to form, via the unstable adduct (211), the paraconic ester (212). 

Both LiBr and Lil have been widely used to effect epoxide-carbonyl rearrangements. These salts differ significantly in reactivity, although no systematic comparison has been made. LiBr in benzene (or toluene) solvent requires a solubilizer (unlike LiC104, for which the epoxide itself is sufficient). Lil does not require a solubilizer, and has been used in several solvents, most often CH2CI2 the possible role that solubilizer or solvent may play in Lil reactions has not been addressed. Lil is often employed as a hydrate, but occasionally anhydrous salt is specified, although the purity of this difficult to dry material may be questioned. 

There are many different variations for epoxide-carbonyl rearrangements that utilize lithium salts. Unfortunately, there are very few controlled comparisons of the sort needed to determine the best choice of LiX, solvent, addend and conditions for a particular application. 

The coordination properties of phosphine oxides has been explored with late transition-metal (Ru, Co, Rh, Ir, Pd, Pt, Cu, and Au),301 303 305 306 310 316 early transition-metal,317 lanthanide,304,318,319 and actinide307,320 ions. One interesting complex is the palladium(II) complex (148) (Scheme 10) which is an extremely rare example of a ds metal center with a tetrahedral geometry.313 Phosphine oxides have found uses in the extraction of alkali, alkaline earth, and actinide metals in catalysis (hydroformylation of alkenes and epoxides, carbonylation of methanol324) and as a useful crystallization aid (Ph3PO).325... 

Epoxide-carbonyl rearrangement.2 Lithium bromide effects facile rearrangement of epoxides to aldehydes and/or ketones in benzene solution. The salt is insoluble in benzene but addition of 1 mole of HMPT or tri-n-butylphosphine oxide per mole of lithium bromide affords a soluble complex which effects the epoxide rearrangement. Evidence suggests a mechanism involving the salt of the bromohydrin as an intermediate. 

Discrete cationic aluminum complexes of Co(CO)4 have unprecedented activity and selectivity for epoxide carbonylation (Scheme 6.109) [14]. Complex 9 carbony-lated propylene oxide with 95% conversion in 1 h under a high pressure of CO. Because (-i-P)- -butyrolactone is of particular interest for polymerization and other asymmetric transformations, (P) -propylene oxide was treated wifh CO and catalyst 9. Propylene oxide was converted to (P)- -butyrolactone wifh >98% retention of configuration. 

Paracyclophane (la) suflers from peracid oxidation readily oxidation of la with MCPBA proceeded at 0 °C to yield the dimer 109 quantitatively [5b], The formation of dimer 109 is explained in terms of [4 + 2] dimerization of the cyclohexadienone 108 which was formed by epoxide-carbonyl rearrangement of the initially formed epoxide 107 (Scheme 20). Naphthalenophane 55a and anthracenophane 56a (s. Scheme 13) were more reactive the MCPBA oxidation completed immediately at — 78°C to give unstable dienones 110 and 111 (Structures 21) [50]. 

An interesting chelation-controlled regioselective ring enlargement, by means of an epoxide-carbonyl rearrangement, i.e. (8) -... 

Types of Catalysts and Scope of Substrates for Epoxide Carbonylation... 

Direct synthesis of succinic anhydrides from epoxides has also been achieved by using the catalyst containing the aluminum Lewis acid and cobalt carbonyl anion (Equation 17.48). The carbonylations of epoxide and p-lactone in the presence of this catalyst were found to occur separately and sequentially, with p-lactone carbonylation occurring only after all of the epoxide had undergone carbonylation. p-Lactone carbonylation was slow in polar or donor solvents. Because epoxide and p-lactone carbonylation occur with opposing solvent dependences, the carbonylation of the epoxide to the anhydride in a single pot depended upon the identification of a solvent (1,4-dioxane) that is sufficiently donating to accelerate epoxide carbonylation, but sufficiently weak in polarity to allow rapid p-lactone carbonylation. 

Efforts to develop aziridine carbonylation have occurred in parallel with efforts to develop epoxide carbonylation. The 3-lactams that are formed by this process are important in medicinal, organic, and polymer chemistry. Important contributions to aziridine carbonylation have been made by Alper, Davoli and Prati, and Coates. " A series of carbonylations of aziridines were developed using [Rh(CO)jCl]j and COj(CO)jas catalyst. More recently, faster rates and expanded scope have been found with catalysts that combine a Lewis acid cation with the [Co(CO)J" anion. 

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. 

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