Carbon formate esters

The mechanism of this reaction has been studied by several groups [133,174-177]. The consensus is that interaction of ester with the phenolic resole leads to a quinone methide at relatively low temperature. The quinone methide then reacts rapidly leading to cure. Scheme 11 shows the mechanism that we believe is operative. This mechanism is also supported by the work of Lemon, Murray, and Conner. It is challenged by Pizzi et al. Murray has made the most complete study available in the literature [133]. Ester accelerators include cyclic esters (such as y-butyrolactone and propylene carbonate), aliphatic esters (especially methyl formate and triacetin), aromatic esters (phthalates) and phenolic-resin esters [178]. Carbamates give analogous results but may raise toxicity concerns not usually seen with esters. 

The addition of Grignard reagents to aldehydes, ketones, and esters is the basis for the synthesis of a wide variety of alcohols, and several examples are given in Scheme 7.3. Primary alcohols can be made from formaldehyde (Entry 1) or, with addition of two carbons, from ethylene oxide (Entry 2). Secondary alcohols are obtained from aldehydes (Entries 3 to 6) or formate esters (Entry 7). Tertiary alcohols can be made from esters (Entries 8 and 9) or ketones (Entry 10). Lactones give diols (Entry 11). Aldehydes can be prepared from trialkyl orthoformate esters (Entries 12 and 13). Ketones can be made from nitriles (Entries 14 and 15), pyridine-2-thiol esters (Entry 16), N-methoxy-A-methyl carboxamides (Entries 17 and 18), or anhydrides (Entry 19). Carboxylic acids are available by reaction with C02 (Entries 20 to 22). Amines can be prepared from imines (Entry 23). Two-step procedures that involve formation and dehydration of alcohols provide routes to certain alkenes (Entries 24 and 25). 

Scheme 10.1 gives some representative examples of laboratory syntheses involving polyene cyclization. The cyclization in Entry 1 is done in anhydrous formic acid and involves the formation of a symmetric tertiary allylic carbocation. The cyclization forms a six-membered ring by attack at the terminal carbon of the vinyl group. The bicyclic cation is captured as the formate ester. Entry 2 also involves initiation by a symmetric allylic cation. In this case, the triene unit cyclizes to a tricyclic ring system. Entry 3 results in the formation of the steroidal skeleton with termination by capture of the alkynyl group and formation of a ketone. The cyclization in Entry 4 is initiated by epoxide opening. 

Epoxides can also be reductively opened to form a radical. An example of an intramolecular cyclization of such a radical has recently been reported <06TL7755>. Treatment of 40 with Cp2TiCl generates an intermediate alkoxy radical, which then adds to the carbonyl of the formate ester. The product, 41, is formed as a 2 1 mixture of isomers at the anomeric carbon. This reaction is one of the first examples of a radical addition to an ester. The major byproduct of this reaction is the exo-methylene compound, 42, arising from a P-hydrogen elimination. 

Hydrogenation of carbon dioxide in the presence of an epoxide generates a mixture of the diol, its formate esters, and the cyclic carbonate. While the reaction has been shown to operate in high yield (1300 TON for the cyclic carbonate Eq. (10)) [93], the fact that a mixture is generated and that the cyclic carbonate could be made more cleanly in the absence of H2 makes the reaction uninteresting for synthesis. Sasaki s group showed that this reaction in the presence of an amine base gives CO rather than cyclic carbonate (Eq. (11)) [94]. The epoxide then serves as a trap for the water. 

Scheme 7.19). Prototropic shift of the initial adduct to produce ROCHCl2 and, subsequently, the formate ester is a less favourable pathway. Alternatively, the carbon monoxide-separated ion-pair can lose a proton leading to an alkene, or cycloadducts derived from further reaction with the carbene. The formation of rearranged products from the reaction of 1 -hydroxymethyladamantane suggests that a relatively unencumbered carbenium cation can also be generated, which leads to a Nametkin rearrangement of the system [4]. 

The formation of formate esters in the hydroformylation reaction (90, 64) may be explained by a CO-alkoxide insertion reaction as well as by the CO-hydride insertion mechanism mentioned above. Aldehydes formed in the hydroformylation reaction can be reduced by cobalt hydrocarbonyl (27) presumably by way of an addition of the hydride to the carbonyl group (90, 2). If the intermediate in the reduction is an alkoxycobalt carbonyl, carbon monoxide insertion followed by hydrogenation would give formate esters (90, 64). 

If the formation of formate esters under hydroformylation conditions involves the carbonylation of an alkoxycobalt carbonyl as suggested previously (90), this would be evidence that cobalt hydrocarbonyl adds the reverse way to acyl groups. Since the formation of formate esters can be explained as well by a CO insertion into a cobalt-hydrogen group followed by alcoholysis, however, the data would be explained best if a cobalt-carbon bond was formed in the hydride reduction of acyl compounds. 

On the other hand, the linear ester 7 can be prepared as the major product by the carbonylation of a 1-alkene in the presence of a formate ester using Pd(Q) with dppb as a catalyst[12], The linear acid 8 is obtained as the main product by using Pd(OAc)2 or even Pd on carbon as a catalyst and dppb as a ligand in DME in the presence of formic acid or oxalic acid under CO pressure[13]. The linear ester 9 is obtained from a 1-alkene as the main product using PdCI2(Ph3P)2 coordinated by SnCl2[14]. 

Organic acid esters carbonates (e.g. ethyl carbonate), formates, acetates (e.g. ethyl acetate, butyl acetate, amyl acetate), propionates, etc., oxalates (e.g. ethyl oxalate), maleates, phtha-lates (e.g. butyl phthalate) carbamates and phenylcarbamates (e.g. ethyl phenylcarbamate). 

Several groups have been successful at the catalytic conversion of carbon dioxide, hydrogen, and alcohols into alkyl formate esters using neutral metal - phosphine complexes in conjunction with a Lewis acid or base (109). Denise and Sneeden (110) have recently investigated various copper and palladium systems for the product of ethyl formate and ethyl formamide. Their results are summarized in Table II. Of the mononuclear palladium complexes, the most active system for ethyl formate production was found to be the Pd(0) complex, Pd(dpm)2, which generated 10/imol HCOOEt per /rniol metal complex per day. It was anticipated that complexes containing more than one metal center might aid in the formation of C2 products however, none of the multinuclear complexes produced substantial quantities of diethyl oxalate. 

It is known from free radical chemistry 6S> that the hydrogen attached to a carbonyl carbon is easily abstracted as a hydrogen atom. In aldehydes and formate esters it is this hydrogen that is abstracted by a free radical ... 

RNH2 or R2NH Formation of substituted urea from carbonic acid esters (Figure 6.39)... 

This reaction is a useful method to prepare alcohols with two identical groups on the carbon bonded to the hydroxy group. Formate esters produce secondary alcohols other esters produce tertiary alcohols. Examples are provided in the following equations. Again, acyl chlorides and anhydrides also give this reaction, but they are seldom used because they offer no advantages over esters. 

Recently, the use of carbon dioxide as a carbon building block [152] has attracted increasing attention. The hydrosilylation of carbon dioxide catalyzed preferably by ruthenium complexes leads to the synthesis of silyl formate esters (Eq. 98) [153]. Results of the reaction of hydrosilylation in supercritical carbon dioxide as a solvent and substrate have recently been reported [154]. 

For the alkyl carbonates and esters such as butyrolactone, CaH2 can be used as the contaminant absorber. EC, PC and BL have to be distilled in vacuum due to their high boiling points. For esters such as methyl formate, P2Os can be used as a contaminant absorber (highly efficient for absorbing protic species). 

For instance, the reduction potential of many solvents depends on the salt used and, in particular, on the cation. The reduction potentials of alkyl carbonates and esters in the presence of tetraalkyl ammonium salts (TAA) are usually much lower than in the presence of alkaline ions (Li+, Na+, etc.). Similar effects were observed with the reduction potential of some common contaminants (e.g., H20, 02, C02). Moreover, the reduction products of many alkyl carbonates and esters are soluble in the presence of tetraalkyl ammonium salts, while in the presence of lithium ions, film formation occurs, leading to passivation of the electrode [3],... 

Surface film formation on noble metal electrodes at reduction potentials was studied extensively with solutions of DME, THF, 2Me-THF, and DN. Basically, these solvents are much less reactive at low potentials than are alkyl carbonates and esters. However, in contrast to ethereal solutions of TBA+ whose electrochemical window is limited cathodically by the TBA+ reduction at around OV (Li/Li+), in Li+ solutions, ether reduction processes that form Li alkoxides occur at potentials below 0.5 V (Li/Li+) [4], It should be emphasized that the onset potential for surface film formation on noble metals in ethereal solutions is as high as in... 

Similar considerations apply to reactions of organomagnesium compounds with formate esters (leading to aldehydes), chloroformates or carbonates (leading to esters) or carbamoyl chlorides (leading to amides) ... 

The carbonylation of alcohols can proceed with formation of carboxylic acid by catalytic insertion of CO into the carbon-oxygen bond. An alternative reaction gives rise to oxalate or formate esters, when the CO is inserted into the oxygen-hydrogen bond. The members of the nickel triad carbonylate alcohols to give each of these products, and they will be discussed separately. 

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