Carbonates and Carbamates

A number of types of organotin derivatives of the phosphorus acids have been prepared, one of the objects being to combine the biological effects of organotin and organophos-phorus compounds. 

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Carbonates and carbamates are reported to be intermediate in stability and easier to prepare than xanthates. " They can conveniently be prepared directly from the alcohol in high yield and give clean conversion to olefins. Cholesta-3,5-diene, for example, can be readily obtained via the phenylcar-bamate (114) or ethylcarbonate (115) of cholesterol. Such esters appear to have been somewhat neglected as synthetic intermediates. 

The reaction of silyl enol ethers with fluoroformates and fluoroformamides, catalysed by the addition of tetra-n-butylammonium fluoride, produces enol carbonates and carbamates in acceptable yields [60],... 

Fig. 9.8. Reaction cascade in the activation of (phosphoryloxy)methyl carbonates and carbamates (X = O or NH, respectively) as potential prodrugs of alcohols, phenols, and amines... 

Fig. 9.8 presents another, more complex type of phosphate prodrugs, namely (phosphoryloxy)methyl carbonates and carbamates (9-26, X = O or NH, resp.) [84], Here, the [(phosphoryloxy)methyl]carbonyl carrier appears quite versatile and of potential interest to prepare prodrugs of alcohols, phenols, and amines. The cascade of reactions leading from prodrug to drug as shown in Fig. 9.8 involves three steps, namely ester hydrolysis, release of formaldehyde, and a final step of carbonate hydrolysis (X = O) or A-decar-boxylation (X = NH). Three model compounds, a secondary alcohol, a primary aliphatic amine, and a primary aromatic amine, were derivatized with the carrier moiety and examined for their rates of breakdown [84], The alcohol, indan-2-ol, yielded a carrier-linked derivative that proved relatively... 

Apart from aryl esters and amides of carboxylic acids, aryl carbonates, and carbamates, other types of organic compound have been reported to undergo PFR. They include oxalates, formiates, sulfonates, sulfonamides, thioesters, selenoesters, and telluroesters. 

L = Sulfonate esters or phosphonate esters 44 L = Carbonates and carbamates Scheme 26 Photoremovable protecting groups 33, 43, and 44. 

Klan et al. proposed that both alcohols and amines are initially released as carbonates and carbamates from 44 these undergo thermal decarboxylation to release CO2 and form their respective alcohols and amines.Furthermore, the chemical yields of the protected amine were reduced since they were involved in electron transfer reactions that decomposed 44. 

Carbonates, Carbamates. The ester portion of both carbonates and carbamates is hydrolyzed to give the monosubstimted carbonic acid, which is unstable and decomposes with loss of carbon dioxide. Thus, carbonates are hydrolyzed to give alcohol and carbon dioxide carbamates are hydrolyzed to yield an alcohol, an amine, and carbon dioxide. 

In special cases, other photochemical reactions can totally suppress an acyl group migration.76,117 The primary photochemical process of 195 is fast equilibration with trans-isomer 196. The crr-isomer 195 then cyclizes in 65% yield to dehydroaporphane (197) in a similar way as does stilbene and its derivatives.118 7>a j-isomer 196 cyclizes in 10-21% yield to dehydroproto-berberine (198).76 The latter reaction is analogous to the already discussed cyclization 110 -> 112 and 134 -> 131 of aromatic carbonates and carbamates. 

K. Buch measured the partial press, of ammonia and carbon dioxide in mixtures of ammonium carbonate and carbamate, and from the results calculated the cone, of the free and bound ammonia and higher carbonate, and of the carbamate and carbonic acid. The hydrolysis and equilibrium constants were then calculated. The hydrolysis constants Kx and K% and the equilibrium constant K3 of the hydrocarbonate to carbamate, were ... 

Enol carbonates and carbamates. Olofson and Cuomo have extended the regio-ipccilic alkylation of ketones (6, 44) to a regiospeeific preparation of enol carbonates and carbamates. The enol carbonates are prepared by reaction of silyl... 

Importantly, TV-carbamoyl derivatives of primary amines obtained from photolabile benzoins exist in varying proportions as cyclic hydroxyoxazolidinone tautomers. Therefore, the preparation of TV-carbamoyl derivatives of benzoins is applicable only for secondary amines and TV-alkylated amino acids. 244 At basic pH unsymmetrical benzoins, such as 3,5-dimethoxybenzoin, their mixed carbonate and carbamate derivatives, tend to equilibrate to the isomeric forms. Nevertheless, in TFA and aqueous solutions at pH 8 the structural integrity is fully maintained. 244 Preparation of 3,5-dimethoxybenzoin-derived carbamates of secondary amines and amino acids can be mediated by either CDI/methyl triflate in ni-tromethane 246 or 4-nitrophenyl chloroformate/DMAP in dry THF. 244 ... 

Allyl esters, carbonates, and carbamates readily undergo C-O bond cleavage upon reaction with palladium(O) to yield allyl palladium(II) complexes. These complexes are electrophilic and can react with nucleophiles to form products of allylic nucleophilic substitution. Linkers based on this reaction have been designed, which are cleavable by treatment with catalytic amounts of palladium complexes [165,166], For the immobilization of carboxylic acids, support-bound allyl alcohols have proven suitable (Figure 3.12, Table 3.7). 

Amides, carbamates, and ureas can be generated by nucleophilic cleavage of resin-bound esters, carbonates, and carbamates, respectively, with amines (Figure 3.20). These reactions only proceed well if sufficiently reactive resin-bound derivatives are used. 

The reactions of C02 with metal-hydroxides, -alkoxides, and -amides to provide metallobicarbonates, -alkyl carbonates, and -carbamates [Eqs. (27)—(29)] in general do not involve activation of carbon dioxide by prior coordination to the metal center. These processes generally entail either... 

The solid ammonium carbonate and carbamate are much more difficult to remove than the hydrates (Townsend and Reid, 1978, p. 100). The natural gas industry has opted for methanol and glycols, which may be injected into pipelines and processes without undesirable side reactions. 

Wulff and collaborators, for instance, reported the preparation of TSA imprinted beads for the hydrolysis of carbonate and carbamate [61, 62], exploiting the amidine (33) functional monomer previously developed by the same group and successfully applied to the bulk format [63]. The polymers were prepared using a suspension polymerisation that produced beads with sizes in the range 8-375 pm, depending on the polymerisation conditions. The pseudo-first order reaction rate of the imprinted beads (Tyrrp/ soin) was enhanced by a factor of 293 for the carbonate hydrolysis and 160 for the carbamate, when compared with the background. 

Carbonate and carbamate derivatives, prepared from the reaction of alcohols and amines with phosgene and /V-hydroxy-2-thiopyridone, provide the corresponding alkoxyl radicals and aminyl radicals respectively, via radical decarboxylation [71-81]. For example, photolytic treatment of carbamate (57) derived from 4-pentenylamine generates the corresponding 4-pentenylaminyl radical, as shown in eq. 8.24. Under neutral conditions in the presence of tert-BuSH, a direct reduction product (A) of 4-pentenylaminyl radical is formed, while, in acidic conditions, cyclized product (C), pyrrolidine, via 5-exo-trig manner from 4-pentenylaminyl radical is formed. Under the latter conditions, the real reactive species is an electrophilic 4-pentenylaminium radical which rapidly cyclized via 5-exo-trig manner. 

Again, we ignore buried amidines such as imidazoles and purines, and note the only such unburied amidine for which the enthalpy of formation is known to the authors is the above jV,jV -diphenylformamidine. (There are some guanidine derivatives for which the enthalpies of formation are known, but these seem inappropriate to use, much as carbonates and carbamates are not usually considered normal carboxylates.)... 

Some monothio-carboxylate, -carbonate and -carbamate complexes have also been synthesized but these all contain P donor ligands (see Section 45.5.4.8.v). 

In spite of numerous failures to prepare vinyl chlorofor-mate and Isopropenyl chloroformate by direct phosgenatlon of either acetaldehyde or acetone, we accepted the challenge to find economical routes to vinylic chloroformates and their derivatives, vinylic carbonates and carbamates. 

Alkenyl chloroformates, especially vinyl and isopropenyl chloroformates, as well as vinylic carbonates and carbamates have found number of valuable applications in various fields. 

Polymerization and copolymerization of vinyl chloroformate (Ref. 133, 134, 135), vinyl carbonates and carbamates (135, 136) using standard radical initiators (e.g. per-oxydicarbonates) to yield high molecular weight polymers and random copolymers is well documented. More recently,... 

Vinyl carbonates and carbamates containing chromophore groups have been prepared and claimed as useful polymerisable photoinitiators for photoreticulable polymers (Ref. 147). 

Polymers and copolymers of vinylic carbonates and carbamates may find interesting applications as aroma and flavours releasing agents. For example, isopropenyl menthyl carbonate has been patented (Ref. 148) as an useful monomer for the manufacture of a smoking composition comprising an admixture of tobacco and a menthol-release agent. Recently, Harwood et. al (Ref. 149) have published a new preparation of enol carbonates including especially isopropenyl menthyl carbonate by selective O-acylation of ketones sodium... 

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