Carbamates as nucleophiles

In sharp contrast, Bartoli showed that the (salen) Co catalyst system could be applied to the kinetic resolution of terminal epoxides with unprotected tert-butyl carbamate as nucleophile with extraordinarily high selectivity factors (Scheme 7.40) [72]. Excellent yields and selectivities are also obtained with use of ethyl, Cbz,... 

Recently, Kureshy et al. [209] have reported the use of highly efficient recyclable Co(III)-salen complexes in ionic liquids in the catalysed KR of aryloxy/terminal epoxides using carbamates as nucleophiles, which provided high regio- and... 

Nitrogen compounds are also effective as nucleophiles in the anodic oxidation of silyl-substituted ethers. The electrochemical oxidation in the presence of a carbamate or a sulfonamide in dry THF or dichloromethane results in the selective cleavage of the C-Si bond and the introduction of the nitrogen nucleophile at the carbon (Scheme 21) [55]. Since a-methoxycarbamates are useful intermediates in the synthesis of nitrogen-containing compounds [44], this reaction provides useful access to such compounds. Cyclic silyl-substkuted ethers such as 2-silyltetrahydrofurans are also effective for the introduction of nitrogen nucleophiles. The anodic oxidation in the presence of a carbamate or a... 

It is well known that oxidation of carbamates leads to the formation of N-acyliminium ions via dissociation of the C-H bond a. to nitrogen. The electrochemical,4 metal-catalyzed,5 and chemical methods6 have been reported in the literature to accomplish this transformation. The transformation serves as a useful tool for organic synthesis, although only compounds of high oxidation potentials such as methanol and cyanide ion can be used as nucleophile. It... 

Jacobsen et al. [48], in 1997 for the first time demonstrated KR of racemic terminal epoxides with water as nucleophile for the production of optically pure epoxides and corresponding 1,2-diols. Since then, various other nucleophiles viz., carboxylic acids, phenols, thiols, amines, carbamates and indols were used in KR to produce optically pure epoxides with concomitant production of corresponding enantioenriched l,2-bifimctional moieties [49-52]. 

Alkyl carbamates are generally more stable towards nucleophiles than amides, and are therefore of limited utility as protective groups. Amines lacking other base-sensitive functionalities can, however, be protected as alkyl carbamates. An illustrative example of the use of ethyl carbamate as a protective group is sketched in Figure 10.11 [188]. 

Anodic oxidation of acylsilane in the presence of various nucleophiles provides the corresponding ester, acid and carbamate as shown in equation 3641. 

The successful utilization of nitrogen-based nucleophiles in the previously described cascade reactions has allowed for the synthesis of complex polycyclic structures from simple and readily available starting materials. The fact that carbamates can participate as nucleophiles has provided the opportunity for development of diastereoselective ring closures onto the halocyclopropane-derived allyl cation. 

Other amidoselenenylation reactions have been described. Salazar [50] reported that the carbamatoselenenylation of alkenes can be effected using hT-(phenylseleno)phthalimide, in the presence of tetrafluoboric acid, and ethyl carbamate as the nucleophile. The reaction is a stereospecific anti addition and works well with monosubstituted and 1,2-disubstituted alkenes. In the case of monosubstituted alkenes mixtures of regioisomers were obtained. 

The use of carbamates as the nucleophile allowed extension of the method to the gold-cataWzed intermolecular hydroamination of conjugated dienes at room temperature. The reaction took place regioselectively as 1,2-addition at the less substituted double bond. 

Clearly the reaction of Scheme 63 can be used not only to deprotect amines but also for Pd(0)-catalyzed allylation of nucleophiles using carbamates as substrates, as proposed by J. Tsuji and co-workers " However, carbonates are preferred since amines liberated when working with carbamates are strongly nucleophilic and can react further. To avoid this side reaction Tsuji proposed the use of 7V,N -diisopropyl carbamates, which hberate the moderate nucleophile diisopropylamine. However, the side reaction is much less important with alcohols. 

Since the initial studies of Tsuji and Trost, it has been known that Tr-allylpalladium complexes react irreversibly with active methylene compounds to form new carbon-carbon bonds. Applying this information, Kunz introduced dimedone and A,A-dimethylbarbi-turic acid (NDMBA) as nucleophiles in the Pd-catalyzed deprotection of allyl carbamates and carbonates. The presumed catalytic cycle for this process is shown in Scheme 4. Dimedone and NDMBA have also been employed as nucleophiles in the deprotection of aUyl esters. Dimethyl malonate has also been used occasionally in allyl carbamate deprotection, although it appears to be less reactive than the other carbon nucleophiles.f ... 

The last class of nucleophiles used to scavenge the n-allylpalladium intermediate is that of sulfur nucleophiles. 2-Thiobenzoic acid has been used as a water-soluble scavenger of 7r-allylpalladium complexes in allyl carbamate deprotections. More recently, aryl sulfinic acids have been demonstrated to very effectively serve as nucleophiles in the Pd-mediated deprotection of allyl esters, carbamates, amines, and ethers. This reagent appears to afford substantially bettCT reactivity than most other classes of nucleophiles. 

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