Recent Developments on the Carbamation of Amines

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Current Organic Chemistry, 2011, 15, 1593-1624 1593

Devdutt Chaturvedi*

Natural Products Chemistry Division, North-East Institute of Science and Technology (CSIR), Jorhat-785006, Assam, India.

Abstract: Organic carbamates represent an important class of compounds showing various interesting properties. They find wide utility in various areas as pharmaceuticals, agrochemicals (pesticides, herbicides, insecticides, fungicides etc.), intermediates in organic synthesis, protection of amino group in peptide chemistry, and as linker in combinatorial chemistry etc. Classical synthesis of carbamates involves use of harmful reagents such as phosgene, its derivatives and carbon monoxide. Recently, various kinds of synthetic methods have been developed for the synthesis of organic carbamates. In the present review, I would like to highlight the recent developments on the synthesis of organic carbamates using variety of reagents.

Keywords: Carbamation, amines, carbamates.

1. INTRODUCTION in organic synthesis [7], for the protection of amino groups in peptide chemistry [8], as linkers in combinatorial chemistry [9] etc. Organic carbamates are the stable class of compounds derived Functionalization of amines as carbamates offers an attractive from the unstable carbamic acid (H N-COOH) by the substitution 2 method for the generation of derivatives, which may have interest- of amino and acid ends through the various kinds of structurally ing medicinal and biological properties [10]. Organic carbamates diverse alkyl/aryl, aryl-alkyl or substituted alkyl/aryl and aryl-alkyl have been extensively used as useful synthons for the synthesis of groups, and are identified by the presence of the linkage O-CO-NH- structurally diverse synthetic intermediates/molecules of biological [1, 2]. When the carbamate linkage is present in a cyclic system, significance [11]. Therefore, considerable interest has been gener- this class of compounds is referred to as cyclic carbamates [3]. ated in the recent past for the development of efficient and safe When the carbamate group is attached with any inorganic atom methodologies for carbamate esters synthesis. either metal or non metal, such compounds are referred to as inorganic carbamates [4]. Organic carbamates have frequently been employed as demandable pharmaceuticals in the forms of drugs and prodrugs [12]. The reaction of carbamation of amines has frequently been util- In recent years, several reports have indicated that carbamate link- ized in the synthesis of organic carbamates which holds unique age present in between the active pharmacophores of various struc- applications in the field of pharmaceuticals [5], agrochemicals (pes- turally diverse molecules increases manifold biological activities of ticides, herbicides, insecticides, fungicides etc.) [6], intermediates semisynthetic/synthetic natural/synthetic molecules [13]. Further-

O R1 OH O OH H3CHN O R2 O N NH O R3 O N O O Physostigmine: Anti-alzheimer drug OH AcO OH O O O

O N N O O

Taxol analogues: anticancer drugs F NH Linezolid: Antibacterial drug N O

Cl S N O NHCH3 O

N O NH2 Cl Carbaryl: Insecticide Capravirine: anti-HIV

Fig. (1). Biologically active drug molecules bearing carbamate linkage.

*Address correspondence to this author at the Natural Products Chemistry Division, North-East Institute of Science and Technology (CSIR), Jorhat-785006, Assam, India; Tel: ------; Fax: 91-376-2370011; E-mail: [email protected]

Carbamates

Inorganic Organic

Metal Non metal Saturated Unsaturated

Symmetrical Unsymmetrical

Aliphatic Aliphatic-aromatic Aromatic Others

Simple Substituted

Simple Activated Cyclic

Simple Functional

Thio Imido Ortho Pyro

Linear Branched Alicyclic Linear-alicyclic

Symmetrical Unsymmetrical Symmetrical Unsymmetrical

Simple Substituted

Fig. (2). Classification of carbamates. more, the role of carbamate linkage have been extensively studied toxic which limits its use. Safer substitutes have been proposed in structurally diverse natural/semisynthetic molecules against vari- such as 1,1,1-Trichloromethylformate (Diphosgene, 2) [17], and ous disease such as anti-cancer, anti-bacterial, anti-fungal, anti- bis-(1,1,1-trichloromethyl) carbonate (Triphosgene 3) [18], which malarial, anti-viral, anti-HIV, anti-estrogenic, anti-progestational, are frequently used in recent years. Depolymerization of 3 into 1 anti-osteoporosis, anti-inflammatory, anti-filarial, anti-tubercular, has widely replaced phosgene by triphosgene, which is relatively anti-diabetic, anti-obesity, anti-convulsant, anti-helminthes, Alz- safer to use [19]. Thus, carbamates 6 synthesis has been achieved heimer disease, CNS and CVS active etc [14, 15, 13, 5]. Some of through the reaction of an amine 4 with an alcohol 5 using either 1, the important biologically active drug molecules bearing carba- or 2 or 3 as source of carbonyl equivalent (Scheme 1). mates have shown in Fig. (1). O O Cl Cl O Cl Cl Cl Cl 2. CLASSIFICATION OF CARBAMATES C C Cl Cl Cl O O O Carbamates can be mainly classified into two groups, namely 1 23Cl Cl Cl inorganic and organic. Depending upon the structural variations on the attached moieties they are further classified as shown in Fig. O O (2). R NH + + R OH 1 2 X X 2 4 NHR1 OR2 3. METHODS OF PREPARATION (either 1 or 2 or 3) 5 6 3.1. Phosgenation Technique Scheme 1. Phosgene (1) is a potentially useful, versatile building block in organic synthesis [16]. It offers the possibility of binding two nu- Chloroformates and isocyanates are intermediates produced cleophilic units to the same carbon atom and such two-component from phosgene, have frequently been employed in the synthesis of system is particularly well suited for the combinatorial synthesis of organic carbamates. Chloroformates [20] 7, can be obtained through carbonate, ureas and carbamate (Scheme 1). Phosgene is extremely the reaction of alcohol/phenol 5 with phosgene 1, reacts with Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1595 amines/substituted amines 4 (i.e. aminolysis) afforded carbamates ketone 11 in the presence of Sn4+ modified Zeolite H (H-SnA) at [21] 6 (Scheme 2). Carbamates synthesis through the chlorofor- room temperature (Scheme 5). mates route has been achieved using different reaction conditions O O such as use of strong bases [22] (NaOH, NaHCO3, Et3N, pyridine 11 and triphenylphosphene), metals [23], ultrasound [24], RNH2 + ClCOOR1 bis(trimethylsilyl)acetamide [21] in combination with azides [23]. 24h, 68-75% 4 7 RNCOOR1 4 12 O O O R1NH2 R2OH + Scheme 5. 5 Cl Cl R O Cl 1 2 7 R2O 6 NHR1 More recently, Kim and Jung have reported [28] a simple and efficient synthesis of carbamates 6, through reacting equimolar Scheme 2. amounts of amines 4, chloroformates 7, and Indium metal (Scheme In recent years, there has been much attention on the synthesis 6). Thus, carbamates of structurally diverse substituted aliphatic, of carbamates using chloroformates as key intermediate. Thus, aromatic, heterocyclic amines were prepared using various kinds of Pandey et al have reported [25] an efficient synthesis of carbamates chloroformates. 6 through the reaction of variety of amines 4 with chloroformates 7 O using catalytic amount of Yttria-Zirconium based Lewis acid cata- O lyst (Scheme 3). In, OR RNH2 + Cl OR RHN 1 Yttria-zirconia based 1 rt, 7-12h, 4 7 6 Lewis acid catalyst 73-91% RNH + ClCOOR1 RNHCOOR 2 1 R = - CH ,-CH=CH , Bn r.t., 5 min-6h, 88-96% 1 3 2 4 7 6 Scheme 6. Scheme 3. Isocyanates [29] 13, obtained through the reaction of phosgene Later on, Mormeneo et al have reported [26] a versatile method 1 and amines 4, react with hydroxy compounds 5 (i.e. alco- for the synthesis of carbamates 6 through an in-situ generated hols/phenols) afforded the corresponding carbamates (Scheme 7) polymer supported chloroformate resin 9. Bis-trichloromethyl car- [30]. Polyurethanes [31], the building blocks of isocyanates have bonate (BTC) has been used as a phosgene equivalent to afford a been used in industry. Synthesis of carbamates starting from isocy- supported chloroformate 9, which on sequential one-pot reaction of anates could be achieved through the use of strong bases [32], and variety of alcohols 5 with amines 4 afforded the corresponding metal halides [33] etc. Carbamates could be converted into isocy- carbamates 6 in high yields (Scheme 4).

O O 5 BTC RCH OH OH 2 O Cl O OCH R Pyr 2 Pyr 10 8 9

R1 75-99% NH R2 4

R1 N OCH2R 6 R2

Scheme 4.

O O

+ R2OH + RNH2 R1-N=C=O 4 - R2OH R O NHR Cl Cl 13 2 1 1 6

Scheme 7.

Raje et al have reported [27] an efficient, one-pot synthesis of anates by thermal decomposition at higher temperatures using dif- N-substituted (3-oxobutyl) carbamates 12, via tandem condensation ferent reaction conditions such as use of metal catalysts [34], of primary amines 4, with methyl chloroformate 7 followed by the chlorocatecholborane/boron halides with triethylamine [35], silanes conjugate addition of the resulting carbamates with methyl vinyl [36], chlorosilanes [37], dichlorosilanes [38], Mitsunobu’s reaction 1596 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

conditions [39], montmorillonite K-10 [40], Bi2O3 [41], and re- NEt4Cl as a cocatalyst that the alcohol participates in the catalytic cently used basic metal oxide nanoparticles [42] etc. cycle, since when it was absent, practically no isocyanate was obtained (Scheme 9). 3.2. Reductive Carbonylation of Nitro-aromatics Several reports have been published by Gladfelter and Cenini The reductive carbonylation of aromatic nitro-compounds 14 to giving more insight into the mechanism of the catalytic cycle of the the corresponding carbamates is an interesting approach towards rhodium and ruthenium catalysts. Gladfelter [48] has proposed a synthesis of carbamates 15 (Scheme 8) [43]. The carbonylation mechanism using Ru(dppe) (CO)3 16 (dppe = bis(diphenyl- reaction of nitro-aromatics is an exothermic reaction and is cata- phosphene)-ethane as a catalyst, in which an aromatic amine 4 is lyzed by palladium, ruthenium, and to a lesser extent rhodium. Fur- suggested as a linker intermediate (Scheme 10). thermore, platinum [44], iridium [45], and iron [46] have also been Very similar results were obtained by Cenini using [(PPh3)2N] reported to be active in this reaction. [Rh(CO)4] 22 as a catalyst (Scheme 11) [49]. Most previously reported catalytic systems for the reductive O carbonylation of aromatic nitro-compounds usually have employed corrosive Lewis acids and or a base [50] such as pyridine or triethy- C lamine in excess amounts. For example, supported palladium is NO 2 NH OR inactive in the absence of a Lewis acid even in the presence of an Cat., CO excess of pyridine, whereas PdCl2 exhibits, good activity in the absence of Lewis acids but requires excess of base. Palladium (II) R' OH complexes of the type [Pd(Py)2Cl2] can catalyze the reaction at low R R Py:Pd ratio but the method requires promotors such as FeCl3 or 14 15 MoCl5 and aprotic solvents such as chlorobenzene [51]. On the other hand reductive carbonylation of aromatic nitro-compounds Scheme 8. could be catalyzed by palladium anchored to montmorillonite [52], For a reductive carbonylation of aromatic nitro-compounds car- supported Pd-1,10-phenanthroline derivatives in the presence of a ried out in alcohol, it might generally be considered that the carba- Bronsted acid [53] Pd complexes with 1,10-phenanthroline deriva- mate 15 is formed by reaction of aromatic isocyanates ArNCO, tives [54], and Pd heteropolyanion is an approach to this problem. with an alcohol R-OH, outside the coordination sphere of the metal In addition to this ruthenium carbonyl complexes such as Ru3 (CO)12 or Ru(CO)3(PPh3)2 are efficient homogeneous catalysts in [47]. However, Cenini has found in the case of Ru3 (CO)12 with

O NH-Ar CO, R'OH CO L M=N-Ar n LnM + L MC=O Ar-HN OR' n COOR' 15

Scheme 9.

P.Ph Ph .P 2 P.Ph 2 Ph .P 2 2 OMe Ru CO + ArNO2 Ru = P.Ph2 -CO OC CO OH Ph .P 2 OC CO 2 O Ru 16 21 N OC CO Ar 17 OMe Ar.N=C=O P.Ph2 Ph2.P + CO +ArNH Ru O 2 N H OC CO Ar Ar-CO-NH.Ar P.Ph O Ph .P 2 20 2 O Ru -MeOH N -ArNH2 P.Ph2 Ph2.P Ar-NH OC CO Ar COOMe 2 -CO2 Ru O 4 COOMe OC CO 18 19 Ar-NH-COOMe 15

Scheme 10. Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1597

[(Rh(CO)4] Ar-NO 2 22 + CO, ArNH2

O O=C O O=C COOMe Rh Rh N O O=C COOMe O=C Ar 26 23

+ 2CO and 2MeOH

O=C O + CO O=C CO2 Rh Rh N O=C N Ar O=C Ar 24 25

Scheme 11. O

NO2 HN OR2

Sn/NH4Cl + ClCOOEt or (Boc)2O MeOH, Ultrasound R 27 1 7 79-93% R1 14 15

Scheme 12. the reductive carbonylation of aromatic nitro-compounds to carba- platinum and alkali metal halides [62], CO, Cu and Rh (Scheme 14) mates if additives such as alkylammonium salts [55] chelating [63]. Pd and Cu halides have also been employed as a catalysts ligands [56] or anilines [57] are used. Palladium [58] has been often during the oxidative carbonylation process [64]. applied as catalyst in the homogeneous and heterogeneous catalyst O systems. Rhodium [59] catalysts also have been applied less often to the reductive carbonylation of nitrobenzene than ruthenium and Metal catalyst R NH + CO + HOR + 1/2 O palladium catalysts. 1 2 2 2 R O NHR 4 5 2 6 1 Chandrasekhar et al have reported [24] an efficient protocol for the synthesis of carbamates 15 through reductive carbonyltion of Scheme 14. aromatic-nitro compounds 14 with either (Boc)2O, or ClCOOEt Use of Iodine promoted Pd-catalyzed [65], and Gold-complex using Sn/NH4Cl system under ultrasound radiation (Scheme 12). with triphenylphosphine [66] have also been reported in the synthe- More recently, Tomkinson and coworkers have reported [60] an sis of carbamates (Scheme 15). efficient protocol for the synthesis of N-aryl N-hydroxy carbamates O 28, through one-pot procedure involving zinc-mediated reduction of 1 2 Pd catalyst nitroarenes 14 in the presence of chloroformates 7 (Scheme 13). R NH2 + CO + HOR + I2 45 R O NHR O 2 6 1 HO Scheme 15. NO N OR 2 2 Mizuno et al have reported [67] the synthesis of 2- Zn/NH4Cl oxazolidinone derivatives in which 2-aminoethanols 29 were easily + ClCOOR2 subjected to the thiocarboxylation with CO promoted by elemental NaOMe, 89-97% 7 sulfur followed by oxidative cyclization with molecular oxygen to R1 R1 14 28 afford the corresponding 2-oxazolidinones 30 in good yields under Scheme 13. mild conditions (Scheme 16).

R1 NH2 R1 3.3. Oxidative Carbonylation of Amines 1. CO, S, K2CO3, DMF, 4h NH

Carbamates 6, have been prepared in good to excellent yields o O 2. O2, 2h, 1atm, 20 C through the reaction of amines 4, alcohols 5, carbon monoxide and R2 OH R2 O oxygen in the presence of novel metal catalysts. The metallic cata- 29 30 lysts used during the oxidative carbonylations are palladium [61], Scheme 16. 1598 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

Wan et al have reported [68] an efficient synthesis of carba- catalytic systems. Synthesis of carbamates 6 through the reaction of mate esters 6, through the oxidative carbonylation of amines 4 with variety of secondary amines 4 with structurally diverse alkyl halides alcohols 5 using polymer-supported palladium manganese 31 was achieved using K2CO3/tetra-n-butylammonium hydrogen bimetallic catalyst (Scheme 17). sulfate (Scheme 22) [73]. This method produces carbamates 6 as a major product along with the minor amount of N-alkylated amines O2 Ar-NHCOOR 32. Ar-NH2 + CO + ROH PVP-PdCl2-MXn O 45 6 R1 R1 (n-C H )NHSO R1 3 4 9 4 C Scheme 17. NH + R .X N OR3 + N.R3 K2CO3 R2 31 R2 R2 Later on, Shi et al have reported [69] a novel synthesis of car- 4 6 32 bamates 6 through the oxidative carbonylation of amines 4 with 2- o alcohols 5 using PdCl2/ZrO2-SO4 catalyst system at 170 C Scheme 22. (Scheme 18). Sodium carbonate have also been used in the synthesis of carbamates [74]. Moreover, this method was not efficient for produc- O2 tion of only O-alkylated carbamates 6 due to the exclusive forma- R-NH + CO + R OH R-NHCOOR 2 1 1 tion of N-alkylated amines 32 (Scheme 23). PdCl2-ZrO2-SO4 4 56 O R1 R1 Na2CO3 R1 3 C NH + R .X N OR3 + N.R3 Scheme 18. R R R 2 31 2 6 2 Shi et al have reported [70] an efficient and clean synthesis of 4 32 carbamates 6 through oxidative carbonylation of aromatic amines 4 Scheme 23. using polymer immobilized gold catalysts (Scheme 19). The role of Cs CO in minimizing the synthesis of N-alkylated O 2 3 2 tertiary amines 32 during the one-pot, efficient synthesis of carba- Ar-NH2 + CO + ROH Ar-NHCOOR mates 6 from the corresponding alkyl halides 31 and amines 4 was 4 5 Au/Polymer 6 first investigated by Butcher [75] (Scheme 24). He found that Cs CO was much better than K CO in yielding better yields of Scheme 19. 2 3 2 3 carbamates from their corresponding alkyl halides and amines. Later on, Shi et al have also reported [71] a high yielding effi- O cient carbonylation of amines 4 with variety of alcohols 5 using R R1 R1 Cs2CO3 1 3 C palladium complex-ionic liquid afforded carbamates 6. The desired NH + R .X N OR3 + N.R3 products could be precipitated by adding water into the resulting R2 31 R2 R2 reaction mixture and the catalysts system could be reused with 4 6 32 slight loss of catalytic activity (Scheme 20). 44-96% 0-19%

O2 Scheme 24. Ar-NH2 + CO + ROH Ar-NHCOOR 4 56Pd(phen)Cl2-[Bmim]BF4 Several bicarbonates have been used for the synthesis of carbamates. Of these sodium bicarbonate (NaHCO3) has found use in Scheme 20. peptide chemistry [76]. Inesi and their coworkers have reported Recently, Mei and coworkers have reported [72] the methyl N- [77] the synthesis of linear carbamates 6 starting from correspond- phenyl carbamates 6 through the oxidative carbonylation of aniline ing primary and secondary amines 4 and alkyl halides 31 using 4, using a series of recoverable Co(salen) in zeolite Y as catalysts tetra-ethylammoniumhydrogen carbonate (Et4NHCO3) as the car- wherein Co(salen) complexes were successfully encapsulated in bonyl source (Scheme 25). The yields of carbamates were affected zeolite Y by a flexible ligand method (Scheme 21). They have stud- by the nature of alkyl halides used. They have further extended ied the catalytic activity of various kinds of Co(salen) over zeolite their methodology for the synthesis of cyclic carbamates 33 starting based catalysts. from the corresponding haloamines 34 using Et4NHCO3.

O2 O Ar-NH2 + CO + ROH Ar-NHCOOR R Co(salen)in zeolite R1 1 4 56 Et4NHCO3 C NH + R3.X N OR3 53-98% Scheme 21. 31 R2 4 R2 6 3.4. Using Metal/Non-Metal Carbonates/ Bicarbonates HN ( )n Carbonates and bicarbonates have been effectively employed Et4NHCO3 Br-CH2(CH2)n-NH2.HBr for providing carbonyl functionality for the preparation of carba- O n = 1, 56% 33 O mates. Variety of metal carbonates such as potassium carbonate n = 2, 42% 34 (K2CO3), sodium carbonate (Na2CO3), and cesium carbonate (Cs2CO3) has been used alone and in combination with different Scheme 25. Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1599

3.5. Syntheses of Carbamates Using Carbon Dioxide Later on, Ishii et al have reported [84] the synthesis of carba- Carbon dioxide has been frequently used in its various condi- mates 6 through the one-pot reaction of primary or secondary ali- tions and forms, as a cheap and safe alternative for the synthesis of phatic amines 4 with ortho-esters 38 using gaseous CO2 (Scheme carbamates [78], carbonates [79], and several other interesting or- 29). This method takes longer reaction times and afforded carba- ganic transformations [80]. Carbamates synthesis using various mates in low yields. forms of carbon dioxide such as gaseous, electrochemical, and supercritical has been achieved in recent years employing diversity of O R reagents and catalytic systems. R1 1 C NH + CO2 + R3C(OR4)3 N OR3 3.5a. Gaseous Carbon Dioxide 35 6 R2 4 38 R2 Although carbon dioxide 35 has low reactivity [81] e.g. with amines 4 it forms unstable carbamic acids 36, which revert to their corresponding starting materials (Scheme 26). Scheme 29.

R-NH2 + CO2 R-NH-COOH Monocarbamates of 1,2-diols 40 have been synthesized [85] 4 35 36 from the corresponding 1,2-epoxides 39 through the reaction of primary and secondary aliphatic amines using gaseous CO2 Scheme 26. (Scheme 30). However, about half of the epoxide is lost due to the However, Yoshida et al have reported [82] the synthesis of car- accompanying nucleophilic ring opening by the amine afforded N- alkylated product 41. bamates 6 starting from CO2 35, amines 4, and unsaturated ethers 37 (Scheme 27). This method limits carbamates synthesis only from Monocarbamates 40 could also be obtained [86] starting from secondary aliphatic amines. Moreover, it requires longer reaction an epoxide 39 using tetrakis(dimethyl)-titanium(IV) and gaseous times (~70-80 h) and afforded low yields (3-12%).

O ~70-80h R2NH + CO2 + CH2=CH-O-CH2CH3 R2N C O OCH CH 3-12% 2 3 43537 6

Scheme 27.

Later on, Yoshida et al have also reported [83] synthesis of car- CO2 (Scheme 31). But this method was not satisfactory due to its bamates 6 through the one-pot reaction of amines 4 with alkyl hal- longer reaction time (~3-4 d), and low yields (5-20%). ides 31 using gaseous CO (Scheme 28). Carbamates obtained in 2 Similarly, chloromethyl oxirane 43 or phenyl oxirane 44 on re- this method are also limited to primary and secondary aliphatic action with CO and aliphatic amines in methanol gave various amines, require longer reaction times (~45 h), and affording 6-25% 2 yields. kinds of substituted carbamates [87] in 2-17% yields (Scheme 32). Later on, better yields have been reported by Yoshida et al O through the reaction of various epoxides 50, with variety of amines

4 using gaseous CO2 35 (Scheme 33) [88] However, this method R1 R1 C ~ 45h suffers from leading to isomeric mixtures. NH + R3 X + CO2 N OR3 6-25% Kojima et al have reported [89] carbamates synthesis from ep- R2 4 31 35 R2 6 oxides 50, amines 4, and CO2 35, wherein the latter was previously fixed on an aluminium porphyrin (Scheme 34). Scheme 28.

R 1 OH OH 22-24 h R NH + CO + O 1 R1 2 + R 42-52% 2 O C N N 4 35 39 40 R 41 O 2 R2 Scheme 30. OH ~ 3-4 days Ti(NMe2)4 + CO2 + O 5-20% 42 O C N 39 40 O Scheme 31. 1600 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

OH O OH O O + RHN O NH R Cl Cl 3 O NHR NO 43 45 46 R O O 47 O Ph O O R' R' Ph + R N O NH R HO O N Ph O N + OH R R' R' R 44 48 49 45 Scheme 32. R1 O R1 O R' R' R' HO HO + CO2 + NH O N N R1 + R2 R'' R'' 50 35 R'' 2 4 R2 R 51 52 Scheme 33. R1 O O R' R' Aluminium porphyrin HO + CO2 + NH O N R1 R2 R'' R'' 50 35 R2 4 51 Scheme 34. O R O R' Ph O MeO RNH-COONH R NO Ph 3 MeO Ph X R' 40-60% 54 53 OH R' 55 Scheme 35. O MeO Ph O MeO MeO Ph Ph OH MeO Br 54 53 MeO

CO2/ H2N-R-NH2 55

O O O R R NH2 ON ON NO + Ph Ph Ph OH OH OH R' 56 57

Where R= (CH2)n, n=1, 2, 3, 4, 5, 6 etc. Scheme 36.

Toda et al have reported [90] the synthesis of cyclic carbamates This reaction led to the formation of bis(2-oxazolidinones) [91] 55 (i.e. oxazolidinones) through the reaction of carbon dioxide with 57 when 2-methoxy 3,3-dimethyl-2-phenyloxirane 54 or -bromo- -bromoacylophenones 53 in the presence of aliphatic primary i-butyrophenone 53 was reacted with CO2 and aliphatic ,- amines 4 in methanol afforded 3-alkyl-4-hydroxy oxazolidone-2 diamines 56 (Scheme 36). derivatives 55 under mild reaction conditions (Scheme 35). Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1601

Reaction of 2-(1-haloalkyl)-oxiranes 58 with carbon dioxide However, this method was useful for the preparation of carba- and aliphatic primary amines gave five-and six-membered cyclic mates of only primary and secondary aliphatic amines. The effect of carbamates 59 (Scheme 37) [92]. several strong bases (CyTMG, TMG, DBU, MTDB, CyTEG, etc) in increasing the nucleophilicity of 45 resulting the formation of O O 45 carbamates 6, was studied by McGhee et al [95] They have demon- R' R' O R'NH-COONH3R' ON ON strated the role of various strong bases in yielding O-alkylated car- Br + bamate products using various kinds of alkylating agents (Scheme R R 41). R 58 OH OH 31 59a O H 59b R' X 2 RNH2 + CO2 RNH-COONH3R N Scheme 37. base 4 35 43 R' O R In the above reaction it was shown that there is an ionic species 40-78% 6 45 involved which is formed when 2 molar ratio of amine 4 was reacted with CO2 (Scheme 38). Scheme 41. O-Allyl carbamates [96] 62 could be obtained by the addition of RNH-COONH R - 2 RNH2 + CO2 3 preformed carbamate ion 60 [R.R’NH-COO H+Base] generated 4 35 45 from various primary and secondary amines 4 and CO2, to a THF Scheme 38. solution of allylic chlorides 61 containing a palladium/phosphine catalyst (Scheme 42). An improvement in the yields of carbamate formation has been Later on, Perez et al. have reported [97] synthesis of N-alkyl achieved by using different basic reagents, which might be helpful carbamates 6 in good to excellent yields through a clean and mild in increasing the nucleophilicity of the ionic species 43. Thus, Hori et al. have reported [93] the synthesis of carbamates 6 through the transcarboxylation of several amines 4 with the previously synthesized DBU-CO2 complex and subsequent O-alkylation by different reaction of primary and secondary amines 4, CO2, and alkyl halides 31 in the presence of a strong proton acceptor like DBU (Scheme alkyl halides 31 (Scheme 43). 39). Cyclic carbamates [98] 66 could be obtained in good yields (33-

1 93%) under mild reaction conditions from amino alcohols 65 and 1 O R R carbon dioxide using phosphorus (III) reagents 63 [i.e. Ph3P, CO2, DBU RX + NH N (PhO)3P] and halogenoalkanes 64 (i.e. CCl4 and CCl3.CCl3) 2 31 R RO R2 (Scheme 44). 4 6 Tominaga et al. have reported [99] an efficient protocol for the Scheme 39. synthesis of 2-oxazolidinones from CO2 and 1,2-amino alcohols 65 catalyzed by n-Bu2SnO afforded 53-94% yields (Scheme 45). Aresta et al. have reported [94] synthesis of carbamates 6 em- Synthesis of cyclic carbamates 67a and 67b from amino alco- ploying ionic species 45 through the alkylation with alkyl halides hols 65 involves sequential carboxylation with carbon dioxide fol- 31 using 18-Crown 6-ether as a phase transfer catalyst (Scheme 40). lowed by a Mitsunobu’s reaction was reported by Dinsmore and Mercer [100]. Unexpectedly, the stereochemical course of the Mit- 31 O H R' X sunobu’s reaction is dependent on whether the carbamic acid inter- 2 RNH + CO RNH-COONH R N 2 2 3 18-Crown-6-ether mediate is N-substituted with hydrogen (retention) or carbon (inver- 4 35 45 R' O R sion) (Scheme 46). 6 We have reported [101] an efficient, one-pot, high yielding pro- Scheme 40. tocol for the synthesis of carbamates 6 through the reaction of vari-

O O R R PdLn R NH + base + CO N C O H base N C O R' 2 R' R' 4 35 60 X 62 61 66-100%

Scheme 42.

O R1 R1 DBU-CO2 complex R-(CH )n-CH X + NH R-(CH )n-CH 2 2. o 2 2. O N R2 5 C, 24 h, 80-98% 31 4 6 R2

Scheme 43. 1602 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

R1 R2 R3 R2 n-Bu2SnO N + CO2 O 180oC, 16h, 53-94% HN OH R3 O

R1 66 65

Scheme 44.

R1 R2 R3 R2 n-Bu2SnO N + CO2 O 180oC, 16h, 53-94% HN OH R3 O

R1 66 65

Scheme 45. R R R1 N N Mitsunobu's reaction + CO2 O O 69-99% R O O RHN OH 3 R3 65 67a 67b Scheme 46. O R1 1 o R Triton-B, CO2, 90-100 C R-(CH2)n-CH2.X + NH R-(CH2)n-CH2. O N R2 3-5 h, 80-98% 31 4 6 R2 Scheme 47.

R1 R1 O R R Dry DMSO, Triton-B, CO2 2 R O.Tos + HN R2 OCN 3 R, 60oC, 2-4 h, 76-98% R 4 , R3 R 68 6 Scheme 48.

R1 R1 O R R 2 Dry DMSO, Basic resin/CO2 R O.Tos + HN R2 OCN 3 R, 100oC, 2.5-4 h, 70-98% R 4 , R3 6 R 68 Scheme 49.

R1 R1 O R R Dry DMSO, Basic resin/CO2 R2 X + HN R2 OCN , o 3 R 100 C, 2-4h, 70-98% R , 4 R3 R 31 6

Scheme 50. ous amines 4 with variety of alkyl halides 31 using benzyltrimethy- variety of alcoholic tosylates 68 with various amines 4 was also lammonium hydroxide (Triton-B)/CO2 system (Scheme 47). reported by our group (Scheme 49) [103]. We have also reported [102] the synthesis of carbamates 6 in The utility of basic resin in the synthesis of carbamates 6 in high yields through the reaction of a variety of alcoholic tosylates high yields from the corresponding alkyl halides 31 and amines 4 68 with various amines 4 using Triton-B/CO2 system (Scheme 48). using gaseous CO2 was also investigated by our group (Scheme 50) Use of Amberlite IRA 400 resin (basic resin) in the synthesis of [104]. carbamates 6 in high yields through the reaction of corresponding Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1603

2 1 R O R CO , Ph P/DEAD 1 2 3 R. (CH2)n. CH2.OH + R NH R. (CH ) . CH .O N R2 2-4h, 80-98% 2 n 2 5 4 6 Scheme 51.

R1 R1 O R4 R4 Dry DMSO, Mitsunobu's reagent/CO2 R2 OH + HN R2 OCN 3 R5 90-100oC, 2-5h, 76-98% R 4 3 R 5 R 6 5

Scheme 52. R 1 Zeolite-Y R1 NH+ CO2+ RCH2CH2CH2Br NCOOCH CH CH R R catalysts 2 2 2 2 4 31 R2 6

Scheme 53. R 1 Zeolite-based R1 NH+CO + RCH CH CH Br 2 2 2 2 NCOOCH2CH2CH2R R catalysts 2 4 31 R2 6

Scheme 54. O R1 R1 KO2/Et4NBr/CO2 HN + MeI N C OMe DMF, rt, 2-7h, R2 31 R2 6 4 38-90% Scheme 55. O R1 R1 KO2/Et4NBr/CO2 HN + MeI N C OMe DMF, MW, 5-7min. R2 31 R2 6 4 77.5-95.5%

Scheme 56.

A direct synthesis in high yields of carbamates 6 from the pri- Carbon dioxide has been converted into carbamates 6 through ’ mary alcohols 5 and amines 4 using Mitsunobu s reagent/CO2 sys- the reaction of various amines 4 with variety of alcohols 5 catalyzed tem has been first reported by our group (Scheme 51) [105]. by tin complexes [111]. The addition of acetals as dehydrating The above method for the preparation of carbamates 6 was fur- agent under high CO2 pressure is the key to achieve high yields ther extended from a variety of primary, secondary and tertiary (Scheme 57). alcohols 5 and amines 4 using Mitsunobu’s reagent/CO2 system by O our group (Scheme 52) [106]. Tin complexes R1NH2 + CO2 + HOR2 The use of zeolite-based catalysts in the synthesis of carbamates o 200 C, 24h, R2O NHR1 6 was investigated by Srivastava et al [107] through the reaction of 4355 24-90% 6 corresponding amines 4, gaseous CO2, and alkyl halides 31 over either titano-silicate molecular sieves or metal phthalocyanine com- Scheme 57. plexes encapsulated in Zeolite–Y. The catalysts could be used with little or no loss in activity (Scheme 53). Sasaki and Dixneuf have first reported [112] synthesis of vinyl carbamates (70, 71, 72) starting from diethylamine 4, and alkynes Srivastava et al have also reported [108] efficient protocol for 69 using CO in presence of a ruthenium catalyst Ru (CO) the synthesis of carbamates 6 using CO mediated by Zeolite-based 2 3 12 2 (Scheme 58).The overall yields of the products is poor in most of organic-inorganic hybrid catalysts (Scheme 54). the reactions. Recently, Singh have reported synthesis of various kinds of Later on, Sasaki and Dixneuf have reported [113] the a direct methyl carbamates [109] 6 through the corresponding amines 4 and synthesis of vinyl carbamates 73 in good yields through direct reac- methyl iodide 31 using tetra-ethyl ammonium bromide- tion of secondary amines 4 with acetylene 69 using gaseous CO in superoxide/CO system (Scheme 55). 2 2 presence of catalytic amount of RuCl .3H O (Scheme 59). Singh and coworkers have further investigated [110] the im- 3 2 Sasaki and Dixneuf have also reported [114] the synthesis of 2- proved and efficient protocol for the synthesis of methyl carbamates oxoalkyl substituted carbamates 75 in good yields through the reac- 6, through the reaction of corresponding amines 4 with methyl io- tion of secondary amines 4, -ethynyl alcohols 74 and CO using dide 31 using tetra-ethyl ammonium bromide-superoxide/CO sys- 2 2 Ru (CO) catalyst (Scheme 60). tem under microwave conditions (Scheme 56). 3 12 1604 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

H H R H Et Ru (CO) , CO O R + HN 3 12 2 O + Et 125-140oC O C N(Et) H O C N(Et)2 69 4 R 2 71 upto 52% 70 + H R O

H O C N(Et) 72 2

Scheme 58. O R RuCl3.3H2O 1 C R2NH + + CO2 N o O 4 90 C, 10-46% 69 R 2 73

Scheme 59.

R3 O R3 R1 Ru (CO) R1 3 12 O N NH + CO2 + OH R 4 80oC, 20h, 4-64% R 4 R2 4 R2 O 74 75

Scheme 60.

R NOCO Bu [Ru]/PR'3 2 R2NH + CO2 + Bu 100oC, 8-48h 469 37-62% 76

[Ru]/PR'3 CH2NHR + CO2 ONR 100oC, 8-48h 77 63-80% O 78

Scheme 61. O R1 R1 Ru complexes O C N HN R + CO2 + R2 3-67% R 69 4 2 R 79

Scheme 62.

Matsudo and coworkers have reported [115] synthesis of enol Shim et al have synthesized [117] carbamates 81 through the carbamates 76 in good yields with high regio and stereoselectivity reaction of amines 80, acetylenic alcohols 74, and carbon dioxide through the reaction of amines 4, terminal alkynes 69 and using using a lanthanide catalyst. Thus, the reaction of perhydro-azepine 4 gaseous CO2 in presence of catalytic amount of ( -cyclooctadiene) 80 with 3,3-dimethyl prop-1-yne-3-ol 74 and CO2 in presence of 6 ( -1,3,5-cyclooctatriene)ruthenium [Ru (COD) (COT)] and tertiary MCl3 (M = Ce, Pr, Nd, Gd) gave carbamates 81 (n = 6) in 20-38% phosphine (Scheme 61). They have further extended their method- yields. They have also prepared the carbamates (n = 4, 5) in 31 and ology for the synthesis of cyclic enol carbamates 78 using N- 21% yields (Scheme 63). substituted propargyl amines 77 and CO2. Dixneuf and coworkers have reported [118] regioselective syn- Later on, Dixneuf et al have also reported [116] the synthesis of thesis of O-1-(1,3-dienyl)carbamates 83 through regioselective ad- vinyl carbamates 79 through the reaction of an alkyne 69 with an dition of CO2 and secondary amines 4 to isopropenylacetylene 82 in 3 amine 4 using gaseous CO2 in presence of various kinds of ruthe- the presence of [Ph2P(CH2)nP.Ph2]Ru( -CH2-C(Me)=CH2)2 cata- nium complexes i.e. RuCl2(PR3)carene) and RuCl2(nor- lyst (Scheme 64). The yields were dependent on the nature of le- bornadiene)(pyridine)2 (Scheme 62). They have studied the cata- gand used in catalyst. They have realized that legand wherein n = 2, lytic role of different Ru-complexes in affording better yields of afforded better yields of the products. The addition is favored in vinyl carbamates. case of secondary cyclic amines. Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1605

R1 O R1

MCl3 (CH )n NH + HO (CH )n NCO 2 R + CO2 2 2 20-38% R2

80 74 81

Scheme 63.

R N O 82 100oC, 2 H NR ] R2NH + CO2 [ R2NCOO 2 2 Ru catalyst, 20h 4 35 45 O 83 upto 80%

Scheme 64.

R R4 O R4 2 R2 [M]/ bipy, CO 2 R1 R1 OH + NH O N R5 100oC, 24h, 30-97% R3 R5 4 R3 O 74 84

Scheme 65.

O R1 R1 O C N [Ru(TMHD)3] R + CO2 + HN R2 100oC, 24h, upto 55% 69 4 R2 R 79

Scheme 66.

Kim and coworkers have reported [119] the synthesis of carba- synthesis 6 in good to excellent yields (58-96%) from various alkyl mates 84 through the reaction of various kinds of propargyl alco- halides 31 and amines 4 using the cesium carbonate/CO2 system hols 74 with variety of amines 4 using gaseous CO2 in presence of (Scheme 67). catalytic amount of [Cu (1) (BF4)2 (1 = 2, 5, 19, 22, tetraaza [6, 6] (1, 1’) ferrocenophane, 1,5-diene) (Scheme 65). O R1 R1 Recently, Bhanage et al have reported [120] synthesis of vinyl CO2, Cs2CO3 N RCH2X + NH carbamates 79 through reaction of various kinds of alkynes 69, 2 2 R DMF, 58-96% RH2CO R amines 4, and CO using ruthenium tris(2,2,6,6-tetramethyl 3,5- 31 2 4 6 heptanedionate) metal complex as a catalyst (Scheme 66).

Carbon dioxide providing the carbonyl functionality for the Scheme 67. synthesis of carbamates is not sufficient itself in yielding high yields of desired carbamates. Therefore, several researchers have Later on, Jung and coworkers have reported [122] synthesis of considered that by adding basic reagents to reaction mixture may carbamates 86 using solid phase Merrifield resin 85 in good to ex- increase the basicity and nucleophilicity of the ionic species 43. cellent yields using cesium carbonate/CO2 and TBAI as phase Consequently, it has been proposed that metal carbonates such as transfer catalyst (Scheme 68).

Na2CO3, K2CO3, Cs2CO3 etc are good basic reagents, which could Later on, they have also reported synthesis of carbamates [123] provide carbonyl functionality in addition to its basic properties. 6 in solution phase through the reaction of various kinds of struc- Based on this concept there are many reports appreared in the re- turally diverse aliphatic, aromatic, and heterocyclic amines 4 with variety of alkyl halides 31 using cesium carbonate/CO system in cent past on the use of metal carbonates/CO2 system for the synthe- 2 sis of carbamates. Thus, Butcher have reported [121] a carbamate presence of a catalytic amount of TBAI (Scheme 69).

O R1

R1 N CO2, Cs2CO3 Cl + NH O R2 R2 TBAI, DMF, 60oC,~24h 55-97% 85 4 86

Scheme 68. 1606 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

O R1 R1 CO2, Cs2CO3, TBAI N RCH2X + NH 2 DMF, 23oC, 47-96% 2 R RH2CO R 31 4 6

Scheme 69. Ph Ph BzCl, CO2, Cs2CO3 O o TBAI, DMF, 23 C, 12h PhCH2O NH COOPh H2N COOPh 8787% 88

BzCl, CO , Cs CO 2 2 3 O o TBAI, DMF, 23 C, 24h COOMe ClH N COOMe PhCH2O NH 2 92% 89 90

Scheme 70.

They have extended their methodology in the synthesis of pep- O R tidomimetic using various kinds of protected amino acids (Scheme 1 Cs2CO3 R1 NH + CO2 + ROH N OR 70). upto 69% R2 5 R A direct synthesis of N-alkyl carbamates 6 from primary amines 4 2 6 4, and alkyl halides 31 using cesium carbonate/CO system has also 2 Scheme 74. been reported by Jung and their coworkers (Scheme 71) [124]. A very recent report for the synthesis of radio-labeled carba- O R 11 mates 6 through the incorporation of [ C] CO2 using various kinds CO2/Cs2CO3,TBAI, N RNH2 + R'X of alkyl halides 31, amines 4 and catalytic amount of DBU was DMF, 23oC, 20h-6 days 4 31 R' O R' reported by Hooker and coworkers (Scheme 75) [128]. 6 52-92% 11 O C Scheme 71. R1 11 DBU, CO2 R1 NH + RCH2Cl OR o N A study on comparative yields of carbamates 6 using different R2 75 C, 10min., 431 R2 metal carbonates and bases on O-as well as N-alkylated products 77% 6 was reported by Shi and Shen (Scheme 72) [125]. They have realized that best yields (> 87%) of carbamates could be achieved using Scheme 75. DBU/CO2 system. 3.5b Electrochemical Carbon Dioxide We have reported [126] a convenient, high yielding, one-pot Inesi and coworkers have first reported [129] the synthesis of synthesis of carbamate esters in high yields (70-90%) from corre- linear carbamates and cyclic carbamates (6 & 34) from the corresponding alcoholic tosylates 68, and amines 4 using the K CO /CO 2 3 2 sponding amines 4, alkyl halides 31 or haloalkylamines 33 using system in the presence of a catalytic amount of tetra-n- electrogenerated-superoxide activated carbon dioxide (O . -/CO ) butylammonium iodide (Scheme 73). This method has been used 2 2 (Scheme 76). for carbamates derived from various aliphatic primary, secondary, and aromatic amines. Later on, Inesi and coworkers have also reported [130] synthesis of carbamates 6 using carbon dioxide through an electrochemi- Recently, Vos and coworkers have reported [127] an efficient cal process (Scheme 77). This synthesis is based on the reaction of and green synthesis of carbamates 6 through the coupling of various amines 4, with the electrochemically generated base 92 (associated amines 4 with variety of alcohols 5 using Cs2CO3/CO2 system with the Et N+ cation) from 2-pyrrolidone 91 followed by sequen- (Scheme 74). 4

CO , base, solvent PhCH X + PhCH NH 2 PhH CO 2 2 2 metal carbonate 2 NHCH2Ph 31 4 6

Scheme 72.

O R1 1 R Dry DMSO, anhyd. K2CO3 NH RCH2.OTos + RCH2. O N 2 o R TBAI, CO2, 90-100 C, 5-6h, 70-90% 68 4 6 R2

Scheme 73. Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1607

O Dunach and Tascedda have reported [134] a new and selective R 1. O /CO 1 2 2 R1 electrochemical procedure for the synthesis of five membered ring NH N R OEt 2 2. EtI R2 cyclic carbamates 94 & 95 involving nickel-catalyzed CO2 incorpo- 4 upto 97% 6 ration into aziridines 93 under mild electrochemical conditions (Scheme 81). Out of several Ni catalysts used, Ni (bipy)3(BF4)2 was 1. O2 /CO2 BrCH2.(CH2)nNH2 CH2.(CH2)nNH shown to be an efficient catalyst for this transformation afforded upto 98% 33 O 100% yields of desired carbamates. O 34 O R1 Scheme 76. e Ni(bpy)3Cl2 R1 NH + CO2 N OEt EtI, upto 82% R R tial addition of CO2 and ethyl iodide, afforded carbamates in high 2 4 2 6 yields. Scheme 81.

e , MeCN, EtN ClO4 1. R2NH Recently, Lu and coworkers have reported [135] a new and ef- R2N-COOEt ficient electrochemical synthesis of carbamates through the electro- 2. CO2 N O N O 6 chemical incorporation of carbon dioxide into amines catalyzed by H 3. EtI NEt4 an electrogenerated Ni complex [Ni(bpy)3Cl2] using tetraethylam- 91 33-89% monium bromide (Scheme 82). 92 O Scheme 77. R1 e Ni(bpy)3Cl2 R1 NH + CO2 N OEt Later on, Inesi et al have also reported [131] an improved elec- EtI, upto 82% R R trochemical synthesis of chiral oxazolidin-2-ones 66 starting from 2 4 2 6 the corresponding chiral 1,2-amino alcohols 65. Subsequent CO2 bubbling and addition of tosyl chloride afforded desired cyclic Scheme 82. carbamates 66 in high yields (Scheme 78). 3.5c. Supercritical Carbon Dioxide O Yoshida et al have first reported [136] the synthesis of carbamates 6 in good yields starting from the corresponding amines 4 H2N OH MeCN-Et4NClO4 HN O and alkyl halides 31 using supercritical CO2/K2CO3 in presence of tetra-n-alkylammonium halides acting as phase transfer catalyst 1. e 66-88% R1 R (Scheme 83). They have also demonstrated the role of different 2 2. CO2 R1 R2 phase transfer catalysts on carbamates synthesis and found that 653. TsCl 66 tetrabutylammonium bromide was best among all in affording high yields of carbamates. Scheme 78. The utility of electrochemically generated cyanomethyl an- O 1 R Onium salt, CO R1 ion/carbon dioxide system in affording high yields of carbamates 6 NH 2 3 + R3X + K2CO3 N C O R using various kinds of amines 4 and alkyl halides 31 was further 2 R 1- 4h,72-94% R2 investigated by Inesi and their coworkers (Scheme 79) [132]. 431 6

O Scheme 83. Later on, Baker and coworkers have reported [137] solvent free H2N OH MeCN-Et NClO 4 4 HN O ruthenium-catalyzed synthesis of vinyl carbamates 79 through the reaction of phenyl acetylene 69 with diethyl amine 4 using superi- 1. e 66-88% R1 R 2 2. CO2 R1 critical CO2 (Scheme 84). They have also studied the effect of tem- R2 653. TsCl 66 perature on the catalytic activity of Ru-complexes and found that the best yields of vinyl carbamates were obtained at 120oC. Scheme 79. O Recently, Inesi and coworkers have reported [133] a new elec- R1 R1 O C N trochemical procedure for the synthesis of carbamates 6 in high RuCl3.xH2O Ph + CO2 + HN yields starting from the corresponding amines 4, and alkyl halides R2 3h, 60-120oC using electrochemical carbon dioxide-saturated room temperature 69 R 4 2 Ph 100% 79 (E& Z) ionic liquid [Bmim]BF4 solutions (Scheme 80). Scheme 84. O R1 e Ni(bpy)3Cl2 R1 Recently, Jiang and coworkers have demonstrated [138] a new NH + CO2 N OEt and efficient protocol for the synthesis of oxazolidinones 96 and EtI, upto 82% R R 2 4 2 6 oxaolones 97 through the cycloaddition reaction of CO2 with propargyl alcohols 74 and amines 4 under supercritical conditions Scheme 80. (Scheme 85). 1608 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

R , R = Alkyl, aryl 1 2 ONR3

32-95% R1 R R1 2 scCO2, CuI 96 OH + R3 NH2 o O 4 60 C, 12-24h R2 R1 = Alkyl, aryl, H 74 R2 = H ONR3 88-96%

R1 97 Scheme 85. O R R1 1 RuCl2{P(OEt)3}4 R O C N R + HN o R2 scCO2, 15h, 80 C 69 4 R2 31-68% 79 (Z) Scheme 86.

R1 NH Pd(OAc)2 O N R2 o scCO2, 50 C, 15h R2 R1 upto 65% O 98 99

Scheme 87.

2RNH + CO 2 2 RNH-COONH3R (eq. 1) 4 45 OMe OMe O O C 45 + O C + MeOH (eq. 2) OMe O C NHR 100 101 OMe (eq. 3) 101 O C + CO2 6 NH2R Scheme 88.

Recently, Ikariya and coworkers have reported [139] the stereo- carbonates can be easily obtained by transesterification of DMC selective synthesis of Z-alkenyl carbamates 79 from the corre- with phenols [143] and long chain high boiling alcohols [144]. The sponding amines 4 and alkynes 56 using CO2-soluble ruthenium- reaction between primary and secondary amines and dialkyl car- P(OEt)3 catalyst under supercritical conditions (Scheme 86). bonates needs a suitable catalyst in order to get satisfactory conver- Very recently, Ikariya and coworkers have also reported [140] sion rates and high selectivities. Strong bases such as alkali metal the synthesis of 5-vinyl 1,3-oxazolidine-2-ones 99, through car- alkoxides, Zn, Co, Sn, Al, and Tin compounds have been widely boxylative transformation of 2,3-allenic amines 98 and CO2 pro- employed as catalysts in the carboalkoxylation of anilines and more moted by palladium catalysts under supercritical conditions generally of aromatic amines [145]. Moreover, Lewis acids, such as (Scheme 87). AlCl3, SnCl2, ZnCl2, Zn(OAc)2.2H2O, FeCl3, or metal(Rh, Ru) complexes have proved to be effective in promoting the conversion 3.5d. Organic Carbonates and Carbon Dioxide of propyl amine and diethyl carbonate selectively to propyl and Organic carbonates constitute an important source for carbonyl ethyl carbamate [142]. functionality during the synthesis of carbamates. Their reaction Primary and secondary aliphatic amines can react with CO2 ac- with amines represents an alternative synthetic route to carbamates cording to equilibrium (eq.1) afforded monoalkylammoniumalkyl- that has gained growing attention in the last few years as a non- carbamate ion [78] 45 that serves as a convenient source of carba- phosgene route to the synthesis of organic carbamates [141]. Nowa- mate moiety in the synthesis of carbamates using DMC 100 days, dimethyl carbonate (DMC) 100 can be produced on a large (Scheme 88) [146]. O-Carbomethoxylation of carbamate anion is scale by oxidative carbonylation of methanol [142]. Other organic the first step eq. 2, to afford a mixed carbamic-carbonic anhydride Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1609

O O O O R O 1 Et N, rt 3 R N N + HN 1 N N O O O R2 5-10h, 50-90% 4 R 102 2 O O O 103 Scheme 89. N O R R N 1 o 1 O rt, 0 C + HN N N OR R 53-98% 462 R 104 O OR 2 Scheme 90.

O O O 102 O o N + N3 Et3N, MeCN, 23 C OH O O O O O 106 107 105 O 69-80%

Et3N, THF H2, 10% Pd/C O O O

O N H O O O 108 Scheme 91. OTBS O TBSO O 102 O O O HO O Et3N, MeCN, rt., 4 h N O O O 110 O 109 O O Et3N, CH2Cl2 H O rt, 3 h 111 65-89% NH2 Ph OTBS O O O H O O N O Ph H O 112 Scheme 92.

RNH-COOCOMe 101. This step could be catalyzed by acidic spe- Later on, Ogura and their coworkers have demonstrated [148] + cies, such as RNH3 , RNH-COOH, present at equilibrium. Selective an efficient protocol for the synthesis of carbamates 6 through the decarboxylation of 101 through expulsion of a CO2 molecule from reaction of corresponding amines 4 with benzotriazole carbonate its carbamic moiety leads to the formation of carbamate 6. 104 (Scheme 90). Organic carbonates have received much attention in recent Use of N,N’-disuccinimidyl carbonate 102 has been further ex- years as cheap and safe alternatives to the non-phosgene routes for plored for the synthesis of carbamates [149] 108 from azides 107 the synthesis of organic carbamates. Therefore, several researchers and mixed carbonates 106 by Ghosh and coworkers (Scheme 91). over the world have become interested to synthesize carbamates Ghosh and coworkers have also reported [150] synthesis of car- through this route. bamates 112 in high yields starting from the corresponding amines Ogura and coworkers have first reported [147] an efficient syn- 111 and a mixed carbonate 110, which is obtained through a reac- thesis of N-succinimidyl carbamates 103 through the reaction of tion of alcohol 109 with DSC 102 (Scheme 92). corresponding amines 4 with N,N’-disuccinimidyl carbonate (DSC) Chiral carbamates 6 have also been synthesized through an en- 102 using triethyl amine (Scheme 89). zymatic alkoxy-carbocylyzation reaction with vinyl carbonates 113 1610 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

O O C CAL + C RO O H2N R 20-72h, 38-45% RO N R H 113 4 upto 98%ee 6

Scheme 93. O HO O O B O C B N CAL, THF O O O O + 115 30oC, 116a HO HO 114 5oC, 116b 116

o RNH2, THF, 80-100 C o PSL, THF, 30 C O

O B HO HO RHN O O B O B o O RNH2, THF, 80-100 C O HO 117 80-100% O O RHN O 119a, 80% 118 117a, 70% 119 119b, 52% 117b, 75%

Scheme 94.

O O P-acid Ar.NH2 + RO OR Ar.HN OR 4 15-76h, upto 98% 6 100, R = Me 120, R = Ph

Scheme 95.

RN3

Me3P O O Ph N -20oC, RT R + RN=PMe t O OBut 3 N OBu 5h, 87-100% H 6 CN 122 121

Scheme 96. and racemic amines 4 using Candida antarctica lipase, CAL was rous acids [Ph2P(O).OH, (PhO)2P(O).OH, (BuO)2P(O).OH/ reported by Pozo and Gotor (Scheme 93) [151]. (BuO)P(O)(OH)2 equimolar mixtures] (Scheme 95). They have Gotor and coworkers have reported [152] the chemoenzymatic further realized that better yields of carbamates were obtained using synthesis of a 2’-deoxynucleoside urethan. 2’(Deoxynucleoside 5’ DPC. and 3’(N-alkyl)carbamates (117 & 119) were synthesized in a two- Urpi and coworkers have reported [154] an efficient protocol step procedure using lipase catalysis in the regioselective vinyloxy for the synthesis of tert-butyl carbamates 6 through reaction of az- carbonylation step (Scheme 94). The regioselectivity of the reaction ides with trimethyl phosphine followed by addition of 2-(tert- depends upon the type of lipase enzyme used. Total regioselectivity butoxycarbonyloxyamino)-2-phenylacetonitrile 121 at -20oC is obtained in the presence of PS lipase and only a small amount of (Scheme 96). the regioselective product is obtained when the reaction is catalyzed Chandrasekhar et al have reported [155] an excellent one-pot by CA lipase. method for the synthesis of carbamates 6 through reaction of azides Aresta et al have reported [153] the synthesis of carbamates 6 with di-tert-butylcarbonates (Boc) 123 using the inexpensive and through the reaction of aromatic amines 4 with DMC 100 or safe hydride source i.e. polymethylhydrosiloxane (PMHS) under diphenyl carbonate (DPC) 120 in the presence of organo phospho- Pd-C catalysis (Scheme 97). Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1611

PHMS / PD-C O heterogeneous catalyst O RN3 + (Boc)2O RNH-Boc EtOH, rt + R-NH2 123 6 OEt 4 upto 98% 72-98% EtO EtO NHR 124 6

Scheme 101. PHMS / PD-C RNH-Cbz + (Boc)2O RNH-Boc EtOH, rt Selva and workers have reported [160] an efficient synthesis of 123 6 carbamates 6 from primary aliphatic amines 4 with dialkyl carbon- 75-98% ates (100 & 124) in supercritical CO2 (Scheme 102). O Scheme 97. o O scCO2 , 130-140 C + RNH The enzyme lipase has also been used as a catalyst in the syn- 1 2 1 OR 4 1 thesis of chiral carbamates [156] 6 starting from racemic amines 4 R O 14-20h, 50-83% RHN OR 100, 124 6 and alkylvinyl carbonates 113 (Scheme 98). Scheme 102.

NH2 O (R) ( )n O O HN Lipase, 72 h, 25oC C C + O + O upto 86% yield, 99% ee H 113 4

O HN (S) C

O 6

Scheme 98.

Lemaire and coworkers have reported [157] efficient synthesis Sodeoka and coworkers have reported [161] a convenient of carbamates 6 through the reaction of amines 4 with DMC 100 method for the synthesis of carbamates 6 through reaction of vari- catalyzed by -Al2O3 (Scheme 99). ous amines 4 with variety of polymer-supported N-hydroxy suc- cinimide substituted carbonates 125 (Scheme 103). PHMS / PD-C RN3 + (Boc)2O RNH-Boc O EtOH, rt O O 123 6 S base 72-98% + R1-NH2 N O OR upto 98% 4 R1HN OR PHMS / PD-C 6 RNH-Cbz + (Boc)2O RNH-Boc EtOH, rt O 125 123 6 75-98% Scheme 103.

Scheme 99. Later on, Christention and coworkers have reported [162] the synthesis of carbamate protected polyamines (130, 131, 132, 134, Chaudhari and coworkers have reported [158] an efficient pro- 135, 136) using alkyl phenyl carbonates (126, 127, 128). This is an tocol for the synthesis of carbamates 6 through the reaction of vari- economical, practical and versatile method for selective Boc, Cbz, ous carbonates (100, 120) with variety of amines 4 catalyzed by and Alloc protection of polyamines. This method allows Boc, Cbz, silicagel (Scheme 100). and Alloc protection of primary amines in the presence of secondary amines by reaction of polyamines with alkylphenyl carbonates. O O Also, this method allows monocarbamation of simple symmetrical Silicagel, 150oC, 1 aliphatic ,-alkanediamines in high yields with respect to the dia- + R NH2 1 2 3- 8h, upto 96% R HN OMe mine. Furthermore, the method allows selective carbamate protec- R2O OR 4 6 tion of a primary amine located on a primary carbon in the presence 100, 120 of a primary amine located on a secondary or a tertiary carbon in excellent yields (Scheme 104). The alkyl-phenyl carbonates inves- Scheme 100. tigated in this study were tert-butylphenylcarbonate (126), benzyl- Carloni and coworkers have reported [159] synthesis of carba- phenylcarbonate (127), and allylphenylcarbonate (128), which in- mates 6 through reaction of diethyl carbonate 124 with variety of troduces the Boc, Cbz, and Alloc protecting groups. amines 4 using a heterogeneous catalyst a hybrid organic-inorganic Later on, Curini and workers have reported [163] that ytterbium material prepared by anchoring TBD to MCM-41 silica. (Scheme triflate, Yb(OTf)3 can be efficiently used for the preparation of car- 101). 1612 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

O O O O O O O O 126 O 127 128

O H ( )m ( )n NH2 126, 127, 128 ( )m ( )n N OPh H2N N RO N N 52-98% H H H O 129 130, 131, 132

H 126, 127, 128 N OR NH2 H N H2N 2 70-97% 133 O 134, 135, 136

Scheme 104. bamates 6, through reaction of various amines 4 with DMC 100 (100, 120, 124) with various amines 4, using various catalysts under solvent free conditions (Scheme 105). (Scheme 109). Out of several catalysts used, n-butyltin oxide was found to be best in affording good yields of the carbamates. O O Yb(OTf)3 O O + RNH2 carboxylation catalyst o MeO OMe 4 80 C, 8h, 61-96% RHN OMe R NH + 100 6 1 2 RO OR 4-24h, upto 93% RO NHR 4 1 6 Scheme 105. 100, 120, 124

Later on, Deng and coworkers have reported [164] the synthesis Scheme 109. of carbamates 6 through the reaction of primary and secondary aliphatic amines 4 with DMC 100 using ionic liquids (Scheme 106). Shen et al have reported [168] a facile synthesis of N-methyl N-aryl carbamates from aromatic amines 4 and DMC 100 in the O presence of potassium carbonate and tetrabutylammonium bromide O 1 R1 Ionic liquid, 170oC, 1h R under solvent free conditions (Scheme 110). + NH N OMe 2 MeO OMe R upto 99% 2 O O 100 4 R 6 Base/PTC R1NH2 + 5-35h, 51-98% NHR Scheme 106. 4 MeO OMe MeO 1 6 Conversion of azides to tert-butyl carbamates [165] 6 could also 100 be synthesized using di-tert-butyl dicarbonate 123, decaborane (20 mol%) and 20% Pd-C at room temperature in methanol (Scheme Scheme 110. 107). Distaso and Quaranta have reported [169] a high yielding syn- Decaborane, 20% Pd-C thesis of carbamates 6 through reaction of various aliphatic amines RN3 + (Boc)2O RNH-Boc 4 and dimethyl carbonate 100 catalyzed by group III metal (Sc, La) 6 123 1.5h-8h, rt, 71-100% triflates, under mild reaction conditions (Scheme 111). Sc(OTf)3 is more effective than La salt. Scheme 107. O O Chandrasekhar and coworkers have reported [166] the one-step (M = Sc, La) conversion of N-benzyl, N-trityl, N-diphenyl amines 137 to tert- M(OTf)3 + RNH2 OMe RHN OMe butyl carbamates 6 using Boc 123 in the presence of polymethylhy- MeO 4 20-48h, upto 84% 6 drosiloxane (Scheme 108). 100 R PMHS-10% Pd(OH)2/C R Scheme 111. N X + (Boc) O R 2 N-Boc 1 rt, 2-8h, 82-92% R 137 123 1 6 Distaso and Quaranta have also reported [170] carbomethylat- X = Bn, Tr, DPM ing reactivity of methyl phenyl carbonate 138 towards aromatic amines 4 in the presence of group 3 metal (Sc, La) triflate catalyst Scheme 108. under mild conditions to afford the corresponding carbamates 6 in Chaudhari and coworkers have reported synthesis of carba- high yields (Scheme 112). They have optimized the effect of the mates [167] 6 through the reaction of various organic carbonates various catalysts at different temperature, time, and molar ratio of Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1613

O O (Sc, La) triflate catalysts + Ar-NH2 Me O O Ph upto 72% Me O N Ar 4 H 138 6

Scheme 112.

O R1 5-10min O + NH R1 R 70-95% O O 2 O N 4 NO2 NO2 NO 2 R2 139 6

Scheme 113.

O O

CO2 (5-200 bar) + RNH2 OMe o RHN OMe MeO 4 130 C, upto 84% 6 100

Scheme 114.

NH2(CH2)nNH2 + 2 DMC MeOOCNH(CH2)nNHCOOMe 100 n = 6-8 Acid functional 141 140 ionic liquids

80oC, 4h

34-98% RNH2 + DMC RNHCOOMe 4 6

Scheme 115.

O O O Catalyst + PhHN NHPh OMe MeO 4h, upto 74% MeO NHPh 100 142 6

Scheme 116. amines on the yields of the carbamates. Sc(OTf)3 is more effective Deng and coworkers have reported [173] synthesis of carba- than La salt. mates 6 and dicarbamates 141 through reaction of variety of ali- Cotarca and workers have reported [171] an efficient synthesis phatic amines 4 and bisamines 140 with dimethyl carbonate 100 of o-nitrophenyl carbamates 6 through the reaction of bis(o- catalyzed by acid functionalized ionic liquids (Scheme 115). They nitrophenyl)carbonate 139 with aliphatic amines 4 under mild reac- have realized that -SO3H functionalized ionic liquids were found to be most effective among the applied ionic liquids. tion conditions (Scheme 113). Later on, Li and coworkers have reported [174] synthesis of Later on, Selva and coworkers have reported [172] a high yield- methyl-N-phenyl carbamates 6 through the reaction of dimethyl ing one-pot synthesis of methyl carbamates 6 from primary ali- carbonate 100 with 1,3-diphenyl urea 142 under atmospheric pres- phatic amines 4 and dimethyl carbonates 100 using supercritical sure (Scheme 116). Among various catalysts used, NaOMe was CO2 (Scheme 114). The pressure of CO2 largely influences both the found to be best in affording high yields of carbamates. reaction conversion and the selectivity towards urethanes. Gener- Later on, Han and Porco have reported [175] an efficient proto- ally, conversion goes through a maximum (70-80%) in the mid- col for the synthesis of structurally diverse carbamates 6 through range (40 bar) and drops at lower and higher pressures, whereas the reaction of various amines 4 with variety of carbonates (100, selectivity is continuously improved (from 50% up to 90%) by an 120, 124 etc) using zirconium (IV) catalyzed exchange process increase of pressure. using 2-hydroxy pyridine as catalytic additives (Scheme 117). 1614 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

O O R1 Zr(O-t-Bu)4 R1 + HN N OR OR o RO R2 80 C, 12h R 4 2 6 N OH R = Me, Et, Allyl, Bn 47-98%

Scheme 117. O O AISBA Ph-NH2 + o PhHN OMe 4 MeO OMe 100 C, 3h, 99% 100 6

Scheme 118.

BnHN OCO2Me DBU-CO2 BnN O

R 80oC, 6-48h R R 51-99% R 143 144

Scheme 119.

O O ZnO-TiO2 Ph-NH2 + OMe MeO OMe 7h, 66.7% PhHN 4 6 100

Scheme 120.

Use of DMC 100 in the synthesis of methyl-N-phenyl carba- O mates 6 using aromatic amines using ordered AISBA-15 catalyst R 1 100, [Bmim]OH R1 was recently reported by Halligudi and coworkers (Scheme 118) N OMe HN [176]. 90oC, 1h R R2 Recently, Yoshida and coworkers have reported [177] an effi- 2 6 cient protocol for the synthesis of 5-vinylidineoxazolidin-2-ones 4 upto 96% 144 by DBU-mediated CO2-fixation reaction of 4-(benzylamino)-2- Scheme 121. butynyl carbonates and benzoates 143 (Scheme 119). Recently, Sureshbabu and Hemantha have reported [180] the Use of DMC 100 was further explored in the synthesis of synthesis of dipeptidyl carbamates 147 through the reaction of an methyl-N-phenyl carbamates 6 from aromatic amines 4 catalyzed amino acid 146 with their synthesized F-moc aminoalkoxy pen- by ZnO-TiO catalyst (Scheme 120) [178]. 2 tafluorophenyl carbonate 145. They have further explored the utility N-Heterocyclic carboxymethylation of amines 4 using DMC of 145 in synthesis of oligopeptidyl carbamates using variety of 100 catalyzed by ionic liquid, afforded corresponding carbamates 6 amino acids (Scheme 122). in high yields was reported by Gao and coworkers (Scheme 121) A most recent report for an efficient preparation of N-tert-butyl [179]. carbamates 6 of various amines 4 using di-tert-butyl carbonate 123

R2 F OMe O R2 F H2N O OMe 146 O Fmoc HN Fmoc HN O N o H O O NMM, 0 C, 20-25min. F O R1 F 58-94% R1 147 145

Scheme 122. Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1615

O O R1 Amberlyst 15 R1 N O NH + O O R2 1-10min., 90-99% R2 4 123 6

Scheme 123.

O N O O H MgSO4 H + H N + EtO O OEt 2 4 rt 150 R R1 1 148 149 35-98% 15min., 130oC

N OEt

OEt

R1 151

Scheme 124.

O O O C OH O O Neat, rt, 1h C O NHR' + R'NH + 2 O NHR' 4 70-95% OH R R R 152 6a 6b

Scheme 125. in presence of Amberlyst 15 under solvent free conditions was re- 3.6. Carbamates Synthesis from Dithiocompounds ported by Pal and their coworkers (Scheme 123) [181]. 3.6a. Using Dithiocarbamates/Thiocarbamates More recently, A direct conversion of various allylic imines 149 to their corresponding -ethoxy carbamates 151 using diethyl pyro- Tandel and coworkers have reported an efficient synthesis of carbonate 150 was reported by Grognec and coworkers (Scheme carbamates 6 from dithiocarbamate 153 through either series of 124) [182]. The imines were synthesized from the corresponding transformations or direct conversion using NaOMe/MeOH afforded aldehydes 148 using allylic amines 4. carbamates 6 (Scheme 126) [184]. The overall yields in carbamates Iwasaki and coworkers have reported [183] the synthesis of hy- from dithiocarbamates during three step is 36%, whereas the direct droxy carbamates 6 (a & b) from cyclic five-membered carbonates conversion afforded 95%. 152 and primary amines 4 at room temperature (Scheme 125). Recently, Fochi and coworkers have reported an efficient protocol for the synthesis of various kinds of carbamates 6 through S S reaction of corresponding thiocarbamates 155 with alcohol 5 in triethylamine (Scheme 127) [185]. R1 NaOMe /MeOH R1 N SR N OR O 50-93% O R2 R2 153 154 R1 Et N R N SMe 3 1 H + ROH N OR 6-25% 4-24h, 41-100% H 5 6 I 155 NaOMe 95% NaOMe 2 DMSO Scheme 127. MeOH 60% O 3.6b. From Carbon-Imido Dithiolates O NaOMe R1 Rajappa and coworkers have reported [186] that carbon-imido R1 N SR dithiolates 156 are important precursors for the synthesis of carba- MeOH R N OR 2 155 mates 6 and can be first converted into S-methyl thiocarbamates R2 6 50-85% [187] 155 using zeolite catalyzed partial hydrolysis. This method therefore provides an alternative route to methyl carbamates 6 Scheme 126. 1616 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

O MeS ZnCl , MeCN-Water, 60oC, 6-10h N.R 2 MeS reflux, 24h upto 98% MeS NHR 156 155

ZnCl2, MeOH-Water o o 60 C, 6h ZnCl2, MeOH PhCH2ONa, THF 30 C, 6h upto 85% 60-80oC, 12h 52-98% O O upto 95%

NHR NHR MeO PhH2CO 6 6

Scheme 128.

[188]. The carbon di-imidodithiolates can also be converted into using NBS and DBU in methanol (Scheme 131). This method has carbamates 6 through one-step synthesis (Scheme 128). been widely useful for the conversion of alkyl and aryl carboxamides 157 to their corresponding methyl carbamates 6 in excellent 3.7. Carbamates Synthesis Through Rearrangement Reactions yields under extremely mild conditions.

3.7a. Hoffmann Rearrangement O O Generally, Hoffmann rearrangement [189] converts primary NBS, DBU R carboxamides to amines using aqueous NaOH and Br . In recent R NH N OMe 2 2 MeOH, reflux, 25min. H years, it has been used for the synthesis of carbamates 6 through the 157 6 involvement of an isocyanate intermediate. Several researchers 89-100% have directed their efforts on this rearrangement reaction in order to achieve an efficient synthesis of carbamates. Scheme 131. Moriarty and coworkers have reported [190] an efficient proto- The synthesis of N-tert-butoxy carbamates 6 from primary car- col for the synthesis of methyl carbamates 6 from primary alkyl and boxamides 157 using copper (II) reagent (prepared from copper (II) aryl carboxamides 157 using hypervalent iodine. They have treated bromide and lithium tertiary butoxide i.e. CuBr2-LiOBu-t) afforded a series of primary alkyl/aryl carboxamides 157 with PhI(OAc)2 in good to high yields, has also been reported by Yamaguchi and their KOH-MeOH at 5-10oC afforded the corresponding methyl carba- coworkers (Scheme 132) [193]. mates 6 in good to excellent yields (Scheme 129). These conditions avoids the use of elemental bromine or heavy metal reagents such O O as Pb(OAc)4, AgOAc, Hg(OAc)2, while taking advantage of the CuBr2-LioBu-t, rt, 2-6h R commercial availability of PhI(OAc)2. N O R NH2 H 36-99% 6 157

Scheme 132. 157 6 Later on, Matsumara and coworkers have reported [194] the Scheme 129. electrochemical synthesis of carbamates 6 from primary carbox- Later on, Huang and coworkers have reported [191] the synthe- amides 157 and the process has been referred to as" Electrochemi- sis of methyl carbamates 6 via a modified Hoffmann rearrange- cally induced Hoffmann rearrangement” which under new solvent ment. They have treated a series of p-substituted aromatic and pri- systems containing a variety of alcohols 5 was developed since the mary aliphatic carboxamides 157 with NBS and NaOMe in metha- reaction proceeds under mild conditions (neutral). An epoxy func- nol heated to reflux for ten minutes for the conversion of the car- tional group in the amide and alcohol remains intact during the boxamides to their corresponding primary amino methyl carba- electrolysis (Scheme 133). mates 6 in nearly quantitative yields (Scheme 130). The mild oxidative conditions of this modified Hoffmann rearrangement are shown O O to be particularly useful for the preparation of p-substituted ani- EI-Hofmann rearrangement + R OH R R NH 1 N OR lines. 2 CH CN, 40-98% 1 1575 3 H 6 O O NBS, NaOMe Scheme 133. R R NH N OMe 2 MeOH, reflux, 10min H Later on, Hiegel and Hogenaur have reported [195] a base cata- 157 6 85-100% lyzed synthesis of N-substituted carbamates 6 through the rear- Scheme 130. rangement of N-chloroamides (Scheme 134). These N-chloramides were obtained by the chlorination of amides 157 using trichloro They have further elaborated [192] the synthesis of methyl car- isocyanuric acid (TCICA). bamates 6 through the involvement of Hoffmann rearrangement Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1617

O O O TCICA O R NH MeONa RHN OMe ROH, m-xylene HN OR' 2 X 6 N3 157 46-98% X heat, upto 86% 161 160 Scheme 134. Scheme 139. Nishikawa and coworkers have reported the synthesis of carbamates 6 through the rearrangement reaction of trichloroacetamides Lebel and Leogane have reported [201] an efficient protocol for 158 using variety of alcohols 5 (Scheme 135) [196]. the preparation of tert-butyl carbamates 6 from the corresponding acid 162. The reaction of carboxylic acid 162 with di-tert-butyl O O dicarbonate 123 and sodium azide allowed the formation of an acyl Na2CO3, azide 159, which undergoes a Curtius rearrangement in the pres- + R1OH R Cl C NHR N OR1 3 CuCl, Bu4NCl ence of tetrabutylammonium bromide and zinc(II)triflate afforded 1585 H 6 upto 83% the corresponding carbamates 6 through the trapping of isocyanate intermediate (Scheme 140). They have also extended the same pro- Scheme 135. tocol for the direct synthesis of carbamates of aromatic amines using aromatic acid [202, 203]. Gogoi and Konwar have reported [197] synthesis of methyl O O carbamates 6 through the modification of Hoffmann rearrangement. Boc2O, NaN3, n-Bu4NBr, Zn(OTf)2 Thus, a series of methyl carbamates 6 were synthesized in good to O-t-Bu excellent yields using NaOCl as an oxidant in the presence of R OH 40-50oC, 16-24h, 57-98% RHN 162 6 KF/Al2O3/ MeOH at reflux conditions (Scheme 136). Scheme 140. O O NaOCl/KF/Al O 2 3 Dussault and Xu have reported a direct conversion of various R NH2 MeOH, reflux, 30min. RHN OMe acid azides 159 to their corresponding carbamates 6 through Curtius 157 6 rearrangement using an alcohol 5 (Scheme 141) [204]. Similar kind 72-95% of approach was adopted by Saigo and coworkers for the synthesis Scheme 136. of fullerene carbamates through reaction of corresponding fullerene acid azide with an alcohol [205]. A most recent method for the synthesis of carbamates 6 starting from the corresponding amines 157 through Hoffmann rearrange- O O ment using microreactor technique has been reported by Ley and EtOH, Benzene their coworkers (Scheme 137) [198]. R N RHN OEt 3 reflux, 47-60% 159 6 O O DBU or NBS, 120oC Scheme 141. + R OH R 1 N OR R NH2 Advion NanoTek LF microreactor 1 Ikegami and coworkers have reported synthesis of carbamates 6 5 H 6 157 32-80% of various sugar and other functionalities using corresponding acid 162. Insitu conversion of acid 162 to the corresponding azides 159 Scheme 137. was achieved using diphenylphosphoryl azides (DPPA), followed 3.7b. Curtius Rearrangement by addition of an alcohol afforded corresponding carbamates (Scheme 142) [206]. They have further explored their methodology Curtius rearrangement involves the pyrolysis of acyl azides 159 for the synthesis of carbamate linked glycoconjugates using various to yield isocyanates 13 (Scheme 138) [199]. Isocyanates 13 can kinds of sugar acids and DPPA [207]. treated with alcohols 5 to afford the corresponding carbamates 6. In recent years, much interest has been developed among the chemists O O to synthesize the carbamates through Curtius rearrangement by trapping of isocyanate intermediate with an alcohol. R1OH, DPPA, Base OH OR R Benzene, reflux RHN 1 O 162 6 O 5 5-30h, upto 94% Heat, Curtius rearrangement R'OH R-N=C=O RHN OR' N Scheme 142. R 3 6 159 13 3.7c. Lossen rearrangement Scheme 138. The Lossen rearrangement is a useful chemical reaction in Richer and Andersen have reported [200] the synthesis of car- which O-activated hydroxamic acids 163 can be converted into the bamates 161 solid supports using polystyrene resin. They have corresponding isocyanates 13 (Scheme 143) [208]. Carbamate 6 can prepared an acid azide derivative 160, which was previously loaded be synthesized through insitu trapping of an isocyanate intermediate on polystyrene resin and treated with appropriate alcohol in m- 13 through an alcohol 5. In recent years, based on the above con- xylene. The carbamates were obtained in excellent yields (Scheme cept researchers become interested to synthesize carbamates using 139). hydroxamic acids through the Lossen rearrangement reaction. 1618 Current Organic Chemistry, 2011, Vol. 15, No. 10 Devdutt Chaturvedi

O O 5 OH Lossen rearrangement R'OH R NH R-N=C=O RHN OR' 163 6 13 Scheme 143.

Recently, Papot and coworkers have reported an efficient syn- 3.8b. Carbamates Synthesis Using Burgess Reagent thesis of carbamates 6 through the reaction of a hydroxamic acid 163 with an alcohol 5 promoted by 2,4,6-trichloro-1,3,5-triazine Burgess reagent [215] 168 is prepared from a reaction of an al- 164 (cyanuric chloride, TCT) in presence of excess of N-methyl cohol 5 with chlorosulfonyl isocyanate 166 and triethyl amine 167 morpholine (NMM) through Lossen rearrangement reaction (Scheme 147), has been shown to be an efficient for the stereo- (Scheme 144) [209]. specific cis-dehydration of secondary and tert. to provide olefins. Primary alcohols do not undergo elimination due to competing (and Cl predominant) displacement reaction to form corresponding methyl carbamates. Several kinds of alcohols have been used in order to O N N O 164 get more efficient Burgess reagent which could afford carbamates OH Cl N Cl R'OH in high yields. In recent years, researchers have directed their ef- R NH + NMM, 0oC to reflux RHN OR' forts to synthesize carbamates through Burgess reagent. 163 5 6 O 67-99% O

Scheme 144. ROH + OCNSO2Cl + Et3N RO N S NEt3 5 167 166 O R = Me, PhCH2 A most recent method for the synthesis of carbamates 6 through 168 the reaction of hydroxamic acids 163 with an alcohol 5 using carbonyl diimidazole (CDI) through Lossen rearrangement was re- Scheme 147. ported by Dube and coworkers (Scheme 145) [210]. Nicolaou and coworkers have reported [216] an efficient, one- O O pot synthesis of methyl carbamates 170 through the corresponding OH CDI, 5-40min. cis-diols 169 using methyl-Burgess reagent 168 (Scheme 148). NH + R1OH R o RHN 5 60 C to reflux OR1 168 163 6 HO OH NHCOOMe Nu 81-99% Et3N-SO2-N-COOMe Scheme 145. R1 R2 Nu (90-95%) 169 R R2 1 170 3.8. Miscellaneous Methods for the Synthesis of Carbamates . 3.8a. Carbamates Synthesis Using Sodium Cyanate Scheme 148. Recently, use of sodium cyanate 165 in the synthesis of primary Later on, Wood and coworkers have reported [217] a novel, carbamates 6 through reaction with alcohol 5 using various kinds of one-step conversion of primary alcohols 5 into carbamate-protected acidic catalysts such as trichloroacetic acid, silica supported- 6 amines using benzyl-Burgess reagent 168 (Scheme 149). sulfuric acid, silica supported-perchloric acid, and Al(HSO4)3 has Conversion of Baylis-Hillman adducts 171 of -amino acids been realized by Modaressi-Alam and coworkers (Scheme 146) into corresponding methyl carbamates 172 using methyl Burgess [211-214]. reagent 168 was reported by Mamaghani and Badrian (Scheme 150) O [218]. Acidic catalyst ROH + NaOCN 3.8c. Synthesis of Carbamates from Oximes o RO NH 5 165 60-70 C, 30-60min 2 6 Goti and coworkers have reported the synthesis of various sub- 60-95% stituted aromatic carbamates 174 through the reaction of variety of Scheme 146.

O R R1 168 1 R-(CH )n-CH OH + NH 2 2 R-(CH2)n-CH2 O N o R2 50-85 C, 16-36h 5 R2 4 80-98% 6 Scheme 149. OH O NHCOOMe O methyl Burgess reagent OMe OMe 20-88% R R 171 172

Scheme 150. Recent Developments on the Carbamation of Amines Current Organic Chemistry, 2011, Vol. 15, No. 10 1619 aromatic oximes 173 with alcohols using methyltrioxorhenium ences during the preparation of manuscript. Author is also thankful (MTO) and urea-hydrogen peroxide (UHP) (Scheme 151) [219]. to Guest Editor of this issue, Prof. Lucio Pellacini for his kind invi- tation and fruitful suggestions. O N OH REFERENCES NH OR3 [1] Adams, P.; Baron, F. A. Esters of carbamic acid. Chem. Rev., 1965, 65, 567- R R1 R R1 MTO, UHP 602. [2] (a) Sutchell, D. P. N.; Satchell, R. S. Acylation by ketenes and isocyanates. R OH, rt A mechanistic comparison. Chem. Soc. Rev., 1975, 4, 231-250; (b) Snieckus, 3 V. Directed ortho metalation: Tertiary amide and O-carbamate directors in synthetic strategies for polysubstituted aromatics. Chem. Rev., 1990, 90, 879- R upto 72% 2 R2 933; (c) Jessop, P. G.; Ikariya, T.; Noyori, R. Homogeneous hydrogenation 174 of carbon dioxide. Chem. 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X-Ray structure of + + [PPN][Rh(CO)2ON[C6H3Cl2)C(O)O]; [PPN =(PPh3)2N ; bipy = 2,2'- This review gives a comprehensive survey regarding the syn- bipyridyl]. J. Chem. Soc. Chem. Comm., 1992, 1467-1468; (d) Jaitner, P.; 6+ thesis of organic carbamates through the various starting materials Rieker, C.; Wurst, K. Aggregation of carbamato ligands around the [Co4O] core. Synthesis and structure of the cluster [Co4O(O2CNC9H18)6] prepared by from the beginning to the recent reports (covering upto December, a novel oxo-transfer reaction of the nitroxyl free radical 2,2,6,6- 2009). Organic carbamates have clearly been demonstrated to be tetramethylpiperidin-1-oxyl with [Co2(CO)8]. J. Chem. Soc. Chem. Comm., extremely useful and stable reagents, exhibiting unique physical, 1997, 1245-1246; (e) Aoki, S.; Kawatani, H.; Goto, T.; Kimura, E.; Shiro, M. A double-functionalized cyclen with carbamoyl and dansyl groups (cyclen = chemical and biological properties. 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In recent years, it has been realized by pesticides in fruits and vegetables by liquid chromatography electrospray various researchers that introduction of carbamate functionality in ionization tandem mass spectrometry using a short column. Anal. Chimica Acta, 2006, 555, 225-232; (c) Ma, J.; Lu, N.; Qin, W.; Xu, R.; Wang, Y.; various biologically active synthetic/natural/semisynthetic mole- Chen, X. Differential responses of eight cyanobacterial and green algal spe- cules increases many fold biological activities. In addition, organic cies, to carbamate insecticides. Ecotoxicol. Environmental Safety, 2006, 63, carbamates have made a great impact in the fields of polymer sci- 268-274. [7] (a) Wills, A. J.; Ghosh, Y. K.; Balasubramanian, S. Synthesis of a polymer- ence, biology and medicine. Organic carbamates have been utilized supported oxazolidine aldehyde for asymmetric chemistry. J. Org. 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Received: 01 March, 2010 Revised: 27 May, 2010 Accepted: 29 May, 2010