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Functionality of Amidines and Amidrazones

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Functionality of Amidines and Amidrazones Special Issue Reviews and Accounts ARKIVOC 2008 (i) 153-194 Functionality of amidines and amidrazones Ashraf A. Aly* and Ahmed M. Nour-El-Din Chemistry Department, Faculty of Science, El-Minia University 61519-El-Minia, A. R. Egypt E-mail: [email protected] Abstract The review summarizes literature dealing with the synthesis of amidines and amidrazones including some of their physical and chemical properties along with their applications in heterocycles synthesis. Keywords: Amidines, amidrazones, heterocycles Contents Introduction 1. Synthesis of Amidines 2. Cyclic Amidines 3. Miscellaneous Amidines 4. Amidines as Building Blocks in Synthesis 4.1. Synthesis of amidrazones and amidoximes 4.2. Synthesis of heterocyclic derivatives 5. Amidrazones 5.1. General properties 5.2. Methods in the synthesis of amidrazones 5.3. Reactions of amidrazones 6. Reaction of Monosaccharides with 2-Pyridylcarboxamidrazone 7. Industrial and Medicinal Applications of Amidrazones References ISSN 1551-7012 Page 153 ©ARKAT USA, Inc. Special Issue Reviews and Accounts ARKIVOC 2008 (i) 153-194 Introduction The amidino group in 1a, which contains an amino nitrogen atom with a free electron pair conjugated with the π-electrons of the C=N double bond, is the bis-nitrogen analogue of carboxylic acids and esters 2.1 It combines the properties of an azomethine-like C=N double bond with an amide like C-N single bond with a partial double bond character as indicated by the resonance form 1b.2,3 NR2 NR2 O R1 R1 R NR3R4 NR3R4 OR 1a 1b 2 Amidines are strong bases (pKa ranges from 5-12). The protonation occurs on the imino nitrogen4,5 leading to symmetrical amidinium ion 3 which is stabilized by resonance as in the isoelectronic carboxylate anion 4. NHR2 NHR2 O O R1 R1 R R NR3R4 NR3R4 O O 4 3 In a strong acidic media, the dication 5 is formed, which has a localized carbon-nitrogen double bond, whereas in a strong alkaline solution the anion 6 can be formed.6-8 2 NHR2 NR2 NR 1 R1 R1 R NHR3R4 NR4 NR4 6 5 ISSN 1551-7012 Page 154 ©ARKAT USA, Inc. Special Issue Reviews and Accounts ARKIVOC 2008 (i) 153-194 1. Synthesis of Amidines Numerous methods have been reported for the preparation of amidines.9 The most common methods of amidine synthesis are from amides, nitriles and/or thioamides. 1.1 Amidines from amides or carboxylic acids Amides can be converted to imidoyl chlorides using phosphorous pentachloride. The forming imidoyl chlorides can react with primary or secondary amines to yield amidines (Scheme 1).10 This method is generally poor for preparing unsubstituted amidines from primary amides, but it is an excellent general method for preparing di- and tri-substituted amidines. Other reagents such as phosphorous oxychloride or thionyl chloride can be employed in the synthesis of imidoyl chlorides but usually lower yields are obtained. O Cl NR' PCl5 R"R"'NH R NHR' RNR' RNR''R"' Scheme 1 Amides can be o-alkylated with triethyloxonium fluoroborate at ambient temperature to yield the corresponding imidic ester fluoroborates, which react with amines to yield the target amidines (Scheme 2).11 O OEt NR" Et OBF ,EtOH,25 o C R"NH 3 4 + - 2 BF4 R NHR' 78-90% R NHR' 71-90% RNHR' R=Ar,Me R" = H, Me R' = H, Me Scheme 2 Direct synthesis of amidines from carboxylic acids and amines can be performed via intermediate amides using polyphosphoric acid trimethylsilyl ester (PPSE), generated in situ from the reaction of phosphorus pentoxide with hexamethyldisiloxane as a condensing agent (Scheme 3).12 ISSN 1551-7012 Page 155 ©ARKAT USA, Inc. Special Issue Reviews and Accounts ARKIVOC 2008 (i) 153-194 O NR' R'NH2 R'NH RCOOH 2 o RNHR'65-88% RNHR' PPSE, CH2Cl2,40 C R=Ar,alkyl R' = Ar Scheme 3 1.2 Amidines from nitriles Nitriles are activated to the intermediate salts in the presence of ethanol and hydrogen chloride under anhydrous conditions. The separated salts are then reacted with amines to generate the amidines shown in Scheme 4.13,14 This method can be also considered as the most widely used procedure for the preparation of unsubstituted amidines. The direct synthesis of amidines from nitriles and amines can only be achieved if nitriles are substituted by electron-withdrawing groups.15 Unreactive nitriles have been used to prepare amidines in the presence of Lewis acids 16 such as AlCl3 or ZnCl2 at elevated temperatures of 150-200 °C. Previously, it was shown that alkylchloroaluminum amides are useful reagents for the conversion of nitriles to amidines under milder conditions.16 It was found that17 addition of alkylchloroaluminum amides, generated conveniently from trimethyl aluminum and ammonium chlorides, to nitriles in warm toluene efficiently afforded the desired amidines in reasonable yields after hydrolysis of aluminum species as shown in Schemes 5 and 6. + - NH2 Cl NH EtOH, HCl R'R"NH RCN ROEt RNR'R'' Scheme 4 Scheme 4 Al(Cl)Me NH MeAl(Cl) NR'R", PhMe, 80 o C N H O RCN 2 60-96% R NR'R'' RNR'R" R=Ar,alkyl Scheme 5 Moss18 demonstrated that reaction of methylchloroaluminum amide with sterically hindered nitriles 7-10 produced the corresponding amidines (Scheme 6). ISSN 1551-7012 Page 156 ©ARKAT USA, Inc. Special Issue Reviews and Accounts ARKIVOC 2008 (i) 153-194 Al(Cl)Me N NH MeAl(Cl)NH ,PhMe,80 o C H2O RCN 2 7-10 R NH2 R NH2 Me CH2 Ph R= , , CH CH Me , 2 2 Ph CH2 78 9 10 64% 70% 56% 27% Scheme 6 A general preparation of amidines from unactivated nitriles has been reported.19 Stoichiometric copper(I) chloride induced addition of various amines to nitriles provided amidines in excellent yields (Scheme 7). NH R'R"NH, CuCl, DMSO or MeOH RCN 80-100% R NR'R'' R=Ph,Me R', R" = H, alkyl Scheme 7 Scheme 7 Scheme 8 illustrates the catalytic use of Ln(III) ions in the addition of primary amines to unactivated nitriles to give intermediate mono-substituted amidines, which can react with another primary amine to yield di-substituted amidines.20 o NH NR" R'NH2,Ln(III),80 C R"NH2 RCN R=Me,Ph R NHR' R" = Ph, alkyl R NHR' R' = Ph, cyclohexyl Scheme 8 Substituted benzonitriles were converted into intermediate benzamidoximes, which are then successfully reduced to benzamidines, via palladium-catalyzed hydrogenolysis in acetic acid/acetic anhydride mixture (Scheme 9).21 Acetic anhydride was found to be a necessary acylating agent in providing useful reaction rates in the hydrogenolysis step. ISSN 1551-7012 Page 157 ©ARKAT USA, Inc. Special Issue Reviews and Accounts ARKIVOC 2008 (i) 153-194 NH OH.HCl, Na CO ,EtOH,HO, 80 o C 2 2 3 2 R R or o N OH NH2OH.HCl,KO-Bu,MeOH,PhMe,t 80 C R' CN R' 60-93% NH2 R = H, Me; R' = H, F, Me, CF3,NMe2,CO2Me R" R'' = H, OMe, Me, Et R" R NH H2, Pd/C, HOAc, Ac2O R' 45-99% .HOAc NH2 Scheme 9 R" Scheme 9 Iron pentacarbonyl was employed to the conversion of amidoximes into amidines via reductive cleavage of the N-O bond (Scheme 10).22 OH N o NH Fe(CO)5,THF,65 C R NHR' R=Ar,Ph R NHR' R' = Ar, Ph, alkyl Scheme 10 Imidates were prepared from nitriles in the presence of thiophenol and HBr (Scheme 11).23 Phenylthioimidates were readily converted to amidine salts in the presence of various amines. + - + NH Br - PhSH, Et O, HBr,0 o C 2 R'R"NH, MeOH,20 o C NH2 Br RCN 2 -PhSH 83-100% RSPh RNR'R'' R = Ar, alkyl 57-100% R'R" = H, Ar and alkyl Scheme 11 Scheme 11 Previously, it was shown that addition of primary amines to ethylthioimidates under acidic conditions provided amidines (Scheme 12).24 ISSN 1551-7012 Page 158 ©ARKAT USA, Inc. Special Issue Reviews and Accounts ARKIVOC 2008 (i) 153-194 HN SEt HNNHR o RNH2, HOAc, CHCl3 , 20 C 60-90% N(C16H33)2 R=Ar,alkyl N(C16H33)2 .2HCl .2HCl Scheme 12 Schäfer used N-acetylcysteine 11 to transform nitriles to thioimido intermediates 12, which reacted with ammonia to give amidine salts 13 (Scheme 13).25 The amidines are then isolated as their acetate salts via ion exchange. O NH O O + HS OH R S OH MeOH NH3 NH - HS O RCN+ HN HN 66-94% R NH2 HN O O 11 12 13 O R = hetero, amino acid and alkyl Scheme 13 An efficient synthesis of amidinoformic acids 18 has been published using benzyl cyanoformate (14) as a synthon. Compound 14 was converted to the thioamide 15 in the presence of hydrogen sulfide (Scheme 14).26 Thioamide 15 is S-alkylated with triethyloxonium tetrafluoroborate to afford thioamidate 16. Addition of one equivalent of amines to 16 furnished amidinoformic esters 17, which after hydrolysis provided with amidinoformic acids 18 (Scheme 14). ISSN 1551-7012 Page 159 ©ARKAT USA, Inc. Special Issue Reviews and Accounts ARKIVOC 2008 (i) 153-194 + - S NH2BF4 H S O CN 2 O Et3OBF4 O PhCH2 PhCH NH SEt 95% 2 2 PhCH2 14 1594% 16 O O O + - NH NH BF o 2 4 - R'R''NH, CH2Cl2,25 C Dowex 1 HO O NHR R'R'' = H, alkyl, Ar PhCH2 NHR 92-95% O O Scheme 14 17 18 Scheme 14 2. Cyclic Amidines Cyclic optically active amidines are strongly basic compounds which can serve as chiral reagents for NMR analysis of enantiomeric mixture of weakly acid compounds, or as ligands in catalysts for enantioselective synthesis.
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