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DOI: 10.1055/s-0030-1258523
Selective Copper-Promoted Cross-Coupling of Aromatic Amines with Alkyl Boronic Acids
Publication History
Publication Date:
27 July 2010 (online)
Abstract
A simple copper-promoted N-monoalkylation of anilines that utilizes alkyl boronic acids as the alkylating partner is presented. The reaction is carried out in refluxing dioxane, and it allows a number of structurally and electronically diverse anilines to be functionalized in a single step. A broad study was carried out to demonstrate the utility of this new methodology for the preparation of phenethylanilines.
Key words
alkylation - copper - cross-coupling - boron - amines
Aromatic amines are biologically active compounds found throughout the pharmaceutical and agrochemical industries. [¹] Hence, the development of new methods for their synthesis is still an active area of research.
Especially appealing is the conversion of widely available primary anilines to the corresponding secondary ones. Direct alkylation is typically accomplished by reaction with an alkyl halide or similar reagent in the presence of a base. It seems to be a simple transformation, but in many cases efficient N-monoalkylation of primary anilines is not possible due to overalkylation, that affords tertiary anilines or quaternary ammonium salts as byproducts. Other methods for the preparation of monoalkylated anilines are based on the use of a temporary protecting group (e.g., carbamate, benzyl) that allows the introduction of a single alkyl group, followed by removal of the protecting group (introducing two additional steps in the process). However, due to their toxic nature, the use of many of the alkylating agents must be avoided, particularly in the last step of the synthesis of APIs and active compounds. Reductive amination with carbonyl derivatives and amide reduction are also used as alternative approaches for the preparation of secondary amines; these are selective methods, but the reaction conditions are somewhat harsh, as strong reducing reagents must be used.
Nowadays, the formation of C-N bonds via cross-coupling reactions is an essential methodology for the preparation of nitrogen-containing compounds. Recently, different transition-metal-promoted reactions have been developed for the synthesis of secondary amines: catalytic systems for N-alkylation of amines with alcohols using Ru, Rh, or Ir catalysts, [²] based on the ‘borrowing hydrogen’ approach. Some of these reactions must be run at high temperature (up to 180 ˚C) or are somewhat restricted regarding amines and alcohols to which the conditions can be applied.
Other approaches rely on the hydroamination of alkenes or alkynes, but in some cases the selectivity is low or the substrate scope is not very extensive. [³]
Copper-promoted carbon-nitrogen bond-forming cross-coupling reactions of NH-containing substrates with arylboronic acids have emerged as a powerful synthetic method since the initial reports by Chan, Lam, and Evans. [4] These reactions typically employ milder reaction conditions than the analogous Ullmann reaction [5] (nucleophiles are usually arylated at r.t.), and their applications have extended to different nucleophiles such as amines, anilines, amides, imines, ureas, carbamates, and aromatic heterocycles. In recent years, the catalytic version of the reaction, [6] and the arylation of amines under base- and ligand-free conditions, [7] have been reported.
The reaction has been applied to different boronic acids, including aryl, vinyl, and heteroaryl compounds. However, alkylboronic acids rarely act as effective reagents for this reaction presumably because of their low reactivity toward transmetalation with copper salts. Until 2008, only two papers reported the use of an alkylboronic acid (cyclopropyl- and cyclohexylboronic acid) in this kind of coupling reactions with NH-containing substrates (restricted to substrates with increased NH acidity such as amides and indoles). [8]
Recently, we have developed a procedure for the selective N-monomethylation of aromatic amines by means of a cross-coupling reaction between anilines and methylboronic acid (Scheme [¹] ). [9]
Scheme 1 Monomethylation of aromatic amines by cross-coupling reaction with methylboronic acid
We wondered if the scope of this method could be extended by the application to other alkylboronic acids different from methyl. In this letter we present a simple and mild copper-promoted N-alkylation of anilines that utilizes such boron reagents.
In our previous work, we applied ‘Design of Experiments’ (DoE) in order to optimize the conditions for the cross-coupling reaction between p-toluidine and methylboronic acid. It transpired that using 2.5 equivalents of the boronic acid, 2.5 equivalents of Cu(OAc)2, and 3.5 equivalents of pyridine in refluxing dioxane, full conversion was achieved, while the dimethylated compound was obtained in only very small amounts.
We decided to apply the same conditions to the cross-coupling of cyclopropyl- and cyclohexylboronic acids, which have been reported to participate in copper cross-coupling-promoted reactions. We presumed that electronic properties of substrates could dramatically influence in the reaction outcome; thus we decided to test the reaction over three anilines ranging from electron-rich to electron-poor (4-tert-butyl-, 4-methoxy-, and 4-chloroaniline). No product was obtained for the cyclopropylboronic acid and conversion for the cyclohexylboronic acid was very low. This prompted us to force reaction conditions, using larger amounts of reagents, and longer reaction times.
For cyclohexylboronic acid, an increase from 2.5 equivalents to 4 equivalents allowed us to isolate the products in moderate to good yield. Nevertheless, cyclopropylboronic acid failed completely to afford coupling product, recovering unreacted anilines.
A broader study was carried out, using nine structurally diverse boronic acids which differed in their chain length and steric hindrance (primary and secondary, cyclic, and branched). Using this standard protocol, we were able to alkylate the three anilines with most of conversions occurring in moderate to excellent yields (Table [¹] ).
The order of addition of reagents was crucial for improving conversions: the copper salt must be added first and an induction period (10-15 min) was needed before the addition of the boronic acid. Otherwise, reactions were not complete and there was a drop in the yield.
There is a marked relationship between the electronic properties of anilines and their reactivity profile. Reaction is slower for electron-poor anilines, and longer reaction times are required to achieve good conversion. In most of the cases, some unreacted 4-chloroaniline was recovered unreacted. Conversely, electron-rich 4-methoxy aniline reacts rapidly, and some of the yields are moderate due to the formation of dialkylated product.
Primary alkylboronic acids such as 1-propyl and 1-butyl (entries 1 and 2) were coupled in good yield. In addition, the presence of an extra methyl group on the chain was well tolerated (entry 3), although boronic acids bearing substitution at the β-position reacted sluggishly, in contrast to the corresponding boronic acids with no substitution. When substituents are located at the γ-position (entries 4-6), no steric effect is observed, and reaction profile is similar to that of unsubstituted alkylboronic acids. Yields for phenethylboronic acid are quite low due to the formation of dialkylated product.
Scheme 2 Alkylation of aromatic amines by copper-mediated cross-coupling reaction with alkylboronic acids
Secondary boronic acids also participate in the reaction, but the outcome depends on the nature of the reagent (entries 7-9). Only cyclohexylboronic acid furnishes the product in good yield. 2-Propylboronic acid provides unclean reactions, with lower yields, and cyclopentylboronic acid only reacts in a mere 6% with the more reactive 4-methoxyaniline.
The optimal amount of reagents seems to be quite dependent on the nature of the substrate. Thus, a four-fold excess of reagents is suitable for electronically unbiased anilines, but not for anilines bearing electron-donating or -withdrawing substituents. For 4-methoxyaniline, short reaction times and minor reagent excesses must be used to avoid the formation of dialkylated product. On the other hand, larger amounts of reagents and longer reaction times must be applied to electron-poor anilines to ensure good conversion. It could be concluded that the general reaction conditions should be adapted for each substrate, and this could possibly be best achieved by using a DoE protocol. [5a]
After achieving good results for the alkylation of anilines with different boronic acids, we further applied this reaction to the N-alkylation of different aromatic amines with one particular boronic acid. We were especially interested in the synthesis of phenethylamine derivatives because of the range of active compounds that present this structure. [¹0]
Despite the importance of N-phenethyl derivatives, there are few methods to incorporate phenethyl groups onto nitrogen atoms. [¹¹] Phenethylation of simple aniline derivatives by reductive amination with phenethylaldehyde generally proceeds in low yield, and the scope is limited. Alkylation with phenethyl halides is usually difficult due to competitive elimination of the halide to give styrenes.
To confirm the generality of our conditions, we evaluated the cross-coupling of phenethylboronic acid with anilines having different electronic and steric demands (Table [²] ). Electron-rich, -deficient, and neutral anilines were all suitable substrates and provided the corresponding phenethylanilines in good to excellent yields (entries 1-9). After some initial experiments, it was found that, for a predetermined amount of Cu(OAc)2 and pyridine (threefold excess for each of them), the optimal amount of boronic acid seemed to be 1.3 equivalents for electron-rich anilines, 1.3-1.5 equivalents for neutral anilines, and 2.5 equivalents for electron-poor anilines.
It was noticed again that, in general, anilines with an electron-donating group afforded better yields than did those with electron-withdrawing or neutral groups (entries 1-9).
Sterically demanding ortho-substituted anilines required slightly longer reaction times, but they were also efficiently alkylated.
The functional-group tolerance is good. Chemoselective coupling of 3-bromoaniline (entry 10) is particularly interesting for several reasons. First, it reveals that the N-coupling process is favorable over possible C Suzuki-type reactions and second, it allows further functionalizations of the product, since it still bears a bromine substituent which is helpful for additional modifications.
The reaction could be also applied to substrates with a thioether moiety that had proven problematic in some cross-coupling reactions (entry 11). Substrates bearing ester and nitrile groups were also transformed to the desired products in good yield (entries 12 and 13).
The 2- and 3-aminopyridines could be alkylated, albeit in lower yield due to the formation of several byproducts (entries 14-16). It should be noted that when 4-aminopyridine was submitted to the reaction conditions no coupling product was isolated. This is in agreement with the results previously obtained with methyl boronic acid. [9] Unexpectedly, aminoquinolines (entries 17 and 18) were alkylated in moderate to good yields.
In conclusion, we have developed a novel system for the alkylation of anilines in good to excellent yields by means of a cross-coupling with alkylboronic acids in the presence of Cu(OAc)2. This represents the first general metal-mediated coupling of a nitrogen nucleophile with alkylboronic acids reported to date. This work expands the utility of the Chan-Lam-Evans reaction to the use of alkylboronic acids, a class of reagent that has been problematic in previous cross-coupling protocols with other nucleophiles. It extends the utility of the copper-promoted cross-coupling reactions in the field of N-functionalization, and it is likely to find considerable application in the synthesis of complex organic molecules. Further studies will focus on the implementation of a protocol catalytic in copper reagent and on the application to other amines different from anilines.
Copper-Promoted Cross-Coupling of Anilines with Alkyl Boronic Acids - General Procedure
Cu(OAc)2 (1.3-4 mmol) was added to a solution of aniline (1 mmol) and pyridine (3-4 mmol) in dioxane (5 mL), and the mixture was refluxed for 15 min. Boronic acid (1.3-4 mmol) was added, and the reaction mixture was allowed to react at reflux until no unreacted aniline was detected or for a maximum time of 10 h. The reaction was allowed to reach r.t., poured into H2O (25 mL) and extracted with EtOAc (40 mL). The organic layer was dried over Na2SO4 (anhydrous), filtered, and concentrated. The crude residue was purified by flash chromatography on SiO2 using EtOAc-hexanes as eluent.
Selected Spectroscopic Data N -(4- tert -Butylphenyl)- N -propylamine (Table 1, Entry 1, R = t -Bu)
Yield 76%. ¹H NMR (250 MHz, CDCl3): δ = 7.20 (d, J = 8.8 Hz, 2 H), 6.58 (d, J = 8.8 Hz, 2 H), 3.07 (t, J = 7.1 Hz, 2 H), 1.64 (q, J = 7.1 Hz, 2 H), 1.28 (s, 9 H), 0.99 (t, J = 7.4 Hz, 3 H) ppm. ¹³C NMR (62.9 MHz, CDCl3): δ = 146.1, 139.7, 125.9, 112.7, 46.0, 33.8, 31.5, 22.8, 11.6 ppm. MS (ESI+): m/z = 192.2 [M + 1].
N -(4-Methoxyphenyl)- N -propylamine (Table 1, Entry 1, R = OMe)
Yield 95%. ¹H NMR (250 MHz, CDCl3): δ = 6.79 (d, J = 8.8 Hz, 2 H), 6.59 (d, J = 8.8 Hz, 2 H), 3.75 (s, 3 H), 3.04 (t, J = 7.1 Hz, 2 H), 1.61 (q, J = 7.1 Hz, 2 H), 0.99 (t, J = 7.4 Hz, 3 H) ppm. ¹³C NMR (62.9 MHz, CDCl3): δ = 151.8, 142.7, 114.8, 113.9, 55.7, 46.7, 22.7, 11.6 ppm. MS (ESI+): m/z = 166.1 [M + 1].
N -(4-Chlorophenyl)- N -propylamine (Table 1, Entry 1, R = Cl)
Yield 42%. ¹H NMR (250 MHz, CDCl3): δ = 7.10 (d, J = 9.0 Hz, 2 H), 6.52 (d, J = 9.0 Hz, 2 H), 3.04 (t, J = 7.1 Hz, 2 H), 1.60 (m, 2 H), 0.99 (t, J = 7.4 Hz, 3 H) ppm. ¹³C NMR (62.9 MHz, CDCl3): δ = 147.0, 128.9, 121.5, 113.6, 45.8, 22.5, 11.6 ppm. MS (ESI+): m/z = 170.1 [M + 1].
N -(4-Methylphenyl)- N -(2-phenylethyl)amine (Table 2, Entry 2)
Yield 71%. ¹H NMR (250 MHz, CDCl3): δ = 7.21-7.35 (m, 6 H), 6.53 (m, 1 H), 6.45 (m, 2 H) 3.39 (t, J = 6.6 Hz, 2 H), 2.91 (t, J = 6.6 Hz, 2 H), 2.27 (s, 3 H) ppm. ¹³C NMR (62.9 MHz, CDCl3): δ = 148.0, 138.9, 129.1, 128.7, 126.3, 118.3, 113.7, 110.0, 45.0, 35.5, 21.6 ppm. MS (ESI+): m/z = 212.2 [M + 1].
N -(2-Phenylethyl)pyridin-3-amine (Table 2, Entry 15)
Yield 36%. ¹H NMR (250 MHz, CDCl3): δ = 8.00 (m, 1 H), 7.01-7.37 (m, 8 H), 3.42 (q, J = 6.9 Hz, 2 H), 2.93 (t, J = 6.9 Hz, 2 H) ppm. ¹³C NMR (62.9 MHz, CDCl3): δ = 138.7, 138.1, 135.3, 128.7, 128.6, 128.0, 127.8, 126.5, 118.9, 44.5, 35.2 ppm. MS (ESI+): m/z = 199.2 [M + 1].
N -(2-Phenylethyl)quinolin-8-amine (Table 2, Entry 17)
Yield 45%. ¹H NMR (250 MHz, CDCl3): δ = 8.69 (m, 1 H), 8.05 (m, 1 H), 7.24-7.43 (m, 7 H), 7.04 (m, 1 H), 6.72 (m, 1 H), 3.59 (m, 2 H), 3.08 (t, J = 7.4 Hz, 2 H) ppm. ¹³C NMR (62.9 MHz, CDCl3): δ = 146.7, 144.5, 139.5, 138.1, 135.9, 128.7, 128.6, 128.5, 127.7, 126.3, 121.3, 113.8, 104.5, 44.9, 35.5 ppm. MS (ESI+): m/z = 249.2 [M + 1].
N -(2-Phenylethyl)quinolin-3-amine (Table 2, Entry 18)
Yield 80%. ¹H NMR (250 MHz, CDCl3): δ = 8.46 (br s, 1 H), 7.97 (m, 1 H), 7.61 (m, 1 H), 7.23-7.45 (m, 8 H), 7.08 (m, 1 H), 3.51 (t, J = 6.9 Hz, 2 H), 3.00 (t, J = 6.9 Hz, 2 H) ppm. ¹³C NMR (62.9 MHz, CDCl3): δ = 145.6, 138.7, 134.2, 128.8, 128.7, 128.6, 128.2, 127.9, 127.8, 127.1, 126.6, 126.0, 125.0, 44.6, 34.8 ppm. MS (ESI+): m/z = 249.1 [M + 1].
Supporting Information for this article is available online at http://www.thieme-connect.com.accesdistant.sorbonne-universite.fr/ejournals/toc/synlett.
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Acknowledgment
We thank the CDTI (Spanish Ministry of Research and Science) for financing this work through the CENIT Genius Pharma project.
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Müller TE.Beller M. Chem. Rev. 1998, 98: 675 - 3b
Beller M.Trauthwein H.Eichberger M.Breindl C.Herwig J.Müller TE.Thiel OR. Chem. Eur. J. 1999, 5: 1306 - 3c
Nobis M.Driessen-Hölscher B. Angew. Chem. Int. Ed. 2001, 40: 3983 - 3d
Utsunomiya M.Kuwano R.Kawatsura M.Hartwig JF. J. Am. Chem. Soc. 2003, 125: 5608 - 3e
Ryu J.-S.Li GY.Marks TJ. J. Am. Chem. Soc. 2003, 125: 12584 - 3f
Johns AM.Utsunomiya M.Incarvito CD.Hartwig JF. J. Am. Chem. Soc. 2006, 128: 1828 - 4a
Chan DMT.Monaco KL.Wang RP.Winters MP. Tetrahedron Lett. 1998, 39: 2933 - 4b
Evans DA.Katz JL.West TR. Tetrahedron Lett. 1998, 39: 2937 - 4c
Lam PYS.Clark CG.Saubern S.Adams J.Winters MP.Chan DMT.Combs A. Tetrahedron Lett. 1998, 39: 2941 - 4d
Lam PYS.Clark CG.Saubern S.Adams J.Averill KM.Chan DMT.Combs A. Synlett 2000, 674 - For reviews on copper-mediated carbon-heteroatom bond formations, see:
- 5a
Ley SV.Thomas AW. Angew. Chem. Int. Ed. 2003, 42: 5400 - 5b
Beletskaya IP.Cheprakov AV. Coord. Chem. Rev. 2004, 248: 2337 - 6a
Collman JP.Zhong M. Org. Lett. 2000, 2: 1233 - 6b
Antilla JC.Buchwald SL. Org. Lett. 2001, 3: 2077 - 6c
Lam PYS.Vincent G.Clark CG.Deudon S.Jadhav PK. Tetrahedron Lett. 2001, 42: 3415 - 7a
Moessner C.Bolm C. Org. Lett. 2005, 7: 2667 - 7b
Lan J.-B.Zhang G.-L.You J.-S.Chen L.Yan M.Xie R.-G. Synlett 2004, 1095 - 7c
Sreedhar B.Venkanna GT.Kumar KBS.Balasubrahmanyam V. Synthesis 2008, 795 - 8a
Tsuritani T.Strotman NA.Yamamoto Y.Kawasaki M.Yasuda N.Mase T. Org. Lett. 2008, 10: 1653 - 8b
Bérnard S.Neuville L.Zhu J. J. Org. Chem. 2008, 73: 6441 - 8c For the use of cyclohexylboronic
acid, see: Boronic Acids
1st ed.:
Hall DG. Wiley-VCH; Weinheim: 2005. Chap. 5. - 9
González I.Mosquera J.Guerrero C.Rodríguez R.Curces J. Org. Lett. 2009, 11: 1677 - 10
Patrick GL. In An Introduction to Medicinal Chemistry 2nd ed.: Oxford University Press; Oxford: 2001. p.69 - 11a
Heutling A.Pohlki F.Bytschkov I.Doye S. Angew. Chem. Int. Ed. 2005, 44: 2951 - 11b
Hamid MHSA.Williams JMJ. Chem. Commun. 2007, 725 - 11c
Munro-Leighton C.Delp SA.Alsop NM.Blue ED.Gunnoe TB. Chem. Commun. 2008, 111 - 11d
Iranpoor N.Firouzabadi H.Nowrouzi N.Khalili D. Tetrahedron 2009, 65: 3893
References
- 1a
Negwer M. Organic-Chemical Drugs and their Synonyms (An International Survey) 7th ed.: Akademie Verlag GmbH; Berlin: 1994. - 1b
Montgomery JH. Agrochemicals Desk Reference: Environmental Data Lewis Publishers; Chelsea MI: 1993. - 1c
The Chemistry of Anilines
Vol.
1:
Rappoport Z. Wiley-Interscience; New York: 2007. - 2a
Watanabe Y.Tsuji Y.Ohsugi Y. Tetrahedron Lett. 1981, 22: 2667 - 2b
Watanabe Y.Tsuji Y.Ige H.Ohsugi Y.Ohta T. J. Org. Chem. 1984, 49: 3359 - 2c
Watanabe Y.Morisaki Y.Kondo T.Mitsudo T.-A. J. Org. Chem. 1996, 61: 4214 - 2d
Jun C.Hwang D.Na S. Chem. Commun. 1998, 1405 - 2e
Abbenhuis RATM.Boersma J.van Koten G. J. Org. Chem. 1998, 63: 4282 ; and references cited therein - 2f
Naota T.Takaya H.Murahashi S.-I. Chem. Rev. 1998, 98: 2599 - 2g
Grigg R.Mitchell TRB.Sutthivaiyakit S.Tongpenyai N. J. Chem. Soc., Chem. Commun. 1981, 611 - 2h
Cami-Kobeci G.Williams JMJ. Chem. Commun. 2004, 1072 - 2i
Fujita K.-I.Yamaguchi R. Synlett 2005, 560 - 2j
Yamaguchi R.Kawagoe S.Asai C.Fujita K.-I. Org. Lett. 2008, 10: 181 - 2k
Hollmann D.Bähn S.Tillack A.Beller M. Angew. Chem. Int. Ed. 2007, 46: 8291 - 2l
Tillack A.Hollmann D.Michalik D.Beller M. Tetrahedron Lett. 2006, 47: 8881 - 2m
Fujita K.Enoki Y.Yamaguchi R. Tetrahedron 2008, 64: 1943 - 2n
Hamid MH.Allen L.Lamb GW.Maxwell AC.Maytum HC.Watson AJA.Williams JMJ. J. Am. Chem. Soc. 2009, 131: 1766 - 3a
Müller TE.Beller M. Chem. Rev. 1998, 98: 675 - 3b
Beller M.Trauthwein H.Eichberger M.Breindl C.Herwig J.Müller TE.Thiel OR. Chem. Eur. J. 1999, 5: 1306 - 3c
Nobis M.Driessen-Hölscher B. Angew. Chem. Int. Ed. 2001, 40: 3983 - 3d
Utsunomiya M.Kuwano R.Kawatsura M.Hartwig JF. J. Am. Chem. Soc. 2003, 125: 5608 - 3e
Ryu J.-S.Li GY.Marks TJ. J. Am. Chem. Soc. 2003, 125: 12584 - 3f
Johns AM.Utsunomiya M.Incarvito CD.Hartwig JF. J. Am. Chem. Soc. 2006, 128: 1828 - 4a
Chan DMT.Monaco KL.Wang RP.Winters MP. Tetrahedron Lett. 1998, 39: 2933 - 4b
Evans DA.Katz JL.West TR. Tetrahedron Lett. 1998, 39: 2937 - 4c
Lam PYS.Clark CG.Saubern S.Adams J.Winters MP.Chan DMT.Combs A. Tetrahedron Lett. 1998, 39: 2941 - 4d
Lam PYS.Clark CG.Saubern S.Adams J.Averill KM.Chan DMT.Combs A. Synlett 2000, 674 - For reviews on copper-mediated carbon-heteroatom bond formations, see:
- 5a
Ley SV.Thomas AW. Angew. Chem. Int. Ed. 2003, 42: 5400 - 5b
Beletskaya IP.Cheprakov AV. Coord. Chem. Rev. 2004, 248: 2337 - 6a
Collman JP.Zhong M. Org. Lett. 2000, 2: 1233 - 6b
Antilla JC.Buchwald SL. Org. Lett. 2001, 3: 2077 - 6c
Lam PYS.Vincent G.Clark CG.Deudon S.Jadhav PK. Tetrahedron Lett. 2001, 42: 3415 - 7a
Moessner C.Bolm C. Org. Lett. 2005, 7: 2667 - 7b
Lan J.-B.Zhang G.-L.You J.-S.Chen L.Yan M.Xie R.-G. Synlett 2004, 1095 - 7c
Sreedhar B.Venkanna GT.Kumar KBS.Balasubrahmanyam V. Synthesis 2008, 795 - 8a
Tsuritani T.Strotman NA.Yamamoto Y.Kawasaki M.Yasuda N.Mase T. Org. Lett. 2008, 10: 1653 - 8b
Bérnard S.Neuville L.Zhu J. J. Org. Chem. 2008, 73: 6441 - 8c For the use of cyclohexylboronic
acid, see: Boronic Acids
1st ed.:
Hall DG. Wiley-VCH; Weinheim: 2005. Chap. 5. - 9
González I.Mosquera J.Guerrero C.Rodríguez R.Curces J. Org. Lett. 2009, 11: 1677 - 10
Patrick GL. In An Introduction to Medicinal Chemistry 2nd ed.: Oxford University Press; Oxford: 2001. p.69 - 11a
Heutling A.Pohlki F.Bytschkov I.Doye S. Angew. Chem. Int. Ed. 2005, 44: 2951 - 11b
Hamid MHSA.Williams JMJ. Chem. Commun. 2007, 725 - 11c
Munro-Leighton C.Delp SA.Alsop NM.Blue ED.Gunnoe TB. Chem. Commun. 2008, 111 - 11d
Iranpoor N.Firouzabadi H.Nowrouzi N.Khalili D. Tetrahedron 2009, 65: 3893
References
Scheme 1 Monomethylation of aromatic amines by cross-coupling reaction with methylboronic acid
Scheme 2 Alkylation of aromatic amines by copper-mediated cross-coupling reaction with alkylboronic acids
