Encyclopedia of Reagents for Organic Synthesis || ( S ...

(S)-α-METHYLBENZYLAMINE 1

(S)-α-Methylbenzylamine

NH2

[2627-86-3] C8H11N (MW 121.20)InChI = 1/C8H11N/c1-7(9)8-5-3-2-4-6-8/h2-7H,9H2,1H3/t7-/m0/

s1InChIKey = RQEUFEKYXDPUSK-ZETCQYMHBE

(resolving agent for carboxylic acids;7–11 determination of enan-tio purity of carboxylic acids;16,17 stereospecific reactions ofcarbonyl compounds;18 reductive amination of carbonyl

compounds29,30)

Alternate Name: (S)-phenylethylamine; (S)-PEA.Physical Data: bp 187 ◦C; d 0.940 g cm−3; [α]D −39◦ (neat).Solubility: readily sol in organic solvents.Form Supplied in: both enantiomers are commercially available.Analysis of Reagent Purity: the enantiomeric purity of the

reagent can be assessed by NMR analysis of the correspondingMosher’s amide.4 Chiral complexing reagents (such as 1,1′-binaphthyl-2,2′-diylphosphoric acid) have also been used in thedirect NMR analysis of the reagent.5,6

Preparative Methods: racemic α-methylbenzylamine has beenresolved utilizing chiral acids such as tartaric acid1 and (S)-(−)-carbamalactic acid,2 among others. Chiral resolution can alsobe effected by enzymatic resolution.36 Several stereospecificsyntheses have been reported.3

Handling, Storage, and Precautions: stable at rt for extendedperiods of time when stored under nitrogen.

Original Commentary

Juan C. JaenParke-Davis Pharmaceutical Research, Ann Arbor, MI, USA

Resolving Reagent for Carboxylic Acids and Other Typesof Compounds. A large number of carboxylic acids have beenresolved via their diastereomeric salts with (S)- or (R)-α-methyl-benzylamine (1). The ready availability of both enantiomers of (1)guarantees access to both enantiomers of the desired acid. Com-pounds (2)–(6) are representative examples of acids obtained inhigh enantiomeric purity.7–11 Alternatively, racemic carboxylicacids have been resolved by covalent derivatization with (1) andseparation of the resulting diastereomeric amides by physicalmeans such as chromatography (eq 1)12 or fractional crystalliza-tion (eq 2).13

Racemic compounds other than carboxylic acids have alsobeen resolved by reaction with enantiomerically pure (1) andseparation of the corresponding diastereomeric mixtures by phys-ical methods. For example, reaction of a racemic β-substitutedγ-butyrolactone with (1) yields a mixture of hydroxy amides,which can be separated by fractional recrystallization andchromatography (eq 3).14 Amide hydrolysis regenerates the

chiral hydroxy acids, which spontaneously cyclize to producethe chiral lactones.

N

N

Me

O

MeHO2C

O

CO2H

MeO

HO2C

PhH

(2) (3) (4)

O

O

CO2HN

O

CO2Me

H

H

CO2H

(5) (6)

N

Pr

CO2H

t-Boc

N

Pr

t-Boc HN

O

Ph

N

Pr

t-BocHN

O

Ph

(1)+

(+)-(1), EDCI

racemic

HOBT, DMF

OH

OH

O

OP

O

N

Ph

OH

OH

LiAlH4

(2)

1. POCl3 Et3N2. (1), Et3N

racemic

3. fract. crystall.

H

O

F3C

O HOO N

H

CF3

Ph

O

F3C

O (3)

1. (1), toluene

racemic

conc. HCl

2. chromatography

The displacement of a variety of leaving groups by (1) producesdiastereomeric mixtures of amines, which can be separated intodiastereomerically pure secondary amines and, following reduc-tive removal of the α-methylbenzyl group, serve as a source ofchiral primary amines (eq 4).15

Avoid Skin Contact with All Reagents

2 (S)-α-METHYLBENZYLAMINE

OTs HPh

AcHN (4)

acetonitrile

racemic

2. recrystall.N

1. (1), K2CO3

Reagent for the Determination of Enantiomeric Purity ofCarboxylic Acids. Amine (1) is frequently used as a derivatiz-ing reagent for determining the enantiomeric purity of carboxylicacids by HPLC, with limits of detection often as low as 1%. Mostcommonly used coupling methods include use of dehydratingagents such as 1,3-Dicyclohexylcarbodiimide (eq 5)16 and themixed anhydride method (eq 6).17

N3

CO2H

HOBT NH

O

Ph

N3

NH

O

Ph

N3

(5)(1), DCC

+

Et CO2H

Br

EtNH

O

Ph

Br

EtNH

O

Ph

Br

(6)

1. i-BuOCOCl NMM, THF

+

2. (1), THF

Stereospecific Reactions of Carbonyl Compounds. One ofthe most frequent uses of both enantiomers of reagent (1) is inpromoting the stereospecific reaction of carbonyl compounds viathe corresponding chiral imines. The transfer of chirality from(1) to the newly formed bonds is generally most effective in cy-clization reactions. Some examples are the Lewis acid-catalyzedcyclization of ω-unsaturated aldehyde imines to produce aminesof high enantiomeric purity (eq 7),18 the enantioselective synthe-sis of γ ,δ-unsaturated aldehydes via the aza-Claisen rearrange-ment of derivatives of (1) (eq 8),19 and the asymmetric Lewisacid-catalyzed aza-Diels–Alder reaction of aldehyde imines withelectron-rich dienes (eq 9).20

CHON

Ph

SnCl4

NH2

(7)H2, Pd/C(1)

91%

90% ee

CHO NH

Ph

PhCHMeCHON

Ph

Ph

OHC Ph

(8)

1. (1)1. TiCl4 toluene

90% ee

2. NaBH4 2. HCl

Ph N Ph

OTMS

OMe

N

O

Ph

Ph N

O

Ph

Ph

(9)

B(OPh)3

CH2Cl2–78 °C

+

98:2

+55%

Enantiomerically pure disubstituted β-lactams are also avail-able by cyclization of acyclic intermediates containing (1) as achiral appendage, which is later removed by catalytic hydrogena-tion (eq 10).21

NO

Bn2N CO2-t-Bu

Ph

NO

Bn2N CO2-t-Bu

Ph

NHO

BocHN

(10)

(1)1. BuLi

100% ee100% de

2. NIS

Examples of highly stereoselective acyclic reactions includethe Zr-mediated coupling of aldehydes with imines of (1) to pro-duce chiral amino alcohol derivatives (eq 11),22 and the additionof cyanide to aldimines of (1) to yield intermediates that can beelaborated into enantiomerically pure α-amino acids (eq 12).23

PhCHOPh N

Ph

PhPh

OH

HN

Ph

(11)1. Cp2Zr(Bu)2

94% de

(1)

2. PhCHO, THF

R N Ph R NH

HO2C

Ph

H2

Pd(OH)2

R NH2

CO2H(12)

76–91% ee

1. CN polymer

2. 6N HCl

Another frequent use of (1) and its enantiomer is the stere-ospecific conjugate addition of carbonyl compounds to α,β-unsaturated systems. Most published examples contain chiralimine derivatives of cyclic ketones, which add to α,β-unsaturatedesters and ketones in a highly stereoselective manner (eqs 13 and14).24,25 When the ketone is not symmetrically substituted, reac-tion usually occurs at the most substituted α-position, includingthose cases where the ketone is α-substituted by oxygen (eq 15).26

High stereoselectivity can also be achieved when the Michael ac-ceptor is other than an unsaturated ketone or ester, such as a vinylsulfone (eq 16).27 Intramolecular variations of this transformationhave also been described (eq 17).28

A list of General Abbreviations appears on the front Endpapers


N

i-Pr

O

Et

O

N O

Et

NPh

i-Pr

i-Pr

N

CO2Me

O

O

(13)

(1) 1. CH2CHCO2Me

88% ee

2. AcOH

NPh NHPh

O

O

O

(14)

91% ee

1.

2. AcOH 88%

O

O

O

NPh

O

O

CO2Me(15)

(R)-(1)1. CH2CHCO2Me2. AcOH, H2O

95% ee

78%

NPh

CO2Me

SO2Ph O

SO2Ph

CO2Me(16)

80 °Cthen AcOH

91% ee

74%

O

CO2MeN

CO2Me

Ph

O

CO2Me

AcOH

(17)

1. (1)2. DMF, 120 °C

ca 100% ee

85%

Stereospecific Reductive Amination of Carbonyl Com-pounds. Catalytic or chemical reduction of chiral imines derivedfrom (1) often proceeds with high diastereoselectivity. Reductiveremoval of the α-methylbenzyl group yields chiral primary amines(eqs 18 and 19).29,30

MeO

OMe

O

i-Bu

MeO

OMe

HN

i-Bu

Ph

OMe

OMe

i-Bu

NH2

(18)

1. (1), NEt3 TiCl4, CH2Cl2

94% de

2. H2, Pd/C EtOH

Pd/C, MeOH100%

NH4HCO3

N

O

OO

N

N

OO

Ph

N

NH2

OO

(19)

(1), TiCl4

cyclohexenePd/C, EtOH

88% ee

90% 90%

Removable Chiral Appendage. Even in reactions that pro-ceed with moderate stereoselectivity, incorporation of a chiralmoiety such as (1) frequently provides an opportunity to easilyseparate diastereomeric products. For example, the introductionof (1) into an imidazolone structure allows the easy separation ofdiastereomers by chromatography. Reductive removal of the chi-ral appendage and imidazolone hydrolysis provides a synthesis ofoptically pure α-amino acids (eq 20).31 In another example, eventhough the conjugate addition of (1) to methyl crotonate proceedswith low stereoselectivity, the diastereomeric conjugates are easilyseparated by chromatography and elaborated to provide opticallyactive β-amino esters (eq 21).32 Similarly, cycloaddition of thealdimine of (1) with a substituted ketene produces a mixture ofβ-lactams, which can be separated by chromatography as a sourceof optically active β-lactams (eq 22).33

N

O

NHCO2Bn

Ph

N

N

OPh

Cbz

HO2C

H2N(20)

(1)

1. Hg(CF3CO2)2

2. NaBH4

3. chromatography



CO2MeCO2Me

HN

Ph

H

CO2Me

NH2

(21)

(1), MeOH

2:3 mixture

1. separate diastereomers

2. H2, Pd/Creflux

NPh

N

BnO

O Ph (22)

BnOCH2COClEt3N, CH2Cl2

48% of a mixtureof diastereomers

chromatography

0 °C

Miscellaneous Uses. Substituted derivatives of (1), e.g. (7),react with α,β-unsaturated carbonyl systems in a highly stereose-lective manner to produce chiral β-aminocarbonyl compounds.34

The lithium amides of a different type of substituted derivatives,e.g. (8), have been used to deprotonate symmetrical ketones, usu-ally cyclic, in a highly stereoselective manner.35

NH

Ph

Ph

NH

Ph Ph

(7) (8)

First Update

Amy C. HartThe Ohio State University, Columbus, OH, USA

Use as Chiral Resolving Agent. Incorporation of (1) intoseveral templates has afforded some novel chiral shift reagents forNMR spectroscopy. Although (1) can also be used as a chiral shiftreagent for carboxylic acids, the analog (9) was more efficient.In fact, the epimer was equally efficacious. Both epimers werereadily available via a three-step sequence (Grignard coupling,triflate formation, and triflate displacement by (1)).37

Me2N HN

Ph

(9)

Macrocyclic system (10) was also developed as a chiral shiftreagent for carboxylic acids. Up to 1 ppm shifts were observed byNMR spectroscopy.38

O O

NH HNPh Ph

(10)

A thiourea-based system was also developed.39 Treatment ofthiophosgene with (1) in the presence of triethylamine providedthe C2 symmetric product (eq 23). The use of this reagent wastested out successfully on phenylglycine.

Ph NH2 Cl

S

Cl Ph NH

NH

S

Ph

Et3N(23)

Use in Catalysis. In the last decade, incorporation of (1) asa chiral ligand for a metal or as a part of an organocatalyst hasflourished, albeit with mixed results. Thiourea (11) was synthe-sized as a potential organocatalyst for the conjugate addition ofhydroxylamines.40 Unfortunately (11) failed to provide productsin greater than 20% yield.

NH

S

NH

Ph

Ar

(11)

More success has been observed when (1) has been incorpo-rated into a ligand for transition metal chemistry resulting in C–Cbond formation. In the iridium-mediated displacement of an al-lylic acetate with dimethyl malonate, a binaphthyl ligand wasdeveloped.41 Treatment of binaphthol with phosphorous trichlo-ride was followed by displacement of the residual chloride with(1) in the presence of triethylamine to yield the phosphoramiditeligand (eq 24).

OH

OH

O

OP NH

Ph

1. PCl3, Et3N

35–69%

(24)2. (1), Et3N

Although the use of the ligand assisted in providing the productin good levels of regioselectivity, the ee was modest (27–64%). Arelated dimethylphosphoramidate in which the nitrogen has beenalkylated with an additional equivalent of (1) gave much improvedee’s, albeit with a substantial lowering in regioselectivity.

(S)-α-Methylbenzylamine has also been incorporated into N-heterocyclic carbenes for catalytic asymmetric π-allyl palladiumchemistry.42 Although (12) could effect the desired transformationin 50% yield and 80% ee, the ethylbenzylamine derivative was



found to be superior. Other N-heterocyclic carbenes containing(1) have also been employed in the copper-catalyzed conjugateadditions of diethylzinc.43

N NR

HN Ph

Ph

Ag

Cl

(12)

A large study of ligands based on cyclohexene oxide incorporat-ing (1) has also been developed for the enantioselective addition ofdiethylzinc to benzaldehyde.44 All the ligands could be accessedin short sequences and high yields. The most promising ligand,(13), provided the desired product in 79% yield and 76% ee.

N

O

Ph

(13)

Other systems have also been developed for the enantios-elective addition of organozincs to aromatic aldehydes. How-ever, most of them provide very modest enantioselectivities.45

Even when attached to privileged ligands such as TADDOL andBINOL, (1) did not provide usable levels of selectivity.46 Dicyclo-hexylcarbodiimide coupling of (1) with (S)-mandelic acid led to aligand that when mixed with titanium isopropoxide and dimethylz-inc provided the desired adduct in 57% ee.47 While encouraging,even better results were observed with the simpler benzylaminederived system.

Use of (1) in asymmetric reduction has seen similar hurdles. Inthe asymmetric reduction of ketones with sodium aluminum hy-dride, the simple diamines (14) and (15) provided no asymmetricinduction.48

NH HNPhPh

Ph NH

NHOH

Ph

5% ee 0% ee(14)

(15)

Cyclohexene oxide derived diamines fared better in the asym-metric hydrosilylation of ketones.49 The diamine ligand wasavailable in three steps from cyclohexene oxide (eq 25).

In the case of hydrosilylation, the ketone can be treated withthe diamine (eq 25), diethylzinc, and polymethylhydroxysilane toyield the chiral alcohol in yields ranging from 72–95% and withselectivities from 11–89% ee (eq 26). Aryl and vinyl ketones pro-vided the best results, although substituted aryls were consistentlyworse in the reaction.

O NPh

NH

NH

Ph

Ph

1. (1), LiClO4

MeCN, ↑↓(1), LiClO4

MeCN, ↑↓

(25)

2. MsCl, Et3N CH2Cl2

Ph

O

R Ph

OH

R

diamine(26)

Et2Zn, PMHS

Excellent results were obtained in the rhodium-catalyzedhydrogenations of unsaturated enamides and esters.50 Thenecessary ligands were obtained by ortho-lithiation of (1) andtrapping as the phosphine (eq 27). Subsequent methylation of thefree amine led to the initial ligand. Chloride displacement froma (S)-BINOL-derived chlorophosphite led to the best catalyst.Treatment of the unsaturated systems with Rh(COD)2BF4,1% (S)-α-methylbenzylamine derived catalyst, and hydrogen(10 bar) generally resulted in ee’s exceeding 99%.

NH2

NH2

PPh2

HCO2Et

NHMe

PPh2

1. BuLi TMSCl Et2O, –35 °C

(27)

2. BuLi ClPPh2 –35 °C

LAH, THF50 °C

Although the above ligand system elegantly exemplified the po-tential of (1), numerous systems led to poor enantioselectivitiesand/or poor yields. A substituted 1,3-aminophosphine incorporat-ing (1) could effectively induce reduction of acetophenone deriva-tives, albeit with very small substrate scope.51 Phosphoramidatescontaining (1) were found to be fairly effective in the reductionof α,β-unsaturated systems with Rh(COD)2BF4.52 The simplifiedphosphoramidate (16), generated for asymmetric hydrogenationof enamides, failed to provide any yield with the free amine.53

When the amine had been alkylated with another equation of (1),the reaction proceeded in 90% yield, albeit with negligible enan-tioselectivity (3% ee).

OP

ONH

Ph

(16)

Diphenylphosphorylated derivatives of (1) were employed inRu-based asymmetric transfer hydrogenations, although modestee’s were once again observed.54 Lower enantioselectivities werealso observed with aminothiol derivatives of (1) in the asymmetricreduction of acetophenone by Ru(II) complexes.55



Miscellaneous. Recent work has employed derivatives of (1)as catalysts for kinetic resolution of N-acyloxazolidinethiones.56

Synthesis of the acylation catalyst began with methylation of theamine followed by formylation of (1) via directed lithiation andsubsequent trapping of the anion with dimethylformamide (eq 28).Treatment of the aldehyde with trifluoromethyl(trimethyl)silaneand tetrabutylammonium fluoride led to the epimeric catalystsin high yields. The kinetic resolutions typically proceeded undermild conditions in 50–60% yields, with the recovered startingmaterial having ee’s up to 99%. Selectivity factors ranged from17 to 32.

Ph NH2NMe2

CHO

NMe2

OH

CF3

NMe2

OH

CF3

1. HCHO HCO2H, 66%

TMS-CF3TBAF92%

(28)2. t-BuLi DMF, 84%

(S)-(α)-Methylbenzylamine was also an effective chiral aux-iliary for a radical cyclization to generate a lactam (eq 29).57

Best diastereomeric ratios (up to 10:1) were obtained with eithertitanium tetrachloride or boron trifluoride diethyletherate at lowtemperatures. More substituted alkenes could also be used.

N

OBrPh

N

O

Ph (29)lewis acid

THF, –78 °C68–74%

Et3B, O2Bu3SnH

Coupling of (1) with dibromoethane followed by cyclizationto the phosphoramidate generated a chiral ligand suitable forcyanohydrin formation (eq 30).58 Treatment of benzaldehydederivatives with trimethylsilyl cyanide, titanium isopropoxide,and the chiral ligand led to the desired cyanohydrins in excellentyields (>90%). Enantioselectivity seemed to vary widely basedon the aromatic group of the aldehyde.

Ph NH2N

NP

OAr

O

Ph

Ph

1. BrCH2CH2Br Et3N, 110 °C

(30)2. ArOP(O)Cl2 Et3N, CH2Cl23. BuLi, THF –78 °C

Solid supported derivatives of (1) were examined in the asym-metric deprotonation of cyclohexene oxide with n-butyllithium.59

The secondary amine (17) provided 67% conversion in 12 h toyield the (S)-allylic alcohol in 91% ee. The methylated version of(17) and (S)-(α)-methylbenzylamine provided the same substratein 19% and 5% ee, respectively.

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