Synthesis and evaluation of a broad range of new chiral phosphine–carbene ligands for asymmetric hydrogenation

A straightforward and gram scale synthesis (six-step synthesis from enantioenriched β-hydroxy esters) of new structurally simple phosphine–carbene ligands bearing a single stereogenic centre has been achieved. Enantioselectivities of up to 60–63% could be achieved in the hydrogenation of methylstilbene and dehydroaminoacids when the reactions were performed under 20–50 bar hydrogen pressure.

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(R)-1-(3-Hydroxybutyl)-3-mesityl-1H-imidazol-3-ium 4-methylbenzenesulfonateC23H30N2O4S[α]D20=-6.0 (c 1.0, MeOH)Source of chirality: (R)-methyl 3-hydroxybutanoateAbsolute configuration: (R)

(R)-3-(2,6-Diisopropylphenyl)-1-(3-hydroxybutyl)-1H-imidazol-3-ium 4-methylbenzene sulfonateC26H36N2O4S[α]D20=-5 (c 0.33, MeOH)Absolute configuration: (R)Source of chirality: (R)-methyl 3-hydroxybutanoate

(S)-1-(3-Hydroxypentyl)-3-mesityl-1H-imidazol-3-ium 4-methylbenzenesulfonateC24H32N2O4S[α]D20=+8.2 (c 1.0, MeOH)Absolute configuration: (S)Source of chirality: (S)-methyl 3-hydroxypentanoate

(S)-3-(2,6-Diisopropylphenyl)-1-(3-hydroxypentyl)-1H-imidazol-3-ium 4-methylbenzene sulfonateC27H38N2O4S[α]D20=+3.6 (c 0.072, EtOH)Absolute configuration: (S)Source of chirality: (S)-methyl 3-hydroxypentanoate

(R)-1-(3-Hydroxy-4-methylpentyl)-3-mesityl-1H-imidazol-3-ium 4-methylbenzene sulfonateC25H34N2O4S[α]D20=+9.4 (c 1.01, MeOH)Absolute configuration: (R)Source of chirality: (R)-ethyl 3-hydroxy-4-methylpentanoate

(R)-3-(2,6-Diisopropylphenyl)-1-(3-hydroxy-4-methylpentyl)-1H-imidazol-3-ium 4-methylbenzenesulfonateC28H40N2O4S[α]D20=+5.1 (c 2.0, EtOH)Absolute configuration: (R)Source of chirality: (R)-ethyl 3-hydroxy-4-methylpentanoate

(R)-1-(3-Hydroxy-3-phenylpropyl)-3-mesityl-1H-imidazol-3-ium 4-methylbenzenesulfonateC28H32N2O4S[α]D20=+9.8 (c 1.0, MeOH)Absolute configuration: (R)Source of chirality: (R)-ethyl 3-hydroxy-3-phenylpropanoate

(R)-3-(2,6-Diisopropylphenyl)-1-(3-hydroxy-3-phenylpropyl)-1H-imidazol-3-ium 4-methylbenzenesulfonateC31H38N2O4S[α]D20=+3.7 (c 0.99, MeOH)Absolute configuration: (R)Source of chirality: (R)-ethyl 3-hydroxy-3-phenylpropanoate

(S)-3-Mesityl-1-(3-(diphenylphosphine borane)butyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC60H47B2F24N2P[α]D20=-1.1 (c 1.005, CHCl3)Absolute configuration: (S)Source of chirality: (R)-1-(3-hydroxybutyl)-3-mesityl-1H-imidazol-3-ium 4-methylbenzenesulfonate

(S)-3-(2,6-Diisopropylphenyl)-1-(3-(diphenylphosphine borane)butyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC63H53B2F24N2P[α]D20=-0.3 (c 1.0, CHCl3)Absolute configuration: (S)Source of chirality: (R)-3-(2,6-diisopropylphenyl)-1-(3-hydroxybutyl)-1H-imidazol-3-ium 4-methylbenzene sulfonate

(R)-3-Mesityl-1-(3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC61H49B2F24N2P[α]D20=-4.4 (c 0.99, CHCl3)Absolute configuration: (R)Source of chirality: (S)-1-(3-hydroxypentyl)-3-mesityl-1H-imidazol-3-ium 4-methylbenzenesulfonate

(R)-3-(2,6-Diisopropylphenyl)-1-(3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC64H55B2F24N2P[α]D20=-5.2 (c 1.0, CHCl3)Absolute configuration: (R)Source of chirality: (S)-methyl 3-hydroxypentanoate

(S)-3-Mesityl-1-(4-methyl-3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC62H51B2F24N2P[α]D20=-8.7 (c 1.005, CHCl3)Absolute configuration: (S)Source of chirality: (R)-ethyl 3-hydroxy-4-methylpentanoate

(S)-3-(2,6-Diisopropylphenyl)-1-(4-methyl-3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC65H57B2F24N2P[α]D20=-7.7 (c 1.0, CHCl3)Absolute configuration: (S)Source of chirality: (R)-ethyl 3-hydroxy-4-methylpentanoate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{(3S)-3-(diphenylphosphino-κP)butyl}-3-mesityl-2H-imidazol-2-ylidene-κC}iridium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC68H55BF24IrN2P[α]D20=-15.0 (c 0.1, CHCl3)Absolute configuration: (S)Source of chirality: (S)-3-mesityl-1-(3-(diphenylphosphine borane)butyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{(3S)-3-(diphenylphosphino-κP)butyl}-3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene-κC}iridium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC71H61BF24IrN2P[α]D20=-11.0 (c 0.1, CHCl3)Absolute configuration: (S)Source of chirality: (S)-3-(2,6-diisopropylphenyl)-1-(3-(diphenylphosphine borane)butyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{(3R)-3-(diphenylphosphino-κP)pentyl}-3-mesityl-2H-imidazol-2-ylidene-κC}iridium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC69H57BF24IrN2P[α]D20=+22.0 (c 0.1, CHCl3)Absolute configuration: (R)Source of chirality: (R)-3-mesityl-1-(3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{(3R)-3-(diphenylphosphino-κP)pentyl}-3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene-κC}iridium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC72H63BF24IrN2P[α]D20=+12.2 (c 0.1, CHCl3)Absolute configuration: (R)Source of chirality: (R)-3-(2,6-diisopropylphenyl)-1-(3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{4-methyl-(3S)-3-(diphenylphosphino-κP)pentyl}-3-mesityl-2H-imidazol-2-ylidene-κC}iridium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC70H59BF24IrN2P[α]D20=+26.0 (c 0.1, CHCl3)Absolute configuration: (S)Source of chirality: (S)-3-mesityl-1-(4-methyl-3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{4-methyl-(3S)-3-(diphenylphosphino-κP)pentyl}-3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene-κC}iridium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC73H65BF24IrN2P[α]D20=+37.0 (c 0.1, CHCl3)Absolute configuration: (S)Source of chirality: (S)-3-(2,6-diisopropylphenyl)-1-(4-methyl-3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{(3S)-3-(diphenylphosphino-κP)butyl}-3-mesityl-2H-imidazol-2-ylidene-κC}rhodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC68H55BF24N2PRh[α]D20=-2.0 (c 0.1, CHCl3)Absolute configuration: (S)Source of chirality: (S)-3-mesityl-1-(3-(diphenylphosphine borane)butyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{(3R)-3-(diphenylphosphino-κP)pentyl}-3-mesityl-2H-imidazol-2-ylidene-κC}rhodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC69H57BF24N2PRh[α]D20=+11.0 (c 0.1, CHCl3)Absolute configuration: (R)Source of chirality: (R)-3-mesityl-1-(3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{4-methyl-(3S)-3-(diphenylphosphino-κP)pentyl}-3-mesityl-2H-imidazol-2-ylidene-κC}iridium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC70H59BF24N2PRh[α]D20=+16.2 (c 0.1, CHCl3)Absolute configuration: (S)Source of chirality: (S)-3-mesityl-1-(4-methyl-3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{(3S)-3-(diphenylphosphino-κP)butyl}-3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene-κC}rhodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC71H61BF24N2PRh[α]D20=-12.2 (c 0.115, CHCl3)Absolute configuration: (S)Source of chirality: (S)-3-(2,6-diisopropylphenyl)-1-(3-(diphenylphosphine borane)butyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{(3R)-3-(diphenylphosphino-κP)pentyl}-3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene-κC}iridium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC72H63BF24N2PRh[α]D20=+20.9 (c 1.115 , CHCl3)Absolute configuration: (R)Source of chirality: (R)-3-(2,6-diisopropylphenyl)-1-(3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

[(1,2,5,6-η)-cycloocta-1,5-diene]{1-{4-methyl-(3S)-3-(diphenylphosphino-κP)pentyl}-3-(2,6-diiopropylphenyl)-2H-imidazol-2-ylidene-κC}iridium tetrakis(3,5-bis(trifluoromethyl)phenyl)borateC73H65BF24N2PRh[α]D20=+24.0 (c 1.00, CHCl3)Absolute configuration: (S)Source of chirality: (S)-3-(2,6-diisopropylphenyl)-1-(4-methyl-3-(diphenylphosphine borane)pentyl)-1H-imidazol-3-ium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate