Synthesis and catalytic activity of group 5 metal amides with chiral biaryldiamine-based ligands

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Synthesis and catalytic activity of group 5 metal amides with chiral

biaryldiamine-based ligands†

Furen Zhang,^aHaibin Song^band Guofu Zi*^a

Received 14th September 2010, Accepted 18th November 2010

DOI: 10.1039/c0dt01229g

A new series of group 5 metal amides have been prepared from the reaction between V(NMe₂)₄or

M(NMe₂)₅(M = Nb, Ta) and chiral ligands, (R)-2,2¢-bis(mesitoylamino)-1,1¢-binaphthyl (1H₂),

(R)-5,5¢,6,6¢,7,7¢,8,8¢-octahydro-2,2¢-bis(mesitoylamino)-1,1¢-binaphthyl (2H₂), (R)-6,6¢-

dimethyl-2,2¢-bis(mesitoylamino)-1,1¢-biphenyl (3H₂), (R)-2,2¢-bis(mesitylenesulfonylamino)-

6,6¢-dimethyl-1,1¢-biphenyl (4H₂), (R)-2,2¢-bis(diphenylthiophosphoramino)-1,1¢-binaphthyl (5H₂),

(R)-2,2¢-bis[(3-tert-butyl-2-hydroxybenzylidene)amino]-6,6¢-dimethyl-1,1¢-biphenyl (6H₂),

(R)-2,2¢-bis[(3,5-di-tert-butyl-2-hydroxybenzylidene)amino]-6,6¢-dimethyl-1,1¢-biphenyl (7H₂),

(R)-2,2¢-bis[(3-tert-butyl-2-hydroxybenzylidene)amino]-1,1¢-binaphthyl (8H₂), (S)-2-(mesitoylamino)-

2¢-(dimethylamino)-1,1¢-binaphthyl (9H), and (R)-2-(mesitoylamino)-2¢-(dimethylamino)-

6,6¢-dimethyl-1,1¢-biphenyl (10H), which are derived from (R) or (S)-2,2¢-diamino-1,1¢-binaphthyl, and

(R)-2,2¢-diamino-6,6¢-dimethyl-1,1¢-biphenyl, respectively. Treatment of V(NMe₂)₄or M(NMe₂)₅(M =

Nb, Ta) with 1 equiv of C₂-symmetric amidate ligands 1H₂, 2H₂, 3H₂, 4H₂, and 5H₂, or Schiff base

ligands 6H₂, 7H₂and 8H₂at room temperature gives, after recrystallization from a benzene, toluene or

n-hexane solution, the vanadium amides (1)V(NMe₂)₂(11), (2)V(NMe₂)₂(14), (3)V(NMe₂)₂(17),

(5)V(NMe₂)₂(22), (6)V(NMe₂)₂(23) and (7)V(NMe₂)₂(24), and niobium amides (1)Nb(NMe₂)₃(12),

(2)Nb(NMe₂)₃(15), (3)Nb(NMe₂)₃(18), (4)Nb(NMe₂)₃(20) and [2-(3-Me₃C-2-O-C₆H₃CHN)-

2¢-(N)-C₂₀H₁₂][2-(Me₂N)₂CH-6-CMe₃-C₆H₃O]NbNMe₂·C₇H₈(25·C₇H₈), and tantalum amides

(1)Ta(NMe₂)₃(13), (2)Ta(NMe₂)₃(16), (3)Ta(NMe₂)₃(19) and (4)Ta(NMe₂)₃(21) respectively, in good

yields. Reaction of V(NMe₂)₄or M(NMe₂)₅(M = Nb, Ta) with 2 equiv of C₁-symmetric amidate ligands

9H or 10H at room temperature gives, after recrystallization from a toluene or n-hexane solution, the

chiral bis-ligated vanadium amides (9)₂V(NMe₂)₂·3C₇H₈(27·3C₇H₈) and (10)V(NMe₂)₂(28), and chiral

2

bis-ligated metallaaziridine complexes (10)₂M(NMe₂)(h -CH₂NMe) (M = Nb (29), Ta (30)) respectively,

in good yields. The niobium and tantalum amidate complexes are stable in a toluene solution at or

below 160 ^◦C, while the vanadium amidate complexes degrade via diemthylamino group elimination at

this temperature. For example, heating the complex (2)V(NMe₂)₂(14) in toluene at 160 ^◦C for four days

leads to the isolation of the complex [(2)V]₂(m-NMe₂)₂(26) in 58% yield. These new complexes have

been characterized by various spectroscopic techniques, and elemental analyses. The solid-state

structures of complexes 12, 13, and 15–30 have further been conﬁrmed by X-ray diffraction analyses.

The vanadium amides are active chiral catalysts for the asymmetric hydroamination/cyclization of

aminoalkenes, affording cyclic amines in moderate to good yields with good ee values (up to 80%), and

the tantalum amides are outstanding chiral catalysts for the hydroaminoalkylation, giving chiral

secondary amines in good yields with excellent ee values (up to 93%).

Introduction

Both the hydroamination and hydroaminoalkylation are highly

^aDepartment of Chemistry, Beijing Normal University, Beijing, 100875,

China. E-mail: gzi@bnu.edu.cn; Fax: +86-10-58802075; Tel: +86-10-

58806051

atom economical processes (100%) in which an amine N–H bond

or an a-C–H bond of a primary or a secondary amine is added to

an unsaturated carbon–carbon bond, leading to the formation

of the nitrogen-containing molecules that are useful building

blocks for the biological, pharmaceutical, agrochemical, and ﬁne

chemical industries.^1–3Therefore, it is not surprising that the

^bState Key Laboratory of Elemento-Organic Chemistry, Nankai University,

Tianjin, 300071, China

† CCDC reference numbers 739122, 775657–775663, 791864–791874 and

800129. For crystallographic data in CIF or other electronic format see

DOI: 10.1039/c0dt01229g

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Dalton Trans., 2011, 40, 1547–1566 | 1547

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development of chiral catalysts for intramolecular asymmetric

alkene hydroamination has received signiﬁcant attention over the

past two decades.⁴Since the pioneering work of Marks and co-

workers in 1992,^5a,cmany chiral catalysts based on lanthanide met-

als, main group metals, group 4 metals and late transition metals

with C₁-symmetric Cp ligands, and chiral non-Cp ligands have

been widely developed for hydroamination/cyclization;^5–14how-

ever, only a small number of successful catalysts have been reported

affording signiﬁcant enantioselectivity (>90% ee),^{9d,12d–f,p,q–s,13e,f,14b}

and those high enantioselective inductions are only observed for

one or two substrates. On the other hand, only one chiral catalyst

for asymmetric hydroaminoalkylation has been reported, and the

best enantioselectivity is only up to 61% ee by a precatalyst

loading of 10%.¹⁵Thus, the development of new chiral catalysts

for the synthesis of chiral amines via alkene hydroamination and

hydroaminoalkylation reactions is still a desirable and challenging

goal.

Recently, we have reported a new series of biaryl-based

chiral non-Cp multidentate ligands, and their group 4 metal

complexes and lanthanide complexes are useful catalysts for

the asymmetric hydroamination/cyclization reaction,¹⁶e.g.,

the group 4 metal amides derived from the C₂-symmetric

binaphthylamidate ligands (R)-2,2¢-bis(mesitoylamino)-1,1¢-

binaphthyl (1H₂) and (R)-5,5¢,6,6¢,7,7¢,8,8¢-octahydro-2,2¢-

bis(mesitoylamino)-1,1¢-binaphthyl (2H₂) are outstanding

chiral catalysts for this transformation, in which excellent

enantioselectivities (up to 93%) with good conversions have

been obtained.¹⁶ⁱEncouraged by the attractive features of

the amidate ligand system and the fact that the tantalum

catalyst system exclusively yields branched regioisomers in

hydroaminoalkylation reactions,³and to our knowledge,

no example of asymmetric hydroamination/cyclization of

unactivated aminoalkenes promoted by group 5 catalysts has

been reported, we have recently started exploring ligands 1H₂

and 2H₂in group 5 chemistry, and found that the vanadium

and tantalum amides with these modular ligands are efﬁcient

catalysts for the asymmetric hydroamination/cyclization or

hydroaminoalkylation, in which good enantioselectivities

(up to 80%) for hydroamination/cyclization and excellent

enantioselectivities (up to 93%) for hydroaminoalkylation have

been obtained, respectively. Most importantly, this is the ﬁrst

example to show that the asymmetric hydroamination/cyclization

of unactivated aminoalkenes can be catalyzed by group 5 metal

complexes, and the ﬁrst example of a highly enantioselective

amine synthesis by catalytic hydroaminoalkylation. Herein, we

report on these observations concerning the chemistry of amidate

ligands 1H₂and 2H₂with group 5 amides, and the applications

of the resulting complexes as catalysts for the asymmetric

hydroamination/cyclization of unactivated aminoalkenes and

the asymmetric hydroaminoalkylation of alkenes.¹⁷For better

understanding and comparison, the C₂-symmetric amidate

ligands (R)-6,6¢-dimethyl-2,2¢-bis(mesitoylamino)-1,1¢-biphenyl

(3H₂), (R)-2,2¢-bis(mesitylenesulfonylamino)-6,6¢-dimethyl-1,1¢-

biphenyl (4H₂) and (R)-2,2¢-bis(diphenylthiophosphoramino)-

1,1¢-binaphthyl (5H₂), C₂-symmetric Schiff base ligands

(R)-2,2¢-bis[(3-tert-butyl-2-hydroxybenzylidene)amino]-6,6¢-di-

methyl-1,1¢-biphenyl (6H₂), (R)-2,2¢-bis[(3,5-di-tert-butyl-2-

1,1¢-binaphthyl (8H₂), and C₁-symmetric amidate ligands

(S)-2-(mesitoylamino)-2¢-(dimethylamino)-1,1¢-binaphthyl (9H)

and

(R)-2-(mesitoylamino)-2¢-(dimethylamino)-6,6¢-dimethyl-

1,1¢-biphenyl (10H) will also be included in this contribution.

Experimental

General methods

Group 5 complexes and catalytic reactions were performed

under an atmosphere of dry dinitrogen with rigid exclusion of

air and moisture using standard Schlenk or cannula techniques,

or in a glovebox. All organic solvents used for the synthesis

of complexes were freshly distilled from sodium benzophenone

ketyl immediately prior to use. (R) or (S)-2,2¢-Diamino-

1,1¢-binaphthyl (>98% ee),¹⁶ⁱ(R)-5,5¢,6,6¢,7,7¢,8,8¢-octahydro-

2,2¢-bis(mesitoylamino)-1,1¢-binaphthyl (>98% ee),¹⁶ⁱ(R)-

2,2¢-diamino-6,6¢-dimethyl-1,1¢-biphenyl (>98% ee),^16h(R)-2,2¢-

bis(mesitoylamino)-1,1¢-binaphthyl (1H₂),¹⁶ⁱ(R)-5,5¢,6,6¢,7,7¢,8,8¢-

octahydro-2,2¢-bis(mesitoylamino)-1,1¢-binaphthyl (2H₂),¹⁶ⁱ(R)-

6,6¢-dimethyl-2,2¢-bis(mesitoylamino)-1,1¢-biphenyl (3H₂),^13e(R)-

2,2¢-bis(diphenylthiophosphoramino)-1,1¢-binaphthyl

(5H₂),^16j

(S) - 2 - (mesitoylamino) - 2¢ - (dimethylamino) - 1,1¢ - binaphthyl

(9H),^16g(R)-2-(mesitoylamino)-2¢-(dimethylamino)-6,6¢-dimethyl-

1,1¢-biphenyl

(10H),^16g

M(NMe₂)₅,¹⁸

2,2-dimethylpent-

4-enylamine,^8a2,2¢-dimethylhex-5-enylamine,^8aand 1-(amino-

methyl)-1-allylcyclohexane^10cwere prepared according to

literature methods. All chemicals were purchased from Aldrich

Chemical Co. and Beijing Chemical Co. used as received unless

otherwise noted. Infrared spectra were obtained from KBr

pellets on an Avatar 360 Fourier transform spectrometer. ¹H

and ¹³C NMR spectra were recorded on a Bruker AV 400

spectrometer at 400 and 100 MHz, respectively. All chemical

shifts are reported in d units with reference to the residual protons

of the deuterated solvents for proton and carbon chemical

shifts. Optical rotations were measured on a Perkin-Elmer 343

polarimeter. HPLC analyses were conducted on a Shimadzu

Series SPD-20A with UV/VIS detector using a Chiralcel OD-H

or AS-H column (length: 25 cm, inner diameter: 4.6 mm, particle

size: 5 mm). Retention time was given in minutes. Melting points

were measured on an X-6 melting point apparatus and were

uncorrected. Elemental analyses were performed on a Vario EL

elemental analyzer.

Syntheses

Preparation

dimethyl-1,1¢-biphenyl

of

(R)-2,2¢-bis(mesitylenesulfonylamino)-6,6¢-

(4H₂). Mesitylenesulfonyl chloride

(2.19 g, 10.0 mmol) was mixed with (R)-2,2¢-diamino-6,6¢-

dimethyl-1,1¢-biphenyl (1.06 g, 5.0 mmol) in dry toluene (30 mL).

Pyridine (2 mL, 25.3 mmol) was added, and the solution was

reﬂuxed for two days. The solvent was removed and the residue

was decomposed with H₂O (20 mL) and extracted with ethyl

acetate (20 mL ¥ 3) and washed with brine (20 mL). The combined

organic layers were dried with anhydrous Na₂SO₄and the solvent

was removed under reduced pressure to give a white solid, which

was further puriﬁed by ﬂash column chromatography (n-hexane–

ethyl acetate = 10 : 1) to give 4H₂as a yellow solid. Yield: 2.40 g

(83%) (Found: C, 66.43; H, 6.17; N, 4.80. C₃₂H₃₆N₂O₄S₂requires

hydroxybenzylidene)amino]-6,6¢-dimethyl-1,1¢-biphenyl

(7H₂)

and (R)-2,2¢-bis[(3-tert-butyl-2-hydroxybenzylidene)amino]-

1548 | Dalton Trans., 2011, 40, 1547–1566

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C, 66.64; H, 6.29; N, 4.86%). M.p.: 246–248 ^◦C.¹H NMR (CDCl₃):

d 7.29 (s, 1H, aryl), 7.25 (t, J = 6.4 Hz, 1H, aryl), 7.19 (d, J =

6.4 Hz, 2H, aryl), 7.04 (d, J = 6.0 Hz, 2H, aryl), 6.96 (s, 4H, aryl),

6.18 (s, 2H, NH), 2.48 (s, 12H, CH₃), 2.33 (s, 6H, CH₃), 1.66

(s, 6H, CH₃). ¹³C NMR (CDCl₃): d 142.8, 139.2, 138.4, 135.5,

134.6, 132.2, 129.4, 126.8, 126.5, 118.3, 22.9, 21.0, 19.5. IR (KBr,

cm^-1): n 3367 (s), 2939 (m), 2859 (m), 1604 (s), 1582 (s), 1464 (s),

1369 (s), 1282 (s), 1151 (s), 1054 (s), 958 (s), 852 (s), 777 (s).

extracted with n-hexane (30 mL ¥ 3). The solvent was then removed

from the ﬁltrate in_◦vacuo, and the resulting semi-solid residue was

distilled at 55–60 C at 10^-2mmHg to give V(NMe₂)₄as a green

solid. Yield: 4.83 g (85%) (Found: C, 42.23; H, 10.63; N, 24.72.

C₈H₂₄N₄V requires C, 42.28; H, 10.65; N, 24.65%).

Preparation of (1)V(NMe₂)₂(11). A toluene solution (10 mL)

of 1H₂(0.28 g, 0.5 mmol) was slowly added to a toluene solution

(10 mL) of V(NMe₂)₄(0.11 g, 0.5 mmol) with stirring at room

temperature. After this mixture was stirred at room temperature

for one day, the solution was ﬁltered and the solvent was removed

under reduced pressure. The resulting red solid was recrystallized

from a benzene solution to give 11 as red microcrystals. Yield:

0.28 g (79%) (Found: C, 73.89; H, 6.71; N, 7.73. C₄₄H₄₆N₄O₂V

requires C, 74.04; H, 6.50; N, 7.85%). M.p.: 178–180 ^◦C (dec.).

IR (KBr, cm^-1): n 2959 (m), 1614 (s), 1538 (s), 1453 (s), 1257 (s),

1089 (s), 1019 (s), 796 (s).

Preparation of (R)-2,2¢-bis[(3-tert-butyl-2-hydroxybenzylide-

ne)amino]-6,6¢-dimethyl-1,1¢-biphenyl (6H₂). Modiﬁed method.¹⁹

3-tert-Butylsalicyladehyde (1.78 g, 10.0 mmol) was mixed with (R)-

2,2¢-diamino-6,6¢-dimethyl-1,1¢-biphenyl (1.06 g, 5.0 mmol) in dry

˚

toluene (30 mL). A few 4 A molecular sieves were added, and

the solution was warmed up to 70 ^◦C and kept for two days at

this temperature. The solution was ﬁltered and the solvent was

removed under reduced pressure to give a brown solid, which was

further puriﬁed by ﬂash column chromatography (n-hexane–ethyl

acetate = 20 : 1) to give 6H₂as an orange solid. Yield: 2.26 g (85%).

M.p.: 104–106 ^◦C. ¹H NMR (C₆D₆): d 13.23 (s, 2H, OH), 8.37 (s,

2H, CH N), 7.27 (t, J = 7.7 Hz, 2H, aryl), 7.16 (m, 4H, aryl),

6.99 (d, J = 6.2 Hz, 4H, aryl), 6.66 (t, J = 7.6 Hz, 2H, aryl), 2.01

(s, 6H, CH₃), 1.24 (s, 18H, CH₃). These spectroscopic data were

in agreement with those reported in the literature.¹⁹

Preparation of (1)Nb(NMe₂)₃(12). This compound was pre-

pared as orange crystals from the reaction of 1H₂(0.28 g,

0.5 mmol) with Nb(NMe₂)₅(0.16 g, 0.5 mmol) in toluene (20 mL)

and recrystallized from a benzene solution by a similar procedure

as in the synthesis of 11. Yield: 0.36 g (90%) (Found: C, 69.15;

H, 6.63; N, 8.72. C₄₆H₅₂N₅NbO₂requires C, 69.08; H, 6.55; N,

8.76%). M.p.: 131–133 ^◦C (dec.). ¹H NMR (C₆D₆): d 7.73 (d, J =

8.2 Hz, 2H, aryl), 7.64 (d, J = 7.8 Hz, 1H, aryl), 7.51 (m, 4H,

aryl), 6.83 (s, 1H, aryl), 6.74 (s, 2H, aryl), 6.60 (s, 2H, aryl), 6.36

(s, 1H, aryl), 6.28 (s, 2H, aryl), 6.12 (d, J = 8.0 Hz, 1H, aryl), 3.38

(s, 18H, Nb(NMe₂)₃), 2.22 (s, 3H, CH₃), 2.01 (s, 3H, CH₃), 1.93

(d, J = 6.2 Hz, 6H, CH₃), 1.86 (s, 3H, CH₃), 1.62 (s, 3H, CH₃).

¹³C NMR (C₆D₆): d 178.2, 159.4, 149.7, 138.9, 137.3, 136.3, 135.6,

131.8, 130.2, 129.7, 129.5, 128.9, 126.5, 125.7, 124.5, 121.7, 47.8,

25.6, 21.7. IR (KBr, cm^-1): n 2962 (m), 2912 (m), 1589 (s), 1464 (s),

1422 (s), 1260 (s), 1089 (s), 1018 (s), 799 (s).

Preparation of (R)-2,2¢-bis[(3,5-di-tert-butyl-2-hydroxybenzyl-

idene)amino]-6,6¢-dimethyl-1,1¢-biphenyl (7H₂). This compound

was prepared as an orange solid from the reaction of 3,5-di-tert-

butylsalicyladehyde (2.34 g, 10.0 mmol) with (R)-2,2¢-diamino-

6,6¢-dimethyl-1,1¢-biphenyl (1.06 g, 5.0 mmol) in dry toluene

(30 mL) in the presence of 4 A molecular sieves at 70 ^◦C

˚

and puriﬁed by ﬂash column chromatography (n-hexane–ethyl

acetate = 20 : 1) using a similar procedure_◦as in the synthesis of

1

6H₂. Yield: 2.71 g (84%). M.p.: 118–120 C. H NMR (C₆D₆):

d 13.49 (s, 2H, OH), 8.36 (s, 2H, N CH), 7.56 (d, J = 2.2 Hz,

2H, aryl), 7.32 (t, J = 7.7 Hz, 2H, aryl), 7.22 (d, J = 7.5 Hz, 2H,

aryl), 7.07 (d, J = 2.2 Hz, 2H, aryl), 6.90 (d, J = 7.8 Hz, 2H, aryl),

2.14 (s, 6H, CH₃), 1.64 (s, 18H, CH₃), 1.36 (s, 18H, CH₃). These

spectroscopic data were in agreement with those reported in the

literature.²⁰

Preparation of (1)Ta(NMe₂)₃(13). This compound was pre-

pared as pale yellow crystals from the reaction of 1H₂(0.28 g,

0.5 mmol) with Ta(NMe₂)₅(0.20 g, 0.5 mmol) in toluene (20 mL)

and recrystallized from a benzene solution by a similar procedure

as in the synthesis of 11. Yield: 0.41 g (92%) (Found: C, 62.08; H,

5.86; N, 7.78. C_◦₄₆H₅₂N₅O₂Ta requires C, 62.23; H, 5.90; N, 7.89%).

Preparation of (R)-2,2¢-bis[(3-tert-butyl-2-hydroxybenzylidene)-

amino]-1,1¢-binaphthyl (8H₂). This compound was prepared as

an orange solid from the reaction of 3-tert-butylsalicyladehyde

(1.78 g, 10.0 mmol) with (R)-2,2¢-diamino-1,1¢-binaphthyl (1.42 g,

1

M.p.: 140–142 C (dec.). H NMR (C₆D₆): d 7.77 (m, 3H, aryl),

7.52 (m, 6H, aryl), 6.94 (s, 2H, aryl), 6.82 (s, 2H, aryl), 6.65 (m,

1H, aryl), 6.47 (m, 1H, aryl), 6.31 (m, 1H, aryl), 3.58 (s, 18H,

Ta(NMe₂)₃), 2.27 (s, 3H, CH₃), 2.13 (s, 9H, CH₃), 1.89 (s, 3H,

CH₃), 1.66 (s, 3H, CH₃). ¹³C NMR (C₆D₆): d 179.5, 156.5, 147.8,

142.4, 139.2, 138.6, 138.0, 136.0, 135.2, 134.0, 133.5, 133.1, 130.6,

128.5, 127.9, 125.2, 124.3, 120.6, 118.9, 44.5, 24.5, 20.4. IR (KBr,

cm^-1): n 2962 (m), 1588 (s), 1464 (s), 1422 (s), 1260 (s), 1090 (s),

1017 (s), 799 (s).

˚

5.0 mmol) in dry toluene (30 mL) in the presence of 4 A molecular

sieves at 70 ^◦C and puriﬁed by ﬂash column chromatography (n-

hexane–ethyl acetate = 15 : 1) using a similar procedure as in the

◦

1

synthesis of 6H₂. Yield: 2.60 g (86%). M.p.: 77¢79 C. H NMR

(C₆D₆): d 13.38 (s, 2H, OH), 8.36 (s, 2H, N CH), 7.96 (d, J =

8.8 Hz, 2H, aryl), 7.90 (d, J = 8.2 Hz, 2H, aryl), 7.60 (d, J = 8.4 Hz,

2H, aryl), 7.32 (m, 4H, aryl), 7.23 (d, J = 7.6 Hz, 2H, aryl), 7.13

(t, J = 7.4 Hz, 2H, aryl), 6.84 (d, J = 6.4 Hz, 2H, aryl), 6.68 (m,

2H, aryl), 1.44 (s, 18H, CH₃). These spectroscopic data were in

agreement with those reported in the literature.²¹

Preparation of (2)V(NMe₂)₂(14). This compound was pre-

pared as red microcrystals from the reaction of 2H₂(0.29 g,

0.5 mmol) with V(NMe₂)₄(0.11 g, 0.5 mmol) in toluene (20 mL)

and recrystallized from a benzene solution by a similar procedure

as in the synthesis of 11. Yield: 0.27 g (75%) (Found: C, 73.20; H,

7.63; N, 7.72. C_◦₄₄H₅₄N₄O₂V requires C, 73.21; H, 7.54; N, 7.76%).

M.p.: 278–280 C (dec.). IR (KBr, cm^-1): n 2959 (w), 2925 (m),

2848 (m), 1612 (s), 1560 (s), 1458 (s), 1419 (s), 1351 (s), 1180 (s),

1106 (s), 1053 (s), 968 (s), 833 (s), 789 (s).

Preparation of V(NMe₂)₄. A THF solution (50 mL) of LiNMe₂

(5.10 g, 100 mmol) was slowly added to a THF solution (10 mL)

of VCl₄(4.82 g, 25 mmol) with stirring at -78 ^◦C. The dark green

solution was slowly warmed to room temperature and stirred

one day. All volatiles were removed in vacuo, and the solid was

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Preparation of (2)Nb(NMe₂)₃(15). This compound was pre-

pared as orange crystals from the reaction of 2H₂(0.29 g,

0.5 mmol) with Nb(NMe₂)₅(0.16 g, 0.5 mmol) in toluene

(20 mL) and recrystallized from an n-hexane solution by a similar

procedure as in the synthesis of 11. Yield: 0.36 g (89%) (Found:

C, 68.42; H, 7.36; N, 8.58. C₄₆H₆₀N₅NbO₂requires C, 68.39; H,

7.49; N, 8.67%). M.p.: 118–120 ^◦C (dec.). ¹H NMR (C₆D₆): d 6.80

(m, 4H, aryl), 6.70 (s, 2H, aryl), 6.55 (s, 2H, aryl), 3.36 (s, 18H,

Nb(NMe₂)₃), 2.73–2.56 (m, 4H, CH₂), 2.35 (s, 6H, CH₃), 2.30 (m,

4H, CH₂), 2.24 (s, 6H, CH₃), 2.01 (s, 6H, CH₃), 1.55 (m, 8H, CH₂).

¹³C NMR (C₆D₆): d 176.4, 158.9, 148.3, 141.8, 138.3, 135.1, 134.1,

133.0, 129.6, 125.0, 120.2, 48.2, 30.2, 29.7, 28.3, 27.0, 26.4, 23.5,

23.2, 22.8, 22.4, 22.1, 21.5. IR (KBr, cm^-1): n 2969 (s), 2924 (s),

1607 (s), 1454 (s), 1368 (s), 1260 (s), 1089 (s), 1020 (s), 938 (s),

796 (s).

C, 58.72; H, 6.66; N, 8.48. C₄₀H₅₂N₅O₂Ta requires C, 58.89; H,

6.42; N, 8.58%). M.p.: 126–128 ^◦C (dec.). ¹H NMR (C₆D₆): d 6.85

(m, 1H, aryl), 6.81 (m, 2H, aryl), 6.76 (s, 4H, aryl), 6.64 (m, 2H,

aryl), 6.54 (m, 1H, aryl), 3.52 (s, 18H, Ta(NMe₂)₃), 2.19 (s, 3H,

CH₃), 2.09 (s, 3H, CH₃), 2.08 (s, 3H, CH₃), 2.05 (s, 3H, CH₃),

2.00 (s, 3H, CH₃), 1.98 (s, 3H, CH₃), 1.86 (s, 3H, CH₃), 1.84 (s,

3H, CH₃). ¹³C NMR (C₆D₆): d 179.1, 157.1, 149.1, 143.9, 141.0,

138.8, 136.5, 135.9, 131.7, 127.6, 125.0, 118.5, 44.7, 25.8, 24.4,

20.9, 20.5, 19.6, 19.5, 18.8, 17.2. IR (KBr, cm^-1): n 2962 (m), 2910

(m), 1578 (s), 1508 (s), 1458 (s), 1260 (s), 1091 (s), 1018 (s), 797 (s).

A few pale yellow crystals suitable for X-ray diffraction analysis

were selected.

Preparation of (4)Nb(NMe₂)₃(20). This compound was pre-

pared as orange microcrystals from the reaction of 4H₂(0.29 g,

0.5 mmol) with Nb(NMe₂)₅(0.16 g, 0.5 mmol) in toluene (20 mL)

and recrystallized from a benzene solution by a similar procedure

as in the synthesis of 11. Yield: 0.29 g (72%) (Found: C, 57.12;

H, 6.44; N, 8.58. C₃₈H₅₂N₅NbO₄S₂requires C, 57.06; H, 6.55; N,

Preparation of (2)Ta(NMe₂)₃(16). This compound was pre-

pared as pale yellow crystals from the reaction of 2H₂(0.29 g,

0.5 mmol) with Ta(NMe₂)₅(0.20 g, 0.5 mmol) in toluene (20 mL)

and recrystallized from an n-hexane solution by a similar pro-

cedure as in the synthesis of 11. Yield: 0.38 g (87%) (Found: C,

62.58; H, 6.86; N, 7.88. C₄₆H₆₀N₅OTa requires C, 62.79; H, 6.87;

◦

1

8.76%). M.p.: 172–174 C (dec.). H NMR (C₆D₆): d 7.10–6.80

(m, 6H, aryl), 6.58 (s, 4H, aryl), 3.50 (s, 18H, Nb(NMe₂)₃), 2.45

(br s, 15H, CH₃), 1.98 (s, 9H, CH₃). ¹³C NMR (C₆D₆): d 139.2,

134.3, 133.8, 132.9, 131.4, 130.0, 127.9, 125.8, 124.3, 46.7, 25.9,

22.4, 19.3. IR (KBr, cm^-1): n 2949 (m), 1617 (s), 1471 (s), 1319 (s),

1166 (s), 979 (s), 748 (s). A few orange crystals suitable for X-ray

diffraction analysis were selected.

◦

1

N, 7.96%). M.p.: 178–180 C (dec.). H NMR (C₆D₆): d 6.80

(m, 4H, aryl), 6.66 (s, 2H, aryl), 6.53 (s, 2H, aryl), 3.59 (s, 18H,

Ta(NMe₂)₃), 2.71–2.56 (m, 4H, CH₂), 2.36 (s, 6H, CH₃), 2.55 (m,

4H, CH₂), 2.06 (s, 6H, CH₃), 1.91 (s, 6H, CH₃), 1.58 (m, 8H, CH₂).

¹³C NMR (C₆D₆): d 175.4, 156.8, 146.7, 135.7, 134.9, 134.3, 133.4,

128.7, 123.7, 121.6, 119.1, 44.6, 29.0, 28.5, 27.1, 25.8, 25.4, 22.3,

21.9, 20.3, 19.7. IR (KBr, cm^-1): n 2962 (m), 2923 (m), 1586 (s),

1465 (s), 1422 (s), 1260 (s), 1090 (s), 1018 (s), 798 (s).

Preparation of (4)Ta(NMe₂)₃(21). This compound was pre-

pared as pale yellow microcrystals from the reaction of 4H₂(0.29 g,

0.5 mmol) with Ta(NMe₂)₅(0.20 g, 0.5 mmol) in toluene (20 mL)

and recrystallized from a benzene solution by a similar procedure

as in the synthesis of 11. Yield: 0.34 g (76%) (Found: C, 51.32;

H, 6.04; N, 8.05. C₃₈H₅₂N₅O₄S₂Ta requires C, 51.40; H, 5.90; N,

7.89%). M.p.: 192–194 ^◦C (dec.). ¹H NMR (C₆D₆): d 7.14–6.83 (m,

6H, aryl), 6.53 (s, 4H, aryl), 3.62 (s, 18H, Nb(NMe₂)₃), 2.41 (br s,

15H, CH₃), 1.97 (s, 9H, CH₃). ¹³C NMR (C₆D₆): d 139.4, 138.1,

137.1, 131.0, 130.1, 128.0, 127.2, 126.8, 126.5, 125.8, 45.6, 21.9,

19.3, 18.7. IR (KBr, cm^-1): n 2955 (m), 1599 (s), 1449 (s), 1297 (s),

1260 (s), 1138 (s), 1098 (s), 1032 (s), 942 (s), 843 (s), 794 (s). A few

pale yellow crystals suitable for X-ray diffraction analysis were

selected.

Preparation of (3)V(NMe₂)₂(17). This compound was pre-

pared as red crystals from the reaction of 3H₂(0.25 g, 0.5 mmol)

with V(NMe₂)₄(0.11 g, 0.5 mmol) in toluene (20 mL) and

recrystallized from a benzene solution by a similar procedure as in

the synthesis of 11. Yield: 0.25 g (78%) (Found: C, 70.97; H, 7.06;

N, 8.53. C₃₈H₄₆N₄O₂V requires C, 71.12; H, 7.22; N, 8.73%). M.p.:

120–122 ^◦C (dec.). IR (KBr, cm^-1): n 2963 (m), 2855 (m), 1609 (s),

1504 (s), 1361 (s), 1260 (s), 1094 (s), 1017 (s), 946 (s), 798 (s).

Preparation of (3)Nb(NMe₂)₃(18). This compound was pre-

pared as orange microcrystals from the reaction of 3H₂(0.25 g,

0.5 mmol) with Nb(NMe₂)₅(0.16 g, 0.5 mmol) in toluene

(20 mL) and recrystallized from an n-hexane solution by a similar

procedure as in the synthesis of 11. Yield: 0.32 g (88%) (Found: C,

66.12; H, 7.14; N, 9.58. C₄₀H₅₂N₅NbO₂requires C, 66.01; H, 7.20;

N, 9.62%). M.p.: 168–170 ^◦C (dec.). ¹H NMR (C₆D₆): d 7.03 (m,

1H, aryl), 6.98 (m, 2H, aryl), 6.87 (s, 4H, aryl), 6.66 (m, 2H, aryl),

6.51 (m, 1H, aryl), 3.42 (s, 18H, Nb(NMe₂)₃), 2.27 (s, 6H, CH₃),

2.16 (s, 6H, CH₃), 2.09 (s, 6H, CH₃), 2.02 (s, 3H, CH₃), 1.99 (s, 3H,

CH₃). ¹³C NMR (C₆D₆): d 172.3, 156.8, 148.2, 137.2, 135.4, 135.2,

134.8, 132.2, 125.9, 125.5, 124.3, 46.1, 25.9, 19.7, 19.2, 19.0, 18.8.

IR (KBr, cm^-1): n 2954 (m), 1538 (s), 1504 (s), 1417 (s), 1361 (s),

1256 (s), 1084 (s), 1014 (s), 959 (s), 793 (s). A few orange crystals

suitable for X-ray diffraction analysis were selected.

Preparation of (5)V(NMe₂)₂(22). This compound was pre-

pared as red crystals from the reaction of 5H₂(0.36 g, 0.5 mmol)

with V(NMe₂)₄(0.11 g, 0.5 mmol) in toluene (20 mL) and

recrystallized from a benzene solution by a similar procedure as in

the synthesis of 11. Yield: 0.35 g (82%) (Found: C, 67.35; H, 5.33;

N, 6.72. C₄₈H₄₄N₄P₂S₂V requires C, 67.51; H, 5.19; N, 6.56%).

◦

M.p.: 112–114 C (dec.). IR (KBr, cm^-1): n 305 (w), 2961 (m),

1588 (s), 1409 (s), 1260 (s), 1090 (s), 939 (s), 799 (s).

Preparation of (6)V(NMe₂)₂(23). This compound was pre-

pared as red crystals from the reaction of 6H₂(0.27 g, 0.5 mmol)

with V(NMe₂)₄(0.11 g, 0.5 mmol) in toluene (20 mL) and

recrystallized from an n-hexane solution by a similar procedure

as in the synthesis of 11. Yield: 0.24 g (73%) (Found: C, 71.68; H,

7.63; N, 8.42. C₄₀H₅₀N₄O₂V requires C, 71.73; H, 7.52; N, 8.36%).

Preparation of (3)Ta(NMe₂)₃(19). This compound was pre-

pared as pale yellow microcrystals from the reaction of 3H₂

(0.25 g, 0.5 mmol) with Ta(NMe₂)₅(0.20 g, 0.5 mmol) in toluene

(20 mL) and recrystallized from an n-hexane solution by a similar

procedure as in the synthesis of 11. Yield: 0.35 g (85%) (Found:

◦

M.p.: 176–178 C (dec.). IR (KBr, cm^-1): n 2961 (m), 1601 (s),

1533 (s), 1448 (s), 1259 (s), 1227 (s), 1076 (s), 1018 (s), 799 (s).

1550 | Dalton Trans., 2011, 40, 1547–1566

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Preparation of (7)V(NMe₂)₂(24). This compound was pre-

pared as red microcrystals from the reaction of 7H₂(0.32 g,

0.5 mmol) with V(NMe₂)₄(0.11 g, 0.5 mmol) in toluene (20 mL)

and recrystallized from an n-hexane solution by a similar pro-

cedure as in the synthesis of 11. Yield: 0.31 g (80%) (Found: C,

73.68; H, 8.63; N, 7.12. C₄₈H₆₆N₄O₂V requires C, 73.72; H, 8.51;

6H, CH₃). These spectroscopic data are in agreement with those

reported in the literature.^13eColorless crystals suitable for X-ray

structural analysis were grown from a diethyl ether solution at

room temperature.

Preparation of (9)₂V(NMe₂)₂·3C₇H₈(27·3C₇H₈). This com-

pound was prepared as red crystals from the reaction of 9H (0.23 g,

0.5 mmol) with V(NMe₂)₄(0.06 g, 0.25 mmol) in toluene (20 mL)

and recrystallized from a toluene solution by a similar procedure

as in the synthesis of 11. Yield: 0.27 g (81%) (Found: C, 80.15; H,

7.23; N, 6.42. C_◦₈₉H₉₄N₆O₂V requires C, 80.33; H, 7.12; N, 6.32%).

M.p.: 170–172 C (dec.). IR (KBr, cm^-1): n 2953 (m), 2860 (m),

1500 (s), 1451 (s), 1299 (s), 1256 (s), 1097 (s), 1018 (s), 972 (s),

793 (s).

◦

N, 7.16%). M.p.: 106–108 C (dec.). IR (KBr, cm^-1): n 2961 (s),

2850 (s), 1611 (s), 1528 (s), 1458 (s), 1425 (s), 1259 (s), 1093 (s),

1020 (s), 799 (s). A few red crystals suitable for X-ray diffraction

analysis were selected.

Preparation of 25·C₇H₈. This compound was prepared as

orange crystals from the reaction of 8H₂(0.30 g, 0.5 mmol)

with Nb(NMe₂)₅(0.16 g, 0.5 mmol) in toluene (20 mL) and

recrystallized from a toluene solution by a similar procedure as

in the synthesis of 11. Yield: 0.34 g (73%) (Found: C, 71.69; H,

6.83; N, 7.80. C₅₅H₆₄N₅NbO₂requires C, 71.80; H, 7.01; N, 7.61%).

M.p.: 166–168 ^◦C (dec.). ¹H NMR (C₆D₆): d 7.74 (s, 1H, CH N),

7.63 (d, J = 8.1 Hz, 1H, aryl), 7.52 (t, J = 8.5 Hz, 2H, aryl), 7.44

(m, 2H, aryl), 7.39 (d, J = 8.5 Hz, 1H, aryl), 7.34 (m, 2H, aryl),

7.12–6.87 (m, 12H, aryl), 6.60 (d, J = 7.8 Hz, 1H, aryl), 6.41 (d,

J = 8.4 Hz, 1H, aryl), 6.35 (t, J = 7.6 Hz, 1H, aryl), 6.27 (m, 1H,

CH(NMe₂)₂), 4.28 (s, 3H, NbNMe), 3.72 (s, 3H, NbNMe), 2.19

(s, 6H, NMe₂), 2.14 (s, 3H, CH₃), 1.79 (s, 6H, NMe₂), 1.69 (s,

9H, CMe₃), 1.57 (s, 9H, CMe₃). ¹³C NMR (C₆D₆): d 162.8, 162.7,

151.5, 147.6, 138.0, 136.5, 133.9, 133.2, 131.0, 130.7, 128.4, 127.6,

125.2, 124.6, 124.3, 123.1, 121.9, 121.6, 121.1, 120.0, 116.1, 115.7,

56.2, 45.9, 45.8, 45.2, 34.0, 30.6, 29.0, 25.9, 21.6. IR (KBr, cm^-1):

n 2961 (m), 1574 (s), 1557 (s), 1455 (s), 1415 (s), 1260 (s), 1090 (s),

1018 (s), 798 (s).

Preparation of (10)₂V(NMe₂)₂(28). This compound was pre-

pared as red crystals from the reaction of 10H (0.19 g, 0.5 mmol)

with V(NMe₂)₄(0.06 g, 0.25 mmol) in toluene (20 mL) and

recrystallized from a toluene solution by a similar procedure as

in the synthesis of 11. Yield: 0.17 g (76%) (Found: C, 73.72; H,

7.55; N, 9.01. C_◦₅₆H₇₀N₆O₂V requires C, 73.90; H, 7.75; N, 9.23%).

M.p.: 214–216 C (dec.). IR (KBr, cm^-1): n 2961 (m), 2849 (m),

1555 (s), 1518 (s), 1398 (s), 1357 (s), 1260 (s), 1173 (s), 1089 (s),

1018 (s), 940 (s), 784 (s).

Preparation of (10)₂Nb(NMe₂)(g²-CH₂NMe) (29). This com-

pound was prepared as orange crystals from the reaction of 10H

(0.19 g, 0.5 mmol) with Nb(NMe₂)₅(0.08 g, 0.25 mmol) in toluene

(20 mL) and recrystallized from an n-hexane solution by a similar

procedure as in the synthesis of 11. Yield: 0.20 g (83%) (Found: C,

70.52; H, 7.19; N, 8.80. C₅₆H₆₉N₆NbO₂requires C, 70.72; H, 7.31;

N, 8.84%). M.p.: 114–116 ^◦C (dec.). ¹H NMR (C₆D₆): d 7.78 (d,

J = 8.0 Hz, 1H, aryl), 7.38 (m, 3H, aryl), 6.98 (m, 3H, aryl), 6.93

(d, J = 7.3 Hz, 1H, aryl), 6.89 (d, J = 8.0 Hz, 1H, aryl), 6.78 (d, J =

8.0 Hz, 1H, aryl), 6.73 (d, J = 7.4 Hz, 1H, aryl), 6.69 (s, 1H, aryl),

6.61 (d, J = 7.5 Hz, 1H, aryl), 6.59 (s, 1H, aryl), 6.44 (s, 1H, aryl),

6.32 (s, 1H, aryl), 3.62 (s, 3H, NbNMe), 3.47 (s, 3H, NbNMe),

3.30 (s, 3H, NbNMe), 2.98 (d, J = 5.2 Hz, 1H, CH₂), 2.87 (d, J =

5.2 Hz, 1H, CH₂), 2.25 (s, 9H, CH₃), 2.19 (s, 3H, CH₃), 2.10 (s,

9H, CH₃), 2.04 (s, 3H, CH₃), 1.86 (s, 6H, CH₃), 1.67 (s, 3H, CH₃),

1.57 (s, 3H, CH₃), 1.02 (s, 6H, CH₃). ¹³C NMR (C₆D₆): d 184.4,

175.8, 154.1, 153.0, 144.1, 144.0, 141.1, 140.0, 139.8, 138.9, 138.8,

138.3, 137.9, 135.7, 134.9, 134.8, 134.7, 134.5,,133.5, 133.1, 129.3,

127.3, 127.0, 126.8, 126.2, 126.1, 125.6, 124.6, 117.6, 116.1, 52.7,

51.4, 48.1, 45.1, 43.9, 43.4, 21.6, 20.9, 20.1, 18.4. IR (KBr, cm^-1): n

2962 (m), 2915 (m), 1560 (s), 1508 (s), 1420 (s), 1364 (s), 1260 (s),

1176 (s), 1138 (s), 1089 (s), 1017 (s), 796 (s).

Preparation of [(2)V]₂(l-NMe₂)₂(26). NMR Scale. In a

nitrogen-ﬁlled glove box, (2)V(NMe₂)₂(14, 15.0 mg, 0.02 mmol)

and C₇D₈(0.7 mL) were introduced sequentially into a J. Young

NMR tube equipped with a Teﬂon screw cap. The reaction

mixture was subsequently kept at 160 ^◦C, and the reaction was

monitored periodically by ¹H NMR spectroscopy. When this

solution was heated for 1 day, a new diamagnetic resonance

attributable to Me₂NNMe₂(singlet at 2.24 ppm)²²appeared, and

this resonance increased on prolonged heating but it did not show

any changes after heating for 4 days. The solution was cooled to

room temperature and allowed to stand for two days, and dark red

crystals were isolated, which were identiﬁed as [(2)V]₂(m-NMe₂)₂

(26) by X-ray diffraction analysis. Yield: 7.9 mg (58%) (Found: C,

74.32; H, 7.15; N, 6.22. C₈₄H₉₆N₆O₄V₂requires C, 74.43; H, 7.14;

◦

N, 6.20%). M.p.: 190–192 C (dec.). IR (KBr, cm^-1): n 2964 (m),

2921 (m), 1607 (m), 1456 (m), 1412 (m), 1260 (s), 1081 (s), 1020 (s),

799 (s).

Preparation of (10)₂Ta(NMe₂)(g²-CH₂NMe) (30). This com-

pound was prepared as pale yellow crystals from the reaction of

10H (0.19 g, 0.5 mmol) with Ta(NMe₂)₅(0.10 g, 0.25 mmol) in

toluene (20 mL) and recrystallized from a toluene solution by a

similar procedure as in the synthesis of 11. Yield: 0.21 g (81%)

(Found: C, 64.58; H, 6.86; N, 7.88. C₅₆H₆₉N₆O₂Ta requires C,

Hydrolysis of 18. Water (0.2 mL, 11 mmol) was added to a

toluene (10 mL) solution of (3)Nb(NMe₂)₃(18; 146 mg, 0.2 mmol)

with stirring at room temperature. After this solution mixture was

stirred at room temperature for 0.5 h, the solvent was removed

under vacuum. The resulting residue was further puriﬁed by ﬂash

column chromatography (hexane–ethyl acetate = 20 : 1) to give

(R)-6,6¢-Dimethyl-2,2¢-bis(mesitoylamino)-1,1¢-biphenyl (3H₂) as

a white solid. Yield: 99 mg (98%). M.p.: 220–222 ^◦C. [a]_D²⁰= -140^◦

◦

1

64.73; H, 6.69; N, 8.09%). M.p.: 118–120 C (dec.). H NMR

(C₆D₆): d 7.95 (d, J = 8.0 Hz, 1H, aryl), 7.80 (d, J = 8.0 Hz, 1H,

aryl), 7.37 (m, 2H, aryl), 6.98 (m, 3H, aryl), 6.93 (d, J = 7.3 Hz,

1H, aryl), 6.88 (d, J = 7.9 Hz, 1H, aryl), 6.77 (d, J = 8.0 Hz, 1H,

aryl), 6.72 (d, J = 7.4 Hz, 1H, aryl), 6.68 (s, 1H, aryl), 6.59 (s, 2H,

aryl), 6.43 (s, 1H, aryl), 6.31 (s, 1H, aryl), 3.77 (s, 3H, TaNMe),

3.72 (s, 3H, TaNMe), 3.41 (s, 3H, TaNMe), 2.97 (d, J = 3.4 Hz,

1

(c 1.0, CH₂Cl₂). H NMR (CDCl₃): d 7.93 (d, J = 8.0 Hz, 2H,

aryl), 7.35 (m, 4H, aryl H and NH), 7.17 (d, J = 7.6 Hz, 2H, aryl),

6.75 (s, 4H, aryl), 2.23 (s, 6H, CH₃), 2.10 (s, 12H, CH₃), 1.97 (s,

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1H, CH₂), 2.87 (d, J = 3.4 Hz, 1H, CH₂), 2.27 (s, 6H, CH₃), 2.25 (s,

3H, CH₃), 2.19 (s, 3H, CH₃), 2.08 (s, 9H, CH₃), 2.03 (s, 3H, CH₃),

1.85 (s, 6H, CH₃), 1.67 (s, 3H, CH₃), 1.56 (s, 3H, CH₃), 1.01 (s, 3H,

CH₃), 1.00 (s, 3H, CH₃). ¹³C NMR (C₆D₆): d 184.8, 175.8, 154.1,

153.0, 143.3, 141.1, 139.8, 139.7, 138.7, 138.6, 138.2, 138.0, 135.9,

135.2, 134.7, 134.4, 133.4,,133.0, 131.5, 129.3, 127.3, 127.0, 126.5,

126.2, 125.6, 125.4, 124.6, 117.3, 116.1, 57.4, 50.3, 48.0, 45.0, 43.9,

42.2, 21.0, 20.8, 20.3, 18.4. IR (KBr, cm^-1): n 2962 (m), 2924 (m),

1610 (s), 1458 (s), 1420 (s), 1364 (s), 1260 (s), 1089 (s), 1018 (vs),

799 (vs).

1.72 (s, 1H, CH), 1.66–1.56 (m, 5H, CH₂), 1.46 (d, J = 12.2 Hz,

1H, CH), 1.23–1.04 (m, 5H, CH₂), 0.83 (d, J = 6.9 Hz, 3H, CH₃).

N-(2-Methyloctyl)-N-phenylbenzamide¹⁵. HPLC (AS-H, 254

nm, hexane–2-propanol 98.5 : 1.5, 1.0 mL min^-1): t_R= 26.1 and

30.8 min. ¹H NMR (CDCl₃): d 7.18 (d, J = 5.7 Hz, 2H, aryl), 7.12

(d, J = 7.5 Hz, 3H, aryl), 7.03 (t, J = 7.5 Hz, 3H, aryl), 6.95 (d, J =

7.6 Hz, 2H, aryl), 3.77 (m, 2H, CHN), 1.68 (m, 1H, CH), 1.32 (m,

10H, CH₂), 0.87 (d, J = 6.7 Hz, 3H, CH₃), 0.79 (t, J = 6.7 Hz, 3H,

CH₃).

(2-(Bicyclo[2.2.1]heptan-2-yl)-3,4-dihydroquinolin-1-(2H)-yl)-

(phenyl)methanone¹⁵. HPLC (AS-H, 254 nm, hexane–2-

propanol 95 : 5, 1.0 mL min^-1): t_R= 8.4 and 12.1. ¹H NMR

(CDCl₃): d 7.19–7.07 (m, 5H, aryl), 7.08 (m, 1H, aryl), 6.92 (t,

J = 7.6 Hz, 1H, aryl), 6.73 (t, J = 7.6 Hz, 1H, aryl), 6.36 (br. s,

1H, aryl), 4.67 (br. s, 1H, CHN), 2.79 (m, 2H, CH₂CH₂CHN),

2.32–2.27 (m, 1H, CH₂CH₂CHN), 2.20 (s, 1H, CHnorb), 2.02 (s,

1H, CHnorb), 1.95–1.92 (m, 1H, CH₂CH₂CHN), 1.54–1.38 (m,

4H, CHnorb), 1.41–1.39 (m, 1H, CHnorb), 1.23 (d, J = 3.8 Hz,

1H, CHnorb), 1.06–0.98 (m, 3H, CHnorb).

General procedure for asymmetric hydroamination/cyclization

In a nitrogen-ﬁlled glove box, precatalyst (0.016 mmol), C₆D₆

(0.7 mL), and aminoalkene (0.16 mmol) were introduced sequen-

tially into a J. Young NMR tube equipped with Teﬂon screw cap.

The reaction mixture was subsequently kept at 140 ^◦C to achieve

hydroamination, and the reaction was monitored periodically by

¹H NMR spectroscopy. The cyclic amine was vacuum transferred

from the J. Young NMR tube into a 25 mL Schlenk ﬂask which

contained 31 mg (0.16 mmol) of (S)-(+)-O-acetylmandelic acid.

The resulting mixture was stirred at room temperature for 2 h and

the volatiles were removed in vacuo. The resulting diastereomeric

salt was then dissolved in CDCl₃and the enantiomeric excesses

were determined by ¹H NMR spectroscopy.^8a

X-Ray crystallography

Single-crystal X-ray diffraction measurements were carried out on

a Bruker Smart APEX II CCD diffractometer at 150(2) K or on

a Rigaku Saturn CCD diffractometer at 113(2) K using graphite

˚

monochromated Mo Ka radiation (l = 0.71070 A) or Cu-Ka

General procedure for asymmetric hydroaminoalkylation

˚

radiation (l = 1.54187 A). An empirical absorption correction was

In a nitrogen-ﬁlled glove box, precatalyst (0.005 mmol), C₇D₈

(0.7 mL), amine (0.10 mmol), and alkene (0.15 mmol) were

introduced sequentially into a J. Young NMR tube equipped with

Teﬂon screw cap. The reaction mixture was subsequently kept

at 130 ^◦C or 160 ^◦C to achieve hydroaminoalkylation, and the

applied using the SADABS program.²³All structures were solved

by direct methods and reﬁned by full-matrix least squares on F²

using the SHELXL-97 program package.²⁴All the hydrogen atoms

were geometrically ﬁxed using the riding model. The disordered

solvents in the voids of the complexes were modelled or removed

by using the SQUEEZE program.²⁵The absolute conﬁgurations of

compounds were determined according to the literature method.²⁶

Program PLATON was used to create HKL5 data ﬁle for the twin

structures.²⁷The crystal data and experimental data for 3H₂, 12,

13, 15–30, and 34 are summarized in Tables 1–4. Selected bond

lengths and angles are listed in Tables 5 and 6.

1

reaction was monitored periodically by H NMR spectroscopy.

The mixture was added to a CH₂Cl₂solution (5 mL) of benzoyl

chloride (140 mg, 116mL, 1.0 mmol) and Et₃N (152 mg, 1.5 mmol)

with stirring at room temperature, and the reaction mixture was

stirred continuously for 2 h. After removal of the solvent, diethyl

ether (10 mL) and 2 M NaOH (10 mL) were added and stirred

for 0.5 h, then extracted with diethyl ether (20 mL ¥ 3) and

washed with brine (20 mL). The combined organic layers were

dried with anhydrous Na₂SO₄, and then ﬁltered. The solvent was

removed, and the resulting residue was further puriﬁed by ﬂash

column chromatography. The enantiomeric excesses of benzamide

derivative were determined by HPLC analysis using a Chiralcel

AS-H or OD-H column.

Results and discussion

Complexes with C₂-symmetric ligands

Group 5 metal amidate complexes can be efﬁciently prepared

via amine elimination reaction between

5 metal amides

and protic amidate ligands. For example, treatment of

V(NMe₂)₄or M(NMe₂)₅(M = Nb, Ta) with 1 equiv of (R)-2,2¢-

bis(mesitoylamino)-1,1¢-binaphthyl (1H₂), (R)-5,5¢,6,6¢,7,7¢,8,8¢-

octahydro-2,2¢-bis(mesitoylamino)-1,1¢-binaphthyl (2H₂), (R)-

6,6¢-dimethyl-2,2¢-bis(mesitoylamino)-1,1¢-biphenyl (3H₂), (R)-

2,2¢ - bis(mesitylenesulfonylamino) - 6,6¢ - dimethyl - 1,1¢ - biphenyl

N-(Bicyclo[2.2.1]heptan-2-ylmethyl)-N-phenylbenzamide¹⁵.

HPLC (AS-H, 254 nm, hexane–2-propanol 98 : 2, 1.0 mL min^-1):

t_R= 37.9 and 49.9 min. ¹H NMR (CDCl₃): d 7.18 (d, J = 6.9 Hz,

2H, aryl), 7.13 (d, J = 7.2 Hz, 3H, aryl), 7.05 (d, J = 7.7 Hz, 3H,

aryl), 6.95 (d, J = 7.6 Hz, 2H, aryl), 3.86 (m, 1H, CHN), 3.61 (m,

1H, CHN), 2.16 (s, 1H, CH), 2.03 (s, 1H, CH), 1.60 (m, 1H, CH),

1.39 (m, 3H, CH₂), 1.22 (m, 1H, CH₂), 1.21 (m, 2H, CH₂), 1.09

(m, 2H, CH₂).

(4H₂)

or

(R)-2,2¢-bis(diphenylthiophosphoramino)-1,1¢-

binaphthyl (5H₂) at room temperature gives, after recrystallization

from a benzene, toluene or n-hexane solution, the vanadium

amides (1)V(NMe₂)₂(11), (2)V(NMe₂)₂(14), (3)V(NMe₂)₂(17)

and (5)V(NMe₂)₂(22), niobium amides (1)Nb(NMe₂)₃(12),

(2)Nb(NMe₂)₃(15), (3)Nb(NMe₂)₃(18) and (4)Nb(NMe₂)₃

(20), and tantalum amides (1)Ta(NMe₂)₃(13), (2)Ta(NMe₂)₃

(16), (3)Ta(NMe₂)₃(19) and (4)Ta(NMe₂)₃(21) respectively,

N-(2-Cyclohexylpropyl)-N-phenylbenzamide¹⁵. HPLC (OD-

H, 254 nm, hexane–2-propanol 99 : 1, 0.5 mL min^-1): t_R= 36.7

and 40.1 min. ¹H NMR (CDCl₃): d 7.16 (d, J = 7.6 Hz, 2H, aryl),

7.12 (d, J = 7.7 Hz, 3H, aryl), 7.06 (t, J = 7.8 Hz, 3H, aryl), 6.94

(d, J = 7.6 Hz, 2H, aryl), 3.91 (m, 1H, CHN), 3.76 (m, 1H, CHN),

1552 | Dalton Trans., 2011, 40, 1547–1566

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Table 1 Crystal data and experimental parameters for compounds 3H₂, 12, 13, 15, and 16

Compound

3H₂

12

13

15

16

Formula

C₃₄H₃₆N₂O₂

504.65

Orthorhombic

P2₁2₁2₁

8.692(1)

17.638(2)

18.832(3)

90

2886.9(6)

4

113(2)

Rigaku Saturn CCD

0.072 (Mo-Ka)

1.161

0.24 ¥ 0.22 ¥ 0.18

1080

4.62 to 55.70

30162

3858 (R_int= 0.0440)

C₄₆H₅₂N₅NbO₂

799.84

Monoclinic

P2₁

11.680(2)

13.465(2)

13.696(2)

90

108.56(1)

90

2041.9(5)

2

113(2)

Rigaku Saturn CCD

0.338 (Mo-Ka)

1.301

0.42 ¥ 0.38 ¥ 0.36

840

3.14 to 55.76

26215

9650 (R_int= 0.0402)

8460

0.89, 0.87

0.030

0.067

C₄₆H₅₂N₅O₂Ta

887.88

Monoclinic

P2₁

11.687(2)

13.451(3)

13.686(3)

90

108.63(3)

90

2038.8(7)

2

113(2)

Rigaku Saturn CCD

2.739 (Mo-Ka)

1.446

0.22 ¥ 0.18 ¥ 0.14

904

3.14 to 55.84

18042

9255 (R_int= 0.0397)

9010

0.70, 0.58

0.035

0.084

C₄₆H₆₀N₅NbO₂

807.90

Monoclinic

P2₁

11.934(2)

14.079(3)

13.534(2)

90

111.01(1)

90

2122.8(6)

2

113(2)

Rigaku Saturn CCD

2.633 (Cu-Ka)

1.264

0.22 ¥ 0.20 ¥ 0.20

856

7.00 to 144.18

21782

6900 (R_int= 0.0448)

6542

0.62, 0.60

0.037

0.100

C₄₆H₆₀N₅O₂Ta

895.94

Monoclinic

P2₁

11.832(2)

14.325(3)

13.481(3)

90

111.28(3)

90

2129.1(7)

2

113(2)

Rigaku Saturn CCD

2.624 (Mo-Ka)

1.398

0.32 ¥ 0.30 ¥ 0.28

920

3.24 to 55.76

22072

9882 (R_int= 0.0529)

7509

0.53, 0.49

0.035

0.085

Formula weight

Crystal system

Space group

˚

a (A)

˚

b (A)

˚

c (A)

a (^◦)

b (^◦)

g (^◦)

3

˚

V (A )

Z

T(K)

Diffractometer

m (mm^-1)

D_c(g cm^-3)

Size (mm)

F(000)

2q range (^◦)

No. of reﬂns, collected

No. of unique reﬂns

No. of obsd reﬂns [I > 2s(I)] 3661

abscorr (T_max, T_min

R [I > 2s(I)]

R_w[I > 2s(I)]

wR₂(all data)

gof

)

0.99, 0.98

0.038

0.095

0.097

1.03

0.068

1.01

0.085

1.12

0.124

1.18

0.086

0.94

Flack c

CCDC

-0.025(18)

775657

-0.001(7)

775658

0.001(11)

775659

-0.016(7)

775660

739122

Table 2 Crystal data and experimental parameters for compounds 17–21

Compound

17

18

19

20

21

Formula

C₃₈H₄₆N₄O₂V

641.73

Monoclinic

P2₁

11.605(3)

10.657(2)

13.937(2)

90

92.28(1)

90

1722.2(6)

2

C₄₀H₅₂N₅NbO₂

727.78

Triclinic

P(-1)

9.757(2)

12.302(3)

15.936(3)

98.09(1)

103.42(1)

90.47(1)

1840.3(6)

2

C₄₀H₅₂N₅O₂Ta

815.82

Triclinic

P(-1)

9.761(2)

12.259(2)

15.969(3)

82.37(1)

77.15(1)

89.89(1)

1845.7(1)

2

C₃₈H₅₂N₅NbO₄S₂

799.88

Monoclinic

P2₁/c

15.136(2)

16.063(2)

17.379(2)

90

113.53(1)

90

3874.1(7)

4

113(2)

Rigaku Saturn CCD

3.900 (Cu-Ka)

1.371

0.22 ¥ 0.20 ¥ 0.16

1680

6.36 to 145.14

41218

7536 (R_int= 0.0458)

7174

0.57, 0.48

0.032

0.082

C₃₈H₅₂N₅O₄S₂Ta

887.92

Monoclinic

P2₁/c

15.181(2)

16.052(2)

17.371(2)

90

113.33(1)

90

3887.1(8)

4

113(2)

Rigaku Saturn CCD

6.583 (Cu-Ka)

1.517

0.22 ¥ 0.20 ¥ 0.18

1808

6.34 to 144.52

40234

7532 (R_int= 0.0456)

7251

0.38, 0.33

0.030

0.077

Formula weight

Crystal system

Space group

˚

a (A)

˚

b (A)

˚

c (A)

a (^◦)

b (^◦)

g (^◦)

3

˚

V (A )

Z

T (K)

113(2)

Diffractometer

Rigaku Saturn CCD Rigaku Saturn CCD Rigaku Saturn CCD

1.237 (Mo-Ka)

1.237

0.28 ¥ 0.26 ¥ 0.24

682

2.92 to 52.00

8487

m (mm^-1)

0.368 (Mo-Ka)

1.313

0.26 ¥ 0.22 ¥ 0.20

768

3.34 to 55.76

31741

8743 (R_int= 0.0402)

7881

0.93, 0.91

0.031

0.081

3.018 (Mo-Ka)

1.468

0.24 ¥ 0.22 ¥ 0.20

832

2.64 to 55.78

22062

8670 (R_int= 0.0344)

8385

0.58, 0.53

0.035

0.086

D_c(g cm^-3)

size (mm)

F(000)

2q range (^◦)

No. of reﬂns, collected

No. of unique reﬂns

8487 (R_int= 0.0000)

No. of obsd reﬂns [I > 2s(I)] 8413

abscorr (T_max, T_min

R [I > 2s(I)]

R_w[I > 2s(I)]

wR₂(all data)

gof

)

0.93, 0.91

0.059

0.194

0.195

1.21

0.083

1.09

0.089

1.21

0.099

1.10

0.078

1.14

Flack c

CCDC

0.08(3)

791864

775661

775662

791865

791866

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The Royal Society of Chemistry 2011

Dalton Trans., 2011, 40, 1547–1566 | 1553

©

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Table 3 Crystal data and experimental parameters for compounds 22–26

Compound

22

23

24

25·C₇H₈

26

Formula

C₄₈H₄₄N₄P₂S₂V

853.87

C₄₀H₅₀N₄O₂V

669.78

Triclinic

P1

11.027(1)

11.422(1)

14.924(2)

85.96(1)

85.29(1)

83.18(1)

1856.6(3)

2

C₄₈H₆₆N₄O₂V

781.99

Triclinic

P(-1)

12.433(4)

15.427(5)

15.628(5)

98.60(1)

112.41(1)

94.67(1)

2708.5(15)

2

C₅₅H₆₄N₅NbO₂

920.02

Triclinic

P1

9.550(1)

10.944(1)

11.425(1)

97.97(1)

93.03(1)

99.57(1)

1162.5(2)

1

C₈₄H₉₆N₆O₄V₂

1355.55

Monoclinic

C2

24.254(5)

21.750(3)

21.964(4)

90

115.94(1)

90

10419(3)

4

113(2)

Rigaku Saturn CCD

1.799 (Cu-Ka)

0.864

Formula weight

Crystal system

Space group

Orthorhombic

P2₁2₁2₁

9.737(1)

19.029(2)

22.878(3)

90

4239.1(8)

4

113(2)

˚

a (A)

˚

b (A)

˚

c (A)

a (^◦)

b (^◦)

g (^◦)

3

˚

V (A )

Z

T (K)

113(2)

150(2)

113(2)

Diffractometer

Rigaku Saturn CCD Rigaku Saturn CCD Bruker Smart CCD

0.447 (Mo-Ka)

1.338

0.24 ¥ 0.22 ¥ 0.20

1780

Rigaku Saturn CCD

0.307 (Mo-Ka)

1.314

0.26 ¥ 0.22 ¥ 0.20

486

m (mm^-1)

0.305 (Mo-Ka)

1.198

0.26 ¥ 0.24 ¥ 0.20

714

0.217 (Mo-Ka)

0.959

0.32 ¥ 0.27 ¥ 0.13

842

D_c(g cm^-3)

Size (mm)

0.34 ¥ 0.32 ¥ 0.28

2880

F(000)

2q range (deg)

No. of reﬂns, collected

No. of unique reﬂns

2.78 to 52.00

34533

8312 (R_int= 0.0438)

3.60 to 65.74

28339

21065 (R_int= 0.0361)

14747

0.94, 0.92

0.037

0.077

0.083

0.93

3.58 to 55.12

15957

15957 (R_int= 0.0000)

9327

0.96, 0.95

0.083

0.219

0.245

0.95

3.82 to 70.24

20069

15623 (R_int= 0.0317)

14011

0.94, 0.92

0.037

0.077

0.080

1.01

4.48 to 145.36

72632

19202 (R_int= 0.0507)

16455

0.63, 0.58

0.036

0.085

0.089

1.00

No. of obsd reﬂns [I > 2s(I)] 7694

abscorr (T_max, T_min

R [I > 2s(I)]

R_w[I > 2s(I)]

wR₂(all data)

gof

)

0.92, 0.90

0.073

0.158

0.162

1.11

Flack c

CCDC

0.03(4)

791867

-0.005(8)

791868

-0.003(14)

791870

0.033(3)

800129

791869

Table 4 Crystal data and experimental parameters for compounds 27–30, and 34

Compound

27·3C₇H₈

28

29

30

34

Formula

C₈₉H₉₄N₆O₂V

1330.64

Monoclinic

P2₁

16.237(5)

10.878(3)

21.013(6)

90

98.07(1)

90

3674.5(18)

2

C₅₆H₇₀N₆O₂V

910.12

C₅₆H₆₉N₆NbO₂

951.08

C₅₆H₆₉N₆O₂Ta

1039.12

Orthorhombic

P2₁2₁2₁

13.166(2)

13.468(2)

32.490(5)

90

C₂₁H₂₃NO

305.40

Triclinic

P1

9.39(2)

9.44(2)

9.48(2)

88.28(9)

81.19(5)

89.14(9)

830(3)

Formula weight

Crystal system

Space group

Orthorhombic

P2₁2₁2₁

13.709(2)

18.865(3)

19.441(3)

90

Orthorhombic

P2₁2₁2₁

13.169(1)

13.430(1)

32.594(1)

90

5764.6(3)

4

113(2)

˚

a (A)

˚

b (A)

˚

c (A)

a (^◦)

b (^◦)

g (^◦)

90

3

˚

V (A )

5027.9(13)

4

113(2)

5761.4(14)

4

113(2)

Rigaku Saturn CCD

1.949 (Mo-Ka)

1.198

0.34 ¥ 0.20 ¥ 0.20

2144

2.50 to 55.74

40645

13699 (R_int= 0.0395)

13138

0.70, 0.56

0.029

0.077

0.078

1.07

Z

T (K)

2

113(2)

Diffractometer

Rigaku Saturn CCD Rigaku Saturn CCD Rigaku Saturn CCD

0.189 (Mo-Ka)

1.203

0.22 ¥ 0.18 ¥ 0.16

1418

3.42 to 55.74

45257

Rigaku Saturn CCD

0.574 (Cu-Ka)

1.222

0.22 ¥ 0.16 ¥ 0.12

328

9.38 to 145.08

11805

4974 (R_int= 0.0458)

4582

0.93, 0.88

0.042

0.111

0.128

1.12

m (mm^-1)

0.244 (Mo-Ka)

1.202

0.32 ¥ 0.26 ¥ 0.24

1948

3.00 to 55.72

52617

11984 (R_int= 0.0479)

11267

0.94, 0.93

0.038

0.083

0.250 (Mo-Ka)

1.096

0.26 ¥ 0.24 ¥ 0.22

2016

2.50 to 55.76

41838

13705 (R_int= 0.0432)

12696

0.95, 0.94

0.043

0.113

D_c(g cm^-3)

Size (mm)

F(000)

2q range (^◦)

No. of reﬂns, collected

No. of unique reﬂns

17377 (R_int= 0.0449)

No. of obsd reﬂns [I > 2s(I)] 15026

abscorr (T_max, T_min

R [I > 2s(I)]

R_w[I > 2s(I)]

wR₂(all data)

gof

)

0.97, 0.96

0.053

0.127

0.133

1.04

0.084

1.05

0.115

1.07

Flack c

CCDC

0.0333(12)

791871

0.000(12)

791872

-0.02(2)

791873

0.000(6)

791874

0.2(3)

775663

1554 | Dalton Trans., 2011, 40, 1547–1566

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The Royal Society of Chemistry 2011

©

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˚

Table 5 Selected bond distances (A) and bond angles (deg) for compounds 12, 13 and 15–21

Compound

12 (Nb)

13 (Ta)

15 (Nb)

16 (Ta)

17 (V)

18 (Nb)

19 (Ta)

20 (Nb)

21 (Ta)

M–N (av.)

M-N(Me₂)₂

2.064(2)

1.974(2)

1.962(2)

2.020(2)

2.185(2)

2.082(2)

2.675(2)

2.064(3)

1.963(3)

1.983(3)

2.017(3)

2.155(3)

2.062(3)

2.656(4)

2.073(4)

2.024(4)

1.972(4)

1.996(4)

2.186(3)

2.076(3)

2.658(4)

2.062(5)

1.960(5)

2.031(5)

1.977(4)

2.163(4)

2.065(4)

2.655(6)

2.028(4)

1.858(4)

1.852(4)

2.053(1)

2.000(1)

1.961(1)

1.972(1)

2.180(1)

2.069(1)

2.652(2)

2.054(3)

1.979(2)

2.002(3)

1.968(2)

2.164(2)

2.051(2)

2.645(3)

2.075(2)

1.935(2)

1.947(2)

2.010(2)

2.052(2)

1.939(2)

1.948(2)

2.008(2)

M–O

2.098(3)

2.072(3)

2.529(5)

2.551(5)

357.9(4)

358.3(4)

M–C (carbonyl)

Sum angle of N(Me₂)₂

360.0(2)

356.3(2)

359.7(2)

74.1(2)

357.5(4)

359.8(4)

359.9(4)

75.0(4)

359.4(5)

356.9(4)

359.9(4)

78.0(4)

359.1(5)

359.2(5)

360.0(5)

78.9(5)

159.2(1)

159.8(1)

158.9(1)

77.4(1)

359.8(3)

359.2(3)

359.2(2)

77.0(3)

359.9(2)

359.6(2)

68.7(2)

359.9(2)

359.7(2)

359.5(2)

68.2(2)

Torsion (aryl-aryl)

62.8(4)

˚

Table 6 Selected bond distances (A) and bond angles (deg) for compounds 22–30

Compound

22 (V)

23 (V)

24 (V)

25 (Nb)

26 (V)

27 (V)

28 (V)

29 (Nb)

30 (Ta)

M–N (av.)

M-N(Me₂)₂

1.907(8)

1.794(6)

1.881(6)

2.038(2)

1.882(2)

1.886(2)

1.969(1)

1.970(1)

2.045(3)

1.880(3)

1.891(3)

1.963(2)

1.951(2)

2.142(2)

1.976(2)

2.086(2)

2.064(2)

2.079(2)

2.078(2)

2.057(2)

1.978(2)

1.865(2)

1.873(2)

2.149(2)

1.981(1)

1.861(1)

1.875(1)

2.141(1)

2.150(1)

2.099(2)

1.977(2)

2.094(3)

1.986(3)

M–O

2.040(1)

2.005(1)

2.166(2)

2.206(2)

2.201(3)

2.621(3)

2.650(3)

359.8(3)

2.202(3)

2.157(3)

2.194(4)

2.611(3)

2.645(4)

359.8(3)

M–C

M–C (carbonyl)

2.489(2)

2.472(2)

2.513(2)

2.528(2)

357.9(1)

358.7(2)

70.2(2)

2.516(2)

2.518(2)

358.7(2)

sum angle of N(Me₂)₂

359.2(6)

359.9(8)

76.4(8)

358.9(2)

359.5(2)

72.4(2)

358.9(3)

359.6(3)

71.4(3)

359.7(2)

53.5(2)

358.2(2)

65.6(1)

torsion (aryl-aryl)

72.5(2)

73.9(2)

72.3(3)

74.7(3)

73.3(3)

71.5(3)

70.6(2)

in good yields (Schemes 1–5). Reaction of V(NMe₂)₄with 1

equiv of the Schiff base ligands (R)-2,2¢-bis[(3-tert-butyl-2-

hydroxybenzylidene)amino]-6,6¢-dimethyl-1,1¢-biphenyl

(6H₂)

or (R)-2,2¢-bis[(3,5-di-tert-butyl-2-hydroxybenzylidene)amino]-

6,6¢-dimethyl-1,1¢-biphenyl (7H₂) also furnishes the expected

vanadium amides (6)V(NMe₂)₂(23) and (7)V(NMe₂)₂(24)

respectively, in good yields (Scheme 6). However, under

similar reaction conditions, treatment of Nb(NMe₂)₅with 1

equiv of the Schiff base ligand (R)-2,2¢-bis[(3-tert-butyl-2-

hydroxybenzylidene)amino]-1,1¢-binaphthyl (8H₂) does not lead

to the expected amide complex (8)Nb(NMe₂)₃, instead, a chiral

solvated niobium complex [2-(3-Me₃C-2-O-C₆H₃CHN)-2¢-(N)-

C₂₀H₁₂][2-(Me₂N)₂CH-6-CMe₃-C₆H₃O]NbNMe₂·C₇H₈(25·C₇H₈)

has been isolated from a toluene solution in 73% yield (Scheme

7), in which the amide complex (8)Nb(NMe₂)₃is degraded by

the dimethylamino group via 1,2-migratory insertion at the imine

unit of 8, presumably due to the electronic effect between the

ligand and metal ion.^4k

The niobium and tantalum amidate complexes are stable

in a C₇D₈solution at or below 160 ^◦C, while the vanadium

amidate complexes degrade via diemthylamino group elimination

at this temperature. For example, the vanadium amidate complex

(2)V(NMe₂)₂(14) is stable in a C₇D₈solution at or below 140 ^◦C,

while heating the complex in C₇D₈solution at 160 ^◦C for four

days leads to the isolation of the vanadium(III) complex [(2)V]₂(m-

NMe₂)₂(26) in 58% yield (Scheme 2). The complexes 11–26 are

stable in dry nitrogen atmosphere, while they are very sensitive

to moisture. They are soluble in organic solvents such as THF,

DME, pyridine, toluene, and benzene, and only slightly soluble in

n-hexane. They have been characterized by various spectroscopic

Scheme 1 Synthesis of complexes 11–13.

techniques and elemental analyses. The ¹H NMR spectra of

niobium complexes 12, 15, 18 and 20, and tantalum complexes

13, 16, 19 and 21 support that the ratio of amino group NMe₂and

ligand anion 1, 2, 3 or 4 is 3 : 1, and exhibit a singlet resonance at

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Scheme 3 Synthesis of complexes 17–19.

Scheme 2 Synthesis of complexes 14–16 and 26.

Scheme 4 Synthesis of complexes 20 and 21.

about 3.35 for niobium amides and 3.55 ppm for tantalum amides,

respectively, attributable to the NMe₂groups. These results are

in line with their ¹³C NMR spectra. The solid-state structures of

complexes 12, 13, and 15–26 have been further conﬁrmed by X-ray

diffraction analyses.

The X-ray diffraction analyses reveal that the amides 12 and

13, 15 and 16, or 18 and 19 are isostructural, respectively. In each

molecule (1)M(NMe₂)₃, (2)M(NMe₂)₃or (3)M(NMe₂)₃, the M⁵⁺

is s-bound to one nitrogen atom and two oxygen atoms from

the ligand anion 1, 2 or 3, and three nitrogen atoms from amino

groups NMe₂in a distorted-octahedron geometry (Fig. 1–3) with

Scheme 5 Synthesis of complex 22.

˚

ranged from 1.961(1) to 2.024(4) A for Nb and from 1.960(5) to

the average M–N distance ranging from 2.053(1) to 2.073(4) A)

˚

for Nb and from 2.054(3) to 2.064(3) A for Ta. The potentially

2.031(5) A for Ta, and the planar geometry around the N(3), N(4)

tetradentate amidate ligand adopts a tridentate binding mode,

which results in the M–O(1) distance being slightly longer than M–

O(2). The solution NMR characterization data are also consistent

with this C₁symmetric binding mode, as the ¹³C NMR spectra

show two signals at about 158.0 ppm and 175 ppm for the carbonyl

group, attributable to an alkoxyimine (kO) binding and a bidentate

(kO, kN) binding, respectively.¹⁵The short M–NMe₂distances

and N(5) nitrogen atoms indicates that the three nitrogen atoms

with sp²hybridization are engaged in N(p_p)→M(d_p) interactions.

The twisting between the aryl rings of torsion angle ranges from

74.1(2) to 78.9(5)^◦, indicating they are almost perpendicular to

each other.

The single-crystal X-ray diffraction analyses show that com-

plexes 20 and 21 are isostructural. In each molecule (4)M(NMe₂)₃,

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Fig. 1 Molecular structures of 12 (M = Nb) and 13 (M = Ta) (thermal

ellipsoids drawn at the 35% probability level).

Scheme 6 Synthesis of complexes 23 and 24.

Fig. 2 Molecular structures of 15 (M = Nb) and 16 (M = Ta) (thermal

ellipsoids drawn at the 35% probability level).

Scheme 7 Synthesis of complex 25.

the potentially tetradentate ligand adopts a bidentate binding

mode, in which the oxygen atoms are far away from the metal

center, the M⁵⁺is s-bound to two nitrogen atoms from the ligand

anion 4 and three nitrogen atoms from three amino groups NMe₂

in a distorted-trigonal-bipyramidal geometry (Fig. 4) with the

twisting between the phenyl rings of torsion angle is 68.7(2)^◦for

Nb, and 68.2(2)^◦for Ta, which is smaller than those found in 18

(77.4(1)^◦) and 19 (77.0(3)^◦).

The single-crystal X-ray diffraction analyses show that there are

two molecules (6)V(NMe₂)₂(23) in the lattice. In each molecule

of (3)V(NMe₂)₂(17), (6)V(NMe₂)₂(23) or (7)V(NMe₂)₂(24), the

V⁴⁺is s-bound to two nitrogen atoms and two oxygen atoms from

the ligand 3, 6 or 7 and two nitrogen atoms from two amino

groups NMe₂in a distorted-octahedron geometry (Fig. 5–7) with

˚

average M–N distance of 2.075(2) A for Nb, and 2.052(2) A)

for Ta, respectively. The short M–NMe₂distances of 1.935(2),

˚

1.947(2) and 2.101(2) A for Nb, and 1.939(2), 1.948(2) and 2.008(2)

˚

A for Ta, and the planar geometry around the N(3), N(4) and

˚

N(5) nitrogen atoms indicate that the three nitrogen atoms with

sp²hybridization are engaged in N(p_p)→M(d_p) interactions. The

the average V–N distance of 2.028(4) A for 17, 2.038(2) A for

˚

23, and 2.045(3) A for 24, respectively, and the average V–O

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Fig. 5 Molecular structure of 17 (thermal ellipsoids drawn at the 35%

probability level).

Fig. 3 Molecular structures of 18 (M = Nb) and 19 (M = Ta) (thermal

ellipsoids drawn at the 35% probability level).

Fig. 6 Molecular structures of 23 (thermal ellipsoids drawn at the 35%

probability level).

Fig. 4 Molecular structures of 20 (M = Nb) and 21 (M = Ta) (thermal

ellipsoids drawn at the 35% probability level).

17, which is smaller than those found in 23 (72.4(2)^◦) and 24

(71.4(3)^◦).

˚

distance of 2.085(3) A for 17, 1.970(1) A for 23, and 1.957(2)

˚

A for 24, respectively. The short V–NMe₂distances of (1.858(4)

The single-crystal X-ray diffraction analysis shows that the

potentially tetradentate ligand in complex 22 adopts a bidentate

binding mode, in which the sulfur atoms are far away from the

metal center, the V⁴⁺is s-bound to two nitrogen atom from

the ligand anion 5 and two nitrogen atoms from two amino

groups NMe₂in a distorted-tetrahedron geometry (Fig. 8) with

˚

and 1.852(4) A) for 17, (1.882(2) and 1.886(2) A) for 23, and

˚

(1.880(3) and 1.891(3) A) for 24 and the planar geometry around

the nitrogen atom of NMe₂indicate that the nitrogen atoms with

sp²hybridization are engaged in N(p_p)→V(d_p) interactions. The

twisting between the phenyl rings of torsion angle is 62.8(4)^◦for

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Fig. 7 Molecular structures of 24 (thermal ellipsoids drawn at the 35%

probability level).

Fig. 9 Molecular structure of 25 (thermal ellipsoids drawn at the 35%

probability level).

twisting between the naphthyl rings of torsion angle is 53.5(2)^◦,

which is smaller than that found in 12 (74.1(2)^◦).

The single-crystal X-ray diffraction analysis shows that there

are two molecules [(2)V]₂(m-NMe₂)₂(26) in the lattice. In each

molecule [(2)V]₂(m-NMe₂)₂, the V³⁺is s-bound to two nitrogen

atoms and two oxygen atoms from the ligand 3 and two doubly

bridging nitrogen atoms from two amino groups NMe₂in a

distorted-octahedron geometry (Fig. 10) with the average V–N

˚

distance of 2.086(2) A, and the average V–O distance of 2.068(2)

˚

A, respectively. These structural data are close to those found in

17. The twisting between the aryl rings of torsion angle is 65.6(1)^◦,

which is smaller than those found in 15 (77.9(5)^◦) and 16 (78.9(5)^◦).

Fig. 8 Molecular structure of 22 (thermal ellipsoids drawn at the 35%

probability level).

˚

the average V–N distance of 1.907(8) A. The short V–N distances

˚

of 1.979(4), 1.972(4), 1.881(6) and 1.794(6) A, and the planar

geometry around the N(1), N(2), N(3) and N(4) nitrogen atoms

indicate that the four nitrogen atoms with sp²hybridization are

engaged in N(p_p)→M(d_p) interactions. These structural data are

very close to those found in V(NMe₂)₄.²⁸The twisting between the

naphthyl rings of torsion angle is 76.4(8)^◦, which is comparable to

those found in 12 (74.1(2)^◦) and 13 (75.0(4)^◦).

The single-crystal X-ray diffraction analysis shows that there

is one 25 and one solvate toluene molecules in the lattice. The

Nb⁵⁺is s-bound to four nitrogen atoms and two oxygen atoms

in a distorted-octahedron geometry (Fig. 9) with the average

Fig. 10 Molecular structure of 26 (thermal ellipsoids drawn at the 35%

probability level).

˚

Nb–N distance of 2.142(2) A and the average Nb–O distance

The crystal data show that the crystal structures of 18–21 and

24 are centrosymmetric with space groups of P(-1) or P2₁/c,

however, no racemization of the chiral ligands has been observed

upon complex formation,^13k,30as deliberate decomposition of

the complexes followed by re-isolation and characterization of

proligand results in identical values for speciﬁc rotations. For

˚

of 2.023(1) A. The short Nb–N(5) distance of 1.976(2) and

the planar geometry around the N(5) nitrogen atom indicate

that this nitrogen atom with sp²hybridization is engaged in

N(p_p)→M(d_p) interactions. The shorter Nb–N(2) distance of is

1.813(2), supporting the formation of an imido–metal bond.²⁹The

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example, hydrolysis of the complex (3)Nb(NMe₂)₃(18) leads to the

re-isolation of the chiral ligand (R)-3H₂in 98% yield (Scheme 3),

which has been further identiﬁed by ¹H NMR and speciﬁc rotation

spectroscopic techniques and X-ray diffraction analysis, indicating

that the few (in most cases, only one or two) racemic crystals

suitable for X-ray diffraction analysis may result from the starting

material, (R)-2,2¢-diamino-6,6¢-dimethyl-1,1¢-biphenyl with 98%

enantiomeric purity.

The single-crystal X-ray diffraction analysis shows that 3H₂

crystallizes ina C₂-symmetric distorted-tetrahedral geometry (Fig.

11). The twisting between the phenyl rings of torsion angle

is 85.5(2)^◦, which is larger than that found in (R)-2-amino-2¢-

(dimethylamino)-6,6¢-dimethyl-1,1¢-biphenyl (81.6(2)^◦).^16g

Scheme 8 Synthesis of complex 27.

ppm, whereas the N-methyl of the metallaaziridine and the methyl

signals of the dimethylamino ligand resonate at about 3.65, 3.50

and 3.35 ppm, respectively. These results are in line with their ¹³

C

Fig. 11 Molecular structure of 3H₂(thermal ellipsoids drawn at the 35%

probability level).

NMR spectra. The solid-state structures of complexes 27–30 have

been further established by X-ray diffraction analyses.

The single-crystal X-ray diffraction analyses show that there

is one molecule (9)₂V(NMe₂)₂(27) and three solvate toluene

molecules in the lattice. In each molecule of (9)₂V(NMe₂)₂(27)

or (10)₂V(NMe₂)₂(28), the substituted Me₂N groups are far away

from the metal center, and the V⁴⁺is s-bound to two nitrogen

atoms and two oxygen atoms from the ligands 9 or 10 and two

nitrogen atoms from two amino groups NMe₂in a distorted-

octahedron geometry (Fig. 12 and 13) with the average V–N

Complexes with C₁-symmetric ligands

Simultaneously, we have also developed a series of group 5

metal amide complexes via amine elimination reaction between

V(NMe₂)₄or M(NMe₂)₅(M = Nb, Ta) and C₁-symmetric

amidate ligands, (S)-2-(mesitoylamino)-2¢-(dimethylamino)-1,1¢-

binaphthyl (9H), (R)-2-(mesitoylamino)-2¢-(dimethylamino)-6,6¢-

dimethyl-1,1¢-biphenyl (10H). Treatment of V(NMe₂)₄with 2

equiv of 9H or 10H at room temperature gives, after recrystal-

lization from a toluene solution, the chiral bis-ligated vanadium

amides (9)₂V(NMe₂)₂·3C₇H₈(27·3C₇H₈) and (10)V(NMe₂)₂(28)

respectively, in good yields (Schemes 8 and 9). However, under

similar reaction conditions, reaction of M(NMe₂)₅(M = Nb, Ta)

with 2 equiv of the amidate ligand of 10H does not lead to the

expected bis-ligated amide complexes (10)₂M(NMe₂)₃; instead,

˚

distance of 1.978(2) A for 27, and 1.981(1) A for 28, respectively,

˚

and the average V–O distance of 2.149(2) A for 27, and 2.146(1) A

for 28, respectively. The short M–NMe₂distances of 1.865(2) and

˚

1.873(2) A for 27, and 1.861(1) and 1.875(1) A for 28, respectively,

and the planar geometry around the nitrogen atoms of NMe₂

groups indicate that both nitrogen atoms with sp²hybridization

are engaged in N(p_p)→V(d_p) interactions. These structural data

are very close to those found in 17. The torsion angles between the

aryl ring are 70.2(2) and 70.6(2)^◦for 27, and 72.5(2) and 73.9(2)^◦

for 28, respectively, which are comparable to those found in

(9)₂Ti(NMe₂)₂(70.4(2) and 69.9(4)^◦) and (10)₂Ti(NMe₂)₂(70.0(2)

and 69.7(2)^◦).^16g

2

chiral bis-ligated metallaaziridine complexes (10)₂M(NMe₂)(h -

CH₂NMe) (M = Nb (29), Ta (30)) have been isolated in good

yields (Scheme 9), in which the amide complexes (10)₂M(NMe₂)₃

is degraded via C–H activation by b-hydrogen abstraction.¹⁵

The niobium and tantalum amidate complexes are stable in

a C₇D₈solution at or below 160 ^◦C, and all the complexes are

stable in dry nitrogen atmosphere, while they are very sensitive

to moisture. These complexes are soluble in organic solvents such

as THF, DME, pyridine, toluene, and benzene, and only slightly

soluble in n-hexane. They have been characterized by various

The single-crystal X-ray diffraction analyses show that 29 and 30

2

are isostructural. In each molecule (10)₂M(NMe₂)(h -CH₂NMe),

the substituted Me₂N groups are far away from the metal center,

and the M⁵⁺is s-bound to two nitrogen atoms and two oxygen

atoms from two ligand anions 10 and one nitrogen atom and one

1

2

spectroscopic techniques and elemental analyses. The H NMR

carbon atom from (h -CH₂NMe) group and one nitrogen atom

spectra of niobium complex 29, and tantalum complex 30 support

that the ratio of amino group NMe₂and ligand anion 10 is 1 : 1,

and clearly show the diagnostic signals for the diastereotopic

metallaaziridine CH₂protons as doublets at about 2.97 and 2.87

from amino group NMe₂in a distorted-hexagonal-pyramidal

geometry (Fig. 14) with the average M–N distance of 2.099(2)

˚

A for Nb and 2.094(3) A for Ta, respectively, and the average

˚

M–O distance of 2.168(2) A for Nb and 2.180(3) A for Ta,

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Fig. 13 Molecular structure of 28 (thermal ellipsoids drawn at the 35%

probability level).

Scheme 9 Synthesis of complexes 28–30.

Fig. 14 Molecular structures of 29 (M = Nb) and 30 (M = Ta) (thermal

ellipsoids drawn at the 35% probability level).

2

Me₂C₆H₃)]₂Ta(NMe₂)(h -CH₂NMe).¹⁵The twisting between the

phenyl rings of torsion angles are 72.3(3) and 74.7(3)^◦for 29 and

73.3(3) and 71.5(3)^◦for 30, which are comparable to those found in

28 (72.5(2) and 73.9(2)^◦), (10)₂Ti(NMe₂)₂(70.0(2) and 69.7(2)^◦),^16g

and (10)₂Zr(NMe₂)₂(71.1(6) and 68.6(6)^◦).^16g

Catalytic activity

To examine the catalytic ability of the complexes 11–30, the

asymmetric hydroamination/cyclization of unactivated termi-

nal aminoalkenes, and the asymmetric hydroaminoalkylation of

alkenes have been evaluated under the conditions given in Tables

7 and 8, respectively.

Fig. 12 Molecular structure of 27 (thermal ellipsoids drawn at the 35%

probability level).

The screened results of the hydroamination/cyclization of 2,2-

dimethylpent-4-enylamine (30a) clearly show that the niobium

amides 25 and 29, and tantalum amide 30, and most vanadium

amides are active chiral catalysts for this transformation at 140 ^◦C

(Table 7, entries 4–21). The mesitoylamidate vanadium complex

11 is the most effective catalyst for this transformation and

gives a good enantioselectivity (up to 76%; Ta_◦ble 7, entry 4).

˚

respectively. The short M–NMe₂distances of 1.977(2) A for Nb

˚

and 1.986(3) A for Ta and the planar geometry around the nitrogen

atom of NMe₂indicate that the nitrogen atom with sp²hybridiza-

tion are engaged in N(p_p)→M(d_p) interactions. These structural

data are very close to those found in complex [Me₃CCON(2,6-

◦

Decreasing the temperature from 140 C to 130 C, the ee value

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Table 7 Enantioselective hydroamination/cyclization catalyzed by group 5 complexes^a

Entry

Catalysts (M)

Substrate

Product

Temp. (^◦C)

Time (h)

Conv. (%)^b

ee (%)^c

1

2

V(NMe₂)₄(V)

Nb(NMe₂)₅

(Nb)

120

24

100

N.A.

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

Ta(NMe₂)₅(Ta)

11 (V)

120

140

130

140

130

140

24

48

160

24

48

24

160

24

100

89

20

N.R.

85

N.A.

76

80

N.A.

68

70

54

N.A.

24

27

28

65

12

9.5

8.0

11 (V)

12 (Nb)

13 (Ta)

14 (V)

17 (V)

20 (Nb)

21 (Ta)

22 (V)

23 (V)

24 (V)

25 (Nb)

26 (V)

27 (V)

28 (V)

29 (Nb)

30 (Ta)

32

96

N.R.

83

100

86

100

85

90

22

23

24

25

26

27

28

29

30

31

32

11 (V)

14 (V)

17 (V)

23 (V)

24 (V)

25 (Nb)

26 (V)

27 (V)

28 (V)

29 (Nb)

30 (Ta)

140

24

100

98

100

92

100

56

54

49

20

16

23

48

16

11

15

12

33

34

35

36

38

39

40

41

42

11 (V)

14 (V)

17 (V)

23 (V)

24 (V)

26 (V)

27 (V)

28 (V)

29 (Nb)

30 (Ta)

140

48

96

90

100

76

90

82

100

32

31

32

14

19

28

12

25

30

42

^aConditions: C₆D₆(0.70 mL), aminoalkene (0.16 mmol), catalyst (0.016 mmol). ^bDetermined by ¹H NMR based on p-xylene as the internal standard.

N.R. = no reaction. ^cDetermined by ¹H NMR of its diastereomeric (S)-(+)-O-acetylmandelic acid salt.^8aN.A. = not applicable.

increases slightly (up to 80%) but the rate falls signiﬁcantly (Table

7, entry 5). When the amidate complexes 14 and 17 are used,

the rate remains good but the ee value falls slightly (Table 7,

entries 8 and 10). The phenolate vanadium complexes 23 and

24 are also active catalysts for this transformation but the ee

value remains moderate (Table 7, 14 and 15). The vanadium(III)

amide 26 can also promote this transformation, giving a good

ee value with good conversion (Table 7, entry 17). When the

bis-ligated amidate vanadium complexes 27 and 28 are used,

the rate increases but the ee value falls signiﬁcantly (Table 7,

entries 18 and 19). Under similar reaction conditions, the amides

V(NMe₂)₄, Nb(NMe₂)₅and Ta(NMe₂)₅, niobium complex 25, bis-

ligated amidate niobium complex 29 and tantalum complex 30

can also promote this transformation (Table 7, entries 1–3, 16,

20 and 21), however, no detectable hydroamination activity is

observed for mono-ligated amidate niobium complexes 12, 15,

18 and 20, tantalum complexes 13, 15, 19 and 21, and mono-

ligated dipenyl_◦phosphoramidate vanadium complex 22 even when

heated at 140 C for one week (Table 7, entries 7, 8 and 11–13),

presumably due to the electronic effect between the ligand anion

and the metal ion, and none of the complexes described above are

effective catalysts for the cyclization of 1-aminopent-4-ene into 2-

methylpyrrolidine, presumably due to a lack of a Thorpe–Ingold

effect³¹from the unsubstituted aminoalkene. The mono-ligated

amidate complexes 11, 14 and 17 are also effective catalysts for

the substrate 31a, giving spiro-pyrrolidine products in excellent

yield with moderate enantioselectivity (Table 7, entries 22–24).

The formation of a six-membered ring can also be accomplished

with our group 5 amidate catalysts (Table 7, entries 33–42),

and a moderate enantioselectivity (up to 42%), mediated by the

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Table 8 Enantioselective hydroaminoalkylation catalyzed by group 5 metal amides^a

Entry

Catalysts (M)

Amine

Alkene

Product

Temp. (^◦C)

Time (h)

Conv. (%)^b

ee (%)^c

1

2

V(NMe₂)₄(V)

Nb(NMe₂)₅

(Nb)

160

48

83 (75)

95 (84)

N.A.

3

4

5

6

7

8

9

10

11

12

13

Ta(NMe₂)₅(Ta)

11 (V)

160

130

160

130

160

130

160

48

160

48

120

48

120

48

120

160

48

100 (96)

N.R.

95 (88)

89 (84)

89 (85)

83 (77)

85

N.A.

85^d

12 (Nb)

13 (Ta)

93^d

16 (Ta)

91^d

16 (Ta)

91^d

19 (Ta)

77^d

19 (Ta)

71

N.R.

98 (95)

82^d

21 (Ta)

N.A.

4.0

30 (Ta)

14

15

16

17

18

19

13 (Ta)

16 (Ta)

19 (Ta)

160

130

160

130

160

130

48

120

48

120

48

89

83

87

81 (75)

82

74

83

76

88

71

82

120

79

20

21

22

23

24

25

13 (Ta)

16 (Ta)

19 (Ta)

160

130

160

130

160

130

48

120

48

120

48

91 (84)

86

79

82

54

63

56

70

52

60

120

76

26

27

28

13 (Ta)

16 (Ta)

19 (Ta)

160

120

35

20

40

40^e

43^e

31^e

^aConditions: C₇D₈(0.70 mL), amine/alkene 1 : 1.5, amine (0.100 mmol), catalyst (0.005 mmol). ^bEstimated by ¹H NMR based on TMS as the internal

standard, isolated yield given in brackets. N.R. = no reaction. ^cDetermined by HPLC analysis of its benzamide derivative. N.A. = not applicable. ^dThe

absolute conﬁguration of its benzamide derivative was established by X-ray diffraction analysis. ^eIn analogy to product 34 and the data reported in the

literature.¹⁵

catalyst 30, has been obtained (Table 7, entry 42). Although

the enantiomeric excesses obtained remain good, it should be

noted that there are only a small number of catalysts for these

reactions that give a signiﬁcant enantioselectivity (>90% ee) at

all,^{9d,12d–f,p,q–s,13e,f,14b}and here, to our best knowledge, we present

the ﬁrst example of the asymmetric hydroamination/cyclization

of unactivated aminoalkenes catalyzed by chiral group 5 metal

complexes in the literature.

The hydroaminoalkylation results clearly show that the tanta-

lumamides are active chiralcatalysts for the hydroaminoalkylation

of N-methylaniline with norbornene (Table 8, entries 6–13). At

160 ^◦C, the mono-ligated complex 16 is the most effective catalyst

for this transformation, giving an excellent enantioselectivity (up

to 91%; Table 8, entry 8) in 85% yield of 34 in 48 h. Decreasing the

temperature from 160 ^◦C to 130 ^◦C, the ee value increases slightly

but the rate falls (Table 1, entries 6–12). At 130 ^◦C, the complex 13

can also promote the hydroaminoalkylation to give a signiﬁcant

ee value (up to 93%; Table 8, entry 7), while the ee value remains

good (Table 8, entry 11) when complex 19 is used. When the bis-

ligated tantalum complex 30 is used, the ee value falls signiﬁcantly

(Table 8, entry 13). The mono-ligated tantalum amides 13, 16 and

19 derived from ligand (R)-1H₂, (R)-2H₂and (R)-3H₂induce the

enantiomeric excess of 34 in exo-(S)-conﬁguration (Fig. 15). This

stereochemical outcome can be rationalized by the model shown

in Fig. 16. It involves an approach of the oleﬁn to the Ta–C bond of

the tantalaaziridine complex,^3e,fin which the approach of the Ta–C

bond to the Si face of the oleﬁn, leading to the exo-(R)-34 isomer,

is hampered by sterically unfavourable interactions between the

bridged-carbon chain of the substrate and the sterically demand-

ing ligand stereodirecting groups, whereas no such interaction

occurs if the oleﬁn approaches with the opposite Re face to give

the observed exo-(S) stereochemistry. The tantalum amides 13, 16

and 19 are also effective catalysts for the hydroaminoalkylation of

unactivated alkenes such as vinylcyclohexane and 1-octene, giving

branched regioisomers in good yields with good enantioselectivity

(Table 8, entries 14–19 and 20–25). However, when the more bulky

amine 1,2,3,4-tetrahydroquinoline is used, both the rate and the

ee values fall signiﬁcantly (Table 8, entries 26–28). Although the

amides V(NMe₂)₄, Nb(NMe₂)₅and Ta(NMe₂)₅, and tantalum

amidate complexes 13, 16, 19 and 30 are effective catalysts

for this transformation, under similar reaction conditions, no

detectable hydroaminoalkylation activity (neither branched nor

linear regioisomer) is observed for niobium amides 12, 15, 18, 20,

25 and 29, tantalum amide 21, or vanadium amides 11, 14, 17, 22,

◦

23, 24, 26, 27 and 28 even when heated at 160 C for one week,

presumably due to the size of the metal ions coupled with the

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the tantalum amides are outstanding chiral catalysts for the

hydroaminoalkylation, giving chiral secondary amines in good

yields with excellent ee values (up to 93%), while the vanadium

amides and niobium amides are not. The nature of the metal

ion coupled with the electronic and steric effect of the ligand

plays an important role in the group 5 complex formation and

their catalytic activity. For example, treatment of M(NMe₂)₅with

C₂-symmetric amidate ligand 3H₂gives the amide complexes

(3)Nb(NMe₂)₃(18) and (3)Ta(NMe₂)₃(19), while reaction of

M(NMe₂)₅with C₁-symmetric amidate ligand 10H affords the bis-

2

ligated metallaaziridine complexes (10)₂Nb(NMe₂)(h -CH₂NMe₂)

2

(29) and (10)₂Ta(NMe₂)(h -CH₂NMe₂) (30), in which the amide

complexes are degraded via C–H activation by b-hydrogen ab-

straction. The complexes 29 and 30 can promote the hydroamina-

tion/cyclization, while complexes 18 and 19 cannot, due to the lack

of an active M–C bond. The mesitoylamidate tantalum complex 19

is an efﬁcient catalyst for the asymmetric hydroaminoalkylation,

while the mesitoylamidate niobium complex 18 and mesitoylsul-

fonylamidate tantalum complex 21 are not, presumably due to the

size of the metal ions and the electronic effects between the ligand

and the metal ion. We are currently focusing on group 5 chemistry

and these transformations, and further efforts will concentrate on

mechanistic investigations, optimization of the ligand architecture

to develop a general class of catalysts combining both high

catalytic activity and a consistent high level of enantioselectivity

for a wide range of substrates with high functional group tolerance

and user-friendliness, and exploration of these catalysts toward

other types of transformations.

Fig. 15 Molecular structure of benzamide derivative of 34 (thermal

ellipsoids drawn at the 35% probability level).

Acknowledgements

This work was supported by the National Natural Science Founda-

tion of China (Grant No. 20972018, 21074013), Beijing Municipal

Commission of Education, the Fundamental Research Funds

for the Central Universities, and Excellent Doctoral Dissertation

Fund of Beijing Normal University.

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Fig. 16 Proposed transition states.

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In conclusion, a new series of group 5 amides have been prepared

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Synthesis and catalytic activity of group 5 metal amides with chiral biaryldiamine-based ligands

DOI: 10.1039/c0dt01229g

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