Titanium hydroamination catalysts bearing a 2-aminopyrrolinato spectator ligand: Monitoring the individual reaction steps

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Titanium hydroamination catalysts bearing a 2-aminopyrrolinato spectator

ligand: monitoring the individual reaction steps†

Katharina Weitershaus,^aBenjamin D. Ward,^aRaphael Kubiak,^bCarsten Mu¨ller,^bHubert Wadepohl,^a

Sven Doye*^band Lutz H. Gade*^a

Received 2nd February 2009, Accepted 25th March 2009

First published as an Advance Article on the web 15th April 2009

DOI: 10.1039/b902038a

-

A series of new titanium half sandwich complexes, containing a 2-aminopyrrolinato ligand {N^XylN} as

the ancillary ligand, have been prepared and are shown to be pre-catalysts for the hydroamination of

-

alkynes. The coordination of {N^XylN} to titanium was achieved by reaction of [Cp*TiMe₃] with the

protioligand N^XylNH giving [Cp*Ti(N^XylN)(Me)₂] (1). Upon reaction of complex 1 with an excess of

t

tert-butylamine, the imido complex [Cp*Ti(N^XylN)(N^tBu)(NH₂Bu)] (2) was formed. The latter

provided the preparative entry to the synthesis of a range of N-aryl substituted imido complexes. Imido

ligand exchange with 2,6-dimethylaniline, 2,4,6-trimethylaniline as well as 2,6-diisopropylaniline gave

the corresponding arylimido complexes 3–5 in clean reactions. Reaction of the titanium imido complex

[Cp*Ti(N^XylN)(N^tBu)(NH₂Bu)] 2 with terminal arylacetylenes, such as phenylacetylene and

t

tolylacetylene, led to C–H activation and the formation of alkynyl/amido complexes, whereas the

arylimido complexes 3 and 5 cleanly underwent {2 + 2} cycloaddition, giving the azatitanacyclobutene

derivatives. A single-crystal X-ray structure analysis of the azatitanacyclobutene

2

[Cp*Ti(N^XylN){k N(2,6-C₆H₃Me₂)CTol CH}] (11) provided the ﬁrst crystallographically characterized

=

Markovnikov cycloaddition product of an imidotitanium complex with a terminal alkyne. The

mechanistic aspects of the hydromanination of alkynes with the new Ti half sandwich complexes were

studied and established a reversible {2 + 2} cycloaddition step and the cleavage of the metallacyclic

intermediate as the rate determining step in the catalytic cycle. The titanium half sandwich imido

complexes were found to be active catalysts for the inter- and intramolecular hydroamination of a

broad range of alkynes and w-aminoalkynes.

Neutral group 4 metal complexes appear to possess a relatively

broad scope. They have been employed for the intramolecular

Introduction

hydroamination of alkynes,²allenes³and alkenes⁴as well as the

intermolecular hydroamination of alkynes⁵and allenes.⁶Primary

aryl- and alkylamines have been successfully used, however,

secondary amines have so far deﬁed this type of transformation

with neutral catalysts.⁷For the reactions of the latter, cationic

Zr- and Ti-complexes have been employed in intramolecular

cyclizations of aminoalkenes.⁸

Among all the varieties of aminations of C–C multiple bonds,

the Ti-catalyzed hydroamination of alkynes,^2,5which generally

gives imines as reaction products has been the most intensely stud-

ied. The currently accepted mechanism is based on the assumption

that—in analogy with the Zr-catalyzed transformation—an imi-

dotitanium species is the active species.^6,9,10The latter is formed

from a suitable precursor complex, such as e.g. [Cp₂TiMe₂] or a

variety of other compounds, all of which have two formally anionic

reactive ligands at the titanium (alkyl-, dimethylamide-, chloride-

The development of catalytic methods for the hydroamination of

non-activated alkenes, allenes and alkynes has received consid-

erable attention in recent years.¹These highly atom economic

processes allow direct access to industrially and biologically

relevant classes of compounds such as amines, enamines and

imines from cheap and readily available starting materials. This has

recently led to an ever increasing range of molecular compounds,

which have been identiﬁed as catalysts for these transformations.¹

However, many of the newly developed processes are limited

in their scope. Whereas rare earth catalysts have been found

to be mainly active in intramolecular hydroamination, other

catalysts—in particular those of the late transition metals—are

frequently limited to the addition of weakly basic substrates

(aniline, sulfonamides, carboxamides, etc.) to alkenes, alkynes and

allenes.

=

ligands), which are transformed to the Ti N multiple bond in a

^aAnorganisch-Chemisches Institut, Universita¨t Heidelberg, Im Neuenheimer

Feld 270, 69120 Heidelberg, Germany. E-mail: lutz.gade@uni-hd.de;

Fax: +49 6221 545 609

ﬁrst reaction step. Alternatively preformed imido complexes may

be used as catalysts.

^bInstitut fu¨r Reine und Angewandte Chemie, Universita¨t Oldenburg,

Carl-von-Ossietzky-Str. 9–11, 26111 Oldenburg, Germany, doye@uni-

oldenburg.de; Fax: +49 4417 983 329

Apart from the choice of the appropriate “reactive” ligands

at the group 4 metal centre, the control of its reactivity by the

ancillary ligand(s) is crucial for the performance of the catalyst.

In recent years, amidinates have been shown to be ideal ancillary

ligands for a variety of early transition metals, including group 3

† Electronic supplementary information (ESI) available: Kinetic investiga-

tions. CCDC reference numbers 718799–718803. For ESI and crystallo-

graphic data in CIF or other electronic format see DOI: 10.1039/b902038a

4586 | Dalton Trans., 2009, 4586–4602

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and lanthanides,¹¹as well as for metals from groups 4¹²and 5.¹³We

have recently reported the synthesis of 2-aminopyrrolinato ligands

in which the amidinato binding unit is exocyclic with respect to

the heterocycle and which have proven to act as particularly robust

spectator ligands.¹⁴Their use in the preparation of a series of

new titanium half sandwich complexes, which are shown to be

pre-catalysts for the inter- and intramolecular hydroamination of

alkynes, will be reported in this work.

Results and discussion

Synthesis of titanium half sandwich complexes bearing a

2-aminopyrrolinato spectator ligand

-

The coordination of the 2-aminopyrrolinato ligand {N^XylN} to

titanium was achieved in a straightforward two-step procedure in

which [Cp*TiMe₃] was generated in situ by reaction of [Cp*TiCl₃]

with three equivalents of methyl lithium and subsequent reaction

with the protioligand N^XylNH. The methane elimination is rapid

and irreversible and, even in the presence of an excess of ami-

dine, occurred only once giving selectively the dimethyl complex

[Cp*Ti(N^XylN)(CH₃)₂] (1) (Scheme 1).

Fig. 1 Molecular structure of [Cp*Ti(N^XylN)(CH₃)₂] (1). Hydrogen atoms

are omitted for clarity (displacem_◦ent ellipsoids set at 40% probability).

˚

Selected distances (A) and angles ( ): Ti–N(1) 2.127(1), Ti–N(2) 2.193(1),

Ti–C(23) 2.136(1), Ti–C(24) 2.128(1), Ti–Cent. 2.045, N(1)–C(4) 1.315(2),

N(2)–C(4) 1.331(2), N(1)–Ti–N(2) 61.72(5), C(23)–Ti–C(24) 88.11(6),

N(1)–Ti–C(23) 81.66(6), N(2)–Ti–C(24) 90.57(6).

2

The bite angle of the k -coordinated amidinato ligand was found

to be 61.72(6)^◦, which is within the range of previously reported

values for this type of ligand system (min: 59^◦, max: 67^◦, mean:

63^◦, for 92 titanium complexes bearing amidinato ligands),¹⁶and

˚

the Ti(1)–N_Amidinatodistances of 2.1269(13) and 2.1927(13) A are

also in the range of previously determined values (min: 1.953,

16

˚

max: 2.275 A, mean: 2.112 A, for 92 examples).

Upon reaction of complex

1

with an excess of tert-

butylamine, aminolyses occurs and the imido complex

[Cp*Ti(N^XylN)(N^tBu)(NH₂Bu)] (2) is cleanly formed (Scheme 1).

t

One molar equivalent of the amine is converted to the formally

dianionic four-electron donor imido ligand whilst a second

equivalent of tBuNH₂coordinates to the metal as a neutral ligand.

The tert-butyl substitutents can be distinguished and assigned

based on the ¹³C NMR chemical shifts of their quaternary carbon

nuclei. The imido signal is detected at 67.1 ppm, whereas the

corresponding nucleus of the amine resonates at 50.4 ppm. The

high frequency shift of the quaternary C atom of the imido

substituent is diagnostic for this structural unit.¹⁷

Scheme 1 Synthesis of [Cp*Ti(N^XylN)(CH₃)₂] (1) and its aminolytic

conversion to [Cp*Ti(N^XylN)(N^tBu))(NH₂Bu)] (2).

t

The two proton signals of the NH₂group of the coordinated

amine are very broad at 295 K but give two well resolved

resonances at 218 K. The doublets of both protons are observed

at 7.28 and 1.80 ppm, indicating a strong shielding anisotropy due

to the aromatic rings in the molecule. The ²J_H–Hcoupling constant

of 11 Hz is as expected for geminal coupling.

In the ¹H and ¹³C NMR spectra of the dimethyl complex

[Cp*Ti(N^XylN)(CH₃)₂] (1), only one titanium-methyl resonance is

2

1

observed indicating a dynamic behaviour (k ¤ k ) (or possibly

an in-place rotation) of the amidinato ligand, which we have

previously observed for bis(amidinato)titanium complexes¹⁴and

which has also been reported and discussed in detail for amidi-

natotitanium half sandwich compounds by Sita et al.¹⁵Variable

low temperature ¹H NMR studies (400 MHz) established the

Synthesis and structural characterization of arylimidotitanium

complexes

◦

coalescence point at -66 C and a DG^πfor the overall process

of 40.9( 0.2) kJ mol^-1.

It was not possible to substitute the coordinated tert-butylamine

in 2 by direct reaction with other donor ligands such as

pyridine or 4-dimethylaminopyridine (dmap). The amine

could only be removed by reaction with the strong Lewis

acid tris(pentaﬂuorophenyl)borane (BArF). The reaction

product, [Cp*Ti(N^XylN)(N^tBu)] (2a), was found to be stable

A single-crystal X-ray diffraction study of 1 conﬁrmed the

proposed piano stool coordination geometry of the complex in

which the Cp*-ligand occupies the apical coordination site [Ti–

˚

Cp(centroid) 2.045 A). Its molecular structure is depicted in Fig. 1

along with selected bond lengths and angles.

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Scheme 2 Imide exchange reactions of [Cp*Ti(N^XylN)(N^tBu)(NH₂Bu)] (2) for the synthesis of arylimido complexes.

t

in solution and could be characterized by NMR spectroscopy.

By addition of pyridine to 2a the corresponding donor-adduct

[Cp*Ti(N^XylN)(N^tBu)(py)] (2b) was obtained quantitatively

(Scheme 2).

If a sterically less crowded aromatic amine such as para-toluidine

is employed in the imide exchange, only the tert-butyl imido ligand

is exchanged whilst the additional molecule of tert-butylamine

remains coordinated to the titanium centre (Scheme 2). The

resulting complex (6a) was not isolated and only characterized in

situ by NMR spectroscopy. By addition of one molar equivalent of

pyridine the coordinated t-BuNH₂is substituted and the resulting

complex 6 was isolated as an orange solid. The ortho-protons of

the coordinated pyridine in 6 are observed at 8.74 ppm, which

is characteristically shifted to higher frequency compared to free

pyridine.²⁰

Complex 2 provided the preparative entry to the synthesis

of a range of N-aryl substituted imido complexes. Such imido

exchange reactions have been reported and extensively exploited

by Mountford and co-workers.¹⁸Imido ligand exchange with 2,6-

dimethylaniline, 2,4,6-trimethylaniline as well as the sterically

demanding 2,6-diisopropylaniline gave the corresponding imido

complexes 3–5 in clean reactions (Scheme 2). As a consequence of

the steric bulk of the 2,6-aryl substituents on the imides, all of the

compounds were isolated without additional coordinated amine.

All three complexes are green diamagnetic solids for which single

crystals could be obtained from hexane solutions. Since only crys-

tals of poor quality were obtained for complex 5 the metric param-

eters of this structure are to be viewed with some caution. However,

its gross structural features are well established and will be related

to those of compounds 3 and 4 which are discussed in detail.

Compounds 3–5, which are displayed in Fig. 2, are piano

stool sandwich complexes. The imido-N–titanium bond lengths of

Reactions of imido complexes with terminal alkynes: C–H

activation vs. {2 + 2} cycloaddition

Reaction of the titanium imido complex [Cp*Ti(N^XylN)(N^tBu)-

t

(NH₂Bu)] (2) with terminal arylacetylenes, such as phenylacety-

lene and tolylacetylene, led to C–H activation and the formation of

alkynyl/amido complexes 7 and 8 (Scheme 3, top). The conversion

is rapid and irreversible. The coordinated tert-butylamine in 2 is

displaced and a proton is transferred from the terminal acetylene

to the imido nitrogen atom.

˚

1.736(1) A (3) and 1.738(1) A (4) are in the range which has been

previously reported for imidotitanium complexes with ancillary

Competing C–H-activation and {2 + 2}-cycloaddition reactions

have been previously reported by Bergman²¹and by us.²²We also

note that C–H-activations induced by group 4 metal imides in

general have been extensively studied by Wolczanski,²³Bergman²⁴

and Scott.⁵ⁿ

˚

amidinato ligands (min: 1.656 A, max: 1.752 A, mean: 1.712 A,

for 14 examples in the CSD). The same applies to the amidinato-

˚

N–Ti distances of 2.091(1) A and 2.106(1) A for 3 and 2.082(12) A

˚

and 2.115(1) A for complex 4. The bond angle Ti–N(3)–C(13) of

168.5(1)^◦in 3 deviates signiﬁcantly from linearity. This distortion

is even more pronounced in complex 4 [164.9(1)^◦] and is attributed

to the steric repulsion between the isopropyl groups on the imido

substituent and the 3,5-xylyl moiety on the amidinato ligand.

Despite the bending of the imides they are nevertheless to be

viewed as 4-electron donors which render the half sandwich species

overall 16 valence electron complexes.¹⁹

The newly formed [NH-tBu]^-ligand could be detected by ¹⁵N-

NMR spectroscopy and resonates at 331 ppm whilst the signal of

the imido nitrogen atom of the reactant was observed at 407 ppm.

The assignment is backed up by ¹⁵N-HMBC spectra, the ¹J(¹⁵N–

¹H) coupling constant in the NH-unit of 7 and 8 is 63 Hz. In

contrast to the observed reactivity of the alkyl substituted imido

complex 2, the arylimido complexes 3 and 5 cleanly underwent

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◦

˚

Table 1 Selected bond distances (A) and angles ( ) for imido compounds

(3), (4) and (5)

3

4

5

Ti–N(1)

Ti–N(2)

Ti–N(3)

Ti–cent.

2.091(1)

2.106(1)

1.736(1)

2.042

2.082(1)

2.115(1)

1.738(1)

2.038

2.084(3)

2.105(3)

1.728(3)

2.036

N(3)–C(13)

N(1)–C(4)

N(2)–C(4)

N(1)–Ti–N(2)

Ti–N(3)–C(13)

1.381(2)

1.324(2)

1.339(2)

64.25(5)

1.378(2)

1.327(2)

1.332(2)

64.74(5)

1.381(5)

1.319(5)

1.335(5)

64.3(1)

168.5(1)

164.9(1)

170.6(3)

Scheme 3 Reactions of imidocomplexes with terminal alkynes: C–H

activation for 2 vs. {2 + 2} cycloaddition for 3 and 5.

arylimides with two molar equivalents of alkyne for four hours

at ambient temperature gave complete conversion to complexes

9–11. Bulky substituents in the ortho positions of the aromatic

substituent of the imido ligand partially suppressed the cycloaddi-

tion, as in the case of complex 4, for which only partial conversion

to a metallacyclic product was observed in the NMR tube.

In situ ¹H-NMR studies of the {2 + 2} cycloadditions depicted

in Scheme 3 revealed the formation of only one of the two possible

regioisomers. The ¹H NMR signal of the methyne proton on the

metallacycle was observed at high frequency (11.1 ppm) for all

three derivatives. The corresponding resonances for metallacycles

leading to the anti-Markovnikov products have been previously

observed shifted by 1–2 ppm to lower frequency.²⁵In order to

establish the details of the molecular structure of the metallacyclic

reaction products, a single-crystal X-ray structure analysis of

compound 11 has been carried out. Its molecular structure is

depicted in Fig. 3 along with the principal bond lengths and

angles. To our knowledge, complex 11 is the ﬁrst crystallograph-

ically characterized Markovnikoff cycloaddition product of an

imidotitanium complex with a terminal alkyne.

Fig.

2

Molecular structures of the aryl imidocomplexes. (a)

[Cp*Ti(N^XylN)(2,4,6-NC₆H₂Me₃)] (3), (b) [Cp*Ti(N^XylN)(2,6-NC₆H₃-

The coordination geometry at the metal centre may be described

as a four-legged piano stool with a Ti-Cp*–cent distance of

(ⁱPr)₂)] (4), (c) [Cp*Ti(N^XylN)(2,6-NC₆H₃Me₂)] (5). Hydrogen atoms are

omitted for clarity (displacement ellipsoids set at 40% probability). Selected

bond lengths (A) and angles ( ) of all three complexes are listed in Table 1.

˚

◦

2.048 A. The Ti–N(1) bond length of the amidinate of 2.203(2) A

is slightly greater than in previous examples whilst the Ti–N(2)

distance has remained essentially unaffected. The slightly non-

symmetrical coordination of the amidinato ligand compared to

the situation in 1, 3 and 5 may be attributed to greater steric

˚

{2 + 2} cycloaddition giving the azatitanacyclobutene derivatives

9–11 (Scheme 3), which are postulated as intermediates in the

catalytic cycle of the hydroamination of alkynes. Reaction of the

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2

Fig.

3

=

Molecular structure of [Cp*Ti(N^XylN){k N(2,6-C₆H₃Me₂)-

CTol CH}] (11). Hydrogen atoms are omitted for clarity (displacement

Fig. 4 The thermodynamics of the equilibrium between the imide 5 and

acetylene and the metallacyclic compound 10 determined by Van’t Hoff

analysis of the equilibrium constants between 45 ^◦C and 70 ^◦C.

ellipsoids set at 40% probability; only the prevailing conformation _◦of

˚

the C₄N ring is shown). Selected bond lengths (A) and angles ( ):

Ti–N(1) 2.203(2), Ti–N(2) 2.095(2), Ti–N(3) 1.959(2), Ti–C(21) 2.051(2),

C(21)–C(22) 1.349(2), C(22)–N(3) 1.424(2), Ti-Cent 2.048, N(1)–C(4)

1.309(2), N(2)–C(4) 1.337(2), C(21)–Ti–N(3) 70.35(7), N(1)–Ti–N(2)

62.16(6), N(3)–Ti–N(1) 87.51(6), N(2)–Ti–C(21) 94.24(7).

only be located when the simpliﬁed system [CpTi(NMe)(lig_qm)] A

was employed.

crowding by the metallacycle, compared with the almost linear

imido ligand. The deviation from symmetrical binding is also

reﬂected in the N(1)–C(4) and N(2)–C(4) bond lengths, which

26

˚

were found to be 1.309(2) and 1.337(2) A, respectively. As for

the metric parameters of the metallacycle, the Ti–N(3) distance

˚

of 1.959(2) and the Ti–C(21) bond length of 2.051(2) A indicate

˚

single bonds whereas the C(21)–C(22) distance of 1.349(2) A is

An analysis of the metric parameters of the reaction products

with the non-simpliﬁed system indicates no signiﬁcant differences

consistent with there being a carbon–carbon double bond.^20,22

This interpretation has been recently conﬁrmed by a DFT study

of a related azatitanacyclobutene derivative.²⁰To minimize the

intra- and inter-ligand repulsion, the 2,6-xylyl substituent on the

metallacycle adopts an orthogonal orientation with respect to the

plane of the four-membered ring.

˚

=

with those using the simpliﬁed system (e.g. T N = 1.678 A for

A and 1.681 A for [Cp*Ti(N^XylN)(N^tBu)] 2a modelled with the

˚

same method), but in the absence of transition states for the more

complex model, our discussion is conﬁned to the simpliﬁed system.

An energy proﬁle diagram is shown in Fig. 5, with the C–H

activation pathway leading to the acetylide product B via transition

state B^‡, and the cycloaddition product C via transition state C^‡.

Both the acetylide B and titanacyclobutene C are viable products,

with the cycloaddition product C being thermodynamically more

favourable. With this simpliﬁed system, the cycloaddition pathway

has the lowest activation barrier, 3.5 kcal mol^-1, compared to

the acetylide formation, which has an activation barrier of

9.3 kcal mol^-1.

Even when allowing for the error in the calculation methods,

this relatively small difference in transition state energies indicates

that the cycloaddition pathway is implicated as both the thermo-

dynamically, as well as the kinetically favoured pathway. Therefore

in the absence of any extenuating circumstances arising from the

additional complexity of the ‘real’ system, compared to the model

system, it would be expected that the reaction of these imido

complexes with acetylenes would yield solely the cycloaddition

product. Although the ‘real’ models did not give rise to well

deﬁned minima on the potential energy surface, the selectivity

can be easily rationalised by a closer inspection of the transition

state structures B^‡and C^‡, which are also illustrated in Fig. 5.

When considering steric arguments, it can be seen that in the

C–H activation transition state B^‡, the non-reacting acetylene

[metallacycle]

ꢀꢀꢁ

[imide] + [acetylene]

[metallacycle] K =

ꢂꢀꢀ

[imide][acetylene]

The metallacyclobutene is in equilibrium with the starting

materials, the imido complex and phenylacetylene. With increasing

temperature the equilibrium is shifted towards the latter. Van’t

Hoff analysis of the equilibrium constants determined between

45 ^◦C and 70 C permitted the determination of the thermody-

◦

namics of this equilibrium (Fig. 4). DS_owas obtained as -173

6 J mol^-1K^-1and DH_oas -64 4 kJ mol^-1.

A DFT study of the reactions of the imido complexes with alkynes

The observed selectivity with regard to cycloaddition vs. C–H acti-

vation is apparently dependent on the imido N-substituent. In light

of this, we sought to rationalise this selectivity by considering the

reaction product and transition state structures for both of these

reaction pathways, which were modelled using DFT (B3PW91)

methods. Unfortunately, calculations of complexes possessing

these amidinate ligands have a tendency to have poorly deﬁned

minima on the potential energy surface;¹⁴the reaction products

were calculated on both simpliﬁed systems and on the non-

simpliﬁed models, however well-deﬁned transition states could

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In addition to explaining the role of the imido substituent in

the cycloaddition vs. C–H activation selectivity, the cycloaddition

transition state C^‡also shows, as alluded to above, that the

most stable arrangement in the ‘real’ system is that in which the

acetylene substituent is located closest to the imido substituent,

and hence furthest from the amidinate aryl substituent, since the

amidinate substituent reduces the longitudal space in that direc-

tion of the amidinate, relative to the orientation of the acetylene

in the cycloaddition transition state. The greater feasibility of this

transition state implies that catalytic hydroamination reactions

using terminal alkynes will be more favoured in the Markovnikov

sense, rather than the anti-Markovnikov; this relative stability is

readily inferred by a consideration of the catalysis studies (vide

infra), in which selectivities of 99 : 1 are observed.

A kinetic analysis of the key steps in the catalytic

hydroamination cycle

The active species in the generally accepted catalytic cycle for

the hydroamination of alkynes is an imido-transition metal

complex.^6,9,10We studied the mechanistic aspects of our catalysts

for the reaction of complex 5 with phenylacetylene since it

proved to be possible to isolate the ﬁrst proposed intermediate

in the catalytic cycle, the azatitanacyclobutene complex 10. As

discussed above, this ﬁrst {2 + 2} cycloaddition step was found

to be reversible, the metallacycle being in equilibrium with the

imide and the acetylene starting materials. In Scheme 4, the

cycloaddition and cycloreversion are characterized by the rate

constants k₁and k_-1. In the presence of primary amine the

metallacyclic compound 10 may undergo aminolysis to generate a

diamido intermediate, which rapidly forms the imido compound

and liberates the reaction product. The intermediate diamido

compound could never be observed by spectroscopic techniques,

so that the disappearance of the metallacycle in the aminolysis is

characterized by the second order rate law (rate constant k₂).

The individual equations for the generation and conversion of

the metallacyclic intermediate 10 are given by:

Fig. 5 Energy proﬁle for the reaction of the computational model

[CpTi(lig_qm) (NMe)] (A) with acetylene (kcal mol^-1). The calculated

transition state structures for C–H activation (B‡) and cycloaddition (C‡)

of A with acetylene are also depicted.

C–H (which would represent the alkyl or aryl substituent in

terminal acetylenes) is directed away from both the imido N-

substituent and the amidinate aryl group (represented by a proton

in the model system). It would therefore be expected that the

transition state energy would be inﬂuenced little, if at all, by

altering the acetylene or imido substituents.

Conversely with the cycloaddition transition state structure C^‡,

the two acetylene protons are both in close proximity to other

co-ligands; one of them is in close proximity to the amidinate

N–H, which in the real system would be replaced by an aryl

group. The remaining acetylene C–H lies in close proximity to

the imido substituent. It can be clearly seen from the molecular

structure shown in Fig. 5 that there is insufﬁcient space for

an acetylene substituent to be located close to the aromatic

amidinate substituent, thereby forcing any acetylene substituent

to reside close to the imido substituent. It is therefore expected

that the relative energy of this transition state will be strongly

dependent on the steric repulsion between these two groups; larger

acetylene substituents coupled with larger imido substituents

will increase the energy of the corresponding transition state.

Since, as noted above, the cycloaddition pathway possesses the

lowest activation barrier by only a relatively small amount, any

increase in the steric repulsion between these two groups will

reduce the difference between the relative activation energies of

the two possible pathways, especially since we have noted that

the transition state energy of B^‡will remain largely unaffected by

alterations in either of these groups. It is perhaps unsurprising

therefore that in the real system, the smaller aryl imido complex

undergoes the expected cycloaddition reaction, whereas in the

signiﬁcantly larger tert-butyl imido complex favours the C–H

activation pathway due to a destabilisation of the cycloaddition

transition state.

k

1

ꢀꢀꢀꢁ

ꢂꢀꢀꢀ

[imide] + [acetylene]

[metallacycle]

k

-1

k

2

[metallacycle] + [xylylamine] ææÆ [enamine] + [imide]

Overall, the rate of formation and disappearance of the metal-

lacycle 10 (MC) is given by:

d[MC]/dt = k₁[I][acetylene] - k_-1[MC] - k₂[MC][amine]

(1)

The formation and degradation of the metallacycle by cyclore-

version as well as its aminolysis were investigated separately. For

the cycloaddition and the determination of the rate constant k₁

only the ﬁrst term in eqn (1) had to be considered since the

reverse reaction, of the metallacycle does not play a signiﬁcant

role at temperatures below 10 ^◦C. Moreover, at the beginning

there was practically no cycloaddition product and thus the

reverse reaction could be neglected. In our investigations we

ﬁrst proved that the formation of the metallacyclic compound

10 is in ﬁrst order in both starting materials (method of initial

rates). Therefore the imidotitanium compound 5 was reacted with

0.5, 1, 2, 3 and 5 molar equivalents of phenylacetylene and the

conversion was monitored by ¹H NMR spectroscopy. The initial

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Scheme 4 Catalytic cycle of the hydroamination of phenylacetylene with 2,6-xylylamine using 5 as catalyst. The primary product of the aminolysis step

following the {2 + 2} cycloaddition is not observed.

rates were plotted against the concentration of acetylene to give

a linear dependence. (Fig. S1 in the ESI).† Subsequently, variable

quantities of the imidotitanium compound 5 (5 to 25 mmol) were

reacted with an excess (20 equiv.) of phenylacetylene. Using again

the method of initial rates, a ﬁrst order behaviour with respect to

the concentration of the imido comlex 5 was found (Fig. S2 in the

ESI).† The second order rate constant k₁was then obtained by

reacting equimolar amounts of imidotitanium compound with

phenylacetylene (20 mmol). The reactions were monitored for

several temperatures between 0 ^◦C and 10 ^◦C, giving the Arrhenius

activation energy (Fig. 6). This measure was taken to suppress the

cycloreversion which operates at higher temperature and allowed

◦

the extrapolation the rate coefﬁcient k₁for the reaction at 20 C

as 1.13 ¥ 10^-2L mol^-1s^-1.

All other reaction steps were studied at 20 ^◦C, because the

cycloreversion and the aminolysis were impractically slow to

monitor beneath 10 ^◦C. Since we were able to isolate the

metallacyclic complex 10, the reverse reaction, which regenerates

the reactants of the ﬁrst step and the aminolysis could be studied

separately. It was thus possible to neglect the rate of formation

of 10, since no imido complex was present at the beginning of

the conversion and allowed the simpliﬁcation of eqn (1), which

assumes the following form:

Fig. 6 Arrhenius plot for the cycloaddition leading to the metallacyclic

compound 10 between 0 ^◦C and 10 ^◦C; determination of the temperature

dependence of the rate constant k₁.

the disappearance of the compound was monitored by ¹H NMR

spectroscopy. The initial rates were plotted vs. the concentration

of complex 10 and from the straight line the rate coefﬁcient k_-1

was obtained as 3.4 ¥ 10^-5s^-1.

The protolytic cleavage of the metallacycle 10 was found to obey

the expected rate law which is ﬁrst order in complex 10 (Fig. S3 in

ESI)† as well as ﬁrst order in the arylamine (Fig. 8). The straight

line in Fig. 8 does not intersect the ordinate at the origin due

to the competing cycloreversion reaction. The second order rate

-d[MC]/dt = k_-1[MC] + k₂[MC][amine]

(2)

The rate of the cycloreversion displayed a ﬁrst order dependence

on the concentration of the metallacycle 10 (Fig. 7), which was

proven by analysis of the initial rates. Therefore 5 to 25 mmol

of the metallacyclic compound 10 were dissolved in benzene and

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Table 2 Intermolecular hydroamination of symmetric alkynes

Isolated

Cat. yield (%)

Entry Alkyne

Amine

1

2

1

2

96

94

3

4

1

2

94

96

5

6

1

2

29

68

7

8

1

2

81

84

Fig. 7 Determination of the ﬁrst order dependence on the concentration

of the cycloreversion of the metallacyclic compound 10. The analysis is

based on the initial rates.

9

10

1

2

8

9

11

12

1

2

9

13

14

1

2

<5

Reaction conditions: alkyne (2.4 mmol), amine (2.4 mmol), 5 mol%

catalyst, 1 mL of toluene, 105 ^◦C, 24 h. Reduction: NaCNBH₃(4.8 mmol),

ZnCl₂(2.4 mmol), 10 mL methanol, 20 ^◦C, 20 h. All yields determined

after puriﬁcation by column chromatography.

Fig. 8 First order dependence of the aminolysis of 10 on the concentra-

tion of arylamine. The analysis is based on the initial rates.

tolerate the elevated temperatures required for the hydroamination

of some of the substrates.

constant k₂for the aminolysis was determined by eqn (3) as 1.58 ¥

10^-3L mol^-1s^-1.

We therefore initially focused on the dimethyl complex 1 and the

imido complex 2 in our study of the catalytic performance of the

Ti half sandwich compounds reported in this paper. The methyl

groups in 1 are readily and irreversibly cleaved by aminolysis

with the primary amine giving the key imido species, which is

preformed in complex 2, making a speciﬁc activation step for the

latter unnecessary. Furthermore, our studies of the stoichiometric

reactivity of 2 delineated above indicated its facile convertibility

to other imido derivatives. The tert-butylamine liberated in the

latter process is volatile and remains only partially dissolved at the

elevated temperatures (105 ^◦C) employed in the catalytic runs.

The hydroamination of diphenylacetylene has been achieved

with both primary aryl amines (entries 1 and 2 in Table 2) as well

as alkyl amines (entries 3 and 4, 7 and 8, Table 2) with high isolated

yields. Only the sterically crowded amines, 2,6-dimethylaniline

(entries 9 and 10, Table 2) and 2,4,6-trimethylaniline (entries 11

and 12, Table 2) gave rise to poor conversion. The two pre-catalysts

1 and 2 did not give rise to any differences in activity, except for the

hydroamination of diphenyacetylene with benzylamine (entries 5

-d[MC]/dt = k_-1[MC] + k₂¢[amine] with k₂¢ = k₂[MC]

(3)

The formation of the metallacyclic compound 10 by the imide

5 and phenylacetylene is thus faster by a factor of 7 than the

aminolysis under the reaction conditions. The cleavage of the

metallacyclic compound 10 by an arylamine is the rate determining

step in this catalytic cycle as has been previously observed for this

type of transformation.

The use of the titanium half sandwich imido complexes as

pre-catalysts in the intermolecular hydroamination of alkynes

Pre-catalysts for the hydroamination of alkynes should be active

for a broad range of substrates, easily synthetically accessible

and convertible into imido complexes since they are the active

species. Moreover, the ligand exchange reaction associated with

the generation of the active catalyst should not interfere in the

subsequent catalytic transformations. Finally, the catalysts should

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Table 3 Intermolecular hydroamination of 1-phenylpropyne

Entry

Alkyne

Amine

Cat.

Isolated yield (%)

Ratio AM : M

1

2

1

2

92

99

2 : 1

3

4

1

78

11

1 : 2

1 : 1.5

5

6

1

2

11

1 : 99

7

8

1

2

12

13

1 : 99

Reaction conditions: alkyne (2.4 mmol), amine (2.4 mmol), 5 mol% catalyst, 1 mL of toluene, 105 ^◦C, 24 h. Reduction: NaCNBH₃(4.8 mmol), ZnCl₂

(2.4 mmol), 10 mL methanol, 20 ^◦C, 20 h. All yields determined after puriﬁcation by column chromatography.

and 6). In this case, the tert-butylimido complex 2 displayed higher

activity than complex 1.

Increasing the steric bulk beyond a certain point may lead to a loss

in catalytic activity as has been shown for 2,6-diisopropylaniline

(entry 12, Table 4). This behaviour is also reﬂected in the

The limitations of this system became apparent in the hydroam-

ination of 3-hexyne with p-toluidine (entries 13 and 14, Table 2).

i

stoichiometric reactions of complex [Ti(N^XylN)(N-2,6-C₆H₃Pr₂)]

◦

Even after a reaction time of 24 h at 105 C no conversion was

4 with phenyl- and tolylacetylene which did not lead to detectable

quantities of the corresponding azatitanacyclobutenes. It therefore

seems to be at this step of the catalytic cycle that the reaction is

suppressed.

observed and it appears that the catalyst at hand is unsuitable for

internal aliphatic alkynes.

On the other hand, in their use in the catalytic hydroamination of

phenylpropyne both catalysts again display very similar activities

and low regioselectivity with sterically undemanding amines.

Whereas hydroamination with p-toluidine (entries 1 and 2, Table 3)

preferentially yields the anti-Markovnikov product, the product

ratio is completely inverted in the corresponding reaction with

cyclopentylamine (3 and 4 in Table 3). This trend is enhanced

for amines with increasing steric bulk, such as 2,6-dimethylaniline

(entries 5 and 6) and 2,4,6-trimethylaniline (7 and 8 in Table 3), for

which the Markovnikov products were obtained quantitatively.

In order to test a possible signiﬁcance of the nature of the imido

ligand in the performance of the catalyst we also investigated

the hydroamination of terminal alkynes with the other imido

complexes 3, 4, 5 and 6 and their corresponding primary amines.

In the reaction of phenylacetylene with primary amines no

signiﬁcant differences in activity and selectivity was observed

for these systems. Only for the coupling of 2,6-dimethylaniline

with phenylacetylene (Table 4, entries 8, 9 and 10) complex 5

was found to be less selective with respect to the Markovnikov

products than the reference systems 1 and 2. Generally, the trend

already established for 1-phenylpropyne was conﬁrmed, that the

regioselectivity is enhanced with increasing steric demand of the

amines (a notable exception is the reaction with benzylamine which

occurs with very high regioselectivity: entries 6 and 7, Table 4).

Upon replacement of phenylacetylene by a terminal aliphatic

alkyne such as 1-octyne, even 2,6-diisopropylaniline may be

coupled in moderate yield (Table 5, entry 12). The steric demand

of the amine leads to a high regioselectivity in this case. In general,

1-octyne was found to be more reactive as terminal alkyne than

phenylacetylene also giving rise to generally higher selectivity of

the hydroamination products (entries 1–3 and 6–12 in Table 5).

The only exception to this trend is benzylamine for which both

yield and regioselectivity in its reactions with 1-octyne are lower

than for phenylacetylene (entries 4 and 5, Table 5).

Intramolecular hydroaminations of x-aminoalkynes

Both complexes 1 and 2 have displayed excellent activities in the

intramolecular hydroamination of several w-aminoalkynes as is

shown in Table 6. After reduction of the primary hydroamination

products, the 2-benzyl-substituted piperidine derivatives were

isolated in high yields.

Conclusions

The introduction of the 2-aminopyrrolinate as second ancillary

ligand in Cp* titanium half sandwich complexes has allowed the

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Table 4 Intermolecular hydroamination of phenylacetylene with primary amines

Entry

Alkyne

Amine

Cat.

Isolated yield (%)

Ratio AM : M

1

2

3

1

2

6

94

93

94

1 : 9

1 : 7

1 : 12

4

5

1

2

<5

—

–

6

7

1

2

79

80

1 : 99

8

9

10

1

2

5

76 imine^a

75 imine^a

86^b

1 : 99

1 : 13

11

12

3

4

80 imine^a

0 : 100

4

—

Reaction conditions: alkyne (2.4 mmol), amine (2.4 mmol), 5 mol% catalyst, 1 mL of toluene, 105 ^◦C, 24 h. Reduction: NaCNBH₃(4.8 mmol), ZnCl₂

(2.4 mmol), 10 mL methanol, 20 ^◦C, 20 h. All yields determined after puriﬁcation by column chromatography.^aReduction incomplete. Isolation of the

imine. ^bReduction with LiAlH₄.

isolation of key intermediates of the catalytic hydroamination of

alkynes. On the other hand, these complexes have displayed good

activities in the catalytic reactions themselves. Using sterically

demanding amines as substrates excellent regioselectivities were

observed in these transformations. Both the imido pre-catalyst 2

as well as the dimethyl precursor 1 display very similar activities

and selectivities, indicating the role of the former itself as a key

intermediate in the catalytic reaction.

Samples for NMR spectroscopy were prepared under argon in

5 mm Wilmad tubes equipped with J. Young Teﬂon valves. NMR

spectra were recorded on Bruker Avance II 400 or Bruker Avance

III 600 NMR spectrometers. NMR spectra are quoted in ppm and

were referenced internally relative to the residual protio-solvent

(¹H) or solvent (¹³C) resonances, or externally to ¹⁵NH₃. Where

necessary, NMR assignments were conﬁrmed by the use of two-

dimensional ¹H–¹H and ¹H–¹³C correlation experiments. ¹⁵N data

1

were obtained by two-dimensional H correlated experiments or

by direct detection using a cryogenically cooled direct-detection

NMR probe (QNP CryoProbe^TM). Microanalyses were performed

by the analytical services in the chemistry department of the

Universita¨t Heidelberg. IR spectra were recorded on a Varian 3100

Exalibur spectrometer as KBr plates. Infrared data are quoted

in cm^-1.

Experimental

All manipulations of air and moisture sensitive species were

performed under an atmosphere of argon using standard Schlenk

and glove box techniques. Solvents were pre-dried over molecular

sieves and dried over Na/K alloy (pentane, diethylether), Na

(toluene) or K (THF, hexanes), distilled and stored over potassium

mirrors in Teﬂon valve ampoules. Deuterated solvents were dried

over K (benzene-d₆, toluene-d₈) vacuum distilled, and stored

under argon in Teﬂon valve ampoules. [Cp*TiCl₃] was prepared

according to a literature procedure.²⁷The protioligand N^XylNH

was prepared as we have previously described.¹⁴Amines were

dried over CaH₂, vacuum distilled and stored in Teﬂon valve

ampoules before use. All other reagents were purchased from

commercial suppliers and used as received unless otherwise stated.

(A) Preparation of the compounds

[Cp*Ti(N^XylN)(CH₃)₂] (1). [Cp*TiCl₃] (1.0 g, 3.5 mmol) was

dissolved in toluene (50 ml), and cooled to -40 ^◦C. Methyllithium

(1.6 M in diethylether, 6.5 mL, 10.5 mmol) was added dropwise

over 30 min. The reaction was stirred for 2 h below -30 ^◦C before

a toluene solution of N^XylNH (658 mg, 3.5 mmol) was added

dropwise. The mixture was stirred for additional 2 h under a

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Table 5 Intermolecular hydroamination of 1-octyne with primary amines

Entry

Alkyne

Amine

Cat.

Isolated yield (%)

Ratio AM : M

1

2

3

1

2

6

87

93

96

1 : 7

1 : 14

1 : 10

4

5

1

2

57

54

1 : 7

1 : 9

6

7

8

1

2

4

91

92

1 : 99

0 : 100

9

10

11

1

2

3

88

87

88

1 : 99

0 : 1000

12

4

42

0 : 100

Reaction conditions: alkyne (2.4 mmol), amine (2.4 mmol), 5 mol% cat. in 1 mL of toluene, 105 ^◦C, 24 h. Reduction: NaCNBH₃(4.8 mmol), ZnCl₂(2.4

mmol), 10 mL methanol, 20 ^◦C, 20 h. Yields determined after column chromatography.

Table 6 Intramolecular hydroaminations

Entry

Aminoalkyne

Cat.

Isolated yield (%)

1

2

1

2

98

95

3

4

1

2

97

5

6

1

2

98

96

Reaction conditions: aminoalkyne (2.4 mmol), 5 mol% cat. in 1 mL of toluene, 105 ^◦C, 6 h. Reduction: NaCNBH₃(4.8 mmol), ZnCl₂(2.4 mmol), 10 mL

methanol, 20 ^◦C, 20 h. Yields determined after column chromatography.

static vacuum while the solution was allowed to warm to ambient

temperature. All volatiles were removed under reduced pressure,

the residue suspended in pentane and the supernatant was removed

by ﬁltration. From the concentrated pentane so_◦lution the desired

product crystallized as an orange solid at -80 C (990 mg, 75%

yield). Suitable crystals for X-ray diffraction were grown from a

concentrated pentane solution at -18 ^◦C. (Found: C, 71.83, H,

9.05, N, 7.02. Calcd for C₂₄H₃₆N₂Ti: C 71.98, H 9.06, N 7.00%).

n_max(KBr)/cm^-1: 2958 (m), 2945 (m), 2912 (m), 2857 (m), 1618

(m), 1601 (m), 1519 (s), 1472 (m), 793 (s). ¹H NMR (399.9 MHz,

benzene-d₆, 293 K): d 6.72 (s, 2 H, ortho-C₆H₃Me₂), 6.60 (s, 1 H,

para-C₆H₃Me₂), 3.27 (m, 2 H, NCH₂), 2.35 (m, 2 H, NCCH₂), 2.20

(s, 6 H, C₆H₃Me₂), 1.92 (s, 15 H, C₅Me₅), 1.55 (m, 2 H, NCH₂CH₂),

1

0.85 (s, 6 H, TiMe₂). ¹³C{ H} NMR (100.6 MHz, benzene-d₆,

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293 K): d 175.0 (NCN), 150.3 (ipso-C₆H₃Me₂), 138.0 (meta-

C₆H₃Me₂), 124.4 (para-C₆H₃Me₂), 124.5 (C₅Me₅), 122.1 (ortho-

C₆H₃Me₂), 66.6 (TiMe₂), 52.2 (NCH₂), 28.7 (NCCH₂), 23.9

(NCH₂CH₂), 21.6 (C₆H₃Me₂), 12.4 (C₅Me₅).

123.3 (para-C₆H₃Me₂), 121.0 (ortho-C₆H₃Me₂), 118.8 (C₅Me₅),

67.3 (NC(CH₃)₃), 54.9 (NCH₂), 33.3 (NC(CH₃)₃), 29.4 (NCCH₂),

24.5 (NCH₂CH₂), 21.6 (C₆H₃Me₂), 11.9 (C₅Me₅).

General procedure for the synthesis of imido complexes

with aromatic substituents on the imido nitrogen: preparation

of [Cp*Ti(N^XylN)(N-2,4,6-C₆H₂Me₃)] (3), [Cp*Ti(N^XylN)(N-2,6-

C₆H₃(ⁱPr)₂)] (4) and [Cp*Ti(N^XylN)(N-2,6-C₆H₃Me₂)] (5). To a

[Cp*Ti(N^XylN)(N^tBu)(NH₂Bu)] (2). [Cp*Ti(N^XylN)(CH₃)₂] (1)

t

(1.6 g, 4 mmol) was dissolved in hexane (20 mL), ^tBuNH₂(2.3 mL,

20 mmol) was added and the reaction mixture heated to 70 ^◦C

for 24 h. The volatiles were removed under reduced pressure and

the crude product was recrystallized from pentane containing

t

solution of [Cp*Ti(N^XylN)(N^tBu)(NH₂Bu)] (2) (1 g, 2 mmol)

in hexane (5 mL) was added the appropriate amine (2 mmol),

and the solution was stirred at room temperature for 30 min.

The imido complexes started to precipitate as green solids, and

after stirring for 1 h at room temperature the supernatant was

removed by ﬁltration to give the product as a green microcrys-

talline solid. ([Cp*Ti(N^XylN)(2,4,6-NC₆H₂Me₃)] (3): 734 mg, 75%

yield, [Cp*Ti(N^XylN)(2,6-NC₆H₃(ⁱPr)₂)] (4): 819 mg, 71% yield,

[Cp*Ti(N^XylN)(2,6-NC₆H₃Me₂)] (5): 685 mg, 68% yield).

2% BuNH₂at -80 ^◦C. The product crystallized as a yellow-

t

orange solid (1.9 g, 95% yield). (Found: C 69.99, H 9.78, N

10.82. Calcd for C₃₀H₄₉N₄Ti: C 70.15, H 9.62, N 10.91%). n_max

(KBr)/cm^-1: 3317 (w), 2959 (s), 2914 (s), 2864 (s), 2833 (m),

1591 (m), 1545 (s), 1454 (m), 1388 (m), 1375 (m), 1265 (m),

1

1234 (m), 1084 (m). H NMR (399.9 MHz, toluene-d₈, 218 K):

2

t

d 7.28 (d, J_H–H= 11.1 Hz, 1 H, NH₂Bu,), 6.82 (s, 2 H, ortho-

C₆H₃Me₂), 6.65 (s, 1 H, para-C₆H₃Me₂), 4.21 (m, 1 H, NCH₂),

3.68 (m, 1 H, NCH₂), 2.49 (m, 1 H, NCCH₂), 2.39 (s, 6 H,

C₆H₃Me₂), 2.19 (m, 1 H, NCCH₂), 2.08 (s, 15 H, C₅Me₅), 1.80

[Cp*Ti(N^XylN)(N-2,4,6-C₆H₂Me₃)] (3). Suitable crystals for X-

ray analysis were obtained from a concentrated hexane solution at

◦

-18 C. (Found: C 73.77, H 8.22, N 8.32. Calcd for C₃₁H₄₁N₃Ti:

2

t

(d, J_H–H= 10.9 Hz, 1 H, NH₂Bu,), 1.72 (m, 2 H, NCH₂CH₂),

C 73.94, H 8.21, N 8.34%). n_max(KBr)/cm^-1: 2910 (m), 2854 (m),

1.40 (s, 9 H, N^tBu), 1.37 (s, 9 H, NH₂Bu). ¹³C{ H} NMR

(100.6 MHz, toluene-d₈, 218 K): d 173.0 (NCN), 154.1 (ipso-

C₆H₃Me₂), 138.0 (meta-C₆H₃Me₂), 122.8 (para-C₆H₃Me₂), 121.0

(ortho-C₆H₃Me₂), 116.8 (C₅Me₅), 67.1 (NC(CH₃)₃), 57.5 (NCH₂),

50.4 (NH₂C(CH₃)₃), 33.4 (NC(CH₃)₃), 30.8 (NH₂C(CH₃)₂), 29.8

(NCCH₂), 25.6 (NCH₂CH₂), 22.0 (C₆H₃Me₂), 12.2 (C₅Me₅). ¹⁵N

NMR (60.8 MHz, benzene-d₆, 295 K): d 407.3 (NC(CH₃)₃, 204.6

(XylNCN), 199.0 (XylNCN), 79.4 (NH₂C(CH₃)₃).

t

1

1593 (s), 1502 (s), 1465 (s), 1309 (s), 1286 (s), 782 (s). H NMR

(399.9 MHz, benzene-d₆, 295 K): d 6.82 (s, 2 H, meta-C₆H₂Me₃),

6.66 (s, 2 H, ortho-C₆H₃Me₂), 6.60 (s, 1 H, para-C₆H₃Me₂), 3.48

(m, 2 H, NCH₂), 2.47 (m, 1 H, NCCH₂), 2.38 (s, 6 H, ortho-

C₆H₂Me₃), 2.25 (s, 6 H, C₆H₃Me₂), 2.22 (s, 3 H, para-C₆H₂Me₃),

2.17 (m, 1 H, NCCH₂), 1.99 (s, 15 H, C₅Me₅), 1.51 (m, 2 H,

1

NCH₂CH₂). ¹³C{ H} NMR (100.6 MHz, benzene-d₆, 295 K):

d 167.8 (NCN), 156.7 (ipso-C₆H₂Me₃), 148.4 (ipso-C₆H₃Me₂),

138.4 (meta-C₆H₃Me₂), 131.4 (ortho-C₆H₂Me₃), 128.5 (meta-

C₆H₂Me₃), 128.2 (para-C₆H₂Me₃), 123.6 (para-C₆H₃Me₂), 121.1

(C₅Me₅), 120.2 (ortho-C₆H₃Me₂), 53.5 (NCH₂), 29.8 (NCCH₂),

24.3 (NCH₂CH₂), 21.6 (C₆H₃Me₂), 21.2 (para-C₆H₂Me₃), 19.9

(ortho-C₆H₂Me₃), 11.5 (C₅Me₅).

NMR tube generation of [Cp*Ti(N^XylN)(N^tBu)] (2a) and

[Cp*Ti(N^XylN)(N^tBu)(Py)] (2b). A solution of [Cp*Ti(N^XylN)-

t

(N^tBu)(NH₂Bu)] (2) (36 mg, 0.07 mmol) in benzene-d₆(0.5 mL)

was added to B(C₆F₅)₃(36 mg, 0.07 mmol). The product was

added to a Teﬂon valve NMR tube, sealed, and characterized

in situ.

[Cp*Ti(N^XylN)(N-2,6-C₆H₃(ⁱPr)₂)] (4). Suitable crystals for X-

ray diffraction were obtained from a concentrated hexane solution

at 10 ^◦C. (Found: C 74.82, H 8.67, N 7.62. Calcd for C₃₄H₄₇N₃Ti: C

74.84, H 8.68, N 7.70%). n_max(KBr)/cm^-1: 3039 (w), 2960 (m), 2913

(m), 1619 (m), 1501 (m), 1459 (m), 1438 (m), 1355 (m), 784 (s). ¹H

NMR (399.9 MHz, benzene-d₆, 295 K): d 7.05 (d, ³J_H–H= 7.6 Hz, 2

H, meta-C₆H₃(ⁱPr)₂), 6.88 (t, ³J_H–H= 7.6 Hz, 1 H, para-C₆H₃(ⁱPr)₂),

6.59 (overlapping s, 3 H, ortho-C₆H₃Me₂, para-C₆H₃Me₂), 3.64

[Cp*Ti(N^XylN)(N^tBu)(Py)] (2a). ¹H NMR (399.9 MHz,

benzene-d₆, 293 K): d 6.64 (s, 2 H, ortho-C₆H₃Me₂), 6.58 (s, 1 H,

para-C₆H₃Me₂), 3.51 (m, 2 H, NCH₂), 2.60 (m, 1 H, NCCH₂), 2.29

(m, 1 H, NCCH₂), 2.24 (s, 6 H, C₆H₃Me₂), 2.00 (s, 15 H, C₅Me₅),

1

1.21 (s, 9 H, NC(CH₃)₃), 1.68 (m, 2 H, NCH₂CH₂). ¹³C{ H}

NMR (100.6 MHz, benzene-d₆, 293 K): d 168.1 (NCN), 149.2

(ipso-C₆H₃Me₂), 138.2 (meta-C₆H₃Me₂), 123.4 (para-C₆H₃Me₂),

120.3 (ortho-C₆H₃Me₂), 119.8 (C₅Me₅), 67.3 (NC(CH₃)), 54.1

(NCH₂), 33.2 (NC(CH₃)), 29.4 (NCCH₂), 24.4 (NCH₂CH₂), 21.6

(C₆H₃Me₂), 11.8 (C₅Me₅). Subsequently, few drops of pyridine

were added and the solvent was removed after 30 min. The residue

was redissolved in benzene-d₆and the NMR spectra of 2b were

recorded.

3

(septet, J_H–H= 6.9 Hz, 2 H, C₆H₃(CH(CH₃)₂)₂), 3.54 (m, 1 H,

NCH₂), 3.40 (m, 1 H, NCH₂), 2.49 (m, 1 H, NCCH₂), 2.26 (m, 1

H, NCCH₂), 2.27 (s, 6 H, C₆H₃Me₂), 1.98 (s, 15 H, C₅Me₅), 1.56 (m,

2 H, NCH₂CH₂), 1.37 (d, ³J_H–H= 6.4 Hz, 6 H, C₆H₃(CH(CH₃)₂)₂),

3

1

1.32 (d, J_H–H= 7.2 Hz, 6 H, C₆H₃(CH(CH₃)₂)₂). ¹³C{ H}

NMR (100.6 MHz, benzene-d₆, 295 K): d 167.0 (NCN), 155.2

(ipso-C₆H₃(ⁱPr)₂), 147.9 (ipso-C₆H₃Me₂), 143.3 (ortho-C₆H₃(ⁱPr)₂),

138.4 (meta-C₆H₃Me₂), 123.4 (ortho- or para-C₆H₃Me₂), 122.3

(meta-C₆H₃(ⁱPr)₂), 121.4 (C₅Me₅), 120.3 (para-C₆H₃(ⁱPr)₂), 119.6

(ortho- or para-C₆H₃Me₂), 52.3 (NCH₂), 30.2 (NCCH₂), 28.4

(C₆H₃(CH(CH₃)₂)₂), 24.6 (NCH₂CH₂), 24.4 (C₆H₃(CH(CH₃)₂)₂),

24.2 (C₆H₃(CH(CH₃)₂)₂), 21.7 (C₆H₃Me₂), 11.5 (C₅Me₅).

[Cp*Ti(N^XylN)(N^tBu)(Py)] (2b). ¹H NMR (399.9 MHz,

benzene-d₆, 293 K): d 8.49 (d, 2 H, ³J_H–H= 4 Hz; ortho-C₅H₅N),

6.95 (t, ³J_H–H= 7.5 Hz, 1 H, para-C₅H₅N), 6.63–6.60 (m, 2 H, meta-

C₅H₅N, overlapping with ortho-C₆H₃Me₂), 6.61 (s, 2 H, ortho-

C₆H₃Me₂, overlapping with Py), 6.58 (s, 1 H, para-C₆H₃Me₂),

3.61 (m, 2 H, NCH₂), 2.38 (broad m, 2 H, NCH₂CH₂), 2.22 (s, 6

H, C₆H₃Me₂), 2.07 (s, 15 H, C₅Me₅), 1.70 (m, 2 H, NCCH₂), 1.23

[Cp*Ti(N^XylN)(N-2,6-C₆H₃Me₂)] (5). Suitable crystals for X-

ray diffraction were obtained from a concentrated hexane solution

at -18 ^◦C. (Found: C 73.60, H 8.02, N 8.70. Calcd for C₃₀H₃₉N₃Ti:

C 73.60, H 8.03, N 8.58%). n_max(KBr)/cm^-1: 3024 (w), 2950

1

(s, 9 H, NC(CH₃)₃). ¹³C{ H} NMR (100.6 MHz, benzene-d₆, 293

K): d 168.8 (NCN), 150.4 (ortho-C₅H₅N), 141.8 (ipso-C₆H₃Me₂),

138.0 (meta-C₆H₃Me₂), 135.7 (para-C₅H₅N), 123.5 (meta-C₅H₅N),

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(m), 2910 (m), 2853 (m), 1596 (s), 1492 (s), 1457 (s), 1291 (s),

General preparative procedure for the synthesis of acetylido-

amido complexes [Cp*Ti(N^XylN)(CCPh)(NH^tBu)] (7) and

[Cp*Ti(N^XylN)(CCTol)(NH^tBu)] (8). To a solution of [Cp*Ti-

(N^XylN)(N^tBu)(NH^tBu)] (2) (327 mg, 0.637 mmol) in toluene (20

mL) was added the appropriate acetylene (70 mL, 0.637 mmol),

producing an immediate colour change to bright orange. After

stirring for 1 h at room temperature the solvent was removed and

the product recrystallized as a bright yellow solid from pentane at

1196 (m). ¹H NMR (600.1 MHz, benzene-d₆, 295 K): d 7.03

3

(d, J_H–H= 7.6 Hz, 2 H, meta-2,6-C₆H₃Me₂), 6.74 (t, J_H–H

=

7.1 Hz, 1 H, para-2,6-C₆H₃Me₂), 6.65 (s, 2 H, ortho-3,5-C₆H₃Me₂),

6.60 (s, 1 H, para-3,5-C₆H₃Me₂), 3.45 (m, 2 H, NCH₂), 2.47

(m, 1 H, NCCH₂), 2.37 (s, 6 H, 2,6-C₆H₃Me₂), 2.24 (s, 6 H,

3,5-C₆H₃Me₂), 2.16 (m, 1 H, NCCH₂), 1.98 (s, 15 H, C₅Me₅),

1.52 (m, 2 H, NCH₂CH₂). ¹³C{ H} NMR (150.1 MHz, benzene-

1

◦

d₆, 293 K): d 167.9 (NCN), 158.6 (ipso-2,6-C₆H₃Me₂), 148.1

(ipso-3,5-C₆H₃Me₂), 138.3 (meta-3,5-C₆H₃Me₂), 131.5 (ortho-2,6-

C₆H₃Me₂), 127.6 (meta-2,6-C₆H₃Me₂), 123.7 (para-3,5-C₆H₃Me₂),

121.2 (C₅Me₅), 120.1 (ortho-3,5-C₆H₃Me₂), 119.2 (para-2,6-

C₆H₃Me₂), 53.4 (NCH₂), 29.6 (NCCH₂), 24.2 (NCH₂CH₂), 21.4

(3,5-C₆H₃Me₂), 19.8 (2,6-C₆H₃Me₂), 11.4 (C₅Me₅).

-80 C. ([Cp*Ti(N^XylN)(CCPh)(NH^tBu)] (7) 219 mg, 63% yield,

[Cp*Ti(N^XylN)(CC-p-Tol)(NH^tBu)] (8) 217 mg, 58% yield).

[Cp*Ti(N^XylN)(CCPh)(NH^tBu)] (7). (Found: C 74.88, H 8.33,

N 7.85. Calcd for C₃₄H₄₅N₃Ti: C 75.12, H 8.34, N 7.73%). n_max

(KBr)/cm^-1: 3332 (w), 3284 (w), 2948 (m), 2910 (m), 1860 (m),

1594 (s), 1558 (s), 1459 (s), 1374 (s), 1287 (s), 1095 (m). ¹H NMR

(399.9 MHz, benzene-d₆, 295 K): d 8.46 (s, 1 H, NH^tBu), 7.59 (d,

³J_H–H= 7.1 Hz, 2 H, ortho-C₆H₅), 7.13 (m, 2 H, meta-C₆H₅), 7.00

(t, ³J_H–H= 7.4 Hz, 1 H, para-C₆H₅), 6.74 (s, 2 H, ortho-C₆H₃Me₂),

6.56 (s, 1 H, para-C₆H₃Me₂), 4.24 (m, 1 H, NCH₂), 3.57 (m, 1 H,

NCH₂), 2.49 (m, 1 H, NCCH₂), 2.27 (s, 6 H, C₆H₃Me₂), 2.13 (m, 1

H, NCCH₂), 2.08 (s, 15 H, C₅Me₅), 1.60 (m, 2 H, NCH₂CH₂), 1.39

NMR tube generation of [Cp*Ti(N^XylN)(N-4-C₆H₄Me)-

(NH₂Bu)] (6a). In the glove box [Cp*Ti(N^XylN)(N^tBu)-

t

(NH₂Bu)] (2) (24 mg, 0.05 mmol) was dissolved in benzene-d₆

(0.5 mL), p-toluidine (5 mg, 0.05 mmol) was added and the NMR

1

spectra were recorded. H NMR (399.9 MHz, benzene-d₆, 293

3

K): d 7.00 (d, J_H–H= 8.0 Hz, 2 H, ortho-or meta-C₆H₄Me),

1

(s, 9 H, NHC(CH₃)₃). ¹³C{ H} NMR (100.6 MHz, benzene-d₆, 295

3

6.89 (d, J_H–H= 7.9 Hz, 2 H, ortho-or meta-C₆H₄Me), 6.69 (s,

K): d 173.1 (NCN), 153.2 (CCPh), 147.9 (ipso-C₆H₃Me₂), 137.7

(meta-C₆H₃Me₂), 131.0 (ortho-C₆H₅), 128.4 (meta-C₆H₅), 125.6

(para-C₆H₅), 122.8 (C₅Me₅), 122.6 (para-C₆H₃Me₂), 120.2 (ortho-

C₆H₃Me₂), 108.8 (CCPh), 60.3 (NHC(CH₃)₃), 53.3 (NCH₂), 31.2

(NHC(CH₃)₃), 30.2 (NCCH₂), 24.5 (NCH₂CH₂), 21.6 (C₆H₃Me₂),

13.2 (C₅Me₅). ¹⁵N NMR (60.8 MHz, benzene-d₆, 295 K): d 331.1

(NH^tBu, ¹J_N–H= 62.6 Hz), 216.2 (NCNXyl), 174.5 (NCNXyl).

2 H, ortho-C₆H₃Me₂), 6.63 (s, 1 H, para-C₆H₃Me₂), 4.18 (m,

1 H, NCH₂), 3.66 (m, 1 H, NCH₂), 2.38 (m, 1 H, NCCH₂),

2.28 (s, 6 H, C₆H₃Me₂), 2.21 (s, 3 H, para-C₆H₄Me), 2.05 (m, 1

H, NCCH₂), 1.97 (s, 15 H, C₅Me₅), 1.61 (m, 2 H, NCH₂CH₂),

1

1.25 (s, 9 H, NH₂C(CH₃)₃), NH₂C(CH₃) not observed. ¹³C{ H}

NMR (100.6 MHz, benzene-d₆, 293 K): d 173.4 (NCN), 153.6

(broad ispo-C₆H₃Me₂), 138.3 (meta-C₆H₃Me₂), 129.4 (ortho- or

meta-C₆H₄Me₂), 123.8 (broad para-C₆H₃Me₂and ortho- or meta-

C₆H₄Me), 121.0 (ortho-C₆H₃Me₂), 118.8 (C₅Me₅), 51.2 (broad

H₂NC(CH₃)₃), 48.0 (broad NCH₂), 32.2 (NH₂C(CH₃)₃), 29.8

(NCCH₂); 23.0 (NCH₂CH₂); 21.8 (C₆H₃Me₂); 20.9 (C₆H₄Me);

11.6 (C₅Me₅), ipso-C₆H₄Me not observed.

[Cp*Ti(N^XylN)(CC-p-Tol)(NH^tBu)] (8). (Found: C 72.06, H

8.31, N 7.13. Calcd for C₃₅H₄₇N₃Ti: C 75.38, H 8.49, N 7.53%,

value for carbon is too low owing to incomplete combustion). n_max

(KBr)/cm^-1: 3429 (w), 2956 (m), 2910 (m), 2859 (m), 1592 (m),

1504 (s), 1512 (s), 1467 (m), 1289 (s), 1201 (m), 1185 (m), 1097 (m).

¹H NMR (399.9 MHz, benzene-d₆, 295 K): d 8.46 (s, 1 H, NH^tBu),

[Cp*Ti(N^XylN)(N-4-C₆H₄Me)(NC₅H₅)] (6). To a solution of

3

7.56 (d, J_H–H= 7.5 Hz, 2 H, ortho-C₆H₄Me), 6.99 (d, J_H–H

=

[Cp*Ti(N^XylN)(N^tBu)(NH₂Bu)] (2) (564 mg, 1.1 mmol) in toluene

t

7.9 Hz, 2 H, meta-C₆H₄Me), 6.75 (s, 2 H, ortho-C₆H₃Me₂), 6.56 (s,

1 H, para-C₆H₃Me₂), 4.29 (m, 1 H, NCH₂), 3.59 (m, 1 H, NCH₂),

2.47 (m, 1 H, NCCH₂), 2.27 (s, 6 H, C₆H₃Me₂), 2.11 (m, 1 H,

NCCH₂), 2.09 (s, 15 H, C₅Me₅), 2.08 (s, 3 H, C₆H₄Me), 1.60 (m, 2

was added a solution of p-toluidine (110 mg, 1.1 mmol) in toluene

dropwise. The dark red solution was stirred for 1 h before pyridine

(0.2 ml, 2.2 mmol) was added. The reaction mixture was stirred

an additional hour at room temperature before all volatiles were

removed under reduced pressure. The residue was washed with

pentane and the product was obtained as orange powder (305 mg,

50% yield). Found C 73.21, H 7.64, N 9.91. Calcd for C₃₄H₄₂N₄Ti:

C 73.63, H 7.63, N 10.10%. n_max(KBr)/cm^-1: 2964 (m), 2911 (m),

1858 (m), 1620 (s), 1598 (s), 1536 (s), 1485 (s), 1309 (s), 1288 (s). ¹H

NMR (399.9 MHz, benzene-d₆, 295 K): d 8.74 (d, ³J_H–H= 5.1 Hz,

2 H, ortho-C₅H₅N,), 7.05–7.00 (m, 4 H, ortho- and meta-C₆H₄Me),

6.85 (s, 2 H, ortho-C₆H₃Me₂), 6.81 (m, 1 H, para-C₅H₅N), 6.60 (s,

1 H, para-C₆H₃Me₂), 6.53 (m, 2 H, meta-C₅H₅N), 3.40 (broad

m, 2 H, NCH₂), 2.22 (s, 3 H, C₆H₄Me), 2.21 (s, 6 H, C₆H₃Me₂),

H, NCH₂CH₂). ¹³C{ H} NMR (100.6 MHz, benzene-d₆, 295 K): d

1

173.1 (NCN), 152.5 (CC-Tol), 148.0 (ipso-C₆H₃Me₂), 137.7 (meta-

C₆H₃Me₂), 135.1 (ipso-C₆H₄Me), 131.0 (ortho-C₆H₄Me), 129.2

(meta-C₆H₄Me), 125.5 (para-C₆H₄Me), 122.8 (C₅Me₅), 122.5

(ortho-C₆H₃Me₂), 120.1 (para-C₆H₃Me₂), 108.6 (CC-Tol), 60.4

(NHC(CH₃)₃), 53.3 (NCH₂), 31.2 (NHC(CH₃)₃), 30.2 (NCCH₂),

24.5 (NCH₂CH₂), 21.7 (C₆H₃Me₂), 21.3 (C₆H₄Me), 13.1 (C₅Me₅).

¹⁵N NMR (60.8 MHz, benzene-d₆, 295 K): d 331.1 (NH^tBu,

¹J_N–H= 62.6 Hz), 216.2 (NCNXyl), 174.5 (NCNXyl).

General procedure for the synthesis of azatitanacycles

1

2

1.99 (s, 15 H, C₅Me₅), 1.50 (m, 2 H, NCH₂CH₂). ¹³C{ H} NMR

Cp*Ti(N^XylN){j N(2,4,6-C₆H₂Me₃)CPh CH}]

(9),

[Cp*Ti-

=

2

(N^XylN){j N(2,6-C₆H₃Me₂)CPh CH}] (10), and Cp*Ti(N^XylN)-

=

(100.6 MHz, benzene-d₆, 295 K): d 172.7 (NCN), 158.5 (ipso-

C₆H₄Me), 152.5 (ortho-C₅H₅N), 151.3 (ipso-C₆H₃Me₂), 137.6

(meta-C₆H₃Me₂), 136.6 (para-C₅H₅N), 129.1 (ortho- or meta-

C₆H₄Me), 123.8 (ortho- or meta-C₆H₄Me), 123.5 (para-C₆H₃Me₂),

123.4 (meta-C₅H₅N), 123.1 (ortho-C₆H₃Me₂), 118.2 (C₅Me₅), 54.2

(NCH₂), 29.3 (NCCH₂), 23.5 (NCH₂CH₂), 21.6 (C₆H₄Me₂), 21.6

(C₆H₃Me₂), 12.0 (C₅Me₅), not observed: para-C₆H₄Me.

2

=

{j N(2,6-C₆H₃Me₂)CTol CH}] (11). To

a stirred solution

of the appropriate imido complex (1.3 mmol) in toluene

(10 mL) was added the appropriate acetylene (2.6 mmol)

dropwise. The solution immediately turned dark brown.

After 4 h all volatiles were removed under reduced pressure

and the crude product was dissolved in pentane whilst

4598 | Dalton Trans., 2009, 4586–4602

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maintaining the temperature below

0

^◦C. The product

Calcd for C₃₉H₄₇N₃Ti: C 77.33, H 7.82, N 6.94%). n_max(KBr)/cm^-1:

crystallized at -30 ^◦C as a deep brown microcrystalline solid.

3024 (w), 2954 (m), 2914 (s), 2856 (m), 1638 (m), 1590 (s), 1503

2

1

([Cp*Ti(N^XylN){k N(2,4,6-C₆H₂Me₃)CPh CH}] (9): 377 mg,

(s), 1467 (s), 1375 (m), 1292 (s), 1181 (m), 1090 (m), 785 (s). H

=

2

48% yield, ([Cp*Ti(N^XylN){k N(2,6-C₆H₃Me₂)CPh CH}] (10):

NMR (600.1 MHz, toluene-d₈, 263 K): d 11.14 (s, 1 H, C CH),

=

2

472 mg, 74% yield, [Cp*Ti(N^XylN){k N(2,6-C₆H₃Me₂)CTol CH}]

7.12 (s, 1 H, meta-2,6-C₆H₃Me₂, overlapping with toluene-d₈) 7.36

=

3

(11): 471 mg, 60% yield).

(d, J_H–H= 8.1 Hz, 2 H, ortho-C₆H₄Me), 6,86–6.77 (m, 2 H,

3

meta-C₆H₄Me and m, 1 H, para-2,6-C₆H₃Me₂), 6.65 (d, J_H–H

=

2

[Cp*Ti(N^XylN){j N(2,4,6-C₆H₂Me₃)CPh CH}] (9). (Found:

=

7.9 Hz, 1 H, meta-2,6-C₆H₃Me₂), 6.52 (s, 3 H, ortho- and para-3,5-

C₆H₃Me₂), 3.53 (m, 1 H, NCH₂), 3.05–3.01 (m, 1 H, NCH₂), 2.59

(s, 3 H, 2,6-C₆H₃Me₂) 2.41–2.36 (m, 1 H, NCCH₂), 2.26 (s, 6 H, 3,5-

C₆H₃Me₂), 2.10 (2,6-C₆H₃Me₂overlapping with toluene-d₈), 1.94–

1.91 (m, 1 H, NCCH₂, overlapping with C₆H₄Me and C₅Me₅), 1.94

(s, 3 H, C₆H₄Me), 1.91 (s, 15 H, C₅Me₅), 1.44 (m, 1 H, NCH₂CH₂),

C 76.94, H 7.81, N 6.58. Calcd for C₃₉H₄₇N₃Ti: C 77.33, H 7.82,

N 6.94%). n_max(KBr)/cm^-1: 2943 (w), 2009 (m), 2855 (m), 1637

(m), 1592 (s), 1534 (s), 1482 (s), 1433 (s), 1308 (m), 1274 (m),

1

788 (s). H NMR (399.9 MHz, benzene-d₆, 295 K): d 11.10 (s,

=

1 H, C CH), 7.46 (m, 2 H, ortho-C₆H₅), 7.03 (m, 2 H, meta-

C₆H₅), 6.93 (s, 1 H, meta-C₆H₂Me₃), 6.90 (m, 1 H, para-C₆H₅),

6.55 (s, 3 H, overlapping ortho- and para-C₆H₃Me₂), 6.44 (s,

1 H, meta-C₆H₂Me₃), 3.57 (m, 1 H, NCH₂), 3.05 (m, 1 H,

NCH₂), 2.48 (m, 1 H, NCCH₂), 2.56 (s, 3 H, para-C₆H₂Me₃),

2.24 (s, 6 H, C₆H₃Me₂), 2.10 (s, 3 H, ortho-C₆H₂Me₃), 2.08 (s,

3 H, ortho-C₆H₂Me₃), 1.99 (m, 1 H, NCCH₂), 1.90 (s, 15 H,

C₅Me₅), 1.46 (m, 1 H, NCH₂CH₂), 1.02 (m, 1 H, NCH₂CH₂).

1.25 (m, 1 H, NCH₂CH₂). ¹³C{ H} NMR (150.9 MHz, toluene-

1

=

d₈, 263 K): d 228.9 (C CH), 174.1 (NCN), 152.2 (ipso-2,6-

=

C₆H₃Me₂), 148.9 (ipso-3,5-C₆H₃Me₂), 147.5 (C CH), 138.1 (meta-

3,5-C₆H₃Me₂), 134.7 (ispo-C₆H₄Me), 132.4 (ortho-2,6-C₆H₃Me₂),

131.0 (para-C₆H₄Me), 129.7 (meta-C₆H₄Me), 129.6 (meta-2,6-

C₆H₃Me₂), 128.7 (meta-2,6-C₆H₃Me₂), 127.2 (ortho-C₆H₄Me),

124.2 (ortho- or para-3,5-C₆H₃Me₂), 122.7 (para-2,6-C₆H₃Me₂),

122.5 (C₅Me₅), 120.8 (ortho- or para-3,5-C₆H₃Me₂), 53.8 (NCH₂),

35.0 (NCCH₂), 30.9 (NCH₂CH₂), 22.2 (meta-3,5-C₆H₃Me₂), 21.6

(para-C₆H₄Me), 20.5 (ortho-2,6- C₆H₃Me₂), 12.5 (C₅Me₅). Second

ortho-2,6-C₆H₃Me₂not observed.

13

1

=

C{ H} NMR (100.6 MHz, benzene-d₆, 295 K): d 228.4 (C CH),

173.7 (NCN), 149.0 (ipso-C₆H₂Me₃), 148.7 (ipso-C₆H₃Me₂), 147.3

=

(C CH), 137.7 (meta-C₆H₃Me₂), 137.4 (ipso-C₆H₅), 131.8 (para-

C₆H₂Me₃), 131.1 (ortho-C₆H₂Me₃), 129.7 (meta-C₆H₂Me₃), 128.6

(meta-C₆H₂Me₃), 127.9 (para-C₆H₅), 127.5 (meta-C₆H₅), 126.9

(ortho-C₆H₅), 123.8 (ortho- or para-C₆H₃Me₂), 122.6 (C₅Me₅),

120.5 (ortho- or para-C₆H₃Me₂), 53.2 (NCH₂), 30.4 (NCCH₂), 23.9

(NCH₂CH₂), 21.7 (C₆H₃Me₂), 20.8 (ortho-C₆H₂Me₃), 20.5 (para-

C₆H₂Me₃), 19.8 (ortho-C₆H₂Me₃), 12.0 (C₅Me₅). Second ortho-

C₆H₂Me₃not observed.

(B) Kinetic Studies of the {2 + 2}-cyloaddition of phenylacetylene

to the imido complex 5, the cycloreversion reaction of the

metallacycle 10 to the imido compound 5 and phenylacetylene and

the aminolysis of the azatitanacyle 10 with 2,6-dimethylaniline

A solution of [Cp*Ti(N^XylN)(N-2,6-C₆H₃Me₂)] (5) (9.8 mg,

20 mmol) and 1,4-dimethoxybenzene (3 mg, internal standard)

in toluene-d₈was transferred into a J. Young NMR tube. After

cooling the solution to 10 ^◦C, 0.5–5 equiv. of phenylacetylene

(10–100 mmol) were added. The tube was transferred to a NMR

spectrometer probe that had been pre-cooled to 10 ^◦C. ¹H NMR

spectra were recorded every 3 min for a period of up to 30 min.

The concentration of the reaction product was plotted against

time, and the conversion curve was line-ﬁtted to a ﬁrst order

exponential decay Aexp(-x/b). The initial rate was estimated from

-A/b, derived from differentiation of the ﬁtted line for x = 0. A

plot of the initial rate versus alkyne concentration indicated a linear

relationship.

2

[Cp*Ti(N^XylN){j N(2,6-C₆H₃Me₂)CPh CH}] (10). (Found:

=

C 76.76, H 7.67, N 7.00. Calcd for C₃₈H₄₅N₃Ti: C 77.14, H

7.67, N 7.10%). n_max(KBr)/cm^-1: 3020 (w), 2960 (m), 2912 (m),

2856 (m), 1636 (m), 1591 (s), 1503 (s), 1485 (s), 1293 (s), 1090

1

(m), 784 (s). H NMR (600.1 MHz, toluene-d₈, 258 K): d 11.08

3

=

(s, 1 H, C CH), 7.44 (d, J_H–H= 8.0 Hz, 2 H, ortho-C₆H₅),

3

7.10 (d, J_H–H= 7.8 Hz, 1 H, meta-2,6-C₆H₃Me₂), 7.04–6.99 (m,

3

meta-C₆H₅, overlapping with toluene-d₈signal), 6.89 (t, J_H–H

=

3

7.2 Hz, 1 H, para-C₆H₅), 6.76 (t, J_H–H= 7.2 Hz, 1 H, para-2,6-

C₆H₃Me₂,), 6.61 (d, ³J_H–H= 7.7 Hz, 1 H, meta-2,6-C₆H₃Me₂), 6.52

(s, 1 H, para-3,5-C₆H₃Me₂), 6.50 (s, 2 H, ortho-3,5-C₆H₃Me₂),

3.51 (m, 1 H, NCH₂), 3.00 (m, 1 H, NCH₂), 2.57 (s, 3 H,

ortho-2,6-C₆H₃Me₂), 2.36 (m, 1 H, NCCH₂), 2.26 (s, 6 H, meta-

3,5-C₆H₃Me₂), 2.05 (s, 3 H, ortho-2,6-C₆H₃Me₂), 1.90 (m, 1 H,

NCCH₂), 1.88 (s, 15 H, C₅Me₅), 1.43 (m, 1 H, NCH₂CH₂), 0.84 (m,

A solution of [Cp*Ti(N^XylN)(N-2,6-C₆H₃Me₂)] (5) (5–25 mmol)

and 1,4-dimethoxybenzene (3 mg, internal standard) in toluene-

d₈was transferred into a J. Young NMR tube. After cooling the

solution to 0 ^◦C, 20 equiv. of phenylacetylene (0.1–0.5 mmol)

were added. The tube was transferred to a NMR spectrometer

probe that had been pre-cooled to 0 ^◦C. ¹H NMR spectra

were recorded every 3 min for a period of up to 30 min. The

concentration of the reaction product was plotted against time,

and the conversion curve was line-ﬁtted to a ﬁrst order exponential

decay Aexp(-x/b). The initial rate was estimated from -A/b,

derived from differentiation of the ﬁtted line for x = 0. A plot

of the initial rate versus titanium concentration indicated a linear

relationship.

1 H, NCH₂CH₂). ¹³C{ H} NMR (150.9 MHz, toluene-d₈, 258 K):

1

=

d 228.2 (C CH), 174.1 (NCN), 152.0 (ipso-2,6-C₆H₃Me₂), 148.9

(ipso-3,5-C₆H₃Me₂), 147.2 (C=CH), 138.1 (meta-3,5-C₆H₃Me₂),

137.6 (ipso-C₆H₅), 132.4 (ortho-2,6-C₆H₃Me₂), 130.9 (ortho-

2,6-C₆H₃Me₂), 129.5 (meta-2,6-C₆H₃Me₂), 128.8 (meta-C₆H₅),

128.5 (para-C₆H₅), 128.3 (meta-2,6-C₆H₃Me₂), 127.2 (ortho-

C₆H₅), 124.7 (para-3,5-C₆H₃Me₂), 123.0 (C₅Me₅), 122.8 (para-

2,6-C₆H₃Me₂), 120.7 (ortho-3,5-C₆H₃Me₂), 53.7 (NCH₂), 30.81

(NCCH₂), 24.4 (NCH₂CH₂), 22.3 (3,5-C₆H₃Me₂), 21.2 (2,6-

C₆H₃Me₂), 20.5 (2,6-C₆H₃Me₂), 12.5 (C₅Me₅).

[Cp*Ti(N^XylN){j N(2,6-C₆H₃Me₂)CTol CH}] (11). Suitable

A solution of [Cp*Ti(N^XylN)(N-2,6-C₆H₃Me₂)] (5) (20 mmol) in

toluene-d₈(0.5 mL) was transferred into a J. Young NMR tube.

After cooling the solution to 0 ^◦C, 5 ^◦C, 7.5 ^◦C or 10 ^◦C a solution

2

=

crystals for X-ray diffraction were obtained from a concentrated

pentane solution at -30 ^◦C. (Found: C 76.77, H 8.19, N 6.46.

This journal is

The Royal Society of Chemistry 2009

Dalton Trans., 2009, 4586–4602 | 4599

©

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of phenylacetylene (20 mmol) in toluene-d₈(0.1 mL) was added.

The tube was transferred to a NMR spectrometer probe that had

been pre-cooled. ¹H NMR spectra were recorded every 3 min for

a period of up to 60 min and then every 10 min for a period of up

to 50 min. (1/[I]) - (1/[I₀]) was plotted against the time (sec). The

second order rate coefﬁcient k₁was obtained from the slope of the

graph. For the Arrhenius plot the rate coefﬁcients were plotted

against 1/T to yield a straight line. The rate coefﬁcient for 20 ^◦C

(1 mL). The resulting mixture was heated to 105 ^◦C for 24 h.

Then the mixture was cooled to room temperature and a mixture

of NaBH₃CN (4.80 mmol) and ZnCl₂(2.4 mmol) in methanol

(10 mL) was added. After this mixture had been stirred at 25 ^◦C for

at least 20 h, dichloromethane and a saturated aqueous solution

of Na₂CO₃were added. The mixture was ﬁltered and the solid

residue was washed with dichloromethane. After extraction, the

organic phase was separated. The aqueous layer was extracted

with dichloromethane and the combined organic layers were

dried over Na₂SO₄. After concentration under vacuum, the

residue was puriﬁed by ﬂash chromatography (light petroleum

ether/EtOAc/SiO₂).

was estimated graphically as 1.13 ¥ 10^-2L mol^-1s^-1.

2

A solution of [Cp*Ti(N^XylN){k N(2,6-C₆H₃Me₂)CPh CH}]

=

(10) (5–25 mmol) and 1,4-dimethoxybenzene (3 mg, internal

standard) in benzene-d₆was transferred into a J. Young NMR

tube. The tube was transferred to a NMR spectrometer probe that

◦

1

had been pre-cooled to 20 C. H NMR spectra were recorded

(D) General method for intramolecular hydroaminations

every 5–10 min for a period of up to 50–100 min. The concentration

An oven dried Schlenk tube equipped with a Teﬂon valve stopcock

and a magnetic stirring bar was transferred into a nitrogen ﬁlled

glovebox and charged with the catalyst (0.12 mmol, 5 mol%).

The aminoalkyne (2.4 mmol) was subsequently added and the

mixture was dissolved in t_◦oluene (1 mL). The resulting solution

was heated for 6 h at 105 C. The mixture was allowed to reach

room temperature before a mixture of NaCNBH₃(4.8 mmol),

ZnCl₂(2.4 mmol) and methanol (10 mL) was added. The mixture

was stirred at room temperature for 20 h. Then dichloromethane

and a saturated aqueous solution of Na₂CO₃were added. After

ﬁltration the residue was washed with dichloromethane. The

organic layer was separated and the aqueous layer was extracted

with dichloromethane. The combined organic layers were dried

over MgSO₄and the solvent was removed under reduced pressure.

The resulting product was puriﬁed by ﬂash chromatography

(EtOAc/3% NH₃in MeOH/SiO₂).

2

of [Cp*Ti(N^XylN){k N(2,6-C₆H₃Me₂)CPh CH}] (10) was plotted

=

against time, and the conversion curve was line-ﬁtted to a

ﬁrst order exponential decay Aexp(-x/b). The initial rate was

estimated from -A/b, derived from differentiation of the ﬁtted

line for x = 0. A plot of the initial rate versus the concentration

2

of [Cp*Ti(N^XylN){k N(2,6-C₆H₃Me₂)CPh CH}] (10) indicated a

=

linear relationship, from which the rate coefﬁcient k_-1was obtained

from the slope of the graph as 3,4 ¥ 10^-5s^-1.

2

To

a

solution

of

the

[Cp*Ti(N^XylN){k N(2,6-

=

C₆H₃Me₂)CPh CH}] (10) (5–50 mmol) and 1,4-dimethoxybenzene

(3 mg, internal standard) in benzene-d₆, 20 equiv. 2,6-

dimethylaniline (0.1–0.5 mmol) were added at 20 ^◦C. The

sample was transferred into a J. Young NMR tube. The tube was

transferred to a NMR spectrometer probe that had been pre-

cooled to 20 C. H NMR spectra were recorded every 3–5 min

for a period of up to 30–50 min. The concentration of the reaction

product was plotted against time, and the conversion curve was

line-ﬁtted to a ﬁrst order exponential decay Aexp(-x/b). The

initial rate was estimated from -A/b, derived from differentiation

◦

1

(E) Computational study

All calculations were performed on the Gaussian 03 suite of

programs.²⁸Calculations were carried out using the hybrid

B3PW91 functional, with the 6–31G(d,p) basis set for all atoms.

Geometry optimisations were initially performed without geome-

try restraints, followed by frequency calculations to ascertain the

nature of the resulting structure (minimum vs. transition state).

Attempts to determine the corresponding structures for the real

(non-simpliﬁed) ligands were successful for the reaction products,

and gave structures which were comparable to the simpliﬁed

structures; however calculation of the transition states with the

non-simpliﬁed systems lead to multiple-order saddle points; single,

well-deﬁned transition states were not located.

of the ﬁtted line for x = 0. A plot of the initial rate versus the

2

concentration of [Cp*Ti(N^XylN){k N(2,6-C₆H₃Me₂)CPh CH}]

=

(10) indicated a linear relationship.

2

To a solution of [Cp*Ti(N^XylN){k N(2,6-C₆H₃Me₂)CPh CH}]

=

(10) (14,8 mg, 25 mmol) and 1,4-dimethoxybenzene (3 mg, internal

standard) in benzene-d₆was added 2,6-dimethylaniline (1–4 mmol)

at 20 ^◦C. The sample was transferred into a J. Young NMR tube.

The tube was transferred to a NMR spectrometer probe that

◦

1

had been pre-cooled to 20 C. H NMR spectra were recorded

every 5 min for a period of up to 60 min and then every

10 min for another hour. The reaction product was plotted against

time, and the conversion curve was line-ﬁtted to a ﬁrst order

exponential decay Aexp(-x/b). The initial rate was estimated

from -A/b, derived from differentiation of the ﬁtted line for

x = 0. A plot of the initial rate versus the concentration of 2,6-

dimethylaniline indicated a linear relationship. The pseudo-ﬁrst

order rate coefﬁcient was obtained from the slope of the line and

the second order rate coefﬁcient k₂was obtained from k₂¢ = k₂¥

[MC] as 1.58 ¥ 10^-3L mol^-1s^-1.

(F) X-Ray crystal structure determinations

Crystal data and details of the structure determinations are listed

in Table 7. Intensity data were collected at low temperature

with an Enraf-Nonius Kappa CCD (complex 1) and a Bruker

AXS Smart 1000 CCD diffractometer (MoKa radiation, graphite

˚

monochromator, l = 0.71073 A). Data were corrected for air

and detector absorption, Lorentz and polarization effects;^29,30

absorption by the crystal was treated with a semiempirical multi-

scan method.^31,32The structures were solved by the heavy atom

method combined with structure expansion by direct methods

applied to difference structure factors³³or by conventional direct

methods^34,35and reﬁned by full-matrix least squares methods

(C) General method for intermolecular hydroaminations

A Schlenk tube equipped with a Teﬂon stopcock and a magnetic

stirring bar was charged with the alkyne (2.40 mmol), the amine

(2.40 mmol), the catalyst (0.12 mmol, 5 mol%) and toluene

4600 | Dalton Trans., 2009, 4586–4602

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The Royal Society of Chemistry 2009

©

View Article Online

Table 7 Details of the crystal structure determinations of the complexes 1, 3, 4, 5 and 11

1

3

4

5

11

Empirical formula

C₂₄H₃₆N₂Ti

400.45

193(2)

0.71073

Triclinic

¯

P1

8.6070(17)

9.2070(18)

15.015(3)

91.00(3)

100.98(3)

106.19(3)

1118.6(4)

2

1.189

0.393

432

0.4 ¥ 0.35 ¥ 0.20

2.3–30.6

-12 ≤ h ≤ 11,

-13 ≤ k ≤ 13,

0 ≤ l ≤ 21

31 528

C₃₁H₄₁N₃Ti

503.57

100(2)

0.71073

Monoclinic

C₃₄H₄₇N₃Ti

545.65

100(2)

0.71073

Triclinic

¯

P1

8.0565(4)

10.2565(5)

18.9495(10)

86.2970(10)

84.8290(10)

79.4270(10)

1531.13(13)

2

1.184

0.306

588

0.30 ¥ 0.25 ¥ 0.20

2.0–31.0

-11 ≤ h ≤ 11,

-14 ≤ k ≤ 14,

0 ≤ l ≤ 27

37 750

C₃₀H₃₉N₃Ti

489.54

100(2)

0.71073

Monoclinic

C₄₄H₅₉N₃Ti

677.84

100(2)

0.71073

Triclinic

¯

P1

M_r

T/K

˚

l/A

Crystal system

Space group

C2/c

P2₁/c

˚

a/A

17.143(2)

25.0707(3)

14.581(2)

90

116.658(2)

90

5600.4(12)

8

1.194

13.8833(13)

9.7449(9)

20.1013(18)

90

100.422(2)

90

2674.7 (4)

4

1.216

9.509(2)

˚

b/A

12.983(3)

17.624(4)

105.994(4)

96.108(4)

108.513(4)

1937.9(7)

2

1.162

0.254

732

0.15 ¥ 0.15 ¥ 0.05

1.8–30.0

-13 ≤ h ≤ 13,

-18 ≤ k ≤ 17,

0 ≤ l ≤ 24

45 869

˚

c/A

a/^◦

b/^◦

g /^◦

3

˚

V/A

Z

D_c/Mg m^-3

m(MoKa)/mm^-1

F₀₀₀

0.329

2160

0.342

1048

Crystal size/mm³

0.40 ¥ 0.10 ¥ 0.10

2.3–30.0

-24 ≤ h ≤ 21,

0 ≤ k ≤ 35,

0 ≤ l ≤ 20

57 812

0.10 ¥ 0.10 ¥ 0.10

2.1–25.0

-16 ≤ h ≤ 16,

0 ≤ k ≤ 11,

0 ≤ l ≤ 23

35 562

q range/^◦

Index ranges (indep. set) h,k,l

Reﬂections measured

Unique [R_int

Max., min. transmission

factors

]

6832 [0.0224]

0.9095, 0.8347

8193 [0.0537]

0.7464, 0.5941

9660 [0.0453]

0.7464, 0.6805

4724 [0.0916]

0.7463, 0.6577

11 314 [0.0547]

0.7464, 0.6762

Data/restraints/parameters

6832/0/253

1.053

0.0397, 0.0991

8193/0/326

1.031

0.0398, 0.0930

9660/0/354

1.079

0.0436, 0.1168

4724/0/316

1.078

0.0525, 0.1140

11 314/9/455

1.112

0.0563, 0.1461

GOF on F²

R indices [F > 4s(F)] R(F),

wR(F²)

R indices (all data) R(F),

0.0494, 0.1036

0.0680, 0.1136

0.0606, 0.1255

0.1078, 0.1490

0.0869, 0.1588

wR(F²)

Largest residual peaks/e A

-3

˚

0.364 and -0.365

0.0501 and -0.397

0.473 and -0.366

0.455 and -0.432

0.932 and -0.548

based on F²against all unique reﬂections.^35,36All non-hydrogen

atoms were given anisotropic displacement parameters. Hydrogen

atoms were input at calculated positions and reﬁned with a riding

model. The C₄N ring in complex 11 was found disordered between

two envelope conformations (reﬁned occupancies 0.65 : 0.35). In

the same structure, bond lengths and angles of the solvent of

crystallization (pentane) had to be restricted to sensible values

due to severe disorder effects.

Pure Appl. Chem., 2004, 76, 507; (m) F. Alonso, I. P. Beletskaya and

M. Yus, Chem. Rev., 2004, 104, 3079; (n) S. Doye, Synlett, 2004, 1653;

(o) A. L. Odom, Dalton Trans., 2005, 225; (p) K. C. Hultzsch, Adv.

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Tada, Chem. Rev., 2008, 108, 3795; (t) J.-J. Brunet and N.-C. Rodriguez-

Zubiri, Eur. J. Inorg. Chem., 2007, 4711.

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Bytschkov, H. Sibeneicher and S. Doye, Eur. J. Org. Chem., 2003, 2888;

(c) C. Li, R. K. Thomson, B. Gillon, B. O. Patrick and L. L. Schafer,

Chem. Commun., 2003, 2462.

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Acknowledgements

We thank the Deutsche Forschungsgemeinschaft (SFB 623) for

funding.

4 (a) J. A. Bexrud, J. D. Beard, D. C. Leitch and L. L. Schafer, Org.

Lett., 2005, 7, 1959; (b) H. Kim, P. H. Lee and T. Livinghouse, Chem.

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4602 | Dalton Trans., 2009, 4586–4602

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Titanium hydroamination catalysts bearing a 2-aminopyrrolinato spectator ligand: Monitoring the individual reaction steps

DOI: 10.1039/b902038a

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