{Phenoxy-imine}aluminum versus -indium complexes for the immortal ROP of lactide: Different stereocontrol, different mechanisms

Article

pubs.acs.org/Organometallics

{Phenoxy-imine}aluminum versus -indium Complexes for the

Immortal ROP of Lactide: Diﬀerent Stereocontrol, Diﬀerent

Mechanisms

Mickael Normand,^†Vincent Dorcet,^‡Evgeny Kirillov,^†,* and Jean-Francois Carpentier^†,*

̧

^†Organometallics: Materials and Catalysis and ^‡Centre de Diﬀractomet

́

rie X, Institut des Sciences Chimiques de Rennes, UMR 6226

́

CNRS-Universite de Rennes 1, F-35042 Rennes Cedex, France

S

* Supporting Information

ABSTRACT: A series of dialkylaluminum and -indium {ON^R}MR′₂

complexes (M = Al, R′ = Me; M = In, R′ = Me, CH₂SiMe₃) stabilized

by a phenoxy-imine {ON^R}⁻ligand platform, with variable R-imino

substituents and functionalized by a bulky o-SiPh₃in the phenoxy

moiety, has been prepared and structurally characterized in solution

and in the solid state. {ON^R}AlMe₂complexes reacted with alcohols, in

particular with alkyl (S)-H-lactate, to generate the corresponding

{ON^R}Al(OR)₂compounds. On the other hand, the indium complexes

{ON^R}InR′₂proved largely inert toward alcohols. When they were combined with an alcohol (iPrOH, BnOH), the {ON^R}AlMe₂

complexes promoted the living (immortal) ring-opening polymerization ((i)ROP) of racemic lactide (rac-LA) with a good

control over the molecular weights and various microstructures, dependent on the R-imino substituent. Complexes having

benzyl-type imino substituents enabled the achievement of signiﬁcant isotacticity (P_mup to 0.80), following grossly the bulkiness

of the aryl moiety. The analogous {ON^R}InR′₂proved similarly active for the (i)ROP of rac-LA in presence of an external

alcohol, but the polymerizations were less controlled and none of the complexes induced stereoselectivity, except one (3a, P_m=

0.70). Kinetic studies revealed diﬀerent rate laws, with an apparent zero-order dependence on monomer for the aluminum

system 1m/iPrOH and a ﬁrst-order dependence on monomer for the analogous indium system 3m/iPrOH. On the basis of the

stoichiometric reactivity of model compounds, two diﬀerent operative ROP mechanisms are suggested, depending on the nature

of the metal center: Al-based complexes proceed through coordination−insertion, while In-based complexes are proposed to

operate through an activated monomer mechanism.

was tentatively proposed to arise, at least in part, from the

larger size of the indium center,¹⁵which might generate less

sterically crowded coordination environments around the active

metal center. It is indeed well-accepted that organometallic

compounds usually play a crucial assistance in chain-end-

controlled ROP processes, with evidence that subtle ligand/

metal modiﬁcations may result in very important eﬀects on

stereoselectivity.¹⁶However, possible fundamental diﬀerences

in the operative ROP mechanism (i.e., coordination−insertion

vs activated monomer pathways) when shifting from aluminum

to the relatively more covalent and less Lewis acidic indium

compounds were also questioned.

Herein we report a series of dialkyl and dialkoxy {phenoxy-

imine}aluminum and -indium complexes and their use as

catalysts/initiators for the (i)ROP of rac-LA. Our aim was to

compare the performance of these {ON}InX₂complexes in

terms of activity, as well as molecular weight and stereo-

chemical control, with that of their aluminum congeners. For

this purpose, we used a phenoxy-imine {ON}⁻ligand platform,

with variable R-imino substituents, functionalized in the

phenoxy moiety by a o-triphenylsilyl moiety,¹⁷in order to

INTRODUCTION

■

Intensive eﬀorts have been devoted over the past two decades

to the development of metal-based catalysts able to achieve the

stereoselective and controlled (immortal) ring-opening poly-

merization ((i)ROP) of rac-lactide (rac-LA).¹In this regard,

aluminum complexes have demonstrated attractive perform-

ance, with the possibility to access PLAs with a wide range of

iso-/heterotacticities, notably with salen,²dialkoxy-diimino,³

salan,⁴or salalen⁵{ONNO}²⁻tetradentate ligand platforms

(Figure 1). On the other hand, aluminum derivatives of

bidentate phenoxy-imine {ON}⁻ligands were also shown to be

eﬀective in the ROP of LA, but the examples of such hemisalen

compounds developed thus far did not oﬀer noticeable

stereocontrol when applied to rac-LA (P_mup to 60%).^6,7

Within the same group of elements, indium(III)-based

initiators⁸⁻¹²have been more recently explored for the ROP

of rac-LA and other cyclic esters (ε-caprolactone, β-

butyrolactone).¹³The catalytic performances of these com-

pounds were contrasted, however; in fact, quite a few systems⁸

revealed valuable activity and high stereoselectivity. In a recent

study, we prepared a series of ﬂuorinated dialkoxy-diimino

{ON^RNO^CF3}InX compounds (X = OiPr, alkyl) and showed

that they are signiﬁcantly less stereoselective than their

{ON^RNO^CF3}AlX analogues in the ROP of rac-LA.¹⁴This

Received: November 28, 2012

A

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bulkiness of the phenyl ring (b−e) directly bonded to the

nitrogen of the imino moiety, and (3) bidentate proligands that

possess a N-benzyl-type group substituted at either the CH₂

linker or the phenyl ring (f, h−k). These proligands a−m were

prepared from 2-hydroxy-5-methyl-3-(triphenylsilyl)-

benzaldehyde¹⁸and the corresponding primary amine, using

two diﬀerent protocols¹⁹(Scheme 1). Compounds a−m were

obtained in 26−62% isolated yields after recrystallization as

analytically pure yellow (red for a) crystalline powders (Table

1

1). They were authenticated by elemental analysis, H, ¹³C-

{¹H}, and ¹⁹F{¹H} (when relevant) NMR spectroscopy (see

the Experimental Section), and X-ray diﬀraction studies for f, e,

and l (see the Supporting Information)

Synthesis of {Phenoxy-imine}aluminum and -indium

Complexes. Dimethylaluminum complexes were prepared in

good yields (50−83%) by treatment of the appropriate Schiﬀ

base proligand with a stoichiometric amount of AlMe₃. The

reaction took place in dry toluene at room temperature with

concomitant elimination of 1 equiv of methane (Scheme 1).

The synthesis of dialkylindium complexes was achieved also by

protonolysis using two diﬀerent precursors, namely [InMe₃]_n

generated in situ from InCl₃and MeLi²⁰and the discrete

homoleptic precursor In(CH₂SiMe₃)₃(Scheme 1).²¹The

former protonolysis reaction proved selective only toward

two proligands, namely {ONN^qui}H (a) and {ON^pipeBn}H (m),

resulting in the corresponding dimethylindium complexes

{ONN^qui}InMe₂(2a) and {ON^pipeBn}InMe₂(2m), isolated in

47% and 77% yields, respectively. When the same reaction was

conducted with the other proligands, complex mixtures of

starting materials and unidentiﬁed products were observed. In

contrast, the reactions of the tricarbyl precursor In(CH₂SiMe₃)₃

with the six proligands tested (a, f−h, l, and m) yielded

selectively the corresponding dicarbylindium derivatives 3a,f−

h,l,m as bright yellow crystalline solids.

Figure 1. Examples of aluminum (pre)catalysts based on tetradentate

{ONNO}⁻and bidentate {ON}⁻ligands for the ROP of racemic

lactide.

The structures of these complexes were established on the

1

basis of H and ¹³C{¹H} NMR spectroscopy and elemental

enhance the hindrance around the metal center and to prevent

any aggregation phenomena (Figure 2). It is shown that

analysis and by single-crystal X-ray diﬀraction studies for

1f,g,m, 2a,m, and 3a.²²

The ¹H NMR spectra of complexes 1a−m, 2a,m, and 3a−m

in C₆D₆at room temperature contain all single sets of sharp

resonances that are consistent with monomeric structures in

solution (as observed in the solid state; vide infra). Expectedly,

in the aluminum and indium complexes bearing a chiral

substituent, {ON^CMetBu}AlMe₂(1l) and {ON^CMetBu}In-

(CH₂SiMe₃)₂(3l), two sets of close but distinct singlets were

observed for the inequivalent methyl and CH₂SiMe₃groups

(see the Experimental Section).

The solid-state molecular structures of aluminum complexes

{ON^pipeBn}AlMe₂(1m) and {ONN^CH2pyr}AlMe₂(1g) are

shown in Figures 3 and 4, respectively (see Figure S2 in the

Supporting Information for that of 1f). Values of relevant bond

distances and angles are given in Table 2. Complexes 1f,m,

which bear bidentate ligands, feature a four-coordinated

aluminum center in a distorted-tetrahedral geometry (bond

angles (C(1_1)−Al−C(1_2) = 122.96(6) and 123.10(4)°,

O(1)−Al−C(1_1) = 109.74(8) and 109.47(11)°, and O(1)−

Al−C(1_2) = 109.03(8) and 109.45(12)°, respectively). On

the other hand, the aluminum center in the ﬁve-coordinated

complex 1g, which has a tridentate ligand, lies in a highly

distorted trigonal bipyramidal geometry (τ = 0.65).²³Of note,

the piperidinyl ring moiety in complex 1m remains in a boat

conformation with no coordination of the nitrogen to the

metallic center. The Al−C (1.951(2)−1.973(3) Å), Al−O

Figure 2. Structure of the {phenoxy-imino}metal platform used in this

study.

modulation of the R-imino substituents in the {ON}AlMe₂

complexes allowed control of the stereoselectivity of the ROP,

from highly isoselective to heteroselective systems, with

improved performance in comparison to the related {phe-

noxy-imine}aluminum systems thus far reported. A change in

the operative mechanism between aluminum- and indium-

based catalysts is also highlighted.

RESULTS AND DISCUSSION

■

Synthesis of Proligands. Three main categories of phenol-

imine proligands (a−m, Table 1)) were explored: (1)

potentially tridentate ({ONN^qui}H (a) and {ONN^CH2pyr}H

(g)) proligands, (2) bidentate proligands that tune the

B

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Table 1. Synthetic Route toward Phenol-(R substituted)imine Proligands {ON^R}H

a

Protocol A: MeOH, HCOOH (cat.), reﬂux, 16 h. Protocol B: benzene, PTSA (cat.), reﬂux, Dean−Stark.

(1.772(2)−1.861(2) Å), and CN (1.9703(16)−2.027(3) Å)

distances in compounds 1f,g,m are in the range of bond lengths

observed in the {phenoxy-imine}aluminum complex Me₂Al[O-

2-tBu-6-{(2,4,6-Me₃C₆H₂)NCH}C₆H₃] reported by Nomura

et al.^4a(1.953(2)−1.956(2), 1.7748(19), and 1.965(2) Å,

respectively).

(3a) are isostructural, and their highly distorted coordination

environments fall just between trigonal bipyramidal and square

pyramidal (τ = 0.51 for both compounds, Figure S4

(Supporting Information) and Figure 5, respectively). The

In−O, In−C, and In−N bond distances of these three

complexes (Table 2) are to some extent larger than those

observed in the ﬁve-coordinated {diaminophenoxy}-

dimethylindium derivative {NN_HO}InMe₂described by

Mehrkhodavandi and co-workers (2.152(2), 2.161(2)−

2.150(2), and 2.355(2)−2.503(2) Å, respectively).²⁴

Complex {ON^pipeBn}InMe₂(2m) crystallizes with the indium

center in a distorted-tetrahedral coordination and is isostruc-

tural with its aluminum congener (see Figure 5 and Figure S3

(Supporting Information)). The ﬁve-coordinated indium

complexes {ONN^qui}InMe₂(2a) and {ONN^qui}In(CH₂SiMe₃)₂

C

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Scheme 1. Synthetic Routes toward {phenoxy-imine}AlMe₂Complexes (Route 1), {phenoxy-imine}InMe₂Complexes (Route

2), and {phenoxy-imine}In(CH₂SiMe₃)₂Complexes (Route 3)

detail on the reactivity of the complexes {ONN^qui}AlMe₂(1a),

{ON^Bn}AlMe₂(1b), and {ONN^CH2pyr}AlMe₂(1c) toward

iPrOH.²⁵We have shown that the last two compounds lead

to the monoalkoxides {ON^Bn}₂Al(OiPr) (5b) and

{ONN^CH2pyr}₂Al(OiPr) (5c), respectively, as the major

products, along with other unidentiﬁed species (Scheme 2).

Unexpectedly, for 1a, which contains an {ONN^qui}⁻ligand

system with an electron-withdrawing N substituent, occurrence

of deleterious (although totally selective) Meerwein−Ponn-

dorf−Verley (MPV) reduction at the imino functionality was

evidenced; [{ONN^qui}{ON^HN^qui}Al] (6a), which contains a

monoanionic {ONN^qui}⁻ligand and another dianionic ligand

({ON^HN^qui}²⁻) that results from the reduction of the latter

Figure 3. Solid-state molecular structure of {ON^pipeBn}AlMe₂(1m).

imino group into an amido moiety, was thus recovered.²⁵On

All hydrogen atoms are omitted for clarity.

the other hand, our attempts to convert dialkylaluminum

complexes 1d,e,h−m into alkoxy complexes, via treatment with

2−5 equiv of iPrOH in a polar solvent (THF) or in aromatic

hydrocarbons (benzene, toluene) over a wide range of

temperatures (−20 to +80 °C), resulted in intractable mixtures

of products and no clear evidence of actual generation of

alkoxide species was gained (Scheme 2); even release of the

proligands was observed in some cases.

Alternative routes toward well-deﬁned aluminum alkoxy type

species using H-lactates instead of isopropyl alcohol were

investigated. Bis(α-alkoxy ester)aluminum derivatives were

synthesized cleanly by reacting {ONN^qui}AlMe₂(1a) with 2

equiv of (1R,2S,5R)-menthyl (S)-lactate and by reacting

{ON^Bn}AlMe₂(1f) as well as {ON^pipeBn}AlMe₂(1m) with 2

equiv of isopropyl (S)-lactate, to give {ONN^qui}Al(menthyl

(S)-lactate)₂(7a), {ON^Bn}Al(iPr (S)-lactate)₂(8f), and

{ON^pipeBn}Al(iPr (S)-lactate)₂(8m), which were isolated in

Figure 4. Solid-state molecular structure of {ONN^CH2pyr}AlMe₂(1g).

All hydrogen atoms are omitted for clarity.

30%, 70%, and 82% yields, respectively (Scheme 3). These

1

complexes were characterized on the basis of 1D and 2D H

and ¹³C{¹H} NMR spectroscopy and elemental analysis.

Repeated attempts to grow X-ray-quality crystals were

unfortunately unsuccessful. Still, these constitute to our

knowledge the ﬁrst examples reported of {phenoxy-imine}

aluminum lactate complexes.²⁶

Reactivity of {Phenoxy-imine}aluminum and -indium

Dialkyl Complexes toward Alcohols. Typical metal-based

catalysts/initiators for the ROP of lactones incorporate one (or

more) alkoxide group which is capable of rapid and eﬃcient

initiation.¹This alkoxide either is initially present as a discrete

alkoxy complex or, more commonly, is formed in situ by the

reaction between an exogenous alcohol and an alkyl(amido)

metal precursor. The latter is actually the route that was

followed earlier by the groups of Nomura and Pellechia, and

more recently by Redshaw, with {phenoxy-imine)AlMe₂

compounds, although the exact nature of the active ROP

species was not addressed.^6a−cRecently, we have reported in

The ¹H NMR spectra of {ON^Bn}Al(iPr (S)-lactate)₂(8f) and

{ON^pipeBn}Al(iPr (S)-lactate)₂(8m) in benzene-d₆at room

temperature are similar (Figures A53 and A56 in the

Supporting Information). The observation of a series of sharp

resonances in both cases is consistent with the existence of

single monomeric species in solution. In particular, the methyl

phenoxy hydrogensused as an NMR probecome out as

D

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Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes 1f,g,m, 2a,m, and 3a

{ON^Bn}AlMe₂

{ON^pipeBn}AlMe₂

{ON^CH2pyr}AlMe₂

{ONN^qui}InMe₂

{ONN^qui}InCH₂SiMe₃

{ON^pipeBn}InMe₂

(1f)

(1m)

(1g)

(2a)

(3a)

(2m)

M−C(1_1)

M−C(1_2)

M−O(1)

M−N(1)

1.951(2)

1.952(2)

1.778(1)

1.970(2)

1.963(3)

1.958(3)

1.772(2)

1.992(2)

1.979(3)

1.973(3)

1.861(2)

2.027(3)

2.184(3)

1.304(3)

87.3(1)

2.161(2)

2.147(2)

2.193(1)

2.339(2)

2.383(2)

1.309(3)

80.32(6)

150.77(6)

101.27(8)

96.68(8)

2.162(2)

2.170(2)

2.198(1)

2.266(2)

2.471(2)

1.325(3)

79.93(5)

149.08(6)

99.87(7)

98.24(7)

2.139(2)

2.140(2)

2.101(1)

2.268(2)

M−N(2)

CN(1)

1.290(2)

93.67(6)

1.299(3)

94.71(9)

1.288(2)

85.33(5)

O(1)−M−N(1)

O(1)−M−N(2)

O(1)−M−C(1_1)

O(1)−M−C(2_2)

161.9(1)

95.8(1)

109.74(8)

109.03(8)

109.5(1)

103.10(8)

101.19(8)

99.1(1)

two quartets for the methine hydrogens of the iPr and lactate

moieties, respectively.

The reactivity of indium complexes was also investigated

(Scheme 4). Dimethyl (2a,m) and bis((trimethylsilyl)methyl)

(3a,f−h,l,m) complexes were thus mixed with 2 equiv of iPrOH

in toluene or THF at reﬂux; similar reactions with iPr (S)-H-

lactate were also attempted. Indium complexes 3f−h,l,m,

whatever the conditions used, appeared to be reluctant to any

protonolysis reaction and remained intact after several hours. In

the case of 2m, partial release of the proligand was the main

reaction observed. On the other hand, both complexes bearing

the 8-quinolyl imino substituent (2a and 3a) speciﬁcally

reacted as their aluminum analogue 1a, via MPV reduction of

the imino functionality, to generate [{ONN^qui}{ON^HN^qui}In]

(9a).²⁵

ROP of rac-Lactide Catalyzed by {Phenoxy-imine}-

aluminum and -indium Initiators: Activity, Living

Immortal Character, and Stereoselectivity. The data

from a series of polymerizations of rac-lactide (rac-LA) by

aluminum complexes 1a−m in toluene solution at 100 °C are

collected in Table 3.

Figure 5. Solid-state molecular structure of {ONN^qui}In(CH₂SiMe₃)₂

(3a). All hydrogen atoms are omitted for clarity.

Aluminum-Based Initiators. When they were associated

with an exogenous alcohol (isopropyl alcohol or benzyl

alcohol), all dimethyl{phenoxy-imine}aluminum complexes

proved to be active in the ROP of rac-LA at 100 °C for a

period of time ranging from 2 to 20 h at a [lactide]/[catalyst/

initiator]/[alcohol] ratio of 100/1/1. The observed molecular

weights of all the polymers produced with the aluminum

complexes employed in this study, i.e., complexes 1a−m, were

found to be close to the theoretical values in the range of

M_n(SEC) = 3300−11900 g mol⁻¹. All the PLAs formed under

those conditions had unimodal and narrow molecular

distributions (M_w/M_n= 1.10−1.24; Table 3, entries 1−22),

except for complexes 1i,j,l, for which polydispersity indices

were somewhat broader (M_w/M_n= 1.51, 1.32, and 1.46,

respectively; Table 3, entries 16, 17, and 19). For a given

Scheme 2. Fate of {ON^R}AlMe₂Compounds 1a−m on

Treatment with Isopropyl Alcohol

1

experiment, monitoring the ROP of rac-LA using 1f by H

NMR spectroscopy showed that the molecular weight increased

with conversion in a linear relationship in [rac-LA]/[Al]/

[iPrOH] ratios of 100/1/1 and 500/1/1 (Figure 6).

The best activity obtained under optimized reaction times

was reached with complex 1f, which featured an apparent

turnover frequency (TOF) of 14 mol of LA (mol of Al)⁻¹h⁻¹

at 100 °C. This value is similar to those achieved with

{phenoxy-imine}aluminum catalyst precursors used with

MeOH as a chain transfer agent (CTA) reported by Pappalardo

et al.^6b(TOF = 1.5 mol of LA (mol of Al)⁻¹h⁻¹at 70 °C) and

those of Nomura et al.^6aor Redshaw et al.^6cusing nBuOH as a

singlets (δ 1.80 and 1.95 ppm, respectively). In compounds

8f,m, the two lactate moieties are magnetically inequivalent, as

indicated by a series of six doublets (some of them partially

overlap), assigned to the methyl groups and two heptets and

E

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Scheme 3. Routes toward {ON^R}bis(alkyl (S)-lactate)aluminum Complexes 7a and 8f,m

promoted by aluminum²⁸and zinc²⁹complexes. Hillmyer and

Tolman²⁸have rationalized this according to a Michaelis−

Menten model in which reversible binding of the cyclic ester to

the metal occurs and, if K_M≪ [cyclic ester] (where K_Mis the

Michaelis constant related to this rapid pre-equilibrium), it can

lead in fact to saturation kinetics in terms of monomer

concentration. Mountford and co-workers found also this

model applicable to their zinc system,²⁹and we assume this is

also the case in the present study.

Scheme 4. Fate of {ON^R}InR′₂Compounds on Treatment

with Isopropyl Alcohol

Of interesting note, the binary catalyst made up of

dimethylaluminum complex 1m in combination with iPrOH

and the single-component system bis(iPr (S)-lactate)aluminum

complex 8m featured roughly the same apparent rate constants

at 110 °C: i.e., k_app= (1.1 0.1) × 10⁻³s⁻¹for 1m/iPrOH

([rac-LA]₀/[Al]/[iPrOH]₀= 500/1/5) vs (1.4 0.1) × 10⁻³

s⁻¹for 8m ([rac-LA]₀/[Al] = 200/1) (Table 4; see Figure S80

in the Supporting Information). This observation is consistent

with relatively rapid and complete generation of alkoxy

initiating species when dimethyl{phenoxy-imine}aluminum

complexes are reacted with isopropyl alcohol and an operative

coordination−insertion polymerization process in both binary

and single-component systems.

Experiments conducted under the so-called “immortal”

polymerization conditions,^1h,iin which the alcohol is added in

excess relative to the number of potential initiating groups of

the complexes (i.e., 2) and acts as a CTA ([rac-LA]₀/[Al]/

[iPrOH]₀= 1000/1/10; Table 3, entries 8, 13, 20, and 22)

evidenced a good degree of control over the polymerization.

The overall good match between calculated (M_n(theor)) and

experimental molecular weights (M_n(SEC) and M_n(NMR))

and the narrow molecular weight distributions ranging from

1.09 to 1.11 establish that adequate conditions were met for

eﬀective living immortal conditions: i.e., the transfer reaction

between dormant hydroxyl-end-capped polymer chains and the

active alkoxy-type polymer chain coordinated onto the

aluminum center proceeded signiﬁcantly more quickly than

propagation.^1h,iAccordingly, analysis by ¹H NMR spectroscopy

in CDCl₃of reprecipitated PLAs obtained under such

conditions revealed the presence of an isopropoxycarbonyl

moiety (doublets at δ 1.25 ppm for the methyl groups and a

multiplet at δ 5.10 ppm for the methine hydrogen, which

mostly overlapped with the PLA signals) or a benzyloxycar-

bonyl moiety (multiplets at δ 7.30 ppm for the aromatics and a

resonance that is overlapped with the PLA signal) at one

transfer agent (TOF = 4−5 mol of LA (mol of Al)⁻¹h⁻¹at 80

and 100 °C, respectively).²⁷

The kinetics of the ROP of rac-LA promoted by binary

systems made up of dimethylaluminum complex 1m in

combination with iPrOH were studied in more detail. All

reactions featured linear plots of conversion vs time to ≥80%

conversion (Figures 7 and 8) (in contrast to semilogarithmic

plots, which were not linear), indicating an apparent zero order

in lactide. Further experiments were conducted with variable

amounts of catalyst precursor and alcohol, to determine the

corresponding orders in these two components. The apparent

rate constants thus determined are reported in Table 4. The

experimental gradient of the least-squares ﬁtted line between ln

k_appvs ln [Al] was 0.97(5) (R²= 0.999) (Figure S78,

Supporting Information), indicating a ﬁrst-order dependence

on catalyst concentration. The same treatment for the isopropyl

alcohol concentration led to a gradient of 0.23(6) (R²= 0.997)

(Figure S79, Supporting Information). Hillmyer and Tolman

have reported similarly close, but not strictly equal, to zero

values for elementary orders in their ternary InCl₃/BnOH/

NEt₃catalyst system and eventually concluded on a formal

zero-order dependence on alcohol and amine.⁸Thus, for the

1m/iPrOH system, we propose the idealized rate expression

−d[rac-LA]/dt = k_app[LA]⁰[Al]¹[iPrOH]⁰.

An apparent zero-order dependence on monomer concen-

tration is unusual but not unprecedented: at least two examples

have been recently disclosed for the ROP of ε-caprolactone

F

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Table 3. (i)ROP of rac-Lactide Promoted by Catalytic Systems Made of Aluminum Complexes 1a−m and an Exogenous Alcohol

b

c

d

e

f

[LA]/[Al]

a

time

(h)

conversn M_n(theor) (g.mol⁻¹) ( M_n(NMR) (g.mol⁻¹) ( M_n(SEC) (g.mol⁻¹) ( M /

^wf

g

entry [Al]

ROH

/[ROH]

(%)

× 10³)

M_n

P_m

1

1a

1b

1c

1d

1e

1f

BnOH

iPrOH

BnOH

iPrOH

BnOH

iPrOH

BnOH

iPrOH

BnOH

iPrOH

BnOH

iPrOH

BnOH

iPrOH

100/1/1

100/1/2

100/1/1

1000/1/10

100:1:1

15

17

20

16

17

16

18

2

69

66

85

82

88

90

78

61

27

39

46

97

29

78

74

79

80

96

95

33

84

61

0

9.7

9.2

nd

4.8

5.7

1.10

1.12

1.13

1.17

1.21

1.14

1.09

1.11

1.08

1.09

1.35

1.11

1.24

1.18

1.51

1.32

1.23

1.46

1.10

1.21

1.11

0.30

0.29

0.32

0.50

0.65

0.56

0.50

2

7.6

nd

3

6.1

4.1

4

11.8

12.8

13.0

11.2

8.8

8.7

nd

9.4

5

10.4

10.1

8.0

6

7.8

nd

7

8

7.2

4.2

5.5

6.8

nd

5.8

nd

9

3.9

3.4

0.70

0.67

0.73

1.00

0.63

0.50

0.76

0.80

0.71

0.42

0.47

0.46

0.40

0.42

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

1f

100/1/1

1000/1/10

100/1/1

1000/1/10

100/1/1

1000/1/10

100/1/0

4

5.6

5.5

1f

6

6.6

6.2

h

1f

16

17

16

18

16

17

15

18

24

18

14.0

4.2

11.9

3.3

1f

3.6

10.1

6.7

12.9

8.1

10.8

nd

1g

1h

1i

11.2

10.7

11.3

11.5

13.8

13.3

4.8

7.3

8.5

11.3

9.9

1j

1k

1l

14.9

10.8

3.8

1l

4.0

nd

1m

7a

12.1

8.8

9.0

nd

5.7

i

7a

58

4.2

nd

3.2

1.30

0.51

a

b

Polymerization conditions unless otherwise stated: reactions performed in slurry/solution, [rac-LA]₀= 2.0 M at 100 °C. Reaction times were not

c

d

necessarily optimized. Monomer conversion determined by ¹H NMR spectroscopy (CDCl₃, 298 K). Calculated molecular weight calculated using

e

M_n(theor) = conversion × [rac-LA]₀/[Al or ROH] × M_LA. Experimental molecular weight determined by NMR from the relative intensities of the

f

main chain and terminal resonances. Experimental molecular weight determined by SEC vs polystyrene standards and corrected by a factor of 0.58.

g

h

1

P_mis the probability of a meso linkage, as determined by homonuclear decoupled H NMR experiments. Experiment using L-lactide as the

i

monomer. Experiment run in bulk at 125 °C.

Figure 6. Plot of M_n(NMR) (g mol⁻¹) vs. rac-lactide conversion (%)

◆

using 1f/iPrOH at [rac-LA]₀/[Al]/[iPrOH]₀ratios of 100/1/1 (

)

Figure 7. Plot of rac-LA conversion (%) vs time (s) with the binary

system 1m/iPrOH at [rac-LA]₀/[Al]/[iPrOH]₀= 500/1/5 ([rac-LA]₀

■

and 500/1/1 ( ). Both solid lines correspond to the calculated M_n

values.

■

▲

●

= 2.0 M) and diﬀerent temperatures: ( ) 90 °C; ( ) 100 °C; ( )

◆

110 °C; ( ) 120 °C.

terminus and of the methine hydrogen in the α-position of the

terminal alcohol CH(OH)CH₃(broadened quartet at δ 4.35

ppm) at the other terminus of the polymer chain (see Figure

S72, Supporting Information).

The microstructure of the formed PLAs was determined by

homonuclear decoupled ¹H NMR spectra of the methine

region.³⁰Interestingly, samples derived from 1a−m were

indicative of the formation of chains with either heterotactic-

or isotactic-biased microstructures. In fact, the nature of the

imino substituent on the phenoxy-imine ligand signiﬁcantly

aﬀects the ability of the catalyst to control monomer

enchainment in the ROP of rac-lactide. Polymerization

reactions conducted with systems based on {ONN^qui}AlMe₂

(1a), {ON^trityl}AlMe₂(1k), and {ON^pipeBn}AlMe₂(1m) led to

heterotactic-enriched PLAs (P_m= 0.30−0.42, entries 1−3, 18,

21 and 22) whereas complexes {ON^ArF}AlMe₂(1c), {ON^Bn}^-

AlMe₂(1f), {ON^CH2Mes}AlMe₂(1h), {ON^PhOMe}AlMe₂(1i),

and {ON^CHPh2}AlMe₂(1j) led to isotactic PLAs (P_m= 0.65−

0.80, entries 5, 9−13, and 15−17). In contrast, {ON^PhiPr2

}

-

G

dx.doi.org/10.1021/om301155m | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

AlMe₂(1b), {ON^Phmorpho}AlMe₂(1e), {ON^CH2pyr}AlMe₂(1g),

and {ON^MeBu}AlMe₂(1l) (P_m= 0.47−0.56, entries 4, 7, 14 and

19−20) were found to be essentially nonstereoselective.

The trend in isoselectivity toward rac-LA can be in part

rationalized for some systems having a benzyl-type imino

substituent. It seems to follow grossly the bulkiness of the aryl

moiety in the benzyl substituent: 3,4,5-trimethoxyphenyl (i, P_m

= 0.80) > mesityl (h, P_m= 0.76) > phenyl (f, P_m= 0.63−073).

However, bulkiness is apparently not the sole parameter that

aﬀects isotacticity in this series of compounds, as demonstrated

by the almost identical isoselectivity of the benzyl (f, P_m= 0.68

0.05) and benzal moieties (j, P_m= 0.71) while the bulkier

trityl substituent induced a reversal in stereocontrol, with a

slightly heterotactic enriched polymer (k, P_m= 0.42, entry 18).

Trying to rationalize the heteroselectivity observed for several

systems appears even much trickier. The highest value was

obtained with the 8-quinolyl substituent (a, P_m= 0.30, entries

1−3), and modestly heterotactic-enriched polymers were

obtained with the piperidinylbenzyl (m, P_m= 0.40−0.42,

entries 21 and 22) and trityl moieties (k, P_m= 0.42, entry 18).

It is noteworthy that bulkiness has no signiﬁcant eﬀect on the

stereoselectivity of systems having a ligand derived from aniline

derivatives, as attested by the narrow range of P_mvalues

obtained with quite diﬀerent substituents on the aryl moiety:

2,6-diisopropyl (b, P_m= 0.50), 3,5-bis(triﬂuoromethyl) (c, P_m=

0.65), 2,6-bis(3,5-bis(triﬂuoromethyl)phenyl) (d, P_m= 0.56),

and 2-morpholinyl (e, P_m= 0.50). Similarly, no stereoselectivity

was observed for the aluminum complex bearing a chiral alkyl

substituent (l, P_m= 0.47, entries 19 and 20).

Figure 8. Plot of rac-LA conversion (%) vs time (s) at 110 °C with the

binary system 1m/iPrOH, with [rac-LA]₀= 2.0 M and [iPrOH]₀=

▲

0.10 M, at diﬀerent concentrations of precatalyst: ( ) [Al] = 4.0 mM;

■

◆

( ) [Al] = 12 mM; ( ) [Al] = 20 mM.

Table 4. Apparent Rate Constants Determined for the ROP

of rac-Lactide with Binary Systems 1m/iPrOH and Single

a

Component 8m

complex

temp (°C)

[rac-LA]₀/[Al]/[iPrOH]₀

k_app(10⁻³s⁻¹

)

1m

8m

90

100

110

120

110

500/1/5

500/1/20

500/1/25

500/3/25

500/5/25

200/1/0

0.44 0.05

0.59 0.04

1.1 0.1

2.1 0.1

2.1 0.3

2.6 0.3

7.8 0.3

12.2 0.9

1.4 0.1

Indium-Based Initiators. The performance of the dimethy-

lindium complexes 2a,m and of the bis((trimethylsilyl)methyl)-

indium complexes 3a,f,g,l,m was next assessed. Representative

results obtained in the (i)ROP of rac-LA in the presence of

exogenous CTAs are reported in Table 5.

a

All reactions were performed with [rac-LA]₀= 2.0 M in toluene.

Table 5. (i)ROP of rac-Lactide Promoted by Binary Systems Made of Isopropyl Alcohol and Indium Complexes 2a,m, and

3a,f,g,l,m

b

c

d

e

f

[LA]/[In]/

a

time

(h)

conversn

(%)

M_n(theor) (×10³g

M_n(NMR) (×10³g

M_n(SEC) (×10³g

M /

^wf

g

entry [In]

[iPrOH]

mol⁻¹

)

mol⁻¹

)

mol⁻¹

)

M_n

P_m

1

2a

100/1/1

3

2.5

24

3

86

98

99

56

93

90

99

93

99

97

98

97

87

91

87

86

67

12.4

14.1

7.2

13.0

13.5

6.6

9.9

13.4

4.4

1.58

1.45

1.96

1.38

1.90

1.33

1.69

1.80

1.53

1.91

1.60

1.12

1.13

1.08

1.07

1.10

1.06

0.50

0.49

nd

2

2a

100/1/1

3

2a

100/1/2

4

2a

500/1/1

40.3

67.0

12.9

14.4

13.4

14.4

70.0

14.1

14.0

12.5

13.1

12.5

12.4

9.7

13.8

33.0

10.1

13.7

14.2

nd

14.5

26.6

9.0

0.52

nd

5

2a

500/1/1

16

2

6

2m

3a

100/1/1

nd

7

100/1/1

17

7.3

nd

8

100/1/1

0.25

0.5

9.1

nd

9

100/1/1

7.2

nd

10

11

12

13

14

15

16

17

500/1/1

16

40.0

12.2

13.3

11.1

14.6

9.7

27.6

11.8

13.7

9.5

0.47

0.48

0.70

0.44

0.54

0.44

0.46

0.52

1000/1/10

15

18

16

19

16

15

3

h

3f

3g

14.4

13.3

12.8

6.8

3h

3l

14.5

7.9

3m

a

b

c

Polymerization conditions unless otherwise stated: [rac-LA]₀= 2.0 M at 80 °C. Reaction times were not necessarily optimized. Monomer

d

conversion determined by ¹H NMR spectroscopy (CDCl₃, 298 K). Theoretical molecular weight calculated using M_n(theor) = conversion × [rac-

e

LA]₀/[In or iPrOH] × M_LA. Experimental molecular weight determined by NMR from the relative intensities of the main chain and terminal

resonances; due to the important uncertainty in relative intensities for high degrees of polymerization, values above 15000 g mol⁻¹must be seen as

f

g

estimates. Experimental molecular weight determined by SEC vs polystyrene standards and corrected by a factor of 0.58. P_mis the probability of

meso linkages, as determined by homonuclear decoupled H NMR experiments. This experiment was duplicated.

h

1

H

dx.doi.org/10.1021/om301155m | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Complexes {ONN^qui}InMe₂(2a) and {ON^pipeBn}InMe₂

(2m), on combination with 1−2 equiv of isopropyl alcohol,

showed interesting performance in the ROP of rac-LA in

toluene solution at 80 °C. Under these conditions, non-

optimized turnover frequencies determined from these batch

experiments reached up to 37 mol of LA (mol of In)⁻¹h⁻¹.

These apparent TOF values²⁷are greater than those of their

aluminum counterparts (especially considering that the latter

were achieved at a higher temperature: i.e., 100 °C) but

somewhat lower than those found for the dinuclear halide-/

ethoxy-bridged indium complex reported by Mehrkhodavandi

et al.²⁴(50 mol of LA (mol of In)⁻¹h⁻¹at 25 °C) or the

dinuclear isopropoxy-bridged indium complex described by

Okuda et al.³¹(49 mol of LA (mol of In)⁻¹h⁻¹at 50 °C). All

the PLAs formed under those conditions had unimodal,

although broadened, molecular distributions (M_w/M_n= 1.33−

1.96) and experimental molecular weights determined by SEC

quite close to the theoretical values calculated on the

assumption of the growth of one macromolecular chain per

added alcohol equivalent (see Figure S73, Supporting

Information). A noticeable deviation from this trend occurred

when the monomer loading was increased from 100 to 500

equiv (Table 5, entries 4, 5, and 10), which led to PLAs with

molecular weights signiﬁcantly lower than those expected,

demonstrating a rather low degree of control over the

polymerization provided by these systems, in contrast to what

could be achieved by their aluminum congeners (vide supra).

We assume that this may reﬂect the occurrence of transfer to

the monomer and/or transesteriﬁcation side reactions under

such conditions, as supported by the rather broad dispersity

values. On the other hand, carrying out the polymerization in

the presence of a larger excess (10 equiv) of CTA (entry 11)

aﬀorded polymers with experimental molecular weights

(M_n(SEC) and M_n(NMR)) that matched well with the

theoretical M_nvalues and with narrow polydispersity indexes.

The bis((trimethylsilyl)methyl)indium complexes 3a,f,g,l,m

were tested with an excess of alcohol in the ROP of rac-LA at a

monomer/catalyst/initiator alcohol ratio of 1000/1/10. All the

systems exhibited good control over the polymerization

parameters with fast and eﬃcient transfer between the growing

polymer chains and the dormant (macro)alcohols, as evidenced

by the narrow molecular weight distribution of the resulting

polymers (M_w/M_n= 1.06−1.13)³²and the agreement between

Table 6. Apparent Rate Constants Determined for the ROP

of rac-Lactide with Binary Systems 3m/iPrOH

a

[rac-LA]₀/[In]/[iPrOH]₀

k_app(10⁻³M⁻¹s⁻¹

)

500/1/5

0.91 0.10

0.68 0.10

1.1 0.2

500/1/10

500/1/25

500/2/10

500/3/10

1.0 0.3

1.84 0.3

a

All reactions were performed at 110 °C with [rac-LA]₀= 2.0 M in

toluene.

reﬂecting the very small diﬀerent rate constants observed at

diﬀerent alcohol concentrations (vide supra). Thus, for the

3m/iPrOH system, we propose the idealized rate expression

−d[rac-LA]/dt = k_app[LA]¹[In]¹[iPrOH]⁰.

Those kinetic diﬀerences between the aluminum and indium

systems, that is, in the former case an apparent zero-order

dependence on monomer and in the latter case a ﬁrst-order

dependence on monomer, were unexpected. Note that this

change does not necessarily imply a change in the operative

mechanism but could simply reﬂect a change in the rate-

determining step. In fact, the rate law determined for the 3m/

iPrOH system can be well accounted for by both coordina-

tion−insertion and activated monomer mechanisms. In the

coordination−insertion mechanism, the ﬁrst-order dependence

on monomer and catalyst classically reﬂects slow internal

nucleophilic attack of the alkoxide (metal-bound) onto the

coordinated monomer, and the zero-order dependence on

alcohol accounts for fast exchange between active (alkoxide)

and dormant (alcohol) polymer chains, as usual for eﬀective

immortal systems. On the other hand, in the latter activated

monomer mechanism, this ﬁrst-order dependence on monomer

and catalyst can be accounted for slow activation (coordina-

tion) of the monomer, with rapid attack of the external

nucleophile, reﬂecting the pseudo zero-order dependence on

alcohol. More decisive information for identifying the probable

mechanism at work comes, in our opinion, from the

nonreactivity of dicarbylindium complexes with alcohols

under conditions similar to or more drastic than those used

in polymerizations and still the controlled formation of alcohol

end-capped polymers.

1

theoretical and observed molecular weights. H NMR spec-

In order to get insights into the reactivity of the dialkyl

complexes under the polymerization conditions, we ﬁrst

monitored by NMR spectroscopy the reaction of dimethylalu-

minum complex 1m with 2 equiv of iPrOH and 2.6 equiv of

rac-LA in C₆D₆. No reaction took place at room temperature as

troscopy of a rather low molecular weight PLA established that

it is selectively capped by isopropoxycarbonyl and hydroxyl end

groups.

Kinetics of the ROP of rac-LA with the 3m/iPrOH system

were determined under conditions identical with those used for

the analogous aluminum 1m/iPrOH system (110 °C, [rac-LA]

= 2.0 M). In contrast to the latter aluminum system, all

reactions performed with the indium system featured a more

usual ﬁrst-order dependence on lactide, as evidenced by linear

semilogarithmic plots (Figures S81−S86, Supporting Informa-

tion). Further experiments were conducted with variable

amounts of catalyst precursor³³and alcohol, to determine the

corresponding orders in these two components. The apparent

rate constants thus determined are reported in Table 6. The

experimental gradient of the least-squares ﬁtted line between ln

k_appvs ln [In] was 0.90(4) (R²= 0.934) (Figures S81−S83),

indicating a ﬁrst-order dependence on catalyst concentration.

The same treatment for the isopropyl alcohol concentration led

to a gradient of 0.28(3) (R²= 0.979) (Figures S84−S86),

1

judged by H NMR spectroscopy, whereas conversion of the

reagents was complete within 16 h at 100 °C, concomitant with

release of methane and formation of the corresponding

bis(polylactidyl)Al species (Figure S87, Supporting Informa-

tion). The complexity of the methine resonances of the opened

lactide units and the presence of four diﬀerent CO moieties

in the ¹H−¹³C HMBC NMR spectrum for that species (Figure

S88, Supporting Information) evidenced the formation of

diastereomeric products. These data are consistent with

formation of a major species in which two isopropyl-terminated

short chains, derived from two ring-opened lactide units, are

bonded to the aluminum center. Most importantly, under

strictly identical conditions, indium complex 3m was found not

to react with iPrOH/rac-LA, as judged by NMR spectroscopy.

Overall, these observations provide strong clues for supporting

I

dx.doi.org/10.1021/om301155m | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

an activated monomer mechanism for the indium-based system,

in contrast to aluminum-based systems.

exogenous alcohol and still the formation of alcohol-capped

PLAs.

In addition, homodecoupled ¹H NMR experiments con-

ducted on PLAs produced with these indium compounds

indicated that most of them lead to atactic polymers (0.44 < P_m

< 0.54); only a modest enrichment in isotactic sequences (P_m=

0.70) was achieved with the complex {ONN^qui}In(CH₂SiMe₃)₂

(3a).³⁰This latter value is in contrast with the observed

heteroselectivity obtained with the aluminum congener

{ONN^qui}AlMe₂(1a; P_m= 0.30). On the other hand, no

stereoselectivity was observed for the analogous complex

{ONN^CH2pyr}In(CH₂SiMe₃)₂(3g; P_m= 0.54). Because the

fate of these alkyl{phenoxy-imine}metal complexes in the

presence of isopropyl alcohol has been shown to be strikingly

diﬀerentformation of MPV imine reduction products 8a and

9a for 1a and 3a, respectively, vs no reaction between alcohol

and 3gand because this fate remains unknown in the

presence of lactide (i.e., under the polymerization conditions),

it is diﬃcult at this stage to discuss possible reasons for these

diﬀerences in stereoselectivity. On the other hand, assuming

that both 2a and 3a polymerize rac-LA via an activated

monomer mechanism, it can be speculated that the signiﬁcant

diﬀerence in stereoselectivity (P_m= 0.50 and 0.70, respectively)

may arise from the higher steric hindrance induced at the metal

center by the CH₂SiMe₃moieties as compared to the methyl

ones.

EXPERIMENTAL SECTION

■

General Considerations. All manipulations were performed

under a puriﬁed argon atmosphere using standard Schlenk techniques

or in a glovebox. Solvents were distilled from Na/benzophenone

(THF, Et₂O) or Na/K alloy (toluene, hexane, and pentane) under

argon, degassed thoroughly, and stored under argon prior to use.

Deuterated solvents were stored over Na/K alloy (benzene-d₆,

toluene-d₈, THF-d₈; >99.5% D, Eurisotop) or CaH₂(CD₂Cl₂) and

vacuum-transferred just before use. CDCl₃was dried over a mixture of

3 and 4 Å molecular sieves. 4-Methyl-6-(triphenylsilyl)-

salicylaldehyde,³⁴proligands {ONN^qui}H,¹⁹{ON^Bn}H,²⁵and

{ON^CH2pyr}H,²⁵precursors In(CH₂SiMe₃)₃,³⁵In{N(SiMe₃)₂}₃,³⁶and

37

In(OiPr)₃(also purchased from Alfa Aesar), and complexes

{ONN^qui}AlMe₂(1a),²⁵{ON^Bn}AlMe₂(1f),²⁵{ONN^CH2pyr}AlMe₂

(1g),²⁵{ONN^qui}InMe₂(2a),²⁵and {ONN^qui}In(CH₂SiMe₃)₂

(3a)²⁵were prepared using literature procedures. rac-Lactide (rac-

LA) was received from Aldrich, and L-lactide (^L-LA) was kindly

provided by Total Petrochemicals. Puriﬁcation of either rac-LA or ^L-

LA required a three-step procedure involving ﬁrst a recrystallization

from a hot, concentrated iPrOH solution (80 °C), followed by two

subsequent recrystallizations in hot toluene (100 °C). After

puriﬁcation, rac-LA and ^L-LA were stored at a temperature of −30

°C in the glovebox.

Instrumentation and Measurements. NMR spectra of com-

plexes were recorded on Bruker AC-200, AC-300, Avance DRX 400,

and AM-500 spectrometers in Teﬂon-valved NMR tubes at 25 °C

unless otherwise indicated. ¹H and ¹³C NMR chemical shifts are

reported in ppm vs SiMe₄and were determined by reference to the

residual solvent peaks. Assignment of resonances for organometallic

CONCLUSIONS

■

Living (immortal) ring-opening polymerization of rac-LA was

assessed with a variety of dialkyl{phenoxy-imine}aluminum and

-indium complexes in combination with an alcohol (iPrOH,

BnOH) as coinitiator. The dimethylaluminum and dicarbylin-

dium complexes polymerized rac-LA in a controlled fashion,

while dimethylindium complexes were found to produce PLA

with somewhat broad molecular weight distributions and

moderate control over the polymerization parameters.

1

complexes was made from 2D H−¹³C HMQC and HMBC NMR

experiments. ¹⁹F chemical shifts were determined (when relevant) by

external reference to an aqueous solution of NaBF₄.

Elemental analyses (C, H, N) were performed using a Flash EA1112

CHNS Thermo Electron apparatus and are the average of two

independent determinations.

Size exclusion chromatography (SEC) analyses of PLAs were

performed in THF (1.0 mL min⁻¹) at 20 °C using a Polymer

Laboratories PL-GPC 50 plus apparatus equipped with two ResiPore

300 × 7.5 mm columns and RI and dual-angle LS (PL-LS 45/90)

detectors. The number-average molecular masses (M_n) and poly-

dispersity indexes (M_w/M_n) of the polymers were calculated with

reference to a universal calibration vs polystyrene standards. Reported

experimental SEC molar mass values (M_n(SEC)) for PLA samples

were corrected by a factor of 0.58, as previously established.³⁸Unless

otherwise stated, the SEC traces of the polymers all exhibited a

unimodal, and usually symmetrical, peak.

Interestingly, dimethylaluminum complexes having benzyl-

type imino substituents enabled the achievement of signiﬁcant

isotacticity (P_mup to 0.80), and the stereocontrol within this

series followed grossly the bulkiness of the aryl moiety. Except

for the dicarbylindium complex 3a (P_m= 0.70), no indium

complexes induced stereoselectivity for the ROP of rac-LA.

This behavior may be tentatively attributed to the lower steric

hindrance induced by the ligand around the indium metallic

center in comparison to the aluminum congener, as a result of

the larger size of the indium center.¹⁵However, changes in

operative mechanisms from Al to In systems might be another

possible explanation.

In fact, kinetic studies revealed diﬀerent rate laws for two

analogous aluminum and indium systems; however, these

studies were inconclusive for discerning unambiguously

between diﬀerent operative mechanisms or diﬀerent rate-

determining steps of a given mechanism. On the basis of

stoichiometric reactivity studies, we propose that aluminum-

based systems operate through a coordination−insertion

mechanism, as demonstrated by the similar performances of

dimethylaluminum complexes in combination with iPrOH in

comparison to the isolated bis(isopropyl (S)-lactate)aluminum

complexes in the ROP of rac-LA. On the other hand, we

suggest that ROP with these indium-based systems proceeds

through an activated monomer mechanism, as supported by the

absence of reaction between dialkylindium complexes with

{ON^Ph(iPr)2}H (b). A solution of 2-hydroxy-5-methyl-3-

(triphenylsilyl)benzaldehyde (1.01 g, 2.56 mmol), 2,6-diisopropylani-

line (0.543 g, 3.07 mmol), and PTSA (cat., ca. 2.0 mg) in benzene (20

mL) was stirred in a Dean−Stark apparatus at reﬂux at 110 °C for 48

h. The reaction mixture was cooled to room temperature, and volatiles

were removed under vacuum. The resulting solid residue was then

recrystallized in methanol at −30 °C to give {ONN^Ph(iPr)2}H (b) as a

1

yellow solid (0.581 g, 41%). H NMR (400 MHz, C₆D₆, 298 K): δ

1.02 (d, ³J_H−H= 6.8 Hz, 12H, CH(CH₃)₂), 1.94 (s, 3H, ArCH₃), 2.90

3

(q, J_H−H= 6.7 Hz, 2H, CH(CH₃)₂), 6.86 (s, 1H, H arom), 7.06 (s,

4H, H arom), 7.14−7.19 (m, 8H, H arom), 7.48 (s, 1H, H arom),

7.87−7.88 (m, 6H, H arom), 7.96 (s, 1H, ArCHN), 13.60 (s, 1H,

ArOH). ¹³C{¹H} NMR (100 MHz, C₆D₆, 298 K): δ 20.14 (ArCH₃),

23.32 (CH(CH₃)₂), 28.43 (CH(CH₃)₂), 118.12, 123.15, 123.39,

125.67, 129.45, 134.91, 135.12, 136.87, 138.71, 143.33, 146.75, 164.98

(ipso-C phenol), 166.96 (ArCHN). Anal. Calcd for C₃₈H₃₉NOSi: C,

82.41; H, 7.10; N, 2.53. Found: C, 82.34; H, 7.24; N, 2.44. Mp: 179

°C.

{ON^ArF}H (c). A solution of 2-hydroxy-5-methyl-3-(triphenylsilyl)-

benzaldehyde (3.00 g, 7.60 mmol), 3,5-bis(triﬂuoromethyl)aniline

J

dx.doi.org/10.1021/om301155m | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

(2.09 g, 9.12 mmol), and formic acid (2 drops of a 98% pure solution)

in methanol (50 mL) was stirred at reﬂux at 70 °C for 18 h. The

reaction mixture was cooled to room temperature, and volatiles were

removed under vacuum until the appearance of a precipitate. The

suspension was then stored at 4 °C for 24 h. The precipitate was

ﬁltered oﬀ, washed with cold methanol (3 × 20 mL), and dried in

1.98 (s, 3H, ArCH₃), 3.36 (s, 6H, m-OCH₃), 3.84 (s, 3H, p-OCH₃),

4.21 (s, 2H, NCH₂Ar), 6.28 (s, 2H, H arom), 6.78 (s, 1H, H arom),

7.15−7.16 (m, 10H, H arom), 7.44 (s, 1H, H arom), 7.88−7.89 (m,

7H, H arom and ArCHN), 13.99 (s, 1H, ArOH). ¹³C{¹H} NMR

(100 MHz, C₆D₆, 298 K): δ 20.01 (ArCH₃), 55.50 (m-OCH₃), 60.13

(p-OCH₃), 62.22 (NCH₂Ar), 105.28, 118.05, 122.32, 129.19, 133.11,

134.14, 135.10, 136.60, 142.16, 153.98, 164.60 (ArCHN), 165.21

(ipso-C phenol). Anal. Calcd for C₃₆H₃₅NO₄Si: C, 75.36; H, 6.15; N,

2.44. Found: C, 75.28; H, 6.13; N, 2.39. Mp: 189 °C.

1

vacuo to give {ON^PhCF3}H (c) as a yellow solid (2.65 g, 58%). H

NMR (200 MHz, C₆D₆, 298 K): δ 2.01 (s, 3H, ArCH₃), 6.70 (d, J_H−H

= 1.5 Hz, 1H, H arom), 6.87 (s, 2H, H arom), 7.21−7.25 (m, 9H, H

arom), 7.55 (d, J_H−H= 2.7 Hz, 3H, H arom), 7.91−7.93 (m, 6H, H

arom and ArCHN), 12.66 (s, 1H, ArOH). ¹⁹F{¹H} NMR (376

MHz, C₆D₆, 298 K): δ − 62.49 (s, 2 CF₃). ¹³C{¹H} NMR (100 MHz,

C₆D₆, 298 K): δ 19.89 (ArCH₃), 117.73, 121.41, 121.44, 121.89,

{ON^CHPh2}H (j). This product was prepared as described above for c

starting from 2-hydroxy-5-methyl-3-(triphenylsilyl)benzaldehyde

(0.542 g, 1.37 mmol), benzhydrylamine (0.302 g, 1.65 mmol), and

formic acid (2 drops of a 98% solution) to give j as a yellow solid

1

123.25, 124.61, 129.50, 131.62−132.61 (q, J_C−F= 33.2 Hz, CF₃),

(0.210 g, 27%). H NMR (400 MHz, C₆D₆, 298 K): δ 1.93 (s, 3H,

134.61, 135.51, 136.52, 144.34, 149.91, 164.44 (ipso-C phenol), 165.28

(ArCHN). Anal. Calcd for C₃₄H₂₅F₆NOSi: C, 67.43; H, 4.16; N,

2.31. Found: C, 68.01; H, 4.36; N, 2.29. Mp: 197 °C.

ArCH₃), 5.20 (s, 1H, NCHPh₂), 6.67 (s, 1H, H arom), 7.01−7.05 (m,

10H, H arom), 7.13−7.16 (m, 9H, H arom), 7.40 (s, 1H, H arom),

7.85−7.90 (m, 7H, H arom and ArCHN), 13.87 (s, 1H, ArOH).

¹³C{¹H} NMR (100 MHz, C₆D₆, 298 K): δ 19.98 (ArCH₃), 76.27

(NCHPh₂), 118.13, 122.17, 127.08, 128.50, 129.21, 134.56, 135.08,

136.64, 142.41, 142.56, 164.54 (ipso-C phenol), 165.18 (ArCHN).

Anal. Calcd for C₃₉H₃₃NOSi: C, 83.68; H, 5.87; N, 2.20. Found: C,

83.69; H, 5.85; N, 2.22. Mp: 176 °C.

{ON^PhCF3}H (d). This product was prepared as described above for c

starting from 2-hydroxy-5-methyl-3-(triphenylsilyl)benzaldehyde

(0.636 g, 1.61 mmol), 3,3″,5,5″-tetrakis(triﬂuoromethyl)[1,1′:3′,1″-

terphenyl]-2′-amine (0.998 g, 1.93 mmol), and formic acid (2 drops of

1

a 98% solution) to give d as a yellow solid (0.342 g, 26%). H NMR

(400 MHz, C₆D₆, 298 K): δ 1.65 (s, 1H, ArCH₃), 6.34 (d, J_H−H= 1.4

Hz, 1H, H arom), 6.85 (d, J_H−H= 1.5 Hz, 1H, H arom), 6.86 (s, 1H, H

arom), 6.90−6.94 (m, 1H, H arom), 7.08 (s, 1H, ArCHN), 7.25 (t,

J_H−H= 3.0 Hz, 8H, H arom), 7.28 (m, 1H, H arom), 7.52 (s, 4H, H

arom), 7.65 (s, 2H, H arom), 7.73 (m, 6H, H arom), 11.77 (s, 1H,

ArOH). ¹⁹F{¹H} NMR (376 MHz, C₆D₆, 298 K): δ −62.57(s, 4 CF₃).

¹³C{¹H} NMR (100 MHz, C₆D₆, 298 K): δ 19.59 (ArCH₃), 116.62,

{ON^trityl}H (k). This product was prepared as described above for c

starting from 2-hydroxy-5-methyl-3-(triphenylsilyl)benzaldehyde

(0.500 g, 1.27 mmol), tritylamine (0.394 g, 1.52 mmol), and formic

acid (2 drops of a 98% solution) to give k as a yellow solid (0.310 g,

38%). ¹H NMR (300 MHz, C₆D₆, 298 K): δ 1.86 (s, 3H, ArCH₃), 6.65

(s, 1H, H arom), 6.97−6.99 (m, 9H, H arom), 7.10−7.18 (m, 16H, H

arom), 7.42 (d, J_H−H= 1.4 Hz, 1H, H arom), 7.88−7.92 (m, 5H, H

arom), 8.00 (s, 1H, ArCHN), 14.48 (s, 1H, ArOH). ¹³C{¹H} NMR

(100 MHz, C₆D₆, 298 K): δ 19.89 (ArCH₃), 79.30 (NCPh₃), 118,16,

122.19, 127.00, 129.20, 129.75, 135.09, 135.17, 136.48, 136.69, 142.66,

144.68, 164.71 (ipso-C phenol), 164.96 (ArCHN). Anal. Calcd for

C₄₅H₃₇NOSi: C, 85.00; H, 5.87; N, 2.20. Found: C, 85.06; H, 5.97; N,

2.09. Mp: 210 °C.

2

120.57 (m, J_C−F= 3.6 Hz, C-CF₃), 121.99, 122.91, 124.70, 125.73,

129.22, 129.99, 130.64, 131.39 (q, ¹J_C−F= 33 Hz, CF₃), 131.21, 134.29,

134.62, 136.40, 17.91, 144.50, 154.47, 164.25 (ipso-C phenol), 170.26

(ArCHN). Anal. Calcd for C₄₈H₃₁F₁₂NOSi: C, 64.50; H, 3.50; N,

1.57. Found: C, 64.49; H, 3.56; N, 1.56. Mp: 230 °C.

{ON^Phmorpho}H (e). This product was prepared as described above

for c starting from 2-hydroxy-5-methyl-3-(triphenylsilyl)benzaldehyde

(2.00 g, 5.07 mmol), 2-morpholine-aniline (1.08 g, 6.08 mmol), and

formic acid (2 drops of a 98% solution) to give e as a yellow solid

{ON^CMetBu}H (l). This product was prepared as described above for c

starting from 2-hydroxy-5-methyl-3-(triphenylsilyl)benzaldehyde (3.00

g, 7.60 mmol), (R)-3,3-dimethyl-2-butylanine (0.923 g, 9.12 mmol),

and formic acid (2 drops of a 98% solution) to give l as a yellow solid

1

(1.70 g, 60%). H NMR (300 MHz, C₆D₆, 298 K): δ 1.99 (s, 3H,

ArCH₃), 2.61 (s, 4H, NCH₂morpholine), 3.45 (s, 4H, OCH₂

morpholine), 6.67−6.75 (m, 2H, H arom), 6.85 (s, 2H, H arom),

7.14−7.22 (m, 11H, H arom), 7.47 (s, 1H, H arom), 7.89 (s, 5H, H

arom), 8.14 (s, 1H, ArCHN), 14.36 (s, 1H, ArOH). ¹³C{¹H} NMR

(125 MHz, CDCl₃, 298 K): δ 20.51 (ArCH₃), 52.18 (NCH₂

morpholine), 66.93 (OCH₂morpholine), 118.38, 118.81, 122.34,

123.28, 127.66, 127.72, 129.29, 129.81, 134.71, 135.21, 135.47, 136.44,

142.78, 146.58, 160.95 (ArCHN), 164.50 (ipso-C phenol). Anal.

Calcd for C₃₆H₃₄N₂O₂Si: C, 77.94; H, 6.18; N, 5.05. Found: C, 77.85;

H, 6.17; N, 5.11. Mp: 211 °C. Crystals suitable for X-ray diﬀraction

studies were grown from slow evaporation of a saturated C₆D₆

solution.

1

(1.88 g, 52%). H NMR (400 MHz, C₆D₆, 298 K): δ 0.68 (s, 9H,

3

C(CH₃)₃), 0.81 (d, J_H−H= 6.4 Hz, 3H, NCHCH₃), 2.02 (s, 3H,

ArCH₃), 2.55 (q, ³J_H−H= 6.0 Hz, 1H, NCHCH₃), 6.87 (d, J_H−H= 1.5

Hz, 1H, H arom), 7.17−7.19 (m, 9H, H arom), 7.43 (d, J_H−H= 1.5 Hz,

1H, H arom), 7.86 (s, 1H, ArCHN), 7.88−7.90 (m, 5H, H arom),

13.92 (s, 1H, ArOH). ¹³C{¹H} NMR (100 MHz, C₆D₆, 298 K): δ

16.93 (NCHCH₃), 20.05 (ArCH₃), 26.10 (C-(CH₃)₃), 33.61

(C(CH₃)₃), 74.23 (NCHCH₃), 118.04, 122.04, 127.06, 129.15,

134.01, 135.20, 136.64, 141.90, 162.95 (ArCHN), 164.74 (ipso-C

phenol). Anal. Calcd for C₃₂H₃₅NOSi: C, 80.46; H, 7.38; N, 2.93.

Found: C, 80.59; H, 7.08; N, 2.95. Mp: 165 °C. Crystals suitable for X-

ray diﬀraction studies were grown from slow evaporation of a saturated

C₆D₆solution.

{ON^CH2mes}H (h). This product was prepared as described above for

c starting from 2-hydroxy-5-methyl-3-(triphenylsilyl)benzaldehyde

(0.701 g, 1.77 mmol), 2,4,6-trimethylbenzylamine (0.318 g, 2.13

mmol), and formic acid (2 drops of a 98% solution) to give h as a

{ON^pipeBn}H (m). This product was prepared as described above for

b starting from 2-hydroxy-5-methyl-3-(triphenylsilyl)benzaldehyde

(2.00 g, 5.07 mmol), 4-amino-1-benzylpiperidine (1.16 g, 6.08

mmol), and formic acid (2 drops of a 98% solution) to give m as a

yellow solid (1.79 g, 62%). ¹H NMR (400 MHz, C₆D₆, 298 K): δ 1.30

(m, 2H, CH₂pip), 1.46 (qd, J_H−H= 10.2 and 3.5 Hz, 2H, CH₂pip),

1.79 (t, ³J_H−H= 10.2 Hz, 2H, CH₂pip), 2.03 (s, 3H, ArCH₃), 2.57 (m,

2H, CH₂pip), 2.70 (m, 1H, CH pip), 3.26 (s, 2H, NCH₂Ph), 6.87 (d,

³J_H−H= 1.5 Hz, 1H, H arom), 7.17−7.25 (m, 10H, H arom), 7.32 (s,

1H, H arom), 7.34 (s, 1H, H arom), 7.45 (d, J_H−H= 1.8 Hz, 1H, H

arom), 7.83 (s, 1H, ArCHN), 7.90 (m, 6H, H arom), 13.80 (s, 1H,

ArOH). ¹³C{¹H} NMR (100 MHz, C₆D₆, 298 K): δ 20.09 (ArCH₃),

33.13 (CH₂pip), 51.65 (CH₂pip), 62.79 (NCH₂Ph), 65.41 (CH pip),

118.17, 122.18, 126.89, 127.07, 128.17, 128.65, 129.18, 133.93, 135.19,

136.66, 139.28, 141.99, 162.76 (ArCHN), 164.68 (ipso-C phenol).

Anal. Calcd for C₃₈H₃₈N₂OSi: C, 80.52; H, 6.76; N, 4.94. Found: C,

80.53; H, 6.56; N, 4.81. Mp: 211 °C.

1

yellow solid (0.580 g, 61%). H NMR (400 MHz, C₆D₆, 298 K): δ

1.91 (s, 9H, CH₃Mes), 2.09 (s, 3H, ArCH₃), 4.35 (s, 2H, NCH₂Mes),

6.57 (s, 1H, H arom), 6.67 (s, 2H, H aromMes), 7.14−7.16 (m, 8H, H

arom), 7.39 (s, 1H, H arom), 7.84−7.89 (m, 7H, H arom and ArCH

N), 14.03 (s, 1H, ArOH). ¹³C{¹H} NMR (100 MHz, C₆D₆, 298 K): δ

19.27 (o-CH₃Mes), 19.97 (p-CH₃Ar), 20.63 (p-CH₃Ar), 54.60

(NCH₂Mes), 118.31, 121.99, 127.05, 129.08, 129.15, 129.87, 134.21,

135.21, 136.65, 136.93, 137.35, 141.92, 163.52 (ArCHN), 165.55

(ipso-C phenol). Anal. Calcd for C₃₆H₃₅NOSi: C, 82.44; H, 6.71; N,

2.66. Found: C, 82.56; H, 6.61; N, 2.68. Mp: 210 °C.

{ON^BnOMe3}H (i). This product was prepared as described above for

c starting from 2-hydroxy-5-methyl-3-(triphenylsilyl)benzaldehyde

(1.00 g, 2.53 mmol), 3,4,5-trimethoxybenzylamine (0.600 g, 3.04

mmol), and formic acid (2 drops of a 98% solution) to give i as a

1

yellow solid (0.536 g, 37%). H NMR (400 MHz, C₆D₆, 298 K): δ

K

dx.doi.org/10.1021/om301155m | Organometallics XXXX, XXX, XXX−XXX

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{ON^Ph(iPr)2}AlMe₂(1b). A solution of {ON^Ph(iPr)2}H (b; 0.096 g,

0.170 mmol) in toluene (2 mL) was added to a solution of AlMe₃

(0.170 mL of a 1.0 M solution in n-hexane, 0.41 mmol) in toluene (1

mL). The reaction mixture was stirred at room temperature for 18 h.

Then, the volatiles were evaporated and the resulting material was

washed with n-hexane (2 × 1 mL) and dried under vacuum to give 1b

as a yellow solid (0.067 g, 65%). ¹H NMR (400 MHz, C₆D₆, 298 K): δ

CH₃Mes), 2.06 (s, 3H, CH₃Mes), 4.38 (d, J_H−H= 1.4 Hz, 2H, CH₂

Mes), 6.29 (d, J_H−H= 1.9 Hz, 1H, H arom), 6.67 (s, 2H, H arom

Mes), 7.21−7.24 (m, 9H, H arom), 7.41 (t, J_H−H= 1.5 Hz, 1H,

ArCHN), 7.48 (d, J_H−H= 2.2 Hz, 1H, H arom), 7.84−7.87 (m, 6H,

H arom). ¹³C{¹H} NMR (125 MHz, C₆D₆, 298 K): δ −10.51

(Al(CH₃)₂), 19.00 (o-CH₃Mes), 19.97 (ArCH₃),20.63 (p-CH₃Mes),

50.91 (NCH₂Mes), 118.15, 125.31, 125.96, 126.06, 129.09, 129.36,

135.35, 136.50, 136.82, 138.33, 138.54, 146.81, 166.73 (ArCHN),

166.87 (C−O−Al). Anal. Calcd for C₃₈H₄₀AlNOSi: C, 78.45; H, 6.93;

N, 2.41. Found: C, 78.58; H, 6.84; N, 2.48.

3

−0.77 (s, 6H, Al(CH₃)₂), 0.69 (d, J_H−H= 6.7 Hz, 6H, CH(CH₃)₂),

3

1.03 (d, J_H−H= 6.6 Hz, 6H, CH(CH₃)₂), 1.72 (s, 3H, ArCH₃), 2.95

3

(m, J_H−H= 6.6 Hz, 2H, CH(CH₃)₂), 6.50 (s, 1H, H arom), 7.00 (s,

1H, H arom), 7.02 (s, 1H, H arom), 7.06−7.08 (m, 1H, H arom), 7.23

(br s, 9H, H arom), 7.64 (s, 1H, H arom), 7.76 (s, 1H, H arom), 7.87

(m, 6H, H arom and ArCHN). ¹³C{¹H} NMR (100 MHz, C₆D₆,

298 K): δ −9.71 (Al(CH₃)₂), 19.67 (ArCH₃), 22.33 (CH(CH₃)₂),

25.59 (CH(CH₃)₂), 28.10 (CH(CH₃)₂), 117.72, 124.07, 126.66,

129.20, 134.97, 136.63, 136.80, 142.19, 142.33, 148.53, 168.28 (C−

O−Al), 172.96 (ArCHN). Anal. Calcd for C₄₀H₄₄AlNOSi: C, 78.78;

H, 7.27; N, 2.30. Found: C, 78.69; H, 7.33; N, 2.33.

{ON^BnOMe3}AlMe₂(1i). This product was prepared as described

above for 1b starting from {ON^BnOMe3}H (i; 0.215 g, 0.374 mmol) and

AlMe₃(0.374 mL of a 1.0 M solution in n-heptane, 0.374 mmol) to

1

give 1i as a yellow solid (0.117 g, 50%). H NMR (400 MHz, C₆D₆,

298 K): δ −0.63 (s, 6H, Al(CH₃)₂), 1.87 (s, 3H, ArCH₃), 3.36 (s, 6H,

2 OCH₃), 3.80 (s, 3H, OCH₃), 4.11 (s, 2H, NCH₂Ar), 6.24 (s, 2H,

H−Ar(OMe)₃), 6.47 (s, 1H, H arom), 7.23 (s, 9H, H arom), 7.46 (s,

1H, ArCHN), 7.53 (s, 1H, H arom), 7.85 (m, 6H, H arom).

¹³C{¹H} NMR (100 MHz, C₆D₆, 298 K): δ −9.99 (Al(CH₃)₂), 19.67

(ArCH₃), 55.66 (m-OCH₃), 59.10 (NCH₂Ar), 60.19 (p-OCH₃),

107.06, 117.96, 125.99, 129.13, 129.75, 135.26, 136.57, 136.70,

147.25, 154.25, 167.26 (C−O−Al), 170.35 (ArCHN). Anal. Calcd

for C₃₈H₄₀AlNO₄Si: C, 72.47; H, 6.40; N, 2.22. Found: C, 72.40; H,

6.34; N, 2.34.

{ON^ArF}AlMe₂(1c). This product was prepared as described above

for 1b starting from {ON^ArF}H (c; 0.300 g, 0.495 mmol) and AlMe₃

(0.248 mL of a 2.0 M solution in n-hexane, 0.495 mmol) to give 1c as

a pale yellow solid (0.200 g, 62%). ¹H NMR (300 MHz, C₆D₆, 298 K):

δ −0.63 (s, 6H, Al(CH₃)₂), 1.93 (s, 3H, ArCH₃), 6.46 (s, 1H, H

arom), 7.06 (s, 1H, H arom), 7.25−7.26 (m, 7H, H arom), 7.30 (br s,

1H, H arom), 7.54 (s, 1H, H arom), 7.64 (s, 1H, H arom), 7.83 (m,

6H, H arom and ArCHN). ¹⁹F{¹H} NMR (188 MHz, C₆D₆, 298

K): δ −63.16 (s, 2 CF₃). ¹³C{¹H} NMR (125 MHz, C₆D₆, 298 K): δ

−9.45 (Al(CH₃)₂, 19.62 (ArCH₃), 117.99, 120.87, 120.90, 122.51,

{ON^CHPh2}AlMe₂(1j). This product was prepared as described

above for 1b starting from {ON^CHPh2}H (j; 0.100 g, 0.178 mmol) and

AlMe₃(0.180 mL of a 1.0 M solution in n-heptane, 0.180 mmol) to

1

give 1j as a yellow solid (0.068 g, 62%). H NMR (500 MHz, C₆D₆,

1

123.89, 126.94, 129.40, 132.81−133.36 (q, J_C−F= 33.8 Hz, CF₃),

298 K): δ −0.72 (s, 6H, Al(CH₃)₂), 1.80 (s, 3H, ArCH₃), 6.08 (s, 1H,

NCHPh₂), 6.36 (d, J_H−H= 1.1 Hz, 1H, H arom), 6.94 (m, 4H, H

arom), 6.99−7.01 (m, 6H, H arom), 7.21−7.22 (m, 9H, H arom), 7.55

(d, J_H−H= 2.3 Hz, 1H, H arom), 7.80 (s, 1H, ArCHN), 7.84−7.86

(m, 6H, H arom). ¹³C{¹H} NMR (125 MHz, C₆D₆, 298 K): δ −9.83

(Al(CH₃)₂), 19.60 (ArCH₃), 70.15 (NCHPh₂), 117.91, 126.05, 128.72,

129.08, 129.10, 135.20, 136.60, 137.12, 138.03, 147.61, 167.55 (C−

O−Al), 171.00 (ArCHN). Anal. Calcd for C₄₁H₃₈AlNOSi: C, 79.97;

H, 6.22; N, 2.27. Found: C, 80.16; H, 6.51; N, 2.13.

134.66, 136.46, 137.61, 147.86, 149.83. 168.55 (C−O−Al), 170.66

(ArCHN). Anal. Calcd for C₃₆H₃₀AlF₆NOSi: C, 65.35; H, 4.57; N,

2.12. Found: C, 65.33; H, 4.58; N, 2.12.

{ON^PhCF3}AlMe₂(1d). This product was prepared as described

above for 1b starting from {ON^PhCF3}H (d; 0.060 g, 0.060 mmol) and

AlMe₃(0.067 mL of a 1.0 M solution in n-heptane, 0.067 mmol) to

1

give 1d as a yellow solid (0.033 g, 52%). H NMR (500 MHz, C₆D₆,

298 K): δ −1.58 (s, 6H, Al(CH₃)₂), 1.84 (s, 3H, ArCH₃), 6.63 (d, J_H−H

= 1.75 Hz, 1H, H arom), 6.79 (s, 1H, H arom), 6.81 (s, 1H, H arom),

{ON^trityl}AlMe₂(1k). This product was prepared as described above

for 1b starting from {ON^trityl}H (k; 0.100 g, 0.157 mmol) and AlMe₃

(0.160 mL of a 1.0 M solution in n-heptane, 0.160 mmol) to give 1k as

6.93 (m, 1H, H arom), 7.13−7.15 (m, 13H, H arom), 7.25 (t, J_H−H

=

3.2 Hz, 1H, H arom), 7.27 (s, 1H, ArCHN), 7.45 (d, J_H−H= 2.3 Hz,

1H, H arom), 7.51 (s, 1H, H arom), 7.53 (s, 3H, H arom), 7.58−7.60

(m, 6H, H arom), 7.71 (m, 3H, H arom). ¹⁹F{¹H} NMR (376 MHz,

C₆D₆, 298 K): δ 62.65 (s, 4 CF₃). ¹³C{¹H} NMR (125 MHz, C₆D₆,

298 K): δ −11.10 (Al(CH₃)₂), 19.54 (ArCH₃), 116.28, 121.45, 122.18,

124.35, 126.53, 127.05, 129.14, 129.99, 130.64, 131.20, 131.47, 131.57,

131.73, 132.00, 134.53, 134.59, 134.62, 136.33, 140.91, 142.66, 144.50,

150.26, 168.64 (C−O−Al), 175.55 (ArCHN). Anal. Calcd for

C₅₀H₃₆AlF₁₂NOSi: C, 63.22; H, 3.82; N, 1.47. Found: C, 63.26; H,

3.81; N, 1.47.

1

a yellow solid (0.058 g, 57%). H NMR (500 MHz, C₆D₆, 298 K): δ

−0.89 (s, 6H, Al(CH₃)₂), 1.75 (s, 3H, ArCH₃), 6.26 (s, 1H, H arom),

6.96−6.99 (m, 7H, H arom), 7.17 (s, 4H, H arom), 7.21−7.23 (m, 7H,

H arom), 7.31 (m, 6H, H arom), 7.59 (s, 1H, H arom), 7.88 (m, 6H,

H arom), 8.11 (s, 1H, ArCHN). ¹³C{¹H} NMR (100 MHz, C₆D₆,

298 K): −8.03 (Al(CH₃)₂), 19.55 (ArCH₃), 81.63 (NCPh₃), 117.72,

126.02, 129.13, 129.74, 130.69, 135.09, 136.69, 143.32, 144.68, 146.56,

164.95 (C−O−Al), 168.43 (ArCHN). Anal. Calcd for

C₄₇H₄₂AlNOSi: C, 81.59; H, 6.12; N, 2.02. Found: C, 81.69; H,

6.32; N, 2.20.

{ON^Phmorpho}AlMe₂(1e). This product was prepared as described

above for 1b starting from {ON^Phmorpho}H (e; 0.300 g, 0.541 mmol)

and AlMe₃(0.270 mL of a 2.0 M solution in n-hexane, 0.541 mmol) to

{ON^CMetBu}AlMe₂(1l). This product was prepared as described

above for 1b starting from {ON^CMetBu}H (l; 0.300 g, 0.628 mmol) and

AlMe₃(0.314 mL of a 2.0 M solution in n-hexane, 0.628 mmol) to give

1l as a bright yellow solid (0.177 g, 53%). ¹H NMR (400 MHz, C₆D₆,

298 K): δ −0.64 (s, 3H, Al(CH₃)), −0.63 (s, 3H, Al(CH₃)), 0.57 (s,

1

give 1e as a yellow solid (0.227 g, 70%). H NMR (300 MHz, C₆D₆,

298 K): δ −0.59 (s, 6H, Al(CH₃)₂), 1.90 (s, 3H, ArCH₃), 2.57 (t,

3

³J_H−H= 4.2 Hz, 4H, NCH₂morpholine), 3.40 (t, J_H−H= 4.2 Hz, 4H,

3

OCH₂morpholine), 6.70−6.71 (m, 2H, H arom), 6.76 (t,J_H−H= 7.6

Hz, 1H, H arom), 6.91 (t, J_H−H= 7.4 Hz, 1H, H arom), 7.22−7.23 (m,

10H, H arom), 7.58 (d, J_H−H= 2.0 Hz, 1H, H arom), 7.83−7.86 (m,

7H, H arom and CHN). ¹³C{¹H} NMR (125 MHz, C₆D₆, 298 K):

δ −9.36 (Al(CH₃)₂), 19.72 (ArCH₃), 51.25 (NCH₂morpholine),

66.43 (OCH₂morpholine), 118.10, 120.74, 123.92, 124.07, 126.53,

127.97, 128.06, 129.22, 125.09, 136.52, 136.77, 140.62, 145.30, 145.29,

168.19 (C−O−Al), 174.00 (ArCHN). Anal. Calcd for

C₃₈H₃₉AlN₂O₂Si: C, 74.72; H, 6.44; N, 4.59. Found: C, 74.59; H,

6.39; N, 4.43.

9H, C(CH₃)₃), 0.90 (d, J_H−H= 6.5 Hz, 3H, NCHCH₃), 1.88 (s, 3H,

ArCH₃), 2.87 (q, ³J_H−H= 6.9 Hz, 1H, NCHCH₃), 6.58 (d, J_H−H= 2.2

Hz, 1H, H arom), 7.11 (s, 1H, H arom), 7.15−7.16 (m, 8H, H arom),

7.55 (s, 1H, H arom, ArCHN), 7.87 (m, 6H, H arom). ¹³C{¹H}

NMR (100 MHz, C₆D₆, 298 K): δ −8.45 (Al(CH₃)), −8.15

(Al(CH₃)), 15.78 (NCHCH₃), 19.76 (ArCH₃), 26.79 (C(CH₃)₃),

35.02 (C(CH₃)₃), 70.52 (NCHCH₃), 117.95, 117.95, 125.93, 129.10,

135.20, 136.63, 136.88, 147.11, 167.47 (C−O−Al), 170.60 (ArCH

N). Anal. Calcd for C₃₄H₄₀AlNOSi: C, 76.51; H, 7.55; N, 2.62. Found:

C, 76.42; H, 7.84; N, 2.43.

{ON^CH2mes}AlMe₂(1h). This product was prepared as described

above for 1b starting from {ON^CH2mes}H (h; 0.100 g, 0.190 mmol)

and AlMe₃(0.190 mL of a 1.0 M solution in n-heptane, 0.190 mmol)

to give 1h as a yellow solid (0.092 g, 83%). ¹H NMR (500 MHz, C₆D₆,

298 K): δ −0.49 (s, 6H, Al(CH₃)₂), 1.73 (s, 3H, ArCH₃), 1.86 (s, 6H,

{ON^pipeBn}AlMe₂(1m). This product was prepared as described

above for 1b starting from {ON^pipeBn}H (m; 0.300 g, 0.530 mmol) and

AlMe₃(0.530 mL of a 1.0 M solution in n-heptane, 0.530 mmol) to

give 1m as a yellow solid (0.208 g, 63%). ¹H NMR (500 MHz, C₆D₆,

298 K): δ −0.55 (s, 6H, Al(CH₃)₂), 1.37 (m, 2H, CH₂pip), 1.55 (td,

L

dx.doi.org/10.1021/om301155m | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

J_H−H= 10 and 2.0 Hz, 2H, CH₂pip), 1.72 (qd, J_H−H= 8.6 and 3.6 Hz,

2H, CH₂pip), 1.96 (s, 3H, ArCH₃), 2.62 (m, 2H, CH₂pip), 2.80 (m,

1H, NCH(CH₂)₂), 3.23 (s, 2H, NCH₂Ph), 6.56 (d, J_H−H= 1.8 Hz, 1H,

H arom), 7.05−7.29 (m, 14H, H arom), 7.38 (s, 1H, ArCHN), 7.55

(dd, J_H−H= 1.9 and 0.5 Hz, 1H, H arom), 7.87 (m, 6H, H arom).

¹³C{¹H} NMR (125 MHz, C₆D₆, 298 K): δ −8.45 (Al(CH₃)₂), 19.75

(ArCH₃), 32.60 (CH₂pip), 52.19 (CH₂pip), 62.56 (NCH₂Ph), 65.59

(NCH), 118.01, 125.79, 127.11, 127.95, 128.28, 128.72, 129.11,

135.25, 136.59, 136.71, 138.58, 147.04, 167.24 (C−O−Al), 168.91

(ArCHN). Anal. Calcd for C₄₀H₄₃AlN₂OSi: C, 77.13; H, 6.96; N,

4.50. Found: C, 76.87; H, 7.06; N, 4.83. Crystals suitable for X-ray

diﬀraction studies were grown from a saturated Et₂O solution layered

with hexanes at room temperature.

{ON^CH2mes}In(CH₂SiMe₃)₂(3h). This product was prepared as

described above for 3f starting from {ON^CH2mes}H (h; 0.119 g, 0.226

mmol) and In(CH₂SiMe₃)₃(0.112 g, 0.226 mmol) to give 3h as a

1

bright yellow solid (0.114 g, 61%). H NMR (400 MHz, C₆D₆, 298

K): δ −0.62 (d, ²J_H−H= 12.6 Hz, 2H, CHHSiMe₃), −0.53 (d, ²J_H−H

=

12.6 Hz, 2H, CHHSiMe₃), 0.00 (s, 18H, Si(CH₃)₃), 1.76 (s, 3H, p-

CH₃Mes), 1.99 (s, 6H, o-CH₃Mes), 2.04 (s, 3H, ArCH₃), 4.33 (s, 2H,

NCH₂Ph), 6.49 (s, 1H, H arom), 6.64 (s, 2H, H arom Mes), 7.10−

7.15 (s, 8H, H arom), 7.40 (s, 1H, H arom), 7.51 (s, 1H, ArCHN),

7.87 (m, 7H, H arom). ¹³C{¹H} NMR (100 MHz, C₆D₆, 298 K): δ

−1.80 (CH₂SiMe₃), 0.00 (Si(CH₃)₃), 17.53 (p-CH₃Mes), 17.58 (o-

CH₃Mes), 18.58 (ArCH₃), 53.09 (NCH₂Ar), 116.02, 121.42, 126.75,

127.53, 134.26, 134.58, 135.86, 136.31, 136.63, 144.44, 166.21

(ArCHN), 170.14 (ArC−O−In). Anal. Calcd for C₄₄H₅₆InNOSi₃:

C, 64.92; H, 6.93; N, 1.72. Found: C, 65.04; H, 6.91; N, 1.66.

{ON^CMetBu}In(CH₂SiMe₃)₂(3l). This product was prepared as

described above for 3f starting from {ON^CMetBu}H (l; 0.229 g, 0.477

mmol) and In(CH₂SiMe₃)₃(0.236 g, 0.477 mmol) to give 3l as a

{ON^pipeBn}InMe₂(2m). To a stirred suspension of InCl₃(0.117 g,

0.529 mmol) in Et₂O (ca. 5 mL) was added a solution of MeLi (1.0

mL of a 1.6 M solution in diethyl ether, 1.58 mmol). The reaction

mixture was stirred for 30 min at room temperature, and a solution of

{ON^pipeBn}H (g; 0.300 g, 0.529 mmol) in toluene (ca. 5 mL) was

added. The resulting reaction mixture was stirred at room temperature

for 18 h. Volatiles were removed in vacuo, and the crude product

washed with n-hexane (ca. 10 mL) to give 2m as a yellow solid (0.290

g, 77%). ¹H NMR (300 MHz, C₆D₆, 298 K): δ −0.17 (s, 6H,

In(CH₃)₂), 1.02 (m, 1H, CH₂pip), 1.59 (m, 3H, CH₂pip), 1.83 m,

1H, CH₂pip), 2.59−2.61 (m, 3H, CH₂pip), 3.24 (s, 2H, NCH₂Ph),

3.29 (m, 1H, CH pip), 6.71 (d, J_H−H= 2.2 Hz, 1H, H arom), 7.10−

7.18 (m, 8H, H arom), 7.22−7.23 (m, 3H, H arom), 7.41 (s, 1H, H

arom), 7.50 (d, J_H−H= 2.4 Hz, 1H, H arom), 7.65 (s, 1H, ArCHN),

7.89−7.91 (m, 8H, H arom). ¹³C{¹H} NMR (166 MHz, C₆D₆, 298

K): δ −5.23 (In(CH₃)₂), 19.53 (ArCH₃), 33.56 (CH₂pip), 51.87

(CH₂pip), 62.61 (NCH₂Ph), 68.95 (CH pip), 117.73, 121.31, 121.87,

123.20, 127.01, 127.18, 127.97, 128.03, 128.30, 128.33, 128.54, 128.80,

129.05, 136.65, 136.68, 136.81, 138.25, 138.57, 146.27, 165.40, 168.88,

172.54 (ArCHN), 172.97 (ArC−O−In). Anal. Calcd for

C₄₀H₄₃InN₂OSi: C, 67.60; H, 6.10; N, 3.94. Found: C, 67.34; H,

6.67; N, 3.34. Crystals suitable for X-ray diﬀraction studies were grown

from a saturated Et₂O solution layered with hexanes at room

temperature.

1

bright yellow solid (0.210 g, 57%). H NMR (400 MHz, C₆D₆, 298

K): δ −0.30 (s, 2H, CH₂SiMe₃), −0.23 (dd, ²J_H−H= 12.6 and 12.5 Hz,

2H, CH₂SiMe₃), 0.06 (s, 9H, Si(CH₃)₃), 0.13 (s, 9H, Si(CH₃)₃), 0.75

(s, 9H, C(CH₃)₃), 1.10 (d, 3H, CH(CH₃), 2.06 (s, 3H, ArCH₃), 2.76

3

(q, J_H−H= 6.6 Hz, 1H, CH(CH₃)), 6.84 (d, J_H−H= 2.2 Hz, 1H, H

arom), 7.23 (br s, 1H, H arom), 7.26 (d, J_H−H= 2.2 Hz, 1H, H arom),

7.29−7.31 (m, 8H, H arom), 7.53 (d, J_H−H= 2.4 Hz, 1H, H arom),

7.68 (s, 1H, ArCHN), 7.89−7.93 (m, 5H, H arom). ¹³C{¹H} NMR

(125 MHz, C₆D₆, 298 K): δ 2.19 (Si(CH₃)₃), 2.32 (Si(CH₃)₃), 2.62

(CH₂SiMe₃), 2.78 (CH₂SiMe₃), 16.38 (CH(CH₃)), 19.83 (ArCH₃),

26.82 (C(CH₃)₃), 35.04 (C(CH₃)₃), 75.47 (CH(CH₃)), 118.03,

123.81, 128.95, 129.23, 136.33, 136.72, 139.21, 147.11, 171.67

(ArCHN), 173.27 (ArC−O−In). Anal. Calcd for C₄₀H₅₆InNOSi₃:

C, 62.72; H, 7.37; N, 1.83. Found: C, 62.14; H, 7.46; N, 1.84.

{ON^pipeBn}In(CH₂SiMe₃)₂(3m). This product was prepared as

described above for 3f starting from {ON^pipeBn}H (m; 0.113 g, 0.200

mmol) and In(CH₂SiMe₃)₃(0.098 g, 0.200 mmol) to give 3m as a

1

bright yellow solid (0.128 g, 75%). H NMR (400 MHz, C₆D₆, 298

K): δ −0.35 (s, 4H, CH₂SiMe₃), 0.05 (s, 18H, Si(CH₃)₃), 1.43 (m, 2H,

CH₂pip), 1.72 (m, 4H, CH₂pip), 2.02 (s, 3H, ArCH₃), 2.68 (m, 3H,

CH₂pip and CH), 3.27 (s, 2H, N−CH₂−Ph), 6.72 (d, J_H−H= 2.2 Hz,

1H, H arom), 7.10 (m, 1H, H arom), 7.16−7.24 (m, 12H, H arom),

7.29 (s, 1H, H arom), 7.31 (s, 1H, H arom), 7.50 (s, 2H, H arom and

ArCHN), 7.83−7.87 (m, 5H, H arom). ¹³C{¹H} NMR (100 MHz,

C₆D₆, 298 K): δ −2.52 (CH₂SiMe₃), 0.24 (Si(CH₃)₃), 17.60 (ArCH₃),

31.52 (CH₂pip), 49.75 (CH₂pip), 60.61 (NCH₂Ph), 65.57 (CH pip),

115.86, 121.24, 126.44, 126.67, 127.00, 134.06, 134.47, 136.34, 136.55,

144.63, 167.18 (ArCHN), 170.65 (ArC−O−Al). Anal. Calcd for

C₄₅H₅₆InN₂OSi₃: C, 64.34; H, 6.72; N, 3.33. Found: C, 64.15; H, 7.01;

N, 3.36.

{ON^Bn}In(CH₂SiMe₃)₂(3f). A solution of {ON^Bn}H (f; 0.097 g,

0.201 mmol) in toluene (8 mL) and In(CH₂SiMe₃)₃(0.099 g, 0.201

mmol) in toluene (2 mL) was stirred at 70 °C for 18 h. Then, volatiles

were removed in vacuo, n-hexane (ca. 10 mL) was vacuum-condensed

in, and the resulting solution was ﬁltered oﬀ. The ﬁltrate was

concentrated under vacuum to give 3f as a yellow solid (0.077 g, 50%).

¹H NMR (400 MHz, C₆D₆, 298 K): δ −0.72 (d, ²J_H−H= 12.6 Hz, 2H,

2

CHHSiMe₃), −0.59 (d, J_H−H= 12.6 Hz, 2H, CHHSiMe₃), 0.00 (s,

18H, Si(CH₃)₃), 2.03 (s, 3H, ArCH₃), 4.16 (s, 2H, NCH₂Ph), 6.71 (s,

1H, H arom), 6.94 (d, J_H−H= 6.9 Hz, 2H, H arom), 7.08−7.10 (m,

4H, H arom), 7.25 (s, 5H, H arom), 7.52 (s, 1H, H arom), 7.55 (s, 1H,

ArCHN), 7.87−7.89 (m, 7H, H arom). ¹³C{¹H} NMR (100 MHz,

C₆D₆, 298 K): δ −1.35 (CH₂SiMe₃), 0.00 (Si(CH₃)₃), 17.80 (ArCH₃),

61.65 (NCH₂Ph), 115.81, 121.46, 126.73, 126.89, 127.04, 133.56,

134.29, 134.67, 136.78, 145.02, 168.91 (ArCHN), 170.70 (ArC−

O−In). Anal. Calcd for C₄₁H₅₀InNOSi₃: C, 63.79; H, 6.53; N, 1.81.

Found: C, 65.94; H, 6.42; N, 1.87.

{ONN^qui}Al((1R,2S,5R)-menthyl (S)-lactate)₂(7a). THF (ca. 10

-

mL) was vacuum-condensed in a Schlenk ﬂask containing {ONN^qui

}

AlMe₂(1a; 0.200 g, 0.347 mmol) and (1R,2S,5R)-menthyl (S)-H-

lactacte (0.158 g, 0.694 mmol). The reaction mixture was stirred at

room temperature for 30 min. Then, the solvent was evaporated in

vacuo, n-pentane (ca. 10 mL) was vacuum-condensed in, and the ﬂask

was stored at −30 °C for 72 h. The precipitate that formed was ﬁltered

oﬀ and washed with pentane (ca. 5 mL). The resulting residue was

{ONN^CH2pyr}In(CH₂SiMe₃)₂(3g). This product was prepared as

described above for 3f starting from {ONN^CH2pyr}H (g; 0.083 g, 0.171

mmol) and In(CH₂SiMe₃)₃(0.085 g, 0.171 mmol) to give 3g as a

1

dried under vacuum to give 7a as an orange solid (0.104 g, 30%). H

NMR (400 MHz, C₆D₆, 298 K): δ 0.11 (d, J_H−H= 6.6 Hz, 3H,

CHCH(CH₃)₂), 0.52 (m, 3H, CHCH₃), 0.59 (m, 9H, CHCH(CH₃)₂

and CH₂), 0.67 (d, J_H−H= 5.7 Hz, 3H, CHCH₃), 0.76 (d, J_H−H= 6.8

Hz, 6H, CHCH(CH₃)₂and CH₂), 0.77 (m, 2H, CH₂), 1.05 (m, 2H,

CHCH(CH₃)₂), 1.18 (d, J_H−H= 6.6 Hz, 3H, OCHCH₃), 1.29 (m, 8H,

CH and CH₂), 1.46 (d, J_H−H= 6.5 Hz, 3H, OCHCH₃), 1.96 (s, 3 H,

ArCH₃), 4.24 (q, J_H−H= 6.9 Hz, 1H, OCHCH₃), 4.30 (m, 2H,

CHCH₃), 4.57 (q, J_H−H= 6.9 Hz, 1H, OCHCH₃), 6.8 (m, 1H, H

arom), 7.00 (m, 1H, H arom), 7.08−7.19 (m, 4H, H arom), 7.29−7.58

(m, 12H, H arom), 8.10 (d, J_H−H= 6.4 Hz, 6H, H arom), 8.67 (s, 1H,

ArCHN), 9.20 (s, 1H, H arom). ¹³C{¹H} NMR (125 MHz, C₆D₆,

298 K): δ 15.64 (CHCH(CH₃)₂), 15.98 (CHCH(CH₃)₂), 19.92

(ArCH₃), 20.33 (CHCH(CH₃)₂), 20.72 (CHCH(CH₃)₂), 21.64

1

bright yellow solid (0.092 g, 70%). H NMR (400 MHz, C₆D₆, 298

K): δ −0.60 (d, ²J_H−H= 12.2 Hz, 2H, CHHSiMe₃), −0.43 (d, ²J_H−H

=

12.2 Hz, 2H, CHHSiMe₃), 0.08 (s, 18H, Si(CH₃)₃), 2.04 (s, 3H,

ArCH₃), 4.00 (s, 2H, NCH₂pyr), 6.26 (d, J_H−H= 7.7 Hz, 1H, H arom),

6.41 (t, J_H−H= 5.7 Hz, 1H, H arom), 6.79−6.81 (m, 2H, H arom),

7.30 (m, 9H, H arom), 7.53 (s, 1H, H arom), 7.61 (s, 1H, ArCHN),

7.97−8.03 (m, 7H, H arom). ¹³C{¹H} NMR (100 MHz, C₆D₆, 298

K): δ −2.09 (CH₂SiMe₃), 0.00 (Si(CH₃)₃), 17.93 (ArCH₃), 58.82

(NCH₂Ph), 115,34, 119.14, 119.54, 120.76, 126.31, 126.71, 134.63,

134.90, 135.54, 136.21, 144.54, 145.48, 153.21, 169.98 (ArCHN),

173.50 (ArC−O−In). Anal. Calcd for C₄₀H₄₉InN₂OSi₃: C, 62.16; H,

6.39; N, 3.62. Found: C, 62.86; H, 5.97; N, 3.60.

M

dx.doi.org/10.1021/om301155m | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

(CHCH₃), 21.71 (CHCH₃), 22.89 (OCHCH₃), 23.03 (CH₂), 23.18

(OCHCH₃), 23.33 (CH₂), 25.59 (CHCH(CH₃)₂), 25.88 (CHCH-

(CH₃)₂), 30.85 (CH₂), 33.91 (CH₂), 34.03 (CH₂), 40.42 (C(O)-

OCH), 40.64 (C(O)OCH), 46.73 (CHCH(CH₃)₂), 46.80 (CHCH-

(CH₃)₂), 68.00 (OCHCH₃), 68.47 (OCHCH₃), 73.41 (CHCH₃),

74.05 (CHCH₃), 114.75, 119.16, 122.06, 123.79, 124.00, 126.60,

128.17, 128.40, 139.07, 141.02, 147.74, 149.08, 162.81 (ArCHN),

173.16 (C−Al−O), 181.10 (CO), 182.69 (CO). Anal. Calcd for

C₆₁H₇₃AlN₂O₇Si: C, 73.17; H, 7.35; N, 2.80. Found: C, 73.42; H, 7.98;

N, 2.72.

were determined by homodecoupling ¹H NMR spectroscopy (methine

region) at 20 °C in CDCl₃on a Bruker AC-500 spectrometer.

Crystal Structure Determinations. Diﬀraction data for proli-

gands e, f, and l and complexes 1f,g,m, 2a,m, 3a, and 4h were collected

at 100(2) K using a Bruker APEX CCD diﬀractometer with graphite-

monochromated Mo Kα radiation (λ = 0.71073 Å). A combination of

ω and ϕ scans was carried out to obtain at least a unique data set. The

crystal structures were solved by direct methods; the remaining atoms

were located from diﬀerence Fourier synthesis followed by full-matrix

least-squares reﬁnement based on F²(programs SIR97 and SHELXL-

97)³⁹with the aid of the WINGX program.⁴⁰In most cases, many

hydrogen atoms could be found from the Fourier diﬀerence analysis.

Other hydrogen atoms were placed at calculated positions and forced

to ride on the attached atom. The hydrogen atom contributions were

calculated but not reﬁned. All non-hydrogen atoms were reﬁned with

anisotropic displacement parameters. The locations of the largest

peaks in the ﬁnal diﬀerence Fourier map calculation and the

magnitudes of the residual electron densities were of no chemical

signiﬁcance. Crystal data and details of data collection and structure

reﬁnement for proligands e, f, and l and complexes 1f,g,m, 2a,m, 3a,

and 4h are summarized in Tables S1−S5 (Supporting Information)

and can be obtained as well, free of charge, from the Cambridge

Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/

cif (CCDC 912295−912304).

{ON^Bn}Al(iPr (S)-lactate)₂(8f). A Young NMR tube was charged

with isopropyl (S)-H-lactate (0.0098 g, 0.074 mmol) and a solution of

{ON^Bn}AlMe₂(1c; 0.020 g, 0.037 mmol) in toluene (0.5 mL). The

reaction mixture was heated to 80 °C for 16 h. Then the volatiles were

removed and the product was dried in vacuo to give a colorless solid.

NMR indicated full conversion of the reagents to the desired product

8f. ¹H NMR (500 MHz, C₆D₆, 298 K): δ 0.76 (d, ³J_H−H= 6.1 Hz, 3H,

3

OCH(CH₃)₂), 0.91 (d, J_H−H= 6.2 Hz, 3H, OCH(CH₃)₂), 0.97 (d,

3

³J_H−H= 6.1 Hz, 3H, OCH(CH₃)₂), 1.00 (d, J_H−H= 6.2 Hz, 3H,

3

OCH(CH₃)₂), 1.32 (d, J_H−H= 6.7 Hz, 3H, AlOCH(CH₃)), 1.56 (d,

³J_H−H= 6.60 Hz, 3H, AlOCH(CH₃)), 1.80 (s, 3H, ArCH₃), 4.41 (q,

³J_H−H= 6.8 Hz, 1H, AlOCH(CH₃)), 4.46 (m, 1H, OCH(CH₃)₂), 4.63

3

(q, J_H−H= 6.9 Hz, 1H, AlOCH(CH₃)), 4.72 (m, 1H, OCH(CH₃)₂),

2

5.14 (d, J_H−H= 15.5 Hz, 1H, NCHHPh), 5.14 (d, J_H−H= 15.5 Hz,

1H, NCHHPh), 6.30 (m, 1H, H arom), 7.00 (d, J_H−H= 7.3 Hz, 2H, H

arom), 7.05 (t, J_H−H= 7.9 Hz, 2H, H arom), 7.19 (m, 7H, H arom),

7.33 (s, 2H, H arom), 7.38 (m, 2H, H arom), 7.59 (s, 1H, ArCHN),

7.85 (m, 5H H arom), 7.90 (m, 1H, H arom). ¹³C{¹H} NMR (125

MHz, C₆D₆, 298 K): δ 19.71 (ArCH₃), 21.05 (OCH(CH₃)₂), 21.11

(OCH(CH₃)₂), 21.35 (OCH(CH₃)₂), 21.38 (OCH(CH₃)₂), 22.18

(AlOCH(CH₃)), 22.79 (AlOCH(CH₃)), 58.00 (NCH₂Ph), 68.09

(OCH(CH₃)₂), 68.32 (AlOCH(CH₃)), 68.47 (AlOCH(CH₃)), 71.56

(OCH(CH₃)₂), 118.84, 125.32, 125.52, 125.59, 128.19, 128.47,

128.76, 128.95, 130.53, 136.30, 136.33, 136.51, 136.59, 136.67,

137.31, 137.51, 145.01, 166.83 (ArC−O−Al), 167.66 (ArCHN),

170.00 (CO).

ASSOCIATED CONTENT

* Supporting Information

CIF ﬁles, ﬁgures, and tables giving X-ray crystallographic data

for proligands e, f, and l and complexes 1f,g,m, 2a,m, 3a, and

4h, molecular structures of proligands e, f, and l and complexes

1f, 2a,m, and 4h, ¹H and ¹³C{¹H} NMR spectra of the

■

S

1

prepared proligands and complexes, representative H NMR

spectra and SEC traces of polymers, and additional kinetics

data. This material is available free of charge via the Internet at

http://pubs.acs.org.

{ON^pipeBn}Al(iPr (S)-lactate)₂(8m). A Schlenk ﬂask was charged

with isopropyl (S)-H-lactate (0.029 g, 0.22 mmol) and a solution of

{ON^pipeBn}AlMe₂(1m; 0.070 g, 0.011 mmol) in toluene (ca. 5 mL).

The reaction mixture was heated to 100 °C for 16 h. Volatiles were

removed, and the solid residue was washed with pentane (ca. 5 mL)

and then dried in vacuo to give 8m as a colorless solid (0.067 g, 71%).

¹H NMR (400 MHz, C₆D₆, 298 K): δ 0.73 (d, J_H−H= 6.2 Hz, 3H,

OCH(CH₃)₂), 0.95 (d, J_H−H= 5.3 Hz, 6H, OCH(CH₃)₂), 1.02 (d,

J_H−H= 6.1 Hz, 3H, OCH(CH₃)₂), 1.26 (d, J_H−H= 6.9 Hz, 3H,

AlOCH(CH₃)), 1.51 (d, J_H−H= 6.7 Hz, 3H, AlOCH(CH₃)), 1.85 (m,

3H, CH₂pip), 1.95 (s, 3H, ArCH₃), 2.02 (m, 1H, CH₂pip), 2.23 (m,

1H, CH₂pip), 2.39 (m, 1H, CH₂pip), 2.87 (m, 3H, CH₂and CH pip),

3.29 (s, 2H, NCH₂Ph), 4.23 (m, 1H, OCH(CH₃)₂), 4.38 (m, 1H,

AlOCH(CH₃)), 4.55 (m, 1H, AlOCH(CH₃)), 4.73 (m, 1H,

OCH(CH₃)₂), 6.60 (s, 1H, H arom), 6.99−7.01 (m, 1H, H arom),

7.09−7.12 (m, 2H, H arom), 7.17−7.18 (m, 9H, H arom), 7.32 (s, 1H,

H arom), 7.34 (s, 1H, H arom), 7.37 (d, J_H−H= 2.0 Hz, 1H, H arom),

7.81−7.83 (m, 7H, H arom and ArCHN). ¹³C{¹H} NMR (100

MHz, C₆D₆, 298 K): δ 18.76 (ArCH₃), 20.00 (OCH(CH₃)₂), 20.05

(OCH(CH₃)₂), 20.40 (OCH(CH₃)₂), 21.15 (AlOCH(CH₃)), 21.88

(AlOCH(CH₃)), 31.42 (CH₂pip), 32.28 (CH₂pip), 52.13 (CH₂pip),

52.26 (CH₂pip), 59.37 (OCH(CH₃)₂), 61.84 (NCH₂Ph), 65.45 (CH

pip), 67.12 (AlOCH(CH₃)), 67.30 (AlOCH(CH₃)), 117.96, 124.27,

124.47, 124.53, 127.70, 127.81, 127.91, 135.27, 135.34, 135.55, 138.36,

143.90, 164.50 (ArCHN), 165.74 (ArC−O−Al), 174.02 (CO).

Anal. Calcd for C₅₀H₅₉AlN₂O₇Si: C, 70.23; H, 6.95; N, 3.28. Found: C,

71.21; H, 7.04; N, 3.78.

AUTHOR INFORMATION

Corresponding Author

*Fax: (+33)(0)223-236-939. E-mail: evgueni.kirillov@univ-

rennes1.fr (E.K.); jean-francois.carpentier@univ-rennes1.fr (J.-

F.C.).

■

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

This work was supported by the Minister

■

̀

e de l’Enseignement

Super

́

ieur et de la Recherche (Ph.D. fellowship to M.N.).

Dedicated in honor of the 80th birthday of Prof. I. P.

Beletskaya.

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{Phenoxy-imine}aluminum versus -indium complexes for the immortal ROP of lactide: Different stereocontrol, different mechanisms

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