Ring-opening polymerisation of rac-lactide mediated by cationic zinc complexes featuring P-stereogenic bisphosphinimine ligands

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Ring-opening polymerisation of rac-lactide mediated by cationic zinc

complexes featuring P-stereogenic bisphosphinimine ligands†

Hongsui Sun, Jamie S. Ritch and Paul G. Hayes*

Received 17th October 2011, Accepted 19th December 2011

DOI: 10.1039/c2dt11954d

The diastereomerically pure P-stereogenic bis(phosphinimine) ligands 4,6-(ArNvPMePh)₂dbf [Ar = 4-

isopropylphenyl (Pipp): rac-4, meso-4; Ar = 2,6-diisopropylphenyl (Dipp): rac-4a; dbf = dibenzofuran]

were synthesised and complexed to zinc using a protonation-alkane elimination strategy. The cationic

alkylzinc complexes thus obtained, RZn[4,6-(ArNvPMePh)₂dbf][B(Ar′)₄] [Ar = Pipp, Ar′ =

C₆H₃(CF₃)₂: rac-6 (R = Et), meso-6 (R = Et), rac-7 (R = Me) meso-7 (R = Me); Ar = Dipp: rac-6a

(R = Et, Ar′ = C₆H₃(CF₃)₂), rac-6b (R = Et, Ar′ = C₆F₅)] were investigated for their competency as

initiators for the ring-opening polymerisation of rac-lactide. The formation of polylactide was achieved

under relatively mild conditions (40 °C, 2–4 h) and the microstructures of the resulting polymers

exhibited a slight heterotactic bias [polymer tacticity (P_r) = 0.51–0.63].

The vast majority of ancillary ligands utilized in PLA syn-

Introduction

thesis are anionic, such as β-diketiminates,⁶tethered alkoxides⁷

or tris(pyrazolyl)borates,⁸and are used to support neutral metal

complexes. We, however, have become interested in the ROP of

lactide catalysed by cationic metal complexes featuring a neutral

ancillary ligand, as little is known about such systems.⁹Further-

more, cationic, coordinatively and electronically unsaturated

metal complexes are expected to encourage facile coordination

of LA. Correspondingly, we recently reported Mg¹⁰and Zn¹¹

complexes of ligands featuring a dibenzofuran (dbf) framework

with either one or two phosphinimine functionalities at the 4-

and/or 6- positions (Chart 1). Rather than the metathetical route

usually employed to introduce anionic ligands, we installed these

neutral donors via a protonation-alkane elimination strategy

whereby the ligand is protonated with an appropriate Brønsted

acid to generate an aminophosphonium salt. This compound can

then undergo an alkane elimination reaction upon exposure to

organometallic reagents, such as ZnEt₂, to directly produce the

desired cationic complex.

Bioplastics produced from plant sources continue to gain

momentum as an alternative to petroleum-derived materials

because of their renewability and biodegradability, as well as the

rising cost of crude oil. One such biomaterial, polylactide (PLA),

can be generated by the ring-opening polymerisation (ROP) of

lactide. Although PLA is not yet commercially cost-competitive

with traditional polyoleﬁn plastics, it has found a growing

number of niche applications. For example, its optical transpar-

ency and ease of processability render it suitable for food con-

tainers and packaging.¹In addition, PLA has also been used by

the medical community as a scaffold for tissue engineering and

drug delivery systems.²

A widely-used industrial catalyst for PLA synthesis is bis(2-

ethylhexanoate)tin(^II), also known as tin(II) octanoate.³Although

this compound, as well as other metal alkoxides, can efﬁciently

initiate the ROP of lactide, signiﬁcant challenges in the develop-

ment of new and improved catalysts still exist. To address some

of these objectives, which include increasing activity and stereo-

selectivity, many single-site catalysts have been developed that

possess modular ancillary ligands useful for tuning the steric and

electronic properties of the metal complex.⁴Many different

metals, representing all blocks of the periodic table, have been

used. Of particular interest to our group are inexpensive, non-

toxic metals such as Ca, Mg and Zn,⁵as such catalysts would not

contaminate the resultant PLAwith toxic transition metal ions.

Lactide possesses two chiral centres, and hence the resultant

polymers may feature different stereochemical arrangements

depending on which form of the monomer is used: (R,R)-, (S,S)-

or meso-lactide (Fig. 1). Physical properties, such as glass tran-

sition temperature, melting point and mechanical strength are

Department of Chemistry and Biochemistry, University of Lethbridge,

4401 University Drive, Lethbridge, AB, Canada, T1K 3M4. E-mail:

p.hayes@uleth.ca; Fax: +1-403-329-2057; Tel: +1-403-329-2313

†Electronic supplementary information (ESI) available: Additional

kinetic plots for complexes rac-6 and meso-6. CCDC reference numbers

849079–849088. For ESI and crystallographic data in CIF or other elec-

tronic format see DOI: 10.1039/c2dt11954d

Chart 1 Two dibenzofuran-based ligands.

Dalton Trans., 2012, 41, 3701–3713 | 3701

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Fig. 1 Lactide stereoisomers.

highly dependent on the degree of crystallinity, which in turn

depends on the distribution of stereocentres in the polymer

chains. Thus, stereochemically-controlled polymerisation of

lactide has become an important concern.⁴In particular, the syn-

thesis of isotactic or heterotactic PLA from racemic lactide (rac-

lactide, a 1 : 1 mixture of (R,R) and (S,S) forms), which allows

for the formation of crystalline, high-melting polymers¹²without

the need for resolution of the lactide starting material into enan-

tiopure form, is a desirable objective. Achieving such stereose-

lectivity is nontrivial, however, as two independent processes

affect the outcome of coordination-insertion polymerisations.

The most prevalent of these two mechanisms is chain-end

control, in which the stereochemistry of the last-inserted

monomer inﬂuences the subsequent insertion. The other process

is known as enantiomorphic site-control, and occurs when the

absolute conﬁguration of the catalyst affects each insertion step.

Notably, these mechanisms are not necessarily mutually exclu-

sive, and therefore, may compete with each other in a given reac-

tion, with one or the other proving dominant for a particular

catalyst system.¹³

Given the dramatic inﬂuence that polylactide microstructure

has on the macroscopic properties of the resultant material it is

not surprising that there is a growing interest in the synthesis of

catalysts for stereoselective polymerisation of lactide. Dijkstra,

Du and Feijen have recently reviewed advances in this area.⁴

Our plan to contribute to this area of research is to use our exist-

ing phosphinimine-functionalized dibenzofuran framework B,

and engender chirality at the phosphorus centres by replacing

one of the phenyl groups of the PPh₂moiety with a methyl

group. As a ﬁrst step towards this goal, we recently reported the

synthesis of several P-stereogenic monophosphinimine ana-

logues of A and their complexes, [{(dbf)MePhPvNAr}ZnR][B

(C₆F₅)₄] (Ar = Dipp, Mes (2, 4, 6-trimethylphenyl); R = Et,

methyl-(S)-lactate), and investigated their activity in the syn-

thesis of PLA.¹⁴These complexes were found to initiate the

ROP of lactide, but the activity was relatively low (T = 80 °C,

9 h), and no stereochemical preference was observed in the

resulting polymers (P_r= ca. 0.5). Inspired by our previous work,

in which it was established that bisphosphinimine dbf-based

ligands elicit substantially higher activity than their monofunc-

tionalized counterparts A,^11b,cwe now describe the synthesis of

cationic alkylzinc complexes supported by pincer ligands featur-

ing two chiral phosphinimine donors.

Scheme 1 Synthetic route to the cationic zinc complexes 6 and 7. i: 2

n-BuLi, (OMen)PhPCl; ii: BH₃·THF; iii: MeLi; iv: HNEt₂; v) ArN₃; vi)

[H(Et₂O)₂][BAr′₄]; vii) ZnR₂. For clarity, only one stereoisomer is

shown for the rac-compounds.

MePhPvNAr (Ar = Dipp, Mes;),¹⁴whereby the ﬁrst step was

lithiation of dibenzofuran with n-BuLi, followed by reaction

with (OMen)PPhCl (OMen = (−)-O-menthyl) and subsequently

BH₃·THF to afford the intermediate (OMen)(dbf)PhP(BH₃).

When this approach was employed using 1,8-dilithiodi-

benzofuran,¹⁵the subsequent reaction with two equiv of

Results and discussion

Ligand synthesis and metal complexation

The syntheses of the desired 4,6-disubstituted ligands

(Scheme 1) started with 4,6-dibromodibenzofuran. This strategy

differs from the synthesis of the singly-substituted ligands (dbf)

3702 | Dalton Trans., 2012, 41, 3701–3713

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chlorophosphine yielded

a

mixture of free dibenzofuran,

(1 : 1) of two enantiomers, and hence no preferential crystalliza-

tion of one stereoisomer was observed.

(dbf)₂PhP(BH₃) and (OMen)₂Ph(BH₃) along with a small quan-

tity of the desired product. A more efﬁcient method for generat-

ing the requisite 1,8-dilithiodibenzofuran was accomplished by a

lithium halogen exchange reaction in which 1,8-dibromodiben-

Removal of the borane protecting groups from 2 by heating in

neat diethylamine, followed by assembly of the phosphinimine

functionalities under standard Staudinger conditions,¹⁶was

performed to yield neutral ligands in one-pot: 4,6-

{MePhPvNAr}₂(dbf) (Ar = Dipp: rac-4a; Ar = Pipp (4-isopropyl-

phenyl): rac-4 and meso-4). These compounds were obtained

as analytically pure solids. Upon deprotection and installation of

the NAr moieties, a shielding effect was observed in the result-

ing complexes (³¹P: δ −15.21 for rac-4a and δ −2.26 to −2.07

for rac- and meso-4). One of the neutral ligands, rac-4a,

was also subjected to X-ray diffraction analysis, revealing PvN

bond lengths of 1.551(1) Å and 1.547(1) Å. These values

closely match those observed in the related ligand 4,6-

(MesNvPPh₂)₂dbf (1.549(1) Å and 1.565(1) Å).^11a

t

zofuran was allowed to react with 2 equiv of BuLi at −78 °C in

THF solution. After 2 h, the reaction mixture was treated with

BH₃·THF, affording a crude product containing three borane pro-

tected diastereomers, 4,6-{(OMen)PhP(BH₃)}₂(dbf) (rac-1: (R,

R,R,R) + (R,S,S,R) and meso-1: (R,S)).‡ Flash chromatography

using benzene/hexanes (1 : 4) as the eluent on silica gel allowed

for separation of the rac and meso isomers to give analytically

pure crystalline, colourless products in 33 and 26% yield,

respectively. Although we were unable to fully separate the two

rac stereoisomers, this still represents a signiﬁcant advancement

for our dbf-based systems, as diastereomeric resolution was not

possible with the monophosphine analogues; exhaustive attempts

using a range of experimental conditions, consistently resulted in

elution of the two diastereomers as an inseparable mixture.¹⁴

The successful separation in this study allowed more detailed

characterisation of these diastereomers. To this end, X-ray

quality single crystals of rac-1 and meso-1 were obtained and

their absolute conﬁgurations were conﬁrmed by X-ray diffraction

studies (vide infra). Interestingly, the (R,R,R,R) and (R,S,S,R)

The ligands 4 were easily protonated with the Brønsted

acids [H(Et₂O)₂][BAr′₄] (Ar′ = C₆H₃(CF₃)₂, C₆F₅) to give

the corresponding aminophosphonium salts [H-4,6-{MePhPv

NAr}₂(dbf)][BAr′₄] (Ar = Dipp: rac-5a (Ar′ = C₆H₃(CF₃)₂),

rac-5b (Ar′ = C₆F₅); Ar = Pipp, Ar′ = C₆H₃(CF₃)₂: rac-5, meso-

5) as white crystalline solids. The protonated ligands 5 exhibit

³¹P{¹H} NMR spectra with single resonances at δ 9.17–9.20 for

Dipp-substituted compounds, and δ 15.20–15.84 for the Pipp-

containing analogues; the presence of single ³¹P NMR reson-

ances for these compounds suggests that, on the NMR timescale

in solution, the NH proton is rapidly exchanging between the

1

forms of rac-1 were distinguishable by H NMR spectroscopy,

as two separate mentholate OCH resonances were observed in a

31 : 69 ratio; unique signals for this group were also observed in

the ¹³C{¹H} NMR spectrum. These data indicate that rac-1 was

partially resolved with a slight diastereomeric excess (38%) in

favour of either (R,R,R,R) or (R,S,S,R) conﬁgurations at phos-

1

two phosphinimine groups. The H signal for this proton was

observed at ca. δ 4.8 (Ar = Dipp) and 6.4 (Ar = Pipp). Upon pro-

tonation, the PvN distances elongated by approximately 5% to

1.624(2) Å and 1.619(3) Å as seen in the structures of rac-5a

and rac-5b, respectively. The other moieties in these compounds

remain relatively unchanged, and in the solid state the proton is

localized on one phosphinimine group, rather than shared

between the two.

1

phorus. The meso isomer only exhibited one H OCH environ-

ment due to signal broadness; however, two sets of OCH

resonances were clearly distinguishable in the ¹³C{¹H} NMR

spectrum.

When rac-1 was subject to recrystallization from benzene/

pentane, a single crystal was selected and determined to be a dis-

ordered mixture of the two stereoisomers (R,S,S,R)-1 (major)

and (R,R,R,R)-1 (minor). Upon successive recrystallizations, the

bulk sample became increasingly enriched with (R,S,S,R)-1,

which suggests that this is the stereoisomer in excess in the

initial product, and that the diastereomeric excess arises from

preferential crystallization. Suitable crystals of the meso form of

1 were also obtained and structurally characterised, which

conﬁrmed the expected connectivity. Efforts to fully resolve the

(R,R,R,R) and (R,S,S,R) stereoisomers have thus far been

unsuccessful.

Removal of the OMen resolving groups was achieved by the

facile reaction of 1 with MeLi in benzene at 40 °C to generate

the products 4,6-{MePhP(BH₃)}₂(dbf) (rac-2 and meso-2) in 59

and 51% yields, respectively. Once again, crystalline products

were obtained. In the case of rac-2, when the reaction was con-

ducted at ambient temperature, the partially methylated by-

product 4-{MePhP(BH₃)}-6-{(OMen)PhP(BH₃)}(dbf) (3) was

also obtained in the reaction mixture in 10% yield. This com-

pound, formed due to incomplete conversion at ambient temp-

erature, was puriﬁed and independently characterized (see

Experimental section for details). The methylated compounds

rac- and meso-2 exhibited ³¹P NMR chemical shifts of δ 9.08

and 9.27, respectively. Rac-2 crystallized as a racemic mixture

The reaction of these salts with ZnMe₂or ZnEt₂furnished

the desired cationic organozinc complexes RZn[4,6-(ArNv

PMePh)₂dbf][B(Ar′)₄] [Ar = Dipp: rac-6a (R = Et, Ar′ = C₆H₃-

(CF₃)₂), rac-6b (R = Et, Ar′ = C₆F₅); Ar = Pipp, Ar′ =

C₆H₃(CF₃)₂: rac-6 (R = Et), meso-6 (R = Et), rac-7 (R = Me)

meso-7 (R = Me)] via alkane elimination. The various alkylzinc

complexes 6 and 7 all feature ³¹P environments which resonate

in the narrow range of δ 24.49–25.71, indicating the electronic

effect that the coordinated metal centre has on P dominates over

the inﬂuence from the different aryl substituents on nitrogen.

These chemical shifts agree well with similar complexes, e.g. the

³¹P NMR chemical shift for the achiral complex MeZn[4,6-

(MesNvPPh₂)₂dbf][B(C₆F₅)₄] appears at δ 23.4 in C₆D₅Br at

ambient temperature.^11bThe ¹¹B{¹H} and ¹⁹F{¹H} NMR signals

for the borate anions in the zinc complexes 6 and 7 and the pro-

tonated ligands 5 were unremarkable and consistent with sym-

metrical, weakly coordinating fragments in solution. Essentially

no change in δ was observed between the protonated ligands and

their corresponding zinc alkyl complexes.

In our previous work, we have found that alkyl substituents on

zinc are poorer initiating groups when compared to methyl-(S)-

lactate.^11cHowever, the reaction of the protonated ligands 5 with

EtZn(methyl-(S)-lactate),¹⁷EtZnOPh, EtZnOⁱPr or reactions

of the zinc complexes 6 and 7 with methyl-(S)-lactate gave

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intractable mixtures of products. Therefore, polymerisation

studies were conducted using the alkylzinc complexes (vide

infra). Further experiments are currently underway to form

alkoxy and amido-substituted zinc complexes.

X-ray crystal structures of three alkylzinc complexes (rac-6a,

meso-6 and meso-7) were obtained (CIF data can be found in the

ESI†). In all three cases, no strong cation–anion interactions

were observed [shortest cation–anion contacts were aromatic

close proximity to each other in the syn geometry. The smaller

Pipp substituents allow access to this coordination mode for

meso-6 and meso-7. The Zn–N distances in these complexes

range from 2.028(2)–2.038(3) Å for the meso complexes, and

2.061(3)–2.063(3) Å for rac-6a. These are in excellent agree-

ment with the values of 2.033(4)–2.044(4) Å determined for the

complex MeZn[4,6-(MesNvPPh₂)₂dbf][BPh₄].^11bThe slightly

elongated values in the rac complex are attributed to the larger

Dipp substituents on nitrogen, which cause steric crowding

around the metal centre.

C

_cation–F_aniondistances: rac-6a (2.93 Å), meso-6 (3.02 Å), and

meso-7 (3.04 Å), cf. sum of van der Waals radii = 3.17^18a] and

the coordination geometry at zinc was found to be essentially tri-

gonal planar (Σ(Zn) = ca. 357.9–359.94°); no signiﬁcant inter-

actions with the dbf oxygen atoms were observed (d(Zn⋯O) =

ca. 2.9–3.4 Å).

Polymerisation studies

This ﬁnding lies in contrast to complexes recently reported by

our group, MeZn[4,6-(PippNvPPh₂)₂dbf][BPh₄],^11c(methyl-S-

lactate)Zn[4,6-(PippNvPPh₂)₂dbf][BPh₄]^11cand (methyl-S-

The Dipp-substituted compounds rac-6a and rac-6b were not

active catalysts for the ROP of rac-lactide, presumably due to the

steric bulk of the Dipp groups on nitrogen. We note that similarly

inactive complexes were obtained with mesityl-substituted bis-

(phosphinimine)dbf ligands.^11bBy comparison, the less bulky

Pipp-substituted complexes showed much more promise in

this area. Thus, in order to ascertain the inﬂuence of stereo-

chemistry at phosphorus on the polymerisation chemistry of

the corresponding metal complexes, the two Pipp-substituted

compounds rac-6 and meso-6 were used in the ROP of rac-

lactide. Despite the absence of methyl-(S)-lactate initiating

groups on zinc, both complexes appeared to be competent ROP

initiators. In fact, the activity of these compounds was substan-

tially higher than that previously reported for the monophosphi-

nimine complex (methyl-(S)-lactate)Zn[4-(DippNvPMePh)dbf]

[B(C₆F₅)₄].¹⁴Using NMR-scale reactions in CDCl₃, conditions

for larger scale polymerisations were determined. Excellent

yields were obtained for the solid polymers, which were then

analysed by triple-detection GPC (Table 1).

At higher loadings, the M_nvalues were much larger than cal-

culated for both initiators; this was also true for the lowest

loading of meso-6. All of the M_wvalues were signiﬁcantly

higher than corresponding M_nvalues which is reﬂected in the

relatively large polydispersity indices (PDI: 1.69–2.62). No

other clear trends in the molecular weight data are apparent.

However, the data do suggest signiﬁcant transesteriﬁcation pro-

cesses and/or initiator degradation occurred during polymeris-

ation, leading to larger than expected chains of polylactide (i.e.

if fewer active sites are present in solution, the propagating

chains will be longer than calculated). Analysis by MALDI-TOF

mass spectrometry yielded further information on the polymer

chain structure. For samples prepared using rac-6 or meso-6,

ions corresponding to the formulae [H(C₆H₈O₄)_nOH] + Na⁺

were identiﬁed, with molecular weights for these ions ranging

from ca. 1000 to >7000 amu. Spacings of 72 amu between ions

were also apparent, suggesting signiﬁcant transesteriﬁcation

occurs during polymerisation. Monitoring the polymerization

reactions in situ by NMR spectroscopy yielded no further insight

into such a process.

lactate)Zn[4,6-(PhNvPPh₂)₂dbf][B(C₆H₃(CF₃)₂)]^11d

where

clear Zn⋯O interactions (2.284(2), 2.336(5) and 2.367(2) Å,

respectively) were found in the solid state. Notably, the methyl-

S-lactate complex was found to be a very active catalyst for the

polymerisation of rac-lactide. This compound exhibits a signiﬁ-

cant Zn⋯O interaction (with a distance in between the sums of

the covalent radii and van der Waals radii of 1.90^18band

2.91 Å,^18arespectively), that is perhaps a reﬂection of a greater

effective positive charge on the zinc centre or more open coordi-

nation sphere. Either of these factors would be expected to aid in

a coordination-insertion type polymerisation mechanism. The

fact that no such interaction was seen for rac-6a, meso-6 or

meso-7 is a potential indicator that they will not be as effective

in lactide polymerisation catalysis.

The structure of meso-6 is depicted in Fig. 2. Interestingly, in

rac-6a the ligand is pseudo-C₂symmetric, with two phosphini-

mine groups coordinated to the metal centre in an anti fashion

(one above the plane of the dbf ligand and one below), while for

the meso complex a syn-coordination is seen, giving a pseudo-C_s

symmetric cation. This difference is presumably due to the pres-

ence of the larger Dipp groups in rac-6a, which would be in

The hydroxy-terminated nature of the obtained polymers (i.e.

the lack of ethyl end groups) suggests that rac-6 and meso-6 are

not true catalysts, but rather, ﬁrst undergo preliminary reactions

to form the actual active species. This has been suggested for the

industrial catalyst tin(^II) octanoate.⁴A cationic zinc hydroxy

species may be a reasonable candidate for the true active species,

Fig. 2 Thermal ellipsoid plot (50% probability) of meso-6; hydrogen

atoms and the [B(C₆H₃(CF₃)₂)₄]⁻anion are omitted for clarity. Selected

bond lengths (Å) and angles (°): Zn1–N1 2.037(3), Zn1–N2 2.038(3),

Zn–C45 1.976(4), N1–P1 1.601(3), N2–P2 1.594(3); N1–Zn1–N2 115.2

(1), N1–Zn1–C45 123.5(1), C45–Zn1–N2 119.8(1).

3704 | Dalton Trans., 2012, 41, 3701–3713

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Table 1 GPC results for polylactide samples prepared using rac-6 and meso-6^a

Initiator

[LA]_o/[Zn]

Isolated yield (%)

Time (h)

Calc. M_n(×10³)

M_n(×10³)

M_w(×10³)

PDI

rac-6

200

400

600

800

1000

200

400

600

800

1000

97.2

98.6

97.2

92.9

89.6

99.3

98.2

99.9

97.7

95.3

2.0

2.7

3.3

3.7

4.0

2.0

2.5

3.2

3.5

3.8

28.0

56.8

84.1

107.1

129.1

28.6

56.6

86.4

93.4

102.5

38.3

65.5

56.3

87.0

71.7

60.5

89.2

208.3

165.2

172.9

70.7

111.8

96.5

187.1

188.1

120.0

211.9

390.7

1.77

1.69

1.85

1.71

2.15

2.62

1.98

2.38

1.87

rac-6

meso-6

112.7

137.4

^aExperimental conditions: methylene chloride, 40 °C, samples precipitated from cold methanol and dried in vacuo.

Table 2 The inﬂuence of solvent on polymer tacticity^a

Initiator

P_r(THF-d₈)

P_r(CDCl₃)

P_r(Benzene-d₆)

rac-6

meso-6

0.63

0.57

0.62

0.51

0.52

0.56

1

^aExperimental conditions: 25 °C, 15 h. H{¹H} NMR spectra recorded

on isolated polymer samples that were redissolved in CDCl₃.

although we have thus far been unable to independently prepare

such a complex. Current efforts are underway to synthesize

alkoxyzinc analogues of rac-6 and meso-6 in order to compare

their structure and catalytic activities. Overall, the mechanism of

polymerisation remains unclear. In situ ³¹P{¹H} and H NMR

experiments were complex and yielded no conclusive data on,

e.g. coordination-insertion events.

1

Fig. 3 Plot of ln[A] versus t for polymerisation using rac-6 at 50 °C.

The microstructure of the polymer samples was probed by

1

homodecoupled H{¹H} NMR spectroscopy. Upon examination

of the methine region, values of P_rwere obtained for polymers

made using rac-6 and meso-6 as initiators and in several different

solvents (Table 2). Overall, a modest bias was observed for het-

erotacticity (P_r= 0.52–0.63 for rac-6 and 0.51–0.57 for meso-6),

indicating some form of chain-end control¹³is occurring during

the polymerisation, but it is clearly not a strong effect. Nonethe-

less, a signiﬁcant improvement is seen when compared with the

monophosphinimine variants, which yielded entirely atactic

samples.¹⁴

Kinetic data were collected using both complexes at various

temperatures. A plot of the natural logarithm of the monomer

concentration versus time for rac-6 is depicted in Fig. 3, and is

representative of other kinetic runs. A noticeable induction

period precedes the linear regime in these reactions, which are

ﬁrst order in [monomer]. Eyring and Arrhenius plots (Fig. 4 dis-

plays the Eyring plot for rac-6; refer to ESI† for other plots)

allowed for extraction of activation parameters. The complex

rac-6 yielded values of ΔH^‡= 47.1 1.5 kJ mol⁻¹, ΔS^‡= −159

4.9 J K⁻¹mol⁻¹, and E_a= 49.7 1.5 kJ mol⁻¹, while meso-6

gave ΔH^‡= 39.1 3.3 kJ mol⁻¹, ΔS^‡= −197 10.5 J K⁻¹

mol⁻¹, and E_a= 41.7 3.3 kJ mol⁻¹. Such data are consistent

with an ordered transition state, and match well with values pre-

viously reported for coordination–insertion mechanisms,¹⁹

although the exact steps of polymerisation in this system are as

of yet unknown.

Fig. 4 Eyring plot [ln(K/T) versus 1000 × 1/T] for complex rac-6.

Experimental

Reagents and general procedures

Manipulations of air- and moisture-sensitive materials and

reagents were carried out under an argon atmosphere using

vacuum line techniques or in a glove box. Solvents used for air-

sensitive materials were puriﬁed using an MBraun solvent puriﬁ-

cation system (SPS), stored in PTFE-sealed glass vessels over

sodium benzophenone ketyl (THF and ether), CaH₂(CH₂Cl₂

and bromobenzene) or “titanocene” (pentane, benzene and

toluene), and freshly distilled at the time of use. Deuterated sol-

vents were dried over sodium benzophenone ketyl (benzene-d₆

This journal is © The Royal Society of Chemistry 2012

Dalton Trans., 2012, 41, 3701–3713 | 3705

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and THF-d₈) or CaH₂(CDCl₃and CD₂Cl₂), degassed via three

freeze–pump–thaw cycles, distilled in vacuo and stored over 4 Å

molecular sieves in glass bombs under argon. NMR spectra were

recorded at ambient temperature with a Bruker Avance II NMR

spectrometer (300.13 MHz for ¹H, 75.47 MHz for ¹³C,

121.48 MHz for ³¹P, 282.42 MHz for ¹⁹F and 96.29 MHz for

¹¹B). Chemical shifts are reported in parts per million relative to

the external standards SiMe₄(¹H), 85% H₃PO₄(³¹P), triﬂuoro-

toluene (¹⁹F) and boron triﬂuoride diethyl etherate (¹¹B); residual

H-containing species in CD₂Cl₂(δ 5.32 ppm) or CDCl₃(δ

7.27 ppm) were used as internal references (¹H). Assignments

were aided by the use of ¹³C{¹H}-DEPT and ¹H–¹³C{¹H}-

HSQC experiments (s = singlet, d = doublet, t = triplet, q =

quartet, sp = septet, m = multiplet, br = broad, ov = overlapping

signals). Elemental analyses were performed using an Elementar

Vario Microcube instrument. The reagents 2,6-dibromodibenzo-

rac-1 and meso-1 suitable for analysis by X-ray crystallography

were prepared by dissolving samples in benzene in 5 mm diam-

eter glass tubes and layering the solution with pentane.

A 31 : 69 ratio of rac-1:(R,R,R,R) : (R,S,S,R) was determined

experimentally; for simplicity data are listed as integrations for

1

an ideal 1 : 1 racemic mixture. H NMR (CDCl₃): δ 8.27–8.06

(m, 4H (R,S,S,R), 4H (R,R,R,R), 1,2,8,9-dbf), 7.65–7.17 (br ov

m, 12H (R,S,S,R) and 12H (R,R,R,R), 3,7-dbf + Ph), 4.37 and

4.18 (br m, 2H (R,S,S,R) and 2H (R,R,R,R), OCH), 2.09–0.9 (ov

m, 30H (R,S,S,R) and 30H (R,R,R,R), OMen + BH₃), 0.75 (d,

³J_HH= 6.2 Hz, 3H (R,S,S,R) and 3H (R,R,R,R), CHCH₃), 0.68

3

(d, J_HH= 6.2 Hz, 3H (R,S,S,R) and 3H (R,R,R,R), CHCH₃),

3

0.31 (d, J_HH= 6.6 Hz, 3H (R,S,S,R) and 3H (R,R,R,R),

3

CHCH₃), 0.13 (d, J_HH= 6.6 Hz, 3H (R,S,S,R) and 3H (R,R,R,

R), CHCH₃). ¹³C{¹H} NMR (CDCl₃): δ: (R,S,S,R) diastereomer,

1

155.95 (s, OC-dbf), 133.38 (s, 3/7-dbf), 133.36 (d, J_CP= 73.2

2

furan,²⁰(OMen)PhPCl (OMen = (−)-menthyl),¹⁴[H(Et₂O)₂]-

Hz, 4,6-dbf), 131.41 (s, aromatic C), 130.84 (d, J_CP= 11.8 Hz,

23

2

[B(C₆H₃{CF₃}₂)₄],²¹[H(Et₂O)₂][B(C₆F₅)₄],²²DippN₃

and

aromatic C), 128.69 (d, J_CP= 11.2 Hz, o-Ph), 125.10 (s, aro-

PippN₃,²⁴and S-MeO₂CC(H)(Me)OZnEt¹⁷were prepared

according to literature methods. Rac-lactide was purchased from

Alfa Aesar and puriﬁed by double recrystallization from toluene

followed by sublimation. Diethylamine (Aldrich) was dried over

4 Å molecular sieves and degassed prior to use. All other

reagents were purchased from commercial sources and used as

received. Flash chromatographic puriﬁcation was run on silica

gel (230–400 mesh, as received and without activation) using a

fritted column (3 × 45 cm). GPC data were collected on a Visco-

tek Triple Detection GPC System outﬁtted with a model 270

Dual Detector Platform (Four Capillary Viscometer and Light

Scattering Detector) and a Refractive Index Detector. Samples

were run in THF at a concentration of 1 mg mL⁻¹. MALDI-TOF

data were collected using an Applied BioSystems Voyager Elite

instrument.

matic C), 123.99 (d, J_CP= 4.5 Hz, dbf), 123.25 (d, J_CP= 11.3

3

2

1

2

Hz, aromatic C), 117.09 (d, J_CP= 58.1 Hz, Ph), 80.81 (d, J_CP

= 2.6 Hz, POC-Men ring), 47.74 (d, J_CP= 5.3 Hz, CHCH

3

(CH₃)₂), 43.54 (s, OCHCH₂CH), 34.34 (s, CH₃CHCH₂CH₂CH),

31.68 (s, CH₂CH(CH₃)CH₂), 25.21 (s, CHCH(CH₃)₂), 22.70 (s,

CH₃CHCH₂CH₂CH), 22.38 (s, CH₂CH(CH₃)CH₂), 20.66 (s,

CHCH(CH₃)₂), 14.93 (s, CHCH(CH₃)₂). (R,R,R,R) diastereomer,

2

156.18 (d, J_CP= 2.3 Hz, OC-dbf), 133.16 (s, 3/7-dbf), 132.42

1

(d, J_CP= 73.2 Hz, 4,6-dbf), 131.85 (s, aromatic C), 131.10 (d,

2

²J_CP= 11.8 Hz, aromatic C), 128.46 (d, J_CP= 10.9 Hz, o-Ph),

3

124.56 (s, aromatic C), 124.24 (d, J_CP= 6.0 Hz, dbf), 123.22

2

1

(d, J_CP= 9.8 Hz, aromatic C), 118.03 (d, J_CP= 60.4 Hz, Ph),

2

3

80.73 (d, J_CP= 2.7 Hz, POC-Men ring), 48.16 (d, J_CP= 6.8

Hz, CHCH(CH₃)₂), 42.75 (s, OCHCH₂CH), 34.09 (s,

CH₃CHCH₂CH₂CH), 31.64 (s, CH₂CH(CH₃)CH₂), 25.63 (s,

CHCH(CH₃)₂), 22.90 (s, CH₃CHCH₂CH₂CH), 22.19 (s,

CH₂CH(CH₃)CH₂), 21.15 (s, CHCH(CH₃)₂), 15.61 (s, CHCH

(CH₃)₂). ³¹P{¹H} NMR (CDCl₃): δ 99.86 (br s). ¹¹B{¹H} NMR

(CDCl₃): δ −39.32 (br s). Anal. Calcd for C₄₄H₆₀B₂O₃P₂: C,

73.35; H, 8.39. Found: C, 73.48; H, 8.29.

Synthesis of rac-1 and meso-1

Dibromodibenzofuran (5.0 g, 15.3 mmol) was dissolved in THF

(ca. 100 mL) and cooled to −78 °C. To the well-stirred solution

n-BuLi (19.6 mL, 31.4 mmol, 1.6 M in hexane) was added

dropwise. The light brown solution became deeper in colour

and precipitates were observed to form. The suspension was

stirred at −78 °C for 1 h, followed by an additional 1 h at 0 °C.

The resulting suspension was cooled to −78 °C again and

(−)-(O-menthyl)chlorophenylphosphine (9.38 g, 31.3 mmol) in

THF (20 mL) was added dropwise. The mixture was left to

warm to ambient temperature overnight, resulting in a light

orange solution. At 0 °C, BH₃·THF (40 mL, 40 mmol, 1 M in

THF) was slowly added and stirring was continued at ambient

temperature for 1 h. To the resulting solution water (20 mL) was

added. The organic layer was separated and the aqueous solution

was extracted with hexanes (3 × 15 mL). The combined organic

fractions were washed with brine and dried over MgSO₄. After

removal of volatiles in vacuo, the residue was subjected to

column separation. Upon elution with benzene/hexanes (1 : 4),

rac-1 (3.6 g, 5.0 mmol, 33%) and meso-1 (2.9 g, 4.0 mmol,

26%) were obtained. After recrystallization of rac-1 from

benzene/hexanes, the ratio of (R,R,R,R) to (R,S,S,R) had changed

to 31 : 69, according to ¹H NMR spectroscopy. Single crystals of

1

meso-1: H NMR (CDCl₃): δ 8.25–8.04 (ov m, 4H, 1,2,8,9-

dbf), 7.75–7.69 (m, 2H, 3,7-dbf), 7.52–7.40 (ov m, 5H, Ph),

7.32–7.22 (ov m, 5H, Ph), 4.38 (m, 2H, POCH-Men ring), 2.03

3

(br m, 2H, OMen), 1.90 (br d, J_HH= 12.0 Hz, 1H, OMen),

1.71–1.03 (ov m, 17H, OMen + BH₃), 0.91 (ov t, ³J_HH= 7.0 Hz,

3

10H, OMen + BH₃), 0.74 (d, J_HH= 6.6 Hz, 6H, CH₃), 0.41 (d,

³J_HH= 6.6 Hz, 3H, CH₃), 0.36 (d, ³J_HH= 6.9 Hz, 3H, CH₃). ¹³

C

{¹H} NMR (CDCl₃): δ 156.30 (s, dbf-OC), 156.16 (s, dbf-OC),

2

1

133.04 (d, J_CP= 15.8 Hz, 3/7-dbf), 132.92 (d, J_CP= 73.2 Hz,

1

2

4/6-dbf), 132.64 (d, J_CP= 77.0 Hz, 4/6-dbf), 132.17 (d, J_CP

=

12.1 Hz, 3/7-dbf), 132.08 (s, aromatic C), 131.81 (s, aromatic

C), 131.73 (s, aromatic C), 131.48 (s, aromatic C), 131.32 (s,

aromatic C), 130.81 (d, J_CP= 12.1 Hz, aromatic C), 128.68 (d,

J_CP= 10.6 Hz, aromatic C), 128.35 (d, J_CP= 11.3 Hz, aromatic

2

3

C), 124.98 (d, J_CP= 30.9 Hz, Ph), 124.26 (d, J_CP= 5.3 Hz,

3

dbf), 124.13 (d, J_CP= 3.8 Hz, dbf), 123.44 (d, J_CP= 9.8 Hz,

aromatic C), 123.20 (d, J_CP= 11.3 Hz, aromatic C), 117.56 (d,

2

¹J_CP= 61.1 Hz, Ph), 81.15 (d, J_CP= 3.0 Hz, POCH-Men ring),

2

3

80.65 (d, J_CP= 3.0 Hz, POCH-Men ring), 48.62 (d, J_CP= 6.8

3

Hz, CHCH(CH₃)₂), 48.25 (d, J_CP= 5.3 Hz, CHCH(CH₃)₂),

43.80 (s, OCHCH₂CH), 42.83 (s, OCHCH₂CH), 34.32

3706 | Dalton Trans., 2012, 41, 3701–3713

This journal is © The Royal Society of Chemistry 2012

View Article Online

4

(s, CH₃CHCH₂CH₂CH), 34.22 (s, CH₃CHCH₂CH₂CH), 31.67

(s, CH₂CH(CH₃)CH₂), 31.60 (s, CH₂CH(CH₃)CH₂), 25.70

(s, CHCH(CH₃)₂), 25.62 (s, CHCH(CH₃)₂), 22.84 (s,

CH₃CHCH₂CH₂CH, (2 coincident signals)), 22.33 (s, CH₂CH

(CH₃)CH₂), 22.14 (s, CH₂CH(CH₃)CH₂), 21.18 (s, CHCH

(CH₃)₂), 20.85 (s, CHCH(CH₃)₂), 15.65 (s, CHCH(CH₃)₂),

15.59 (s, CHCH(CH₃)₂). ³¹P{¹H} NMR (CDCl₃): δ 100.71 (br

s). ¹¹B{¹H} NMR (CDCl₃): δ −39.48 (br s). Anal. Calcd for

C₄₄H₆₀B₂O₃P₂: C, 73.35; H, 8.39. Found: C, 73.40; H, 8.38.

(d, J_CP= 2.3 Hz 1/9–dbf), 124.44 (d, J_CP= 2.3 Hz, 1/9–dbf),

3

124.25 (d, J_CP= 4.5 Hz, dbf), 124.09 (d, J_CP= 10.6 Hz,

m-Ph), 123.89 (s, aromatic C), 123.74 (d, ³J_CP= 10.6 Hz, m-Ph),

1

116.92 (d, J_CP= 61.9 Hz, 4/6–dbf), 114.16 (d, J_CP= 52.8 Hz,

2

4/6–dbf), 80.82 (d, J_CP= 3.0 Hz, POCH-menthyl ring), 49.13

(d, ³J_CP= 6.0 Hz, CHCH(CH₃)₂), 43.82 (s, OCHCH₂CH), 34.22

(s, CH₃CHCH₂CH₂CH), 31.72 (s, CH₂CH(CH₃)CH₂), 25.46 (s,

CHCH(CH₃)₂), 22.77 (s, CH₃CHCH₂CH₂CH), 22.28 (CH₂CH

(CH₃)CH₂), 21.12 (s, CHCH(CH₃)₂), 15.20 (s, CHCH(CH₃)₂),

11.10 (d, J_CP= 41.5 Hz, PCH₃). ³¹P{¹H} NMR (CDCl₃): δ

1

100.40 (br s), 8.65 (br s). ¹¹B{¹H} NMR (CDCl₃): δ −38.39

(br s). Anal. Calcd for C₃₅H₄₄B₂O₂P₂: C, 72.44; H, 7.64. Found:

C, 72.05; H, 7.44.

Synthesis of rac-2

To a solution of rac-1 (3.0 g, 4.16 mmol) in benzene (40 mL),

MeLi (6.5 mL, 10.4 mmol, 1.6 M ether solution) was added

dropwise. The reaction mixture was stirred at 40 °C for 14 h, and

then 10 mL of water was added. The light brown solution

became colourless upon addition of the water. The organic layer

was separated and the aqueous solution was extracted with

diethyl ether (2 × 10 mL). The combined organic fractions were

washed with brine and dried over MgSO₄. The solution was con-

centrated and rac-2 was crystallized and isolated by ﬁltration.

Hexanes was added to the mother liquid and a second crop of

crystals was obtained (combined yield: 1.08 g, 2.45 mmol,

59%). X-ray quality single crystals were obtained from layering

Synthesis of meso-2

Meso-2 was prepared via a similar procedure as that described

for rac-2, from meso-1 (2.9 g, 4 mmol) and MeLi (1.6 M in

ether, 5.53 mL, 8.85 mmol) to afford a colourless solid (0.9 g,

1

3

2.0 mmol, 51%). H NMR (CDCl₃): δ 8.14 (d, J_HH= 7.7 Hz,

2H, 1,9-dbf), 7.79–7.65 (ov m, 6H, m-Ph + p-Ph), 7.51–7.27

2

(ov m, 8H, o-Ph + 2,8-dbf + 3,7-dbf), 1.84 (d, J_PH= 10.4 Hz,

6H, PCH₃), 1.70–0.50 (br m, 6H, BH₃). ¹³C{¹H} NMR

2

(CDCl₃): δ 156.73 (s, dbf-OC), 132.89 (d, J_CP= 9.9 Hz, o-Ph),

2

3

131.95 (d, J_CP= 9.9 Hz, 3,7-dbf), 131.61 (d, J_CP= 2.2 Hz,

1

hexanes onto a benzene solution of rac-2. H NMR (CDCl₃): δ

1

3

m-Ph), 129.77 (d, J_CP= 57.4 Hz, ipso-Ph), 129.23 (d, J_CP

=

3

8.19 (d, J_HH= 7.8 Hz, 2H, 1,9-dbf), 8.04 (dd, J_HH= 7.8 Hz,

4

10.5 Hz, 2,8-dbf), 124.63 (d, J_CP= 2.2 Hz, p-Ph), 124.22

³J_PH= 13.0 Hz, 2H, 3,7-dbf), 7.56–7.36 (ov m, 12H, 2,8-dbf +

4

3

(d, J_CP= 4.5 Hz, dbf), 124.05 (d, J_CP= 9.9 Hz, dbf), 114.58

2

Ph), 1.59 (d, J_PH= 10.5 Hz, 6H, CH₃), 1.50-0.40 (br m, 6H,

1

(d, J_CP= 52.8 Hz, 4,6-dbf), 11.37 (d, J_CP= 40.8 Hz, CH₃).

³¹P{¹H} NMR (CDCl₃): δ 9.27 (br s). ¹¹B{¹H} NMR (CDCl₃):

δ −37.65 (br s). Anal. Calcd for C₂₆H₂₈B₂OP₂: C, 70.96; H,

6.41. Found: C, 70.85; H, 6.40.

BH₃). ¹³C{¹H} NMR (CDCl₃): δ 156.68 (d, J_CP= 2.3 Hz, dbf-

2

3

OC), 134.10 (d, J_CP= 13.6 Hz, o-Ph), 131.24 (d, J_CP= 2.3

Hz, m-Ph), 131.02 (d, J_CP= 9.8 Hz, 3,7-dbf), 130.16 (s, dbf),

129.16 (d, J_CP= 10.6 Hz, aromatic C), 125.14 (d, J_CP= 2.3 Hz,

2

aromatic C), 124.18 (d, J_CP= 10.5 Hz, aromatic C), 124.06 (s,

1

aromatic C), 113.57 (d, J_CP= 52.8 Hz, 4,6-dbf), 10.54 (d, J_CP

= 40.0 Hz, CH₃). ³¹P{¹H} NMR (CDCl₃): δ 9.08 (br s). ¹¹B

{¹H} NMR (CDCl₃): δ −37.21 (br s). Anal. Calcd for

C₂₆H₂₈B₂OP₂: C, 70.96; H, 6.41. Found: C, 70.64; H, 6.38.

Identiﬁcation of the by-product 3: When the reaction was con-

ducted at 23 °C and worked up as described above, column

separation yielded rac-2 (for same scale as above, 0.73 g,

1.66 mmol, 40%) and the mono-methylated compound 3

(0.24 g, 0.42 mmol, 10%). X-ray quality single crystals were

obtained from layering hexanes onto a benzene solution of 3.

Structural details are contained within the ESI† CIF ﬁle. ¹H

NMR (CDCl₃): δ 8.26–8.19 (m, 2H, dbf), 8.12–8.09 (m, 1H,

dbf), 7.96–7.89 (m, 1H, dbf), 7.71–7.65 (m, 2H, dbf/Ph),

7.61–7.27 (ov m, 10H, dbf/Ph), 4.34 (m, 1H, POCH-Men ring),

Synthesis of rac-4

Rac-2 (365 mg, 0.83 mmol) was suspended in diethylamine

(15 mL) and the mixture was heated to 50 °C. After 2 h the sol-

ution was clear and colourless. After a total time of 15 h, the

volatiles were removed under vacuum (9 × 10⁻³torr) at 50 °C.

Benzene (15 mL) was transferred to the reaction ﬂask, then

excess PippN₃(401.4 mg, 2.49 mmol) was added. An immediate

evolution of gas (presumably N₂) was observed. The light

yellow solution was stirred at ambient temperature for 15 h and

all volatiles were removed in vacuo. The residue was then heated

to 100 °C under vacuum (9 × 10⁻³torr) to remove the by-

product Et₂NH·BH₃. Upon crystallization from benzene/pentane,

pure rac-4 was obtained as a beige solid (380 mg, 0.56 mmol,

3

2

1

3

2.12 (d, J_HH= 12.0 Hz, 1H, OMen), 1.87 (d, J_PH= 10.8 Hz,

67%). H NMR (CD₂Cl₂): δ 8.24 (d, J_HH= 7.7 Hz, 2H, 1,9-

3

3H, PCH₃), 1.67–1.37 (ov m, 8H, OMen), 1.12 (q, J_HH= 12.0

dbf), 8.09 (dd, J_HH= 7.6 Hz, J_PH= 12.8 Hz, 2H, 3,7-dbf),

Hz, 2H, OMen), 0.97–0.81 (ov m, 7H, OMen + BH₃), 0.61 (d,

7.55 (ov m, 8H, dbf + Ph), 7.41 (m, 4H, Ph), 6.84 (d, ³J_HH= 7.9

³J_HH= 6.9 Hz, 3H, CH(CH₃)₂), 0.34 (d, ³J_HH= 6.9 Hz, 3H, CH

Hz, 4H, m-Pipp), 6.57 (d, J_HH= 8.1 Hz, 4H, o-Pipp), 2.72 (sp,

3

2

(CH₃)₂). ¹³C{¹H} NMR (CDCl₃): δ 156.82 (d, J_CP= 3.0 Hz,

³J_HH= 6.9 Hz, 2H, CH(CH₃)₂), 1.73 (d, J_PH= 13.2 Hz, 6H,

2

3

dbf–OC), 155.71 (br s, dbf–OC), 133.82 (d, J_CP= 3.0 Hz, 3/7–

PCH₃), 1.14 (d, J_HH= 6.9 Hz, 12H, CH(CH₃)₂). ¹³C{¹H}

2

dbf), 133.00 (s, dbf), 132.75 (d, J_CP= 12.1 Hz, 3/7-dbf),

NMR (CD₂Cl₂): δ 155.79 (d, ²J_CP= 3.3 Hz, dbf-OC), 149.35 (d,

³J_CP= 3.3 Hz, dbf-quaternary), 138.11 (s, p-Pipp), 133.49 (d,

4

2

131.98 (d, J_CP= 2.3 Hz, p-Ph), 131.68 (d, J_CP= 9.8 Hz, o-

Ph), 131.22 (d, ⁴J_CP= 2.3 Hz, p-Ph), 130.99 (d, J_CP= 12.1 Hz,

o-Ph), 130.26 (d, J_CP= 75.5 Hz, ipso–Ph), 129.03 (d, J_CP

²J_CP= 6.1 Hz, 3,7-dbf), 132.54 (d, J_CP= 95.9 Hz, ipso-Ph),

2

1

3

=

132.34 (d, J_CP= 2.7 Hz, 2,8-dbf), 131.23 (d, J_CP= 9.9 Hz,

3

2

4

10.6 Hz, 2/8–dbf), 128.86 (d, J_CP= 10.6 Hz, 2/8–dbf), 125.32

m-Ph), 129.46 (d, J_CP= 12.1 Hz, o-Ph), 127.14 (d, J_CP= 1.1

Dalton Trans., 2012, 41, 3701–3713 | 3707

This journal is © The Royal Society of Chemistry 2012

View Article Online

Hz, m-Pipp), 125.42 (d, ⁴J_CP= 3.0 Hz, 1,9-dbf), 124.54 (s, ipso-

Hz, m-Ph), 129.34 (d, ²J_CP= 12.1 Hz, o-Ph), 127.16 (s, m-Pipp),

125.25 (d, J_CP= 2.2 Hz, 1,9-dbf), 124.65 (d, J_CP= 6.1 Hz,

4

3

4

1

Pipp), 124.40 (d, J_CP= 9.8 Hz, p-Ph), 122.88 (d, J_CP= 18.7

1

3

Hz, o-Pipp), 116.54 (d, J_CP= 88.3 Hz, 4,6-dbf), 33.67 (s, CH

ipso-Ph), 124.25 (d, J_CP= 9.5 Hz, 2,8-dbf), 122.92 (d, J_CP=

1

(CH₃)₂), 24.57 (s, CH(CH₃)₂), 14.91 (d, J_CP= 78.5 Hz, PCH₃).

19.3 Hz, o-Pipp), 117.19 (d, J_CP= 85.3 Hz, 4,6-dbf), 33.67

³¹P{¹H} NMR (CD₂Cl₂): δ −2.26 (s). Anal. Calcd for

C₄₄H₄₄N₂OP₂: C, 77.86; H, 6.53; N, 4.13. Found: C, 77.39; H,

6.71; N, 4.08.

(s, CH(CH₃)₂), 24.58 (s, CH(CH₃)₂), 15.35 (d, J_CP= 74.3 Hz,

1

PCH₃). ³¹P{¹H} NMR (CD₂Cl₂): δ −2.07 (s). Anal. Calcd for

C₄₄H₄₄N₂OP₂: C, 77.86; H, 6.53; N, 4.13. Found: C, 77.42; H,

6.54; N, 4.20.

Synthesis of rac-4a

Synthesis of rac-5a

Rac-2 (400 mg, 0.91 mmol) was dissolved in diethylamine

(10 mL) and heated to 50 °C for 17 h. All volatiles were

removed in vacuo at 9 × 10⁻³torr and an oily residue was

obtained. Benzene (15 mL) was added, followed by slow

addition of DippN₃(462.5 mg, 2.25 mmol). Gas evolution was

immediately observed. The light yellow solution was stirred for

12 h at ambient temperature, then all volatiles were removed

in vacuo, yielding a light yellow residue. Pentane (10 mL) was

added and the suspension was stirred, followed by ﬁltration and

washing with pentane to yield an off-white solid (470 mg,

0.62 mmol, 68%). X-ray quality single crystals were obtained

from layering heptane onto a methylene chloride solution of

Rac-4a

(200

mg,

0.26

mmol)

and

[H(Et₂O)₂]⁺

[B(3,5-{CF₃}₂C₆H₃)₄]⁻(265 mg, 0.26 mmol) were dissolved in

benzene (3 mL) and the yellowish solution was stirred for 1 h.

Volatiles were removed in vacuo and pentane (5 mL) was added

to the residue. The resultant colourless solid was collected by

ﬁltration and washed with pentane (2 × 5 mL) (390 mg,

0.24 mmol, 91%). X-ray quality single crystals were obtained

from layering heptane onto a methylene chloride solution of

1

3

rac-5a. H NMR (CD₂Cl₂): δ 8.38 (d, J_HH= 7.8 Hz, 2H, 1,9-

dbf), 7.85–7.68 (ov m, 16H, 3,7-dbf + o-C₆H₃(CF₃)₂–Ph), 7.55

(s, 4H, p-C₆H₃(CF₃)₂)), 7.42–7.36 (br m, 2H, p-dipp), 7.19 (br

1

3

5

rac-4a. H NMR (CD₂Cl₂): δ 8.29 (dd, J_HH= 7.5 Hz, J_PH

1.2 Hz, 1H, 1-dbf), 8.24 (d, J_HH= 7.5 Hz, 3H, 9 + 3,7-dbf),

=

m, 4H, m-Ph), 7.07 (br ov m, 6H, m-dipp + p-dipp), 4.79 (br s,

1H, NH), 2.81 (sp, ³J_HH= 6.6 Hz, 4H, CHMe₂), 2.00 (d, J_PH

=

3

2

3

4

3

7.59 (dt, J_HH= 7.8 Hz, J_PH= 1.5 Hz, 2H, 2,8-dbf), 7.47–7.30

12.3 Hz, 6H, PCH₃), 0.92 (d, J_HH= 6.6 Hz, 12H, CHMe₂),

3

(ov m, 10H, Ph), 6.90 (d, J_HH= 7.5 Hz, 4H, m-dipp), 6.72 (td,

0.66 (d, ³J_HH= 6.6 Hz, 12H, CHMe₂). ¹³C{¹H} NMR (CD₂Cl₂):

³J_HH= 7.5 Hz, J_PH= 2.1 Hz, 2H, p-dipp), 3.15 (sp, J_HH= 6.9

δ 162.29 (q, 1 : 1 : 1 : 1, J_BC= 49.8 Hz, ipso-B[C₆H₃(CF₃)₂]₄),

156.41 (s, dbf-OC), 145.60 (br ov, ipso-dipp and o-dipp), 135.34

(s, o-C₆H₃(CF₃)₂), 134.30 (br, p-dipp), 131.00 (d, J_CP= 11.3

6

3

1

3

Hz, 4H, CHMe₂), 1.55 (d, J_PH= 12.6 Hz, 6H, PCH₃), 0.96 (d,

³J_HH= 6.9 Hz, 12H, CHMe₂), 0.86 (d, J_HH= 6.9 Hz, 12H,

3

CHMe₂). ¹³C{¹H} NMR (CD₂Cl₂): δ 155.62 (d, J_CP= 5.3 Hz,

Hz, m-Ph), 130.45 (ov, ipso-Ph), 130.28 (d, J_CP= 13.6 Hz,

2

dbf-OC), 144.39 (s, o-dipp), 143.09 (d, J_CP= 6.8 Hz, ipso-

o-Ph), 129.40 (qq, ²J_CF= 31.4 Hz, ³J_CB= 2.7 Hz, m-C₆H₃(CF₃)₂),

dipp), 135.22 (d, ¹J_CP= 95.8 Hz, ipso-Ph), 133.20 (d, ³J_CP= 3.8

129.39 (ov, quaternary dbf), 127.58 (br s, 1,9-dbf), 125.52

4

2

1

Hz, m-Ph), 131.72 (d, J_CP= 2.3 Hz, p-Ph), 130.42 (d, J_CP

10.6 Hz, 3,8-dbf), 129.17 (d, J_CP= 12.1 Hz, o-Ph), 124.89

(s, p-dipp), 124.36 (d, J_CP= 6.8 Hz, quaternary dbf), 124.05

(d, J_CP= 9.0 Hz, 2,8-dbf), 122.93 (d, J_CP= 1.5 Hz, m-dipp),

=

(d, J_CP= 10.6 Hz, 3,7-dbf), 125.14 (q, J_CF= 272.5 Hz, CF₃),

2

3

124.75 (d, J_CP= 6.0 Hz, 2,8-dbf), 124.42 (br s, m-dipp and

3

p-Ph), 118.02 (br sp, J_CF= 4.5 Hz, p-C₆H₃(CF₃)₂), 29.39 (s,

3

4

CH(CH₃)₂), 24.00 (s, CH(CH₃)₂), 23.41 (s, CH(CH₃)₂), 10.96

4

1

3

119.57 (d, J_CP= 3.0 Hz, 1,9-dbf), 118.60 (d, J_CP= 109.4 Hz,

4,6-dbf), 28.84 (s, CH(CH₃)₂), 24.00 (s, CH(CH₃)₂), 23.78

(d, J_CP= 66.6 Hz, PCH₃). One aromatic carbon signal was not

observed. ³¹P{¹H} NMR (CD₂Cl₂): δ 9.17 (br s). ¹¹B{¹H} NMR

(CD₂Cl₂): δ −6.61 (s). ¹⁹F{¹H} NMR (CD₂Cl₂): δ −62.83 (s).

Anal. Calcd for C₈₂H₆₉BF₂₄N₂OP₂: C, 60.53; H, 4.27; N, 1.72.

Found: C, 60.75; H, 4.26; N, 1.89.

(s, CH(CH₃)₂), 15.84 (d, J_CP= 69.4 Hz, PCH₃). ³¹P{¹H} NMR

1

(CD₂Cl₂): δ −15.21 (s). Anal. Calcd for C₅₀H₅₆N₂OP₂: C,

78.71; H, 7.40; N, 3.67. Found: C, 78.68; H, 7.68; N, 3.77.

Synthesis of meso-4

Synthesis of rac-5b

Meso-4 was prepared via a similar procedure as that described

for rac-4. Thus, meso-2 (283 mg, 0.643 mmol), diethylamine

(15 mL) and PippN₃(238.5 mg, 1.48 mmol), yielded a colour-

less solid (310 mg, 0.46 mmol, 71%). ¹H NMR (CD₂Cl₂): δ

Rac-5b was prepared via a similar procedure as that described

for rac-5a from rac-4a (208 mg, 0.27 mmol) and [H(Et₂O)₂]⁺[B

(C₆F₅)₄]⁻(210 mg, 0.27 mmol) to afford a colourless solid

(380 mg, 0.26 mmol, 96.6%). ¹H NMR (CD₂Cl₂): δ 8.41 (d,

³J_HH= 7.8 Hz, 2H, 1,9-dbf), 7.90–7.68 (ov m, 8H, dbf + Ph),

3

8.20 (d, J_HH= 7.8 Hz, 2H, 1,9-dbf), 7.90 (dd, J_HH= 7.6 Hz,

3

³J_PH= 12.7 Hz, 2H, 3,7-dbf), 7.76 (dd, J_HH= 7.6 Hz, J_PH

=

7.42 (br t, J_HH= 7.5 Hz, 2H, aromatic H), 7.21 (br ov m, 4H,

12.3 Hz, 4H, m-Ph), 7.53–7.36 (ov m, 8H, 2,8-dbf + o-Ph +

aromatic H), 7.07 (br ov m, 6H, dipp), 4.78 (br s, 1H, NH), 2.82

3

p-Ph), 6.84 (d, J_HH= 8.1 Hz, 4H, m-Pipp), 6.59 (d, J_HH= 8.1

(sp, J_HH= 6.6 Hz, 4H, CH(CH₃)₂), 1.98 (d, J_PH= 12.3 Hz,

3

Hz, 4H, o-Pipp), 2.72 (sp, J_HH= 6.9 Hz, 2H, CH(CH₃)₂), 2.08

6H, PCH₃), 0.92 (d, J_HH= 6.8 Hz, 12H, CH(CH₃)₂), 0.68 (d,

2

3

(d, J_PH= 13.2 Hz, 6H, PCH₃), 1.14 (d, J_HH= 6.9 Hz, 12H,

³J_HH= 6.8 Hz, 12H, CH(CH₃)₂). ¹³C{¹H} NMR (CD₂Cl₂): δ

156.41 (s, dbf-OC), 150.31 (br, aromatic quaternary C), 147.12

(br, aromatic quaternary C), 145.65 (br, aromatic quaternary C),

140.47 (br m, B(C₆F₅)₄), 138.34 (br m, B(C₆F₅)₄), 137.14 (br m,

B(C₆F₅)₄), 135.14 (br m, B(C₆F₅)₄), 134.43 (br s, Ph), 131.70

CH(CH₃)₂). ¹³C{¹H} NMR (CD₂Cl₂): δ 156.28 (d, J_CP= 2.7

2

Hz, dbf-OC), 149.55 (d, J_CP= 3.3 Hz, ipso-Pipp), 137.97

(s, p-Pipp), 133.04 (s, quaternary dbf), 132.78 (d, ²J_CP= 6.1 Hz,

3,7-dbf), 132.25 (d, ⁴J_CP= 2.8 Hz, p-Ph), 131.59 (d, ³J_CP= 10.5

3708 | Dalton Trans., 2012, 41, 3701–3713

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1

(ov, aromatic C), 131.02 (d, J_CP= 6.6 Hz, aromatic C), 130.30

(d, J_CP= 12.8 Hz, aromatic C), 128.85 (s, aromatic C), 127.68

(br s, 1,9-dbf), 127.65 (ov, aromatic C), 125.56 (d, J_CP= 10.6

Hz, aromatic C), 124.83 (br s, aromatic quaternary C), 124.43 (s,

dipp), 29.40 (s, CH(CH₃)₂), 23.98 (s, CH(CH₃)₂), 23.42 (s, CH

³J_CP= 9.8 Hz, 2,8-dbf), 125.14 (q, J_CF= 272.5 Hz, CF₃),

124.77 (d, ²J_CP= 6.0 Hz, ipso-Pipp), 123.68 (d, ³J_CP= 12.7 Hz,

3

o-Pipp), 118.03 (sp, J_CF= 3.87 Hz, p-C₆H₃(CF₃)₂), 112.37 (d,

¹J_CP= 89.1 Hz, 4,6-dbf), 33.80 (s, CH(CH₃)₂), 24.28 (s, CH

(CH₃)₂), 12.95 (d, J_CP= 70.9 Hz, PCH₃). ³¹P{¹H} NMR

1

(CH₃)₂), 10.94 (br d, J_CP= 74.0 Hz, PCH₃). ³¹P{¹H} NMR

(CD₂Cl₂): δ 15.84 (s). ¹¹B{¹H} NMR (CD₂Cl₂): δ −6.60 (s).

¹⁹F{¹H} NMR (CD₂Cl₂): δ −61.93 (s). Anal. Calcd for

C₇₆H₅₇BF₂₄N₂OP₂: C, 59.16; H, 3.72; N, 1.82. Found: C,

59.60; H, 3.74; N, 1.91.

1

(CD₂Cl₂): δ 9.2 (br). ¹¹B{¹H} NMR (CD₂Cl₂): δ −16.67 (s). ¹⁹

F

{¹H} NMR (CD₂Cl₂): δ −133.08 (br, 8F, o-C₆F₅), −163.62

3

(t, J_FF= 21.2 Hz, 4F, p-C₆F₅), −167.48 (br, 8F, m-C₆F₅). Anal.

Calcd for C₇₄H₅₇BF₂₀N₂OP₂: C, 61.59; H, 3.98; N, 1.94.

Found: C, 60.70; H, 4.00; N, 2.19.

Synthesis of rac-6

Rac-5 (100 mg, 64.8 μmol) was dissolved in methylene chloride

(1.5 mL) and excess diethylzinc (0.1 mL) was added. The sol-

ution was stirred at ambient temperature for 15 min and the vola-

tiles were removed in vacuo. After washing with pentane (3 ×

2 mL), the thick residue was dried under reduced pressure to

give a white solid (100 mg, 60.9 μmol, 94%). ¹H NMR

(CD₂Cl₂): δ 8.37 (d, ³J_HH= 7.7 Hz, 2H, 1,9-dbf), 7.72–7.45 (ov

Synthesis of rac-5

Rac-5 was prepared via a similar procedure as that described for

rac-5a, using rac-4 (345 mg, 0.508 mmol) and [H(Et₂O)₂]⁺

[B(3,5-{CF₃}₂C₆H₃)₄]⁻(515 mg, 0.508 mmol) to obtain a colour-

less solid (680 mg, 0.44 mmol, 87%). ¹H NMR (CD₂Cl₂): δ

8.34 (d, ³J_HH= 7.6 Hz, 2H, 1,9-dbf), 7.73–7.45 (ov m, 26H, dbf

3

+ Ph + C₆H₃(CF₃)₂), 6.91 (d, J_HH= 7.9 Hz, 4H, m-Pipp), 6.62

m, 26H, dbf + Ph + C₆H₃(CF₃)₂), 6.80 (d, J_HH= 8.0 Hz, 4H,

3

(d, J_HH= 8.1 Hz, 4H, o-Pipp), 6.42 (br s, 1H, NH), 2.74 (sp,

m-Pipp), 6.48 (d, J_HH= 8.0 Hz, 4H, o-Pipp), 2.74 (sp, J_HH=

2

³J_HH= 6.9 Hz, 2H, CH(CH₃)₂), 2.09 (d, J_PH= 12.9 Hz, 6H,

6.9 Hz, 2H, CH(CH₃)₂), 2.14 (d, J_PH= 12.7 Hz, 6H, PCH₃),

PCH₃), 1.12 (d, J_HH= 6.8 Hz, 12H, CH(CH₃)₂). ¹³C{¹H}

1.14 (d, ³J_HH= 7.0 Hz, 12H, CH(CH₃)₂), 0.64 (t, ³J_HH= 8.0 Hz,

3

1

3

NMR (CD₂Cl₂): δ 162.33 (q 1 : 1 : 1 : 1, J_BC= 49.8 Hz, ipso-B

[C₆H₃(CF₃)₂]₄), 156.92 (s, dbf-OC), 143.36 (s, p-Pipp), 141.50

(s, quaternary dbf), 135.36 (s, o-C₆H₃(CF₃)₂), 134.60 (d, J_CP

3H, CH₃CH₂Zn), −0.22 (q, J_HH= 8.0 Hz, 2H, CH₃CH₂Zn).

¹³C{¹H} NMR (CD₂Cl₂): δ 162.29 (q 1 : 1 : 1 : 1, J_BC= 50.0

1

4

5

=

Hz, ipso-B[C₆H₃(CF₃)₂]₄), 156.73 (s, dbf-OC), 144.48 (d, J_CP

= 3.0 Hz, p-Pipp), 142.80 (d, J_CP= 6.0 Hz, quaternary dbf),

2

3

2.8 Hz, p-Ph), 132.18 (d, J_CP= 7.5 Hz, 3,7-dbf), 132.04 (d,

4

³J_CP= 11.3 Hz, m-Ph), 130.24 (d, ²J_CP= 13.2 Hz, o-Ph), 129.42

135.35 (s, o-C₆H₃(CF₃)₂), 134.56 (d, J_CP= 2.8 Hz, p-Ph),

2

3

2

(qq, J_CF= 31.79 Hz, J_CB= 3.0 Hz, m-C₆H₃(CF₃)₂), 128.03 (s,

131.99 (d, ²J_CP= 10.6 Hz, o-Ph), 130.76 (d, J_CP= 8.3 Hz, 3,7-

4

1

2

m-Pipp), 127.78 (d, J_CP= 2.8 Hz, 1,9-dbf), 125.99 (d, J_CP

=

dbf), 130.17 (d, J_CP= 12.7 Hz, m-Ph), 129.42 (qq, J_CF= 30.9

Hz, J_CB= 2.3 Hz, m-C₆H₃(CF₃)₂), 127.93 (d, J_CP= 1.5 Hz,

1,9-dbf), 127.55 (d, J_CP= 101.0 Hz, ipso-Ph), 126.90 (ov

m-Pipp), 126.26 (d, J_CP= 11.0 Hz, o-Pipp), 125.86 (d, J_CP=

9.9 Hz 2,8-dbf), 125.15 (q, J_CF= 272.5 Hz, CF₃), 124.74

(d, J_CP= 6.1 Hz, ipso-Pipp), 118.02 (sp, J_CF= 4.0 Hz,

p-C₆H₃(CF₃)₂), 113.64 (d, J_CP= 101.9 Hz, 4,6-dbf), 33.84

3

4

107.17 Hz, ipso-Ph), 125.45 (d, ³J_CP= 10.4 Hz, o-Pipp), 125.14

1

2

1

(q, J_CF= 272.5 Hz, CF₃), 124.79 (d, J_CP= 10.4 Hz, ipso-

3

Pipp), 123.33 (d, J_CP= 12.8 Hz, 2,8-dbf), 118.03 (sp, J_CF

3.9 Hz, p-C₆H₃(CF₃)₂), 112.13 (d, J_CP= 90.8 Hz, 4,6-dbf),

=

1

2

3

33.81 (s, CH(CH₃)₂), 24.28 (s, CH(CH₃)₂), 13.06 (d, J_CP

=

1

71.7 Hz, PCH₃). ³¹P{¹H} NMR (CD₂Cl₂): δ 15.20 (s). ¹¹B{¹H}

NMR (CD₂Cl₂): δ −6.57 (s). ¹⁹F{¹H} NMR (CD₂Cl₂): δ −61.89

(s). Anal. Calcd for C₇₆H₅₇BF₂₄N₂OP₂: C, 59.16; H, 3.72; N,

1.82. Found: C, 59.27; H, 3.87; N, 1.91.

1

(s, CH(CH₃)₂), 24.29 (s, CH(CH₃)₂), 13.05 (d, J_CP= 67.9 Hz,

PCH₃), 12.55 (s, CH₃CH₂Zn), 2.23 (s, CH₃CH₂Zn). ³¹P{¹H}

NMR (CD₂Cl₂): δ 24.98 (s). ¹¹B{¹H} NMR (CD₂Cl₂): δ −6.61

(s). ¹⁹F{¹H} NMR (CD₂Cl₂): δ −61.95 (s). Anal. Calcd for

C₇₈H₆₁BF₂₄N₂OP₂Zn: C, 57.25; H, 3.76; N, 1.71. Found: C,

57.44; H, 3.88; N, 1.78.

Synthesis of meso-5

Meso-5 was prepared via a similar procedure as that described

for rac-5a, from meso-4 (240 mg, 0.354 mmol) and [H

(Et₂O)₂]⁺{B[3,5-(CF₃)₂C₆H₃]₄}⁻(358 mg, 0.354 mmol) to yield

Synthesis of rac-6a

1

a beige solid (440 mg, 0.29 mmol, 81%). H NMR (CD₂Cl₂): δ

In a glovebox, rac-5a (60 mg, 0.0369 mmol) was dissolved in

bromobenzene (0.5 mL), and diethylzinc (excess) was added

dropwise. After stirring the solution for 5 min, pentane (3 mL)

was added and an oil formed in the bottom of the reaction

vessel. The pentane layer was separated and the oil was washed

with pentane (3 × 3 mL), yielding a solid which was dried in

vacuo (50 mg, 0.029 mmol, 78.9%). X-ray quality single crystals

were obtained from layering heptane onto a methylene chloride

solution of rac-6a. ¹H NMR (CD₂Cl₂): δ 8.43 (d, ³J_HH= 7.8 Hz,

2H, 1,9-dbf), 7.72 (br ov m, 10H, dbf + Ph), 7.55 (br ov m, 8H,

Ph + B[C₆H₃(CF₃)₂]₄), 7.41–7.27 (br m, 8H, B[C₆H₃(CF₃)₂]₄),

7.13 (m, 2H, p-dipp), 7.00 (br m, 4H, m-dipp), 3.05 (br m, 4H,

3

8.35 (d, J_HH= 7.4 Hz, 2H, 1,9-dbf), 7.73–7.36 (m, 26H, dbf +

Ph + C₆H₃(CF₃)₂), 6.89 (d, ³J_HH= 8.3 Hz, 4H, m-Pipp), 6.62 (d,

3

³J_HH= 7.9 Hz, 4H, o-Pipp), 6.39 (br s, 1H, NH), 2.73 (sp, J_HH

2

= 6.9 Hz, 2H, CH(CH₃)₂), 2.17 (d, J_PH= 13.0 Hz, 6H, PCH₃),

3

1.12 (d, J_HH= 6.8 Hz, 12H, CH(CH₃)₂). ¹³C{¹H} NMR

1

(CD₂Cl₂): δ 162.30 (q, 1 : 1 : 1 : 1, J_BC= 49.8 Hz, ipso-B

[C₆H₃(CF₃)₂]₄), 157.04 (s, dbf-OC), 143.35 (s, p-Pipp), 141.58

4

(s, quaternary dbf), 135.34 (s, o-C₆H₃(CF₃)₂), 134.55 (d, J_CP

=

2.8 Hz, p-Ph), 131.98 (d, J_CP= 10.6 Hz, o-Ph), 131.94 (d, ²J_CP

= 4.5 Hz, 3,7-dbf), 130.18 (d, J_CP= 13.2 Hz, m-Ph), 129.42

(qq, J_CF= 30.9 Hz, J_CB= 2.8 Hz, m-C₆H₃(CF₃)₂), 128.86 (s,

ipso-Ph), 127.96 (s, m-Pipp), 127.76 (br, 1,9-dbf), 125.44 (d,

2

3

2

3

CH(CH₃)₃), 2.08 (d, J_PH= 11.4 Hz, 6H, PCH₃), 0.90–0.30 (br

This journal is © The Royal Society of Chemistry 2012

Dalton Trans., 2012, 41, 3701–3713 | 3709

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3

m, 24H, CH(CH₃)₃), 0.01 (t, J_HH= 7.8 Hz, 3H, ZnCH₂CH₃),

CH₃CH₂Zn), −0.33 (q, J_HH= 8.1 Hz, 2H, CH₃CH₂Zn). ¹³C

−0.80 (br m, 2H, ZnCH₂CH₃). ¹³C{¹H} NMR (CD₂Cl₂): δ

162.29 (q, 1 : 1 : 1 : 1, J_BC= 49.8 Hz, ipso-B[C₆H₃(CF₃)₂]₄),

{¹H} NMR (CD₂Cl₂): δ 162.30 (q 1 : 1 : 1 : 1, J_BC= 49.6 Hz,

1

ipso-B[C₆H₃(CF₃)₂]₄), 156.78 (s, dbf-OC), 144.57 (d, ⁵J_CP= 3.3

1

3

157.41 (s, dbf-OC), 146.16 (d, J_CP= 4.8 Hz, dbf-quaternary),

Hz, p-Pipp), 142.69 (d, J_CP= 6.0 Hz, quaternary dbf), 135.34

4

135.33 (s, o-C₆H₃(CF₃)₂), 134.29 (br s, p-Ph), 132.11 (br ov m,

(s, o-C₆H₃(CF₃)₂), 134.42 (d, J_CP= 2.8 Hz, p-Ph), 131.78 (d,

o-Ph + dbf), 130.34 (ov, dbf), 130.14 (d, ²J_CP= 12.8 Hz, m-Ph),

³J_CP= 10.5 Hz, m-Ph), 130.74 (d, J_CP= 8.3 Hz, 3,7-dbf),

2

3

2

129.38 (qq, J_CF= 31.9 Hz, J_CB= 2.7 Hz, m-C₆H₃(CF₃)₂),

127.51 (d, J_CP= 2.2 Hz, 1,9-dbf), 126.35 (d, J_CP= 2.6 Hz,

o-dipp), 125.99 (d, J_CP= 10.6 Hz, 2,8-dbf), 125.43 (s, p-dipp),

125.12 (q, J_CF= 272.5 Hz, CF₃), 124.68 (d, J_CP= 6.8 Hz,

130.11 (d, J_CP= 12.7 Hz, o-Ph), 129.40 (qq, J_CF= 31.3 Hz,

³J_CB= 2.8 Hz, m-C₆H₃(CF₃)₂), 128.14 (d, J_CP= 104.0 Hz,

4

3

1

3

ipso-Ph), 127.85 (br ov, 1,9-dbf and m-Pipp), 126.25 (d, J_CP

=

1

2

3

11.6 Hz, o-Pipp), 125.99 (d, J_CP= 9.4 Hz, 2,8-dbf), 125.12 (q,

1

2

ipso-dipp), 124.34 (d, J_CP= 101.1 Hz, ipso-Ph), 118.02 (sp,

¹J_CF= 272.5 Hz, CF₃), 124.80 (d, J_CP= 5.3 Hz, ipso-Pipp),

³J_CF= 4.0 Hz, p-C₆H₃(CF₃)₂), 115.33 (d, J_CP= 100.0 Hz, 4,6-

118.01 (sp, J_CF= 4.5 Hz, p-C₆H₃(CF₃)₂), 113.52 (d, J_CP=

1

3

1

dbf), 28.93 (s, CH(CH₃)₂), 24.74 (s, CH(CH₃)₂), 13.28 (d, J_CP

101.1 Hz, 4,6-dbf), 33.79 (s, CH(CH₃)₂), 24.27 (s, CH(CH₃)₂),

12.59 (s, CH₃CH₂Zn), 12.11 (d, ¹J_CP= 67.1 Hz, PCH₃), 2.58 (s,

CH₃CH₂Zn). ³¹P{¹H} NMR (CD₂Cl₂): δ 24.98 (s). ¹¹B{¹H}

NMR (CD₂Cl₂): δ −6.61 (s). ¹⁹F{¹H} NMR (CD₂Cl₂): δ −61.96

(s). Anal. Calcd for C₇₈H₆₁BF₂₄N₂OP₂Zn: C, 57.25; H, 3.76; N,

1.71. Found: C, 57.37; H, 3.81; N, 1.77.

= 167.5 Hz, PCH₃), 6.57 (s, CH₂CH₃), −0.05 (s, CH₂CH₃). ³¹P

{¹H} NMR (CD₂Cl₂): δ 24.49 (br s). ¹¹B{¹H} NMR (CD₂Cl₂):

δ −6.61 (s). ¹⁹F{¹H} NMR (CD₂Cl₂): δ −62.84 (s). Anal. Calcd

for C₈₄H₇₃BF₂₄N₂OP₂Zn: C, 58.64; H, 4.28; N, 1.63. Found: C,

58.15; H, 4.24; N, 1.57.

Synthesis of rac-6b

Synthesis of rac-7

Rac-6b was prepared via a similar procedure as that described

for rac-6a (using rac-5b, 76.0 mg, 0.05 mmol) to afford a white

Rac-7 was prepared via a similar procedure as that described for

rac-6, from rac-5 (100 mg, 64.8 μmol) and dimethylzinc (1.2 M

toluene solution, 0.27 mL) to yield a beige solid (84 mg,

1

solid (74.4 mg, 4.84 mmol, yield 92%). H NMR (CD₂Cl₂): δ

3

1

3

8.44 (d, J_HH= 7.8 Hz, 2H, 1,9-dbf), 7.69 (dt, J_HH= 7.8 Hz,

³J_PH= 2.4 Hz, 2H, 2,8-dbf), 7.63–7.46 (br ov m, 4H, Ph),

7.46–7.20 (br ov m, 8H, dbf + Ph), 7.13 (dt, ³J_HH= 7.5 Hz, ⁶J_PH

= 2.1 Hz, 2H, p-dipp), 7.01 (br ov m, 4H, m-dipp), 3.06 (br m,

51.84 μmol, 80%). H NMR (CD₂Cl₂): δ 8.40 (dt, J_HH= 7.9

5

Hz, J_PH= 1.3 Hz, 2H, 1,9-dbf), 7.72–7.41 (ov m, 26H, dbf +

3

Ph + C₆H₃(CF₃)₂), 6.80 (d, J_HH= 7.9 Hz, 4H, m-Pipp), 6.47

(dd, ³J_HH= 8.4 Hz, ⁴J_PH= 2.2 Hz, 4H, o-Pipp), 2.74 (sp, ³J_HH

=

2

4H, CH(CH₃)₂), 2.08 (d, J_PH= 11.4 Hz, 6H, PCH₃), 1.20–0.18

6.9 Hz, 2H, CH(CH₃)₂), 2.13 (d, J_PH= 12.5 Hz, 6H, PCH₃),

1.14 (d, ³J_HH= 6.9 Hz, 12H, CH(CH₃)₂), −1.10 (s, 3H, ZnCH₃).

3

(br ov m, 24H, CH(CH₃)₂), 0.01 (t, J_HH= 8.1 Hz, 3H,

ZnCH₂CH₃), −0.79 (br m, 2H, ZnCH₂CH₃). ¹³C{¹H} NMR

(CD₂Cl₂): δ 157.40 (s, dbf-OC), 150.24 (br, aromatic C), 147.15

(br, aromatic C), 146.26 (s, aromatic C), 141.11 (br, B(C₆F₅)₄),

140.39 (br, B(C₆F₅)₄), 138.49 (br, B(C₆F₅)₄), 136.98 (br, B

(C₆F₅)₄), 135.17 (br, aromatic C), 134.27 (s, aromatic C), 132.24

¹³C{¹H} NMR (CD₂Cl₂): δ 162.36 (q 1 : 1 : 1 : 1, J_BC= 49.7

1

5

Hz, ipso-B[C₆H₃(CF₃)₂]₄), 156.73 (s, dbf-OC), 144.65 (d, J_CP

3

= 3.3 Hz, p-Pipp), 142.61 (d, J_CP= 6.6 Hz, quaternary dbf),

4

135.35 (s, o-C₆H₃(CF₃)₂), 134.53 (d, J_CP= 3.3 Hz, p-Ph),

2

132.03 (d, ²J_CP= 10.5 Hz, o-Ph), 130.95 (d, J_CP= 7.7 Hz, 3,7-

2

(br, aromatic C), 130.15 (d, J_CP= 12.8 Hz, o-Ph), 127.55 (s,

dbf), 130.11 (d, J_CP= 12.7 Hz, m-Ph), 129.41 (qq, J_CF= 31.9

4

3

1,9-dbf), 126.34 (s, aromatic C), 126.00 (d, J_CP= 10.6 Hz,

Hz, J_CB= 2.9 Hz, m-C₆H₃(CF₃)₂), 128.86 (s, m-Pipp), 127.86

4

1

m-dipp), 125.41 (s, 2,8-dbf), 124.74 (s, aromatic C), 115.32 (d,

(d, J_CP= 1.7 Hz, 1,9-dbf), 127.58 (d, J_CP= 102.4 Hz, ipso-

¹J_CP= 96.6 Hz, 4,6-dbf), 28.93 (s, CH(CH₃)₂), 24.73 (s, CH

Ph), 126.20 (d, J_CP= 11.3 Hz, o-Pipp), 126.14 (d, J_CP= 9.1

3

1

2

(CH₃)₂), 14.63 (d, J_CP

=

36.2 Hz, PCH₃), 12.16 (s,

Hz, 2,8-dbf), 125.15 (q, J_CF= 272.5 Hz, CF₃), 124.77 (d, J_CP

= 6.0 Hz, ipso-Pipp), 118.03 (sp, J_CF= 3.8 Hz, p-C₆H₃(CF₃)₂),

113.46 (d, J_CP= 101.1 Hz, 4,6-dbf), 33.85 (s, CH(CH₃)₂),

CH₃CH₂Zn), 6.57 (s, CH₃CH₂Zn). ³¹P{¹H} NMR (CD₂Cl₂): δ

3

24.65 (br s). ¹¹B{¹H} NMR (CD₂Cl₂): δ −16.67 (s). ¹⁹F{¹H}

1

3

1

NMR (CD₂Cl₂): δ −133.05 (br, 8F, o-C₆F₅), −163.69 (t, J_FF

=

24.30 (s, CH(CH₃)₂), 13.14 (d, J_CP= 68.7 Hz, PCH₃), −11.75

19.7 Hz, 4F, p-C₆F₅), −167.53 (br, 8F, m-C₆F₅). Anal. Calcd for

C₇₆H₆₁BF₂₀N₂OP₂Zn: C, 59.41; H, 4.00; N, 1.82. Found: C,

58.39; H, 4.00; N, 1.78.

(s, CH₃Zn). ³¹P{¹H} NMR (CD₂Cl₂): δ 25.71 (s). ¹¹B{¹H}

NMR (CD₂Cl₂): δ −6.61 (s). ¹⁹F{¹H} NMR (CD₂Cl₂): δ −61.95

(s). Anal. Calcd for C₇₇H₅₉BF₂₄N₂OP₂Zn: C, 57.00; H, 3.67; N,

1.73. Found: C, 57.13; H, 3.77; N, 1.79.

Synthesis of meso-6

Synthesis of meso-7

Meso-6 was prepared via a similar procedure as that described

for rac-6, using meso-5 (100 mg, 64.8 μmol) and diethylzinc

(0.05 mL, excess), yielding a white solid (94 mg, 57.7 μmol,

Meso-7 was prepared via a similar procedure as that described

for meso-6, from meso-5 (100 mg, 64.8 μmol) and dimethylzinc

(excess, 324 μmol, 1.2 M toluene solution, 0.27 mL) to yield a

1

3

89%). H NMR (CD₂Cl₂): δ 8.42 (d, J_HH= 7.9 Hz, 2H, 1,9-

dbf), 7.73–7.52 (ov m, 22H, dbf + Ph + C₆H₃(CF₃)₂), 7.42 (m,

1

beige solid (103 mg, 63.5 μmol, 98%). H NMR (CD₂Cl₂): δ

3

4H, m-Ph), 6.76 (d, J_HH= 8.1 Hz, 4H, m-Pipp), 6.44 (dd, J_HH

8.44 (d, J_HH= 7.9 Hz, 2H, 1,9-dbf), 7.72 (br ov m, 10H, o-Ph

3

= 8.5 Hz, J_PH= 2.1 Hz, 4H, o-Pipp), 2.71 (sp, J_HH= 6.8 Hz,

+ p-Ph + dbf), 7.60–7.48 (ov m, 12H, C₆H₃(CF₃)₂), 7.40 (m,

2

3

2H, CH(CH₃)₂), 2.17 (d, J_PH= 12.7 Hz, 6H, PCH₃), 1.12 (d,

4H, m-Ph), 6.74 (d, J_HH= 8.3 Hz, 4H, m-Pipp), 6.42 (dd, J_HH

3

³J_HH= 6.8 Hz, 12H, CH(CH₃)₂), 0.68 (t, J_HH= 8.1 Hz, 3H,

= 8.5 Hz, J_PH= 2.0 Hz, 4H, o-Pipp), 2.70 (sp, J_HH= 6.9 Hz,

3710 | Dalton Trans., 2012, 41, 3701–3713

Table 3 Selected crystal and reﬁnement data for compounds rac-1, meso-1, rac-2, 3, rac-4a, rac-5a, rac-5b, rac-6a, meso-6 and meso-7

rac-1·0.67

(C₅H₁₂

)

meso-1

rac-2

3

rac-4a

rac-5a

rac-5b

rac-6a

meso-6

meso-7

₇₇H₅₉BF₂₄N₂O

Chemical C_47.33H₆₈B₂O₃P₂C₄₉H₇₂B₂O₃P₂C₂₆H₂₈B₂OP₂C₃₅H₄₄B₂O₂P₂C₅₃H₅₉N₂OP₂C₈₂H₆₉BF₂₄N₂OP₂C₇₄H₅₇BF₂₀N₂OP₂C₈₅H₇₅BCl₂F₂₄N₂OP₂Zn C₇₈H₆₁BF₂₄N₂

C

Formula

Weight

Crystal

System

Space

Group

a/Å

b/Å

c/Å

α (°)

β (°)

OP₂Zn

1636.41

P₂Zn

1622.38

768.58

Monoclinic

P2₁

792.67

Monoclinic

P2₁

440.04

Monoclinic

P2₁

580.26

Monoclinic

P2₁

801.96

1627.14

Monoclinic

P2₁/n

1442.97

Monoclinic

P2₁/n

1805.49

Monoclinic

P2₁/n

Triclinic

ˉ

P1

ˉ

P1

ˉ

P1

12.0455(15)

9.0733(11)

21.256(3)

90

96.742(2)

90

9.1131(6)

17.7459(11)

15.5104(10)

90

102.5560(10) 106.329(2)

90

2448.4(3)

2

0.933

10818/468

9.0916(15)

26.778(5)

10.3509(17) 12.610(3)

8.866(2)

15.561(4)

10.682(3)

12.265(4)

18.315(6)

102.736(4)

93.563(4)

103.759(4)

2256.7(12)

2

25.9495(19)

15.9645(12)

39.031(3)

90

103.7470(10)

90

15706(2)

8

0.972

17.8759(16)

19.0247(17)

39.597(4)

90

99.0010(10)

90

13301(2)

8

0.840

23.695(3)

16.3001(17)

23.842(3)

90

114.8180(10)

90

8358.1(15)

4

1.095

15.008(3)

15.591(3)

16.299(3)

89.306(2)

76.703(2)

85.884(2)

3701.7(11)

2

15.1464(9)

15.4468(9)

16.3669(9)

88.5290(10)

76.3750(10)

86.1190(10)

3712.8(4)

2

90

109.266(2)

90

1642.3(6)

2

1.059

7512/376

γ (°)

90

2418.3(7)

4

1.191

11011/567

V/Å³

Z

2307.0(5)

2

GOF on F²0.891

0.928

10280/533

1.029

16897/1017

1.085

16850/1092

Reﬁned

Data/

10776/468

36047/2239

30607/1786

19174/1214

Parameters

R₁[I >

2σ(I)]^a

0.0862

0.0352

0.0765

0.00(4)

0.0617

0.1403

0.44(11)

0.0289

0.0857

0.03(5)

0.0401

0.0967

0.0563

0.1685

0.0611

0.1839

0.0568

0.1610

0.0652

0.2149

0.0463

wR₂{all

0.2275

00.1398

data}^a

Flack

parameter

−0.08(15)

2

2 2 1/2

^aR₁= Σ||F_o| − |F_c||/Σ |F_o|; wR₂= [Σw(F_o²− F_c)/Σw(F_o) ]

.

View Article Online

2

2H, CH(CH₃)₂), 2.12 (d, J_PH= 12.5 Hz, 6H, PCH₃), 1.12 (d,

density maps and reﬁned freely) were placed in calculated pos-

itions and reﬁned using a riding model.

³J_HH= 6.9 Hz, 12H, CH(CH₃)₂), −1.22 (s, 3H, CH₃Zn). ¹³C

{¹H} NMR (CD₂Cl₂): δ 162.31 (q 1 : 1 : 1 : 1, J_BC= 49.8 Hz,

Crystal data are summarized in Table 3. No special consider-

ations were required for the reﬁnement of the compounds, except

rac-1, in which disordered pentane solvent molecules were

present. The electron density related to the molecule was

removed using SQUEEZE.³⁰Some of the crystals featured disor-

dered organic substituents, in particular the ﬂuorinated borate

anions. Where possible these groups were modelled isotropically

over two positions, although in a few cases no suitable model

could be found, resulting in large thermal displacement

parameters.

1

ipso-B[C₆H₃(CF₃)₂]₄), 156.96 (s, dbf-OC), 144.69 (s, p-Pipp),

142.60 (d, J_CP= 4.9 Hz, quaternary dbf), 135.35 (s,

o-C₆H₃(CF₃)₂), 134.46 (s, p-Ph), 132.00 (d, J_CP= 11.2 Hz,

m-Ph), 130.99 (d, J_CP= 8.3 Hz, 3,7-dbf), 130.05 (d, J_CP

12.7 Hz, o-Ph), 129.51 (qq, J_CF= 31.3 Hz, J_CB= 3.2 Hz,

m-C₆H₃(CF₃)₂), 127.86 (d, J_CP= 107.7 Hz, ipso-Ph), 127.82

3

2

=

2

3

1

3

(br ov, 1,9-dbf and m-Pipp), 126.14 (d, J_CP= 9.4 Hz, o-Pipp),

125.14 (q, ¹J_CF= 272.5 Hz, CF₃), 124.82 (d, ²J_CP= 6.0 Hz, 2,8-

2

3

dbf), 124.51 (d, J_CP= 54.0 Hz, ipso-Pipp), 118.03 (sp, J_CF

4.2 Hz, p-C₆H₃(CF₃)₂), 113.32 (d, J_CP= 98.9 Hz, 4,6-dbf),

=

1

33.82 (s, CH(CH₃)₂), 24.29 (s, CH(CH₃)₂), 12.34 (d, J_CP

=

Conclusions

67.9 Hz, PCH₃), −10.97 (s, CH₃Zn). ³¹P{¹H} NMR (CD₂Cl₂): δ

25.57 (s). ¹¹B{¹H} NMR (CD₂Cl₂): δ −6.61 (s). ¹⁹F{¹H} NMR

(CD₂Cl₂): δ −61.97 (s). Anal. Calcd for C₇₇H₅₉BF₂₄N₂OP₂Zn:

C, 57.00; H, 3.67; N, 1.73. Found: C, 57.07; H, 3.69; N, 1.69.

In summary, a new series of P-stereogenic neutral bisphosphini-

mine ligands, and cationic alkylzinc complexes thereof, have

been synthesized and fully characterised. The successful diaster-

eomeric resolution of these ligands represents an advancement in

the dbf ligand design, as the monophosphinimine derivatives

formed inseparable rac/meso mixtures. Two complexes in par-

ticular, rac-6 and meso-6, were shown to initiate the ring-

opening polymerisation of rac-lactide to yield polymer samples

with a modest heterotactic bias and with signiﬁcantly higher

activity than our ﬁrst-generation initiators. Although signiﬁcant

trans-esteriﬁcation side reactions led to high polydispersity

indices, high conversion was attained. Furthermore, while

mechanistic details remain elusive, kinetic data suggest that rac-

6 and meso-6 are in fact pre-catalysts, which react in solution to

form the active species. To help test this hypothesis, future

studies will focus on the preparation of hydroxy, alkoxide and

lactate derivatives of the alkylzinc complexes presented herein

and investigations of their competency as catalysts for PLA syn-

thesis. To improve the stereocontrol of polymerisation, future

ligand variants may incorporate larger chiral groups to increase

steric differentiation. In addition, placement of the chiral func-

tionality closer to the metal centre (e.g. on nitrogen rather than

phosphorus) is also anticipated to enhance chiral induction.

NMR scale polymerisation of rac-lactide with rac-6 and meso-6

In a glovebox, rac-lactide (72.1 mg, 0.5 mmol) and an appropri-

ate amount of initiator (e.g. M/I = 200, 4.1 mg, 2.5 μmol) were

dissolved in CDCl₃(0.5 mL) in an NMR tube. The tube was

sealed with a rubber septum, and placed in an oil bath (40 °C).

The polymerisation was monitored every 30 min until >95%

1

conversion was observed by H NMR spectroscopy. These con-

ditions were then used for the large scale polymerisation reac-

tions. For determination of activation parameters, data were

recorded at the appropriate temperatures, which were corrected

by using the equation T_actual= 1.111T₀− 1.8505.

Large scale polymerisation of rac-lactide with rac-6 and meso-6

In a glovebox, rac-lactide (144.1 mg, 1 mmol) and an appropri-

ate amount of initiator (e.g. M/I = 200, 8.2 mg) were dissolved

in methylene chloride (1 mL) in a scintillation vial. The solution

was stirred for an appropriate period of time (using NMR scale

conditions). This yielded a thick glue-like substance. In air, the

crude product was washed with cold methanol (2 × 5 mL), and

dried in vacuo (9 ×10⁻³torr) yielding the solid polymer.

Acknowledgements

The authors wish to acknowledge ﬁnancial support from

NSERC (Canada), Canada Foundation for Innovation, Canada

School of Energy and Environment and GreenCentre Canada.

Thanks are given to Dr. Craig Wheaton for performing elemental

analyses of new compounds, Mr. Tony Montina for expert tech-

nical assistance, GreenCentre Canada for GPC analysis, and the

Mass Spectrometric Facility at the Department of Chemistry,

University of Alberta, for MALDI-TOF characterization of PLA

samples.

X-ray crystallographic studies

X-ray crystallography. A high quality crystal of each com-

pound was coated in Paratone oil and mounted on a glass ﬁber.

Data were collected at low temperature (173 K) with ω and φ

scans on a Bruker Smart Apex II diffractometer using graphite-

monochromated Mo-Kα radiation (λ = 0.71073 Å) and Bruker

SMART software.²⁵Unit cell parameters were calculated and

reﬁned from the full data set. Cell reﬁnement and data reduction

were performed using the Bruker APEX2 and SAINT programs,

respectively.²⁶Reﬂections were scaled and corrected for absorp-

tion effects using SADABS.²⁷The structures were solved by

direct methods with SHELXS²⁸and reﬁned by full-matrix least-

squares techniques against F²using SHELXL.²⁹All non-hydro-

gen atoms were reﬁned anisotropically. All hydrogen atoms

(except the N–H atoms, which were located from the electron

References

‡Due to the chiral mentholate substituents on phosphorus, the terms

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are also denoted as (R,R,R,R) and (R,S,S,R) in-text, where the ﬁrst and

last R’s refer to the stereochemistry of the carbon centre in the OMen

group closest to the phosphorus atom.

3712 | Dalton Trans., 2012, 41, 3701–3713

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Dalton Trans., 2012, 41, 3701–3713 | 3713

Article Doi

Ring-opening polymerisation of rac-lactide mediated by cationic zinc complexes featuring P-stereogenic bisphosphinimine ligands

DOI: 10.1039/c2dt11954d

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Article abstract of DOI:10.1039/c2dt11954d

Full text of DOI:10.1039/c2dt11954d

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