Magnesium and zinc complexes of functionalised, saturated N-heterocyclic carbene ligands: carbene lability and functionalisation, and lactide polymerisation catalysis

Arnold, Polly L.; Casely, Ian J.; Turner, Zoë R.; Bellabarba, Ronan M.; Tooze, Robert B.

doi:10.1039/b907034f

Cited by 117 publications

(85 citation statements)

References 67 publications

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“…In order to confirm the above and prove that living polymerization is possible with Me 2 GaOR(IMes), the following experiment was performed. First, the sequential polymerization of 25 equiv of rac-LA at r.t with 5 was performed in order to synthesize Me 2 GaO-(PLA) 25 (S)-melac(IMes), including short PLA chains instead of (S)-melac ligand. Subsequent polymerization of 25 equiv, 75 equiv, or 275 equiv of rac-LA at −20°C was performed (Table 3, entries 23−25) and revealed in each case PLA with monomodal weight distribution and M n close to the expected value calculated from the rac-LA/catalyst ratio.…”

Section: ■ Introductionmentioning

confidence: 99%

Dialkylgallium Alkoxides Stabilized with N-Heterocyclic Carbenes: Opportunities and Limitations for the Controlled and Stereoselective Polymerization of rac-Lactide

et al. 2015

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The structure of a series of Me 2 GaOR(NHC) complexes with N-heterocyclic carbenes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes) and 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes)) have been characterized using spectroscopic and X-ray techniques and discussed in view of their reactivity in the polymerization of rac-lactide (rac-LA). Both structure studies and density functional theory (DFT) calculations show the significant influence of NHC and OR on the structure of investigated complexes and has indicated that the Ga−C NHC bond (32.6− 39.6 kcal mol −1 ) is strong enough to form stable Me 2 GaOR-(NHC) complexes in the form of monomeric species. The reactivity of Me 2 Ga((S)-OCH(Me)CO 2 Me)(SIMes) (1) and Me 2 Ga((S)-OCH(Me)CO 2 Me)(IMes) (5) toward Lewis acids such as CO 2 and GaMe 3 has resulted in breaking of the Ga−C NHC bond with the formation of (NHC)CO 2 and Me 3 Ga(NHC) (8 and 10) and [Me 2 Ga(μ-(S)-OCH(Me)CO 2 Me)] 2 . Different results have been obtained for l,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene (SIPr), which coordinates more weakly to gallium, as demonstrated by the Ga−C NHC bond strength for model Me 3 GaSIMes, Me 3 GaIMes (8), and Me 3 GaSIPr (10) adducts. The reaction of SIPr with [Me 2 Ga(μ-OR)] 2 has not allowed for the breaking of Ga 2 O 2 bridges and the formation of monomeric Me 2 GaOR(SIPr) complexes, contrary to SIMes and IMes. In the case of the reaction with [Me 2 Ga(μ-(S)-OCH(Me)CO 2 Me)] 2 , the ionic compound [Me 2 Ga(OCH(Me)-CO 2 )] − [SIPrH] + (9) has been isolated. The investigated Me 2 GaOR(NHC) complexes are highly active and stereoselective in the ring-opening polymerization of rac-lactide from −20°C to room temperature, due to the insertion of rac-LA exclusively into the Ga−O alkoxide bond, leading to isotactically enriched polylactide (PLA) (P m = 0.65−0.78). It has been shown that the polymerization of lactide at low temperature is influenced by the chelate interaction of (S)-OCH(Me)CO 2 Me or (OCH(Me)C(O)) 2 OR resulting from the primary insertion of rac-LA into the Ga−O alkoxide bond, with the Ga center, which can be responsible for the low control over the molecular weight of the obtained PLA. The latter effect can be eliminated by the initial synthesis of Me 2 Ga((PLA) n OR)(NHC) with short PLA chains, which allows for controlled polymerization. Although the adverse chelate effect can be also eliminated by the polymerization of rac-LA at room temperature, the stereoselectivity of rac-LA polymerization is strongly affected by transesterification reactions. Out of investigated Me 2 GaOR(SIMes) and Me 2 GaOR(IMes) complexes, only the latter allowed for the immortal ring opening polymerization of rac-LA in the presence of i PrOH.

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Section: ■ Introductionmentioning

confidence: 99%

Dialkylgallium Alkoxides Stabilized with N-Heterocyclic Carbenes: Opportunities and Limitations for the Controlled and Stereoselective Polymerization of rac-Lactide

et al. 2015

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“…On the other hand, rac-LA could insert, even at low temperature, into In-CNHC bond of Me2GaOR(NHC), considerably weaker in comparison with Ga-CNHC [3]. Analogously, the insertion of lactide into the M-CNHC bond of Mg alkoxides with alkoxide ligands possessing NHC termini [16], Me3Al(NHC) adducts [21], or yttrium and titanium complexes [22], have been reported so far.…”

Section: Introductionmentioning

confidence: 99%

“…While in the case of corresponding gallium complexes, a relatively strong Ga-C NHC bond led to stable complexes [1,2], in the case of mostly unstable dialkylindium alkoxides with NHCs, a significantly weaker In-C NHC bond was observed [3]. Consequently, in the ring-opening polymerization with Me 2 GaOR(NHC), Ga-C NHC bond remained intact, resulting in the exclusive insertion of rac-LA into Ga-O alkoxide bond, similar to Zn [14][15][16], Al [17], as well as Ti and Zr [18][19][20], metal alkoxides with NHCs, which have been reported to polymerize rac-LA due to the insertion into [17], as well as Ti and Zr [18][19][20], metal alkoxides with NHCs, which have been reported to polymerize rac-LA due to the insertion into M-Oalkoxide bond. On the other hand, rac-LA could insert, even at low temperature, into In-CNHC bond of Me2GaOR(NHC), considerably weaker in comparison with Ga-CNHC [3].…”

Section: Introductionmentioning

confidence: 99%

Probing the Effect of Six-Membered N-Heterocyclic Carbene—6-Mes—on the Synthesis, Structure and Reactivity of Me2MOR(NHC) (M = Ga, In) Complexes

et al. 2018

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Abstract:The investigation of the reactivity of six membered N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-1-ylidene (6-Mes) towards dialkylgallium and dialkylindium alkoxides/aryloxides has shown that both steric hindrances and donor properties of 6-Mes significantly influence the strength of M-C 6-Mes bond, as well as the formation, structure and reactivity of Me 2 MOR(6-Mes) (M = Ga, In) complexes. While the reactions of simple dimethylgallium alkoxides with 6-Mes lead to the formation of stable monomeric Me 2 Ga(OCH 2 CH 2 OMe)(6-Mes) (1) and Me 2 GaOMe(6-Mes) complexes, the analogous Me 2 InOR(6-Mes) are unstable and disproportionate to methylindium alkoxides and Me 3 In(6-Mes) (2). The use of bulky alkoxide ligand-OCPh 2 Me or aryloxide ligand-OC 6 H 4 OMe allowed for the synthesis of stable Me 2 M(OCPh 2 Me)(6-Mes) (M = Ga (3) and In (4)) as well as Me 2 M(OC 6 H 4 OMe)(6-Mes) (M = Ga (5) and In (6)). The structures of 1-6 have been determined using both spectroscopic methods in solution and X-ray diffraction studies, which confirmed the effect of both steric hindrances and donor properties of 6-Mes on their structure and catalytic properties in the ring-opening polymerization (ROP) of rac-lactide.

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“…[17][18][19][20][21][22] NHC-zinc complexes may act as polymerization catalysts. [23][24][25][26] However, all the reported NHC-based zinc compounds are neutral and C2-bound. Considering the unique application of C4-bound NHC [i.e., Scheme 1.…”

mentioning

confidence: 99%

“…The ZnÀC distances in 2·DME [1.994(10)-2.105 (9) ] are comparable to those of typical ZnÀC single bonds in NHCzinc complexes. [23][24][25][26]35] NBO analysis [33] suggests considerable dicationic character of each zinc atom. The zinc natural charges range from + 1.…”

mentioning

confidence: 99%

NHC‐Stabilized Triorganozincates: Syntheses, Structures, and Transformation to Abnormal Carbene–Zinc Complexes

et al. 2012

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The chemistry of organozinc compounds has been inextricably entwined with the development of organic synthetic methods since Franklands seminal discovery of zinc alkyls in 1849. [1,2] Compared to diorganozinc reagents, the corresponding anionic zincate derivatives are considerably better nucleophiles. [3][4][5][6][7][8] Consequently, triorganozincates [R 3 Zn]M (M = alkali metals) and tetraorganozincates [R 4 Zn]M n (M = alkali metals, n = 2; M = alkali earth metals, n = 1) have been extensively utilized in organic transformations such as halogen-metal exchanges, [9][10][11][12] nucleophilic additions, [1,2] deprotonative metalations, [6][7][8] and epoxide ring-openings. [9,10] Recently, Mulvey et al. employed TMP-zincates (TMP = 2,2,6,6-tetramethylpiperidide) in developing a zincationanion trapping strategy [13] and in realizing the unusual transformation of a diamine into an unsaturated diazaethene through a lithium/zinc bimetallic system. [14,15] The advantages of organozincates may be attributed to their intrinsic "bimetallic" character. Indeed, due to their synergic reactivity, bimetallic systems often outperform their monometallic components and have shown greater potential in organic transformations. [6] Lithium zincate investigations reveal that not only the Li/Zn ratio [3,4,9] but also the substituents [5] may have remarkable effects on their reactivity. Thus, the TMP ligand has contributed to the renaissance of metallation chemistry involving organozincate compounds. [6][7][8] We show here that extensions of the ligand systems can result in organozincates with novel reactivity.While N-heterocyclic carbenes (NHCs) have become ubiquitous ligands for organic [16] and transition-metal [16] catalysis, they are also capable of stabilizing highly reactive main group molecules. [17][18][19][20][21][22] NHC-zinc complexes may act as polymerization catalysts. [23][24][25][26] However, all the reported NHC-based zinc compounds are neutral and C2-bound. Considering the unique application of C4-bound NHC [i.e., Scheme 1. Synthesis of NHC-stabilized triorganozincates 2 and 3.

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Magnesium and zinc complexes of functionalised, saturated N-heterocyclic carbene ligands: carbene lability and functionalisation, and lactide polymerisation catalysis

Cited by 117 publications

References 67 publications

Dialkylgallium Alkoxides Stabilized with N-Heterocyclic Carbenes: Opportunities and Limitations for the Controlled and Stereoselective Polymerization of rac-Lactide

Dialkylgallium Alkoxides Stabilized with N-Heterocyclic Carbenes: Opportunities and Limitations for the Controlled and Stereoselective Polymerization of rac-Lactide

Probing the Effect of Six-Membered N-Heterocyclic Carbene—6-Mes—on the Synthesis, Structure and Reactivity of Me2MOR(NHC) (M = Ga, In) Complexes

NHC‐Stabilized Triorganozincates: Syntheses, Structures, and Transformation to Abnormal Carbene–Zinc Complexes

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