On the Search for NNO-Donor Enantiopure Scorpionate Ligands and Their Coordination to Group 4 Metals

Abstract

The preparation of new chiral bis(pyrazol-1-yl)methane-based NNO-donor scorpionate ligands in the form of the lithium derivatives [Li(bpzb)(THF)] [1; bpzb = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide] and [Li(bpzte)(THF)] [2; bpzte = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-p-tolylethoxide] or the alcohol ligands (bpzbH) (3) and (bpzteH) (4) has been carried out by 1,2-addition reactions with trimethylacetaldehyde or p-tolualdehyde. The separation of a racemic mixture of the alcohol ligand 3 has been achieved and gave an enantiopure NNO alcohol-scorpionate ligand in three synthetic steps: (i) 1,2-addition of the appropriate lithium derivative to trimethylacetaldehyde, (ii) esterification and separation of diastereoisomers 5, (iii) saponification. Subsequently, the enantiopure scorpionate ligand (R,R)-bpzmmH {6; R,R-bpzmmH = (1R)-1-[(1R)-6,6-dimethylbicyclo[3.1.1]2-hepten-2-yl]-2,2-bis(3,5-dimethylpyrazol-1-yl)ethanol} was obtained with an excellent diastereomeric excess (>99% de) in a one-pot process utilizing the aldehyde (1R)-(-)-myrtenal as a chiral substrate to control the stereochemistry of the newly created asymmetric center. These new chiral heteroscorpionate ligands reacted with [MX(4)] (M = Ti, Zr; X = NMe(2), O(i)Pr, OEt, O(t)Bu) in a 1:1 molar ratio in toluene to give, after the appropriate workup, the complexes [MX(3)(kappa(3)-NNO)] (7-18). The reaction of Me(3)SiCl with [Ti(NMe(2))(3)(bpzb)] (7) or [Ti(NMe(2))(3)(R,R-bpzmm)] (11) in different molar ratios gave the halide-amide-containing complexes [TiCl(NMe(2))(2)(kappa(3)-NNO)] (19 and 20) and [TiCl(2)(NMe(2))(kappa(3)-NNO)] (21 and 22) and the halide complex [TiCl(3)(kappa(3)-NNO)] (23 and 24). The latter complexes can also be obtained by reaction of the lithium compound 1 with TiCl(4)(THF)(2) and deprotonation of the alcohol group of 6 with NaH, followed by reaction with TiCl(4)(THF)(2) in a 1:1 molar ratio, respectively. Isolation of only one of the three possible diastereoisomers of the complexes 19 and 22 revealed that chiral induction from the ligand to the titanium center took place. The structures of these complexes were elucidated by (1)H and (13)C{(1)H} NMR spectroscopy, and the X-ray crystal structures of 3-7, 12, and 24 were also established. Finally, we evaluated the influence that the chiral center of the new heteroscorpionate complexes has on the enantioselectivity of the asymmetric epoxidation of allylic alcohols.

Coordination from Heteroscorpionate Ligand Towards Pd(II) via Pd⋅⋅⋅Hδ−C(sp3) Interaction: Structural and Catalytic Studies

“…Complexes 2 and 3 catalyzed the coupling of aryl bromides (Table 3, entries 5-14), with conversions from 70 to 91 % having phenyl boronic acid as coupling partner. The coupling of electron-poor substrates (4-bromobenzaldehyde, 4-bromoacetophenone and 4-nitrobromobenzene) with phenyl boronic acid delivered yields from 80 to 91 % (Table 3, entries 5-10) and with electron-rich substrates (4-bromoanisole and 4-bromobenzene) the cross-coupling product reached 70-82 % conversions (Table 3, entries [11][12][13][14].…”

“…Subsequently, the effect of the alkene as substrate in the Heck reactions was also examined using less activated olefin such as styrene. The Heck coupling was performed with a series of aryl bromides and styrene (Table 5, entries [10][11][12][13][14]. In all cases, the reaction proved regioselective, yielding the psubstituted stilbene product and only traces of the corresponding gem-isomer were detected in less than 5 % detected by NMR spectroscopy.…”

“…The results show that the formation of the coupling product depends slightly on the nature of the substituent on the aryl bromides. For instance, aryl bromides with electron withdrawing substituents have been coupled in good yields in comparison to electron donating groups (Table 5, entries [12][13][14]. On the other hand, electron-rich alkenes as substituent are considered more difficult than alkenes with electron deficient substituent analogues for Mizoroki-Heck cross-coupling, showing that the complex 3 is active and tolerate deactivated olefin generating values of TON (182) with a catalyst load of 0.5 mol %.…”

“…Compound 1 was prepared as reported and was used without a previous purification. [12] Characterization of 1. White solid.…”

“…[3][4][5][6][7][8][9][10] Bis(pyrazol-1-yl)methane-based NNO-donor scorpionate ligands are widely desired due their ability to coordinate different metal centers via two coordination mode, with or without the deprotonation of the OH group; a) where the two nitrogen and the oxygen atoms link the metal center via a coordination bonds and b) where the metal center is coordinatively binded to two nitrogen and covalently bound to the oxygen atom (Figure 1). [11][12][13][14][15][16][17][18][19][20] On the other hand, intramolecular CÀ H•••M agostic or anagostic interactions were described as responsible for favoring the coordination chemistry of CÀ H bonds. This important interaction in organometallic chemistry can stabilize complexes that are implicated in several catalytic reactions and represent an opportunity to gain insight into transition-metalmediated CÀ H bond activation reactions.…”