Chapter 2. High Oxidation State Mo-NHC Alkylidene Complexes: Functional-Group
2.2 Result and Discussion
2.2.3 Reactivity of molybdenum imido alkylidene NHC complexes during
that this catalyst, in contrast to 4 and 7, contains a small fraction of the anti-alkylidene.
If these systems behave like existing Schrock catalysts (Scheme 21), then an involvement of the anti-isomer in conjunction with a fast syn–anti interconversion should give rise to high trans contents.[38] As outlined by Schrock et.al., a fast syn–anti interconversion requires the presence of electron-donating anionic ligands, for example, alkoxides. In fact, NHCs are strongly electron-donating and thus fulfill this requirement. However, at this point, this explanation is only a reasonable hypothesis and the rates of interconversion need to be measured by photolysis experiment. In due course, theoretical studies of these novel Mo-imido alkylidene NHC complexes during polymerization might provide a more accurate scenario of the active species in terms of energy profiles.
2.2.3 Reactivity of molybdenum imido alkylidene NHC complexes during
Table 5. Homo- and cross-metathesis results with 3.
Substrates Solvent T [°C]/t [h] TON E/Z allyltrimethylsilane CH2Cl2 rt, 4 h 0 - allyltrimethylsilane C2H4Cl2 80 °C, 4 h 500 72/28
1-hexene CH2Cl2 rt, 4 h 20 80/20
1-hexene C2H4Cl2 80 °C, 4 h 200 80/20 allyltrimethylsilane
+1-hexene C2H4Cl2 80 °C, 4 h 500 85/15
Catalyst:substrate = 1:500 (mol/mol), internal standard for GC-MS = n-dodecane. rt = room temperature.
Figure 44. Substrates for RCM, HM, CM and ethenolysis.
Table 6. Productivity of catalysts 5-27 in RCM[a].
Cat. diethyl diallylmalonate
diallyl diphenylsilane 1,7-octadiene
N,N-diallyl-p-tosylamide
diallylmalodinitrile diallyl ether
5 175 620 140 180 190 220
6 90 490 920 160 70 245
7 3,200[b] 390 4,100[b] 420 360 690
10 500[b] 580 3,300[b] 365 100 850
11 65 210 350 110 65 85
14 0 0 80,000[c]
100,000[d]
- - -
16 700 960 1000
87,100[e]
700 390 600
18 80 530 230 0 0 250
19 130 440 130 80 0 160
20 50 240 120 60 85 70
21 0 130 160 0 0 55
22 0 770 790 160 70 860
24 0 231 213 159 - 177
25 - 450 85 - - -
26 - 70 150 - - -
27 155 170 40 - - 200
[a] Condition unless otherwise stated: ClCH2CH2Cl, 80 °C, 4 h, catalyst:substrate = 1:1000 (mol/mol); [b] ClCH2CH2Cl, 80 °C, 4 h, catalyst:substrate = 1:5,000 (mol/mol);
[c] ClCH2CH2Cl, room temperature, overnight, catalyst:substrate = 1:100,000 (mol/mol); [c] ClCH2CH2Cl, room temperature, 1 h, catalyst:substrate = 1:100,000 (mol/mol); [d] ClCH2CH2Cl, room temperature, 1 h, catalyst:substrate = 1:500,000(mol/mol); [e] ClCH2CH2Cl, 80 °C, 4 h, catalyst:substrate = 1:140,000 (mol/mol); internal standard for GC-MS = n-dodecane.
Among all catalysts investigated, complexes 7, 10, 14, 15, 16 and to some extent 22 stand out in productivity (TON). While 14, 15, 16 and 17 are cationic complexes, but 7, 10, 11, 12, 13, 22 and 26 have one electron-withdrawing group and one triflate bound to molybdenum. Notably, as outlined in Table 6 and 7, all catalysts show distinct differences in HM and RCM of the chosen substrates, whereas the cationic catalysts
14, 15 and 16 exhibits outstanding activity for pure hydrocarbons. All together, this supports our proposed mechanism and finding that Mo-imido alkylidene NHC complexes must either be cationic to be highly active in olefin metathesis reactions or have one very good leaving group such as triflate. Only in this case, a cationic complex can form in the presence of substrate, i.e. of an olefin (vide Scheme 20).
TON in the HM of 1-hexene, styrene and 1-octene were in the range of 45,000-210,000. Similarly, the TON of 14 for the RCM of 1,7-octadiene was 100,000. The cationic complex 15 also deserves special attention. It allows for turnover numbers of 192,000 (95% E) in the homometathesis (HM) of 1-octene (room temperature, 1,2-dichlorethane) using a ratio of 15:1-octene = 1:680,000. At 80 °C, a TON of 323,000 (93% E) is achieved even at a lower ratio of 15:octene = 1:500,000. In the HM of 1-nonene, a TON of 352,000 (92% E) was achieved at room temperature (1,2-dichlorethane) using a ratio of 15:1-nonene of 1:570,000. At 80°C, a TON of 545,000 (90 % E) was achieved using a ratio of 15:1-nonene of 1:1,000,000. Also worth to be mentioned, the cationic complex 16 is also active in the RCM of ester-, silane-, amide-, nitrile- and ether-containing substrates (Table 6). In that regardamide-, 16 strongly differs from 14 and 15 which do not tolerate any functional group and nicely demonstrates the impact of changing one triflate to pentafluorophenolate. Furthermore, the TON of 16 for the RCM of 1,7-octadiene was 87,100.
Table 7. Productivity for catalysts 5, 6, 7, 14-16 during homo-metathesis (HM).
Complex allyltrimethylsilane 1-hexene styrene 1-octene 1-nonene
5 520(60) 340 (100) 60 (100) 680 (85) -
6 435(55) 490 (100) 80 (100) 560 (85) -
7 - 790 (100) 200 (100) - -
14 - 140,000[d]
(100)
45,000[d,e]
(100)
210,000[d]
(86)
-
15 - - - 192,000[f]
(95) 323,000[g]
(93)
352,000[h]
(92) 545,000[i]
(92)
16 800 (88) - - 75,000[j]
(92)
-
[a] Condition unless otherwise stated: ClCH2CH2Cl, 80 °C, 4 h, catalyst:substrate = 1:1000 (mol/mol); [c] ClCH2CH2Cl, room temperature, overnight, catalyst:substrate = 1:100,000 (mol/mol); [c] ClCH2CH2Cl, room temperature, 1 h, catalyst:substrate = 1:100,000 (mol/mol); [d] ClCH2CH2Cl, room temperature, 1 h, catalyst:substrate = 1:500,000(mol/mol); [e] along with approximately 10% 1,3-diphenylprop-1-ene; [f]
ClCH2CH2Cl, room temperature, 4 h, catalyst:substrate = 1:680,000(mol/mol); [g]
ClCH2CH2Cl, 80 °C, 4 h, catalyst:substrate = 1:500,000 (mol/mol); [h] ClCH2CH2Cl, room temperature, 4 h, catalyst:substrate = 1:570,000 (mol/mol); [i] ClCH2CH2Cl, 80
°C, 4 h, catalyst:substrate = 1:1,000,000 (mol/mol). [j] ClCH2CH2Cl, 80 °C, 4 h, catalyst:substrate = 1:140,000 (mol/mol). Values in parentheses refer to the E fraction;
-: reaction was not carried out; internal standard for GC-MS = n-dodecane.
Interestingly, complexes 10 and 11 show different activities, despite comparable pKa
values of the conjugated acids of the anionic ligands (pKa, water, C6F5OH = 5.5; pKa, (CF3)3COH = 5.2). Similar accounts for 11 and 22, which both contain one nonafluoro-tert-butoxide and one triflate ligand but two different imido ligands (2,6-dimethylphenylimido vs. 3,5-dimethylphenylimido). The activities of catalysts 19 and 20, both of them bear a small aryl-imido ligand, were found to be comparable to those of 5 and 6. Surprisingly, catalysts 19 and 20 that contain 3,5-dimethylphenylimido ligand are more reactive than 5 and 6, at least for the homometathesis of methyl oleate. With methyl oleate, 19 allows for turnover numbers of 240 in
1,2-dichloroethane at T = 80 °C for 4 h. All that underlines the importance of subtle changes in catalyst structure on catalytic behavior. In the ethenolysis of cis-cyclooctene with catalyst 14 (COE, Table 8) TONs upto 30,000 were obtained at 30 bar using non-purified ethylene.
Table 8. Ethenolysis of cis-cyclooctene (COE) with catalyst 14.
Pressure [bar] T [°C] Time [h] Conversion [%] TON COE:14
5 rt 3 34 6,800 20,000:1
50 rt 6 15 15,000 100,000:1
30 80 6 30 30,000 100,000:1
Solvent = toluene (ca. 50 mL); internal standard for GC-MS = n-dodecane; catalyst was added in CH2Cl2; ethylene (99%) was used.
Another important finding is that the bis(triflate) complexes were found to be highly temperature resistant. Accordingly, elevated temperature can be used to promote dissociation of one triflate, thereby enhancing catalyst activity. And in fact, complex 19, which is stable in solution at high temperature (Figure 46), shows impressive productivity in case reactions run at 140 °C in 1,2-dichlorobenzene using a catalyst:substrate ratio of 1:20,000 (t = 4 h). TONs for the RCM of diallyldiphenylsilane, diethyl diallylmalonate and N,N-diallyl-p-toluolsulfonamide increased from 440, 130 and 80 to 9600, 1600 and 4200, respectively. High productivity was also found in the self-metathesis (SM) of methyl oleate (TON = 2,500) (Table 9).
Next, correlated the coalescence temperature, Tc, with catalyst activity. If the proposal that the five-coordinate complexes undergo Berry-type pseudorotation is valid, this interconversion, which passes through an SP configuration or vice versa, must result in chemically and magnetically equivalent triflate groups. This results in the coalescence of the parent two signals for the triflates. Consequently, Tc should be indicative for the temperature at which a bistriflate complex becomes active. Table 10 summarizes the results in selected RCM reactions obtained at room temperature for complexes 18, 19 and 20, covering Tc values between -3 and 130 °C. Clearly, 19 having the lowest Tc value shows the highest productivity while 18 with highest Tc
value remains inactive in RCM or delivers much lower TONs than 19 and 20 do.
Figure 45. VT 1H-NMR spectra (400 MHz, 1,2-dichlorobenzene-d4) of 6.
Figure 46. VT 1H-NMR spectra (400 MHz, 1,2-dichlorobenzene-d4) of 19.
Table 9. Productivity of catalysts 5, 6 and 19 in RCM at high temperature[a].
[a]Condition unless otherwise stated: 1,2-dichlorobenzene, 140 °C, 4 h, catalyst:substrate = 1:20,000 (mol/mol); internal standard for GC-MS = n-dodecane.
From the catalytic data presented here, one can conclude that molybdenum imido alkylidene NHC complexes are indeed activated through release of one anionic ligand, thereby forming a cationic complex. Consequently, to achieve high productivity, they need to bear one good leaving group, e.g., a triflate and one strongly electron-withdrawing anionic ligand, e.g., -OC6F5. Alternatively, they can be prepared in their cationic form with one strongly electron-withdrawing anionic ligand at molybdenum as realized in 16. This proposal fits the high productivities of complexes 7, 10, 11, 14-16 and 22 but also the low productivities of complexes 3, 4, 5, 6, 18, 19, 25. Although complexes 19 and 20 showed better activity than 5 and 6 in most cases. The reason might be combined electronic and steric effects of the 3,5-dimethylimido ligand, which is also reflected by the 19F-NMR spectra of these complexes. At room temperature only one signal is observed.
substrate catalysts time [h] TON
diallyl diphenylsilane 5 4 10,500
diallyl diphenylsilane 6 4 8,500
diallyl diphenylsilane 19 4 9,600
diethyl diallylmalonate 5 4 1,500
diethyl diallylmalonate 6 4 1,200
diethyl diallylmalonate 19 4 1,600
methyl oleate 5 4 0
methyl oleate 6 4 0
methyl oleate 19 4 2,500
diallylamine 5 4 0
diallylamine 6 4 0
diallylamine 19 4 0
N,N-diallyl-p-toluolsulfonamide 5 4 900
N,N-diallyl-p-toluolsulfonamide 6 4 1,230
N,N-diallyl-p-toluolsulfonamide 19 4 4,200
2-allylpent-4-enoic acid 5 4 0
2-allylpent-4-enoic acid 6 4 0
2-allylpent-4-enoic acid 19 4 0
Table 10. Productivity of catalysts 18, 19 and 20 in RCM at room temperature[a].
# 18 19 20
Tc (°C) 130 -3 60
diethyl diallylmalonate 0 300 120 diallyl diphenylsilane 90 450 100
1,7-octadiene 90 80 75
N,N-diallyl-p-tosylamide 0 150 0
diallylmalodinitrile 0 60 0
diallyl ether 0 160 45
[a] 1,2-dichloroethane, room temperature, 4 h, catalyst:substrate = 1:1,000(mol/mol).
Finally, a comparison of the Z-selectivity of catalysts 5, 11, 19-22 provides an insight into the structure of the transition state. Clearly, catalysts 19 and 22 bearing the “small”
3,5-dimethylphenylimido-ligand displayed higher Z-selectivity in CM than catalysts 5, 10, 11, 16 and 18 based on the 2,6-dimethylphenylimido-ligand (37 and 34 vs. 12- 28% Z, respectively, Table 11). A comparable Z-selectivity is also obtained with catalyst 19 (32% Z) also bearing the 3,5-dimethylphenylimido-ligand. The highest Z-value (40%) obtained so far was realized with catalyst 21 bearing the 3,5-dimethylphenylimido and the large “terphenoxide” ligand.
Table 11. Productivity of catalysts in the CM of 1-hexene with 1-dodecene.
Catalyst 5 10 11 16 18 19 20 21 22
TON 150 1,060 820 1,200 430 320 260 1,100 820
Z/E 15/85 18/82 12/88 18/82 28/72 37/63 32/68 40/60 34/66
1,2-dichloroethane, T = 80 °C, t = 4 h, catalyst:substrate = 1:2,000.
This strongly points towards a TBP transition state in which the metallacyclobutane is trans to the NHC (Figure 47). Such proposal is also in line with calculations on Mo- imido alkylidenes, which suggest approach of the olefin and formation of the molybdacyclobutane trans to the strongest σ-donor,[39-41] which is here the NHC. In that regard, our novel Mo-imido alkylidene NHC complexes behave similar to the MAP
catalysts[15,35,42-45] published by the Schrock group. Clearly, further ligand tuning is required to push the Z-content up to > 90%.
Figure 47. TBP molybdacyclobutanes with all substituents pointing towards the smaller imido groups. X = -OTf, -O-2,6-Ph2-C6H3O, -OC(CF3)3; R = Mes.