Polymer Chemistry SUPPORTING INFORMATION

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1 Electronic Supplementary Material (ESI) for Polymer Chemistry. This journal is The Royal Society of Chemistry 2015 Polymer Chemistry Thiol-Maleimide Click Chemistry: Evaluating the Influence of Solvent, Initiator, and Thiol on the Reaction Mechanism, Kinetics, and Selectivity. Brian H. Northrop*, Stephen H. Frayne, Umesh Choudhary Wesleyan University, Department of Chemistry Hall-Atwater Laboratories, 52 Lawn Ave., Middletown, CT 06459, USA phone: SUPPORTING INFORMATION (44 pages) I. Ternary Thiol-Maleimide Prodecures and Spectra S2-S7 II. Amine-Maleimide Procedures and Spectra S8-S10 III. Calculation of Nucleophilicity N Indices S11 IV. Model for Deprotonation of 1 and 7 by DMF S12-S14 V. Ion Pair Energies vs. S H Distances S15 VI. Calculated Rate Constants S16-S18 VII. Kinetic Comparison of Initiators in DMF S19 VIII. Stationary Point Coordinates and Free Energies S20-S43 IX. Supporting References S44 S1

2 I. General Methods. Chemicals were purchased from Aldrich, Acros, TCI America, or Cambridge Isotope Labs and used as received. All 1 H NMR spectra were recorded with a Varian Mercury (300 MHz) or Varian Unity Plus (400 MHz) spectrometer using residual solvent as the internal standard. All chemical shifts are quoted using the δ scale and all coupling constants are expressed in Hertz (Hz). General procedure for ternary thiol-maleimide reactions in CDCl 3. Ternary reactions between thiophenol (7), 1-hexanethiol (HT), and N-methyl maleimide (NMM) were carried out in CDCl 3 in the presence of three different initiators: Et 3 N, DBU, or DMPP. In each case 7 (30 mg, 0.27 mmol), HT (32 mg, 0.27 mmol), and NMM (30 mg, 0.27 mmol) were added to a 2-dram vial. To each vial was added 0.2 ml of a stock solution of Et 3 N (0.135 M), DBU (0.135 M or M), or DMPP ( M) in CDCl 3 to initiate the reaction. Each mixture was stirred at ambient temperature for a minimum of 30 minutes and then diluted with additional CDCl 3 and used directly for 1 H NMR spectroscopic analysis to determine the percent formation of thiophenol addition product versus 1-hexanthiol addition product. Spectra of ternary reactions were compared to 1 H NMR spectra of pure thiophenol addition product (Figure S1) and 1-hexanethiol addition product (Figure S2). The chiral methine signals and the N-methyl signals of the two different addition products were well isolated and therefore used to calculate the relative percentages of the two addition products. General procedure for ternary thiol-maleimide reactions in DMF. Ternary reactions of 7, HT, and NMM were similarly run in anhydrous DMF under three different scenarios: initiator free, 0.1 equiv Et 3 N, and 0.1 equiv DBU. Stock solutions of Et 3 N (0.135 M) and DBU (0.135 M) in anhydrous DMF were prepared so that both the initiator and DMF would be introduced at the same time to avoid preemptive DMFcatalyzed initiation of thiol-maleimide reactions prior to addition of the initiator. Equimolar quantities of 1-hexanethiol, thiophenol, and N-methyl maleimide were added to a 2-dram vial followed by the addition of DMF either with or without an initiator as described. The ternary reactions were stirred at ambient temperature for at least 30 minutes to ensure complete reactions. Mixtures were then concentrated under reduced pressure and the resulting residue was placed under high vacuum for 30 minutes. The reaction mixtures were diluted with CDCl 3 and used for 1 H NMR spectroscopic analysis to determine the percent formation of thiophenol addition product versus 1-hexanthiol addition product. S2

3 e,f a d e f d Hc S b O O N a c b b ppm Figure S1. Thiophenol addition product obtained upon reaction of 7 with NMM in CDCl 3 in the presence of 0.1 equiv Et 3 N. a' l k j i h g O S H c' N b' O a' j,k l c' b' g b' h i ppm Figure S2. 1-Hexanethiol (HT) addition product obtained upon reaction of HT with NMM in CDCl 3 in the presence of 0.1 equiv Et 3 N. S3

4 ppm Figure S3. 1 H NMR spectrum following the ternary reaction of 7, HT, and NMM initiated by 0.1 equiv Et 3 N in CDCl ppm Figure S4. 1 H NMR spectrum following the ternary reaction of 7, HT, and NMM initiated by 0.1 equiv DBU in CDCl S4

5 ppm Figure S5. 1 H NMR spectrum following the ternary reaction of 7, HT, and NMM initiated by 0.01 equiv DBU in CDCl ppm Figure S6. 1 H NMR spectrum following the ternary reaction of 7, HT, and NMM initiated by 0.01 equiv DMPP in CDCl 3. S5

6 ppm Figure S7. Ternary reaction between NMM, HT, and 7 in DMF in the absence of an initiator. Reaction conditions: N-methyl maleimide (150 mg, 1.35 mmol), 1-hexanethiol (160 mg, 1.35 mmol), thiophenol (150 mg, 1.35 mmol), and 1.0 ml of anhydrous DMF. Residual DMF is present in the spectrum ppm Figure S8. Ternary reaction between NMM, HT, and 7 in DMF in the presence of 0.1 equiv Et 3 N. Reaction conditions: N-methyl maleimide (150 mg, 1.35 mmol), 1- hexanethiol (160 mg, 1.35 mmol), thiophenol (150 mg 1.35 mmol), and 1.0 ml of the Et 3 N stock solution. S6

7 ppm Figure S9. Ternary reaction between NMM, HT, and 7 in DMF in the presence of 0.1 equiv DBU. Reaction conditions: N-methyl maleimide (30 mg, 0.27 mmol), 1- hexanethiol (32 mg, 0.27 mmol), thiophenol (30 mg 0.27 mmol), and 0.2 ml the DBU stock solution. S7

8 II. Nucleophilic addition of amine bases to NMM in CDCl 3. To a 0.1 M CDCl 3 solution of N-methyl maleimide (50 mg, 0.45 mmol) was added either hexylamine (46 mg, 0.45 mmol), diethylamine (33 mg, 0.45 mmol) or triethylamine (46 mg, 0.45 mmmol) and the reaction mixtures were stirred overnight at ambient temperature. Separately, to a 0.1 CDCl 3 solution of N-methyl maleimide (50 mg, 0.45 mmol) was added triethylamine (46 mg, 0.45 mmmol) and tert-butanol (33 mg, 0.45 mmol) and the reaction mixture stirred overnight at ambient temperature. Each reaction mixture was then diluted with CDCl 3 and used for 1 H NMR spectroscopic analysis to determine the extent of nucleophilic addition of each amine to NMM. Scheme S1. Spectroscopic investigation of the ability of hexylamine, diethylamine, and triethylamine to nucleophilically add to NMM in CHCl 3. NH 2 NH O N ~95% O O O N CHCl K N H N O O N 84% N With and without t-buoh as a proton source N O N O 0% ppm Figure S10. 1 H NMR spectrum of indicating approximately 95% addition of hexylamine to NMM in CDCl 3. S8

9 ppm Figure S11. 1 H NMR spectrum indicating approximately 84% addition diethylamine to NMM in CDCl ppm 1.78 Figure S12. 1 H NMR spectrum indicating no addition of triethylamine to NMM in CDCl S9

10 ppm Figure S13. 1 H NMR spectrum indicating no addition of triethylamine to NMM in CDCl 3 in the presence of 1.0 equiv tert-butanol (a non-nucleophilic proton source). S10

11 III. Calculation of Nucleophilicity N Indices. The nucleophilicity of each thiolate anion was calculated using the formula: N = E HOMO(Nu) (ev) E HOMO(TCE) (ev) (1) where tetracyanoethylene (TCE) was taken as a reference. Nucleophiliciy N indices calculated using equation (1) have shown good general agreement with the nucleophilicity scales developed by Mayr and co workers based on experimentally measured rate constants of various nucleophiles. S1 To keep consistent with all other computational results presented in the current work, E HOMO values for each thiol and TCE were calculated at the M06-2X/6-311G(2D,P)//B3LYP/6-31+G(D) level. Table S1. Calculated values of E HOMO for TCE and thiolates of 1-7 used to calculate nucleophilicity N indices. TCE a E HOMO N (ev) a E HOMO values are given in Hartrees. S11

12 IV. Model for the Deprotonation of 1 and 7 by DMF. As discussed in the text highly polar solvents such as DMF are capable of initiating thiol-michael reactions in the absence of an initiator. In such cases it is DMF itself that deprotonates the thiol to give a reactive thiolate and a solvated proton. The energetics of this process was modeled according to the reaction scheme summarized below: Scheme S2. General model for transfer of a proton from a thiol to a dimer of DMF. (a) Formation of an ion pair: H 3 C H N CH 3 O R S H H CH 3 N O CH 3 H 3 C H 3 C N H O R S H H CH 3 N O CH 3 (b) Formation of isolated thiolate anion and solvated proton: SH R R + + H H 3 C CH O H 3 CH H N N 3 C 3 O H O N H H 3 C O N CH 3 CH H 3 H 3 C S The free energy required to form an ion pair between a given thiol and DMF (Scheme S2a) was calculated by first optimizing a complex between the starting thiol along with two explicit DMF molecules, all in a solvent model for DMF. The suprastructure of the complex was fully conformationally searched using the program Maestro S2 and the global energy minimum was optimized to full convergence using the program Gaussian09. S3 Similarly, the ion pair complex between a thiolate and a proton solvated by two molecules of DMF was conformationally searched and optimized in a solvent model for DMF. Two molecules of DMF were found to be necessary to form a stable ion pair complex. The inclusion of a third molecule of DMF did not influence the free energies calculated. If the three species are not optimized together then a large entropic penalty is incurred upon bring together three separate molecules (the thiol and two equivalents of DMF) into one ion pair complex. It is therefore necessary to optimize all three species together in both their neutral and ion pair arrangements. The model is justified because there should not be an entropic cost associated with bringing molecules of DMF near the thiol when DMF is present throughout as the solvent medium. The free energy required to form a free thiolate anion and a solvated proton in DMF was calculated as shown in Scheme S2b. The primary difference between the two models being that in the ion pair the thiolate anion is stabilized by the nearby protonated DMF dimer whereas in Scheme S2b the thiolate is modeled as infinitely separated from the protonated DMF dimer, though both species are in an implicit solvent model for DMF. S12

Optimized structures and energetics for proton transfer between 1 and DMF to give (a) an ion pair and (b) free ions as described above.

13 (a) ΔH (ion pair) = 9.6 kcal/mol ΔG (ion pair) = 14.4 kcal/mol (b) + + ΔH (free ions) = 15.3 kcal/mol ΔG (free ions) = 19.4 kcal/mol Figure S14. Optimized structures and energetics for proton transfer between 1 and DMF to give (a) an ion pair and (b) free ions as described above. (a) ΔH (ion pair) = 3.5 kcal/mol ΔG (ion pair) = 8.0 kcal/mol (b) + + ΔH (free ions) = 7.0 kcal/mol ΔG (free ions) = 10.6 kcal/mol Figure S15. Optimized structures and energetics for proton transfer between 7 and DMF to give (a) an ion pair and (b) free ions as described above. S13

14 Figure S16. Comparison of the DMF-catalyzed addition of methyl mercaptan (1) to NMM versus the DMF-catalyzed addition of phenylthiol (7) to NMM in the absence of an initiator. The difference in predicted kinetics reflects the difference in the calculated free energy required for DMF to deprotonate 1 versus 7 (Figures S14 and S15 on the previous page). S14

15 V. Ion Pair Free Energies vs. S H Distances. The free energy of forming an ion pair between thiols 1-7 and Et 3 N was found to correlate approximately with the S H hydrogen bond distance in each ion pair (Figure S17). Two notable outliers are methyl mercaptan (1) and cysteine methyl ester (6): the S H distance observed in the ion pair 1 /Et 3 NH + was found to be shorter than the trend (1.98 Å) while the S H distance in the ion pair 6 /Et 3 NH + was found to be longer than the trend (2.03 Å). The likely explanation for methyl mercaptan as an outlier is its size, as 1 represents the smallest thiol possible. Cysteine methyl ester has a notably long S H distance because the sulfur anion is able to hydrogen bond with the NH 2 moiety of 6 as shown in Figure S18 below. With the two outliers the correlation has an R 2 value of 0.816, if thiols 1 and 6 are excluded from the set the correlation increases to R 2 = S H Distance (Å) ΔG of Ion Pair Formation (kcal/mol) 6 Figure S17. General correlation between the free energy of ion pair formation between thiols 1-7 and Et 3 N and the S H hydrogen bond distances in each ion pair Figure S18. Global free energy minimized structure of the 6 /Et 3 NH + ion pair with hydrogen bonding interactions between S H ammonium and S H amide indicated by dotted lines. Distances are given in Å. S15

16 VI. Calculated Rate Constants. Rate constants were calculated at K using the transition state free energies (ΔG ) for every elementary step of each reaction mechanism according to conventional activated complex theory: k (T) = (k B T/h)e (- ΔG /RT). Two general classes of reactions were considered to be barrierless: (i) the association of free thiolate and a protonated base to give an ion pair, and (ii) the association of a protonated base and thiol-maleimide addition product (Scheme S3). Scheme S3. Generic examples of the two classes of reactions that were considered be barrierless in all kinetic models. (i) Barrier ΔG S H Initiator R R S + H Initiator Ion Pair Barrierless Free Ions (ii) R S O O N H Initiator Barrier ΔG Barrierless R S O O N + H Initiator The rate constant for the above barrierless processes was taken to be k = 6.21x10 12 M -1 s -1 (i.e. k B T/h). All other calculated rate constants are summarized in the Tables below. Table S2. Calculated rate constants for all forward and reverse elementary steps of every mechanism discussed in the main text. Process solvent initiator thiol k forward k reverse Formation of a CHCl 3 Et 3 N x x10 12 thiolate/initiator CHCl 3 Et 3 N x x10 12 ion pair CHCl 3 Et 3 N x x10 8 (e.g. 1 Et 3 NH + ) CHCl 3 Et 3 N x x10 11 CHCl 3 Et 3 N x x10 11 CHCl 3 Et 3 N x x10 12 CHCl 3 Et 3 N x x10 11 CHCl 3 EtNH x x10 12 CHCl 3 Et 2 NH x x10 12 CHCl 3 DBU x x10 11 DMF EtNH x x10 11 DMF Et 2 NH x x10 11 DMF Et 3 N x x10 11 DMF DBU x x10 12 DMF DMF x x10 12 DMF DMF x x10 5 S16

17 Separation of CHCl 3 Et 3 N x x10 12 thiolate/initiator into CHCl 3 Et 3 N x x10 12 free ions CHCl 3 Et 3 N x x10 12 (e.g. 1 and Et 3 NH + ) CHCl 3 Et 3 N x x10 12 CHCl 3 Et 3 N x x10 12 CHCl 3 Et 3 N x x10 12 CHCl 3 Et 3 N x x10 12 CHCl 3 EtNH x x10 12 CHCl 3 Et 2 NH x x10 12 CHCl 3 DBU x x10 12 DMF EtNH x x10 12 DMF Et 2 NH x x10 12 DMF Et 3 N x x10 12 DMF DBU x x10 12 DMF DMF x x10 12 DMF DMF x x10 12 Propagation Step CHCl 3 Et 3 N x x10 10 thiolate/initiator ion pair EtSH Et 3 N x x10 9 adding to NMM π-bond DMF Et 3 N x x10 10 (e.g. TS13) CHCl 3 EtNH x x10 9 CHCl 3 Et 2 NH x x10 9 CHCl 3 DBU x x10 9 Chain Transfer Step CHCl 3 Et 3 N x10 7 proton abstraction from EtSH Et 3 N x10 6 the thiol DMF Et 3 N x10 7 (e.g. TS17) CHCl 3 EtNH x10 7 CHCl 3 Et 2 NH x10 6 CHCl 3 DBU x10 7 Chain Transfer Step CHCl 3 Et 3 N x10 8 proton abstraction from EtSH Et 3 N x10 8 initiator conjugate acid DMF Et 3 N x10 9 (e.g. TS15) CHCl 3 EtNH x10 9 CHCl 3 Et 2 NH x10 9 CHCl 3 DBU x10 11 Separation of ammonium CHCl 3 Et 3 N x x10 12 ion from thiol-maleimide EtSH Et 3 N x x10 12 addition product DMF Et 3 N x x10 12 (e.g. 11 Et 3 NH + > CHCl 3 EtNH x x Et 3 NH + ) CHCl 3 Et 2 NH x x10 12 CHCl 3 DBU x x10 12 Propagation Step CHCl x x10 9 free thiolate adding to the EtSH x x10 9 NMM π-bond (e.g. TS8) DMF x x10 10 S17

18 Nucleophilic addition CHCl 3 EtNH x x10 11 of initiator to the π-bond CHCl 3 Et 2 NH 2.80x x10 11 of NMM (e.g. TS19) CHCl 3 Et 3 N 6.92x x10 12 CHCl 3 DBU 1.12x x10 9 CHCl 3 DMPP 7.73x x10 7 EtSH Et 3 N 1.17x x10 11 DMF Et 3 N 3.80x x10 11 Chain transfer step CHCl 3 EtNH x10 3 proton abstraction from 1 CHCl 3 Et 2 NH x10 4 by the zwitterionic CHCl 3 Et 3 N x10 4 intermediate (e.g. TS21) CHCl 3 DBU x10 7 CHCl 3 DMPP x10 5 EtSH Et 3 N x10 4 DMF Et 3 N x10 5 S18

19 VII. Kinetic Comparison of Initiators in DMF. Figure S19. Plot of alkene conversion versus time for the addition of methyl mercaptan (1) to NMM as catalyzed by ethylamine (green trace), diethylaine (red trace), triethylamine (blue trace), and DBU (orange trace) in DMF. In each case the only pathway available for initial thiolate formation is through the direct deprotonation of 1 by each N-centered base. Kinetic modeling indicates that, in DMF, all four N-centered bases are able to directly deprotonate enough of 1 to initiate the catalytic thiol-maleimide cycle. Inclusion of the ion pair addition, nucleophile-initiated, and DMF-catalyzed pathways would further increase the rate of alkene conversion for each initiator. S19

20 VIII. Stationary Point Coordinates and Free Energies. Stationary points were optimized and frequencies calculated at the B3LYP/6-31+G(D) level of theory while single point electronic energies were calculated at the M06-2X/6-311G(2D,P) level on B3LYP-optimized geometries as described in the text. Geometries of all stationary points are given in Cartesian coordinates below. Atoms are given either by atomic number of atomic symbol. Free energies are given in Hartrees. For structures that were optimized in multiple solvents the free energy of each optimized structure in each solvent is given. N-methyl maleimide (NMM) C C C N H O O C H H H C H G CHCl3 : G EtSH : G DMF : Methyl mercaptan (1) C H H H S H G CHCl3 : G EtSH : G DMF : Methane thiolate (1 ) C H H H S G CHCl3 : G EtSH : G DMF : TS8 C C C N H O O C H H H C H S C H H H G CHCl3 : G EtSH : G DMF : C C C N H O O C H H H C H S C H H H G CHCl3 : G EtSH : G DMF : TS10 C C C N H O O C S20

21 H H H C H S C H H H S C H H H H G CHCl3 : G EtSH : G DMF : C C C N H O O C H H H C H S C H H H H G CHCl3 : G EtSH : G DMF : Et 3 N C C N C C C C H H H H H H H H H H H H H H H G CHCl3 : G EtSH : G DMF : Et 3 NH + C C N C C C C H H H H H H H H H H H H H H H H G CHCl3 : G EtSH : G DMF : TS12 S C H H H N C H H C H H C H H H C H S21

22 H H C H H H C H H H G CHCl3 : G EtSH : G DMF : /Et 3 NH + ion pair C C N C C C C H H H H H H H H H H H H H H H H S C H H H G CHCl3 : G EtSH : G DMF : TS13 C C C N H O O C H H H C H S C H H H N C H H H C H H H C H H C H H H C H H C H H H G CHCl3 : G EtSH : G DMF : C C C N H O O C H H H C H S C H H H N C H H H C H H S22

23 H C H H C H H H C H H C H H H G CHCl3 : G EtSH : G DMF : TS15 C O C H H H C C H H N C H H C H H H C H H C H H C H H H C H H H S C H H H H C N O G CHCl3 : G EtSH : G DMF : TS16 C O C H H H C C H H N C H H C H H H C H H C H H C H H H C H H H S C H H H H C N O G CHCl3 : TS17 C C C N H O O C H S23

24 H H C H S C H H H N C H H H C H H H C H H C H H H C H H C H H H H S C H H H G CHCl3 : G EtSH : G DMF : Product 11 Et 3 NH + C C C N H O O C H H H C H S C H H H N C H H H C H H H C H H C H H H C H H C H H H H G CHCl3 : G EtSH : G DMF : TS19 C C C N O O C H H H C H H N C H H C H H H C H H C H H H C H S24

25 C H H H H G CHCl3 : G EtSH : G DMF : C C C N O O C H H H C H H N C H H C H H H C H H C H H H C H C H H H H G CHCl3 : G EtSH : G DMF : TS21 C C C N O O C H H H C H H N C H H C H H H C H H C H H H C H C H H H H H S C H H H G CHCl3 : G EtSH : G DMF : C C C N H O O C H H H C H C H H H N C H H H C H S25

26 H C H H H C H H C H H H G CHCl3 : G EtSH : G DMF : EtNH 2 C H H N H H C H H H G CHCl3 : G DMF : EtNH 2 C H H N H H C H H H H G CHCl3 : G DMF : TS12 between EtNH 2 and 1 N H H C H H C H H H S C H H H H G CHCl3 : G DMF : /EtNH + 3 ion pair N C C H H H H H H S C H H H H H G CHCl3 : G DMF : Et 2 NH C H H N H C H H H C H H C H H H G CHCl3 : G DMF : Et 2 NH 2 C H H N H C H H H C S26

27 H H C H H H H G CHCl3 : G DMF : TS12 between Et 2 NH and 1 N C H H C H H H C H H C H H H H S C H H H H G CHCl3 : G DMF : /Et 2 NH 2+ ion pair N C C C C H H H H H H H H H H H S C H H H H G CHCl3 : G DMF : DBU N C N C C C C C C C C H H H H H H H H H H H H H H H H G CHCl3 : G DMF : DBUH + N C N C C C C C C C C H H H H H H H H H H H H H S27

28 H H H H G CHCl3 : G DMF : TS12 between DBH and 1 C C C C N C C C N C C H H H H H H H H H H H H H H H H H S C H H H G CHCl3 : G DMF : /DBUH + ion pair N C N C C C C C C C C H H H H H H H H H H H H H H H H H S C H H H G CHCl3 : G DMF : DMPP C H H H C H H H C C C C H C H C H H H P G CHCl3 : G DMF : DMPPH + C H H H C H H H C C C C S28

The Thiol-Michael Reaction: From First Principles to Applications in Macromolecular Synthesis

Wesleyan University The Thiol-Michael Reaction: From First Principles to Applications in Macromolecular Synthesis by Stephen Hale Frayne A dissertation submitted to the Faculty of Wesleyan University in