TF-BiphamPhos Catalyzed Reactions of Alkylidene

Article pubs.acs.org/Organometallics

Cu(I)/TF-BiphamPhos Catalyzed Reactions of Alkylidene Bisphosphates and Alkylidene Malonates with Azomethine Ylides: Michael Addition versus 1,3-Dipolar Cycloaddition Meiyan Wang,† Chun-Jiang Wang,*,‡ and Zhenyang Lin*,† †

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China ‡ College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, People's Republic of China S Supporting Information *

ABSTRACT: Cu(I)/TF-BiphamPhos catalyzed reactions of alkylidene bisphosphates and alkylidene malonates with azomethine ylides have been investigated with the aid of density functional theory calculations at the B3LYP level. Michael addition and 1,3-dipolar cycloaddition were calculated. For reactions of alkylidene bisphosphates, the Michael addition pathway is both kinetically and thermodynamically more favorable than 1,3-dipolar cycloaddition. However, for reactions of alkylidene malonates, the 1,3-dipolar cycloaddition pathway is kinetically and thermodynamically more favorable than Michael addition. In the reactions of alkylidene bisphosphates, the significant repulsion between the two bulky phosphonate groups of the alkylidene bisphosphates and the phenyl substituent of the azomethine ylides suppresses 1,3-dipolar cycloaddition and favors Michael addition. In the reactions of alkylidene malonates the less bulky ester groups in the alkylidene malonates allow 1,3-dipolar cycloaddition to occur.

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INTRODUCTION Asymmetric carbon−carbon bond formation is one of the most important reactions in organic synthesis, providing synthetically and biologically significant chiral compounds. The catalytic asymmetric reactions of activated alkenes with carbon-centered nucleophiles are one practical route to obtain chiral compounds.1 The azomethine ylides have been usually used as the carbon-centered nucleophiles. For the asymmetric reactions of activated alkenes with azomethine ylides, two types of reactions, Michael addition and 1,3-dipolar cycloaddition, are possible depending on the catalyst system used. The Michael addition of activated alkenes with azomethine ylides2 provides a facile access to the unnatural α-amino acid derivatives, which are useful compounds in drug synthesis.3 The 1,3-dipolar cycloaddition of activated alkenes with azomethine ylides2a,4 is one of the most powerful and atom-economic strategies to stereoselectively construct five-membered nitrogen heterocycles which are useful reagents in organic synthesis. Recently, Wang et al. reported a new family of chiral TFBiphamPhos ligands for the transition-metal-catalyzed asymmetric reactions of various activated alkenes with azomethine ylides.5 Using the chiral TF-BiphamPhos ligands, different activated alkenes react with the same azomethine ylides, giving different products, Michael addition products or 1,3-dipolar cycloaddition products. The Cu(I)/TF-BiphamPhos catalyzed Michael addition of alkylidene bisphosphates with azomethine ylides provides unnatural α-amino acid derivatives in good yields with excellent diastereoselectivity and enantioselectivity5b (Scheme 1). However, the 1,3-dipolar cycloaddition of © 2012 American Chemical Society

alkylidene malonates with azomethine ylides catalyzed by Cu(I)/TF-BiphamPhos (also Ag(I)/TF-BiphamPhos) gives five-membered nitrogen heterocycles (Scheme 1),5c although the Michael adducts are the common products of most asymmetric reactions involving alkylidene malonates.6 It is interesting that in the reaction of alkylidene bisphosphates Michael addition takes place, while in the reaction of alkylidene malonates 1,3-dipolar cycloaddition occurs. In this paper, we will report our theoretical calculations to understand the reaction mechanisms and to delineate the factors that influence the reaction pathways. We hope to compare theoretically the reactions of the two reagents, alkylidene bisphosphates and alkylidene malonates, to explain why alkylidene bisphosphates undergo Michael addition while alkylidene malonates undergo 1,3-dipolar cycloaddition.

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COMPUTATIONAL DETAILS

Molecular geometries of the model complexes were optimized without constraints via DFT calculations using the Becke3LYP (B3LYP)7 functional. The 6-31G** basis set was used for the N, O, and P atoms coordinated to the Cu metal center, the P and O atoms of the PO groups, and the C and H atoms involved in bond-forming and -breaking processes. The 6-311G* Wachters-Hay basis set8 was used for Cu. For all other atoms, the 6-31G basis set was used. Frequency calculations were carried out at the same level of theory to identify all of the stationary points as minima (zero imaginary frequencies) or Special Issue: Copper Organometallic Chemistry Received: May 18, 2012 Published: August 31, 2012 7870

dx.doi.org/10.1021/om300435s | Organometallics 2012, 31, 7870−7876

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Scheme 1

transition states (one imaginary frequency) and to provide enthalpies at 298.15 K that include zero-point energy corrections. Intrinsic reaction coordinates (IRC)9 were calculated for the transition states to confirm that such structures indeed connect two relevant minima. To examine the basis set dependence, we employed a much larger basis set to carry out single-point energy calculations for several selected structures. In the larger basis set, 6-311G** was used for those atoms directly bonded to the metal center, the P and O atoms of the PO groups, and those C and H atoms involved in bond-forming and -breaking, 6-311G* for Cu, and 6-31G** for all other atoms. We found that the basis set dependence is not significant, in particular when we consider the relative reaction barriers. For example, using the smaller basis set, the enthalpies of TSMichael‑addition and TScycloaddition relative to the intermediate 2 were calculated to be 14.7 and 8.9 kcal/ mol, respectively, while using the larger basis set the relative enthalpies were calculated to be 17.3 and 11.5 kcal/mol, respectively. Similarly, using the smaller basis set, the enthalpies of TSMA‑Michael‑addition and TSMA‑cycloaddition relative to the intermediate 2MA were calculated to be 16.9 and 5.5 kcal/mol, respectively, while using the larger basis set the relative enthalpies were calculated to be 18.1 and 7.5 kcal/mol, respectively. We also consider the solvent effects by performing single-point selfconsistent reaction field (SCRF) calculations based on the polarizable continuum model (PCM)10 using UFF radii. Dichloromethane (DCM) was used as the solvent, corresponding to the experimental conditions. The results show that the solvent effect is also not significant. The solvation-corrected enthalpies of TSMichael‑addition and TScycloaddition relative to the intermediate 2 were calculated to be 17.0 and 12.2 kcal/mol, respectively. The solvation-corrected enthalpies of TSMA‑Michael‑addition and TSMA‑cycloaddition relative to the intermediate 2MA were calculated to be 18.2 and 8.2 kcal/mol, respectively. All calculations were performed with the Gaussian 03 software package.11

Scheme 2

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RESULTS AND DISCUSSION In our calculations, N-benzylidene glycine methyl ester (BE) was employed as an azomethine ylide. Tetramethyl alkylidene bisphosphonate (BP) and dimethyl benzylidene malonate (MA) were used as the models for alkylidene bisphosphates and alkylidene malonates, respectively, while the experimental ligand TF-BiphamPhos was used.5b,c For the Michael addition, our calculations were based on the proposed catalytic cycle shown in Scheme 2. A copper(I) complex (1) having (S)-TF-BiphamPhos and a deprotonated N-benzylidene glycine methyl ester as the ligands is the active species, since the deprotonated N-benzylidene glycine methyl ester ligand can be easily formed in the presence of the base K2CO3. The deprotonated N-benzylidene glycine methyl ester has been considered as the ligand in the reactions of Nbenzylidene glycine methyl ester with various activated alkenes catalyzed by Cu,5b,12 Ag,5c−e,j,13 Zn,14 Ni,15 Ca,16 Au,17 or Fe18 complexes. An electrophilic attack of a tetramethyl alkylidene bisphosphonate (BP) molecule at the metallo-ring methine carbon of the deprotonated N-benzylidene glycine methyl ester

ligand in 1 gives the intermediate 2, in which a hydrogenbonding interaction between a phosphonate PO group and the NH2 group of the chiral ligand occurs. From the intermediate 2, the hydrogen-bonded proton then migrates to the phosphonate-bonded carbon to give 3Michael‑addition, having the Michael addition product as a ligand. Finally, an exchange of an N-benzylidene glycine methyl ester (BE) molecule for the Michael addition product molecule regenerates the active species 1. As mentioned in the Introduction, the reaction of alkylidene malonates with azomethine ylides generates a 1,3-dipolar cycloaddition product, instead of a Michael addition product using the same catalyst system Cu(I)/(S)-TF-BiphamPhos. Therefore, the catalytic cycle for 1,3-dipolar cycloaddition was also proposed and presented in Scheme 2. The first step for 1,3dipolar cycloaddition should be the same as that for Michael addition, generating intermediate 2. From the intermediate 2, the phosphonate-bonded carbon nucleophilically attacks the C atom of the CN double bond to form the intermediate 3cycloaddition having the 1,3-dipolar cycloaddition product as a 7871


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ligand. Similar to Michael addition, an exchange of an Nbenzylidene glycine methyl ester (BE) molecule for the 1,3dipolar cycloaddition product molecule regenerates the active species 1. The active species 1 and its isomer 1′ were calculated (Figure 1). 1 is 1.7 kcal/mol lower in enthalpy than its isomer 1′. When

suggesting that the less stable isomer 1′ is not the active species in the reactions. Using 1 as the active species, the enthalpy profiles shown in Figure 2 were calculated for the catalytic cycles of Michael addition and 1,3-dipolar cycloaddition. We found that it is a barrier-free process for the electrophilic attack of a tetramethyl alkylidene bisphosphonate molecule at the metallo-ring methine carbon of the deprotonated N-benzylidene glycine methyl ester ligand. The electrophilic attack gives the intermediate 2 which has a hydrogen-bonding interaction (O- - -H = 1.678 Å) between a phosphonate PO group and the NH2 group of the chiral ligand and a carbanionic center formed at the phosphonate-bonded carbon. Apparently, the hydrogen-bonding interaction between a phosphonate PO group and the NH2 group of the chiral ligand in intermediate 2 and the hypervalent interaction between the carbanionic center and the phosphorus atoms of the two phosphonate groups enhance the stability of the intermediate structure. From the intermediate 2, there are two pathways, Michael addition and 1,3-dipolar cycloaddition. The Michael addition occurs via the transition state TSMichael‑addition with a barrier of 14.7 kcal/mol, giving 3Michael‑addition, indicating that the proton

Figure 1. Calculated structures of the active species 1 and its isomer 1′.

1′ was assumed as the active species, the reaction intermediates formed were found to be much less stable (vide infra),

Figure 2. Enthalpy profiles calculated for the Cu(I)/(S)-TF-BiphamPhos catalyzed reaction of tetramethyl benzylidene bisphosphonate (BP) with N-benzylidene glycine methyl ester (BE). The calculated relative enthalpies (ΔH) are given in kcal/mol. 7872


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Figure 3. Calculated structures of the intermediate 2 and its isomers 2X (X = A−G). The calculated relative enthalpies and solvation-corrected relative enthalpies (in parentheses) are given. From these different intermediate isomers, different configurations of the Michael addition product can be generated.

migration occurs relatively easily from the NH2 group of the chiral ligand to the phosphonate-bonded carbon, consistent with the significant role of the NH2 moiety of the chiral TFBiphamPhos ligand found in experiments. 5 b From 3Michael‑addition, an exchange of an N-benzylidene glycine methyl ester (BE) molecule for the Michael addition product molecule regenerates the active species 1 and gives the Michael addition product ProdMichael‑addition. The exchange of N-benzylidene glycine methyl ester for the Michael addition product molecule involves a very complicated bond-breaking and -forming process. Computationally, it is almost impossible to locate the relevant transition states for such a complicated process.

Because of the complicated bond-breaking and -forming process, we expect that the exchange should have an appreciable overall barrier. Therefore, in Figure 2 we place the “assumed transition state” for the exchange process 3Michael‑addition + BE → 1 + ProdMichael‑addition above the transition state TSMichael‑addition. Here, the phrase “assumed transition state” is purposely in quotes because 3Michael‑addition + BE → 1 + ProdMichael‑addition is expected to be a multistep process. The “assumed transition state” corresponds to the highest transition state for this multistep process. It should be also pointed out here that the difficulties in locating the relevant transition states would not jeopardize our qualitative 7873


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Figure 4. Enthalpy profiles calculated for the Cu(I)/(S)-TF-BiphamPhos catalyzed reaction of dimethyl benzylidene malonate (MA) with Nbenzylidene glycine methyl ester (BE). The calculated relative enthalpies (ΔH) are given in kcal/mol.

phosphonate groups of the tetramethyl alkylidene bisphosphonate and the phenyl substituent of the N-benzylidene glycine methyl ester in their structures. The significant repulsion leads to a remarkably long distance calculated for the newly formed C−C bond, 1.695 Å in 3 cycloaddition and 1.636 Å in Prodcycloaddition. One may argue the possibility that the “assumed transition state” for the exchange process 3cycloaddition + BE → 1 + Prodcycloaddition could be lower than that for the exchange process 3Michael‑addition + BE → 1 + ProdMichael‑addition (Figure 2). Even with this and other possible scenarios, the Michael addition shown in Figure 2 is still the one experimentally observed because formation of 1,3-dipolar cycloaddition Prodcycloaddition is thermodynamically not possible: i.e., 1 + Prodcycloddition is higher in enthalpy than 2 + BE as discussed above. Other isomers of the intermediate 2 are possible when different ligand arrangements are considered. With (S)-TFBiphamPhos as the ligand, the intermediate 2, from which the experimental (2R,3S) configuration of the Michael addition product can be obtained, has the optimal ligand arrangement and is the most stable isomer (Figure 3). Other relevant isomers, from which other configurations (including the opposite enantiomer and other diastereomers) of the Michael

insights into the mechanistic aspect of the reactions. More discussion of this aspect will be given below. From the intermediate 2, the 1,3-dipolar cycloaddition can proceed via the transition state TScycloaddition to give the intermediate 3cycloaddition. From 3cycloaddition, an exchange of Nbenzylidene glycine methyl ester for the 1,3-dipolar cycloaddition product molecule regenerates the active species 1. Again, following the similar argument above, we expect an appreciable overall barrier for this exchange process. 3cycloaddition + BE and 1 + Prodcycloaddition are respectively higher in enthalpy than 3Michael‑addition + BE and 1 + ProdMichael‑addition. Therefore, it is reasonably expected that the “assumed transition state” for the exchange process 3cycloaddition + BE → 1 + Prodcycloaddition should also be in parallel higher than that for the exchange process 3Michael‑addition + BE → 1 + ProdMichael‑addition (Figure 2). On the basis of the enthalpy profiles shown in Figure 2, we can conclude that the Michael addition pathway is both kinetically and thermodynamically more favorable. The 1,3dipolar cycloaddition pathway is less favorable because 3cycloaddition and Prodcycloaddition are relatively unstable. The instability of 3cycloaddition and Prodcycloaddition, when respectively compared with 3Michael‑addition and ProdMichael‑addition, is clearly related to the significant repulsion between the two bulky 7874


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groups in the dimethyl benzylidene malonate, which have smaller repulsive interactions with the phenyl group of the Nbenzylidene glycine methyl ester, allow the 1,3-dipolar cycloaddition to occur. The more bulky phosphonate groups in the tetramethyl alkylidene bisphosphonate, which have greater repulsive interactions with the phenyl group of the Nbenzylidene glycine methyl ester, suppress the 1,3-dipolar cycloaddition and give the Michael addition product.

addition product could be obtained, were also calculated. The results of the calculations show that these isomers, which do not have an optimal ligand arrangement, are less stable than the intermediate 2, by at least 5 kcal/mol in enthalpy. The experimental observation5b of the Michael addition product with an exclusive diastereoselectivity suggests that the reaction indeed does not occur via these less stable intermediates. The Michael addition and 1,3-dipolar cycloaddition pathways for the related reaction of dimethyl benzylidene malonate with N-benzylidene glycine methyl ester were also calculated, and the enthalpy profiles are shown in Figure 4. Interestingly, the 1,3-dipolar cycloaddition pathway in this case was found to be both kinetically and thermodynamically more favorable than the Michael addition pathway. These results are consistent with the experimental observation5c that reaction of alkylidene malonates with azomethine ylides generated a 1,3-dipolar cycloaddition product, instead of a Michael addition product. In this case, the 1,3-dipolar cycloaddition product is thermodynamically more stable than the Michael addition product. Our calculations support that it is the less bulky ester groups in the dimethyl benzylidene malonate that allow the 1,3-dipolar cycloaddition to occur and that introducing two more bulky phosphonate groups suppresses the 1,3-dipolar cycloaddition, in agreement with the experimental conjecture.5b It should be emphasized one more time that the complexities in the ligand exchange processes involved in the reactions prevent us from calculating the relevant transition states. However, our arguments based on the thermodynamic stabilities of the relevant intermediates and final products allow us to qualitatively discuss the favorable reaction pathways and explain the experimental observations for the Cu(I)/TFBiphamPhos catalyzed reactions of alkylidene bisphosphates and alkylidene malonates with azomethine ylides studied in this paper.

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ASSOCIATED CONTENT

S Supporting Information *

Text giving the complete ref (11) and tables giving Cartesian coordinates and enthalpies for all of the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.-J.W.); [email protected] (Z.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Research Grants Council of Hong Kong (HKUST603711 and HKU1/CRF/08) and the National Natural Science Foundation of China (20972117, 21172176).

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REFERENCES

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CONCLUSIONS The reaction mechanisms of Cu(I)/TF-BiphamPhos catalyzed reactions of tetramethyl alkylidene bisphosphonate and dimethyl benzylidene malonate with N-benzylidene glycine methyl ester have been studied using the density functional B3LYP method. The Michael addition and 1,3-dipolar cycloaddition pathways were calculated. In the reaction of tetramethyl alkylidene bisphosphonate with N-benzylidene glycine methyl ester, the Michael addition pathway is both kinetically and thermodynamically more favorable than 1,3dipolar cycloaddtion. In the reaction of dimethyl benzylidene malonate with N-benzylidene glycine methyl ester, the 1,3dipolar cycloaddition pathway is kinetically and thermodynamically more favorable. These results are consistent with the experiments that Michael addition and 1,3-dipolar cycloaddition products were obtained for reactions of alkylidene bisphosphates and alkylidene malonates, respectively. Our calculations revealed that the Michael addition of tetramethyl alkylidene bisphosphonate with N-benzylidene glycine methyl ester involves an electrophilic attack of tetramethyl alkylidene bisphosphonate at the metallo-ring methine carbon of the deprotonated N-benzylidene glycine methyl ester, followed by a proton migration from the NH2 group of the chiral ligand to the phosphonate-bonded carbon to give the Michael addition product. In the reaction of dimethyl benzylidene malonate, the 1,3dipolar cycloaddition occurs, different from that in the reaction of tetramethyl alkylidene bisphosphonate. The less bulky ester 7875


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