Chemistry and catalysis advances in organometallic chemistry and catalysis
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2T (Scheme 13.12) [43]. Ligation of nitriles to a metal center changes features of the reaction mechanism. In the case of metal-free reactions, the mechanism is concerted and highly synchronous, that is, the reaction occurs in one step via formation of one cyclic five- membered transition state (TS), and the changes of chemical bonds directly involved in the process take place simultaneously. The estimated degree of asynchronicity of the reactions between nitrones and uncomplexed nitriles is only 5–15% [43, 45–48]. In contrast, the coordination of nitriles to the metal (Pt or Pd) results in a dramatic increase of the reaction asynchronicity to 19–49%, and, in some cases, the TS of the reaction may become acyclic [47]. At the same time, the global mechanism of the DCA usually remains concerted. 13.3.1.3 Effect of the Metal Oxidation State The oxidation state of the metal-activator may significantly change the reactivity of nitriles toward the DCA with nitrones. A higher oxidation state provides a higher activation. This effect is accounted for by the fact that the metal in a high oxidation state shifts the electron density from the C ≡N group more efficiently and, hence, lowers the LUMO NCR energy and enhances the positive charge on the nitrile C atom more significantly, compared to the metal in a lower oxidation state. Indeed, the LUMO NCMe
energy drops from –0.95 eV in 1T METAL-MEDIATED [2 + 3] CYCLOADDITION TO NITRILES AND ISOCYANIDES: THEORETICAL STUDIES 179 O N C N C Me Me [M] [M] C N Me C N + O − Me H Me + H Me [M] O N C H Me Me + – −NCMe ΔG s ≠
s ≠ 24.3 26.1 28.8
22.6 48.8
36.2 M = Pt
[M] = trans-[MCl 2 (NCMe)] E b (M–N) M = Pd Scheme 13.12 Cycloaddition and substitution paths of the reaction between nitrone MeCH =N(Me)O and coordinated acetonitrile. Gibbs free energies of activation and metal–NCMe bond energies are indicated in kilocalories per mole. to –1.12 eV in 2T and the atomic charge increases from 0.47 e in 1T to 0.53 e in 2T [45, 46]. The activation energy of the nitrone coupling with the Pt(IV) complexes trans-[PtCl 4 (N
2 ] (R
= Me, C≡CH, CH=CH 2 ) was calculated to be 7.95–11.54 kcal/mol lower than the activation barrier of the reaction with corresponding Pt(II) species, trans-[PtCl 2 (N ≡CR) 2 ] [45–48]. 13.3.1.4 Effect of Substituents in Nitrile Introduction of a strong electron-acceptor group R (Ph, CH 2
, CHCl 2 , CF 3 ) into the nitrile molecule N ≡CR provides a similar effect on the reactivity of nitriles as their coordination to a metal, that is, it activates N ≡CR toward DCA. Such an effect has the same driving force, that is, the lowering of the LUMO NCR
energy upon introduction of this substituent. However, the ligation of N ≡CR to the metal appears to be more efficient way of the nitrile activation than the variation of substituents. For instance, the activation barrier of the reaction CH 2 =N(Me)O + N≡CR decreases by 7.52 kcal/mol ongoing from R = Me to CF 3 (activation by a factor of 3.2 × 10 5 ) while the drop of the barrier upon the coordination of N ≡CMe to Pt(IV) reaches 20.40 kcal/mol (activation by a factor of 9.1 × 10 14 ) [45, 46, 48, 49]. The replacement of the Me with Ph group in N ≡CR provides even lower activation: only 0.9 kcal/mol (by a factor of 4.6) [45]. Meanwhile, the greatest effect on the reactivity of N ≡CR may be achieved when the nitrile is doubly activated (by metal and electron-acceptor substituent). The calculated activation energy of the reaction CH 2 =N(Me)O + trans- [PtCl 2 (NCMe)(N ≡CCF 3 )] is 4.34 kcal/mol (the N ≡CCF 3 ligand participates in the DCA) versus 27.65 kcal/mol for the reaction CH 2 =N(Me)O + N≡CMe [45, 49]. 13.3.1.5 Effect of the Nature of the Nitrone The nature of the nitrone plays an important role in the DCA reactions with nitriles. First, theoretical DFT calculations predicted that acyclic C-alkyl nitrones (e.g., MeCH =N(Me)O) are more reactive toward N ≡CMe than C-aryl nitrones (4-RC 6 H
)CH =N(Me)O (R = Me, OMe) by 2.9–3.2 kcal/mol (a factor of 134–222) [45]. Second, the acyclic nitrone MeCH =N(Me)O in the usual Z-configuration is more inert than the cyclic nitrone with a purely hydrocarbon ring 4T (Fig. 13.3) by 2.68 kcal/mol (by a factor of circa 100) [25, 46]. This effect is accounted for by the fixation of cyclic nitrones in the more strained and, therefore, more reactive, E-configuration. Third, among various cyclic nitrones (4T–8T), those with a heteroatom (O or N) at the position 1 of the cycle (5T and 8T) are the most reactive, whereas the cyclic nitronate 7T is the most inert. The reactivity of 4T and 6T is similar because the oxygen heteroatom in
activation by a metal. For example, the activation of 7T by Pt(IV) is 14.65 kcal/mol, while 5T is activated by 20.40 kcal/mol. Nevertheless, the activation energy of the reaction of 7T bound to Pt(IV) (17.0 kcal/mol) is still sufficiently low for the realization of DCA. This theoretical prediction was fully confirmed later by experiment (see Reference 50). 13.3.1.6 Effect of the Solvent In accord with the quantum-chemical calculations [45–48], solvent effects inhibit the DCAs of nitrones to nitriles and decrease the thermodynamic stability of the cycloadduct owing to a higher stabilization of the reactants’ level compared to the levels of TS and product. As a consequence, lower-polar solvents are predicted to 180 METAL-MEDIATED [2 + 3] DIPOLAR CYCLOADDITION TO SUBSTRATES WITH CN TRIPLE BOND: RECENT ADVANCES N O + – N O O + – N O O N O O + – N HN O + – 4T 5T 6T 7T 8T + –
Cyclic nitrones used in theoretical calculations. O N N C R [M] [M]
N C R C N + O − + O N N C R [M] 1,2,4-Oxadiazoline 1,2,5-Oxadiazoline Scheme 13.13 Two regioisomeric pathways of the cycloaddition of nitrones to nitriles. be preferable for these reactions over higher-polar solvents, although other factors, for example, solubility, should be taken into account. 13.3.1.7 Reaction Selectivity Cycloaddition of nitrones to nitriles is completely regioselective, affording 1,2,4- oxadiazolines (Scheme 13.13) exclusively. Theoretical calculations indicate that the activation barrier of formation of the 1,2,5-regioisomers is 25–26 kcal/mol higher compared to that of the 1,2,4-isomers [48]. At the same time, if the nitrile molecule bears another functionality (e.g., C =C or C≡C bonds), the reaction may occur via different chemoisomeric pathways (cycloaddition at the CN or C =C/C≡C group). Two main factors affect the chemoselectivity: (i) coordination of N ≡CR to a metal and (ii) nature of the substituents. In the case of metal-free reactions, the DFT calculations predict that the DCA of nitrones to N ≡CCH=CH
2 or N
≡CC≡CH should occur exclusively at the C =C or C≡C bond [47]. Coordination of these nitriles to Pt(II) and, in particular, to Pt(IV), dramatically facilitates the DCA at the C ≡N bond relative to that at the C=C/C≡C bond. However, the reaction at the CC bond still remains more preferable. At the same time, the DCA to phenyl-substituted nitrile in the model complexes trans-[PtCl 2 (L)(N ≡CCH=CHPh)] (L = NCCHCH 2 , NCMe) takes place already at the C ≡N bond [47, 51]. Such a switch of the chemoselectivity is accounted for by (i) the steric repulsion between the bulky Ph groups and (ii) the loss of conjugation in phenylnitrone and phenylcyanoalkene upon formation of the TS. Both these effects are more important for the CC than for CN additions and disfavor the reaction along the former channel. 13.3.2 Cycloaddition of Other Dipoles to Nitriles 13.3.2.1 Other dipoles of allyl-anion type Besides nitrones, the DCA of 11 other dipoles of allyl-anion type to free and Pt-bound nitriles has been theoretically studied (Scheme 13.14) [48]. The calculations at the B3LYP level demonstrated that three types of these dipoles, that is, azimines (NHN(Me)NH), azoxy compounds (NHN(Me)O), and nitro compounds (ON(Me)O) are inert toward either free or complexed nitriles owing to high activation energies and strongly positive G of reactions. Nitroso oxides (NHOO), nitrosimines (NHONH), azomethine imines (CH 2 N(Me)NH), and ozone (OOO) are slightly more reactive with N ≡CMe than nitrones, while carbonyl oxides (CH 2 OO), carbonyl imines (CH 2 ONH), and carbonyl and azomethine ylides (CH 2 OCH 2 and CH
2 N(Me)CH
2 ) are the most reactive dipoles. Coordination of N ≡CMe to Pt(II) and Pt(IV) significantly activates nitriles toward CH 2 N(Me)NH (by a factor of 1.7 × 10 6
× 10 9 ). The activation is moderate in the reactions with CH 2 N(Me)CH 2 and CH
2 ONH (by a factor of 3 × 10 3
CH 2 OCH 2 , CH
2 OO, and NHONH (by a factor of 7–230). However, such rather low reaction acceleration is not crucial because of the high reactivity of all these dipoles toward free nitriles. Finally, the DCAs of NHOO and OOO are inhibited by the ligation of N ≡CMe due to inverse electron demand character of these processes.
FINAL REMARKS 181 Z Y X N C Me N C Me X Y + Z − + X, Z = CH 2 , NH, O
Y = NMe, O Scheme 13.14 Cycloaddition of 1,3-dipoles of allyl-anion type to nitriles. 13.3.2.2 Nitrile Oxides The reaction between nitrile oxides (RC ≡NO)—1,3-dipoles of the propargyl/allenyl-anion type—and nitriles (Route II, Scheme 13.1) belongs to the normal electron demand processes controlled by the HOMO dipole
–LUMO nitrile
interaction and the coordination of N ≡CR to a metal center should accelerate this reaction. Indeed, the Gibbs free energy of activation for the DCA of MeC ≡NO decreases from 26.07 kcal/mol to 22.50 kcal/mol on going from free N ≡CMe to the Pt(IV) complex 3T [52]. However, such an activation (by a factor of 415) is incomparable with the acceleration of the nitrone DCA (by a factor of 2.4 × 10
14 [45, 46]). The effect of nitrile coordination to the Pt(II) or Pd(II) centers is even more unexpected. In fact, free acetonitrile and the Pt(II) or Pd(II) complexes 1T and 2T have very similar reactivities toward MeC ≡NO, while the ligation of N≡CC≡CH and N≡CCH=CH 2 to Pt(II) even inhibits the reaction at the CN bond. In contrast to kinetic behavior, the oxadiazole products of the nitrile oxide DCA are much more thermodynamically stable than the oxadiazoline products of the nitrone DCA ( G of reaction being ( −36)–(−40) kcal/mol versus (
−5)–(−22) kcal/mol) and the aromatic character of oxadiazoles (but not oxadiazolines) is responsible for this effect.
13.3.2.3 Azides Despite the practical importance of this reaction leading to tetrazoles (Route III, Scheme 13.1), the number of recent theoretical studies of the metal-assisted DCAs of azides (RN
3 , N
3 − ) to nitriles is limited. This reaction usually controlled by both types of the FMO interaction (HOMO dipole
–LUMO nitrile
and HOMO nitrile
–LUMO dipole
) [53]. At the same time, the coordination of the azide ion to Zn(II) in complexes [Zn (H 2
) n N 3 ] + either has no effect on the activation barrier (when n = 3) or inhibits the reaction by 1.8 kcal/mol (when n = 5). However, when nitrile or both reactants are coordinated to a metal (e.g., Zn(II) or Sn(IV)), the activation barrier decreases by 2–10 kcal/mol compared to the metal-free reactions [54, 55].
There are only few theoretical publications on the DCA of nitrones to isocyanides (C ≡NR)—species isomeric and isoelectronic to nitriles (Scheme 13.1) [56–58]. The calculations at the DFT (B3LYP) level predict that the coordination of C ≡NMe to Pt(II), Pt(II), Pt(IV), and Re(IV) centers in complexes trans-[MCl n (C≡NMe)
2 ] (M
= Pd (9T), Pt (10T), n = 2; M = Pt (11T), Re (12T), n = 4) results in an activation of isocyanide by 9.5–14.2 kcal/mol (by a factor of 9.3 × 10
6 –2.6
× 10 10 ), Pt(IV) being the best metal center. Moreover, the coordination enhances the thermodynamic stability of the derived N, N-disubstituted oxadiazoline heterocycles by 6.1–10.9 kcal/mol. At the same time, the DCA of another nitrone molecule at the second C ≡NMe ligand in complexes 9T–12T requires a higher activation barrier and, hence, is less favorable compared to the first DCA. All these results explain why free isocyanides are completely inert toward DCA with nitrones, while Pd-isocyanide complex yielded corresponding mono-DCA product (Scheme 13.10). The DCA of nitrones to isocyanides belongs to normal electron demand processes, and the activation of C ≡NR by metal centers has the same nature as in the case of nitriles, that is, there is a lowering of the LUMO CNR
energy and an increase of the charge on the C atom of the C ≡N group. The mechanism of this reaction is concerted, highly synchronous for free C ≡NR, and asynchronous for metal-bound isocyanides. 13.4 FINAL REMARKS In this chapter, we examined the recent data on DCA to metal-bound nitriles and isocyanides that allow the generation of new heterocyclic structures. In case of cycloaddition to metal-bound nitriles [1, 5, 6, 10], the obtained heterocyclic ligands can be often isolated as free species, and this approach represents an alternative (to a pure organic) route for their preparation. At 182 METAL-MEDIATED [2 + 3] DIPOLAR CYCLOADDITION TO SUBSTRATES WITH CN TRIPLE BOND: RECENT ADVANCES the same time, transformation of metal-bound isocyanides [7, 9] furnishes new types of N-heterocyclic carbenes, including chiral derivatives, that are not accessible via other synthetic protocols [39]. We also demonstrated that theoretical methods provide a powerful tool for the investigation of 1,3-DCAs to substrates bearing the C ≡N bond. The quantum chemical calculations are indispensable for the study of the reaction mechanism and they were successfully applied for the analysis of the reactivity, selectivity, and driving forces of these processes, for the understanding of reasons of the activation effect upon coordination of the substrate to the metal center, and for the interpretation and explanation of such important factors controlling these reactions as the nature of the metal center, dipolarophile, and dipole molecules. ACKNOWLEDGMENTS This work has been partially supported by the Fundac¸˜ao para a Ciˆencia e a Tecnologia (FCT), Portugal, its PPCDT program (FEDER funded), and through the research projects PTDC/QUI-QUI/098760/2008 and PTDC/QUI-QUI/109846/2009. MLK and KVL thank FCT and IST for the research contract within the Ciˆencia 2007 and 2008 programs, respectively. REFERENCES 1. Kukushkin, V. Yu.; Pombeiro, A. J. L. Chem. Rev. 2002, 102 , 1771 and references therein. 2. D¨omling, A. Chem. Rev. 2006, 106 , 17. 3. D¨omling, A.; Ugi, I. Angew. Chem. Int. Ed. 2005, 39 , 3168. 4. D¨omling, A.; Wang, W.; Wang, K. Chem. Rev. 2012, 112 , 3083. 5. Bokach, N. A.; Kuznetsov, M. L.; Kukushkin, V. Yu. Coord. Chem. Rev. 2011, 255 , 2946. 6. Bokach, N. A.; Kukushkin, V. Yu. Russ. Chem. Bull. 2006, 55 , 1869. 7. Michelin, R. A.; Pombeiro, A. J. L.; Guedes da Silva, M. F. C., Coord. Chem. Rev. 2001, 218 , 75. 8. Pombeiro, A. J. L.; Guedes da Silva, M. F. C.; Michelin, R. A. Coord. Chem. Rev. 2001, 218 , 43. 9. Luzyanin, K. V.; Pombeiro, A. J. L. In Carbene Complexes Derived from Metal-Bound Isocyanides; Nenajdenko, V., Ed.; Isocyanide Chemistry; Wiley-VCH Verlag GmbH: Weinheim, 2012; Chapter 15, p 531. 10. Bokach, N. A. Russ. Chem. Rev. 2010, 79 , 89. 11. Boyarskiy, V. P.; Luzyanin, K. V.; Kukushkin, V. Yu. Coord. Chem. Rev. 2012, 256 , 2029. 12. Pombeiro, A. J. L.; Guedes da Silva, M. F. C. J. Organomet. Chem. 2001, 617–618 , 65. 13. Wagner, G.; Pombeiro, A. J. L.; Kukushkin, V. Yu. J. Am. Chem. Soc. 2000, 122 , 3106. 14. Wagner, G.; Haukka, M.; da Silva, J. J. R. F.; Pombeiro, A. J. L.; Kukushkin, V. Yu. Inorg. Chem. 2001, 40 , 264. 15. Wagner, G.; Haukka, M. J. Chem. Soc., Dalton Trans. 2001, 2690. 16. Desai, B.; Danks, T. N.; Wagner, G. Dalton Trans. 2004, 166. 17. Sarju, J.; Arbour, J.; Sayer, J.; Rohrmoser, B.; Scherer, W.; Wagner, G. Dalton Trans. 2008, 5302. 18. Wagner, G.; Marchant, A.; Sayer, J. Dalton Trans. 2010, 7747. 19. Bokach, N. A.; Krokhin, A. A.; Nazarov, A. A.; Kukushkin, V. Yu.; Haukka, M.; da Silva, J. J. R. F.; Pombeiro, A. J. L. Eur. J.
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