Planar transition structures in the epoxidation of alkenes. A DFT study on the reaction of peroxyformic acid with norbornene derivatives

Journal of Organic Chemistry

Volumn 67, Issue 24, 2002, Pages 8519-8527

  Freccero, Mauro ^a   Gandolfi, Remo ^a   Sarzi Amadè, Mirko ^a   Rastelli, Augusto ^b  

^a UNIVERSITY OF PAVIA   (Italy)

^b UNIVERSITY OF MODENA AND REGGIO EMILIA   (Italy)

Author keywords

[No Author keywords available]

Indexed keywords

ACTIVATION ENERGY; CHEMICAL ANALYSIS; CHEMICAL BONDS; ORGANIC ACIDS; SUBSTRATES;

TRANSITION STRUCTURES;

OLEFINS;

ALKENE DERIVATIVE; ETHYLENE; FORMIC ACID DERIVATIVE; NORBORNENE DERIVATIVE;

ARTICLE; CHEMICAL BOND; CHEMICAL STRUCTURE; EPOXIDATION; PHASE TRANSITION; STEREOCHEMISTRY; VALIDATION PROCESS;

EID: 0037195659     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo026141w     Document Type: Article

References (54)

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40. 2 angle changes by 30° on passing from the B3LYP/6-31G(d) to the B3LYP/6-311+G(d,p) structure, the energy obtained by single-point B3LYP/6-311+G(d,p) calculation on the lower level structure is very similar (only 0.3 kcal/mol higher) to that of the fully optimized B3LYP/6-311+G(d,p) structure. That is, distortion away from an ideal spiro orientation is quite easy.
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46. 7 (in all these TSs the O-H bond is closer to the longer forming bond). B3LYP/6-31G(d) IRC analysis, on the product side, indicates that the final epoxide (hydrogen-bonded to the formic acid) is formed in a nonconcerted one-step process. It resides higher in energy than TS 4. However, we were not able to locate this TS with the higher basis set: 6-311+G(d,p) optimization starting from the 6-31G(d) geometry led to the planar TS 4
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48. (a) According to Lowe "the word concerted is meaningless unless we state what the two processes (the reference processes) are that occur simultaneously", and when "one process takes place first and the second one commences afterwards", the mechanism must be labeled "nonconcerted".35 Consequently, the process under study is nonconcerted as far as the timing of the two oxirane C-O bond formations is concerned. (b) Two reviewers emphasized that it might be better to keep the term "concerted" or "nonconcerted" for the absence or the presence, respectively, of an intermediate before the product formation. We feel that the terms "one-step" and "stepwise" are more adequate to describe these two possibilities while the terms "concerted" (that can be either "synchronous" or "asynchronous") and "nonconcerted" properly describe the bond timing. A concerted process necessarily corresponds to a one-step stereospecific (configuration retention) mechanism that follows, in the case of pericyclic reactions, the Woodward-Hoffmann (W-H) rules. A nonconcerted pathway can be either a stepwise (with possible configuration loss) or a one step pathway. It is interesting to emphasize that the stereochemical outcome of one-step nonconcerted processes is not controlled by W-H rules as nicely exemplified by the preference for the orbital symmetry forbidden stereochemistry exhibited by the 1,5-sigmatropic shift of substituted norcardiene systems (Kless, A.; Nendel, M.; Wilsey, S. Houk, K. N. J. Am. Chem. Soc. 1999, 121, 4524).
49. (a) According to Lowe "the word concerted is meaningless unless we state what the two processes (the reference processes) are that occur simultaneously", and when "one process takes place first and the second one commences afterwards", the mechanism must be labeled "nonconcerted".35 Consequently, the process under study is nonconcerted as far as the timing of the two oxirane C-O bond formations is concerned. (b) Two reviewers emphasized that it might be better to keep the term "concerted" or "nonconcerted" for the absence or the presence, respectively, of an intermediate before the product formation. We feel that the terms "one-step" and "stepwise" are more adequate to describe these two possibilities while the terms "concerted" (that can be either "synchronous" or "asynchronous") and "nonconcerted" properly describe the bond timing. A concerted process necessarily corresponds to a one-step stereospecific (configuration retention) mechanism that follows, in the case of pericyclic reactions, the Woodward-Hoffmann (W-H) rules. A nonconcerted pathway can be either a stepwise (with possible configuration loss) or a one step pathway. It is interesting to emphasize that the stereochemical outcome of one-step nonconcerted processes is not controlled by W-H rules as nicely exemplified by the preference for the orbital symmetry forbidden stereochemistry exhibited by the 1,5-sigmatropic shift of substituted norcardiene systems (Kless, A.; Nendel, M.; Wilsey, S. Houk, K. N. J. Am. Chem. Soc. 1999, 121, 4524).
50. Lowe, J. P. J. Chem. Educ. 1974, 51, 785-786.
51. Interestingly, the nonconcerted C-O bond formation in a onestep process via a planar TS, disclosed by IRC analysis, was anticipated by Rebek et al., on the basis of intuitive chemical reasoning.9b A mechanism that corresponds to a nonconcerted one-step process via a planar TS was also suggested by Hanzlik et al. for the peroxy acid epoxidation of styrene derivatives.6b
52. A word of caution is in order as far as the chemical significance of IRC paths is concerned. One must always keep in mind that the meaning and theoretical status of IRC points is different from that of stationary points (minima and first-order saddle point) and avoid temptation to attribute excessive chemical significance to them. The IRC coordinate corresponds to the minimum energy path located in mass-weighted coordinates and is the path traced by a classical particle sliding from a saddle point down to a minimum with zero kinetic energy. It is a path on the potential energy surface, not a true trajectory: real molecules have kinetic energy and will not follow the intrinsic reaction path. Anyway, at present, IRC represents the more convenient description of the reaction mechanisms as they are qualitatively treated by organic chemists.
53. Stabilization of the positive charge in 6 by σ delocalization is supported by the observation that it is 6.0 kcal/mol more stable than the corresponding protonated epoxide, while the protonated epoxide deriving from tetramethylethene resides 1.3 kcal/mol lower in energy than the related open α-hydroxy carbocation.
54. Freccero, M.; Gandolfi, R.; Sarzi-Amadè M.; Rastelli, A. Tetrahedron 2001, 57, 9843.