Violations to the principle of least motion: the shortest path is not always the fastest

Reis, Marta Castiñeira; Alajarı́n, Mateo; Marín‐Luna, Marta

doi:10.1039/d2cp00700b

Cited by 2 publications

(2 citation statements)

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“…Scheme 5 shows the three resonance structures of the pentadienyl cation and the resonance hybrid. It also shows Pauling bond order calculations indicating that, if the ethoxide ion could freely move along the conjugated cation, the principle of least nuclear motion would favor reactions at C 8 [ 58 , 59 ]. Because all three major photoproducts involve reaction of the ion pair at C 6 , it appears that the reaction is controlled by the proximity of the nascent ethoxide anion to the positive site [ 60 ].…”

Section: Discussionmentioning

confidence: 99%

Photochemistry and Photophysics of Cholesta-5,7,9(11)-trien-3β-ol in Ethanol

et al. 2023

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Cholesta-5,7,9(11)-trien-3β-ol (9,11-dehydroprovitamin D3, CTL) is used as a fluorescent probe to track the presence and migration of cholesterol in vivo. We recently described the photochemistry and photophysics of CTL in degassed and air-saturated tetrahydrofuran (THF) solution, an aprotic solvent. The zwitterionic nature of the singlet excited state, 1CTL* is revealed in ethanol, a protic solvent. In ethanol, the products observed in THF are accompanied by ether photoadducts and by photoreduction of the triene moiety to four dienes, including provitamin D3. The major diene retains the conjugated s-trans-diene chromophore and the minor is unconjugated, involving 1,4-addition of H at the 7 and 11 positions. In the presence of air, peroxide formation is a major reaction channel as in THF. X-ray crystallography confirmed the identification of two of the new diene products as well as of a peroxide rearrangement product.

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Section: Discussionmentioning

confidence: 99%

Photochemistry and Photophysics of Cholesta-5,7,9(11)-trien-3β-ol in Ethanol

et al. 2023

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“…Since then, much research has gone into the isolation of persistent triplet − and singlet carbenes. − Various mechanisms of carbene dimerization have been investigated, and sometimes hotly debated, over the decades. While triplet carbenes are thought to dimerize via a least-motion pathway, involving a coplanar approach, singlet carbene dimerization is believed to follow most often a non-least-motion pathway, in which the molecules approach each other in a perpendicular fashion, the lone pair interacting with the empty p-orbital of the second (Figure a). − For singlet acyclic diaminocarbenes, as well as heterocyclic and N-heterocyclic carbenes (NHCs), the direct non-least-motion pathway was long thought to be the preferred mechanism for dimerization. − In the 1960s Wanzlick proposed an equilibrium between monomeric NHCs and their tetraaminoethylene dimers, based on the fact that reactions of the latter with a variety of electrophiles yield products formally derived from the monomeric NHC. , The so-called “Wanzlick equilibrium” seemed confirmed by successful crossover experiments and later by direct observation of these equilibria in solution in the case of benzo-fused NHCs. , In most other cases, however, experimental evidence revealed the involvement of proton catalysis, the free carbene undergoing C–C coupling with its protonated salt to form a protonated dimer, which is then deprotonated to the corresponding alkene (Figure b). − …”

Section: Introductionmentioning

confidence: 99%

Experimental and Computational Study of a Confirmed Borylene-to-Diborene Dimerization

Heinz,

Arrowsmith,

Schweizer

et al. 2023

J. Am. Chem. Soc.

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While the dimerization of heavier group 13 carbene analogues to the corresponding alkene analogues is known and relatively well understood, the dimerization of dicoordinate borylenes (LRB:, L = neutral donor; R = anionic substituent) to the corresponding diborenes (LRB�BRL) has never been directly observed. In this study we present the first example of a formal borylene-to-diborene dimerization through abstraction of a labile phosphine ligand from the tricoordinate hydroborylene precursor (CAAC)(Me 3 P)BH (CAAC = cyclic alkyl(amino)carbene) by bulky Lewis-acidic dihaloboranes (BX 2 Y, X = Cl, Br, Y = aryl, boryl), generating the corresponding dihydrodiborene (CAAC)HB�BH-(CAAC) and (Me 3 P)BX 2 Y as the byproduct. An in-depth experimental and computational mechanistic analysis shows that this seemingly simple process (2 LL′BH + 2 BX 2 Y → LHB�BHL + 2 L′BX 2 Y) is in fact based on a complex sequence of finely tuned processes, involving the one-electron oxidation of and PMe 3 abstraction from the borylene precursor by BX 2 Y, multiple halide transfers between (di)boron intermediates and BX 2 Y/[BX 3 Y] − , and multiple one-electron redox processes between diboron intermediates and the borylene precursor, which make the reaction ultimately autocatalytic in [(CAAC)(Me 3 P)BH] •+ . The findings suggest that [LBXR] • boryl radicals are more likely coupling partners than dicoordinate LRB: borylenes in the reductive coupling of base-stabilized LBX 2 R boranes to LRB�BRL diborenes.

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Violations to the principle of least motion: the shortest path is not always the fastest

Abstract: The reaction between two molecules is usually envisioned as following a least-motion path with both molecules travelling minimum distances to meet each other. However, the reaction path of lowest activation...

Cited by 2 publications

References 93 publications

Photochemistry and Photophysics of Cholesta-5,7,9(11)-trien-3β-ol in Ethanol

Photochemistry and Photophysics of Cholesta-5,7,9(11)-trien-3β-ol in Ethanol

Experimental and Computational Study of a Confirmed Borylene-to-Diborene Dimerization

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