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Related Concept Videos

¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

1.9K
The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene...
1.9K
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

1.1K
Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the...
1.1K
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

1.5K
The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
1.5K
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

1.1K
Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
1.1K
¹H NMR Signal Multiplicity: Splitting Patterns01:13

¹H NMR Signal Multiplicity: Splitting Patterns

5.3K
When protons A and X are coupled, their nuclear spin energy levels are slightly modified. This is because the energy required to excite proton A to a spin state parallel to proton X is slightly different from the energy required for it to become anti-parallel to spin X. Consequently, there are two possible excitation frequencies for A (A1 and A2), depending on the spin state of X, and vice versa. The mutual nature of coupling implies that the difference between frequencies A1 and A2, indicated...
5.3K
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

1.0K
Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
1.0K

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What is a Cross-Coupling? An Argument for a Universal Definition.

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A universal definition for cross-coupling reactions is needed. Broadening the definition to the union of two molecular entities forming a covalent bond encourages exploration of novel reactivity and future developments in synthetic chemistry.

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Area of Science:

  • Organic Chemistry
  • Synthetic Chemistry
  • Catalysis

Background:

  • Cross-coupling reactions have seen significant advancements, yet a unified definition remains elusive.
  • Existing definitions often rely on historical palladium-catalyzed reactions (Suzuki, Negishi, Heck), overlooking earlier work.
  • This lack of a clear definition hinders the categorization and exploration of new cross-coupling methodologies.

Purpose of the Study:

  • To propose a broadened, universal definition for cross-coupling reactions.
  • To encourage a wider scope of research and discussion in the field of cross-coupling.
  • To inspire innovation by moving beyond traditional synthetic or mechanistic classifications.

Main Methods:

  • Literature review of historical and modern cross-coupling reactions.
  • Analysis of existing definitions and their limitations.
  • Development of a new, inclusive definition based on fundamental bond-forming principles.

Main Results:

  • Identified inconsistencies and limitations in current definitions of cross-coupling reactions.
  • Proposed a new definition: the union of two distinct molecular entities in a covalent-bond-forming process.
  • Highlighted examples of reactions that challenge conventional definitions.

Conclusions:

  • A universal definition is crucial for advancing cross-coupling chemistry.
  • The proposed definition encompasses a broader range of reactions, fostering innovation.
  • This perspective aims to stimulate dialogue and inspire future developments in cross-coupling transformations.