Oxanorbornadiene (OND) is a bicyclic organic compound with an oxygen atom bridging the two opposing saturated carbons of

1,4-cyclohexadiene 1,4-Cyclohexadiene is an organic compound with the formula C6H8. It is a colourless, flammable liquid that is of academic interest as a prototype of a large class of related compounds called terpenoids, an example being γ-terpinene. An isomer of ...

. OND is related to all-carbon bicycle norbornadiene.

Synthesis

While unsubstituted OND is known, the most useful OND derivatives are dialkyl OND-2,3-dicarboxylates, readily obtainable by a Diels–Alder

cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more Unsaturated hydrocarbon, unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of th ...

between furans and acetylenedicarboxylates such as DMAD.

Properties

OND-2,3-dicarboxylates (thereafter referred to as ''OND'') display unusually high reactivity towards

organic azide An organic azide is an organic compound that contains an azide (–) functional group. Because of the hazards associated with their use, few azides are used commercially although they exhibit interesting reactivity for researchers. Low molecular w ...

s and thiols. OND–thiol adducts are prone to retro-Diels–Alder fragmentation, which occurs with widely variable rates.

Reactions with Azides

ONDs react with

s to yield a mixture of four products, arising from initial 1,3-dipolar cycloaddition onto either of the two

olefins In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

, followed by a retro-Diels–Alder reaction (a cycloelimination reaction) to form 1,2,3-

triazoles A triazole is a heterocyclic compound featuring a five-membered ring of two carbon atoms and three nitrogen atoms with molecular formula C2H3N3. Triazoles exhibit substantial isomerism, depending on the positioning of the nitrogen atoms within the ...

and furans. The intermediate triazolines avoid detection because of a very strong thermodynamic drive to collapse into two aromatic products. The relative preference of attack on either double bond is governed by the electronic nature of the azides. Electron-rich

aliphatic In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, or ...

azides, ''e.g.'' benzyl azide, react preferentially via their HOMO orbital. Interaction of the azide

HOMO ''Homo'' () is the genus that emerged in the (otherwise extinct) genus ''Australopithecus'' that encompasses the extant species ''Homo sapiens'' ( modern humans), plus several extinct species classified as either ancestral to or closely relate ...

with

LUMO In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for ''highest occupied molecular orbital'' and ''lowest unoccupied molecular orbital'', respectively. HOMO and LUMO are sometimes collectively called the ''frontie ...

orbital of the OND, localized on the electron-poor C-2 and C-3, leads the products consistent with path A. Electron-poor aryl azides, such as ''p''-nitrophenyl azide, react, to a significant extent, via their LUMO orbitals, interacting with OND HOMO (C-5 and C-6), leading to higher amounts of path B products. The dipolar reactivity of OND with azides is unusually high for olefins, and even exceeds that of parent electron-poor alkyne DMAD.^{/sup>

References

{{reflist

Oxygen heterocycles
Heterocyclic compounds with 2 rings}