Diazo chemistry controlling the selectivity of olefin ketonisation by nitrous oxide

Abstract

The thermal reaction of olefins with nitrous oxide was recently put forward as a promising synthetic ketone source. The 1,3-dipolar cycloaddition of N(2)O to the C=C double bond, forming a 4,5-dihydro-[1,2,3]oxadiazole intermediate, was predicted to be the first elementary reaction step. This oxadiazole can subsequently decompose to the desired carbonyl product and N(2)via a hydrogen shift. In this contribution, Potential Energy Surfaces are constructed at the reliable G2M level of theory and used to evaluate thermal rate constants by Transition State Theory. Compelling theoretical and experimental evidence is presented that an oxadiazole intermediate not only can undergo a hydrogen shift, but eventually also a methyl- or even an alkyl-shift. Special emphasis is also given on a hitherto neglected decomposition of the oxadiazole via a concerted C-C and N-O cleavage. For some substrates, such as internal olefins, this diazo route is negligibly slow, compared to the ketone path, leaving no marks on the selectivity. For cyclopentene the diazo cleavage was however found to be nearly as fast as the desired ketone route. However, the diazo compound, viz. 5-diazopentanal, reconstitutes the oxadiazole much faster upon ring-closure than it is converted to side-products. Therefore, a pre-equilibrium between the diazoalkanal and the oxadiazole is established, explaining the high ketone yield. On the other hand, for primary alkenes, such a concerted C-C and N-O cleavage to diazomethane is identified as an important side reaction, producing aldehydes with the loss of one C-atom. For these substrates, the bimolecular back-reaction of the C(n-1) aldehyde and diazomethane is too slow to sustain an equilibrium with the oxadiazole; diazomethane rather reacts with the substrate to form cyclopropane derivatives. The overall selectivity is thus determined by a combination of H-, methyl- or alkyl-shift, and the eventual impact of a diazo cleavage in the oxadiazole intermediate.

Synthesis of functionalized liquid rubbers from polyisoprene

“…Since the carbonyl groups are formed by an oxidation process, this reaction type was called the “carboxidation.”12, 16, 17 The carboxidation proceeds by a nonradical mechanism via the 1,3‐dipolar cycloaddition of N 2 O to the alkene double bond with intermediate formation of 1,2,3‐oxadiazoline cycle (reaction 1)13, 14, 18: where R 1 and R 2 are the alkyl radicals or hydrogen atoms. This mechanism was convincingly supported by the quantum‐chemical modeling of the N 2 O interaction with some alkenes 19–21. The carboxidation can be applied to alkenes of different types (aliphatic, cyclic, heterocyclic, etc.…”

“…The carboxidation can be applied to alkenes of different types (aliphatic, cyclic, heterocyclic, etc. ), opening a new way for the preparation of carbonyl compounds 15, 21, 22…”

4,5‐Dihydro‐1,2,3‐oxadiazole: A Very Elusive Key Intermediate in Various Important Chemical Transformations

“…Our experiments with suitably substituted N-nitrosoureas using thallium(I) alkoxides as bases under apolar conditions answer important questionso nt he existence and the secondary products of 4,5-dihydro-1,2,3-oxadiazole. 4,5-Dihydro-1,2,3-oxadiazoles [1] of type 1 are postulated to be intermediates in the synthesis of ketones from alkenes under drastic conditions, [2,3] the decomposition of N-nitrosoureas at physiological pH, [4] andt he alkylation of DNA and other relevant molecules in vivo [5] (Schemes 1a nd 2). We becamei nterested in 1 from ap rocess used by BASF Ludwigshafenwherein N 2 O( aw aste product and highly potent greenhouse gas) is utilized to synthesize cyclopentanone and ketonesw ith twelve-membered rings [2c] (30 000 tons per year) by meanso f 1,3-dipolar cycloaddition of N 2 Ot ot he alkene.…”

Continuous Thermal Oxidation of Alkenes with Nitrous Oxide in a Packed Bed Reactor

“…Previous literature results for similar reactions show E a = 21 (experimental) 9 or 24 (computational) 15 kcal/mol for cyclopentene oxidation and 27 or 28 kcal/mol (computational) 13,15 for cyclohexene. The initial rate of trans-7-tetradecene oxidation has also been investigated experimentally and found to be of interim reactivity between cyclopentene and cyclohexene.…”