Chemistry of cyclic aminooxycarbenes

Abstract

A series of oxazolidin-2-ylidenes and one tetrahydro-1,3-oxazin-2-ylidene, generated by thermolysis of Δ3-1,3,4-oxadiazolines in benzene at 90 °C, were intercepted by insertion into the OH bond of phenols. In two cases the initial products rearranged to N-(2-aryloxyethyl)-N-methylformamides. The activation energy for rotation about the amide CN bond of those ultimate products was measured as 20.4 kcal/mol. The aminooxycarbenes reacted with two equivalents of methyl or phenyl isocyanate to give spiro-fused hydantoins. Major products from the reactions of the N-carbonyl carbenes with dimethyl acetylenedicarboxylate or with methyl propiolate were 2-oxazolines resulting from apparent acyl transfers from N to C in the proposed dipolar intermediates; minor products of 1:2 (carbene:trap) stoichiometry were also observed. Keywords: nucleophilic carbene, aminooxycarbene, oxadiazoline, amide rotation, oxazolidine.

Cycloelimination of Imidazolidin‐2‐ylidene N‐Heterocyclic Carbenes: Mechanism and Insights into the Synthesis of Stable “NHC‐CDI” Amidinates

“…[11] Relatively unstable N,O-heterocyclic carbenes are known to undergo [3+ +2]-cycloelimination to generate ethylene and isocyanates;i nt hese cases NHC-(isocyanate) 2 adducts have been observed. [12] The Corey-Winter reaction, whereby alkenes are synthesized through expulsion of CO 2 or CS 2 ,i sp roposed to proceed through highly unstable O,O-or S,S-heterocyclic carbenes, respectively. [13] Furthermore, cycloeliminations of tetrazole, pentazole, thiotriazole, and 1,2,4-triazole derivatives have been noted as well.…”

Diaminocarbenes; Calculation of Barriers to Rotation about Ccarbene−N Bonds, Barriers to Dimerization, Proton Affinities, and 13C NMR Shifts

“…Stable carbenes with adjacent heteroatoms other than nitrogen are restricted to the unique phosphinocarbenes, , e.g., 5 , an aromatic thiazol-2-ylidene, 6 , related to the thiamine intermediate . Warkentin has made important studies of aminooxy- and dialkoxycarbenes, , and several stable aminooxy- and aminothiocarbenes, e.g., 7a and 7b , have been generated in our laboratories …”

Unexpected Insertion Reaction of Dimethoxycarbene with Imidazole-2(3H)-thiones

“…15 Insertion of nucleophilic carbenes 1 into O-H bonds were reported as inter10a and intramolecular processes. 16 Thermal decomposition of 2b (R = 4-MeOC 6 H 4 ) in benzene in the presence of tBuOH gave the orthoester 4 in 73% yield (Scheme1). …”