Organic Syntheses via Transition Metal Complexes. 95.¹ Rearrangement of (Cyclobutenyl)carbene Tungsten Complexes to 1-Tungsta-1,3,5-hexatrienes

Abstract

(Cyclobutenyl)carbene tungsten complexes are shown to rearrange to 1-tungsta-1,3,5-hexatrienes. Azabicyclo[4.2.0]octadiene (bis)carbene tungsten complexes 3a − c, which served as model compounds in these studies, were generated by [4 + 2]- and [2 + 2] domino cycloaddition of 2 equiv of (1-alkynyl)carbene complex (CO)5WC(OEt)C⋮CPh (1) to alkenyl imidates RCHCHC(OEt)NR1 (2a − c) (R = Ph, Me; R1 = Me, i-Pr). Compounds 3 are stable in the solid state at 20 °C but rearrange in solution to 1-metalla-1,3,5-hexatrienes 7a − c by ring opening of the cyclobutene ring and subsequent 1,3 hydrogen migration. Compounds 3a and 7b were characterized by crystal structure analyses.

1,3-Dioxycyclopentadienes from (1-Alkynyl)carbene Tungsten Complexes - Domino Cyclization/Cycloaddition Reactions

“…[9] Compounds 12 exhibit spectroscopic features typical of (4-oxycyclobutenyl)carbene tungsten complexes. [14,15] Most notably, the 13 C NMR shift of the WϭC unit is observed in the range characteristic of nonconjugated 1-tungsta-1,3,5-hexatrienes, e.g. 12a: δ ϭ 319.4, thus indicating an apparently strong distortion of the (WϭC)-(CϭC) unit (which was indeed confirmed by the crystal structure analysis of compound 12b) and consequently little π-conjugation.…”

Biscarbene complexes from the reactions of O-ethyl lactim and 1-alkynyl Fischer carbene complexes of chromium and tungsten

“…Probing into the formation mechanism of a Fischer biscarbene complex and reactivity of the M@C bonds in it are two major aspects in exploration of Fischer biscarbene chemistry. In a previous paper co-authored by Aumann and Yu (one of the authors of this paper), et al [6], it was reported that reactions of 1-alkynyl Fischer carbene complexes (OC) 5 W (b)) with O-methyl lactims $(CH 2 ) n -N@C(OCH 3 )$ (n = 3, 4) afforded biscarbene complexes of type 3 (Scheme 1), in which the stability of a tungsten biscarbene complex over silica gel was studied, while further investigation of its chromium analogues failed due to complicated transformations of the chromium biscarbene complex under the same conditions. Thus, a more detailed and controlled study has been expected to probe into the formation and reactivity of complexes of type 3.…”

“…Rearrangement of a tungsten biscarbene complex of type 3 occurred over silica gel to give compound of type 4 in 21% yield [6], and isomerization of another tungsten biscarbene complex was also observed [5]. The chromium biscarbene complex, i.e., 3a, was applied for a comparison study with its tungsten analogue [6].…”

“…Until recently, limited work has been directed in this aspect [4]. Azabicyclic [4.2.0]octadiene biscarbene complexes of tungsten [5], azabicyclic [n.2.0] alkenyl biscarbene complexes of chromium and tungsten and related reactions of the tungsten biscarbene complexes [6] were reported, respectively. Unusually stable biscarbene complexes were obtained from the reactions of 2-alkenyl-2-oxazoline and 1-alkynyl Fischer carbene complexes of chromium and tungsten from our laboratories [7a].…”

Chemoselective Stepwise Demetalation of Unusually Stable Fischer Biscarbene Complexes by Domino [4+2]/[2+2] Cycloaddition of 2-Isopropenyl-2-oxazoline to 1-Alkynyl Fischer Carbene Complexes of Chromium and Tungsten

“…The first type of Aumann's biscarbene complexes are not stable and decomposed by water, and the second are rearranged to ring-opened isomers on silica gel during column chromatography. 13,14 Alkenyl oxazolines excluding 2 were reported as achiral substrates for cyclopropanation with 1-alkenyl and 1-alkyl chromium Fischer carbene complexes. 15 Keeping in mind the structural similarity between alkenyl imidates and 2, we studied the reactions of 1 and 2.…”