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

Abstract

Domino [4+2]/[2+2] cycloaddition of 2-isopropenyl-2-oxazoline 2 to 1-alkynyl Fischer carbene complexes (CO) 5 MdC(OEt)CtCPh 1 (a, M ) Cr; b, W) in a 1:2 molar ratio afforded unusually stable biscarbene complexes 3a and 3b containing a novel four-, five-, and sixmembered tricyclic core in 99.6% and 45.2% yields, respectively. Chain-opening -aminoalkenyl monocarbene complex 4b and -amidoalkenyl monocarbene complex 5b of tungsten were also isolated from the cycloaddition upon treatment of the reaction mixture of 1b and 2 on silica gel. Partial and full oxidation of 3a,b with pyridine N-oxide underwent efficient chemoselective stepwise demetalation to afford the corresponding monocarbene complexes 6a,b and organic diester 7, respectively, under mild conditions. The X-ray crystallographic study revealed the presence of a four-, five-, and six-membered tricyclic core in compounds 3, 6, and 7, and the methyl and oxazolindinyl groups derived from oxazoline 2 are positioned syn with respect to the azabicyclo[4.2.0]octadiene bicyclic moiety, which is oriented in the opposite direction. X-ray crystal structural data are reported for the bis-and monocarbene complexes 3a, 5b, and 6b as well as for diester 7.

Oxidative demetalation of Fischer alkoxy carbene complexes with stoichiometric pyridine N-oxide and NaBH4-promoted demetalation of Fischer iminocarbene complexes with sulfur and selenium

“…Over the last two decades Fischer monocarbene complexes [1] and recently biscarbene complexes [2] have been used to synthesize a wide range of organic products. Dö tz benzannulation [3] and cyclopropanation [4] reactions are employed to construct functionalized phenol derivatives and cyclopropanes, respectively.…”

“…In most cases, oxidation appears to be a convenient method to convert Fischer alkoxy carbene complexes into the corresponding ester products. Oxidants such as pyridine N-oxide (PNO) [2,18], dimethyl sulfoxide (DMSO) [12b,12c,19], dimethyldioxirane (DMD) [20], ceric ammonium nitrate (CAN) [18c,18f,19a,21], PhIO [22], and air [21c,23] have been used for this purpose. For Fischer aminocarbene complexes, harsh reaction conditions are usually required and only activated aminocarbene complexes can be oxidatively demetalated with PNO [18e].…”

“…For Fischer aminocarbene complexes, harsh reaction conditions are usually required and only activated aminocarbene complexes can be oxidatively demetalated with PNO [18e]. Other reagents such as iodosobenzene (PhIO) [22], Ca(ClO) 2 [24], iodine [24], NaBO 3 /KI [24], CAN [25], H 2 O 2 with methyltrioxorhenium (MTO) [26], and m-chloroperoxybenzoic acid [27], have occasionally been reported to oxidatively demetalate Fischer aminocarbene complexes. Although oxidative demetalation is usually required in organic synthesis with Fischer carbene complexes, no systematic investigation has been well-documented in this aspect.…”

“…For reduction of Fischer aminocarbene complexes, several rare examples are presented in Scheme 1 in which the present reduction methodology demonstrates a potential route to demetalate Fischer aminocarbene complexes, but more detailed and extensive investigation is expected in this aspect. In the course of our ongoing investigation on organic synthesis via Fischer carbene complexes [2], removal of the metal moieties from the newly formed carbene complex products has often been required, which led us to pursue a systematic study on demetalation of different types of Fischer alkoxy carbene complexes as well as other methods to demetalate aminocarbene complexes.…”

Multi-component reactions involving group 6 Fischer carbene complexes: a source of inspiration for future catalytic transformations

“…56 As it happens when alkenyl imidates are employed as nucleophiles, 57 the initially formed iminium carbonyl metalates XLVI undergo a cyclization to the dihydropyridyl carbene complexes XLVII. These compounds evolve to the final products by a [2+2] cycloaddition with another equivalent of 3d.…”