Negishi Cross-Coupling Reaction as a Simple and Efficient Route to Functionalized Amino and Alkoxy Carbene Complexes of Chromium, Molybdenum, and Tungsten

Abstract

Synthesis of functionalized phenyl, 2-thienyl, and N-methyl-pyrrol-2-yl Fischer carbene complexes of chromium and tungsten was achieved via a Negishi cross-coupling reaction of metalated aminocarbenes in the presence of a palladium catalyst. The reverse approach, i.e., coupling bromocarbenes with organozinc reagents, is also feasible and is particularly useful with alkoxycarbene complexes, metalation of which is not possible. Both these procedures are also suitable for the cross-coupling of molybdenum amino- and alkoxycarbene complexes. The presented methodology enables an access to the new amino and alkoxy carbene complexes bearing aldehyde, ketone, nitrile, and ester functionality.

Modular Synthesis of Fischer Biscarbene Complexes of Chromium

“…The attempts to couple an excess (2.4 equiv) of the zincated 1a with easily accessible dibromo derivatives such as 1,4-dibromobenzene, 1,3-dibromobenzene, 4,4′-dibromo-1,1′-biphenyl, 2,5-dibromothiophene, 3,4-dibromothiophene, and 1,1′-dibromoferrocene in cross-coupling reactions have failed to provide any carbene product under the above conditions. However, Pd­(Xantphos)­Cl 2 , which proved to be effective also in previous cross-couplings of carbene complexes, gave better results. Thus, 1,3-dibromobenzene, 4,4′-dibromo-1,1′-biphenyl, and 2,5-dibromothiophene afforded the desired biscarbenes 6 in low to medium yields (Scheme and Table , entries 2–4) accompanied by variable amounts of the corresponding products of monocoupling ( 5-Br ) and homocoupling ( 4a ) together with starting 1a (see the Experimental Section).…”

“…The opposite approach, the coupling of metalated 1a with the tungsten derived bromocarbene 2b , has been more fruitful, giving the expected biscarbene complex 12 in 55% yield (Scheme ). This result is consistent with our previous findings that the coupling of 2b with organozinc reagents proceeded easily . Subsequent conversion of 12 to the bisaminocarbene 13 was also smooth.…”

“…The solvents were dried and degassed by standard procedures; silica gel (Merck, Geduran Si 60, 63–200 μm) was used for column chromatography. Pentacarbonyl­[( N , N -dimethylamino)­(2-thienyl)­carbene]­chromium­(0) ( 1a ), pentacarbonyl­[( N , N -dimethylamino)­(2-furyl)­carbene]­chromium­(0) ( 1b ), pentacarbonyl­[( N , N -dimethylamino)­(5-bromo-1-methyl-2-pyrrolyl)­carbene]­chromium­(0) ( 1c ), pentacarbonyl­[( N , N -dimethylamino)­(4-bromophenyl)­carbene]­chromium­(0) ( 1d ), pentacarbonyl­[ethoxy­(5-bromo-2-thienyl)­methylene]­chromium(0) ( 2a ), and pentacarbonyl­[ethoxy­(5-bromo-2-thienyl)­methylene]­tungsten(0) ( 2b ) were prepared by the reported procedures; other compounds were purchased.…”

“…These complexes have been mostly prepared in the classical way, by the introduction of a carbene moiety to the preformed skeleton. , Recently we have shown that aminocarbene complexes containing aromatic or heteroaromatic groups can be lithiated. The subsequent reaction with electrophiles allows the preparation of functionalized carbene complexes and, after transmetalation to zinc, Negishi cross-coupling reactions . Herein we report that the cross-coupling methodology can be also used for the synthesis of different types of biscarbene complexes, including those containing both alkoxy- and aminocarbene moieties, in which two carbene groups are connected with a (hetero)­aromatic bridge.…”

The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

“…614) and pyrrolyl-substituted complexes could be lithiated through halogen-metal exchange at low temperature. The ability to engage the organolithium intermediates in palladium-catalyzed coupling processes was also reported [939]. Conversion of lithiated carbene complex derivatives (e.g.…”