Carbene complexes alkynyl Fischer, cycloaddition

A kinetic study of the 1,3-dipolar cycloadditions of alkynyl Fischer carbene complexes with nitrones showed tirst-order kinetics for both nitrones and the alkynyl carbene complexes. The 1,3-dipolar cycloaddition of chiral non-racemic Fischer... 

The [2+2] cycloaddition of alkynyl Fischer carbene complexes (e.g., 549) with 3,4-dihydro-2//-pyran 455 to form cyclobut-l-enyl esters 550 has been described (Scheme 101) <1999J(P1)197>. 

Alkynyl Fischer carbene complexes 64 (R = H) in the presence of a nitrone undergo a cascade cycloaddition/cyclopropanation process to afford 4-isoxazolines such as 65 in good yields. Under the same conditions, substituted complexes 64 (R = Me, Ph) gave naphthalenes 66 through a cycloaddition/metathesis process <07OL4143>. 

The reaction of a,(3-unsaturated Fischer carbene complexes 42 with cyclic enamines 43 gives cyclopentannulation products 46 with complete regioselectivity by a formal [3-1-2] cycloaddition (Scheme 5.11) [20]. The formal [3-1-2] cycloaddition is assumed to occur in a stepwise manner via the 1,4-addition of cyclic enamines 43 to the alkynyl carbene complexes 42 (formation of iminium intermediate 44), the intramolecular cyclization to the formed iminium moiety from the carbene carbon (formation of intermediate 45), and the hydrolysis reaction. When (—)-8-phenylmenthyl derivatives of Fischer carbene complexes 42 are reacted with indoles 43, the corresponding indolinones 46 are obtained with >97% ee. 

The formal 84-2-cycloaddition reaction between alkynyl Fischer carbene complexes (141) and tropothione (142) produced 3a//-cyclohepta[fi]thiophene carbene complexes (144) in a stepwise process involving antiaromatic zwitterionic intermediates (143) (Scheme 44). ... 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

Aminothiophenol 37 and 277 formed an isolable adduct 278 (X = S) that on heating was transformed to a mixture of complex 283 (10%) and the demetallated benzothiazepinone 284 (51%) (Scheme 50) <2003CEJ4943>. Similarly, monocyclic 1,4-oxazepinone derivatives of tungsten and chromium have been prepared by a domino [4+2]/[2+2] cycloaddition reaction of 1-alkynyl Fischer carbenes with oxazolines <20050M302>. 

The [3+2] cycloaddition of Fischer alkynyl carbene complex 1 with diazomethane was first reported in 1973 (Scheme 5.1) [9]. The reaction initially generates a pyrazole carbene complex 2 as an intermediate, followed by the nucleophilic addition of the second diazomethane to the carbene center, and the denitrogenation occurs to give the A-metalated pyrazole 3. When trimethylsilyldiazo-methane (TMSCHN2) is used instead of diazomethane, the reaction of the initially formed [3+2] cycloadduct with TMSCHN2 was suppressed to give the pyrazole carbene... 

The reactions of Fischer alkynyl carbene complexes 2 with nitrones 21 regioselectivity give dihydroisoxazole carbene complexes 22 in excellent yields (Scheme 5.6) [13]. The electron-withdrawing property of the metal-carbene complexes 20 is confirmed by the density functional theory calculations on the [3+2] cycloaddition [14], The dihydroisoxazole carbene complexes 22 are readily... 

SCHEME 5.2 [3+2] Cycloaddition of Fischer alkynyl carbene complexes 4 with TMSCHN2... 

A. A -Bicychc pyrazolidin-3-ones 31, potentially bioactive heterocycles, are also constructed by the [3+2] cycloaddition of Fischer alkynyl carbene complexes 28 with azomethine imines 29 via isolable metal-carbene intermediates 30 (Scheme 5.8) [17]. 

Heterocyclic 1,3-dipoles 32-34 also react with Fischer alkynyl carbene complexes to give pyrazolyl- and pyrrolyl-carbene complexes (Scheme 5.9) [18]. These reactions proceed along with elimination of carbon dioxide, isocyanate, and sulfur from the initially formed [3-1-2] cycloaddition products. When Fischer alkynyl carbene complexes 35 are treated with 1,3-dipole 33, pyrrolylcarbene complexes 36 are obtained. 

Azides react with Fischer alkynyl carbene complexes via copper-catalyzed or copper-free [3+2] cycloaddition ( click reaction) [19]. The copper-free click reaction... 

A combination of a Diels-Alder and a Fisher carbene-cyclopentannulation is described as the last example in this subgroup. Thus, Barluenga and coworkers used a [4+2] cycloaddition of 2-amino-l,3-butadienes 4-115 with a Fischer alkoxy-arylalky-nylcarbene complex 4-116 this is followed by a cyclopenta-annulation reaction with the aromatic ring in 4-116 to give 4-117 (Scheme 4.25) [36]. An extension of this domino process is the reaction of 4-118 with 2equiv. of the alkynyl carbene 4-119 containing an additional C-C-double bond (Table 4.2) [37]. The final product 4-120, which was obtained in high yield, is formed by a second [4+2] cycloaddition of the primarily obtained cyclopenta-annulated intermediate. 

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