Tungsten complexes with aminos

Reaction of unsaturated chromium and tungsten carbene complexes 407 with piperidazine provides amino-carbene complexes 408 and 409. Although various solvents such as CH2C12 benzene, ether, and THF can be used in the reaction, the yields of the desired products 410 are 32-59%, and considerable amounts of by-products 409 are formed. The carbene complexes 408 are rather stable and can be stored in a refrigerator. Oxidation of both complexes with iodoso-benzene affords oxo derivative 410 in 70% (M = Cr) and 41% (M =W) yields (Scheme 64) <1994CL777>. 

The molybdenum and tungsten complexes catalyze reactions of soft nucleophiles, such as malonates, related 1,3-dicarbonyl compoimds, and nitroalkanes. Azlactones are also soft carbanions, and Trost has shown that complexes formed from molybdenum and the bis(pyridine) ligands catalyze enantioselective and diastereoselective allylation of azlactones with allylic phosphates to form quaternary amino acids (Equation 20.40). In these reactions, the nucleophile adds to the more substituted position of the allylic electrophile, and a stereocenter is formed at both the allyl carbon and the azlactone carbon. One route to the protease inhibitor tipranavir by the molybdenum-catalyzed allylation with 1,3-dicarbonyl compounds was demonstrated by Trost (Equation 20.41), and the Merck process group used related allylation chemistry with Trost s bis(pyridine) ligand to prepare the cyclopentanone precursor to various analogs of tipranavir (Equation 20.42). 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

When the donor character of the amino substituent at the transition-metal-carbene carbon atom is reduced, it should be possible to influence the thermal stability and reactivity in favor of the desired cyclopropanation process. Indeed, pyrrol-1 -ylcarbene complexes 18 of chromium, molybdenum and tungsten do exhibit the desired reactivity. In the last step, the pyrrole ring of 19 can be converted to the NH2 function in 20 after oxidative cleavage with ozone.In this respect, the pyrrole heterocycle represents a synthetic equivalent of the amino function. 

Dinitrogen Complexes and Nitrogen Fixation.—Nitrogenase from Azotobacter vinelandii has been purified and the molybdoferrodoxin protein crystallized as needles.A molybdenum-containing peptide of mol. wt. ca. 1000 has been isolated from this system and shown to contain at least ten naturally occurring amino-acids, with leucine and isoleucine being most prominent molybdenum is present as both Mo " and Tungsten has been incorporated into... 

The cycloaddition reactions of chiral 2-amino-l, 3-dienes with Fischer-type carbene complexes have been examined by Barluenga et al. Reactions with tungsten vinyl carbene complexes A and with boron-nitrogen-chelated chromium complexes B lead very selectively to cyclohexanone derivatives. However, when employing chromium vinyl carbene complexes in acetonitrile at room temperature in these reactions, cycloheptadienes were obtained selectively by cyclopropanation and subsequent Cope rearrangement. 

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