TpRe tandem addition

In order to probe the tolerance of the molybdenum(O) aromatic complexes to electrophilic/acidic environments, a tandem addition sequence was attempted for the complex TpMo(NO)(MeIm)(q -naphthalene) (114) (Fig.25) [17]. In a strategy similar to that used with the TpRe(CO)(MeIm)(q -naphthalene) analog,(see below) an acetonitrile solution (-35 °C) of 114 was exposed sequentially to triflic acid, l-methoxy-2-methyl-l-trimethylsiloxypropene, and an amine base. The 1,2-dihydronaphthalene complex 116 was isolated in virtually quantitative yield. No evidence of free naphthalene or 1,4-addition product was observed. Stirring the reaction mixture with exposure to air resulted in an 80% overall yield of 2-(l,2-dihydro-naphthalen-2-yl)-2-methyl-propinoic acid methyl ester (117) following TLC purification [17]. 

Another way to obtain enantio-enriched tandem addition products from racemic coordination diastereomers is to use solid-state induced control of TpRe(CO)(L) ( -aromatic) stereoisomers where the aromatic substrate is prochiral. This method is effective for L = Melm, Bulm, py or PMes and f/ -aromatic = anisole, 3-methylanisole... 

In the presence of triflic or trifluoroacetic acid, a tandem addition occurs with TpRe(CO)(L)(f/ -furan) and methanol at —40 °C (L = BuNC, PMes, py or Melm). " Unlike the typical addition of an electrophile to free furan, which is favored at the 2-position, electrophilic addition occurs at the 3-position of the %-bound furan to give a rhenium-stabilized -3H-furanium. Nucleophilic attack anti to the metal at the 2-position gives TpRe(CO)(L)(2-methoxy-2,3-dihydrofuran), and the products form as two diastereomers that differ in the relative orientation of the... 

Scheme 50. Tandem addition to TpRe(CO)(PMe3)( 7 -furan) providing two diastereomers each resulting in 2,3-dihydrofuran with the ring O syn to PMe3 ([Re] = TpRe(CO) L = BuNC, PMe3,...

The stability of cationic Re(I) arenium complexes (see above) renders the arenium ligand susceptible to a second controlled transformation with a nucleophile. Thus, an electrophile can be added to a dihapto-coordinated aromatic ligand followed by a nucleophile in a stepwise tandem addition. For example, TpRe(CO)(MeIm)... 

The coordination of toluene to form TpRe(CO)(MeIm)(// -toluene) results in a mixture of four regio- and stereoisomers.The addition of HOTf and MMTP to TpRe(CO)(MeIm)(f/ -toluene) results in a tandem addition to the coordinated toluene to produce c/y-3-dimethylcarbomethoxymethyl-6H-hydro-l,4-cyclohexa-diene in 62% isolated yield. Despite the mixture of Re isomers in the parent solution, this is the only organic product isolated from the reaction. Likely due to the additional steric bulk of the methyl group, Michael acceptors or acetals were not incorporated at C3. 

TpRe(CO)(L)(f/ -naphthalene) complexes undergo tandem addition reactions with control of regioselectivity dependent upon the identity of At 20 °C,... 

Scheme 57. 1,4-Tandem addition of DMM/MMTP using TpRe(CO)(L)(f7 -naphthalene) (L = PMes,...

Scheme 55. Tandem electrophile/nucleophile addition to the y -benzene ligand of TpRe(CO) (MeIm)(f/ -benzene) (MMTP = l-methoxy-2-methyl-l-(trimethylsiloxy)propene).

---