TpRe complex

Metal fragments of the form (TpRe(CO)(L), in which L = Melm or NH3, have the ability to bind substituted benzenes in an 2-fashion. The fragment having L = Melm has seen more application because of difficulties associated with the synthesis and isolation of aromatic complexes having L = NH3. Anisoles, phenols, and naphthalenes all form thermally stable -complexes when stirred in excess (10-25 equiv.) with TpRe(CO)(MeIm)( /2-benzene) in THF. The complexes are typically isolated by precipitation from hexanes. The benzene complex can be prepared by direct reduction of the Re(III) precursor, TpRe(MeIm)(Br)2, with Na° in benzene under one atmosphere of CO [35]. 

In solution, naphthalene complexes of the form TpRe(CO)(L)( /2-naphthalene) exist as mixtures of diastereomers A and B (Table 15). Only when L = PMe3 (entry 1) does the unbound ring of the naphthalene show a thermodynamic preference for quadrant a (98A). For all... 

Another early success of the (TpRe(CO)(MeIm) fragment was the promotion of Diels-Alder cycloaddition reactions with benzene and anisole. Complex 101 [TpRe(CO)(MeIm)( 2-benzene)] undergoes an endo-selective Diels-Alder reaction with N-methylmaleimide to afford the bound bicyclo[2.2.2]octadiene complex 102 in 65 % yield (Scheme 12) [40]. Oxidation of 102 yields the bicyclo[2.2.2]octadiene 103 and/or the bicyclo[2.2.2]octenone 104 depending upon the choice of oxidation conditions. 

The more activated TpRe(CO)(MeIm)(r/2-anisole) complex has demonstrated a slightly broader range of reactivity by undergoing cycloadditions with both N-methylmaleimide (Scheme 13) and dimethylacetylene dicarboxylate (DMAD) (Scheme 14). Analysis of these reactions is complicated by the fact that the initial anisole complex exists as a 3 1 mixture of... 

TpRe(CO)(PMe3) produces a variety of stable rj complexes of the type TpRe(CO)(PMe3)() -E) (E = cyclohexene, cyclopentene, naphthalene, phenanthrene, thiophene, 2-methylthiophene, furan, or acetone). They are obtained by reduction of the Re complex TpRe(CO)(PMe3)(OTf) with Na/Hg in the presence of the unsaturated ligand. 

The TpRe(CO)(PMc3) fragment is isoelectronic with [CpRe(NO)(PPh3)]+ and yet considerably more electron rich. This stronger tt basic character is reflected in its capacity to bind naphthalene, diasterospecifically, in a r] -coordination mode, to prefer ) -thiophene over ) -thiophene coordination and form stable tt complexes with acetone rather than the more common a-bonded species in, for example, [CpRe(NO)(PPh3 )(acetone)]+. 

Harman has developed an analog of the CpRe(NO)(L)+ systems using hydrido(tris)pyrazolylborate, Tp, complexes. The [TpRe(CO)(PMc3)] moiety is more electron rich than its counterpart and will even bind naphthalene in an jj -fashion. Ethylene rotation in TpRe(CO)(L)(ene) has a barrier of 40.0 kJmol as determined by SST experiments. Substantial variation of barriers correlated with basicity were noted with different L groups L = t-BuNC, 32 kJmol estimated from extensive... 

CpRe(CO)2(CH4) system is smaller than that for the TpRe(CO)2(CH4) complex with AAH = 6.6 kcal/mol. 

Reactions between metal complexes and organic substrates in the condensed phase usually begin with the eoordination of these reactants. Due to 7t- and n-electrons present in the molecules of substances frequently employed in catalytic processes (such as olefins, acetylenes, carbon monoxide) they are capable of forming rather stable complexes with transition metals. For example, the complex TpRe(CO)2(THF) (Tp = hydridotris(pyrazolvl)borate THF = tetra-hydrofiiran) reacts with a variety of aromatic molecules to form stable binuclear complexes of the form TpRe(CO)2 2(fi-r LTi -L), where L = fiiran, Al-methyl-pyrrole, or naphthalene [3a] ... 

Fig.4 The asymmetric environment of TpRe(CO)(L)(arene) complexes and the assignment of quadrants...

Naphthalene forms stable complexes with [TpRe(CO)(L) when L=f-BuNC, PMcj, py, DMAP, or Melm. In toluene and an excess of naphthalene, the direct reduction of TpRe(L)Br2 (L=py or DMAP) with Na° under a CO atmosphere will generate the py or DMAP naphthalene complex in -40% yield [16]. Generation of the Re(I) PMe3 and f-BuNC naphthalene systems is not as straightforward. They are synthesized by the oxidation of the corresponding Re(I) olefin complexes to Re(II) complexes, liberation of the olefin by heating, and reduction of the Re(II) species in the presence of naphthalene [16]. 

The more activated TpRe(CO)(MeIm)(q -anisole) complex has demonstrated a slightly broader range of cycloaddition reactivity. The anisole complex 110 cychzes with DM AD to give the diastereomeric bound bicyclo [2.2.2] octatriene 111 isolated as a 1 1 ratio of coordination diastereomers (Fig. 23). Oxidation of these complexes liberates the corresponding triene 112 as well as the disubstitut-ed anisole 113, which presumably is generated with acetylene from the cycloreversion of 112. 

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]. 

Schrock and co-workers have described the synthesis of a rhenium neopentyli-dyne complex containing the tris(pyrazolyl)borate ligand from Re( = C Bu) (CH2 Bu)3(OTf). Treatment of this complex with excess pyridine afforded the colourless, six-coordinate rhenium neopentylidene/neopentylidyne complex Re( = C Bu)(OTf)(CH2 Bu)( = CH Bu)(py)2], which was subsequently subject to ligand substitution by Na[Tp], producing the thermally stable, 18-electron complex TpRe( = C Bu)(CH2 Bu)( = CH Bu) (Scheme 27). Rhenium is one of the three... 

Scheme 1. Trapping of TpRe(O) by dioxygen ( 0 = 0) and reaction of complex 1 with perfluoroacetone in the presence of dioxygen.

Photolysis of 7 in the presence of DMSO (DMSO = dimethylsulfoxide), py or NCMe yields the phenoxide complex TpRe(OPh)(Cl)(L) (L = O, py or NCMe) (Scheme These transformations likely occur via an intramolecular [l,2]-shift... 

Isolable transition metal complexes containing hydride and terminal oxo ligands are rare however, Tp Re( = 0)(H)X (X = Cl, H or OTf) and TpRe( = 0)(H)Cl have been synthesized, isolated and characterized. Reactions of Tp Re( = 0)(H) OTf (12) with unsaturated substrates (e.g., ethene, propene or acetaldehyde) result in insertion of C = C or C = 0 bonds into the Re-H bond to yield Tp Re( = 0)(R) (OTf) (R = ethyl or propyl) or Tp Re( = 0)(0Et)(0Tf) (Scheme 6). Oxidation of 12 with pyridine-iV-oxide or DMSO produces Tp Re( = 0)3, acid and free pyridine or dimethylsulfide, respectively. A likely mechanism involves initial oxidation of 12 to produce [Tp Re( = 0)2H][0Tf] (13) followed by the formation of Tp Re( = 0) (OH)(OTf) (14) via a 1,2-migration of the hydride to an oxo ligand (Scheme 6). Reaction of 14 with a second equivalent of oxidant in the presence of base yields Tp Re( = 0)3 (15). Direct deprotonation of 13 is noted as less likely than the pathway shown in Scheme 6 due to the lack of precedent for acidity of related rhenium hydride systems. 

Re(V) oxo-amido complexes of the type TpRe( = 0)(NRR )Cl are formed upon reaction of 4 with several primary or secondary amines (NRR = NHEt, NH"Pr, NH Pr, NH2, NEt2, NHPh, NH-/ -tolyl or piperidyl) (Scheme 11). The Re complexes with secondary amido ligands are more robust and less reactive than the... 

Scheme 11. Reactivity of TpRe( = 0)Cl2 (4) with amines to produce Re(V) amido complexes.

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