TpRe

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. 

Scheme 12. The TpRe(CO)(Melm) -promoted Diels-Alder reaction of benzene.

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... 

The TpRe(CO)(L) fragment coordinates -aromatics more strongly. While TpRe(CO)2 binds naphthalene, furan, and A-methylpyrrole in a labile f fashion,... 

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... 

Figure 5 Diastereomer interconversion in TpRe(CO)(MeIm)(thiophene)...

Time course and covariate model TPRE=THETA(1) /... 

SCHEME 11.33 Comparison of calculated energetics for methane oxidative addition to CpRe(CO)2 and TpRe(CO)2 (energies given in kcal/mol). 

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

Fig. 5. SEM pictures for rubrene on mica substrate Ts = 230°C, TwaU = 240°C, Ta = 120°C, Tpre-heat= 90°C.

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