Summary: Anisóle, when complexed by pentaammin-
eosmium(II), undergoes a [4 + 2] cycloaddition with
N-methylmaleimide which is shown to be nonconcerted.
Abstract: A series of alkylated phenols (phenol, p- and o-cresol, 4-methoxy- and 3,4-dimethoxyphenol, 5,6,7,8-tetrahydro-
2-naphthol, and ¡8-estradiol) are complexed with pentaammineosmium(II) and treated with a variety of Michael acceptors
(e.g., MVK, methyl acrylate, acrylonitrile, jV-methylmaleimide, and 3-butyn-2-one). Under both acidic and basic
reaction conditions, a single conjugate addition takes place either ortho or para to the hydroxy group to generate stable
2,4- or 2,5-cyclohexadien-1 -one complexes, respectively. For phenol the conjugate addition occurs exclusively at C(4)
for all Michael acceptors investigated. For cases where C(4) is substituted, the regioselectivity depends on reaction
conditions and the nature of the electrophile, but examples of selective addition at both C(4) and C(6) are reported.
When the sp*123 carbon of the resulting cyclohexadienone is a methine, the ligand may be rearomatized and removed from
the metal by moderate heating (70-80 °C). When the sp3 carbon is quaternary, stable dienone products may be
obtained by oxidative removal of the metal with Ce(IV) or DDQ, and a number of substituted arene and dienone
products are synthesized by this methodology in good overall yield (54-83%).
Reduction of [Re02(en)2]+, [Re02(NH3)4]+, or [Re02(ampy)2]+ (en= ethylenediamine; ampy= 2-(aminomethyl)-
pyridine) in HC1 (aq) produces [ReCl2(en)2]+, [ReCl2(NH3)4]+, and [ReCl2(ampy)2]+ respectively. Chloride
substitution on Zrnnr-[ReCl2(en)2]+ by pyridine, isonicotinamide, nicotinamide, 4-picoline, 4-(dimethylamino)-
pyridine, and n-propylamine is accomplished with accompanying isomerization to a cis geometry. The crystal
structures of [ReCl2(en)2](PF6) (triclinic; £1 (No. 2); a= 8.717(1) k,b= 12.618(2) A, c= 6.523(1) A, a=
98.44(2)°,ß= 102.66(1)°, y= 78.50(1)°; > = 2.543 g/cm3 4;Z= 2), [ReCl2(ampy)2](BPh4),(triclinic;£1 (No.
2); a= 12.334(1) A, b= 17.345(2) A, c= 8.918(1) A, a= 91.30(3)°, ß= 94.75(3)°, y= 73.29(2)°; * <* =1.551
g/cm3; Z= 2), and [ReCl(picoline)(en)2](PF6)2, (monoclinic; P2\/c (No. 14); a= 9.233(3) A, b= 17.824(5) A,
c= 13.979(5) A, ß= 108.15(3)°; Aaicd= 2.202 g/cm3; Z= 4) are reported, and various equilibrium constants
are determined which pertain to the relative affinity of Re(II) and of Re(III) for -acid ligands. Parallels are drawn
to the established -bases [Ru(NH3)5]2+ and [Os(NH3)5]2+.
The reduction of Os(NH3)5(OTf)3 in D20 or H20 over zinc amalgam gives rise to a species that retains the ability
to form (NH3)50sn adducts with the -acidic ligands acetone, 7V,iV-dimethyluracil (DMU), and acetonitrile for
periods of hours in the absence of reducing agent. The i?2-DMU complex was shown not to be in equilibrium with
free DMU, in contrast to a previous report. Kinetic trapping studies are described that demonstrate the following:
(1) an “active” source of (NH3)50sn other than the aquo complex is formed and decomposes by a first-order process
with a half-life of about 3 hat room temperature in D20; (2) the rate of the decomposition process increases markedly
at lower pH; and (3) the decomposition rate in H20 is about twice that in D20. No direct spectroscopic or
electrochemical evidence of species having osmium-hydrogen bonds was observed at neutral pH.