1. Am. Chem. SOC.1989, 1 1 1 , 5485-5487 Table I. Oxidation and Reduction Potentials, Their Numerical Sums. and the Longest Wavelength Absorption Maximum of Quinones 1, 2. 3, 4 Compared with a Reference Compound,
Tetra-rerr-butyldiphenoquinone ( I I ) electrochemical propertieso quinone 11 1
2 3 4
E2OX
€,Ox
E,'""'
€,led
-0.52 f l . 2 0 -0.46 f 1 . 4 4 +0.91 -0.31 +1.07 f 0 . 6 3 -0.26 f 0 . 8 9 f 0 . 5 5 -0.20
-0.89 -0.60 -0.39 -0.30 -0.26
1.65 1.22 0.89 0.75
absorptn maxb
E F m h nm (log 1.81 1.83
1.37 1.15
t)
proposes that the Lewis acid complex I-B enhances facial selectivity by favoring approach of the nucleophile on a trajectory that brings it in greater proximity to the chiral substituent than in the case of the uncomplexed aldehyde (l-A).) This same kind of analysis has been successfully applied to the addition of nucleophiles to chiral ketones6b and chiral thionium ions.6c
(4.85) (4.90) (4.50) (4.77) 830 (4.45)
+
Scheme I1
i
Nu
1-A
NU
1.8
In a previous report from our labor at or^,^ it was observed that the enolate of methoxyl stabilized carbene complex 2 would add to dl-2-phenylpropanal that had been pretreated with titanium tetrachloride to give the aldol adduct 4 as an 8.1 mixture of diastereomers (l.u).*-" The carbene complex 2 can serve as a OMe
[I -41'
Nu
Nu -
420 558 678 785
"Obtained bq cyclic voltammetry vs S C E with 0.1 M Et4NC104 at room temperature (scan rate, 50 mV/s; solvent, I , 11 in M e C N , 2 in CH2C12;3, 4 in EtCN): €Ox and Ercdvalues were calculated by aver€C,)/2. * I n aging the anodic and cathodic peak potentials: € = E; MeCU.
L2-41"
5485
(co),c~+ CH3
1) nBuLi, THF, -78" C
2)
2 equiv
-
2
[I -41'
(1
