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As summarized in Scheme 3, a variety of substrates 6 bearing
achieving high levels of ees in Cu-catalyzed hydrosilylation reac-
tions.11a These results echoed earlier observations on highly enan-
tioselective reductions of ortho-substituted diaryl ketones with Ru-
catalyzed asymmetric transfer hydrogenations and chiral
oxazaborolidines-catalyzed reductions, respectively.9,11bef Fur-
thermore, these magical ‘ortho-effects’ on asymmetric inductions
could not be easily accommodated by conventional steric rationales
as in these systems, in order to stay in line with a self-consistent
stereochemical model, the apparently bulkier ortho-substituted
aryl rings must be assumed to be of smaller sizes when compared
to unsubstituted aryl rings. However, these seemingly abnormal
stereochemical behaviors were readily explained when polariz-
ability electronic effects were considered. As illustrated through
structure 7q, the bond rotation incurred by minimizing the re-
pulsion between ortho-substituent R and the alkene double bond
a more polarizable substituent at the left side (in blue) and a less
polarizable substituent at the right side (in red) were examined.
The simple polarizability ranking principles employed here are
exactly the same as those used more conventionally in inferring the
softness of a group in the classical HSAB theories,5 and conform to
the local chemical environment in which the group under concern
resides. The polarizability ranking for many common organic
groups had been illustrated previously.4 Relevant to this work are
mainly the following two polarizability sequences: for benzenes
electronically differentiated by substituents on their aromatic
rings: benzene ring with electron-donating substituent>benzene
ring itself>benzene ring with electron-withdrawing substituent;
and for the central carbons in alkyl groups, strained alkyls (cyclo-
propyls) >unstrained alkyls.
From the results listed in Scheme 3, it is experimentally evident
that in each case the two substituents on 6 competed sensitively in
responding to their relative local polarizabilities. For those 1,1-
p-cloud would inevitably interfere with the local benzene ring’s
parallel alignment with regard to the reacting double bond, thereby
effectively reducing its ring polarizability influence on the reaction
ee-determining state. Consequently, regardless of the electron-
donating or -withdrawing nature of the R substituent, the aryl
ring bearing an ortho-substituent exerts less stereochemical control
than that of unsubstituted aryl ring. In accord with this polariz-
ability effect, in each case of diols 7ret, the less bulkier benzene
ring functions effectively as PL group leading to appreciable mag-
nitudes of ees.
diaryl substituents bearing more
p-conjugation (7a) or an
electron-donating substituent (7b and 7c), these substituents
possess higher local benzene ring polarizabilities thus function as
stereochemistry-controlling PL groups; for those 1,1-diaryl sub-
stituents bearing less
p-conjugation (7d) or an electron-
withdrawing substituent (7e, 7f, 7g, and 7h), the corresponding
local benzene rings here themselves behave as PL groups. In diols 7i,
7k, and 7l flanked by both an electron-donating and -withdrawing
group-substituted benzene rings, the benzenes of higher
p-elec-
tron densities predominate in local ring polarizabilities. In diol 7j
where both benzene rings are substituted with electron-donating
groups, the one with stronger electron-donating ability (SMe-vs-
OMe) leads the stereochemical course.10 At this point these com-
parisons on polarizabilities are qualitative in nature, there is cer-
tainly no basis for conducting quantitative analysis. But for
structurally highly comparable systems, such as 7b,c,e,f, and 7j,k,l,
in each series a clear trend of ees increasing with the increases of
polarizability distinction between the two aryl substituent local
benzene rings was observed. Cyclopropyl-containing diols 7nep
represent a fascinating class of polarizability-tuned structures. Al-
though the three-membered ring strain-induced bent carbon-
ecarbon
s
-bond electronic densities in a cyclopropyl group are less
-system, they are consid-
Scheme 4. Polarizability consequence of steric ‘ortho-effects’.
polarizable than those of an aromatic
p
erably more polarizable than those in unstrained cyclic or linear
alkyls. Thus, from 7n to 7o,p a reversal of the role of the cyclopropyl
substituent in directing asymmetric induction was observed in
responding to its local polarizability ranking. Furthermore, as the
polarizability difference between phenyl and cyclopropyl is smaller
than that between phenyl and isopropyl, the ee in 7n is lower than
that in 7m, despite the fact that steric size difference in 7m is less
conducive for achieving asymmetric induction than that in 7n.
These are important results exemplifying that, although there is no
doubt that steric effects are important stereochemical factors, they
can be overwritten by competing polarizability effects. Similarly, in
7o,p polarizability controls also outweigh counter-acting steric ef-
fects: the local carbon (highlighted in bold) polarizability of
cyclopropyl ring is higher than that of the bulkier cyclohexyl in 7o,
and methylcyclohexyl in 7p, thus although the magnitudes of ees
eroded with the increasingly developed unfavorable steric in-
terferences from 7o to 7p, the sense of asymmetric induction re-
mains unchanged.
It should be emphasized again that, since the two aryl or ali-
phatic substituents in these examined 1,1-disubstituted alkenes
have significantly smaller polarizability and steric size differences
than those commonly observed in alkenes possessing both an aryl
and an aliphatic groups, there is simply no basis for achieving high
ees herein. But despite the moderate-to-low ees recorded with
dihydroxylations compiled in Schemes 3 and 4, we believe that the
uncovered sensitive dependence of the senses of asymmetric in-
ductions on the aryl ring electronic polarizabilities is conceptually
very significant and novel. Since the updated Sharpless AD ste-
reochemical model2 had already emphasized the synergistic effects
in enantio-controls between the trans-alkene substituents at the
SW and NE quadrants (Scheme 1), the revealed dependence readily
suggests that mapping the polarizabilities of the SW-NE trans-
substituents pair with those of the competing NW-SE trans-pair
should be a productive and predictive means for rationalizing the
general stereochemical outcomes in Sharpless AD processes. In
other words, as summarized in Scheme 5, as long as the local po-
larizabilities of the SW-NE trans-substituents pair outweigh those
of the NW-SE trans-substituent pair, the corresponding alkenes
must be dihydroxylated with the same top-face trajectory with the
Some 1,1-diaryl-alkenes with ortho-substituents were also ex-
amined (Scheme 4). This type of substrates merit particular atten-
tion because they stand out prominently among an extensive range
of diaryl substrates (such as diaryl ketones) that generally perform
very poorly in asymmetric catalysis when such ortho-substituents
are absent. For examples, as already highlighted by some recent
reports, a series of ortho-substituted diaryl ketones were capable of
AD-mix-b reagent, or conversely, the bottom-face selection with