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tion with 1.0 mol% Mo, a straight line was obtained that cor-
roborates that the reaction is first order with respect to molyb-
denum (see Figures S8 and S9 in the Supporting Information).
The divergence in the partial order of molybdenum, observed
for high and low benzyl alcohol concentrations, is attributed to
the aforementioned disproportionation of benzyl alcohol at
high concentration.
Table 2. Reactivity of functional groups in a benzylic position under the
conditions optimised for the transfer HDO of benzyl alcohol.[a]
Entry
Substrate
Product
PhCH3
Yield [%]
1
2[b]
3
PhCH2OH
PhCH2OH
PhCHO
PhCH(OCH3)2
PhCOOCH3
PhCH2OCH(CH3)2
PhCH2NH2
93
60
73
4[b,c]
5
61
ꢀ0[d]
Dilution experiments were carried out by varying the start-
ing concentrations of PhCH2OH, yet leaving the ratio of
PhCH2OH/AHM constant. Hence, the required catalyst turnover
number to reach full conversion remained unchanged. Reac-
tions starting with 50, 20, 8, and 4 mmol of PhCH2OH were fol-
lowed over time (see Figure S10 in the Supporting Informa-
tion). The effect of diluting the reaction mixture became appar-
ent when using 8 and 4 mmol of substrate, for which the con-
sumption of PhCH2OH slowed and the formation of PhCHO
was no longer observed.
6[c]
7
8
13[e]
57
8
PhCH2SH
9
10
PhCH(OH)CH3
PhCH(O)CH3
PhCH2CH3
57
59
[a] Unless otherwise noted, 100 mmol of substrate was used and the
standard reaction conditions were employed with conversions of higher
than 90%. [b] No Bu4NOH was added. [c] An aliquot of 50 mmol of the
substrate was added with 50 mmol of para-methylbenzyl alcohol. [d] The
major product was isopropyl benzoate. [e] The conversion was 84%; the
major product was not identified.
A deuterium-labelling experiment was conducted to ascer-
tain the role of disproportionation during the reaction. The
transfer HDO reaction of 50 mmol of PhCD(OH)CH3 was sub-
jected to the standard reaction conditions in iPrOH. Significant
disproportionation would result in scrambling of the deuteri-
um atom in the a position, thus leading to the products
PhCHDCH3, PhCH2CH3, and PhCD2CH3. Nevertheless, the major
product was always PhCHDCH3, and minor products a-[D1]-sty-
rene, (1-deutero-1-isopropoxyethyl)benzene, and meso- and
rac-2,3-bideutero-2,3-diphenylbutane were also detected. This
outcome excludes the possibility that a significant proportion
of the benzylic alcohol undergoes disproportionation to ethyl-
benzene and acetophenone with concomitant transfer hydro-
genation of acetophenone to 1-phenylethanol and subsequent
HDO to ethylbenzene. On this basis, we conclude that the
main reaction is a transfer HDO that involves iPrOH and benzyl
alcohol, with disproportionation of benzyl alcohol being of
minor importance.
Hammett study
To gain further insight into the nature of the rate-determining
step of the reaction, a Hammett study was carried out in line
with earlier work[27–30] (see the Experimental section for details).
The Hammett study was conducted at a concentration low
enough to ensure that disproportionation did not affect the
result. A series of competition experiments were undertaken in
which the relative rates for the transfer HDO of para-substitut-
ed benzylic alcohols to benzyl alcohol were determined. In
a typical experiment, samples were extracted during the HDO
of a mixture of 4 mmol of a para-substituted benzylic alcohol
and 4 mmol of benzyl alcohol, and the reaction progress (i.e.,
the conversion of the benzylic alcohols) was determined by GC
by using hexadecane as an internal standard. The para-substi-
tuted benzylic alcohols used were para-methylbenzyl alcohol,
para-(methylthio)benzyl alcohol, para-chlorobenzyl alcohol,
para-fluorobenzyl alcohol, and para-(trifluoromethyl)benzyl al-
cohol. para-Methoxybenzyl alcohol was excluded from the
analysis as para-methoxybenzyl isopropyl ether was produced
in addition to para-methoxytoluene.
Functional-group reactivity
The versatility of the catalyst was investigated by testing the
reactivity toward other functional groups in the benzylic posi-
tion (Table 2). The secondary alcohol 1-phenylethanol was
transformed into ethylbenzene in a yield of 57%, whereas the
carbonyl compounds benzaldehyde and acetophenone under-
went transfer hydrogenation and subsequent transfer HDO to
form toluene and ethylbenzene in yields of 73 and 59%, re-
spectively; furthermore, only trace amounts of styrene were
observed from 1-phenylethanol and acetophenone. In the ab-
sence of a base, benzaldehyde dimethyl acetal was trans-
formed into toluene in a yield of 61%. Benzoic acid was con-
verted into the ester isopropyl benzoate under the reaction
conditions; neither methyl benzoate nor benzyl isopropyl
ether displayed any significant reactivity (Table 2, entries 5 and
6). Benzylamine and benzylmercaptan underwent transfer HDN
and HDS in low and moderate yields, respectively (Table 2, en-
tries 7 and 8), thus indicating that these reaction conditions
might be applicable for the removal of benzylic thiol groups.
In a competition study, the reaction order in each compo-
nent will be the same for both substrates under the assump-
tion that the substrates follow the same mechanism. Provided
that the reaction is first order with respect to the substrate,
the relative reactivities of the two benzylic alcohols can be de-
termined by plotting ln(X0/X) (where X is the concentration of
the para-substituted benzylic alcohol and X0 the initial concen-
tration) against ln(H0/H) (where H is the concentration of the
benzyl alcohol and H0 the initial concentration) (Figure 3). The
relative reactivity krel was obtained as the slope of the line
(Table 3).
The values of krel were used to construct the Hammett plot
in Figure 4. Albeit none of the Hammett parameters provide
a good correlation, s+ and the Creary constant s C appear to
C
C
show some correlation. The parameter s C is based on the rate
of the thermal rearrangement of methylenecyclopropanes to
isopropylidenecyclopropanes. This reaction probes the stabiliz-
&
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Chem. Eur. J. 2016, 22, 1 – 12
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ÝÝ These are not the final page numbers!