Alkanes to nitriles and α-iminoesters. Polyoxotungstate photocatalytic radical chain initiation

Article abstract of DOI:10.1039/a805036h

Irradiation of W₁₀O₃₂^4- or PW₁₂O₄₀^3-, alkanes and methyl cyanoformate in CH₃CN solution produces either the corresponding nitriles or α-iminoesters with high selectivity, depending on the temperature, via a mechanism involving two roles for the polyoxotungstate.

Full text of DOI:10.1039/a805036h

Alkanes to nitriles and a-iminoesters. Polyoxotungstate photocatalytic radical
chain initiation
Zhanmiao Zheng and Craig L. Hill*
Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA. E-mail: chill@emory.edu
Received (in Bloomington, IN, USA) 30th June 1998, Accepted 22nd September 1998
Irradiation of W10O3242 or PW12O4032, alkanes and methyl
hν
Pox
RH
R•
Pox
*
(1)
(2)
cyanoformate in CH3CN solution produces either the
corresponding nitriles or a-iminoesters with high selectivity,
depending on the temperature, via a mechanism involving
two roles for the polyoxotungstate.
+
Pox
*
R•
+
H+(Pred
RC(=N•)CO2CH3
RC(=NH)CO2CH3
RC(=NH)CO2CH3
CO2 CH3•
)
+
NCCO2CH3
(3)
RC(=N•)CO2CH3
RC(=N•)CO2CH3
RC(=N•)CO2CH3
+
+
RH
H+(Pred
RCN
CH4
+
R•
(4)
)
+
Pox
(5)
While a host of C–H bond activation or functionalization
methods developed in the last few years have provided a wealth
of mechanistic information and some unusual or unprecedented
transformations, few of the reactions have been of significant
synthetic value. Organometallic systems that activate C–H
bonds rarely lead to functionalization and are usually not
catalytic in the metal complex,1–5 while more conventional
radical and electrophilic systems usually have selectivity or
compatibility difficulties.4,5 We report here the first catalytic
conversion of unactivated C–H bonds to two desirable groups in
high selectivity: nitriles and a-imino-acetic acid esters (hence-
forth iminoesters) via polyoxotungstate photocatalysis.6,7 Ni-
triles are widely used in synthesis,8 and a-ketoesters (or acids),
readily derived from hydrolysis of the iminoesters, have a rich
photochemistry and could be used to render hydrocarbon
materials, including polyethylene, sunlight degradable and
biodegradable.9 Nitriles have been generated in moderate to
good yields via organic radicals generated from conventional
precursors (not generated catalytically by alkane C–H bond
cleavage), by trapping with tert-butylisocyanide,10 or with aryl
and alkylsulfonyl cyanides.11 Alkanes photolyzed in the
presence of ClCN12 or heated with consumption of considerable
conventional radical initiator (10–24 mol% benzoyl peroxide)
in the presence of methyl cyanoformate also yield nitriles.13 The
latter reaction forms some iminoester but yields were not given.
There appear to be no good routes to iminoesters from
unactivated C–H bonds.
+
+
(6)
CH3•
CH3•
+
+
RH
NCCO2CH3
+
R•
CH3C(=N•)CO2CH3
CH3C(=NH)CO2CH3
CH4 CO2
(7)
(8)
CH3C(=N•)CO2CH3
CH3C(=N•)CO2CH3
RC(=NH)CO2CH3
+
RH
+
R•
(9)
CH3CN
+
+
(10)
(11)
under rxn
conditions
RCN
(Pox and Pred are oxidised and reduced polyoxotungstate, respectively.)
same dominant C–H bond cleaving species is operable in the
presence of MCF: (1) the tertiary/primary (3°/1°) C–H cleavage
ratios ( > 200 for 2,3-dimethylbutane and cis-1,2-dimethylcy-
clohexane), (2) the lack of reactivity of tert-butylbenzene (all
primary C–H) and (3) the loss of stereochemistry during
functionalization of cis-1,2-dimethylcyclohexane. The coupling
and disproportionation (alkene only detectable) products in
Table 1 are more consistent with radicals than other organic
intermediates. Three lines of evidence indicate the title
processes, unlike most polyoxotungstate photocatalyzed alkane
functionalizations, involve radical chains. First, there is sig-
nificant inhibition by radical inhibitors. This is seen for
production of both reduced polyoxotungstate and organic
products. For example, the ratio of rates for production of
iminoester from 2,3-dimethylbutane using W10O3242, without
and with 2.0 mM hydroquinone (HQ) inhibitor, kno HQ/kwith HQ
> 100. This ratio without and with 2.0 mM 2,6-di-tert-
butylphenol (BHT), kno BHT /kwith BHT = 5.7 ± 0.2. For HQ and
BHT, respectively, ca. 10 and < 2% of the light is absorbed by
the inhibitor; the rest by W10O3242. Second, the quantum yields
42
Irradiation (l > 280 nm) of CH3CN solutions of W10O32
or PW12O4032 containing one of a variety of alkanes and methyl
cyanoformate (MCF) produces the corresponding nitriles at T =
90 °C and iminoesters at T = 22 °C. Significantly, the
selectivities for these products and turnovers of the poly-
oxotungstate vary inversely with the concentration of the
polyoxometalate and selectivities approach quantitative values
at [polyoxotungstate] < 0.05 mM. Table 1 gives the products
from many reactions using 1.5 mM polyoxotungstate, a
concentration not optimal for selectivity but one permitting by-
product quantification needed for mechanism elucidation. At
low [polyoxotungstate], the selectivities for nitriles (high T) and
iminoesters (low T) exceed those of all literature reactions.
Significantly, as both nitriles and iminoesters are of comparable
or lower reactivity than the alkanes themselves, these reactions
may be of preparative value. For example, the iminoesters
derived from 2,3-dimethylbutane and cis-1,2-dimethylcyclo-
hexane were isolated in 59 and 67% yields respectively (at 28
and 61% conversions of alkane).
for both nitrile and iminoester reactions exceed 1.0 when
42
[polyoxotungstate] < 0.05 mM. Third, W10O32
inhibits
iminoester formation at high [W10O3242]. In contrast, photo-
42
catalytic C–H cleavage by W10O32
generally exhibits
42
conventional kinetics (reaction first order in W10O32 and in
alkane).6,7
Inhibition at high [W10O3242] most likely reflects redox
capture of radical by polyoxotungstate.6
The two propagation steps, eqn. (3) and (4), both of which are
precedented in studies involving conventionally generated
radicals,12 sum to the net reaction for production of iminoester.
Several additional experiments establish that the mechanism for
nitrile formation is eqn. (1)–(3), (6) and (7) [eqn. (8)–(10) are
minor processes]: first, quantification of CO2 (via BaCO3) and
CH4 (via GC/TCD) products indicates that [nitrile] ~ [CO2] ~
[CH4]; second, when the reactions are run in N,N-dimethylace-
tamide (DMA), PhCN, or PhCl versus CH3CN, very little
CH3CN [eqn. (10)] and little or no methyl pyruvate derivatives
[eqn. (9)] are formed; third, negligible CH3CH3 from coupling
of CH3· is observed in CH3CN (interestingly [CH3CH3]/[CH4]
~ 1 in DMA). Control experiments demonstrated that reduced
These and other data discussed below rule out many
mechanisms and are consistent with eqn. (1)–(10) for this
chemistry. Previous studies (product, kinetics, spectroscopic
and others) establish that the oxygen-to-tungsten charge-
transfer excited states of polyoxotungstates abstract hydrogen
atoms, eqn. (1) and (2).6,7 The chemoselectivities and re-
gioselectivities exhibited by the products in Table 1 indicate the
Chem. Commun., 1998, 2467–2468
2467

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Table 1 Functionalization of alkanes with methyl cyanoformate via
polyoxometalate photocatalysisa
NH
CN
CO2CH3
Polyoxo-
tungstateb
t/h;
Organic product (%) of detected
1
2
1a
2a
1b
2b
e
Substrate T/°Cc productsd (turnovers, based on Pox
)
NH
CN
1
2
1a
1b
CO2CH3
Na4W10O32
Q4W10O32
Q3PW12O40
Na4W10O32
Q4W10O32
16; 22 < 1
16; 22 < 1
16; 22 < 1
8; 90 78 (11)
75 (27)
90 (9.1)
99 (0.3)
6 (0.9)
2c
3c
NH
CN
8; 90 61 (5.3) 21 (1.8)
CO2CH3
2a
2b
2c
Na4W10O32
Q4W10O32
Na4W10O32
Q4W10O32
16; 22 < 1
16; 22 < 1
8; 90 78 (20)
51 (8.4) 11 (1.9)
51 (5.6) < 1
3
3a
3b
4b
3d
NH
3 (0.8)
< 1
CN
8; 90 59 (3.0) 20 (1.0) < 1
CO2CH3
3
4
3a
16; 22 2 (1.0)
16; 22 < 1
16; 22 < 1
4a
16; 22 < 1
16; 22 < 1
8; 90 76 (18)
3b
3c
3d
2 (1.0)
2 (1.9)
Na4W10O32
Q4W10O32
Q3PW12O40
75 (36)
80 (25)
< 1
< 1
4
5
4a
5a
4c
50 (0.4) 50 (0.4) < 1
4b 4c
51 (8.4) 11 (1.9)
51 (5.6) < 1
Na4W10O32
Q4W10O32
Na4W10O32
Q4W10O32
2 (0.5)
< 1
8; 90 71 (9.2) 16 (2.0) < 1
PhCH3
PhCH2CH2Ph
5
6
7
5a
6
6a
Na4W10O32
Na4W10O32
8; 90 100 (1.0)
6a
8; 90 100 (0.5)
PhC(CH3)3
7
Na4W10O32
8; 90 No reaction
a
Acetonitrile solutions of polyoxotungstate (1.5 mM) and alkane (0.5 M)
and MCF (0.25 M) were irradiated (550 W medium-pressure Hg lamp;
Pyrex filter, l > 280 nm). The average conversion (based on MCF) was
4 Activation and Functionalization of Alkanes, ed. C. L. Hill, Wiley, New
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b
15% to simplify product distributions for mechanism elucidation. Q =
tetra-n-butylammonium. c t = irradiation time. d All new compounds were
purified and their compositions and purities ( > 98%) confirmed by 1H
NMR, high resolution MS, and elemental analysis. Moles of indicated
product/moles of total organic products as determined by GC. e Turnovers
= moles of indicated product/moles of polyoxotungstate.
W10O3242 does not react with either MCF (no CN2 is formed)
or organic products (iminoester or nitrile). Finally, the yields of
iminoester and nitrile correlate strongly (inversely) with each
other between 22 and 90 °C, and iminoester does not convert to
nitrile under the reaction conditions [eqn. (11) is not operative].
These 2 points suggest that both products derive from a
common intermediate, most likely the iminyl radical,
RC(NN·)CO2CH3.
7 (a) T. Yamase and T. Usami, J. Chem. Soc., Dalton Trans., 1988, 183;
(b) D. Attanasio and L. Suber, Inorg. Chem., 1989, 28, 3779; (c) E.
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Photobiology, ed. W. M. Horspool and P.-S. Song, CRC Press, Boca
Raton, FL, 1995, ch. 39, pp. 471–483; (b) J. Guillet, in Degradable
Polymers, ed. G. Scott and D. Gilead, Chapman & Hall, London, 1995,
ch. 12.
We thank the National Science Foundation (Grant CHE-
9412465) for support of this research.
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1 B. A. Arndtsen, R. G. Bergman, T. A. Mobley and T. H. Peterson, Acc.
Chem. Res., 1995, 28, 154.
2 R. H. Crabtree, Chem. Alkanes Cycloalkanes, 1992, 653.
3 R. H. Crabtree, S. H. Brown, C. A. Muedas, C. Boojamra and R. R.
Ferguson, Chemtech, 1991, 21, 634.
Communication 8/05036H
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Chem Commun., 1998, 2467–2468