Stereoselective oxidation of alkanes with: M -CPBA as an oxidant and cobalt complex with isoindole-based ligands as catalysts

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Stereoselective oxidation of alkanes with m-CPBA

as an oxidant and cobalt complex with isoindole-

based ligands as catalysts†

Cite this: RSC Adv., 2016, 6, 93756

*

Oksana V. Nesterova, Maximilian N. Kopylovich and Dmytro S. Nesterov

Two complexes with isoindole-core ligands of general formula [M{C₆H₄C(NH₂)NC(ONCMe₂)₂}₂](NO₃)₂(M

¼ Co for 1 and M ¼ Ni for 2) were studied as catalysts for the mild stereoselective alkane oxidation with m-

chloroperbenzoic acid (m-CPBA) as an oxidant and cis-1,2-dimethylcyclohexane (cis-1,2-DMCH) as a main

model substrate. Complex 1 disclosed a pronounced activity, with high retention of stereoconﬁguration of

substrates (>98% for cis-1,2-DMCH) and highest cis/trans ratio of tertiary alcohols (products) of 56, under

mild conditions. The best achieved yields of tertiary cis-alcohols were of 13.7 and 50.5%, based on the

substrate (cis-1,2-DMCH) and the oxidant (m-CPBA) respectively. Kinetic experiments, high bond and

stereoselectivity parameters, kinetic isotope eﬀect of 7.2(2) in the oxidation of cyclohexane, and

incorporation of ¹⁸O from H₂¹⁸O support the involvement of Co^IV]O high-valent metal–oxo

intermediates as main C–H attacking species.

Received 2nd June 2016

Accepted 23rd September 2016

DOI: 10.1039/c6ra14382b

www.rsc.org/advances

contain transition metals (for instance, iron in the case of P450

or Rieske dioxygenase), which facilitate the alkane oxidation via

formation of e.g. metal–oxo high-valent species.⁸Attempts to

Introduction

Alkanes are cheap and available raw materials for the synthesis

of valuable chemicals.¹However, due to the inertness of C–H

and C–C bonds, the modication of alkanes generally requires

strong activators (e.g. oxidizing agents) and harsh conditions.

This usually leads to low chemo-, regio- and/or stereo-

selectivities and tedious isolation and purication of desired

isomers.²Moreover, a synthetic approach may fail completely if

the substrate contains impurities, as usually occurs in real raw

materials. As a result, while some particular protocols for direct

selective functionalization of C–H bond have already contrib-

uted to ne organic synthesis,³more general approaches to the

selective C–H activation in alkanes remain a challenge.

Elaboration of eﬀective catalytic systems is a common

approach, and it has long been known that various alkanes can

be selectively hydroxylated under relatively mild conditions

using peroxides as oxidants and coordination compounds as

catalysts.⁴The studies in this eld were inspired by impressive

eﬀectivity of such enzymes as cytochrome P450,⁵Rieske dioxy-

genase⁶or methane monooxygenase (MMO),⁷able to insert

oxygen into C–H bond with exciting stereoselectivity under

ambient conditions.⁶Active centres of the enzymes usually

simulate the mechanism of enzymatic activation led to devel-

opment of a number of catalytic systems,^4ae.g. with iron and

manganese complexes as catalysts and peroxides as terminal

oxidants.^6,9Although some of the man-made catalytic systems

exhibit high selectivity, only few of them are suitable for prac-

tical applications for ne organic synthesis,¹⁰mainly due to low

yields of products and instability of the catalysts. Hence, new

eﬀective catalytic systems for the direct selective transformation

of alkanes continue to be actively searched for.

Apart from the catalyst per se, an oxidant and activation

additives are essential components of the catalytic systems, and

m-chloroperbenzoic acid (m-CPBA)¹¹is a renown terminal

oxidant for the alkanes modication with enhanced selectivity,

as was rstly demonstrated by using iron¹²catalysts. Later on,

an intriguing activity of cobalt complexes in a stereospecic

hydroxylation of alkanes with m-CPBA has been also reported.¹³

However, all the above mentioned systems possess rather low

yields of products based on alkane substrate (typically less than

2%), while this parameter is critical for most of practical

applications.

We decided address this point, and herein we report on the

catalytic activity of cobalt complex with isoindole-based ligands

in the selective hydroxylation of various alkanes, mainly

substituted cycloalkanes, with m-CPBA as an oxidant. The

studied here cobalt-based catalytic system allowed to reach

yields of products up to 16%, based on the alkane substrate,

with high stereo and bond selectivity. A nickel complex with the

´

Centro de Quımica Estrutural, Instituto Superior Tecnico, Universidade de Lisboa, Av.

Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: dmytro.nesterov@tecnico.ulisboa.pt

† Electronic supplementary information (ESI) available: ESI-MS spectrum of 1, EI

mass spectra of reaction products from the oxidation of cis-1,2-DMCH in the

presence of CCl₃Br, accumulations of products in the course of oxidation of

normal and deuterated cyclohexane, EI mass spectra of normal and deuterated

chlorocyclohexanes, kinetic tting details. See DOI: 10.1039/c6ra14382b

93756 | RSC Adv., 2016, 6, 93756–93767

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same ligand was also tried for comparative purposes, but 25 mm), one having an EI-MS (electron impact) detector and the

demonstrated much lesser activity.

other one with a FID detector, was used for analyses of the

reaction mixtures. The following GC sequence has been used:

50 ^ꢀC (3 min), 50–120 ^ꢀC (8 degrees per minute), 120–300 ^ꢀC (35

Experimental

Materials and methods

ꢀ

degrees per minute), 300 C (3.11 min), 20 min total run time;

200 ^ꢀC injector temperature. For analysis of oxidation products

of highly-boiling compounds, such as adamantane or decahy-

dronaphthalene, diﬀerent conditioning was employed: 50 ^ꢀC (3

min), 50–150 ^ꢀC (30 degrees per minute), 150–300 ^ꢀC (14

The chemicals (Aldrich) were of AR reagent grade and used as

received. All experiments were carried out in air. UV-vis spectra

were recorded using Perkin-Elmer Lambda 35 spectrometer in

a 200–800 nm spectral range. Mass spectra were obtained using

LCQ Fleet mass spectrometer with an ESI source (Thermo

Scientic). Coordination cobalt(^II) and nickel(II) compounds

with isoindole-core ligands [M{C₆H₄C(NH₂)NC(ONCMe₂)₂}₂](-

NO₃)₂(M ¼ Co for 1 and M ¼ Ni for 2, Fig. 1) were prepared

according to the published protocols,¹⁴by reuxing the mixture

of cobalt/nickel nitrate hexahydrate M(NO₃)₂$6H₂O, phthalo-

nitrile and acetoxime in acetone for 8 h. The complexes

precipitate from reaction mixture, and their isolation involves

simple ltration with subsequent washing. The Co(^II) complex 1

was almost exclusively used in all the described here studies

since the nickel catalyst 2 has not demonstrated appreciable

activity in the studied transformations.

ꢀ

degrees per minute), 300 C (2.95 min), 20 min total run time;

200 ^ꢀC injector temperature. Helium was used as the carrier gas

with a constant 1 mL per minute ow. All EI-MS spectra were

recorded with 70 eV energy.

Experiments with H₂¹⁸O

The ¹⁸O-labelled water (97% of ¹⁸O, Sigma-Aldrich) was added

prior the addition of m-CPBA. The ¹⁶O/¹⁸O compositions of

oxygenated products (tertiary cis- and trans-alcohols formed

from the cis-1,2-dimethylcyclohexane substrate) were deter-

mined by the relative abundances of mass peaks at m/z ¼ 128/

130.

Catalytic oxidation of alkanes

Mass-spectrometry

The reactions were typically carried out in air in thermostated

cylindrical vials with vigorous stirring. Firstly, acetonitrile

solution of CH₃NO₂(used as GC internal standard of 0.2 M nal

concentration) was mixed with the solid complex 1, and then

liquid substrate (alkane) was added. In the case of solid ada-

mantane, it was placed into the vial prior the addition of

acetonitrile solution. Solid m-CPBA oxidant was dissolved in

acetonitrile (typically 30 mg in 1 mL of CH₃CN) and added

dropwise within 10 seconds to a warm (40 ^ꢀC) solution of other

components (CAUTION: the combination of m-CPBA with

organic compounds at elevated temperatures may be explo-

sive!). Samples were quenched at room temperature with excess

of solid PPh₃according to developed methods¹⁵and directly

analysed by GC and GC-MS techniques.

The identication of peaks at chromatograms was made by

comparison of respective EI mass-spectra with those from the

NIST v.2.0f database (PerkinElmer TurboMass v.5.4.2.1617

soware). The mass-spectra patterns of some products (tertiary

cis- and trans-1,2-dimethylcyclohexanoles, tertiary cis- and trans-

1,4-dimethylcyclohexanoles) were not found in the NIST data-

base, and identication of these products was made by

comparing with the reported previously mass-spectra.¹⁶

Results and discussion

Synthesis and crystal structures of complexes 1 and 2

The preparation of analogous Co(^II) and Ni(II) complexes 1 and 2

with two O,O⁰-(3-amino-1H-isoindole-1,1-diyl)bis(propan-2-one

oxime) ligands (Fig. 1) is straightforward and involves cheap

and available starting materials (phthalonitrile, acetoxime and

metal nitrate).¹⁴

Gas chromatography

A Perkin-Elmer Clarus 600 gas chromatograph, equipped with

The overall structural congurations of 1 and 2 are similar

(Fig. 1 and 2) and will be presented on the example of the

complex 1. The single crystal X-ray analysis showed that 1

consists of [Co{C₆H₄C(NH₂)NC(ONCMe₂)₂}₂]²⁺(Fig. 2) and NO^3ꢂ

ions joined together by strong hydrogen bonds into extended

supramolecular chains. The cobalt(^II) atoms in 1 lie on the

centre of symmetry and have slightly distorted octahedral

coordination environments with a N₆donor sets formed by

nitrogen atoms from isoindole-core ligands. The Co–N

two non-polar capillary columns (SGE BPX5; 30 m ꢁ 0.32 mm ꢁ

˚

distances are in the range from 2.070(5) to 2.186(5) A. The cis N–

Co–N bond angles vary from 75.54(19) to 104.46(19)^ꢀ. The

strong H-bonds, N–H/O, involving nitrogen atoms from the

amino groups of polydentate ligands of the cations and oxygen

atoms from the uncoordinated nitrates [N4–H1/O4; D/A ¼

ꢀ

˚

Fig. 1 Complexes attempted as catalysts.

2.925(6) A, D–H/A ¼ 166.43(1) ; N4–H2/O3; D/A ¼ 2.801(4)

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Fig. 2 The crystal structure of [Co{C₆H₄C(NH₂)NC(ONCMe₂)₂}₂]²⁺in

1, showing the atom numbering. H atoms are omitted for clarity.

Fig. 4 The packing of the supramolecular chains in 1 viewed down the

c axis. Colour scheme: Co, pink; O, red; N, blue; C, grey.

numerous characteristic absorption peaks in the 250–340 nm

range, depending on substituents. Therefore, the bands

observed for the complex 1 (Fig. 5) can be attributable to tran-

sitions within the electronic structure of the ligand, coordinated

to the metal centre.

The UV absorption exhibits a slight intensity increase within

the time (Fig. 5, inset). No isosbestic points and no shis of

absorption peaks were observed during the 50 min time period.

Further, no peaks appeared in the visible range (400–800 nm)

within the time period studied, while it is known that acetoni-

trile complexes of cobalt absorb at 613 and 679 nm.²¹From

these observations we suppose that the isoindole ligands

Fig. 3 The fragment of the supramolecular chain in 1 showing H-

bonding interactions between neighboring Co(^II) cations. The

hydrogen atoms of the molecules are omitted for clarity. Colour

scheme: Co, pink; O, red; N, blue; C, grey.

ꢀ

˚

A, D–H/A ¼ 146.24(1) ] are responsible for the formation of the

overall 1D structure (Fig. 3 and 4).

The bulky ligand structure in 1 prevents the formation of the

hydrogen bonds between adjacent chains making impossible

the further increasing of the overall dimensionality of the

supramolecular polymer (Fig. 3). The shortest Co/Co separa-

˚

tion within the H-bonded chain is 11.168(2) A, while the nearest

˚

Co/Co distance between chains is 9.335(2) A.

The complexes are stable in solid state but slowly degrade

upon their dissolution in some solvents, e.g. alcohols, trans-

forming into more stable phthalocyanines and related prod-

ucts.¹⁷The instability is an obvious drawback of a catalyst,

which, however, can be avoided by immobilization or inclusion

onto/into a support,¹⁸if signicant improvement of a catalytic

process is recognized.

The electrospray mass-spectra (ESI-MS) of acetonitrile solu-

tions of 1 revealed few peaks, the strongest of which were

identied as dication of 1 and its derivatives having one coor-

dinated nitrate anion (Fig. S1†). The latter suggests the possi-

bility of partial decoordination of isoindole-based ligand to

bind nitrate anion. The UV/vis spectrum of the solution of 1 in

acetonitrile exhibits negligible absorption in visible region (l >

400 nm), but showing three clear absorption peaks at 323, 299

and 276 nm, as well as one shoulder at 249 nm (Fig. 5), in the UV

Fig. 5 UV spectra of an acetonitrile solution of 1 (1.6 ꢁ 10^ꢂ4M),

recorded each 5 min after dissolution of the sample. Inset shows

changes in the absorbance at selected wavelengths, plotted as

region. The isoindole¹⁹and its derivatives²⁰are known to exhibit a function of time.

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remain coordinated to the cobalt centre and no “free” cobalt

ions appeared in solution. However, one cannot exclude the

presence of reversible decoordination–coordination of the

oxime groups of the ligands.

Catalytic oxidation of alkanes with m-CPBA

The selective oxidation of C–H bond should avoid formation of

long-living radicals, a subject of epimerization (Scheme 1), and

hence the insertion of oxygen should occur shortly aer

abstraction of hydrogen atom (radical formation), as it happens

in the oxygen-rebound mechanism of the P450 enzyme⁵and

many model complexes.²²In this view, oxidation of cis-1,2-

dimethylcyclohexane (cis-1,2-DMCH, Scheme 1) is a useful

model reaction to test both the stereo- and bond selectivities.²³

Retention of stereoconguration of methyl groups in cis-1,2-

DMCH indicates an absence of the long-living alkyl radicals,²⁴

emerging from the hydrogen abstraction by poorly selective O-

centred radicals (for example, hydroxyl, arylperoxyl or tert-

butoxyl ones), which in their turn arise from the oxidants (H₂O₂,

m-CPBA, tBuOOH, etc.).

Scheme 2 The oxidation of cis-1,2-DMCH substrate leads to tertiary

cis-(retention) and trans-(inversion) alcohols (main products), as well

as the products of 2^ꢀC–H bonds oxidation (mainly ketones).

In accordance with the above considerations, we studied the

oxidation of cis-1,2-DMCH with m-CPBA, catalysed by

1

(Scheme 2), and found the tertiary (3^ꢀ) cis-alcohol to be the main

product. The by-products are tertiary trans-alcohol and

oxygenates (ketones) emerged through the activation of

secondary (2^ꢀ) C–H bonds of cis-1,2-DMCH. As known,^13bin

such a type of chemistry ketones usually appear as result of

overoxidation of secondary alcohols, hence, we assume that the

ketones are formed via the respective alcohols (Scheme 2),

rather than by direct oxidation of C–H bonds of the alkane.

Accumulation of the tertiary alcohols and their cis/trans ratio

in time upon the oxidation of cis-1,2-DMCH with m-CPBA, cat-

alysed by 1, are shown in Fig. 6. As can be seen, lower initial

concentration of 1 ([1]₀) of 3.3 ꢁ 10^ꢂ4M (Fig. 6, blue squares)

leads to better yields and the cis/trans selectivities (Fig. 6,

bottom); in this case the retention of stereoconguration (RC)

was found to be >98% (Fig. 6, bottom, inset). The best achieved

yields of tertiary cis-alcohol were 13.7 and 50.5% based on the

substrate and oxidant, respectively. The dependence of the

initial reaction rate W₀on the concentration of 1 was found to

be linear (Fig. 6, top, inset), pointing out that the mechanism of

action of the complex 1 is the same within the studied

concentration range. The maximum achieved initial reaction

Fig. 6 Top: accumulations of tertiary oxygenates (sum of tertiary cis-

and trans-alcohols) over time in the oxidation of cis-1,2-DMCH (0.1 M)

with m-CPBA (0.027 M) catalysed by complex 1 (3.3 ꢁ 10^ꢂ4and 1 ꢁ

10^ꢂ3M, blue squares and red circles, respectively) in CH₃CN at 40 ^ꢀC.

Inset: dependence of the initial reaction rate (W₀) on [1]₀. Bottom: the

cis/trans ratio of the respective tertiary alcohols (main plot) and

retention of stereoconﬁguration (RC; inset) as functions of time

(experimental conditions and colours are the same).

rate W₀of 1.2 ꢁ 10^ꢂ4M s^ꢂ1corresponds to turnover frequency,

TOF, of 430 h^ꢂ1

.

Chlorobenzene is a by-product formed by the homolytic

Scheme 1 Stereotransformations of cis-1,2-DMCH.

splitting of the O–O bond of m-CPBA and decarboxylation of the

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accumulation curves of chlorobenzene for the tests with [1]₀¼

3.3 ꢁ 10^ꢂ4and 1 ꢁ 10^ꢂ3M (Fig. 7) show that in both cases

nearly the same concentrations of chlorobenzene are reached,

ca. 8.7 ꢁ 10^ꢂ4M, but with slightly diﬀerent accumulation rates

W_PhCl. The dependence of W_PhClon [1]₀is close to linear (Fig. 6,

inset), guring out the catalyst 1 as an origin of the chloro-

benzene formation. From these data, as well as from the high

retention index (cis/trans ratio > 50, RC > 98%, Fig. 6, bottom),

we assume that the main reaction pathway proceeds via the

heterolytic cleavage of the O–O bond of coordinated m-CPBA

with formation of a high-valent cobalt–oxo species (Scheme 3,

middle-bottom part). Coordination of m-CPBA and the

following formation of cobalt–oxo species (Scheme 3) require

the presence of two free sites in the coordination sphere of

cobalt atom in 1. From the structure of the complex 1 (Fig. 1 and

2) one may suppose these could appear through the decoordi-

nation of two oxime groups from diﬀerent ligand molecules

(Fig. 2).

Scheme 3 Proposed general processes, occurring in the course of

oxidation of alkane H–CR₃bond by m-CPBA catalysed by 1. Bold and

thin lines show the main and side cycles (heterolytic and homolytic

splittings of an O–O bond, respectively). Dash lines show the catalyst-

free processes. Starting components (1 and m-CPBA) are highlighted

by colour backgrounds. m-CBA states for m-chlorobenzoic acid.

The introduction of ¹⁸O from H₂¹⁸O into hydroxylated

products is an evidence of involvement of high-valent metal–oxo

species, as they exchange oxygen with water.^13a,30To check this

possibility, we performed the oxidation of cis-1,2-DMCH with m-

CPBA catalysed by 1 in the presence of H₂¹⁸O (0.8 M). Incor-

poration of ¹⁸O into the tertiary cis- and trans-alcohols are

depicted in Fig. 8, top. As can be seen, both tertiary alcohols

reach 3.5% of ¹⁸O. The observed incorporation of the labelled

oxygen in the nal products (Fig. 8, top) is in accord with the

proposed general mechanism with participation of high-valent

m-chlorobenzoate radical (Scheme 3, top).²⁵The rate of decar-

boxylation (ca. 10⁶s^ꢂ1) is known^25a,26to be comparable to that

for H abstraction by the m-chlorobenzoate radical. Hence, one

can assume that half of m-chlorobenzoate is nally transformed

into chlorobenzene while another half reacts with the alkane

substrate. As result, a simple estimation of the homolysis

contribution to the overall catalytic process can be achieved. For

example, the reported considerable amounts of chlorobenzene

(more than 50% based on the products) formed upon the

oxidation of cycloalkanes with m-CPBA, catalysed by pyr-

idylalkylamine complexes of cobalt,²⁷nickel²⁸and ruthenium,²⁹

reveal a homolytic mechanism in the respective process.

In contrast to the above cases, the oxidation catalysed by 1

reveals much lower amount of chlorobenzene, not exceeding

3.3% based on m-CPBA and 8% based on cis-1,2-DMCH. The

Fig. 7 Accumulation of chlorobenzene over time in the oxidation of Fig. 8 Top: incorporation of ¹⁸O into tertiary cis- and trans-alcohols

cis-1,2-DMCH (0.1 M) with m-CPBA (0.027 M) catalysed by complex 1 as a function of time in the oxidation of cis-1,2-DMCH (0.1 M) with m-

(3.3 ꢁ 10^ꢂ4and 1 ꢁ 10^ꢂ3M, blue squares and red circles, respectively) CPBA (0.027 M) catalysed by complex 1 (3.3 ꢁ 10^ꢂ4M) in the presence

in CH₃CN at 40 ^ꢀC. Inset: dependence of the initial rates of chloro- of H₂O (0.8 M) in CH₃CN at 40 ^ꢀC. Bottom: accumulation of the ¹⁸O-

18

benzene accumulation on [1]₀.

labelled tertiary cis- and trans-alcohols.

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cobalt–oxo species as oxidants (Scheme 3). The relatively low

incorporation level of 3.5% can be explained by slow rate of the

oxygen exchange between Co^IV]O and H₂O (Scheme 3), if

compared to that of hydrogen abstraction from cis-1,2-DMCH.

Cobalt high-valent metal–oxo intermediates are known to be

hardly trapped (in contrast to, for example, iron–oxo species),

what is, particularly, due to their high reactivity and, as

a consequence, short lifetime.³¹A similarly low incorporation of

¹⁸O into the cyclohexanol (5%) was observed earlier for oxida-

tion of cyclohexane with m-CPBA, catalysed by cobalt porphyrin

complex.^13a

When reaction is performed in the presence of D₂O instead

of H₂¹⁸O (other conditions are the same) the D-labelled alcohols

are observed at ca. 20% level, nearly equal for cis- and trans-

alcohols. This result is not unexpected as the alcohols normally

undergo isotopic exchange with heavy water.

Fig. 10 Fragments of the chromatograms, showing main reaction

products (tertiary cis- and trans-alcohols) as well as by-products in the

course of cis-1,2-DMCH oxidation with m-CPBA, catalysed by 1, in the

presence (top) or absence (bottom) of CCl₃Br. –X means –OH or ]O.

The accumulation of the main reaction product, tertiary cis-

alcohol, was not aﬀected by D₂O. However, the overall cis/trans

ratio was found to be twice lower (Fig. 9) due to enhanced

amounts of the by-product – tertiary trans-alcohol. Similar eﬀect

The mass-spectra of the peaks I–IX (Fig. S2–S8†) were mainly

was observed for H₂¹⁸O, but in that case both cis- and trans- not recognized by the NIST database, except of III and IV.

products were aﬀected, lowering the nal stereopurity of the Tentative considerations of the mass-spectra (Fig. S2 and S3†)

tertiary alcohols. Thus, the water has a suppression eﬀect on the allowed to assume that peaks I and II may be related to chlo-

overall catalytic activity of the system, probably inuencing the rinated C₈moieties, probably cis- and trans-1-chloro-1,2-

ratio of heterolytic/homolytic O–O splitting pathways towards dimethylcyclohexanes. The peak III, that overlaps with the

the homolytic one (Scheme 3).

weak group of 2^ꢀproducts (Fig. 10), was recognized as 1,2-

To distinguish the participation of the alkyl radicals in the dimethylcyclohexene. Formation of a double bond is possible

oxidation of cis-1,2-DMCH (Scheme 3), we checked how the from the tertiary alkyl radical, attacked at the second tertiary

respective radical trap, CCl₃Br, would inuence the catalytic carbon.³²3-Bromochlorobenzene (IV) was also detected, as ex-

process. The radical trap concentration was equal to that of pected from interaction of aryl radical (Scheme 3) with CCl₃Br.

substrate (0.1 M), while [1]₀¼ 3.3 ꢁ 10^ꢂ4M and other condi- The strong peaks V–IX with similar mass-spectra (Fig. S4–S8†)

tions were as stated in Fig. 6 caption. When CCl₃Br was added to were assigned to be the isomers of brominated dimethylcyclo-

reaction mixture before addition of the m-CPBA oxidant, the cis/ hexanes, where V and VI are tertiary cis- and trans-1-bromo-1,2-

trans ratio of tertiary alcohols aer 1 h reaction time was low- dimethylcylohexanes, respectively, and VII–IX are secondary cis-

ered to 15, what is much lower comparing to that (56) without 1-bromo-2,3-

the addition of the radical trap (Fig. 6, bottom, blue squares). (Fig. 10).

and

cis-1-bromo-3,3-domethylcyclohexanes

The yields of alcohols in the presence of CCl₃Br were also lower

The generally accepted mechanism for hydrogen abstraction

(11% based on the m-CPBA; compare to 57% in the trap-free with a high-valent metal–oxo species (as in some iron contain-

test). Formation of 2^ꢀproducts (Scheme 2) was almost sup- ing enzymes, such as P450) proceeds via formation of a short-

pressed, while a number of new products have been detected living alkyl radical, cCR₃.²²This radical immediately interacts

(Fig. 10).

with the same M–O centre to give the nal hydroxylated product

(Scheme 3). The high rate of this process is the origin of ster-

eoselectivity of a whole process, because the radical collapses

before its epimerization occurs. However, some catalysts oper-

ating via the high-valent metal–oxo species are known to form

large quantities of brominated alkanes when reactions are

performed in the presence of CCl₃Br.^22,33This can be explained

by interaction of CCl₃Br either with alkyl radical inside the

“transition state” (intermediate) or with a free alkyl radical in

solution.

In case of the cis-1,2-DMCH/1/m-CPBA/CCl₃Br catalytic

system, the notable cis/trans ratio and yield of tertiary cis-

alcohol retain even in the presence of CCl₃Br. On the other

hand, tertiary cis- and trans-brominated products V and VI are

observed in a nearly equal quantities and the ratio of 3^ꢀ: 2^ꢀ

brominated products (assuming V + VI as tertiary and VII + VIII

+ IX as secondary ones) is low, close to 4 : 1, while the system cis-

Fig. 9 Inﬂuence of water (0.8 M) on the CT ratio of tertiary alcohols

(products).

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the cis-1,2-DMCH substrate are ca. 20% higher when reaction is

performed in the presence of CCl₃Br. From all these data we

assume that reaction of cobalt-alkyl radical intermediate with

CCl₃Br (Scheme 4) results in release of a free cCCl₃radical. The

latter can abstract hydrogen from alkane substrate,²⁶forming

CHCl₃(detected by chromatography) and a free alkyl radical.

This explains the absence of stereoselectivity in the brominated

tertiary products as well as the elevated overall yields (sum of

hydroxylated and brominated 1,2-DMCH) in the presence of

CCl₃Br. It also can be presupposed that alcohols are formed

through the intermediates (ca. 20%), which did not react with

CCl₃Br.

Scheme 4 Interaction of the “transition state” (intermediate) with

CCl₃Br releases an alkyl cCR₃radical with subsequent abstraction of

bromine from CCl₃Br. The formed cCCl₃radical abstracts hydrogen

from alkane to give chloroform and a free alkyl radical.

Apart from cis-1,2-DMCH, the selective oxidation of other

alkane substrates was performed (Tables 1–3), and it was found

that generally the stereospecicity and bond selectivity of the 1/

m-CPBA catalytic system depends on the spatial conguration

1,2-DMCH/1/m-CPBA exhibits 3^ꢀ: 2^ꢀratio of alcohols of 30 : 1

or higher. Moreover, the overall yield of products (sum of

hydroxylated and brominated alkanes) as well as conversion of

Table 1 Reaction products of oxidation of tertiary and secondary C–H bonds of various alkanes with the 1/m-CPBA system, cis/trans ratios of

the respective tertiary alcohols, bond selectivities and overall yields^a

cis/trans

3^ꢀ: 2^ꢀ

Substrate

Products 3^ꢀ

Products 2^ꢀ

Yield^b(%)

56

35 : 1

16

(a)

(b)

28^ꢂ1

11 : 1

11

21 : 1

9

(c)

20^ꢂ1

11 : 1

11

(d)

46

c

—

19

(e)

40^ꢂ1

17 : 1

12

(f)

a

[Alkane]₀¼ 0.1 M, [m-CPBA]₀¼ 0.027 M, [1]₀¼ 3.3 ꢁ 10^ꢂ4M in CH₃CN at 40 ^ꢀC, 1 h reaction time. ^bYields based on the substrate (sum of 3^ꢀand 2^ꢀ

products). ^cSecondary products peaks are overlapped by a large peak of m-chlorobenzoic acid (m-CBA).

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Table 2 Reaction products in the course of n-heptane oxidation of with 1/m-CPBA system^a

Products 2^ꢀ

(2) : (3) : (4)

Yield^b(%)

Substrate

(2)

(3)

(4)

1.8 : 2.5 : 1.0

4

a

Conditions are as stated in Table 1 footnote. ^bYield based on the substrate (sum of 2^ꢀalcohols and respective ketones).

Table 3 Reaction products in the course of adamantane oxidation of with 1/m-CPBA system^a

3^ꢀ: 2^ꢀ

Substrate

Products 3^ꢀ

Products 2^ꢀ

Yield^b(%)

21 : 1

19

a

Conditions are as stated in Table 1 footnote. ^bYields based on the substrate (sum of 3^ꢀand 2^ꢀproducts).

of substrate. The 3^ꢀ: 2^ꢀbond selectivities vary from 11 : 1 to In the oxidation of adamantine, the yields provided by the

35 : 1, being also dependent on the conguration of alkane catalyst 1 are moderate but comparable to those with nickel

skeleton (Table 1). The highest cis/trans ratio of tertiary alcohols catalysts with tripodal N-donor ligands (11–14, Table 4),

is observed for cis-1,2-DMCH (Table 1, a), while cis-1,4-DMCH although showing better 3^ꢀ: 2^ꢀselectivity. Apart from that, the

was found to show the lowest retention of stereoconguration catalytic systems based on complexes 11–14 (Table 4) produce

(Table 1, c). A tentative explanation is the close proximity of an large amounts of undesirable chlorobenzene by-product

electron rich tertiary carbons in cis-1,2-DMCH, that may facili- (comparable to those of main products), while the 1/m-CPBA

tate hydrogen abstraction. A weak regioselectivity between catalytic system allows to eliminate this drawback.

diﬀerent methylene groups of a linear alkane, n-heptane, was

Competitive oxidation of equimolar mixture of normal and

observed (Table 2): while the C–H bond dissociation energies of deuterated cyclohexane was performed to determine the kinetic

(2), (3) and (4) methylene groups of n-heptane (Table 2) are isotope eﬀect (KIE) value ([C₆H₁₂]₀¼ [C₆D₁₂]₀¼ 0.1 M, [1]₀¼ 3.3

nearly equal,³⁷the 1/m-CPBA system was shown to be slightly ꢁ 10^ꢂ4M, other conditions were as in stated in Fig. 6 caption).

more active towards oxidation of (2) and (3) methylene groups, The accumulations of both normal and deuterated products

rather than (4) ones (Table 2). Oxidation of adamantane under (sums of respective alcohol and ketone) exhibit linear character

similar conditions (Table 3) revealed 21 : 1 normalized 3^ꢀ: 2^ꢀup to 4 min reaction time (Fig. 11, inset). The ratio of their

bond selectivity. This value is slightly higher than that observed slopes, aer linearization, gives KIE value of 7.0(3). However,

for oxidation of adamantane with m-CPBA catalysed by nickel non-linear dependences of cyclohexanols and cyclohexanones

and cobalt complexes (Table 4).

accumulation (Fig. 11) suggest that the consecutive reaction Cy

The parameters of the stereoselective oxidation of cis-1,2- / Cy–OH / Cy]O takes place (where Cy means cyclohexane).

DMCH (cis/trans ratio and yields of products) exhibited by the In this model, reaction rate constants k₁and k₂describe

catalyst 1 are among the highest ones reported to date (Table 4). oxidation of cyclohexane and cyclohexanol, respectively. The

Additionally, the complex 1 is able to perform stereoselective data in the time region from 0 to 4 min (Fig. 11) were tted to

oxidation of alkanes without any promoters. A series of analytical expression,⁴⁰derived for such kinetic process (see the

porphyrin complexes (2–4, 8, Table 4) reveal pronounced ESI†). The tting procedure gave k_1H¼ 3.14(6) ꢁ 10^ꢂ4, k_2H

¼

retention of stereoconguration, but the reported yields of 7.9(3) ꢁ 10^ꢂ3, k_1D¼ 4.4(1) ꢁ 10^ꢂ5and k_2D¼ 2.6(2) ꢁ 10^ꢂ3s^ꢂ1

.

products are close to 1%, much lower to that reported here. The From these values one can see that oxidation of the alcohols (k₂

same yields of products were observed if metal salts (5–7, Table constants) occurs 25 and 58 times faster than oxidation of the

4) were tested as catalysts. Although cobalt perchlorate exhibi- alkanes (k₁constants), for C₆H₁₂and C₆D₁₂, respectively,

ted the bond selectivity and cis/trans ratio better than those for leading to a large range of alcohol/ketone (A/K) ratios (Fig. 12).

the complex 1, the overall yields of products did not exceed 2%. From these data the KIE k_H/k_Dvalues for oxidation of

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Table 4 Selected homogeneous catalytic systems for oxidation of cis-1,2-DMCH and adamantane with m-CPBA

3^ꢀ: 2^ꢀ

ratio

Catalyst

Substrate

Yield^a, %

TON^b

cis/trans ratio^c

Ref.

1

2

3

4

5

6

7

8

1

cis-1,2-DMCH

Adamantane

15.5

1.5

1.1

1.3

1.8

0.3

0.7

0.3

18.9

<1

46

15

11

13

18

3

56

>55

>40

>60

>81

>10

>25

>72

—

35 : 1

11 : 1

10 : 1

34 : 1

41 : 1

10 : 1

8 : 1

58 : 1

21 : 1

18 : 1

12 : 1

17 : 1

11 : 1

12 : 1

—

[Fe(L²)](ClO₄)

[Mn(L²)](ClO₄)

[Co(L²)](ClO₄)

Co(ClO₄)₂

13b

12a

—

13a

34

35

36

Fe(ClO₄)₂

Mn(ClO₄)₂

7

1

[Fe(L³)Cl]

9

1

57

3.1

584

639

595

320

10

11

12

13

14

[Co(L²)](SF₃CO₃)

[Ni(L⁴)(CH₃CN)₂](BPh₄)₂

[Ni(L⁵)(CH₃CN)₂](BPh₄)₂

[Ni(L⁶)(CH₃CN)](BPh₄)₂

[Ni(L⁷)(OAc)(H₂O)]BPh₄

29.2

32.0

29.8

4.2

—

a

Yields based on the substrate (sum of 3^ꢀand 2^ꢀproducts). ^bTurnover numbers, moles of products (sum of 3^ꢀand 2^ꢀproducts) per mol of catalyst.

c

Moles of tertiary cis-alcohol per mol of trans-alcohol. H₂L¹¼ salicylidene-2-ethanolamine. L²¼ meso-tetrakis(pentauorophenyl)porphinato

dianion. L³¼ meso-tetrakis(2,6-diuorophenyl)porphinato dianion. L⁴¼ N,N-dimethyl-N⁰,N⁰-bis(quinolin-2-ylmethyl)ethane-1,2-diamine. L⁵

¼

N,N-dimethyl-N⁰,N⁰-bis(pyrid-2-ylmethyl)ethane-1,2-diamine. L⁶

¼

N-methyl-N,N⁰,N⁰-tris(pyrid-2-ylmethyl)-ethylenediamine. L⁷

¼

tris(2-

pyridylmethyl)amine.

cyclohexane and cyclohexanol can be estimated as 7.2(2) and ¼ 1.31(5) ꢁ 10^ꢂ4, k_2H¼ 7.0(4) ꢁ 10^ꢂ3, k_1D¼ 1.93(9) ꢁ 10^ꢂ5and

3.1(2), respectively. Such a KIE values suggest that (1) hydrogen k_2D¼ 2.3(3) ꢁ 10^ꢂ3s^ꢂ1(Fig. S9 and S10†). The dependences of

abstraction is a rate-determining step and (2) cyclohexanol is k₁rate constants values on the cyclohexane concentration are

oxidized via abstraction of hydrogen atom of carbon rather than linear for both normal and deuterated cyclohexane (Fig. 12,

hydroxyl group. The KIE k_1H/k_1Dof 7.2(2) supports the proposed inset). In contrast, the k₂constants (oxidation of cyclohexanol to

reaction pathway that involves high-valent cobalt–oxo species cyclohexanone) are nearly the same as for 0.1 M test (see above).

(Scheme 3). However, at the same time, such a high KIE value These results are in agreement with the model where oxidation

does not rule out the possibility of radicals as attacking species, of alkane is a limiting step and ketone is formed through the

particularly metal–oxo radicals or selective O-centred radicals, alcohol oxidation.

such as aroylperoxyl ones.²⁶

A comparable value of 8(1) of KIE k_1H/k_1Dwas observed

Similar KIE value of 6.8(4) was obtained when twice lower earlier for oxidation of alkanes with m-CPBA, catalysed by cobalt

concentrations of C₆H₁₂and C₆D₁₂were used (0.05 M each): k_1Hsalts and cobalt porphyrins (2 and 3, Table 5) with the yields of

products up to 3%. The yields of oxygenates (sum of alcohol and

ketone) exhibited by the system 1/m-CPBA in the oxidation of

C₆H₁₂(0.1 M) did not reach 5%. This is comparable to the yields

of secondary products (up to 1.6%), observed in the course of

Fig. 11 Accumulations of oxidation products (alcohols and ketones,

circles and squares, respectively) over time in the oxidation of equi-

molar mixture (0.1 M each) of normal (solid symbols) and deuterated

(open symbols) cyclohexane with m-CPBA (0.027 M) catalysed by

complex 1 (3.3 ꢁ 10^ꢂ4M) in CH₃CN at 40 ^ꢀC. Inset shows accumu- Fig. 12 Drop of the alcohol/ketone (A/K) ratio in the course of

lations of sums oxidation products (alcohol and ketone) of normal competitive oxidation of equimolar (0.1 M each) C₆H₁₂/C₆D₁₂mixture.

(black squares) and deuterated (red circles) cyclohexane. Solid lines are Inset shows the concentration dependence of k₁for the oxidation of

the respective ﬁts (see main text).

normal and deuterated cyclohexane.

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Table 5 Selected homogeneous catalytic systems for oxidation of

cyclohexane with m-CPBA

catalytic system. Therefore, brominated products, observed as

a result of reaction in the presence of CCl₃Br (see above, Fig. 10,

Scheme 4), appear from the “caged” alkyl radicals, but not free

ones. These results are in conformity with the proposed reac-

tion mechanism, operating via a short-lived alkyl radicals

(Schemes 3 and 4).

Catalyst

KIE^a

A/K^b

3-C^c

Ref.

1

2

3

4

5

6

8

9

10

11

12

13

14

15

16

1

7.2(2)

8(1)

n.r.

4.3

0.4–2.8

4.9

1.0

1.2

13.0

6.3

—

n.r.^d

n.r.

n.r

n.r.

+

—

n.r.

+

—

[Co(L²)](SF₃CO₃)

Co(ClO₄)₂

13a

13b

27

29

28

An uncommon feature of the catalytic system 1/m-CPBA was

formation of chlorocyclohexane by-product as a result of the

oxidation of C₆H₁₂(Fig. S12†), C₆D₁₂or equimolar mixture

thereof. From the +11 m/z isotopic shis of molecular ions in

mass-spectra of normal and deuterated chlorocyclohexanes

(Fig. S13†) one may conclude that the chlorocyclohexane is

formed from the alkane substrate, but not via hydrogenation of

chlorobenzene (Scheme 3). From the selected catalytic investi-

gations (Table 5) there are two reporting formation of chlor-

ocyclohexane (15 and 16) during cyclohexane oxidation with m-

CPBA. However, in both cases the catalytic systems contain

a source of chloride: the system 15 (Table 5) operates in

a CH₃CN/CH₂Cl₂medium, while the catalyst 16 contains chlo-

ride anion in its structure. The latter was postulated to produce

chlorinated product by oxidative ligand transfer mechanism,⁴²

with long-lived alkyl radicals as intermediates. In the case of the

catalyst 1 the only source of chlorine is m-CBPA or chloroben-

zene by-product (Scheme 3, Fig. S11†).

[Co(L²)](ClO₄)

[Fe(L²)](ClO₄)

[Mn(L²)](ClO₄)

[Co(L³)(OAc)](BPh₄)

[Ru(L⁴)Cl₂](ClO₄)

[Ru(L⁵)Cl₂](ClO₄)

[Ni(L⁶)(NO₃)](BPh₄)

[Ni(L⁷)(OAc)₂(H₂O)]

[Ni(L⁵)(OAc)(H₂O)](BPh₄)

[Ni(L⁸)(CH₃CN)](BPh₄)₂

[Fe₂O(L⁹)(OAc)₂](ClO₄)₂

[Fe(L¹⁰)Cl]

5.4

0.2–2.6

0.8–2.1

46.0

6.6

8.5

10.6

2.4

3.1

n.r.

2.8

n.r.

3.2

+

36

35

38

39

n.r.

1.7

n.r.

a

Kinetic isotope eﬀect (k_H/k_D) in the oxidation of equimolar mixture of

b

normal and deuterated cyclohexane. Cyclohexanol/cyclohexanone

c

d

ratio. Detection of 3-caprolactone.

Not reported. L²

¼

meso-

N,N-bis(2-

tetrakis(pentauorophenyl)porphinato dianion; L³

¼

pyridylmethyl)-N-[(1-^tBu-1H-1,2,3-triazol-4-yl)methyl]amine; L⁴¼ bis(4-

ethoxycarbonyl-2-pyridylmethyl)(2-pyridylmethyl)amine; L⁵

¼

tris(2-

pyridylmethyl)amine; L⁶¼ tris[2-(2-pyridyl)ethyl]amine. L⁷¼ N-benzyl-

bis(2-pyridylmethyl)amine; L⁸¼ N-methyl-N,N⁰,N⁰-tris(pyrid-2-ylmethyl)-

ethylenediamine. L⁹¼ 1,2-bis[2-di(2-pyridyl)-methyl-6-pyridyl]ethane.

H₂L¹⁰¼ N,N⁰-o-phenylenebis(3,5-di-tert-butylsalicylideneimine).

In general, the combination of KIE and A/K values exhibited

by the 1/m-CPBA catalytic system are closest to complexes of

cobalt (2 and 3, Table 5) and ruthenium (9). For the cobalt

catalysts 2 and 3 participation of high-valent cobalt–oxo inter-

mediates was proposed, aer ¹⁸O studies.^13bComplex 9 was also

presumed to oxidize via Ru^IV]O/Ru^III–Oc species, on the basis

cis-1,2-DMCH oxidation (Tables 1 and 4) and other substrates

containing tertiary C–H bonds, showing that the present system

is not much active towards activation of secondary C–H bonds.

The alcohol/ketone (A/K) parameter, oen used to charac-

terize oxidative catalytic systems, ranges from 0.4 to 2.8 for

C₆H₁₂and from 1.5 to 7.1 for C₆D₁₂, depending on the reaction

time (Fig. 12). Obviously, the A/K ratio tends to decrease with the

time and, therefore, for the present catalytic system no denite

A/K value can be derived. This behaviour diﬀers from most of

the reported catalytic systems for cyclohexane oxidation with m-

CPBA, for which constant A/K values are reported (Table 5).

Oxidation of pure cyclohexane (0.1 M) gave the same A/K ratios

and yields of products, as the oxidation of C₆H₁₂/C₆D₁₂mixture.

Formation of 3-caprolactone, appearing through the Baeyer–

Villiger reaction of cyclohexanone with m-CPBA, was reported

for many catalytic systems oxidizing cyclohexane with m-CPBA

(Table 5), particularly for some cobalt complexes with pyridyl

ligands (8, Table 5). In the case of 1/m-CPBA catalytic system no

3-caprolactone was detected (Fig. S12†).

Recently it was shown that cyclohexyl hydroperoxide, Cy–

OOH, can be directly detected in a catalytic mixture using a non-

polar GC column and characterized by mass-spectroscopy.⁴¹

Cyclohexyl hydroperoxide is formed by the reaction of a free

cyclohexyl radical with O₂and is a marker for a free-radical

reaction.¹⁵In the present case, no Cy–OOH was detected in

GC-MS chromatograms, even at trace levels (Fig. S12†), for both

normal and deuterated cyclohexane oxidation (samples were

injected without quenching by PPh₃). The absence of Cy–OOH

rules out the presence of free alkyl radicals in the 1/m-CPBA

18

of O labelling and Raman spectroscopy;²⁹this complex also

did not produce 3-caprolactone. Coordination compound of

ruthenium with slightly diﬀerent structure (10, Table 5) was

concluded to oxidize cyclohexane via a free-radical pathway.²⁹

A

series of nickel complexes 11–14 (Table 5) show common

features, such as formation of 3-caprolactone. For the complex

14 participation of high-spin Ni^II–Oc attacking species was

postulated on the basis of DFT calculations.³⁵

As mentioned above (11–14, Table 4; 11–14, Table 5), some

complexes of nickel are active catalysts in the oxidation of

alkanes with m-CPBA, thus, we also studied the catalytic activity

of the isostructural to 1 nickel complex 2. The accumulation of

tertiary cis-alcohol in the course of cis-1,2-DMCH (0.1 M)

oxidation with m-CPBA (0.027 M) catalysed by the complex 2 (3.3

ꢁ 10^ꢂ4M) revealed a nearly linear dependence with a low

reaction rate W ¼ 1 ꢁ 10^ꢂ7M s^ꢂ1(Fig. 13, top). Interestingly,

both tertiary alcohols exhibit signicant rise of concentrations

in the rst minutes of reaction; then the concentration of trans-

alcohol remains unchanged within the reaction time studied

(Fig. 13, top). This may point to the presence of some rapid

processes at initial stage with the mechanism diﬀerent from the

described above. Interestingly, although the reaction is slow,

the reaction rate remains almost unchanged within 24 h

(Fig. 13, bottom, inset). Aer 24 h the cis/trans ratio of 29.3

(Fig. 13, bottom) was observed with negligible amounts of

chlorobenzene formed (ca. 1.8% based on the m-CPBA). The

yields of products were 11.2 and 41.4% based on the substrate

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stereospecic oxidation of alkanes with m-CPBA. We demon-

strated that the combination of m-CPBA with the cobalt catalyst

may stereoselectively oxidize saturated tertiary C–H bonds with

the high yields of alcohols, based on substrate. The kinetic data,

isotopic labelling and reaction with radical trap let us assume

that the main reaction pathway proceeds without the formation

of free alkyl radicals, most probably involving the high-valent

metal–oxo species Co^IV]O, which abstract the hydrogen from

the alkane C–H bond in a mechanism, similar to the oxygen

rebound one. This assumption is in accord with a high kinetic

isotope eﬀect of 7.2(2) observed in the oxidation of cyclohexane,

while no cyclohexyl hydroperoxide (typical product in a free

radical cyclohexane oxidation) was detected. Isostructural to 1

nickel catalyst 2 exhibits lower than 1 and somehow uctuating

activity. As continuation of this study, we plan to immobilize the

catalysts with their possible stabilization and recovery.

Acknowledgements

˜

ˆ

This work was supported by Fundaçao para a Ciencia e Tecno-

logia (FCT), Portugal, projects PTDC/QEQ-QIN/3967/2014, UID/

QUI/00100/2013 and “Investigador 2013” (IF/01270/2013/

CP1163/CT0007) and fellowships SFRH/BPD/63710/2009

(O.V.N.) and SFRH/BPD/99533/2014 (D.S.N.). The authors

acknowledge the IST Node of the Portuguese Network of Mass-

spectrometry for the ESI-MS measurements.

Fig. 13 Top: accumulations of tertiary oxygenates (tertiary cis- and

trans-alcohols, blue circles and black squares, respectively) over time

in the oxidation of cis-1,2-DMCH (0.1 M) with m-CPBA (0.027 M)

catalysed by complex 2 (3.3 ꢁ 10^ꢂ4M) in CH₃CN at 40 ^ꢀC. Solid red line

shows the linear ﬁt of the cis-alcohol accumulation data. Bottom: the

cis/trans ratio of the respective tertiary alcohols in the reaction time

span 0–7 h (main plot) and 0–24 h (inset).

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RSC Adv., 2016, 6, 93756–93767 | 93767

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Stereoselective oxidation of alkanes with: M -CPBA as an oxidant and cobalt complex with isoindole-based ligands as catalysts

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