Highly efficient redox isomerisation of allylic alcohols catalysed by pyrazole-based ruthenium(IV) complexes in Water: Mechanisms of bifunctional catalysis in water

FULL PAPER

DOI: 10.1002/chem.201103374

Highly Efficient Redox Isomerisation of Allylic Alcohols Catalysed by

Pyrazole-Based Ruthenium(IV) Complexes in Water: Mechanisms of

Bifunctional Catalysis in Water

Luca Bellarosa,^[b]Josefina Dꢀez,^[a]Josꢁ Gimeno,*^[a]Agustꢀ Lledꢂs,*^[b]

Francisco J. Suꢃrez,^[a]Gregori Ujaque,^[b]and Cristian Vicent^[c]

Dedicated to Dr. Hubert Le Bozec on the occasion of his 60th birthday

Abstract: The catalytic activity of ruth-

enium(IV)

([Ru(h³:h³-C₁₀H₁₆)Cl₂L];

rapidly (10–60 min, turn-over frequen-

cy=750–3000 h^ꢀ1) and, in some cases,

at 358C in 60 min. The nature of the

aqueous species formed in water by

this complex has been analysed by

ESI-MS. To analyse how an aqueous

medium can influence the mechanism

of the bifunctional catalytic process,

DFT calculations (B3LYP) including

one or two explicit water molecules

and using the polarisable continuum

model have been carried out and pro-

vide a valuable insight into the role of

water on the activity of the bifunctional

catalyst. Several mechanisms have

been considered and imply the forma-

tion of aqua complexes and their de-

protonated species generated from

C₁₀H₁₆=2,7-dimethylocta-2,6-diene-1,8-

diyl, L=pyrazole, 3-methylpyrazole,

3,5-dimethylpyrazole, 3-methyl-5-phe-

nylpyrazole, 2-(1H-pyrazol-3-yl)phenol

or indazole) and ruthenium(II) com-

[Ru(h³:h³-C₁₀H₁₆)Cl

₂ACTHNGUTERNNU(G pyrazole)]. Differ-

ent competitive pathways based on

outer-sphere mechanisms, which imply

hydrogen-transfer processes, have been

analysed. The overall isomerisation im-

plies two hydrogen-transfer steps from

the substrate to the catalyst and subse-

quent transfer back to the substrate. In

addition to the conventional Noyori

outer-sphere mechanism, which in-

volves the pyrazolide ligand, a new

plexes ([RuACHTUNGTRENNUNG

(h⁶-arene)Cl₂(3,5-dimethyl-

pyrazole)]; arene=C₆H₆, p-cymene or

C₆Me₆) in the redox isomerisation of

allylic alcohols into carbonyl com-

pounds in water is reported. The

former show much higher catalytic ac-

tivity than ruthenium(II) complexes. In

particular, a variety of allylic alcohols

have been quantitatively isomerised by

mechanism with

a hydroxopyrazole

complex as the active species can be at

work in water. The possibility of for-

mation of an enol, which isomerises

easily to the keto form in water, also

contributes to the efficiency in water.

using [Ru(h³:h³-C₁₀H₁₆)Cl

₂ACTHUNRTGNE(GNU pyrazole)]

as a catalyst; the reactions proceeded

faster in water than in THF, and in the

absence of base. The isomerisations of

monosubstituted alcohols take place

Keywords: allylic alcohols · cooper-

ating ligands · isomerisation · ruthe-

nium · water chemistry

Introduction

[a] Dr. J. Dꢀez, Prof. Dr. J. Gimeno, F. J. Suꢁrez

Departamento de Quꢀmica Orgꢁnica e Inorgꢁnica

Instituto Universitario de Quꢀmica Organometꢁlica “Enrique Moles”

Laboratorio de Quꢀmica Organometꢁlica y Catꢁlisis

(Unidad Asociada al C.S.I.C.)

The search for new catalytic methodologies in aqueous

media has emerged as one of the most competitive strat-

egies in modern organic synthesis. The development of new

efficient transformations in water under mild reaction condi-

tions is especially attractive, not only for economic reasons,

but also in terms of environmental interest.^[1]The main

drawback of such methodology is the general insolubility of

organic compounds in water, although it is well documented

that enhanced reactivity is sometimes found.^[1f]

Organometallic homogeneous catalysis has become a

powerful and versatile methodology in modern organic syn-

thesis. Conventionally, appropriate selection of the metal

and the steric and/or electronic features of the ancillary li-

gands have been used to control the activity and selectivity

of the catalyst. Amongst the catalytic methodologies devot-

ed to mimic the exceptional activity of metalloenzymes

(combining the coordination properties of metals and hydro-

gen bonds),^[2,3]metal–ligand bifunctional catalysis has pre-

Universidad de Oviedo

33071 Oviedo (Spain)

Fax : (+34)985-103-446

E-mail: jgh@uniovi.es

[b] Dr. L. Bellarosa, Prof. Dr. A. Lledꢂs, Dr. G. Ujaque

Departament de Quꢀmica

Universitat Autꢃnoma de Barcelona

Bellaterra

08193 Barcelona (Spain)

E-mail: agusti@klingon.uab.es

[c] Dr. C. Vicent

Serveis Centrals d’Instrumentaciꢂ Cientꢀfica

Universitat Jaume I

Avenida Sos Baynat s/n

12071 Castellꢂ (Spain)

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/chem.201103374.

Chem. Eur. J. 2012, 18, 7749 – 7765

ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

7749

sented a new powerful strategy for achievement of enhanced

reactivity and selectivity in organometallic catalysis.^[4,5]

Typical bifunctional catalysts are characterised by the

presence of ligands able to cooperate directly in the bonding

and activation of the substrate. Transformation is through

hydrogen bonding and subsequent hydrogen transfer. This

type of secondary substrate–catalyst interaction usually pro-

vides lower-energy intermediates or transition states, which

gives rise to rate accelerations. Genuine examples of this

class of catalyst are ruthenium (Noyoriꢅs^[5a,4e–g]and

Shvoꢅs^[5d,g,j]catalysts) and rhodium complexes^[5e,f](Figure 1),

tions proceeded faster in water than in THF. We speculated

that the higher polarity of water relative to THF favours the

dissociation of the chloride ligands, which gives rise to aqua

complexes of type D and E (Figure 2). In the presence of a

Figure 2. Aqua complexes (D), (E) and transient intermediates (F), (G)

involved in the isomerisation catalysed by 1.

base, deprotonation processes—which probably involve the

incoming allylic alcohol—are also more favourable in aque-

ous media and lead to transient alkoxy-allyl active species F

(Figure 2). Under neutral conditions, water exchange proc-

esses and coordination of the allylic alcohol G (Figure 2)

can be considered. Recently, DFT calculations on the mech-

anism, inclusive of the coordination of water molecules in

the intermediate active species, have been performed.^[9]The

theoretical energy profile for the catalytic cycle and overall

energy barrier is in accordance with the relatively high tem-

perature required for the reaction to occur (758C).

However, despite these interesting experimental and theo-

retical results some questions still remain, which arise from

the following drawbacks: 1) no catalytic activity is observed

at temperatures below 608C, 2) the catalytic activity in the

absence of base is much lower, 3) no hydrogen-bonding in-

teractions between water molecules and the substrate were

considered.

Figure 1. Bifunctional catalyst transition-states: a) Noyori type, b) Shvo

type, c) Grꢆtzmacher type.

which are active in hydrogenation and transfer hydrogena-

tion of unsaturated substrates R²R¹C=X (X=O or NR).

The exceptional catalytic performances are rationalised on

the basis of an outer-sphere mechanism that enables transfer

of a hydrogen atom to the substrate by cooperative action

ꢀ

between the metal and either the N H moiety of an amine

ꢀ

ligand (Figure 1, A and C) or the C OH group of a cyclo-

pentadienyl ring (Figure 1, B).

The crucial role of these cooperative effects, based on

ligand–substrate hydrogen interactions, on the enhanced cat-

alytic activity is well documented. Thus, a series of efficient

catalytic transformations have been reported in which the

catalytically active species show intramolecular hydrogen in-

teractions between the substrates and free donor atoms of

the ligands. In particular, important contributions to the

high catalytic activity of some transformations performed

either in aqueous media, or in processes where water acts as

a reagent, are also known.^[6]A huge number of water sol-

vent molecules are present in aqueous media, hence, hydro-

gen bonds can be established and participate in proton-

transfer processes, and even coordinate to the metal. As a

result, water could affect the catalytic activity and mode of

action of the bifunctional catalysts. However, a detailed un-

derstanding at the microscopic level of the mechanism of

metal–ligand bifunctional catalysis in water has not yet been

achieved.

Following our interest in this transformation, and encour-

aged by the improved catalytic activity in an aqueous

medium,^[8–10]we wanted to investigate whether the combina-

tion of the cooperative role of water and the presence of a

bifunctional catalyst could enhance the catalytic activity. We

envisioned that 1,2-azole derivatives might serve as cooper-

ating ligands^[11]in a similar way to Noyoriꢅs amino-amido

ꢀ

complexes, which bear ligands with a-N H protic groups

(Scheme 1).^{[4a,e–g,12,13]}In fact, it is well known^[12]that pyra-

zole-type ligands are prone to deprotonation and lead to a

medium-dependent equilibrium of pyrazolide complexes.

We have reported that bis

ACHTUNGTNER(NUNG allyl)-ruthenium(IV) dimer

[{Ru(h³:h³-C₁₀H₁₆)

ACHTUNGTRENNUNG

2,6-diene-1,8-diyl)^[7]in the presence of base is a highly

active catalyst for the redox isomerisation of allylic alcohols

into carbonyl compounds, both in THF and water, and pro-

vides outstanding turn-over frequencies (TOF) and turnover

numbers (TON).^[8]Remarkably, it was found than the reac-

Scheme 1. 1,2-Azole derivatives as cooperative ligands.

Herein, we report a new, highly efficient methodology for

the isomerisation of allylic alcohols^[14]in water assisted by

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Chem. Eur. J. 2012, 18, 7749 – 7765

Redox Isomerisation of Allylic Alcohols

FULL PAPER

bifunctional catalysis. Remarkably, in contrast to the catalyt-

ic activity found for 1, the new hydrosoluble ruthenium(IV)

bifunctional catalysts that contain pyrazole-based ligands

(Scheme 2) allow the process to be performed at 358C and

in the absence of base.^[15]

Synthesis of complexes 1 f and 2c: Treatment of 1 with inda-

zole in CH₂Cl₂at room temperature afforded mononuclear

complex 1 f. Complex 2c was similarly obtained from

(h⁶-C₆Me₆)}₂] by heating with an equivalent

[{RuClACHTNUTRGENN(UG m-Cl)ACHUTNGTRENNUGN

amount of 3,5-dimethylpyrazole in toluene at reflux temper-

ature for 12 h. Both complexes were isolated (64 and 55%,

respectively) as air-stable orange solids (Scheme 3).

IR and NMR spectroscopic data and elemental analysis

for compounds 1a–f and 2a–c are in agreement with the

proposed formulations (see the Experimental section for de-

1

tails). In particular, H and ¹³C NMR spectra of 1 f and 2c

display signals for the h⁶-C₆Me₆, h³:h³-2,7-dimethylocta-2,6-

diene-1,8-diyl and h⁶-arene ligands that compare well with

those previously reported for analogous complexes.^[16a–e]No-

tably, the proton and carbon resonances of the 2,7-dimethy-

locta-2,6-diene-1,8-diyl group in the NMR spectra are in ac-

cordance with the formation of a simple equatorial adduct

Scheme 2. Ru^IV-catalysed isomerisation of allylic alcohols in water.

with C₂symmetry, in which the two halves of the bisACHTUNGTRENNUNG(allyl)

DFT studies have been performed on competitive path-

ways that involve putative bifunctional catalytic species with

the participation of aqua complexes. New outer-sphere

mechanisms are proposed for the isomerisation of allylic al-

cohols into carbonyl derivatives in the absence of base.

Some general ideas about the effect of solvation in a metal–

ligand bifunctional catalytic system arise from this mecha-

nistic study.

ligand are in equivalent environments.^[7]In addition, the

structures of 1 f and 2c were unequivocally confirmed by

single crystal X-ray diffraction. The molecular structures

with selected bond distances and angles are depicted in Fig-

ures S1 and S2 (see the Supporting Information).^[17]

Motivated primarily by the use of compounds 1a–f and

2a–c as catalysts for isomerisation reactions in both organic

and aqueous media we initially addressed their solution

properties and chemical speciation. In general, 1a–f and 2a–

c are soluble in acetone and CH₂Cl₂; conductivity measure-

ments in acetone indicate that all of the complexes are neu-

tral derivatives. ESI mass spectroscopy of solutions of neu-

tral complexes 1a–f and 2a–c in CH₂Cl₂revealed the pres-

ence of species with the general formula [MꢀCl]⁺as the

base peak (see Figure S3 in the Supporting Information).

Metal–halide bond-breaking is a well-documented ionisation

mechanism under ESI conditions for neutral metal–halide

complexes.^[18]The molar conductivity (L_M) of complexes 1a

and 2a–c in water (the rest of the complexes are scarcely

soluble in water, which prevented the conductivity measure-

ments) had values consistent with the presence of an equili-

brium between neutral and 1:1 electrolytes (L_M=75–

126 W^ꢀ1cm²mol^ꢀ1). These aqueous solutions are acidic with

pH values in the range 3.25–4.49, indicative of proton disso-

ciation, either from a coordinated molecule of water or the

Results and Discussion

Ruthenium(IV) complexes [Ru(h³:h³-C₁₀H₁₆)Cl₂L] (1a–f;

L=pyrazole-based ligand) were used for the catalytic stud-

ies (Scheme 3). The catalytic activity of analogous rutheniu-

ꢀ

pyrazole type N H group. Detailed insights into the chemi-

cal speciation of 1a and 2a–c in water were obtained by in

situ ESI mass spectrometry; the results are in excellent

agreement with the conductivity data. Typically, aqueous

solutions of compounds 1a and 2a (as representatives of

each series) were stirred and aliquots were extracted at

spaced time intervals, diluted with water to a final concen-

tration of 5ꢇ10^ꢀ4m (based on the initial Ru concentration)

and directly introduced to the mass spectrometer. Illustra-

tive ESI mass spectra of compound 1a are provided in Fig-

ure S3a (see the Supporting Information) at t=0 and 1.5 h.

Initially (t=0 h), the ESI spectrum shows two major peaks

identified as [1aꢀCl+H₂O]⁺(m/z=359) and [1aꢀCl]⁺(m/

z=341), which indicate that chloride cleavage and subse-

Scheme 3. Synthesis of complexes 1a–f and 2a–c.

m(II) complexes [RuACTHNUTRGNEUNG

(h⁶-arene)Cl₂L] (2a–c) were also stud-

ied for comparative purposes. All complexes were synthes-

ised from the commercially available dimeric precursors 1

or [{RuClACHTUNGTRENNUNG(m-Cl)ACHTUNGTRENNUNG

(h⁶-arene)}₂] by reaction with the appropri-

ate ligands in procedures previously reported for 1a,^[16a,b]

1b–e,^[16c]2a^[16d]and 2b.^[16e]

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A. Lledꢂs, J. Gimeno et al.

quent water coordination at the vacant site takes place rap-

idly (Scheme 4). After 1.5 h, new species that arise from the

1a (0.2 mol% Ru) and KOtBu (0.4 mol%) octan-3-one was

formed almost quantitatively (TOF=333 h^ꢀ1

;

Table 1,

entry 1). This catalytic performance can be compared with

those reported previously for analogous ruthenium(IV)

complexes [Ru(h³:h³-C₁₀H₁₆)Cl₂L] (TOF [h^ꢀ1]: 353, L=CO;

333, L=PACHTUNGTRNEUNG

(OMe)₃; 500, L=NH₂Ph)^[8]and [Ru(h³:h²:h³-

C₁₂H₁₈)Cl₂] (TOF=429 h^ꢀ1).^[19]

A remarkable drop in the catalytic activity of 1a was ob-

served when the reaction was performed in the absence of

base; only 56% yield was achieved after 24 h (Table 1,

entry 2; TOF= 12 h^ꢀ1versus 333 h^ꢀ1for 1a). Under these

reaction conditions, the precursor to 1a, dimer 1, also gives

rise to an inefficient catalytic transformation with a low

TOF value (20 h^ꢀ1; Table 1, entry 3). Therefore, as for the

reported mononuclear catalysts [Ru(h³:h³-C₁₀H₁₆)Cl₂L] (see

above), the use of a base as a co-catalyst appears to be cru-

cial when the reaction is performed in aprotic solvents, such

as THF. This fact indicates that the formation of the active

species is favoured by the action of the base in the dissocia-

tion of the chloride ligand (Scheme 1), as well as the depro-

tonation of the incoming allylic alcohol.

To take advantage of the solubility of complexes 1a and

2a–c in water and the easy proton and chloride dissociation

processes (Scheme 4), we decided to study the isomerisation

of 1-octen-3-ol catalysed by 1a in aqueous media as a reac-

tion model. To our delight, the reaction proceeds at a higher

rate than observed in THF, not only in the presence of the

base, but also in its absence. Quantitative transformations to

octan-3-one were achieved in 10–15 min (TOF=2000 and

3000 h^ꢀ1, respectively; Table 1, entries 4 and 5). This behav-

iour is in accordance with the formation of active species

that arise from proton and chloride dissociations, much

more favoured in the aqueous reaction media than in THF.

In fact, the addition of small quantities of water to the reac-

tion performed in THF gives rise to a remarkable rate en-

hancement (Table 1, entry 2^[d]). Notably, the higher catalytic

activity observed in the absence of the base (Table 1, en-

tries 4 and 5; TOF=2000 versus 3000 h^ꢀ1) is probably due

Scheme 4. Equilibrium formation of cationic species detected by ESI-MS

of an aqueous solution of 1a at t=0 and 1.5 h.

evolution of the former aqua species were detected in the

ESI mass spectrum. Cationic chloride-free species, namely

[1aꢀClꢀHCl]⁺(m/z=305) and the corresponding mono-

and di-solvated species, namely [1aꢀClꢀHCl+H₂O]⁺(m/

z=323) and [1aꢀClꢀHCl+2H₂O]⁺(m/z=341), were also

detected. Note that species [1aꢀCl]⁺and [1aꢀClꢀHCl+2-

H₂O]⁺overlap, however, they could be unambiguously iden-

tified on the basis of their distinctive collision-induced disso-

ciation (CID) spectra (see the Supporting Information).

Because proton dissociation of coordinated water mole-

cules is a feasible process, favoured by the presence of a pyr-

azolide ligand, aqua-hydroxo species can exist in equilibri-

um. Schematic drawings of plausible species experimentally

detected in the aqueous chemical speciation of 1a are pro-

vided in Scheme 4. Although attempts to isolate or detect

any of the aqua and aqua-hydroxo complexes by NMR spec-

troscopy have been unsuccessful, the complexes are relevant

as catalytically active species in the isomerisation of allylic

alcohols in aqueous medium (see below). Aqueous

chemical speciation for compound 2a was compara-

Table 1. Ruthenium-catalysed isomerisation of 1-octen-3-ol into octan-3-one.^[a]

ble (apart from minor differences in the relative in-

tensities of the detected species) to that found for

1a, as judged by ESI-MS analysis (see Figure S3b

in the Supporting Information).

Entry

Solvent

Catalyst

Co-catalyst

[0.4 mol%]

T

t

Yield

[%]^[b]

[8C]

Isomerisation of 1-octen-3-ol into octan-3-one cata-

lysed by complex 1a in THF or water: We have

previously reported that mononuclear rutheniu-

m(IV) complexes [Ru(h³:h³-C₁₀H₁₆)Cl₂L] (L=CO,

PR₃, N-donor ligands) are active catalysts in the iso-

merisation of 1-octen-3-ol into octan-3-one in

THF.^[8]To assess the catalytic activity of the ruthe-

nium(IV) complexes described herein, we firstly

checked the behaviour of hydrosoluble complex 1a

as a catalyst precursor (Table 1). When a solution

of 1-octen-3-ol in THF (0.2m) was heated at reflux

1

THF

H₂O

–

1a

1

1a

1

KOtBu^[c]

–

75

55

35

75

90min

24h

15min

10min

40min

50min

60min

6h

>99

56

96

>99

99

2^[d]

3^[e]

4

KOtBu

5

–

6^[e]

7^[f]

8

[Ru(h³:h²:h³-C₁₂H₁₈)Cl₂]

1a

9

10

98

3

90min

[a] Reactions performed under N₂atmosphere with 1-octen-3-ol (4 mmol, 0.2m), sub-

strate/Ru=500:1. [b] Yield of octan-3-one determined by GC. [c] Actual co-catalyst

tBuOH/KOH. [d] THF:H2O [mL]/time [h]/yield [%]. i) 20:0/24/53; ii) 19:1/24/61;

temperature for 90 min with a catalytic amount of iii) 18:2/24/84; iv) 17:3/24/96; v) 16:4/3/97; vi) 15:5/1/99. [e] Ref. [8]. [f] Ref. [19].

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Redox Isomerisation of Allylic Alcohols

FULL PAPER

to competitive coordination of the base with the allylic alco-

hol for the vacant site. The catalytic performance is better

than that for dimer 1 (Table 1, entry 6) and analogous cata-

lyst [Ru(h³:h²:h³-C₁₂H₁₈)Cl₂] (Table 1, entry 7) in water. In

addition, although the isomerisation rate decreases with

temperature, good catalytic activity is also observed at

558C, and even at 358C (Table 1, entries 8 and 9, respective-

ly). In contrast, no catalytic activity is observed for 1 at tem-

peratures lower than 608C.^[8]It is interesting to note that no

isomerisation is observed when the reaction is performed in

the absence of water (Table 1, entry 10), which reveals the

key role of water in the catalytic transformation.^[20]As far as

we know, complex 1a is the most active ruthenium catalyst

in the absence of base reported to date.^[21]Significantly, only

Isomerisation of 1-octen-3-ol into octan-3-one catalysed by

complexes 1a–f and 2a–c in water: The catalytic activity of

ruthenium(IV) complexes 1b–f and of h⁶-arene rutheniu-

m(II) complexes 2a–c (for comparative purposes) has also

been tested in water and in the absence of base under analo-

gous reaction conditions (Figure 4). Complexes 1b–f are

one further catalytic system, that is, [RhACHTNUTRGENNUG(cod)CAHTUNGTREN(NUGN MeCN)₂]BF₄/

PTA (cod=1,5-cyclooctadiene; PTA=1,3,5-triaza-7-phos-

phaadamantane), has been reported to be active at ambient

temperature and in the absence of base.^[15p]

The reaction profile of the isomerisation is characterised

by an induction period of several minutes (Figure 3). This is

consistent with the slow formation (after 1.5 h at RT) of

Figure 4. Isomerisation of 1-octen-3-ol into octan-3-one catalysed by com-

plexes 1a–f and 2a–c in water. Reactions performed under N₂atmos-

phere with 1-octen-3-ol (4 mmol, 0.2m); substrate/Ru=500:1; yield of

octan-3-one determined by GC; TOF [(mol product/mol Ru)/time]

values were calculated at the time indicated in each case.

good catalysts; quantitative transformations were achieved

in only 10–15 min (TOF=2000–2970 h^ꢀ1). In contrast, much

lower catalytic activity is shown by 2a–c (TOF=96–

250 h^ꢀ1). The relatively small differences in activity amongst

the ruthenium(IV) catalysts indicates that there is no influ-

ence of the pyrazole-based ligands on the catalytic efficien-

cy. Nevertheless, a slightly higher catalytic activity is found

for those ligands substituted with electron-releasing groups

(1b,c).

Figure 3. Isomerisation of 1-octen-3-ol into octan-3-one catalysed by com-

plex 1a (1 mol%) in water (reaction performed under N₂atmosphere at

558C with 1-octen-3-ol (4 mmol, 0.2m in H₂O). Yield of octan-3-one as a

function of time. [first charge ( ): TOF=119 h^ꢀ1(50 min), 89 h^ꢀ1

&

(25 min); after pre-warming the catalyst solution at 758C for 5 min and

ꢀ1

*

subsequent addition of 1-octen-3-ol at 558C ( ): TOF=600 h (10 min),

912 h^ꢀ1(5 min)].

aqua- and hydroxo cationic complexes (detected by ESI-MS

of 1a in water) which confirms that they can act as active

catalytic species (Scheme 4). Remarkably, when the aqueous

solution of the precursor catalyst 1a (1 mol%) is pre-heated

at 758C for 5 min a quantitative and rapid transformation is

attained at 558C (15 min, TOF=600 h^ꢀ1), which proves that

the active catalysts formed in water show a high efficiency.

Notably, a remarkable drop in the catalytic activity was

observed (79% yield after 12 h) when an excess of H-

Isomerisation of substituted allylic alcohols C(H)R¹=

CR²CH(OH)R³catalysed by complex 1a in water: Catalyst

1a shows the highest catalytic activity of the series and is

also very efficient in the isomerisation of a variety of allylic

alcohols, which proves the wide scope of this catalytic trans-

formation (Table 2).

As expected, and in accordance with other catalytic redox

isomerisations of allylic alcohols, there is a strong depend-

ence upon the substitution of the olefinic bond.^[15]Thus,

monosubstituted allylic alcohols (Table 2, entries 1–7) are

readily isomerised with a catalyst loading of 0.2 mol%

(TOF=490–3000 h^ꢀ1), whereas higher catalyst loading (5–

10 mol%) and longer reaction times are required for 1,1

and 1,2-disubstituted allylic alcohols (Table 2, entries 8–11)

to attain total conversion. Similar activity is also observed

ACHTUNGTRENNUNG[BF₄]·Et₂O was added to the reaction, which suggests that

the formation of the active species is disfavoured in strongly

acidic media.^[22]This is consistent with theoretical calcula-

tions (see below) that support kinetically favoured pathways

in which the key intermediates are aqua- and hydroxo spe-

cies stabilised by hydrogen-bonding interactions.

Chem. Eur. J. 2012, 18, 7749 – 7765

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A. Lledꢂs, J. Gimeno et al.

Table 2. Isomerisation of substituted allylic alcohols catalysed by complex 1a in water at 758C.^[a]

both in THF and water, which

provides a thermodynamic driv-

ing force for the isomerisation.

Because conductivity measure-

ments and ESI-MS data reveal

the presence of cationic species

in water (Scheme 4), we have

studied the mechanism with the

assumption that different active

species are generated in aque-

ous medium (see below). Dif-

ferent mechanisms are dis-

Entry

Substrate

Product

Ru

[mol%]

t

Yield

[%]^[b]

AHCTUNGTRENNUNG

1

2

3

4

5

0.2

10min

30min

60min

20min

>99

91

98

99

cussed as

a function of the

nature of the active species.

The favoured mechanism for

the isomerisation has also been

computed for the transforma-

tion of 2-cyclohexenol to cyclo-

hexenone (Table 2, entry 12).

6

7

8

0.2

5

20min

60min

89

>99

95

9

5

60min

99

Active catalysts: Conductivity

measurements of 1a, along with

10

11

5

60min

20 h

61^[c]ESI-MS spectra in water,

showed the presence of an

99

83

equilibrium between neutral

and cationic species

10

(Scheme 4). On the basis of this

experimental information, we

have estimated the relative

Gibbs energies of the possible

neutral and cationic species

formed in water by Cl^ꢀ/H₂O

ligand exchange and/or dissoci-

ation and pyrazole deprotona-

tion to avoid formation of dica-

12

13

10

15 h

120min

[a] Reactions performed under N₂atmosphere with allylic alcohol (1 mmol, 0.2m). [b] Yields determined by

GC. [c] 14% of a,b-unsaturated aldehyde (cinnamaldehyde) also obtained.

for the trisubstituted allylic alcohol 3-methyl-2-buten-1-ol,

which is transformed into 3-methyl-butyraldehyde in 83%

yield after 2 h (Table 2, entry 13).

tionic species (Scheme 5). To calculate these values, Gibbs

energies in water of Cl^ꢀ, water and a proton have been

added when required. Accurate, experimentally derived

values for the aqueous solvation Gibbs energy of a proto-

n^[23a]and a chloride ion^[23b]have been used (ꢀ264.0 and

ꢀ74.6 kcalmol^ꢀ1, respectively). Energies should be consid-

ered an estimation given the inherent difficulties in calcula-

tion of the solvation energies of charged species.^[24]In the

first row (Scheme 5), coordinative saturated species, which

include the precursor catalyst 1a and cationic aqua com-

plexes generated through chloride–water exchange process-

es, are shown. To provide the vacant site required to receive

a hydride ion, a vacant position should be created in the co-

ordination sphere of 1a. Thus, chloride or water dissociation

is required. Possible aqua-active unsaturated species are

shown in the second row (Scheme 5). Optimised geometries

of these complexes are provided in the Supporting Informa-

tion. The optimised geometry of 1a fits well with the corre-

sponding X-ray crystal structure.^[16c]

Computational study of the isomerisation mechanism: The

experimental rate enhancement of catalyst 1a in neutral

aqueous medium relative to THF (approximately nine-fold;

Table 1, entry 1 versus 5) points to a slight difference be-

tween the actual overall energy barriers (1–2 kcalmol^ꢀ1)

that could be smaller than the accuracy of computational

methods for this complex reaction in water. With this as-

sumption, the main goal of the theoretical study was to ana-

lyse how solvation in water can affect the mechanism of a

bifunctional catalytic process and, in particular, to investi-

gate the plausibility of outer-sphere mechanisms via active

aqua complexes as intermediates. The theoretical study has

been performed with the pyrazole complex 1a as the precur-

sor catalyst, 2-propen-1-ol as the substrate (Table 2, entry 6)

and the ESI-MS detected species as an experimental input

for the active catalyst. For comparative purposes, the proc-

ess in THF has also been computed. The reaction product,

propanal, is 16.3 kcalmol^ꢀ1more stable than 2-propen-1-ol,

The results outline an easy initial substitution of chloride

by a water ligand (see Scheme 4 and the MS results in the

Supporting Information) and a more difficult further evolu-

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Chem. Eur. J. 2012, 18, 7749 – 7765

Redox Isomerisation of Allylic Alcohols

FULL PAPER

freely between the two basic centres. We have also explored

the possibility of a trans-hydroxo complex (t_OH-I₀) that

arises from solvent-mediated proton exchange between the

trans-aqua and pyrazolide ligands of t_water-I₀. Structure

t_OH-I₀(Scheme 5) is 8.1 kcalmol^ꢀ1more stable the corre-

sponding trans-aqua complex t_water-I₀(20.1 versus

28.2 kcalmol^ꢀ1) and of similar stability to c_OH-I₀(20.1

versus 18.3 kcalmol^ꢀ1). The similitude of the experimentally

determined pK_avalues of pyrazole and water supports the

possible existence of hydroxo-pyrazole complexes^[26]and

they should be considered as potential active species in

water. In fact, such species have been detected by ESI-MS

(Scheme 4).

In solution in rigorously dried THF, Cl-I₀is the only cata-

lytically active species possible. The DG_THFfor the reaction

(1a!Cl-I₀+HCl) is 19.8 kcalmol^ꢀ1. However, this value is

ꢀ28.5 kcalmol^ꢀ1when a base is included in the equilibrium

Scheme 5. Possible active species formed from 1a in water by Cl^ꢀ/H₂O

ligand exchange and/or dissociation and pyrazole deprotonation and

their calculated relative Gibbs energies [kcalmol^ꢀ1].

(1a+

ACHTUNGTRENNUNG

crucial role of the addition of a base in the generation of

the active catalyst in THF.

tion of the system toward the unsaturated species from

which the reaction can begin. Formation of all of the poten-

tial active species in water is endergonic and should involve

a slow, but irreversible, step from 1a to the active catalyst to

account for the experimental induction period (Figure 3).

However the barrier of this step cannot be calculated prop-

erly because of the limitations in describing solution proc-

esses entailing charge separation.

As for 1a, the trans-diaqua complex is somewhat more

stable than the cis. This trans-diaqua complex is actually an

aqua-hydroxo complex. When optimisation of this species is

started from the trans-diaqua complex, a proton is spontane-

ously transferred to the pyrazolide ring and the result is a

trans-aqua-hydroxo pyrazole complex. With regards to the

unsaturated complexes (second row in Scheme 5), one mini-

mum is obtained for the chloride complex (Cl-I₀), whereas

two minima have been found for the aqua complex—they

differ in the cis- or trans disposition of the aqua ligand with

respect to the pyrazolide ligand (Scheme 5). In the trans

minimum (t_water-I₀) the coordinated water is on the oppo-

site site of the pyrazolide unit (N-Ru-O_w=153.68), whereas

in the cis minimum (c_water-I₀) the coordinated water is

closer to the pyrazolide (N-Ru-O_w=83.58). The cis isomer is

11.9 kcalmol^ꢀ1more stable than the trans (16.3 versus

28.2 kcalmol^ꢀ1relative to 1a). The occurrence of a cis mini-

Bifunctional catalysis with pyrazole as a cooperating ligand:

In a conventional Noyori-type mechanism the pyrazolide

ligand participates directly in the bond activation as a coop-

erating ligand; a reversible chemical transformation occurs

to release and accept a proton from the substrate.^[5e]In this

mechanism, the neutral unsaturated complex with a chloride

ligand (Cl-I₀) is the active species and water is only acting as

the solvent with no direct intervention into the process.

Thus, no explicit water molecules are involved in the calcu-

lations and the solvent is introduced as a continuum polaris-

able medium with a dielectric constant (e)=78.4. In the dis-

cussion below, the reaction pathway catalysed by this com-

plex is referred to as the chloride route and intermediates

and transition states found along this pathway as Cl-I and

Cl-TS, respectively.

Detailed catalytic cycle for the isomerisation of allylic al-

cohols into carbonyls compounds along the chloride route is

outlined in Scheme 6. Transition states for the hydrogen-

transfer steps with selected structural parameters are shown

in Figure 5. The associated Gibbs energy profile is presented

in Figure 6 (Cl-I₀and 2-propen-1-ol at infinite distance is

taken as the zero-point energy). Optimised structures of all

minima and transition states along the catalytic cycle are in-

cluded in the Supporting Information.

The overall isomerisation implies two hydrogen-transfer

steps, from the substrate to the catalyst and back to the sub-

strate (Scheme 6). In the first hydrogen-transfer step the un-

saturated active species Cl-I₀, which acts as the dehydrogen-

ated form of the catalyst, is suitable to receive a hydride ion

(red) at the metal site and a proton (blue) at the N-pyrazo-

lide site from 2-propen-1-ol, to give rise to acrolein and the

reduced form of the catalyst (intermediate Cl-I₄). Dehydro-

genation of the allyl alcohol proceeds by a concerted mecha-

nism through transition state Cl-TS₂. Initially, the alcohol

coordinates through the oxygen lone pair to the 16electron

catalyst; this k¹-O intermediate (Cl-I₁) is further stabilised

ꢀ

mum is related to the presence of a strong stabilising O_w

H···N hydrogen bond (H···N=1.60 ꢈ; Scheme 5). Calcula-

tions suggest a similar stability in water of the neutral chlor-

ide species Cl-I₀and the cationic mono-aqua species cis_wa-

ter-I₀(14.7 versus 16.3 kcalmol^ꢀ1, respectively).

ꢀ

The short O_wH···N distance found in c_water-I₀led us to

investigate the stability of a structure in which a proton has

been transferred from the aqua ligand to the pyrazolide

ring. We found structure c_OH-I₀(Scheme 5) with pyrazole

and hydroxo ligands, is almost degenerate in energy at only

2.0 kcalmol^ꢀ1above the aqua complex (18.3 versus 16.3 kcal

ꢀ1

ꢀ

mol ). Moreover, the barrier for the O_wH to N-pyrazolide

proton transfer is very low,^[25]a proton may move almost

Chem. Eur. J. 2012, 18, 7749 – 7765

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7755

A. Lledꢂs, J. Gimeno et al.

Figure 5. Transition states for the hydrogen-transfer steps in the isomeri-

sation of 2-propen-1-ol into propanal (Cl-I₀is assumed as the active spe-

cies in the chloride route). Hydrogen atoms in the bis

been omitted for clarity.

ACHTUNGTREN(NUGN allyl) ligand have

ꢀ

ening of the C₁H bond (1.13 ꢈ) that points toward the

metal. At this stage, a seven-membered ring has been

Scheme 6. Proposed catalytic cycle (chloride route) for the isomerisation

of 2-propen-1-ol into propanal with pyrazole as a cooperating ligand (Cl-

I₀is the active species).

ꢀ ꢀ

formed by the catalyst and the substrate (Ru N N in front

ꢀ1

ꢀ

ꢀ ꢀ

of H C₁O H). From this structure, around 10 kcalmol

above the most stable intermediate Cl-I₁, the concerted and

synchronous transfer of hydride ion to the metal centre and

a proton to the nitrogen atom (via transition state Cl-TS₂,

approximately 7 kcalmol^ꢀ1above intermediate Cl-I₃) leads

to the reduced form of the catalyst and acrolein (Cl-I₄). In

ꢀ

by a N···H O hydrogen bond (O H···N=1.61 ꢈ) and is the

most stable intermediate during the reaction (Figure 6).^[27]

Because of the saturated nature of intermediate ligand Cl-I₁,

dissociation is required for the reaction to proceed, which

considerably raises the energy of subsequent species.

ꢀ

ꢀ ꢀ ꢀ

Cl-I₄, acrolein is almost coplanar with the H Ru N N H

ꢀ

unit. The orientation of the H C₁=O group in acrolein

For this reason, and based on accumulated experience in

bifunctional catalysis,^[4,5]we explored an outer-sphere mech-

anism that entailed cooperative action of the metal and the

pyrazolide ligand. The k¹-O intermediate must rearrange for

the reaction to occur. The presence of a hydrogen-bond ac-

ceptor in the catalyst helps this rearrangement to take place.

Dissociation of oxygen results in a 16 electron intermediate

(Cl-I₂) stabilised only by a hydrogen-bond interaction of the

hydroxyl proton with the nitrogen atom of the pyrazolide

allows simultaneous establishment of a carbonyl–HN hydro-

ꢀ

gen bond (C=O···H N=1.87 ꢈ) and C H···H Ru dihydro-

gen bond (H···H=2.06 ꢈ)^[28]with the catalyst.

In the second half of the cycle, the two hydrogen atoms

must be transferred back to the substrate. Initially, we con-

sidered direct transfer to the C₂and C₃positions of acrolein

to reduce the C=C bond (3,4 addition). Hydrogenation of

the C=C bond of acrolein by the 3,4-addition mechanism is

a two-step outer-sphere ligand-assisted reaction,^[29]which

can be considered a modified stepwise version of a bifunc-

tional metal–NH ligand Noyori-type mechanism.^[30]From

Cl-I₄, suitable rotation of acrolein around the C=O···H axis

is necessary to make the terminal of the carbon–carbon

double bond face the catalyst (Cl-I₅). In Cl-I₅, the

RuꢀH···HꢀC dihydrogen bond is formed between the hy-

dride moiety and a proton attached to the terminal carbon

ꢀ

ligand (O H···N=1.88 ꢈ, Ru···O H=3.75 ꢈ). Bifunctional

catalysis requires transfer of both a proton and a hydride

ion to the catalyst; for this reason, a later rearrangement of

the substrate around the catalyst is necessary to reach the

ꢀ

intermediate Cl-I₃. In Cl-I₃, both a ruthenium s-C₁H inter-

ꢀ

action and an O H···N hydrogen bond (O H···N=1.75 ꢈ)

are present. The s-CH interaction is reflected in the length-

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Chem. Eur. J. 2012, 18, 7749 – 7765

Redox Isomerisation of Allylic Alcohols

FULL PAPER

path (Cl-TS_3(3,4)). From transi-

tion state Cl-TS_3(1,4), the reac-

tion evolves toward a loose Cl-

I₀–enol adduct, then the enol is

released to the solvent and

easily transformed into the final

carbonyl compound.

The Gibbs energy profile in

water (DG_water, Figure 6) illus-

trates that the energetics of the

reaction are perfectly within

the accepted limit imposed by

the temperature of the reaction

(758C). If the 1,4-addition is in

operation then the highest

point in the chloride route (Cl-

TS₂) is found 20.6 kcalmol^ꢀ1

above the reactants and corre-

sponds to the dehydrogenative

halfway point of the isomerisa-

tion process. The energy profile

in water (DE_water), presented in

the Supporting Information, is

very similar to the DG_water; the

main difference is that the Cl-I₁

intermediate becomes the most

Figure 6. Gibbs energy [kcalmol^ꢀ1] profile in solution (water and THF) for the isomerisation of 2-propen-1-ol

into propanal (Cl-I₀is assumed as the active species in the chloride route). DG_THF⁽c), DG_THF(1,4)(b),

DG_water(c), DG_water(1,4)(b); the upper values refer to THF, the lower to water.

atom. Further reorientation of acrolein is required to place

the bond to be hydrogenated (C₂=C₃) in front of the hydride

(red) and the NꢀH proton (blue). This configuration is

reached in transition state Cl-TS_3(3,4). Contrary to the sub-

strate dehydrogenation, the direct C=C hydrogenation of

the aldehyde is a two-step process via the transition states

Cl-TS₃and Cl-TS₄. From Cl-I₅, the hydride is first transfer-

red to C₃(via transition state Cl-TS_3(3,4)) to give rise to an

enolate anion in Cl-I₆.^[31]At this point, the proton transfer

stable species. In the DE_waterprofile, Cl-TS₂lies 19.0 kcal

mol^ꢀ1above Cl-I₁.

The neutral intermediate Cl-I_ois the only possible active

species in THF solution, therefore, the chloride route is the

only mechanism of bifunctional catalysis that can be at work

in THF. However, the easy water-catalysed enol–keto iso-

merisation (see the Supporting Information)^[33]required in

the 1,4-addition pathway would be more difficult in THF.

We have recalculated the Gibbs energy profile to consider

the reaction in THF (DG_THF; polarisable continuum model

(PCM), e_THF=7.58). The results are incorporated into

Figure 6. The effect of changing the solvent in this mecha-

nism is very small: the DG profiles in water and THF are

very similar. The overall barrier for the chloride route in

THF is 20.4 kcalmol^ꢀ1if we consider that the 1,4-addition is

still operative in this solvent, and 21.3 kcalmol^ꢀ1if direct C=

ꢀ

from N H to the C₂carbon atom has not yet started (N

ꢀ

H=1.05 ꢈ, C₂H=1.91 ꢈ).

Proton transfer takes place in the subsequent step, via Cl-

TS₄, to give propanal and recovered catalyst. The barrier for

the proton-transfer step is very low (1.6 kcalmol^ꢀ1); this

highlights the shallow-minimum character of intermediate

Cl-I₆, which quickly evolves to Cl-I₇. At this stage, propanal

has been formed and the weak interaction between the

product and catalyst encourages the final dissociation of the

former. The removal of propanal frees up the catalyst (Cl-

I₀) and a new cycle can begin.

C

hydrogenation of acrolein (3,4-addition) is assumed

(Figure 6).

In the chloride route, the active specie Cl-I₀is a neutral

complex and conductivity measurements of 1a and ESI-MS

data have shown the presence of cationic species in aqueous

solution (Scheme 4 and Figure 4). Below we report the

mechanistic study in water with the assumption of different

active species than in THF.

The presence of water could provide an alternative mech-

anism for the reduction of a,b-unsaturated carbonyl species

such as acrolein. Instead of direct reduction of the C=C

bond by a 3,4-addition mechanism to give propanal, the re-

action could proceed through a 1,4-addition mechanism to

producing enol 1-propen-1-ol,^[32]which can be easily isomer-

ised to propanal in water.^[33]We have explored this alterna-

tive pathway for the second hydrogen-transfer step in the

chloride route. From Cl-I₅, we have found a concerted tran-

sition state for the 1,4-addition (Cl-TS_3(1,4); Figure 5), at

2.5 kcalmol^ꢀ1below the highest point on the 3,4-addition

Bifunctional catalysis with hydroxide as

a cooperating

ligand: Given that the ESI-MS measurements showed the

presence of cationic species without chloride ligands and

calculations disclosed hydroxo complexes with stabilities

comparable to that of Cl-I₀(Scheme 5), we have also ex-

plored reaction pathways catalysed by hydroxo complexes

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7757

A. Lledꢂs, J. Gimeno et al.

(hydroxo route). Independent reaction pathways for the cis-

and trans active species have been investigated and com-

pared. Intermediates and transition states found along these

pathways are labelled with the prefixes OH-I and OH-TS,

respectively. Both the cis- and trans-hydroxo routes have

similar steps. The trans-hydroxo route has a considerably

lower overall barrier than the cis-hydroxo route and it is dis-

cussed in detail in this section. Results for the cis-hydroxo

route are given in the Supporting Information.

The catalytic cycle catalysed by the trans-hydroxo species

t_OH-I₀is depicted in Scheme 7. Transition states of the hy-

drogen-transfer steps together with selected structural pa-

rameters are shown in Figure 7. The corresponding Gibbs

energy profile is presented in Figure 8 (t_water-I₀and 2-

propen-1-ol at infinite distance is taken as zero-point

energy). Optimised geometries of all of the intermediates

and transition states are collected in the Supporting Infor-

mation.

Dehydrogenation of the allylic alcohol takes place via

transition state t_OH-TS₂(Figure 7), which is similar to

those found in the chloride and cis-hydroxo routes (Cl-TS₂

and c_OH-TS₂; see Figure 5 and the Supporting Informa-

tion, respectively). Through the delivery of a proton and a

Figure 7. Transition states for the hydrogen-transfer steps in the isomeri-

sation of 2-propen-1-ol into propanal (t_OH-I₀is assumed as the active

species in the trans-hydroxo route). Hydrogen atoms in the bisACHTUNGTRENNUNG(allyl)

ligand have been omitted for clarity.

hydride moiety to the catalyst, the intermediate t_OH-I₃is

obtained, in which acrolein has been formed and the former

hydroxo ligand has become an aqua ligand in the reduced

form of the catalyst. As in the chloride route, Noyoriꢅs bi-

functional concerted mechanism takes place, in which a hy-

dride ion and a proton are simultaneously transferred to the

ruthenium and the basic centre (O from the hydroxo

ligand). However, it is important to note that the formation

of these transition states involves a lower Gibbs energy bar-

rier (15.3 kcalmol^ꢀ1from t_OH-I₀for the trans-hydroxo

route versus 20.6 and 22.4 kcalmol^ꢀ1

ACTHNUTRGNEUNGfrom zero-point energy

in the chloride and cis-hydroxo routes, respectively). We

have also explored the possibility that this step takes place

from t_water-I₀. In this case, the basic nitrogen centre in pyr-

azole is the proton-acceptor site instead of the hydroxo

group oxygen atom in t_OH-I₀. The reaction proceeds

through transition state t_water-TS₂(see the Supporting In-

formation). The barrier found (17.4 kcalmol^ꢀ1) is slightly

higher than that for t_OH-I₀. In addition t_water-I₀is much

less stable than t_OH-I₀(Scheme 5). All of these factors

make the trans-hydroxo species t OH-I₀the most likely

active species. Starting both from t_OH-I₀and t_water-I₀,

the dehydrogenative step results in the same intermediate,

t_OH-I₃(the reduced catalyst with hydride, pyrazole and

trans water ligands, and hydrogen-bonded acrolein). Thus,

the second half of the reaction is common to both active

species.

Two factors contribute to the decrease in the Gibbs

energy for the allylic alcohol dehydrogenation in t_OH-TS₂:

1) the presence of a more basic ligand (hydroxide), which

Scheme 7. Proposed catalytic cycle (trans-hydroxo route) for the isomeri-

sation of 2-propen-1-ol into propanal in water (t_OH-I₀is the active spe-

cies).

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Redox Isomerisation of Allylic Alcohols

FULL PAPER

Figure 8. Gibbs energy [kcal mol^ꢀ1] profile in water for the isomerisation of 2-propen-1-ol into propanal (t_OH-I₀is assumed as the active species in the

trans-hydroxo route). Path O (c), path N (b).

facilitates the proton transfer; 2) the pyrazole proton, which

promotes the hydride transfer through a dihydrogen bond

the system. For t_OH-TS₂we considered two possibilities:

1) the water molecule participates directly in the proton

transfer from 2-propen-1-ol to the hydroxo ligands [inside

path, t_OH-TS₂ACHTGUNTERNNU(G water-in)]; 2) the water molecule is only hy-

drogen bonded in the transition state [t_OH-TS₂(water-out)]

ꢀ

(H···H N=2.21 ꢈ).

The dehydrogenation of 2-propen-1-ol produces acrolein

in its most stable s-trans conformation, hydrogen bonded to

the reduced form of the catalyst, which bears a hydride,

water and pyrazole ligand (t_OH-I₃). In the second half of

the isomerisation cycle, the C₂=C₃bond of acrolein must be

hydrogenated by the reduced form of the catalyst (hydride

intermediate t_OH-I₃). Similarly to the chloride- and the

cis-hydroxo routes,^[34]the 1,4-addition that initially gives the

enol tautomer of propanal is favoured considerably over the

direct 3,4-addition. However, the presence of two protic hy-

(Scheme 8).

ꢀ

drogen atoms (the pyrazole N H and aqua O_wH) in t_OH-

I₃opens two different paths for the proton transfer to acro-

ꢀ

lein in the 1,4-addition: transfer of the O_wH to O (path O)

ꢀ

and the N H to O (path N), which generate transition states

t_OH-TS_3Nand t_OH-TS_3O, respectively. The energy barri-

ers are 16.8 and 10.3 kcalmol^ꢀ1for path O and path N, re-

spectively (Figure 8), with protonation from pyrazole

(path N) favoured over protonation from the aqua ligand.^[35]

The relatively low energy barrier of 15.3 kcalmol^ꢀ1found

for the trans-hydroxo route in dehydrogenative part of the

cycle is the rate-determining step for the overall process.

Recent DFT calculations have disclosed the strong effects

that explicit water molecules could have on proton-transfer

steps in transition-metal-catalysed reactions.^[6j–l]Directly re-

lated to our work is the report of water-accelerated asym-

metric transfer hydrogenation of ketones, in which the tran-

sition state is stabilised by hydrogen bonding of one water

molecule to the ketone oxygen atom.^[5h]We have investigat-

ed this possibility in the transition states of both hydrogen-

transfer steps of the trans-hydroxo route (t_OH-TS₂and

t_OH-TS_3N) by inclusion of an explicit water molecule in

Scheme 8. Transition states for the hydrogen-transfer steps in the trans-

hydroxo route with an additional water molecule.

Geometries of the transition states with one explicit water

molecule are presented in the Supporting Information. The

calculated barriers with respect to t_OH-I₀hydrogen-

bonded to a water molecule and 2-propen-1-ol are 14.9 and

18.3 kcalmol^ꢀ1for the transition state of the dehydrogena-

tive step with the water outside and inside, respectively

(Figure 8).

By comparison of these values to those found with a PCM

description of water as the solvent (15.3 kcalmol^ꢀ1), these

calculations show that, although direct participation of a

water molecule as a bifunctional catalyst in the proton trans-

fer does not accelerate the reaction, the process could be

further favoured by hydrogen-bonding interactions between

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7759

A. Lledꢂs, J. Gimeno et al.

the reactive system and water medium. For the acrolein hy-

drogenation step we have only analysed the influence of an

external explicit water molecule in the transition state of the

most favourable pathway (1,4-addition, path N). Inclusion of

this additional water molecule decreases the barrier from

10.3 to 9.1 kcalmol^ꢀ1(t_OH-TS_3N(water-out), Scheme 8).

Our calculations show mechanisms that involve active

aqua complexes with water or hydroxo cooperative ligands

could be competitive in hydrogen-transfer processes that in-

volve bifunctional catalysis in water. When it is taken into

account that in water the trans-hydroxo active species

(t_OH-I₀) is found 5.4 kcalmol^ꢀ1above Cl-I₀, the trans-hy-

droxo and chloride routes have almost the same barrier for

the dehydrogenative half of the reaction (20.3 (14.9+5.4)

versus 20.6 kcalmol^ꢀ1), whereas the barrier for the 1,4-hy-

drogenation process is lower for the trans-hydroxo route

(14.5 (9.1+5.4) versus 18.5 kcalmol^ꢀ1). When the limitations

inherent to the calculations are considered, the general pic-

ture that arises from the calculated barriers for the particu-

lar reaction studied is that both the trans-hydroxo and chlor-

ide routes can be at work for the catalytic activity of pyra-

zole-based ruthenium(IV) complexes in the redox isomerisa-

tion of allylic alcohols in water. One important aspect that

cannot be properly addressed with calculations is the con-

centration of the active species in the reaction medium. It

seems likely that the concentration of the charged aqua- or

hydroxo complexes could be higher than that of the neutral

chloride active species in water.

Calculations also highlight a slightly lower barrier for the

reaction in water relative to THF. Our best values give a dif-

ference in the overall energy barriers of 1 kcalmol^ꢀ1in

favour of the water process (20.3 and 21.3 kcalmol^ꢀ1in

water and THF, respectively). Although the lowered Gibbs

energy barrier may justify the experimentally observed in-

crease in reaction rate (Table 1, entries 1 and 5), care should

be taken to avoid over-interpretation of this agreement. In-

certitude about the relative energies of the active species in

solution or the simple description of explicit water–substrate

interactions prevent a more conclusive answer because of

computational limitations.

entry 12). We have chosen this substrate to analyse the

effect on the reaction barrier of blocking the 1,4-hydrogena-

tion (Scheme 9).

Scheme 9. Transition states for the hydrogen-transfer steps in the trans-

hydroxo route for the isomerisation of 2-cyclohexenol.

The complete catalytic cycle for the isomerisation of 2-cy-

clohexenol to cyclohexanone in the trans-hydroxo route has

been investigated (see the Supporting Information). In

water, cyclohexanone is 17.1 kcalmol^ꢀ1more stable than 2-

cyclohexenol, representative of a similar thermodynamic

driving force for isomerisation of 2-propen-1-ol into propa-

nal (ꢀ16.3 kcalmol^ꢀ1). However, the Gibbs energy barriers

for the hydrogen-transfer steps are considerably higher than

for 2-propen-1-ol. Thus, the transition state for the dehydro-

genative step (ch-TS₁, Scheme 9) is higher than that for 2-

propen-1-ol (22.3 versus 15.3 kcalmol^ꢀ1), but the largest dif-

ference is found in the 1,4-hydrogenation of 2-cyclohexe-

none (formed in the first hydrogen-transfer step) (i.e.

path O; transition state ch-TS_2O). The barrier is now 36.5

versus 10.3 kcalmol^ꢀ1for acrolein. Because this path in-

volves

a unfavourable cis conformation, hydrogenation

should proceed through a 3,4-addition (transition state ch-

TS_2N, Scheme 9) with a Gibbs energy barrier of 23.9 kcal

mol^ꢀ1. Thus, when the 1,4-addition is blocked and the enol

formation is hampered, the reaction must proceed through

the 3,4-addition path, which also entails a higher energy bar-

rier than that of 2-propen-1-ol.

The role of water in the isomerisation process: Calculations

have shown that use of water as the solvent opens new path-

ways for the bifunctional catalysis of hydrogenation/dehy-

drogenation processes, which can compete with the usual

mechanism that involves the metal and a cooperating nitro-

gen ligand. Water can coordinate to the metal but can also

participate in proton-transfer processes. The presence of two

ligands with similar pK_avalues (coordinated pyrazole and

water)^[26]in the ruthenium coordination sphere permits the

formation of hydroxo species that could play a key role in

bifunctional catalysis in water. The theoretical studies reveal

that hydroxo species (generated from pyrazole and the anal-

ogous ruthenium(IV) precursors 1a–f explored in this work)

are acting as metal–ligand bifunctional catalysts and disclose

concerted outer-sphere mechanisms, which favour overall

lower energy barriers. The following features are remarka-

ble in the formation of the active species:

The importance of the 1,4-addition pathway—the 2-cyclohex-

enol substrate: Calculations presented so far deal with the

isomerisation of the 2-propen-1-ol substrate. Experimental

results for this substrate show a fast isomerisation in water

catalysed by 1a (Table 2, entry 6). The theoretical study of

the reaction mechanism has revealed that for both the chlor-

ide- and hydroxo routes the second half of the reaction has

a considerably lower barrier when the acrolein hydrogena-

tion takes place by a 1,4-addition mechanism by which the

enol is formed initially instead of by a direct C=C reduction.

Comparison of the transition states for both the 1,4- and

3,4-addition paths indicates an s-cis conformation for the

former, but s-trans for the 3,4-addition. In a cyclic allylic al-

cohol, such as 2-cyclohexenol, the s-cis conformation is not

possible. Interestingly, higher catalyst loading and longer re-

action times are required to attain total conversion (Table 2,

7760

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Chem. Eur. J. 2012, 18, 7749 – 7765

Redox Isomerisation of Allylic Alcohols

FULL PAPER

1) The ability of precursor catalysts 1a–f to undergo Cl^ꢀ/

H₂O exchange and/or deprotonation processes facilitated

in water by the high solvation Gibbs energy of both the

chloride anion and the proton released in this solvent to

give rise to unsaturated species (Scheme 5).

2) The coordinating ability of water, which generates new

catalytic species (aqua complexes) and, as a result, opens

new pathways for the transformation. The coordination

of water to the ruthenium centre considerably increases

the acidity of the water protons, therefore, hydrogen-

bonding ability and further proton transfer are en-

hanced.^[36]

3) The role of coordinated water and pyrazole as cooperat-

ing ligands, which are responsible for the deprotonated

active species. The presence of pyrazolide or hydroxo

groups (Lewis base sites) together with the coordinative

unsaturation of the metal (Lewis acid site) generates

Noyori-type active species; a useful strategy for catalysis

of the hydrogen-transfer processes required in the iso-

merisation of the substrates.

ESI-MS techniques and CID experiments furnished valua-

ble information on the nature of the species present in solu-

tion, even though such species are transient or formed in

very small amounts.^[39a,b]ESI mass spectrometry has been

used to obtain mechanistic insights into a number of impor-

tant chemical processes, such as homogeneous Ziegler–

Natta polymerisation of ethane,^[39c]Pd-mediated cyclisation

reactions^[39d]or Pd-catalysed Heck and cross-coupling reac-

tions.^[39e]However, to the best of our knowledge, their use

to investigate bifunctional catalysis has remained unex-

plored.

In the present case, the use of ESI mass spectrometry and

CID experiments has allowed us to obtain a complete pic-

ture of the chemical speciation of the catalysts, both in or-

ganic and aqueous media, that has ultimately been used as

experimental input for theoretical descriptions. In this con-

text, DFT theoretical calculations in water have been car-

ried out and provide a valuable insight into the role of

water in the catalytic activity of the bifunctional catalyst.

Several mechanisms that imply the formation of aqua com-

plexes and their deprotonated variants (both involve coordi-

ꢀ

nated water and the N H proton of the pyrazole ligand)

When all of these features are taken into account, in water

the trans-hydroxo route (Figure 8) appears to be competitive

with the usual Noyori mechanism (chloride route). In the

trans-hydroxo route the highest barrier is associated with

the t_OH-TS₂transition state, responsible for the allylic al-

cohol dehydrogenation to give acrolein. It is apparent that

deprotonation of the coordinated water molecule prevails

have been considered. Different competitive pathways

based on outer-sphere mechanisms and indicative of hydro-

gen-transfer processes have been analysed. The overall iso-

merisation implies two hydrogen-transfer steps from the

substrate to the catalyst and subsequent transfer back to the

substrate. In addition to the conventional Noyori outer-

sphere mechanism, which involved the nitrogen ligand, a

new mechanism with the hydroxo pyrazole species t-OH-I₀

(Scheme 7) as the active species can be at work in water.

In summary, the theoretical insights reported in the pres-

ent work are expected to help us understand other transi-

tion-metal-catalysed reactions in water that involve hydro-

gen-transfer steps. This is, in fact, one further example of

the important role of cooperating ligands (pyrazole and

water) in catalysis,^[5e]which may be particularly relevant to

rationalise and design other metal catalysts in an aqueous

medium.

ꢀ

versus deprotonation of N H for the generation of the

ꢀ

Lewis basic site. This fact allows the subsequent N H hy-

drogen atom transfer in the concerted 1,4-addition step via

transition state t_OH-TS_3Nand completes the double hydro-

gen transfer to acrolein to give the enol. It is important to

note that this reduction step through path N has a consider-

ably lower barrier than path O (10.3 versus 16.8 kcalmol^ꢀ1),

which allows discrimination of the most favourable overall

mechanism.

Conclusion

Experimental Section

The neutral complexes [Ru(h³:h³-C₁₀H₁₆)Cl₂L] (1a–f; L=

pyrazole-type ligands) catalyse the isomerisation of allylic

alcohols in aqueous media. The isomerisation takes place

rapidly at low catalyst loadings (0.2 mol%) at 758C and, in

the case of 1-octen-3-ol, at 358C in 60 min (1 mol% cat.).^[37]

As far as we know, this processes, along with that recently

reported for a rhodium(I) catalytic system,^[38]are the most

efficient to date for the isomerisation of allylic alcohols in

water in the absence of base. Because the process proceeds

efficiently in the absence of a base as a co-catalyst, this

methodology provides an appealing experimental approach

in the current context of the development of sustainable cat-

alytic transformations. Remarkably, the catalytic activity is

much lower in THF and also requires the use of a base as a

co-catalyst.

All manipulations were performed under an atmosphere of dry nitrogen

by using a vacuum line and standard Schlenk techniques. Solvents were

dried by standard methods and distilled under nitrogen before use. All

reagents were bought from commercial suppliers and were used without

further purification, with the exception of 1,^[40][{Ru(h⁶-arene)

ACTHNUTRGNENUG ACHTUNGTRENNUNG(m-Cl)Cl}₂]

(arene=C₆H₆, p-cymene, C₆Me₆),^[41]1a^[16a,b](conductivity in water at

208C: 74.7 W^ꢀ1cm²mol^ꢀ1), 1b–e,^[16c]2a^[16d](conductivity in water at 208C:

126 W^ꢀ1cm²mol^ꢀ1

)

and 2b^[16e](conductivity in water at 208C:

126 W^ꢀ1cm²mol^ꢀ1), which were prepared by literature methods. IR spec-

tra were recorded on a PerkinElmer 1720-XFT spectrometer. The con-

ductivities were measured at RT from solutions in water (c~

10^ꢀ3moldm^ꢀ3) with a Jenway PCM3 conductimeter. pH measurements

were performed with a WTW Microprocessor pH meter equipped with a

SenTix50T electrode. Elemental analyses were carried out with a Perki-

nElmer 2400 microanalyser. GC measurements were made on Hewlett–

Packard HP6890 equipment equipped with

a HP-INNOWAX cross-

linked poly(ethyleneglycol) (30 m, 250 mm) or Supelco Beta-Dex 120

Chem. Eur. J. 2012, 18, 7749 – 7765

ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

7761

A. Lledꢂs, J. Gimeno et al.

(30 m, 250 mm) column. GC-MS measurements were performed on an

(0.4 mol%, if required) were dissolved in THF or water (20 mL), and the

reaction mixture was stirred at 758C for the time indicated in Table 1

and Table 2. The course of the reaction was monitored every five minutes

by analysis of an aliquot of the mixture by GC. The identity of the resul-

tant saturated carbonyl compounds was assessed by comparison with

commercially available pure samples (Aldrich Chemical Co. or Acros Or-

ganics) and by their fragmentation pattern after GC-MS analysis.

Agilent 6890N instrument coupled to an Agilent 5973 mass detector (EI,

1

70 eV) and fitted with a HP-1 MS column. H and ¹³C NMR spectra were

recorded on a Bruker DPX-300 instrument at 300 or 75.4 MHz, respec-

tively, with SiMe₄as a standard. DEPT experiments were carried out for

all of the compounds reported. The numbering for the proton and carbon

atoms of the 2,7-dimethylocta-2,6-diene-1,8-diyl skeleton is as follows:

X-Ray crystal structure determination of compounds 1 f and 2c: Crystals

suitable for X-ray diffraction analysis were obtained, in both cases, by

slow diffusion of n-hexane into a saturated solution of the compound in

CH₂Cl₂. The most relevant crystal and refinement data are collected in

Table S1 (see the Supporting Information).

For 1 f, data collection was performed at 200(2) K on an Oxford Diffrac-

tion Xcalibur Nova single crystal diffractometer by using Cu_Karadiation

(l=1.5418 ꢈ). Images were collected at a 65 mm fixed crystal–detector

distance (oscillation method) with 18 oscillation and 4 s exposure time

per image. Data collection strategy was calculated with the program Cry-

sAlis Pro CCD.^[42]Data reduction and cell refinement was performed

with the program CrysAlis Pro RED.^[42]An empirical absorption correc-

tion was applied by using the SCALE3 ABSPACK algorithm as imple-

mented in the program CrysAlis Pro RED.^[42]

ESI mass spectrometry: A Quattro LC QhQ (quadrupole–hexapole–

quadrupole) mass spectrometer with an orthogonal Z-spray electrospray

interface (Waters, Manchester, UK) was used. The drying and nebulising

gas was nitrogen at a flow of 300 and 50 Lh^ꢀ1, respectively. The tempera-

ture of the source block was set to 1008C and the interface to 1208C.

The capillary voltage was set at 3.5 kV in the positive-scan mode and the

cone voltage was adjusted to a low value (typically U_c=5–15 V) to con-

trol the extent of fragmentation in the source region. Solutions of the

sample (cꢁ5ꢇ10^ꢀ4m) in the appropriate solvent were introduced to the

ESI source through a fused-silica capillary by syringe pump at a flow rate

of 10 mLmin^ꢀ1. The chemical composition of each peak obtained in the

full-scan mode was assigned by comparison of the isotope experimental

and theoretical patterns with the MassLynx 4.0 program. Collision-in-

duced dissociation (CID) experiments were performed with argon at var-

ious collision energies (E_lab=3–15 eV). The most intense precursor peak

of interest was mass selected with Q1 (isolation width of 1 Da), interact-

ed with argon in the hexapole cell and scanned with Q2 to monitor the

ionic fragments.

For 2c, diffraction data were recorded at RT on a Nonius KappaCCD

single crystal diffractometer by using Mo_Karadiation (l=0.71073 ꢈ).

Images were collected at a 40 mm fixed crystal–detector distance (oscilla-

tion method) with 18 oscillation and 20 s exposure time per image. Data

collection strategy was calculated with the program Collect.^[43]Data re-

duction and cell refinement were performed with the programs HKL

Denzo and Scalepack.^[44]A semi-empirical absorption correction was ap-

plied by using the program SORTAV.^[45]The software package

WINGX^[46]was used for space group determination, structure solution,

and refinement. The structure for the complex 2c was solved by Patter-

son interpretation and phase expansion by using DIRDIF.^[47]The crystal

structure of 1 f was solved by direct methods by using the program SIR-

92.^[48]Anisotropic least-squares refinement was carried out with

SHELXL-97.^[49]All non-hydrogen atoms were anisotropically refined.

The coordinates of the hydrogen atoms for 2c were geometrically located

and their coordinates were refined riding on their parent atoms. The co-

ordinates of the hydrogen atoms for 1 f were found from different Fouri-

Complex 1 f: At RT, a stoichiometric amount of indazole (0.64 mmol)

was added to a solution of complex 1 (0.200 g, 0.32 mmol) in CH₂Cl₂

(20 mL). The mixture was stirred for 10 min, the solvent was removed

under vacuum and the resultant orange solid was washed with hexane

(3ꢇ10 mL) and dried in vacuo (64%, 175 mg). ¹H NMR (300 MHz,

CD₂Cl₂): d=2.36 (s, 6H; 2ꢇCH₃), 2.45 (m, 2H; H₄and H₆), 3.12 (m, 2H;

H₅and H₇), 4.38 (s, 2H; H₂and H₁₀), 4.50 (s, 2H; H₁and H₉), 5.24 (m,

2H; H₃and H₈), 7.20 (t, J=7.5 Hz, 1H; H₅L), 7.43 (t, J=7.8 Hz, 1H; H₆

L), 7.52 (d, J=7.8 Hz, 1H; H₇L), 7.76 (d, J=7.5 Hz, 1H; H₄L), 8.90 (s,

1H; H₃L), 12.20 ppm (s, 1H; NH); ¹³C NMR (75 MHz, CD₂Cl₂): d=21.0

(s; 2ꢇCH₃), 36.7 (s; C₄and C₅), 77.0 (s; C₁and C₈), 95.2 (s; C₃and C₆),

109.8 (s; C₇L), 120.6 (s; C₅L), 121.5 (s; C₄L), 123.6 (s; C_3aL), 128.0 (s;

C₆L), 130.7 (s; C₂and C₇), 138.6 (s; C₃L), 141.0 ppm (s; C_7aL); IR

(KBr): n˜ =662 (m) 745 (vs), 867 (m), 950 (s), 1239 (m), 1352 (vs), 1382

(m), 1436 (m), 1506 (m), 1624 (s), 2852 (w), 2913 (m), 3295 cm^ꢀ1(s); ele-

mental analysis calcd (%) for RuC₁₇H₂₂Cl₂N₂: C 47.89, H 5.20, N 6.57;

found: C 47.89, H 5.27, N 6.46.

er maps and included in a refinement with isotropic parameters. The

1

2

function minimised was ([Sw

(F_oꢀF_c²)/Sw

E

2

AHCTUNGTRENNUNG

(F_o)+(aP)²+bP] (a and b values are collected in Table S1, see the Sup-

2

²(F_o) from counting statistics and P=(max-

porting Information) with s ACHTGNURTENNUGN

2

AHCTUNGTRENNUNG

(F_o, 0)+2F_c²)/3.

Atomic scattering factors were taken from the International Tables for

X-Ray Crystallography.^[50]The crystallographic plots were made with

PLATON.^[51]

CCDC-850286 (1 f) and 850287 (2c) contain the supplementary crystallo-

graphic data for this paper. These data can be obtained free of charge

from The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-

c.uk/data_request/cif.

Computational details: All of the theoretical studies were performed by

DFT calculations. Beckeꢅs three-parameter exchange functional (B3),^[52]

in conjunction with the Lee–Yang–Parr correlation functional (LYP),^[53]

was employed as implemented in Gaussian 03.^[54]In all geometry optimi-

sations the effective core potential LANL2DZ, along with its associated

basis set, complemented by a series of f polarisation functions was em-

ployed for Ru,^[55]the 6-31G(d) basis set was used for all of the non-hy-

(h⁶-C₆Me₆)}₂] (134 mg, 0.2 mmol)

Complex 2c: A mixture of [{RuClACTHNUGTRENNUG(m-Cl)HCATUNGTRENNGUN

and 3,5-dimethylpyrazole (0.64 mmol) in toluene (6 mL) was heated at

reflux temperature for 12 h. The solvent was removed under vacuum and

the resultant orange solid was recrystallised from CH₂Cl₂/hexane and

dried in vacuo (55%, 94 mg). ¹H NMR (300 MHz, CDCl₃): d=2.07 (s,

18H; C₆Me₆), 2.29 (s, 3H; 3-Me), 2.38 (s, 3H; 5-Me), 5.98 (s, 1H; CH),

11.01 ppm (s, 1H; NH); ¹³C NMR (75 MHz, CDCl₃): d=10.7 (s; 3-Me),

14.6 (s; 5-Me), 15.5 (s; 6ꢇCH₃), 91.0 (s; 6ꢇC_ipso), 107.5 (s; CH), 142.6 (s;

C₃L), 153.8 ppm (s; C₅L); IR (KBr): n˜ =553 (w), 614 (w), 659 (m), 696

(m), 740 (w), 799 (s), 833 (w), 1024 (s), 1043 (m), 1069 (m), 1149 (m),

1242 (w), 1274 (vs), 1381 (s), 1419 (m), 1457 (m), 1570 (vs), 2920 (m),

3018 (m), 3135 (m), 3217 cm^ꢀ1(vs); elemental analysis calcd (%) for

RuC₁₇H₂₆Cl₂N₂: C 47.44, H 6.09, N 6.51; found: C 47.29, H 6.01, N 6.47;

drogen atoms, the 6-31GACTHNUTRGNEUGN(d,p) basis set for the hydrogen atoms of the

aqua and pyrazole ligands, as well as the substrates, and the 6-31G basis

set for the rest of the hydrogen atoms.^[56]Vibrational frequencies were

computed to characterise the transitions states and the energy minima, as

well for the calculation of gas-phase Gibbs energies. The connections be-

tween transition states and their corresponding reactants and products

were checked by fully optimising structures derived from the transition

states with a small displacement following the transition vector in both

directions.

conductivity (water, 208C): 124 W^ꢀ1cm²mol^ꢀ1

.

General procedure for the catalytic isomerisation of allylic alcohols into

carbonyl compounds: In a sealed tube, the appropriate allylic alcohol

(4 mmol), ruthenium catalyst precursor (0.2–10 mol%), and co-catalyst

The energies in solution (DE_sol) of all species were then estimated by the

application of single-point calculations at the gas-phase optimised sta-

7762

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ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2012, 18, 7749 – 7765

Redox Isomerisation of Allylic Alcohols

FULL PAPER

tionary points, with the extended basis set 6-311+ +G

(d,p) on all atoms,

Di Tommaso, J. A. Iggo, C. R. A. Catlow, J. Bacsa, J. Xiao, Chem.

Eur. J. 2008 , 14 , 769 9 –7 715; i) D. Di Tommaso, S. A. French, A. Za-

notti-Gerosa, F. Hancock, E. J. Palin, C. R. A. Catlow, Inorg. Chem.

2008 , 47 , 267 4 –2 687; j) A. Comas-Vives, G. Ujaque, A. Lledꢂs, J.

Mol. Struct. TEOCHEM 2009 , 903 , 12 3 –1 32; k) H. Zhang, D. Chen,

Y. Zhang, G. Zhang, J. Liu, Dalton Trans. 2010 , 39 , 197 2 –1 978;

l) H.-Y. T. Chen, D. Di Tommaso, G. Hogarth, C. R. A. Catlow,

Dalton Trans. 2011 , 40 , 40 2 –4 12.

with the exception of ruthenium, which was described as LANL2DZ+f,

by using the PCM with standard UA0 solvation spheres,^[57]either in THF

(e=7.58) or H₂O (e=78.4). Gibbs energies in solution (DG_sol) were cal-

culated by addition of the gas-phase Gibbs energy corrections of the

solute to the energies in solution. All of the energies presented in the

text are DG_sol. DE_solprofiles are shown in the Supporting Information.

[6] Among other examples in which hydrogen-bonding effects have

been addressed, we note: i) Hydration of nitriles catalysed by the

complex [RuHACTHNUTRGENNUG ACHTUGNTREN(NNGU 1,1-bis(diphenylphosphino)methane)], in

(h⁵-C₉H₇)

Acknowledgements

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tivates the attack of water: a) W. K. Fung, X. Huang, M. L. Man,

S. M. Ng, M. Y. Hung, Z. Lin, J. Am. Chem. Soc. 2003 , 125 , 1153 9 –

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by the complex [Ru(acetylacetonato)₂L₂] (L=k-(P)-3-diphenylphos-

phinoisoquinolone), which contains a ligand that features C=O and

Financial support from the Spanish MICINN (projects CTQ2011-23336,

CTQ2006-08485/BQU and ORFEO Consolider-Ingenio 2010 CSD2007-

00006) is gratefully acknowledged. CESCA is acknowledged for provid-

ing computational resources. The authors are also grateful to the Serveis

Centrals d’Instrumentaciꢂ Cientꢀfica (SCIC) of the Universitat Jaume I

for provision of mass spectrometry facilities.

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aerobic oxidation of alcohols catalysed by [Pd

G

N

₂ACHTUNGTRENNUNG(H₂O)]

(IiPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)

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ˇ

cently been addressed, see: T. Smejkal, D. Gribkov, J. Geier, M.

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Angew. Chem. Int. Ed. 2009, 48, 551–554; T. Smejkal, B. Breit,

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Chem. Soc. 2002 , 124 , 1510 4 –1 5118; b) C. P. Casey, G. A. Bikzhano-

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tallic complexes, see: a) A. P. Sadimenko, S. S. Basson, Coord.

Chem. Rev. 1996 , 147 , 24 7 –2 97; b) L. A. Oror, M. A. Ciriano, C.

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Chem. 2009, 4913–4925.

[12] For a recent mini-review on the chemistry of pyrazole complexes

ꢀ

with a N H group in the a position relative to the metal as novel

metal–ligand cooperative bifunctional catalysts, see: S. Kuwata, T.

Ikariya, Chem. Eur. J. 2011 , 17 , 354 2 –3 556.

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7763

A. Lledꢂs, J. Gimeno et al.

[13] A remarkable example in enzymatic catalysis is the deprotonation

of water during amide hydrolysis by serine proteases, in which the

basic imidazole of a histidine side-chain also acts as proton-acceptor

sites: a) T. E. Creighton, Proteins: Structures and Molecular Proper-

ties (Ed.: W. H. Freeman), New York, 1993, p. 418; b) J. C. Fontecil-

la-Camps in Bioorganometallics: Biomolecules, Labeling, Medicine

(Ed.: G. Jaouen), Wiley-VCH, Weinheim, 2006, p. 353; c) k-(P)-het-

eroditopic P,N-ligands, such as 2-diphenylphosphinepyridine and

analogous imidazolylphosphines, are known to enhance intramolecu-

lar hydrogen-atom transfer via the uncoordinated nitrogen atom

(see Ref. [6b–d], ii); d) F. A. Jalꢂn, B. R. Manzano, A. Caballero,

M. C. Carriꢂn, L. Santos, G. Espino, M. Moreno, J. Am. Chem. Soc.

2005, 127, 15364–15365.

[21] For other ruthenium-catalysed isomerisations in aqueous solution

(758C; all of them requiring the use of KOtBu or Cs₂CO₃as a co-

catalyst; TOF values in parentheses) see: a) [Ru(h³:h²:h³-C₁₂H₁₈)Cl₂]

(2000 h^ꢀ1);^[19]

c) [RuCl₂(h⁶-C₆H₅OCH₂CH₂OH)(L¹)]

1200 h^ꢀ1);^[15m](p-cymene)(k-(P)-L²)]

d) [RuCl₂A

(PPh₃)_n(OCH₂CH₂NMe₃)_n, n=1,

(THPA)

(h⁶-arene)], THPA=2,4,10-trimethyl-1,2,4,5,7,10-hexaaza-3-

phosphatricyclo-[3.3.1.1]decane, arene=C₆H₆, p-cymene, 1,3,5-

b) [Ru{(h³:h³-C₁₀H₁₆)

G

(2000 h^ꢀ1);^[8]

PPh₃

(3–

N

G

L²=

2

(396–594 h^ꢀ1);^[15h]e) [RuCl₂-

A

ACHTUNGTRENNUNG

ACTHNUTRGNEUNG

C₆H₃Me₃, C₆Me₆(29–132 h^ꢀ1);^[15i]f) [RuCl₂{P(OEt)₃}(h⁶-p-cymene)]

(46 h^ꢀ1, 358C);^[15k]g) TOF values (strongly dependent of pH of the

aqueous phase) have been reported in the range of 77–2226 h^ꢀ1for

water-soluble complexes Na

sulfonatophenyl-diphenylphosphine),

Na₄A[{RuCl(m-Cl)(C=C=CPh₂)(mtppms)₂}₂],

(mtppms)₂];^[15m]

h) [Ru{6,6ꢅ-Cl₂(2,2ꢅ-bipyridine)}

(180 h^ꢀ1; 908C).^[15q]

₄ACHTUGTNRNENUG[{RuCl₂ACHTNUGTRENNNUG

(mtppms)₂}₂] (mtppms=meta-

Na[RuCp(CO)(mtppms)₂],

Na₂A[RuCpCl-

(H₂O)₂)](OTf)₂

G

ACHTUNGTRENNUNG

[14] For reviews on this catalytic process, see: a) R. Uma, C. Crꢊvisy, R.

Grꢊe, Chem. Rev. 2003 , 103 , 2 7 –5 2; b) R. C. van der Drift, E. Bouw-

man, E. Drent, J. Organomet. Chem. 2002 , 650 , 1 –2 4; c) V. Cadier-

no, P. Crochet, J. Gimeno, Synlett 2008, 1105–1124.

N

G

R

G

CHTUNGTRENNUNG

N

G

N

ACHTUNGTRENNUNG

[22] Optimisation of the pH by using aqueous potassium hydrogen phos-

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(ꢀ71.0 kcalmol^ꢀ1) is in reasonable agreement with the experimental

value (ꢀ74.6 kcalmol^ꢀ1) (see Ref. [23b]).

[25] This transition state is found 3.5 kcalmol^ꢀ1above c_water-I₀in the

DE_waterprofile but in the Gibbs energy profile in water it is placed

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coordination to [M

the pK_aof water (15.74) becomes 6.9 when coordinated in the com-

(NH₃)₅]³⁺(M=Cr, Co, Ru).^[26b,c]In the same way

ACHTUNGTRENNUNG

[26d]

plex [Ru(h⁶-benzene)

G

(H₂O)]²⁺

.

It has also been reported

that, in the presence of water, arene–ruthenium amido complexes

give the amino-hydroxo metal complex.^[26e]The formation of a Rh

ꢀ

OH moiety by abstraction of a proton from the coordinated water

molecule to the basic nitrogen centre of the PTA ligand in water-

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ꢀ

shorter intermolecular N H···Cl1 distance (2.41 ꢈ), which points to

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7764

www.chemeurj.org

ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2012, 18, 7749 – 7765

Redox Isomerisation of Allylic Alcohols

FULL PAPER

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Received: October 26, 2011

Revised: March 20, 2012

Published online: May 15, 2012

Chem. Eur. J. 2012, 18, 7749 – 7765

ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

7765

Article Doi

Highly efficient redox isomerisation of allylic alcohols catalysed by pyrazole-based ruthenium(IV) complexes in Water: Mechanisms of bifunctional catalysis in water

DOI: 10.1002/chem.201103374

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