Magnesium ion mediated protonation of imines in acetonitrile

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T&ah&m Vol. 41, No. 13, pp. 2145 10 27SJ.1985 Printed in Mt Britain.

MAGNESIUM ION MEDIATEDPROTONATION OF IMINES IN ACETONITRILE

JOHANNESC.C. VAN NIEL’ and UPENDRAK. PANDIT* Organic Chemistry Nieuwe Achtergracht

Laboratory, University of Amsterdam, 129, 1018 US Amsterdam, The Netherlands and

DERKJ. STUFKENSand GERARDC. SCHOEMAKER Inorganic Chemistry Laboratory, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands (Received in UK 18 March 1985) to a solution of imine Abstract - Addition of magnesium perchlorate PhCH=N-C H (1) and dlimine PhCH=N(CH) N=CHPh (4), in acetonitrile, results in the f!&tTon of the corresponding2i&nium saits due to traces of water present in the solvent. This conclusion is based upon infrared spectra of deuterated iminium salt8 and mixtures of solutions of imines, imlnium salts, imines and magnesium perchlorate in acetonitrile, containing varying amounts of water. The results suggest that magnesium ion catalyzed reactions of imines in CH3CNinvolve iminium intermediates. Several

model studies

drogenaaes

of reduction

have employed metal

ions

continues

sium

ion

to be the subject

catalysis

that

‘H NMRstudy

displacements

between the free

imines

of mixtures shifts

and their of

trace

content

of at least

acetonitrile,

in that

of imines of

coordination

ester

it

(2,6-dimethyl-3,5_diethoxycarbonyl-

became necessary

to investigate

the be-

solvent.

of type 1 and Mg(~Zlo~)~, in acetonitrile,

imine protons

(l+N-1

complexes

amounts of water,

workers’

0.2 qM/l.

containing

various

between magnesium ions,

that

are not correlatable

with qagneeium .tons5.

present

with

indicated equilibria

These findings

in the acetonftrile,

&(H20),12+ To derive

support

the protonated

red spectroscopy.

for

The present

‘H NMRspectra

amounts of water, imines

led us

upon mixtures

of

+

:C:N-

in vet report

suggest

dried

of mixtures that

acetonitrile of

imines

the following

has a water and Mg(C104)2,

equilibria

are estab-

and water.

the abovementioned

imine species,

even the most carefully

Measurements of

Mg2+ +

of

by Hantzsch

and magnesium perchlorate.

It has been shown by several

lished

imines

in acetonitrile4,

ion syatem,

of chemical

to examine the influence imines

of

an NADHmodel),

of the imine-magnesium

A preliminary

which are mediated by pyridine nucleotfde linked dehyas electrophilic catalysts 2,4 . The precise role of the metal of debate 293 . In connection with a study of the mechanism of magne-

of the reduction

-1,4_dlhydropyridine, haviour

reactions

Ions

nH20 = e

CHg(H20),12+ &Ii-

equilibria,

C~(H~O)~_,OH~+

it was necessary

CH3CN, upon addition

describes

+

the results

to demonstrate

of Mg(ClO,,),. and conclusions

the existence

This was done via of this

infra-

spectroscopic

study. Experimental The IR spectra were measured on a Nicolet 7199 B FT-IR spectrometer with a liquid nitro en cooled MCT detector In an 0.01 cm NaCl liquid sample cell, with a resolution better than 1 cm- f . 2145

in

J. C. G.

2746

VAN

NIRLet al.

.. . 0’ +

2% .. 0

-ln

ed _ --I

a.

2741

Magnesium ion maliataJ protonation of imincs in acetonitrile

Iminium salts were prepared by adding a dilute solution of the imine or diimine in anhydrous diethyl ether to a cooled (OOC) mixture consisting of an equimolar amount of perchloric acid (70% in water) and a calculated amount of acetic anhydride (for removal of water) in diethyl ether. The precipitate was filtered off, b suction, washed with anhydrous ether and dried in a heated desiccator over P 0 Yields were 90 and i; 0% for the mono imine and dilmine, respectively. The neutral imine 1 and diim 15 n i were prepared according to literature procedures4. N-Benzylidene-1-aminobutane.HClO4

(2):

Calculated for C H NO Cl: C: 50.48%; H: 6.16%; N: 5.35%; 0: 24.46%; Cl: 13.55%. H: 6.17%; N: 5.2+%;‘& 24.35%; Cl: 13.51%. (4H, q, N-CH2-CH2-CH2), 3.91 NMR(100 MHz, CD CN): 6 0.97 (3H, t, CH ), 1.27-1.89 -7.60-8.08 (5H, rn? Ar-H), 8.80 (lH, 8, CA-N). N,N’-Dibenzylidene-1,3-diaminopropane.2HC104

(2H, t,

N-CH2),

(5):

Calculated for C H N 0 Cl : C: 45.25%; H: 4.47%; N: 6.21%: Cl: salt.0.62 N: 6.12%; Cl: 15!39$? 28 cu Pated for the blsperchlorate 6.06%; Cl: 15.33%. Results

Found: C: 50e45%;

15.71%. Found: C. 44.15%; H: 4.55%; H20: C: 44.15%; H: 4.63%; N:

and discussion

The I&spectra frequencies

of

imlne 1 and its

are listed

n-BuN=CHPh

perchlorate

+

n-BuN=CHPh

n-Bu;:CHPh H c104-

1

salt

I and their

2 are shown In Fig.

(EN)

stretching

in the Table. H H PhCH=t$CH2)3+N=CHPh

PhCH=N(CH2)3N.CHPh

2c104-

D c104-

2

4

2

I

Table -1 v(C=N) In cm

compound

The band at

1646 cm-l

literature7’8.

n-BuN=CHPh

(1)

1647

n-BUN+(H) =CHPh

(2)

1682

n-Bui(D)=CHPh

(1)

1665

PhCH.t(CH2)3N=yPh

(4)

1646

PhCH=NH)(CH2)3N(H)=CHPh(2)

1681

is attributed

When the spectrum

to the (C=N) stretching

teronated

salts, 2 and 2, a displacement of -1 (CZ;~H)~*’ and 1665 cm (C=sD), respectively.

teronated brations are,

salt

is in accordance

of the perchlorates,

unfortunately,

trile,

neat acetonitrile

This shift

expected

-1 cm region

expected

in the regions

(Fig.

the spectra observed

in going

I),

according

to the

of the protonated

from 1646 cm-’

to

from the protonated

and deu-1 1682 cm

to the deu-

from the mass effect. The N-H and N-D vi-1 9 and 1800 cm-1 , respectively, 2325-2500 cm

in the spectra. of

the spectra

of

(a)

a solution

of ng(C10102

in acetonl-

and (c) magnesium perchlorate and imine -1 1 dissolved in acetonitrile. The absorptions at 3629 and 3543 cm in spectrum IIb are assignable to 10 -1 the antisymmetric and symmetric stretching frequencies of H20, respectively . A band at 1632 cm (not

(b)

the C=N band is

with the shift

too weak to be visible

Pig. II shows the 2900-3700

vibration

of imine L is compared with

presented

trum IIa

in IIb)

to which water had been added,

corresponding

the broad absorption

complexed

to the magnesium ion.

M!I(H,O)6

++.(C104-)2]1’a

absorptions

due to free

to the bending vibration of H20 lo, is also -1 around 3407 cm is ascribed to the O-H stretching An analogous

and aqueous

When imine 1 Is added to the solution first

solutions

water are not present

band is

observed.

In spec-

vibrations

of water

observed

in the spectra of llb,12 . It Is noteworthy of magnesium salts

In spectrum

of Mg(C104)2,

the absorption

coordination sphere (3407 cm-‘) of ?@2+ disappears -1 mumat 3622 cm appears (Fig. 11~). From the sharpness

that

IIa.

and, of

this

in its

due to water molecules place,

in the

a new band with a maxi-

band and from the fact

that

this

.

2748

J. C. G.

VAN

NIEL et

RI.

Magnesium ion mediated protonatioo of imines in acetonitrik band 1s not accompanied

by the appearance

the new band at 3622 cm-’ water

From the coordination

that of

sphere

the OH stretching

in C%(H20),_,

is

be pointed

3698 cm-’

3622 cm-’

and ls

in view of

of a proton

cm-’

region

of spectrum

contains

salt

no extra 15

to magnesium

that

tentatively

assigned

the lack

of

In Mg(OHJ2 exhibits

to this

the 3407 cm-’

they are also

an absorption

-1 1646 and 1683 cm

bands at

to mode band. It

at

, which can be as-

1550-1600

-1 arise cm

From

Found in spectra Ib and Ic. ElgnificantlY, -1 belonging to an imine coor1560-1615 cm

in the region

.

of 1 (Fig.

Ib)

Combined with the results

(C=NH+)

is

mediate

the protonation

was Further

since

follows

band in Fact comes close

The two bands in the region

absorptions

It has been shown in the spectrum formed.

exhibits

it

by Kg(H20)n+* to the imine substrate.

vibration

respectively.

the imine and the corresponding the spectrum

III

lo,

the imine does not displace

absorption

of n must be small

the OH stretching

to v(C=N) and v(C=h),

dinated

This

consequently,

of NaOH (3637 cm-‘)13

formed upon donation

out that

at 3543 and 1632 cm-’

water and,

“.

The 1600-1800 signed

of Mg’+.

vibration

(OH)]+ where the value

species

‘he latter should

of absorptions

ls not due to free

2149

attested

of

in the absence

described

the imlne bond via

by deliberately

that

adding

above it

the water

of Mg(C104)2 no iminium salt

can be concluded

present

in the solvent

water to an acetonitrile

that

magnesium ions

acetonitrile.

solution

of imine

This

1 and

exhibited both v(C=N) (1646 cm-‘) and -1 showed a variation ; the 1682 cm band exhibiting -1 absorption. When, in the aforementioned an enhancement with a concommitsnt decrease of the 1642 cm -1 appeared, as expected. experiment water was replaced by deuterium oxide, the C=fiD band at 1665 cm M(C104)2,

v(C=iH)

whereupon the infrared

(1682 cm-‘)

We have searched Infrared Fails

spectra

to reveal

arising

are presented

in the region

imine la expected 15.1n a second

From coordination

of

the imine to magnesium

I$E(C~O~)~, in CD3CN, was titrated

of 1 plus solutions

a new absorption

the mixture

of complexes

a solution

of the resulting

of

intensities

For the presence

In one experiment,

ions.

spectrum

whose relative

in Fig.

(1560-1651

cm-‘)

IV.

with the imlne

Inspection

of

where v(C=N) of

(1).

the spectra

the metal complexed

Mg(C104j2 was replaced by Rg(EtOH)6(C104)2, which has 16 spectra of a solution conbeen shown to Form a 1:2 complex with 2-benzoylpyridine . Once again, taining

increasing

experiment,

amounts of imine 1 (Fig.

V) showed no absorption

which could

be attributed

to a

An Interesting observation in Fig. V is to the magnesium cation. -1 upon increase in imine concentration (Va -+Vb the decrease in the absorption at 3350 cm -1 17 , the broad absorption at 3350 cm-1 is assigned Vc-Vd). Since Free ethanol absorbs at 3531 cm direct

coordination

to v(OH) of ethanol

of the imine

coordinated

Its

to magnesium ions.

decrease

can be rationalized

partial deprotonation of the hexakisethanol complex. The ethanol within 2+ E(g acts as a BrQnsted acid towards the imine in the polar acetonltrile supported

by the presence

In an extension “may

be expected

that

infrared

spectral

of

a relatively

of the present

Since

Spectra

of

diimine

imlnes

studies

might lead

to information his

in the Table.

maxima of v(C=N) (1646 cm-‘)

absorption

of magnesium ions

with t4g2* in view of

4 and the corresponding

its

-1

at

1683 cm

on diimine

bidentate

perchlorate

salt

From the data it

it

evident

that

bands of

was hoped

ion interaction.

2 are shown In Fig. is

and v(C=NH) (1681 cm-‘)

!_l was examined.

character,

on an imine-magnesium

.

VI.

The assign-

the positions

the lmines

of the

and the di-

are very similar.

In Fig.

VII the spectra

diimine

+ l@(C104j2

MgB(C104J2 (Fig. of the band at the case present

the influence

to coordinate

ment of the bands is described absorption

study

weak but significant

in terms of

the coordination sphere of 18 18 is . This conclusion

of

of

(a)

MgC104J2 in CD3CN, (b)

+ H20 are presented.

Uhile

VIIB) is due to v(C=N), addition -1 1683 cm . which has been ascribed

the diimine,

in the mixture.

magnesium perchlorate In the region

the bonded water are observable.

3290-3700

The absorption

diimine

the prominent of water

(Fig.

to the C=h

mediates cm-’

of

in the presence absorption VIIc) stretching

the protonation the spectrum,

at 3407 cm”

gradually

of Hg(C104j2

in the presence causes

a distinct

vibration. of

enhancement

Thus, also

the base,

via

the corresponding disappears,

and (c)

of

while

in

the water changes that

at

For

J. C. G.

2750

3622 cm-’

CM~(H,O)~_,(OH)~+

VAN

Nnx

er al.

becomes prominent. Prom these spectra It cannot be concluded whether

only one or both the C=N groups of the diimine are protonated. As the (C=N) stretching frequencies of imine and diimine are similar and of the corresponding lminium and dilminlum salts are almost the same, it follows, that the imine and the imlnium groups in the dlimine molecule do not interact intramolecularly.

Conclusions The spectroscopic study shows that magnesium perchlorate mediates the protonation of imines of types 1 and ", in acetonitrile, via the water present in that solvent. The equilibrium between the lmines and their corresponding iminium salts is rapidly established. Since trace amounts of water are associated with acetonitrile, these results have salient implications for the mechanisms of magnesium ion catalyzed reactions of iminea In acetonltrile. The role of magnesium ions in the reduction of imines by Hantzsch ester (an NADH model), in particular, will be discussed in a forthcoming paper.

Acknowledgement This work was carried out in part under the auspices of the Netherlands Foundation of Chemical Research (SON) and with financial support from the Netherlands Organization of Pure Research (ZWO).

References . To whom correspondence should be addressed.

1. 2a. b. 3. 4a. b. 5. 6. 7.

8. 9. 10. lla. b. 12. 13. 14. 15a. b. 16. 17.

18.

Taken in part from the forthcoming dissertation of J.C.C. van Niel. D.J. Creighton and D.S. Sigman, J. Am. Chem. Sot. 2, 6314 (1971). R.A. Case and U.K. Pandit, J. Am. Chem. Sot. lOJ, 7059 (1979). A. Ohno, S, Yasui and S. Oka, Bull. Chem. Sot. Jpn 53, 2651 (1980). U.K. Pandit, H. van Dam, and J.B. Steevens, Tetrahedron Lett. 913 (1977 1. J.B. Steevens and U.K. Pandit, Tetrahedron 2, 1395 (1983). Results to be published in a forthcoming paper. D.R. Burfleld, K.-H. Lee, and R.H. Smithera, J. Org. Chem. 42, 3060 (19'77). D. Dolphin and A. Wick, Tabulation of infrared spectral data. J. Wiley h Sons, New York, 1977, p. 116-122. L.J. Bellamy, Infrared spectra of complex molecules, 2nd ed., Msthuen & Co. Ltd., London, 1966, p. 267-271. Ref. 8, p. 260. J.R. Scherer, Advances in infrared and Raman spectroscopy (Ed. R.J.H. Clark and R.E. Hester), Vol. V, Heyden h Son Ltd., London, 1978, p. 149-216. S.N. Andreev and H.P. Smlrnova, Zhur. Fiz. Khim. 5, 1793 (1972). L.D. Shcherba and A.M. Sukhotin, Zhur. Fiz. Khim. 2, 2401 (1959). I.D. Kuntz Jr., and C.J. Cheng, J. Am. Chem. Sot. 97, 4852 (1975). K. Nakamoto, Infrared spectra of inorganic and coordination compounds, 2nd ed., Uiley Interscience, New York, 1963, p. 78. Ref. 13, p. 82. H.R.P. van Vliet, H. van Beck, G. van Koten, and K. Vrieze, Inorg. Chem., In press. N.N. Greenwood, J.W. Akitt, W. Errington, T.C. Gibb, and B.P. Straughan, Spectroscopic properties of inorganic and organometallic compounds. Vol. I, The Chemical Society, 1980, p. 189. R.A. Case, G. Boxhoorn, and U.K. Pandit, Tetrahedron Lett. 2, 2889 (1976). The i.r. spectrum of a dilute solution of ethanol in acetonitrile was recorded by us (not shown in a figure). The accurate position of the absorption due to v(OH) of alcohols is known to be strongly influenced by the solvent. See e.g.: G.M. Barrow, J. Phys. Chem. 2, 1129 (1955). It was shown by Olmstead et al. that alcohols may exhibit an unexpected strongly acidic behaviour In aprotic polar solvents (W.N. Olmstead, 2. Margolin, and P.J. Bordwell, J. Org. Chem. 3, 3295 (1980).