H Y D R O G E N B O N D S A N D R O T A T I O N A L I S O M E R I S M

O F ~ - A N D f l - H Y D R O X Y P H O S P H O R Y L C O M P O U N D S

R . R . S h a g i d u l l i n , E . P . T r u t n e v a , N. I . R i z p o l o z h e n s k i i , a n d F . S. M u k h a m e t o v

UDC 541.571.9:541.62:547.1' 118

The data on the spec t ra l study of the s t ruc tu re of hydroxyphosphoryl compounds a re ambiguous [1-3]. The purpose of this work was a compara t ive investigation of the in t ra - and in te rmolecula r hydrogen bond in a - a n d f l-hydroxyphosphoryl compounds, according to the absorpt ion bands of the basic vibrations of the hydroxyl groups ~OH in the IR spec t ra . We synthesized compounds (I) and (II), possess ing a -hydroxy l s with r e spec t to the P = O group and (HI), (IV), possess ing fl-hydroxyls (Table 1).

In the spec t ra of the pure substances and thei r concentrated solutions in CC14, broad intense bands of the OH groups involved in an in te rmolecular hydrogen bond a re observed - v~PH Ctp~ [3]). As dilu-

tion with the solvent i nc rea se s , shoulders that can be assigned to d imer assoc ia tes [3] - vg H - appear on

the bands of the "po lymers" on the h igher - f requency side and grow. Subsequently, with decreas ing concen- t ra t ion of the substances , the intensi ty of the bands of the "po lymers" and "d imers" drops while at higher f requencies uOH of the monomer molecules appear . The f requencies and re la t ive intensit ies of the la t te r molecules ' do not depend on the degree of dilution. When C ~ 10 -4 M, monomer molecules predominate in solution. The f requencies of the maxima of all the indicated format ions a re cited in Table 1. The in te r - pre ta t ion of vOH of the monomers for assignment to an in t ramolecula r hydrogen bond was pe r fo rmed on the basis of a compar ison of them with one another and with the data of [1-3].

According to this in terpre ta t ion , monomer molecules of ~-hydroxylphosphoryl compounds exist chief- ly in the form of an equil ibr ium of two types of conformers (the mutual orientat ion of the P = O and C - O bonds in the Newman projec t ions is skewed [1]):

The conformational equil ibrium in ~-hydroxy-compounds can be schemat ical ly depicted as follows:

I I I

_ 2 - "

TABLE 1. Frequencies (cm -1) of the Maxima of Hydroxyl Absorp- tion in the IR Spectra of Solutions of ~ - and fl-Hydroxyphosphoryl Compounds in CC14

Compound V

(C~HsO)~P(O)CtI~OH (I) ~33t5 (C~Hs0)C~HsP(O)CH20H (1 I) [ ~ 3275 (C2HsO)~P(O)CH~CH~OIt (III) 3390 (C~HsO)C~HaP(O)CH~CH20H (IV) 3340

d ~'OH

" vOH of monomers

I

--~3430 I 3590 J 36i5 ~3400 3580 36t2 ? 3490 3600

~3400 3465 3600

? ~3630

3640 3640

Arbuzov Institute of Organic and Physical Chemis t ry , Kazan' Branch of the Academy of Sciences of the USSR. Trans la ted f rom Izvest iya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1302-1304, June, 1974. Original a r t ic le submitted August 14, 1973.

�9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 Y~est 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, inany form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the pablisher for $15.00.

1'226

Cl') % ~ / ~

i(N / 32gg 3r

L_

J500 ~, c m -1

Fig. l. IR spectra of solutions of (1)-(IV) in CCI 4. Solid curves: C ~ 1.2 �9 i0 -4 (l = 146 ram), dashed curves: 0.1-0.2 M (/= 0.07 ram).

The most important for the substantiation of the co r r ec tn e s s of the in terpre ta t ion cited is the ap- pearance in the spec t ra of f l-hydroxyphosphoryl compounds of pronounced bands of the f ree hydroxyl (~for~ e) (Fig. 1). The posit ion of these bands is in full agreement with the f requency of the f r ee p r i m a r y alcohol group, taken in the l i t e ra tu re [4], and there is no doubt of the in terpre ta t ion . The absence, as a ru le , of analogous bands inthe spec t ra of a -hydroxy-compounds (see [1, 3] and the spec t rum of (I) in Fig. 1) can be explained by the entropy fac tor . In f i -hydroxy-compounds, on account of the possibi l i ty of supplementary rotat ion around the C - C bond, the re la t ive number of possible confo rmers without H-bonds is substant ial ly higher . Moreover , evidently the large separat ion of the bands in thei r spec t ra also plays a ro le .

As a resu l t of the c lear identification o f , f ree vOH , the assignment of all the other bands of the monomers in the spec t ra of (HI) and (IV) [and by analogy, also (I) and (II)] to an in t ramolecula r hydrogen bond with two possible pro ton acceptor cen te rs in the molecules seems unambiguous. In the t rans i t ion f rom (I) to (If) and from (Ill) to (IV), the frequencies UPH, udH, and vOH'free (OHo . . O=P) (see Table i) decrease in

agreement with the increasing basicity of the P=O group. In this case the frequencies uOH (OH... OR) and UOH (free) remain practically unchanged, which also corresponds to their assignment.

From the data cited it follows that an intramoleeular hydrogen bond closing a six-membered ring ( both with the phosphoryl and with the ester oxygen) in hydroxyphosphoryl compounds is stronger than that closing a five-membered ring. This series is similar to the order of the strength of intramoleeular hydrogen bonds in diols and opposite to that proposed in [2]. In the latter study the proper degrees of dilu- tion of the solutions were not reached, and the conclusions were based on the values of UOH of "polymers," which are actually lower for (I) and (If) than for (HI) and (IV). The absorption band of the "polymers~ of fi-hydroxy-compounds appears appreciably earlier, i.e., at lower dilutions than for ~-hydroxy substances (see Fig. i). The greater strength of the intermoleeular associations of (1) and (If) may be partially ex- plained by the increased acidity of the a-hydroxyl proton as a result of the inductive influence of the P=O group. The latter effect evidently is also responsible for the decrease in - free for (II), although the reser-

vOH ration should be made that this baud is observed only the form of a shoulder~

E X P E R I M E N T A L M E T H O D

The diethyl e s t e r of ~-hydroxymethylphosphonic acid (I) and the ethyl e s t e r of ~ -hydroxymethy le thy l - phosphinic acid (II) were produced according to the method of [6]. The diethyl e s t e r of f l -hydroxyethyl- phosphonic acid (III) was produced according to the method of [7]. The ethyl e s t e r of f i -hydroxyethylethyl- phosphinic acid, produced analogously to (Ill), had bp 88-89 ~ (0.006 ram); n~ 1.4565; d~ 0 1.1027. Found: P 18.52; C 43.21; H 9.21%; MR 40.97. C6HIhO3P. Calculated: P 18.65; C 43.33; H 9.03%; IV[R 41.17.

The IR spectra were taken on a UR-20 instrument at a rate of scanning 25 cm-I/min, slit program 4. The calibration was verified according to the absorption spectrum of atmospheric moisture. During the process of the experiment, vessels with P205 were placed in the cuvette compartment to remove atmospher- ic moisture. The solutions were photographed in demountable curvettes 146 mm long. An analogous cuvette with solvent was placed in the reference beam. The solvent was thoroughly dried and redistiiled over P20~. All the substances investigated were freshly prepared, after redistillation. The solutions were prepared directly before photography in a chamber filled with inert gas.

The authors are grateh]l to M. G. Zimin for providing substances (I) and (If).

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C O N C L U S I O N S

1. In solutions of a - and fl-hydroxyphosphoryl compounds in CC14, molecules of these substances mutually associated by means of intermolecular hydrogen bonds and nonassociated (monomers) can exist.

2. The presence of a conformational equilibrium was noted for the monomers. Conformers stabilized by an intramoleeular hydrogen bond with the oxygens of the phosphoryl or es ter groups, and for the fl-hy- droxyphosphoryl compounds, also with a free OH bond, participate in the equilibrium.

3. The s ix-membered rings formed by intramolecular hydrogen bonds in fl-hydroxyphosphoryl com- pounds are stronger than the analogous f ive-membered rings in oz-hydroxyphosphoryl compounds.

1.

2. 3. 4. 5. 6. 7.

L I T E R A T U R E C I T E D

Yu. Yu. Samitov, R. R. Shagidullin, F. S. Mukhametov, and N. I. Rizpolozhenskii, Materials of the Scientific Conference of the A. E. Arbuzov Institute of Organic and Physical Chemistry, Academy of Sciences of the USSR [in Russian], Kazan' (1970), p. 69; R. R. Shagidullin, Yu. Yu. Samitov, F. S. Mukhametov, and N. I. Rizpolozhenskii, Izv. Akad. Nauk SSSR, Ser. Khim., 1604 (1972). G. Aksness and K. Bergessen, Acta Chem. Scand., 18, 1586 (1964). E. J. Matrosov and M. J. Kabachnik, Spectrochim. Acta, 28A, 313 (1972). L. Bellamy, New Data on the IR Spectra of Complex Molecules [in Russian], Mir (1971), p. 108. L. P. Kyhn, J. Amer. Chem. Soc., 74, 2492 (1952). C. C. Overgberger and E. Sarlo, J. Organ. Chem., 26, 4711 (1961). B. Ackerman, T. A. Jordan, and D. Swern, J. Amer. Chem. Soc., 78, 6025 (1956).

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