D I P O L E M O M E N T S O F O R G A N O P H O S P H O R U S C O M P O U N D S

COMMUNICATION 2. BENZOYLPHOSPHONATES, BENZOYLPHOSPHINATES,

AND BENZOYLPHOSPHINE OXIDES

E. A. I s h m a e v a , M. G. Z i m i n , R . M. G a l e e v a , a n d A. N. P u d o v i k

UDC 539.143.43+661.718.1

It was shown ea r l i e r that the carbony[ group in e s t e r s of acetyl , and benzoylphosphonic acids is dis t inguished by high reac t iv i ty , due to the in teract ion of the carbonyl and diethoxyphosphono groups [1-6]. We were in t e re s t ed in es tabl i sh ing the s t e r i c s t r u c t u r e of such compounds and in evaluating the degree of in te rac t ion of the subst i tuents with the diethoxyphosphone groups. For this purpose we de te rmined the dipole momen t s of diethyl e s t e r s of subst i tuted benzoylphosphinic acids , ethyl e s t e r s of e thyl- and phenyl- benzoylphosphinic acids , and die thyl- and diphenylbenzoylphosphine oxides. Some of these compounds were desc r ibed e a r l i e r [3, 7, 8] ; o thers we synthesized accord ing to the Arbuzov reac t ion f r o m t r ie thyl phos- phite and chlor ides of subst i tuted benzoic acids. Diethyl- and diphenytbenzoylphosphine oxides were p r o - duced analogously f r o m benzoyl chloride and ethyl e s t e r s of d ie thyl- or diphenylphosphinous acids .

The IR sp ec t r a of diethyl e s t e r s of subst i tuted benzoylphosphonic acids contain absorpt ion bands (cm-1): 1650-1670 (C = O), 1260-1270 (P = O), 1020-1030 ( P - O - C ) , 1360 and 1540 (NO 2 group) [9]; the IR spec t r a of d ie thyl - and diphenylbenzoylphosphine oxides contain the bands (cm-1): 1190-1230 (P = 0) , 1670-1720 (C = O), 1450 (P-C6Hs) , 1040-1060 (weak) (P-C2Hs). For all the compounds, exaltat ion of the mo lecu l a r r e f r ac t i ons is observed , which ag ree s with the l i t e r a tu re data for the diethyl e s t e r of benzoyl phosphonic acid [7] and ce r ta in subst i tuted benzoylphosphonic e s t e r s [3]. The c h a r a c t e r i s t i c s of the c o m - pounds obtained a r e cited in Table 1.

To es tab l i sh the s t e r i c s t ruc tu re of benzoylphosphonates , benzoylphosphinates , and benzoylphos- phine oxide, it is n e c e s s a r y to c la r i fy the question of the mutual a r r a n g e m e n t of the carbonyl and phosphoryl groups. Fo r diethyl e s t e r s of benzoylphosphonic acids a s t ruc tu re with a r r a n g e m e n t of the C = O and P = O bonds in mutual ly perpendicu la r planes was p roposed in [5], consider ing the fact that the s y s t e m is s tabi l ized by conjugation of the ~-orb i t a l of the C = O group and the p -e l ec t rons of the phosphoryl oxygen. According to other data [10], the mos t profi table is a f iat a r r a n g e m e n t of the v -e l ec t ron ic s y s t e m and phos- phoryl group; two rota t ional i s o m e r s with s - c i s - and s - t r a n s - a r r a n g e m e n t of the carbonyl and phosphoryl groups a r e poss ib le in this case. The poss ib le s t r u c t u r e s a r e p resen ted in Fig. 1.

To r e so lv e this quest ion we cons idered the dipole moment s of d ie thyl- and diphenylbenzoylphosphine oxides. The va lence angles at the P a tom were taken to be the s ame as in [11]; the angles at the carbonyl C a tom were cons idered to be 120 ~ The momen t of the P = O bond was a s sumed to be 2.98 D with compo- nents along the coordinate axes mx P =O = 1.26 D, m ~ =O = 0, mz p =O = 2.70 D. The group momen t s of C2H 5 - P and C6H 5- P, calcula ted f r o m the exper imenta l dipole momen t s of t r ie thylphosphine (average value 1.42 D) [12, 13] and t r iphenylphosphine [14] proved equal to 1.09 D, and f rom this, for m ( H - C s p 2 ) = 0.70 D [15], the momen t of the C s p - P bond is equal to 0.39 D. The dipole moment of the carbonyl group, ca lcu- la ted f r o m the expe r imen ta l dipole moment of the acetophenone, is equal to 1.94 D.

The calcula ted dipole momen t s for die thyl- and diphenylbenzoylphosphine oxides a r e the s ame as a r e s u l t of the equality of the group moments of C2Hs-P and C~Hs-P , and for each of the s t ruc tu re cited in Fig. 1, they a re equal to 6.25, 4.59, and 1.73 D, r e spec t ive ly . The exper imenta l dipole momen t s a r e equal to 2.28 (diethylbenzoylphosphine oxide) and 2.69 D (diphenylbenzoylphosphine oxide), tf the min.:tmum of the

V. I. Ul 'yanov-Len in Kazan ' State Univers i ty . T rans l a t ed f r o m Izves t iya Akademii Nauk SSSR, Ser iya Khimieheskaya , No. 3, pp. 538-543, March, 1971. Original a r t i c l e submit ted July 1, 1969.

�9 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

473

TABLE 1. Diethyl Es ters of Substituted Benzoylphosphonic Acids and Diethyl(diphenyl)benzoylphosphine Oxides X-CsHtCOP(O)R 2

X R

p-C1 OC2H5 m-Cl [ 0C2H5 m-Br [ OC~H~ p-NO~ [ OC~H~ H C~H5 H CsH5

Y~ld.

80 60 70 20 66

105

Bp, ~ (p, d~O mm Hg)

i50(2) 1,2486 t46.(2) / 1,2560 [ 164 (2) ] 1,4325

i45~ (0 I ~,t22o-- . t55--156(6) [ t67-t68 0,5) 1

n~ - ~ calcu-lfound, Empirical iouna lated ~ formula

t,524tl67,58 65,24 1t,39" CllH~4CIO4P t,5170/66,56 65,24 tt,03 68,04 9,48 CnHI~CIO4P t ,5342169,55 CnH,,BrO,P

t,5405158,76 5832 14,43 CnHuNOoPcnHI~02P t,6385 / -- -- 10,t0 C19Hxs0~P

P cal- cula- ted, %

1t,2i tl,2t 9,65

14,76 t0,13

* Found: C1 12,69%. Calculated: 12.83~ I" Mp 51.5~ found: N 5.02; H 4.84; C 47.02%. $ Dense vaseline-like substance.

Calculated: N 4. 88; H 4.88; C 45.99~0.

potential energy of the molecule cor responds to a perpendicular a r rangement of the C = O and P = O bond, then in any case the dipole moment of the molecule should be equal to 4.59 D; if, however, the sys tem is stable with a flat a r rangement of the carbonyl and phosphoryl groups, then the moment can take values f rom 1.73 to 6.25 D. The experimental dipole moments of phosphine oxides correspond to equilibrium of the s - cis and s - t r an s conformations, containing 94% of the s - t r an s form for diethylbenzoylphosphine oxide and 88~c for diphenylbenzoylphosphine oxide. The ra t io of s - c i s and s - t r ans conformers in the investigated compounds was determined according to the equations

2 ~ts_cisns-ci s -[- ~ts.transns-trans= ~l~xp

ns_ci s + ns-tran s = I

It is in terest ing to follow the influence of the surroundings of the P atom and the carbonyl C atom on the s te r ic s t ruc ture of the compounds in the ser ies : diethyl es te r s of aceto- , benzoyl- , and p-methy l - benzoylphosphonic acids, e thyl es te r s of ethyl- and phenylbenzoylphosphinic acids, and t r ie thyl - and di- phenylbenzoylphosphine oxides. The dipole moments of diethyl es te r s of phosphonic acids were calculated for a flat a r rangement of the carbonyl and phosphoryl groups, just as in oxides of diethyl- and diphenyl- benzoylphosphines, using the moment of the diethoxyphosphono group 2.30 D, with an orientation such that the components along the coordinate axes (see Fig. 1, R = OCzHs) a re equal to m x = 0.72 D, my = 0, m z = 2.19 D [11]. The group moment of CH3-Csp2 was calculated (1.06 D) f rom the experimental dipole mo- ment of toluene [16]. The dipole moments of the conformers calculated on the basis of these data a re cited together with the experimental dipole moments in Table 2.

The content of s - t r a n s conformers is also cited in Table 2. In all cases the s - t r ans conformation is the most profitable, which is due to the substantial repulsion of the s t rongest P = O and C = O dipoles in the case of a c is -or ienta t ion of them. In diethyl e s t e r s of aceto- , benzoyl- , and p-methylbenzoylphos- phonic acids, the ra t io of the conformers is the same. This indicates that the radical added to the carbonyl group does not influence the energy of the conformers . In phosphine oxides, a change in the volume of the rad ica l at the phosphorus also does not significantly change the rat io of the conformers . However, an ap- preciable difference is observed in the ra t ios of the conformers for diethyl es te rs of phosphonic acids and phosphine oxides; therefore we were interested in investigating mixed derivatives, namely, ethyl es te r s of ethyl- and phenylbenzoylphosphinic acids. In this case the dipole moment of the molecule is determined by rotat ion of the ethoxyl group around the P = O bond. As was shown by the data for phospholenes [17], ethyl e s t e r s of phosphinic acids exist preferent ia l ly in gauche conformations with respec t to P = O. In the case under consideration, the dipole moment t~A and t~B of the two gauche conformations A and B (Fig. 2) (rotation of the ethyl group in the direct ion of the benzoyl group or substituent at the phosphorus) differ. We have suggested that both fo rms are equiprobable.

In the calculation we used the geometr ica l pa rame te r s and moments of the bonds and groups cited above. The calculated dipole moments are cited in Table 3. For ethyl es te rs of ethyl- and phenylbenzoyl- phosphinie acids, the calculated moments coincide as a resu l t of the equality of the group moments of C2H 5 - P and C6Hs--P. The experimental dipole moments are equal to 2.44 D for the ethyl es ter of ethylbenzoyl- phosphinic acid and 2.72 D for the ethyl es te r of ethylbenzoylphosphinic acid and 2.72 D for the ethyl es ter of phenylbenzoylphesphinic acid. The content of the s - t r an s conformers is 0.96 and 0.91, respect ively . In these compounds the s - t r ans conformation is rea l ized a lmost exclusively, just as in phosphine oxides.

474

Et Et 0%. o/, o4,

l 2 3 A B

Fig. l Fig. 2

Fig. 1. Possible orientation of the carbonyl and phosphoryl groups: 1) s -c i s ; 2) C = O • P = O; 3) s - t r ans .

Fig. 2. Possible orientation of ethoxyl groups in phosphinates.

TABLE 2

Compound

CH3CO--P (0) (0C~H@e C~HaC0--P (0) (OC~H@~ p-CH~C6H4C0-- P (0) (0Cells)2

I c alc, D

D s-cis s-trans ~exp,

4,84 0,78 3,t0 4,5t 0,50 2,76 4,84 0,25 2,88

ns-trans

0,60 0,63 0,67

T A B L E 3 . . . . ! Orientationofl

rlema~l~ 1 ethoxy groups I of the ben- - ' Paw D zoylgroup PA, D ~B' D i

s-cis . . . . . 6,20 5,64 5,93 s-trans . . . 2,59 1,62 2,t6

To evaluate the degree of interact ion of the substituents in the phenyl r ing with the diethoxyphosphono group, we determined the dipole moments of diethyl es te r s of substituted benzoylphosphonic aicds: p -ch lo ro- , p -b romo- , p -n i t ro- , p-methoxy- , m-cb lo ro - , m - b romo- , and o-ch loro-der iva t ives . It was assumed that in all eases the ra t io of the s - c i s and s - t r ans conformers remains the same as in the diethyl es te r of unsubstituted benzoylphosphonic acids (with the exception of the o -ch loro-der iva t ives , in which s ter ic interact ion of

chlorine and the a toms of the diethoxyphosphono group may be manifested). A substituent in the p a r a - p o - sition may enter into conjugation with the carbonyl and diethoxyphosphono groups. Therefore , we might expect deviations of the dipole moments f rom additivity. For e s te r s containing substituents in the uncon- jugated meta-posi t ion of the phenyl ring, with coplanari ty of the phenyl and sys tem of bonds O = C - P = O preserved , var ious orientations of the substituents a re possible: in the syn-posi t ion to the carbonyl group (I) or in the anti-posi t ion (II)

X _oo ~ / ~ -- ~ ~ \ P (0) (OCeH~)~ P (0) (OC~Hsh X

(I) (II)

The s implest case is when the substituent introduced is regula r (chloro-, b romo- , nitro groups). The moments of the Csp2 -X bonds were calculated according to an additive scheme, based on the data of [15], f rom the experimental dipole moments of substituted benzenes [16]: C-C1 0.89 D, C--Br 0.86 D, C - N O 2 3.30 D. The moments of the bonds in the methoxy group were cited in [11]. Following [11], we have sug- gested that s t ruc tures with differently oriented methoxy groups should be present in the mixture in equal amounts.

On the basis of the data cited we calculated the theoret ical dipole moments of para-subs t i tu ted de- r ivat ives , which a re cited together with the experimental dipole moments in Table 4.

The difference of the experimental and calculated dipole moments represen t s the moment of in te r - action, due to t r ans fe r of e lect rons f rom the donor substituent in the phenyl or competitive conjugation of the electron acceptor nitro group. It can be shown that the moments of the interaction should be positive, since the direct ion of the displacement of electrons in both cases is close to the direction of the summary moment of the molecule. As it follows f rom Table 4, the excess of the experimental dipole moments over those calculated is r a the r substantial. Evidently the substituent in the benzene r ing is in a single sys tem of conjugation with the C = O and P = O groups. A compar ison of the moments of the interaction in diethyl e s t e r s of p-subst i tuted benzoylphosphonic acids with the rat io of the experimental and calculated moments

475

TABLE 4

Compound

p-CIC6H4COP (0) (OC~H~)2 p-BrC~H,COP (0) (0C~Ha)~ p-GHa0C~H4COP (0) (OC~H~)~ p-N0~C~H4COP(0) (0C.,H~)~

~oa1~ 2,t6 2,79 2,t6 2,80 [ 2,6t 3,35 3,t4 ~,84

/xp = gexp "~calc, D

0,63 0,64 0,74 1,70

o f p-subst i tuted acetophenones gives a basis for believing that the phosphono group part icipates substantially in the conjugation with the ~-e lec t ron system. In p-methoxy- acetophenone, the moment of interaction is equal to 0.24 D [18], while in p -ch loro- , and p-bromoacetophenones the experimental moments a re less than those calculated according to an additive scheme. This agrees with the conclusion of the presence of interaction of the carbonyl and diethoxyphosphono groups in benzoylphosphonates, drawn in a study of the UV spect ra of these compounds [6].

For rec ta-der iva t ives we determined the content of the conformers (I) and (II). The ra t io of the con- format ions, due to rotat ion around the C - P bond, was assumed to be the same as for the diethyl es ter of benzoylphosphonic acid. The calculated moments of the conformers , together with the experimental dipole moments of the meta-der iva t ives , a re cited in Table 5. F r o m Table 5 it is evident that for both compounds the conformation in which halogen atoms a re far thest f rom the voluminous diethoxyphosphono group is the mos t stable, and this effect is more pronounced for the bromo-der iva t ives .

The dipole moments of four possible s t ruc tures were calculated for the diethyl es te r s of o -ch lo ro - benzoylphosphonic acid: s - c i s (I) 5.87, s - t r ans (I) 1.51, s - c i s {II) 4.94, and s - t r an s (II) 2.08 D. The ex- per imental dipole moment 3.11 D in this case does not cor respond to equilibrium of the conformations (I) and (II) at the ra t io of s - c i s and s - t r ans conformers determined above (~I calc = 3.75, PII calc = 3.42 D). Evidently the C1 atom in the o-posi t ion has an influence on the rotat ion around the C - P bond, and perhaps on the rotat ion of the ethoxyl groups at the P atom as well. In addition, in this case participation of the C1 atom in the conjugation is possible. The lower value of the experimental dipole moment in comparison with those calculated for the conformations (I) and (II), is evidence that the re la t ively nonpolar S- t rans fo rms a re rea l ized to a g rea te r degree.

EXPERIMENTAL METHOD

Synthesis of Diethyl Esters of Substituted Benzoylphosphonic Acids. To 0.03 mole of the chloride of

the corresponding benzoic acid, we added 0.04 mole of triethylphosphite in a stream of anhydrous CO 2 at 35-40 ~ . Immediately after the end of the reaction, ethyl chloride was removed and the reaction mixture redistilled under vacuum. The characteristics of the compounds obtained are cited in Table I.

Synthesis of the Diethyl Ester of p-Nitrobenzoylphosphonic Acid: In a three-necked flask, equipped with a mixer, dropping funnel, and reflux condenser, we placed 33.5 g of p-nitrobenzoylchloride, dis- solved in 200 ml of abs. ether, and added dropwise 30 g of triethyl phosphite in i00 ml abs. ether. Drop- wise addition was conducted at a temperature no higher than 30 ~ for 8 h with mixing. On the following day the solvent was removed and the light-yellow oil formed subjected to redistillation under vacuum (to avoid explosion, only a third of the reaction mass should be distilled off). The diethyl ester of p-nitrobenzoic acid (7 g), after two redistillations, was washed with hexane and identified; bp 145 ~ (i mm), mp 51.5 ~

Found: 5.02~c. CIIHI406NP. Calculated" N 4.88%.

Synthesis of Diethyl(diphenyl)benzoylphosphine Oxides. To an ether solution of the ethyl ester of diethyl(diphenyl)phosphinic acid we added dropwise an eqnimolar amount of benzoyl chloride at a tempera- ture no higher than 30 ~ . After the end of the reaction, the ether and ethyl chloride were removed, and the mass remaining redistilled under vacuum. All the operations were performed in an atmosphere of an- hydrous CO 2. The characteristics of the compounds obtained are cited in Table i.

T A B L E 5

Compound

m-C1C~HaCOP (O) (OCeHs)e

m-BrCoHaCOP (O) (OC2Hs)e

Orienta-

tion of

phenyl

I II

Conformation

S-eig s . t rans

4,6t t,10 3,35 1,8t 4,61 1,07 3,38 1,78

~av, D

2,93 2,49 2,92 2,49

gexp, D

2,77

2,88

0,6 0,4 0,9 0,1

476

TABLE 6. Coefficients of Calculation Equations and Experimental Dipole Moments of the Compounds RCOP(O) (R') (R")

R R' R" Solvent c~ Y Po, crn3 ~, D

CH~ C~H5 p-CH3C6H4 p-C1C6H4 p-BrC6Ha p-CH3OC~Ha p-NO~C~Ha m~IC~H~ m-BrCsH~ o-CIC~H~ C~Hs C~H~ C~H~ C~Hs

OC:Hs OC~H5 OC~Hs OC~H~ OC~H~ OC~H5 OC~H.~ OC~Hs OC~Hs OC~Hs C2Hs C6Hs OC~Hs OC2H~

OC~H5 OC2H~ OC~Hs OC~H5 OC~H5 0C2H5 0C2H5 OC~H5 0C2H5 OC~H5 CsH5 C6H5 C~Hs CsH5

CC|4 CCla CC14 CC14 CC14 CCh CCH~ CC14 CC14 CCI, CCI4 C~H~ CC14 CCI~

10,152 6,257 6,556 5,736 5,000 8,2t6 8,930 5,9t2 5,366 6,973 8,725 2,8t4 5,424 5,840

--0,t75 0,018 0,27i 0 270 o:25~1 0,3t01 0,077] 0,222 0,330 0,187 0,374 0,i90 0,306 0,588

t96,812 t55,375 169,8 t59,528 t60,66i 228,754 477,8 ~66,74i t70,i47 t98,232 t06,528 t50,870 t22,000 t51,460

3,t0 2,76 2,88 2,79 2,80 3,35 4,84 2,77 2,88 3,11 2,28 2,69 2,44 2,72

Determinat ion of the Dipole Moments. The dipole moments were determined in CC14 and benzene at 25 ~ The die lect r ic permeabi l i t ies of the solutions were determined on an IDM-2 instrument [19], the in- dices of re f rac t ion on an IRF-23 re f rac tomete r . The orientation polarization was calculated according to Guggenhe im-Smi th method [10]. Table 6 presents the coefficients of the calculation equations and the r e - sultant data.

We should like to express our thanks to A. N. Vereshchagin for his valuable advice and aid in the work.

CONCLUSIONS

1. Diethyl esters of certain substituted benzoylphosphonic acids and diethyl(diphenyl)benzoylphos- phine oxides were synthesized, and their dipole moments were determined.

2. The steric structure of the investigated compounds was established.

3. Interaction of the substituents in the benzene ring of diethyl esters of benzoylphosphonic acids with the diethoxyphosphono group was detected.

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

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477