Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 0261 307

6. Dipole moments of compounds containing double bonds

291

With monosubstituted amides, attention was focused on the conformation on the N C bond. This can usually be determined from dipole moments192, sometimes even the conformation on the next bond can be estimated with some probability193. The same applies to disubstituted amides with unequal substituents: in the case of compound 80 the substituents were connected to a heterocycle194; the conformation ap (80) was practically the only one present. Acylated amidines 81 represent one of the most complex problems solved by means of dipole moments combined with the Kerr constant195. In addition to the configuration on CDN, one has to determine conformations on five single bonds. Of these, three (on the N Car and C Car bonds) do not affect but affect mK. When certain angles were estimated on the basis of model compounds, the final solution was that conformation on N C(O) is nonplanar while on N C(N) it is planar and gives rise to an equilibrium of two conformers. The problem certainly would be worth elaborating by means of further experimental methods.

	O
CH3	C
	N
	N
	(80)
		CH3		CH3
CH3	CH3	C		C
		N	N	O

C

N N

C

CH3 O

CH3

(81a)

(81b)

There is some confusion concerning the conformation of substituted ureas196,197, when one difficulty is again in the small solubility. For a dipole moment study combined with IR spectroscopy, derivatives of 82 were selected196 which have only one axis of rotation. The mesomeric moment is of less importance and could be estimated in advance. Then conformation was solved in favor of the sp form 82. In hydroxyureas 83 there is one rotational axis more and the conformation found198 should be considered as approximate.

Dipole moments of hydroxamic acids were originally interpreted199 in terms of the conformational equilibrium of 84a and 84b. When some proof of the hydrogen bond were received from IR spectroscopy, the interpretation was preferred138,200 that the only form present is 84a and the enhanced dipole moment is due to the component H like in the structurally similar ketones (46) or peroxy acids (73). The effect of conjugation, m, is much smaller and can be estimated from model compounds without depreciating the value of m. Compared to the named molecules, the hydrogen bond in hydroxamic acids is of intermediate strength between 46 and 73; see the values of H. A very near value of was

292	Otto Exner

calculated for formohydroxamic acid201: within the framework of bond moments, of formohydroxamic and benzohydroxamic acid should be equal. However, the molecule of the former should be not quite planar according to MP2(full)/6-31CGŁŁ calculations201. In N,O-diacylhydroxylamines (85) the main problem was the steric arrangement. Although there are a priori three axes of possible rotation, the conformation was obtained unambiguously and relatively precisely200 in agreement with some X-ray structures and with the previous estimate199. A great advantage for the dipole moment analysis is the possibility of exploiting substitution from two sides.

A cumulated system NDCDO occurs in isocyanates. Measurements of dipole moments of para-substituted phenyl isocyanates202 served to determine the magnitude and direction of the bond moment. Since it lies at a small angle (ca 21°) to the Car N bond, its direction is not obtained with great accuracy and is not sensitive to the conformation on this bond. No attempt was made to analyze the group moment into components, as it would be probably of little value. The same problems are encountered with other heterocumulenes; see Section VI. Vinyl isocyanate exists in the gas phase in two conformers, 86a,b. Their dipole moments are equal within experimental error32: conjugation is evidently negligible and hence independent of conformation.

			X
CH3	O			O
N	C		N	C
CH3	N	X	H	N	X
	H			O
				H
	(82)			(83)
	O	H		O
X	C		X	C
	NH	O		NH	O
					H
	mH 2.16
	(84a)			(84b)
		O	O
	X	C	C	X
		NH	O

(85)

6. Dipole moments of compounds containing double bonds				293
	O
	C		O
			C
H	N	H	N
C	C	C	C
H	H	H	H
m exp	2.14	m exp	2.12
(86a)		(86b)

E. Conjugated Systems X−C=O

In acyl halides, any n conjugation can be only very weak, nevertheless an attempt was made to evaluate it from dipole moments of para-substituted benzoyl chlorides and bromides203. Only in the latter was an electron transfer from bromine toward carbon found (1.8 D), which cannot be interpreted as a sign of n conjugation but rather as diminished polarity of the C Br bond. On the other hand, the conjugation of the COX groups with the benzene ring is stronger in the case of chlorides. Oxalyl fluoride exists in the gas phase in the equilibrium204 of 87a and 87b. The dipole moment of the latter was calculated with relative success.

F	O		F	F	O	O
C	C		C	C	Cl C	Cl C
O	F		O	O	O R	O
						R
(87a)			(87b)		(88a)	(88b)

The structural unit of acyl chlorides is also contained in the molecule of chloroformates. Their conformation had long been controversial since the agreement of calculated and experimental dipole moments was not sufficient. The literature has been reviewed previously2; of particular importance were two dipole-moment papers205,206 preferring the unnatural sp conformation 88b. (Notation sp corresponds to ap in common esters.) The difficulty lies in that the resulting is relatively small and the bond-moment scheme is not exactly valid due to the proximity of strongly polar bonds. Recently the problem was attacked by two approaches with conforming results3,207. In the first, X-ray analysis of 4-nitrophenyl chloroformate gave conformation ap (88a) and IR spectroscopy furnished proof that the conformation in solution is not different3. Then the experimental dipole moments of aryl chloroformates206 were analyzed in terms of bond moments3. The difference from the anticipated value from the formula 88a does not correspond to an electron transfer from O toward DO or from Cl toward DO, but it should be formally represented by reduction of the CDO bond moment and increasing the C Cl bond moment. A second approach207 was based on dipole moments and Kerr constants of methyl, trichloromethyl and aryl chloroformates, when the same conformation (88a) was established. In this way, the claim208 based on IR and NMR spectra, that chloroformates exist in an equilibrium of 88a and 88b was disproved.

294	Otto Exner

VI. C=S BONDS

Generally the same problems are encountered as with the CDO bonds. Dipole moments are usually larger mainly because the CDS bond is longer; also the n conjugation is stronger209 due to the greater ability of sulfur to accommodate a negative charge210. Dipole moments of the simplest compounds, i.e. thioformaldehyde, thiopropionaldehyde and thioacetone, are not always accessible experimentally but were calculated on a rather high level211 213 (also the seleno analogue CH2DSe212,214). The compounds with cumulated double bonds (89) were both calculated213 and measured in the gas phase156,215 (also the seleno analogue CH2DCDSe216). Their dipole moments are in this case lower than those of the corresponding oxygen derivatives 63 but show similar alternation along the series. In contrast to 63 the sulfur derivatives are all linear156. The unstable compound 90 with an electron sextet217 is an analog of 66 and has a considerably higher . The calculated dipole moments of pyrothione and thiapyrothione (90) are larger than their oxygen analogues218: the main reason was seen in the longer CDS bond. Tropothione has also a larger dipole moment219 than tropone (4.42 compared to 3.72 D).

S

H2 C(

C )n

S

C

C

C

S

X

mexp 1.647, 1.021,

2.064

mexp 3.704

X = O, S

(89)

(90)

(91)

In the sulfonium ylide 92 and similar compounds220, the central C S bond may acquire considerable double-bond character 92a $ 92b . The calculated bond lengths correspond merely to a single bond, but reveals strong back-donation. The conformation of the derivative 93 was also estimated from the dipole moment220.

					H		H
O	O−		O	O	O		− O
S	+		S	CH3		+ S	CH3
	CH3
	CH3			CH3			CH3
m exp	5.46				m exp		4.56
(92a)			(92b)			(93)

In the conjugated systems X CDS attention was focused mainly on the conformation around the C X bond. Calculations preferred the sp form (94a) for dithiocarboxylic acids221, thionesters7 and dithioesters221; this is the same form as preferred in normal esters (3a) but the energy difference against the ap form is lower. Explanation of this preference is still sought in the interaction of dipoles7, although this has been shown to

6. Dipole moments of compounds containing double bonds

295

be insufficient2. Older dipole-moment studies of thiolbenzoates185 and thionbenzoates222 were also directed toward determining conformation. They were later recalculated209 to evaluate also the n conjugation 94a $ 94b. A particularly strong conjugation was found209 in thiobenzamides and N,N-dimethylthiobenzamides (95). Conjugation in several systems XDC Y with n conjugation can be now compared quantitatively; in Table 1 they are expressed in terms of the mesomeric dipole moment mm. The agreement of different methods is not so bad considering the approximate character of several assumptions. Some regularities emerge which could be a priori expected. Conjugation is stronger when the donor group is more basic (NMe2 > NH2 > SMe > OMe) and when the acceptor group is a stronger acceptor (DO > DNH). The most efficient acceptor group is DS, but this is due at least partly to the longer CDS bond and only less importantly to the ability of sulfur to accommodate the negative charge. Most interesting, but somewhat puzzling, are the directions of mm. The theoretical expectation should be from Y toward X and this direction was taken as reference (angle D 0°) in Table 1. Most strange is this angle in t-butyl esters and thiolesters, but the absolute values are small and not so dependable. In amidoximes the values of evidently differ as a function of the configuration of the chain.

S		S −	S		S −
R C		R C	R C		R C
	+			+
X R		X R	N(CH3 )2		N(CH3 )2
(94a)		(94b)	(95a)		(95b)

In thioamide vinylogues (96) the conjugation was proved in a qualitative sense from the enhanced values of dipole moments223. The derivatives of furane (97) were investigated by several experimental methods224: while the conformation on C N followed mainly from IR spectra, dipole moments served to determine conformation of the furan ring on the Car C bond but the result was not convincing (due also to some misprints).

TABLE 1. Mesomeric dipole moments ( m) and relative importance of the polar mesomeric structure (p) in n conjugated systems C(DX) Y

X	Y	ma	m b		mŁ pc		m d
O	OCH3	0.4	0.2	(25° )
	OC(CH3)3		0.5	(52° )
	NH2		0.9	(19° )
	N(CH3)2	1.7	1.4	(26° )	1.9	(0.21)	1.32	(19° )
	SC2H5	0.3	0.5	( 44° )
S	OC2H5	1.2	1.0	( 19° )
	NH2		1.9	(0° )				( 2° )
	N(CH3)2	3.2	2.55	(12° )	3.7	(0.36)	2.49
Se	N(CH3)2	3.7		( 25°° )	4.1	(0.39)
NCH3-(E)	N(CH3)2	2.1	0.9		1.9	(0.18)
NOH-(Z)	NH2		0.8	( 14 )
NOH-(E)	N(CH3)2		1.2	( 49° )

a Method of calculation (2), Section V.D., References 96,97,256.

b In parentheses, angle to the Y. . .X direction, method (3), Section V.D. References 128 and 95. c The moment mŁ is proportional to m, method (4), Section V.D. Reference 89.

d In parentheses, angle to the Y. . .X direction, method (5), Section V.D. Reference 99.

296		Otto Exner
	NR2		H
		S	N CH3
		C	C
		O	O
		N CH3	S
	S	H
(96)		(97a)	(97b)

In substituted thioureas196 and hydroxythioureas198, attention was focused on the conformation while conjugation was accounted for by means of small correction terms. The resulting conformation was the same as in the oxygen analogues, 82 and 83, respectively. Dipole moments of 3-aza-1-thiabutadienes, of which 98 is an example, were calculated225 for the two conformations 98a and 98b, applying small correction terms for the possible conjugation. The configuration on the CDN bond was known from X-ray crystallography and other proofs. The sp conformation 98a is clearly prevailing in solution. Calculated dipole moments of sulfur derivatives of carbamic acid were compared226:of NH2CSOH is greater than of NH2COOH, and that of NH2CSSH is greater than that of NH2COSH.

C6 H5	S	C6 H5	S
C			C
N	H	H	N
	C	C
	N(CH3 )2	N(CH3 )2
calc	4.29	calc	6.69
	calc	4.59
(98a)		(98b)

Heterocumulenes involving a CDS bond were investigated rather extensively. On simple ethyl isothiocyanate the NDCDS group moment was found at an angle of 29° to the C N single bond (in the gas phase227). When individual heterocumulenes are compared228, this angle seems to increase slightly from NCO to NCS and NCSe: for isoselenocyanates it was determined228 from aryl derivatives 99 in the standard way as in the case of isocyanates202 and isothiocyanates229. In the same paper228 also conjugation of NCSe with the benzene ring was quantitatively estimated: the value of mm is between 0.6 and 0.8 D for all heterocumulenes. The nonaxial position of the NCS dipole allows in principle determination of the conformation of this group with respect to certain polar neighbors. However, the deviation from the C N bond is small and the results must be hardly precise. In the case of cinnamoyl isothiocyanates the conformation on C C(O) and (O)C N bonds was at question: the suggested result230 is given by the formula 100. In chalcone derivatives 101 we have a similar steric arrangement231 as far as the NCS group is in the para position. When it is in meta, an additional axis of rotation

6. Dipole moments of compounds containing double bonds

297

arises and the problem seems to exceed the possibilities of the approach. In arylsulfonyl isothiocyanates232 (102) there is only one axis of rotation and the dipoles are great, but the small angle of the NCS group moment is still a problem and the conformation is merely an approximation.

X	N	X		CH
	C			CH
		Se
				τ = 60°
	(99)			(100)
X	CH	O		X
	CH	C
		N	C	S
	(101)
	O	O
	C6 H5		C6 H5
O	S	O S		O
CH3	C S	CH3	C	S
	O
	CH3			CH3
CH3		CH3
	mexp 4.83	m exp	4.68
	(103a)	(103b)

O

C τ

N

C

S

O

S Oτ

N

C

S

τ = 80°

(102)

CH2 S

O

mexp 2.994

(104)

Interesting structures with a formally tetravalent sulfur in which the CDS bond gives rise to two stable configurations were already mentioned previously2. They were augmented by the derivatives 103 and analogous sulfoxides and sulfides. Here dipole moments were used only as a supporting method233 in combination with 1H NMR spectra: configuration on CDS was based only on the latter; helped in determining the conformation on C S. Since it was evaluated only in a less efficient way (without exploiting substitution),

298	Otto Exner

the conformations 103a and 103b are only approximate. In any case it is certain that the nonplanar arrangement SDC S C is in contrast with the planarity in 94. For the simplest compound of this class (104) the dipole moment was measured by MW234.

VII. C=P BONDS

The chemistry of phosphorus compounds was reviewed with particular attention to dipole moments235,236. Compounds with a CDP bond present the same problems as the CDN bond: of course the number of derivatives is restricted and difficulties may arise with their stability. The simplest compound CH2DPH was investigated by MW in the gas phase237. From its dipole moment, 0.896 D, the PDC bond moment of 0.02 D was deduced238, which however was based on different bond moments than used here. In any case this bond is only slightly polar.

H	C6 H5			H	C6 H5
C	P			C	P −
(CH3 )2 N			(CH )	N +
		3 2

calc 1.34

calc 3.43

(105a)			(105b)
			Si(CH3 )3
(CH3 )3 Si O	C6 H5		O	C6 H5
	C P		C	P
(CH3 )3 C			(CH3 )3 C
calc	2.28		calc	2.32
exp	2.29
(106a)			(106b)

Conjugation is possible from either end of the CDP bond. A series of compounds with the N CDP systems was investigated by dipole moments239. From the difference of experimental and that calculated from bond moments, n conjugation was deduced, for instance in 105a $ 105b; the mesomeric dipole moment was not explicitly evaluated. The E configuration of 105 was known a priori from spectroscopy. On the other hand, no perceptible n conjugation was found in 106. Its conformation cannot be deduced from dipole moments and is probably hardly rigid239.

VIII. OTHER DOUBLE BONDS

This section exceeds the subject of this Volume, nevertheless it is included in order to cover the same range of compounds as previously2. Mentioned in particular are compounds of interest in the theory of dipole moments and those presenting similar problems as with the CDX bonds.

6. Dipole moments of compounds containing double bonds

299

The NDN bond is formally nonpolar in the bond-moment system; in symmetrical molecules it is nonpolar really. The nonzero dipole moments of symmetrical azo compounds 107 were explained27 by participation of mesomeric formula 107b. This explanation is unacceptable. The symmetrical formula 107c must participate to the same extent as 107b and the resulting dipole moment is zero according to equation 3 extended to three or more structures. The effective dipole moment would be zero only if calculated similarly as in equation 4, but this is not possible since a mesomeric transition must always be faster than any physical process. Apparent dipole moments of some symmetrical molecules are not yet completely understood26; evidently all cases cannot be explained in the same way. For compounds 107 an abnormally high atom polarization is probable, but formation of CT complexes is also possible. When X D OCH3 or COOC2H5 the evident explanation is in unsymmetrical conformations like 108: exchange of conformation is no more infinitely fast and the effective dipole moment is given by equation 4.

X	N	X	+	N
	N	X		N−	X
X = CH3	mexp 1.23
X = Br	mexp 2.47
	(107a)			(107b)
		CH3
X	N−	O		N	CH3
	N	+ X		N	O
				mexp 5.97
	(107c)			(108)

In azoxy compounds (1, 2) both the configuration on NDN and electron distribution within the conjugated system can be dealt with5. The compounds were chosen as an example of the two possible approaches in Section I.

The NDO bond can cause conformational problems in conjugated systems. According to MW, the stable conformation of nitrosoethene240 is ap (109) while nitrosoethane represents a mixture of eclipsed conformations sp (110a) and ac (110b) with the former prevailing241. Of course, conformation has here a minute effect on since it concerns only the hydrocarbon residue. (Within the framework of the bond-moment scheme, would be not affected.) In nitrosomethane242 itself D 2.320, but its direction seems doubtful. According to in solution243, the conformations ap and sc of 2-nitro-2- nitrosopropane (111) are populated almost equally. Nitrous acid exists in a mixture of sp and ap conformers244; in alkyl nitrites only ap was revealed245. Similarly as with the compounds 109 and 110, measurement of represented only a small supplement of the whole analysis and the results were not discussed. This is common in MW work.

	300	Otto Exner
	N O		O		N O
	CH2 C		N
				CH3 CH2
	H	CH3	CH2
	exp 2.77	exp	2.398	exp 2.288
	(109)	(110a)			(110b)
		NO2		NO2
	CH3	C
			CH3
				C
	CH3
		N	CH3		O
				N
		O
	calc	1.65	calc	4.47
			exp	3.48
	(111a)		(111b)

Of the compounds with a NDS bond, thionylamines are known only in the Z configuration (112). Previous reasoning based essentially on dipole moments2 was reinvestigated and the configuration confirmed also by other methods246. The same configuration was found for thiothionylamines 113 and the dipole moments were analyzed in terms of bond moments247. Compared to thionylamines, the negative charge is more dissipated here, also to nitrogen; the SDS bond is highly polar. Further derivatives with a NDS bond (114) are symmetrical and cannot exist in different configurations. The dipole moments were analyzed248 in terms of a variable SDN bond moment, but a more natural interpretation would probably be a through-conjugation of electron-attracting substituents (NO2) with the lone electron pair on nitrogen, expressed by m.

	S
	N	S
	(CH3 )3 C	CH3
N		X	N
	S			S(CH3 )2
O	C(CH3 )3
mexp 1.95	mexp 1.51	X = F	mexp	5.53
		X = NO2	mexp	10.1
(112)	(113)		(114)