ORGANIC MASS SPECTROMETRY, VOL. 21, 283-287 (1986)

Mass Spectrometric Studies of Heterocyclic Organophosphorus Compounds: 1,3,2-Benzodioxaphospholes

Guo Hangzhou and Kinghunt Tsao Institute of Pharmacology and Toxicology, 11 Tai-Pin Road, Beijing, China

Li Yu-Gui and Wangjian Institute of Elemeto-Organic Chemistry, Nankai University, Tianjin, China

The mass spectral behaviour of a new type of organophosphorus compound, 1,3,2-benzodioxaphospholes, under 70 eV electron impact has been studied by means of high and low resolution mass spectrometry as well as by BIE and B2/E linked scans. The influence of different substituents in the molecule (0, S or N) on the mass spectra is investigated. The reason for isomerhation between oxygen and sulphur within a molecule studied by mass spectrometry is discussed.

INTRODUCTION

1,3,2-Benzodioxaphospholes belong to a new type of organophosphorus compound132 having antibacterial and cholinesterase-inhibiting properties; they may also be potential insecticides. Compared wih most of the phosphorus insecticides used now, they work more rapidly and leave less contamination in the environ- ment. In this paper, 30 of these compounds were studied by means of normal mass spectrometry and B/E and B2/E linked scans.

RESULTS AND DISCUSSION

Compounds studied in this paper can be divided into three groups, according to whether substituent Y = 0, S or N. The main ions, and their relative abundances,

of all 30 of the compounds studied are shown in Table 1.

The mass spectral behaviour of the first group of compounds (1-11) is as follows:

(a) All the compounds show abundant molecular ions, and for some of them this is the base peak.

(b) The base peaks fall into three types: (i) The McLaff erty rearrangement ion is the base

peak if Z is an alkyl group, in this case it is convienent for the six-membered planar ring system to transfer an alkyl hydrogen and eliminate a neutral particle (Scheme 1).

(ii) When Z is a substituted phenyl group with a substituent in the para position contributing to the conjugated system, the molecular ion will be the base peak. Thus, in compound 7, the lone electron pair of bromine can conjugate with the II2 electron to form a TI: system. Such a II system will make the molecule

R

1,3,2-Benzodioxaphospholes

1 Y = O , R = H , Z=FZ-C;H~ 2 Y = 0, R = Me, Z = iso-C,H, 3 Y = O , R = H , Z=p,o-Me,C,H,

5 Y = 0, R = Me, Z = p,m-CI,C,H, 6 Y = 0, R = Me, Z = o-BrC,H, 7 Y = O , R = H , Z=p-BrC,H, 8 Y = 0, R = H, Z = o-BrC,H, 9 Y = 0, R = Me, Z = o-NO,C,H,

11 Y = 0, R = H, Z =p-MeOC,H, 12 Y = S , R = M e , Z = M e 13 Y = S , R = M e , Z = E t 14 Y = S, R = Me, Z = n-C3H,

4 Y = O , R = H , Z=m-CIC,H,

10 Y = O , R = H , Z=p-CNC,jH.q

15 Y = S , R = H , Z=n-C,H,

16 Y=S, R=Me, Z=n-C,H, 17 Y = S, R = Me, Z = n-C,H,, 18 Y = S , R = H , Z = C , H , 19 Y=S, R=Me, Z=C,H, 20 Y = N , R = H , Z=Me, 21 Y = N , R=H, Z=Et, 22 Y = N , R=Me, Z=Me, 23 Y = N , R = M e , Z=Et , 24 Y = N, R = H, Z = (iso-C,H,), 25 Y = N, R = H, Z = (iso-C,H,), 26 Y = N , R = M e , Z = H , Et 27 Y = N, R = H, Z = H, iso-C,H, 28 Y = N, R = Me, Z = H, iso-C,H, 29 Y = N, R = H, Z = H, n-C,H, 30 Y = N , R = H , Z=H,p-CIC,H,

0030-493X/86/O50283-05$05 .OO 0 1986 by John Wiley & Sons, Ltd.

Received 24 July, 1985 Reuised/accepted 1 November 1985

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MASS SPECTROMETRIC STUDIES OF 1,3,2-BENZODIOXAPHOSPHOLES 285

SH 1 + CHZzCH-R'

rnlz 188 (R = H, Y =0) rnlz 202 (R = CH3, Y = 0)

mlz 204 (R = H, Y = S) rnlz 218 (R = CH,, Y = S)

Scheme 1

more stable by electron delocalization with the px-dx bond between the phosphorus and the sulphur atoms.3 Compounds 3, 4, 10 and 11 also belong to this case.

(iii) [M - XI' is the base peak when X is a halogen atom in the o-position in the substituted phenyl group. Although the lone electron pair of the halogen atom in this position could overlap with IT: as well as in the para and metu positions, it also sterically hinders the rotation of the C-0 and P-0 single bonds. When samples are introduced into the mass spectrometer, the thermal motion of molecules and rotation and vibration within a molecule are increased and the steric hindrance effects are ~ t ronger .~ So the X. radicals are easy to lose, as in compounds 6, 8 and 9.

(c) All compounds of group I give the characteristic ions shown in Table 1, due to structural similarity. The genesis of these ions in each compound was similar, as proved by BlE and B2/E linked scans. Compound 7, for example, gives the fragmentation process shown in Scheme 2.

A marked feature of the group I compounds is the isomerization between sulphur and oxygen. Ions of rn l z 155 or rnlz 169 ( R = H or CH3, respectively) shown in Table 1 are composed of C,H,OSP and C7H60SP, proved by high resolution measurements. In the process of synthesis, no such isomerization takes place, as proved by infrared spectroscopy. This implies that the isomerization occurred after the

samples were introduced into the mass spectrometer, and the evidence shows that only oxygen at positions 2 and 3 undergoes isomerization, while oxygen con- nected with Z group is seldom affected.

Compounds 12-19 belong to group 11. The molecular ions of this group can clearly be seen in the spectra. If a McLafferty rearrangement can take place, the rearranged ion must be the base peak, otherwise rnlz 139 or mlz 153 will fill this role. Structurally, the main difference between the two groups is the replacement of an oxygen atom by a sulphur atom, which consequently made their mass spectra different. From Table 1 we found that despite the variation of the Z group, the ion of rnlz 139 (or rnlz 153) is always either the base peak or a peak with an abundance slightly less than that of the base peak. The McLafferty rearrangement in group 1 compounds refers to the transfer of the double bond between P and S to 0, with the simultaneous rupture of a C-0 single bond and the migration of a hydrogen atom. Since the P=O double bond is more stable than that of P==S, the transfer of the double bond lowers the energy of the molecular ion and makes the rearrangement a more favourable process. In compounds of Group 11, such rearrangements involve the transfer of the double bond from one sulphur to another (Scheme 1). Evidently, such transfers do not lower the energy of the molecular ion. Compared with the formation of rnlz 139 or rnlz 153, it seems that

rnlz 63 rnlz 155 rn lz 108

rnlz 342 [MI+'

rnlz 171 rnlz 139 rnlz 92 mlz 64

rc ' I -1. n ?+'

mlz 80 rnlz 52 rnlz 63 rnlz 108

Scheme 2

286 G. HANGZHOU, K. TSAO, L. YU-GUI AND WANGJIAN

139

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the two processes are energetically similar. So, the probability for both fragmentations is similar if the McLafferty rearrangement can take place. As a result, the ion of rnlz 139 (or mlz 153) will always be either very abundant or the base peak.

From Table 1, we found no ion of mlz 155 (or rn lz 169) in the group I1 compounds, where the isomerization between 0 and S atoms would occur. The two groups can be distinguished easily from each other by the existence or not of this ion in the mass spectra.

Certainly, all compounds in group I11 containing one nitrogen atom in the molecule exhibit molecular ions of odd mass number. In most cases ions of rn lz 139 or rnlz 153, or immonium ions, provide the base peak. As most of the nitrogen-containing compounds always show abundant immonium ion peaks, the P-N single bond is easy to cleave under electron bombardment. A distinguishing feature in the mass spectra of this group occurs when Z is an alkyl group. A peak at [M - 33]+ is seen due to the loss of 'HS radical. It is worthwhile considering the mechanism of this loss. In the three groups (Y = 0, S and N, respectively), the bond length pf P-N (c 1.49A is shorter than that of P-0 (1.67A) and P-S (1.85 A ) ,5 meanwhile the bond angle of LPNC is smaller than that of LPOC and LPSC (Fig. 1). Sterically, this results in the hydrogen from the alkyl group being bonded to nitrogen more closely than to the sulphur atom, causing the loss of 'HS from the molecular ion. If the alkyl group contains more than two carbon atoms, the probability of 'HS loss is increased.

If Z is a phenyl group, such as in compound 30, it exhibits no [M-HS]+ ion. Evidently, this can be explained by the fact that the aromatic hydrogen is difficult to lose because of higher bond energy; additionally, the IIS'O p-C1C6H4-NH- system can render the double bond between S and P atoms more stable, so that no loss of 'HS occurs. On the contrary, the abundance of molecular ion is increased. Figure 2 shows the bar spectra of compounds 10, 16 and 24, typical for each of the three groups.

An interesting problem arises as to why the isomerization between sulphur and oxygen can only occur in group I compounds. This can be explained by the structure of the molecule and the bond energy.6 After the molecular ion is formed in the ion source it tends to lose its surplus energy by rearrangement and bond cleavage, since the molecules of group I are composed of C-0, C-C and P-0 single bonds and C==C and P=S double bonds. The P-0 single bond is

5 0 1 1513 mlz

rnlz

2 18 Ib) I I

50 1001 150 200 250 388 mlz

Figure 2. Mass spectra of compounds 10 (a), 16 (b) and 24 (c).

weaker than any other bonds in the molecule. Certainly, further fragmentation of the molecular ion will first cleave the P-0 single bond. Energetically, there are no great differences among the three P-0 single bonds in the molecule. The cleavage may occur at either bond 1, 2 or 3 (Scheme 3). In the former case, we detected an ion of mlz 171 or mlz 185 ( R = H or CH3). In the latter case, [M]+'-+a. The formation of ion a causes the phosphorus-containing five-membered ring to be destroyed. Thus, the sulphur atom can reach a position close to oxygen (b) , due to the rotation of the P-0 single bond, exchange with each other (c), and further rotate to a stable position with ring closure (d). It has been demonstrated that this isomerization can only occur in molecular ions. In contrast, further fragmentations of the molecular ions of group I1 and I11 arose from cleavage of the P-S and P-N single bonds, which are the weakest in the molecules. Thus, the phosphorus- containing five-membered ring remains rigid, so that the sulphur double bonded with phosphorus could not get close enough to oxygen for isomerization to occur. Metastable peaks showed that only rnlz 171, rn lz 204

(R = H), N and [M - HS]+ ions could be formed

from the molecular ions of group I1 and 111.

+ ,Z 'Z

MASS SPECTROMETRIC STUDIES OF 1,3,2-BENZODIOXAPHOSPHOLES 287

cleave bond 1 I

mlz 171 or mlz 185

a b

I d C

Scheme 3

CONCLUSION

It is clear that the heterocyclic organophosphorus compounds, 1 ,3,2-benzodioxaphospholes, all exhibit similar characteristic ions, generated from their similar moieties in the molecules. The isomerization between sulphur and oxygen can only occur when the phosphorus-containing five-membered ring is dest- royed in the molecular ion; this became the marked feature of group I compounds in their mass spectra. When Y = S, m / z 139 or mlz 153 will always be the base or very abundant peak. The distinguishing character in the mass spectra for group I11 is the existence of the [M - HS]+ ion. Evidently, R being H or CH3 did not influence fragmentation.

EXPERIMENTAL

mass spectrometer controlled by a PDP11/34 compu- ter, at a resolving power of 900 and 10000, respectively. Samples were introduced via a direct probe. Source temperature was c. 180°C. B/E and B2/E linked scans were performed on a VG MICROMASS ZAB-2F mass spectrometer. In all instances, the accelerating voltage was 8 kV and the electron energy was 70eV. These samples had not been previously described, except 27 and 29. They were purified by recrystallization and distillation and showed no detectable impurities in field desorption mass spectrometry. All compounds studied in this work were synthesized by the method described in Refs 1 and 2.

Acknowledgements

The electron impact spectra measurements were obtained by

and exact mass using a MAT 711

The authors are grateful to Professor Lu Yongquan and Mr Tang Bingsheng, Instrument Centre, Academy of Military Medical Sciences, for their support and technical assistance.

REFERENCES

1. Li Yu-Gui, Guo Hangzhou et al., Acta Chim. Sin. 42, 353 4. W. L. F. Armarego, Stereochemistry of Heterocyclic

2. Li Yu-Gui, Guo Hangzhou et a/., Acta Chim. Sin. 42, 1192 5. D. E. C. Corbridge, Phosphorus, Elsevier Scientific

3. D. Barton and W. D. Ollis, Comprehensive Organic 6. K. Schofield, Organic Chemistry, Ser. 2, Vol. 4, p. 310,

(1984).

( 1984).

Chemistry, Vol. 2, Part 10, Pergamon Press, Oxford (1978).

Compounds, Part 2, p. 340, John Wiley, New York (1977).

Publishing Company, Amsterdam (1978).

Butterworths London (1975).