Mass spectrometric fragmentation reactions of dialkylselenides 1

Mass spectrometric fragmentation and especially rearrangement reactions of dialkyl selenides after electron ionization will be discussed.


Introduction
In contrast to the ample investigations of the electron impact induced fragmentation behavior of aliphatic ethers and thioethers little is known about that of the corresponding selenides [2].So far, the mass spectra of several R 2 Se (R = CH 3 to n-C 4 H 9 ) [3,4] and CH 3 SeR (R = C 2 H 5 to n-C 4 H 9 ) [5] representatives have been published, but suggested fragmentation paths were not confirmed by labelling studies.These results convey the following general picture: -The relative abundance of M +• is higher than for the analogous O or S compounds.Due to the lower ionization energy the percentage of molecular species with insufficient amounts of internal energy for fragmentation is higher.-Cleavage of one Se-R bond is a preferred process, but for higher alkyl groups the charge stays preferentially with the alkyl residue (for R 2 Se with R C 2 H 5 loss of • SeH from RSeH + may contribute to this ion); only CH 3 Se + is of medium to high abundance.
α-cleavage is not favored (only CH 3 SeCH + 2 is of medium abundance), also (with increasing chain length) β-and higher cleavages are of minor importance.
-RSeH +• is of high abundance and -as appropriate m * show -may decompose further by the loss of • SeH yielding R + .-H 2 Se +• from (CH 3 ) 2 Se (rel.int.> 70%) is (at least partially) formed directly from M +• as appropriate m * for this compound and for its d 6 -analog show.Although the observation of a m * does not necessarily imply that the neutral (C 2 H 4 ) is lost as an entity, elimination of two CH 2 -groups or any other combination of neutrals not comprising a C 2 -unit would be highly unlikely for energetic reasons.Whatever rearrangement may precede the elimination of C 2 H 4 (e.g., [6]; regarding the structure of {CH 3 Se} + see below.For (C 2 H 5 ) 2 Se H 2 Se + still has a rel.int. of 44%, but for higher dialkylselenides it becomes neglegible.
To get information regarding possible rearrangement processes during fragment formation of selenoethers n-butyl-methyl and n-butyl-ethyl selenides were synthesized with deuterium labels in every position, as well as an α-13 C-analog.The results obtained at 70 eV and after collision induced decomposition will be presented.

Instrumentation
The mass spectra were measured with a H-SQ 30 tandem (BEqQ) instrument (Finnigan-MAT, Bremen, Germany) using a EI/CI ion source (temp.150 • C, ionization energy 70 eV).All spectra are a mean of 10-15 scans.Collision activation (CA) was effected in the first (rf-only) quadrupole, Ar pressure (measured at the housing) 6 × 10 −4 Pa resulting in an intensity reduction of the parent ion to about 30%, translation energy of the ions entering the collision quadrupole 70 eV, averaging of 12-16 spectra.
An indirect inlet system for volatile compounds was not available.To get nevertheless a constant sample stream to the ion source the vacuum lock was used as a storage volume: 1-2 µl sample were deposited in an Al crucible the opening of which was squeezed tight and which was introduced into the vacuum lock with the direct probe.After pumping for 2-3 s residual air had been removed.The pump valve was closed and that to the ion source was opened carefully just to maintain a constant stream of substance into the source (3 × 10 −4 Pa, up to 1 h).After each analysis the vacuum lock was baked out.
All mass spectra are normalized for 80 Se and corrected for natural 13 C. NMR spectra were obtained with AM 300, AC 300 or AC 80 spectrometers (Bruker, Rheinstetten, Germany), 300 or 80 MHz for 1 H, 75.5 MHz for H-decoupled 13 C; solvent CDCl 3 , shifts δ ppm.

Syntheses
n-butyl methyl selenide (1).To 50 ml NH 3 condensed in a cooled (−78 • C) 100 ml 3-neck flask equipped with dropping funnel and gas inlet 9.4 g dimethyl diselenide and in small pieces ca.2.3 g Na are added until the blue color persists.NH 3 is allowed to vaporize, under a N 2 -stream 50 ml degassed H 2 O and subsequently 13.7 g n-butyl bromide are added.After refluxing for 1 h the selenide is extracted with 50 ml ether and after drying with MgSO 4 purified by distillation over a Vigreux column.Yield 8.5 g (56%), bp 138 Labelled compounds.The labelled butyl bromides were prepared by standard methods from the corresponding labelled butanols by reaction with PBr 3 .1,1-[ 2 H] 2 -butanol was obtained by reduction of butyric acid anhydride with LiAlD 4 .LiAlD 4 reduction of propionic acid anhydride gave 1,1-[ 2 H] 2 -propanol which was treated with PBr 3 to give 1,1-[ 2 H] 2 -propyl bromide, from which by a Grignard reaction 2,2-[ 2 H] 2 -butyric acid was obtained and subsequently reduced with LiAlH 4 .LiAlD 4 -reduction of acetic acid anhydride gave 1,1-[ 2 H] 2 -ethanol which was transformed to 1,1-[ 2 H] 2 -ethyl bromide and to the corresponding Grignard reagent, which was reacted with ethylene oxide to give 3,3-[ 2 H] 2 -butanol.In the Table 1 Molecular ion region of 1 and 2 and of their labelled analogs (normalized for 80 Se and corrected for natural 13  same way 4,4,4-[ 2 H] 3 -butanol was obtained starting from CD 3 COOD.CD 3 Br and C 2 D 5 Br were synthesized from commercial CD 3 OH and C 2 D 5 OH, respectively.1-[ 13 C]-butanol was prepared by reaction of n-propyl Grignard reagent with 13 CO 2 obtained from Ba 13 CO 3 (90%).The labelled selenides were checked by their 1 H-NMR and mass spectra.The molecular ion regions (normalized for 80 Se and corrected for natural 13 C) are given in Table 1.In can be seen that for the compounds prepared by reaction of labelled ethyl Grignard reagent with ethylene oxide appreciable overdeuteriation is observed which was evident also in for the labelled butyl bromides.

Loss of the smaller alkyl group
The [M-CH 3 ] + -ion of n-butyl methyl selenide (1) (Fig. 1) stemms solely from the loss of the CH 3 -Se-group; the CD 3 -Se-group of 1e is lost while all the labels of the butyl chain are retained (Table 2).Elimination of the terminal methyl from the butyl chain plays apparently no role.This is not the case for the competition between ethyl loss from ethyl-n-butyl selenide (2) (Fig. 2) (m/z 137) by cleavage of the C,Se-bond and by degradation of the butyl chain (see below) which occur in a ratio of ∼ 2 : 3 (see Table 3, 2e).Extensive studies with thioethers led to the conclusion [7] that alkyl loss is not a simple C,S-bond cleavage, but rather a two-step process initiated by a 1,2-hydrogen shift with subsequent loss of the other alkyl group yielding the more stable protonated thioaldehyde (R-CH=SeH + ) rather than RCH 2 S + .It is, therefore, likely that the [M-• R] + -ions of 1 and 2 have the structure of a protonated butaneselenal (cf.also below).
The loss of C 2 H 5 from the Se-ethyl group is accompanied to a minor extent by that of C 2 H 4 (m/z 138) probably giving ionized butaneselenol.The absence of m/z 143 and the presence of m/z 139 for 2e demonstrates that the Se-ethyl is involved (cf.below the loss of C 4 H 8 ) in accordance with the observation that 1 does not lose C 2 H 4 , while for diethyl selenide [M-C 2 H 4 ] +• is the main process [3].

Cleavage of the Se-butyl bond
The main processes for 1 and 2 involve the cleavage of the C(butyl),Se-bond.Loss of C 4 H 8 is of high abundance for 1 (m/z 96) and the main process for 2 (m/z 110).That the smaller alkyl group is retained in these reactions follows from the appropriate mass shifts for 1e (m/z 99) and 2e (m/z 115).The analogs labelled in the butyl chain show that a hydrogen is preferentially transferred back from the β-and γ-positions yielding distonic ions which lose C 4 H 8 and result in ionized selenols.
In contrast to the elimination of the smaller alkyl group loss of the entire butyl chain is of secondary importance.From the spectra of 1e and 2e it follows that H-losses from the smaller alkyl group (further decomposition of the selenol formed by the loss of C 4 H 8 ) do not contribute to a major extent.
As mentioned in the introduction for higher alkyl groups the charge stays to an appreciable extent with the alkyl group (m/z 57) after cleavage of the C,Se-bond.This ion is accompanied by m/z 56 formed by the loss of RSeH from M +• .Mass shifts of the labelled analogs show that the hydrogen stems from various positions of the butyl chain.An exact evaluation of the shift data is not possible since m/z 55 can be formed from different precursors (cf.below) and thus the contributions to the masses from the various processes overlap.

Cleavages of the butyl chain 3.3.1. α-cleavage (loss of propyl)
In contrast to the O-and S-analogs α-cleavage is not the main process for Se ethers.The spectra of the labelled analogs show that this process occurs essentially without any H migrations.

β-cleavage (loss of ethyl)
The almost complete shifts in the spectra of 1a and 1b from m/z 123 to 125 and for 1f to m/z 124 show that the α-and β-CH 2 -groups are retained.The contribution to m/z 124 in the case of 1c and 1d are due to the over-deuteriation mentioned in Section 2. Thus the γ-, δ-ethyl group is lost without H-scrambling.For 2 the results are the same as far as the butyl group is concerned, but superimposed is the loss of the Se-ethyl group (see above).
For the [M-• C 2 H 5 ] + -ions several structures can de discussed, such as a − d/d .Some information can be derived from the CA spectra, though one must keep in mind that by this method long-lived low-energy species are selected which may be the result of internal rearrangement processes.They possibly differ in their structure from species obtained in the ion source from high-energy ions fragmenting further before reaching the collision region.From the C 3 H 7 Se + -ions of different mechanistic origin the α-cleavage product c of 2 (see above) shows a distinct fragmentation pattern upon CA, which can be correlated with its assumed structure: Both, loss of C 2 H 4 (m/z 95, "onium"-reaction) and C 2 H + 5 (m/z 29) are of high abundance.The genesis of the main fragment C 3 H + 5 (m/z 41) implies the loss of H 2 Se and hence a rearrangement (cf. the discussion in the introduction).
Clearly different from the CA pattern of c, but unexpectedly identical are those of the [M-• C 2 H 5 ] + ion of 1 and of sec-butyl methyl selenide (3).The latter one should be formed by α-cleavage and hence have the structure d.The formation of the main fragment C 3 H + 5 (m/z 41) would have been anticipated from the behavior of c (rearrangement accompanying the loss of H 2 Se), but the loss of C 2 H 4 (m/z 95) is not self-evident unless one assumes a loss of CHCH 3 with a concomitant H-migration to give CH 2 =SH + .In any case, an equilibrium between d and c can be excluded due to the absence of m/z 29.
For the β-cleavage product of the S-analog of 1 a thiacyclopropane structure corresponding to e was suggested [8].Structure d would explain the loss of C 2 H 4 and the identical CA spectra of 1a and 1b (the α-and β-CD 2 -groups would have become identical).However, the loss of C 2 H 3 D (m/z 96) in requires H-rearrangements preceding or accompanying the fragment formation.An explanation would be an isomarization to a common structure/a mixture of common structures prior to CA fragmentation (as it is discussed for various {C 3 H 7 S} + ions [9,10]) or an isomerisation of 1 to 3 as it was shown for n-butyl amine [11] (Scheme 1).In this case the β-cleavage product of 2 would have the structure b rather than b .Why the loss of C 2 H 4−n D n is of low abundance only, cannot be explained.

Conclusions
α-cleavage dominant for N-, O-and S-analogs is of minor importance an consequently the subsequent reactions as "onium" or McLafferty decomposition are essentially absent.The prominent processes of Se-ethers involve cleavage of the C,Se-bonds where the charge may remain to an appreciable extent with

5 (
C 2 D 5 SeC 4 H 9 (2e) offers a unique possibility to differentiate between two isomeric structures: The ions formed by the loss of C 2 D 5 (m/z 137) and of C 2 H 5 (m/z 142) are far enough apart so that there is now interference with ions containing heavy isotopes of Se.From Table 4 it can be seen that [M-• C 2 D 5 ] + (which possesses most probably structure a) yields m/z 55 (C 4 H + 7 ) as the main fragment accompanied by C 2 H + 5 and C 2 H + 3 (m/z 29 and 27).For [M-• C 2 H 5 ] + in contrast C 2 D + m/z 34) is the most abundant fragment accompanied by C 2 D + 3 (m/z 30) while the C 4 -region comprises mainly C 4 H 4 D 3 to C 4 H 2 D 5 (m/z 58-60).The two ions clearly differ in their structure.

Table 2
Mass spectra of 1 and its labelled analogs (normalized for 80 Se and corrected for 13 C) Fig. 2. Mass spectrum of n-butyl ethyl selenide (normalied for 80 Se and corrected for natural 13 C).

Table 3
Mass spectra and 2 and its labelled analogs (normalized for 80 Se and corrected for 13 C)

Table 4
H 2 D 2 (m/z 95) and the formation of C 3 H 4 D (m/z 42) in addition to C 3 H 3 D 2 (m/z 43)