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Nature of the УSilicon -EffectФ in Allyltrimethylsilane.

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The R spectrum of liquid ( I ) is complex owing to the low
C, symmetry assumed for each of the possible structures.
However, it does rule out an ionic structure
[(CH,),P] OCH;, as well as significant concentrations
of free ylide and methanol, whose presence could be expected from eq. (1).In contrast, real analogies to (covalently
bonded) (CH3),SbOCH,[61 are apparent.
+
we derive a clear argument in favor of Si-C hyperconjugation from the photoelectron (PE) spectra of trimethylvinylsilane (I) and trimethylallylsilane ( Z j and approximate
quantum-chemical calculations (CNDO/2 method)[,].
Fig. 1 shows the PE spectra of ( I ) and (2)I4] and Fig. 2
the experimental vertical ionization potentials ( I P ) and the
With a large excess of methanol, ( I ) gives a coloriess crystalline adduct of composition (CH,),POCH, -2C13,OH
and of unknown structure (m. p. 62-64 “C).
I
(CH,),P=CH, and ethanol give the colorless distillable
compound tetramethyl(ethoxy)phosphorane (b. p. 50°C/
30 torr) which exhibits similar spectroscopic properties to
( I ) . The reaction products from isopropanol and tertbutanol decompose slowly at room temperature to give
phosphane oxides. Similar reactions occur in the case of
trimethylsilanol. The last-named alcohols, however, also
give highly stable adducts having a greater alcohol content :
(CH3),POC,H,~2C,H,0H ;
(CH,),BOCH(CH,), -(CH,),CHOH ; and
Ill
14
(CH3),POC(CH,)3.(CH,),COH.
18
PO
18
20
I P Lev]--,
Addition of phenol to (CH,), +CH,
gives crystalhe
(CH,),POC,H, (m.p. 71-7S°C, decomp.) whose NMR
spectrum contains a doublet for the (CH,),P group at
25°C in CH,Cl,. The proton exchange analogous to eq. (1)
obviously becomes slower with increasing acidity of the
alcohol. In contrast, the exchange is accelerated with
increasing stability of the ylide : solutions of colorless
crystalline triphenyl(methy1)methoxyphosphorane formed on addition of CH,OH to (C6H,),P=CH, are weakly
tinged with the yellow color of the starting ylide, and the
CH,P doublet in the ‘H-NMR has coalesced to a singlet.
Received: October 27,1971 [Z 556b IE]
German version: Angew. Chem. 84, 166 (1972)
6
0
10
14
12
16
IP M I Fig. 1. Photoelectron spectra of trimethylvinylsjlane ( I ) and trimethylallylsilane (2).
[I]G. W Fenton and C . K . Ingold, J. Chem. SOC.1929, 2342; L. H e y
and C . K . Ingold, ibid. 1933, 531.
[2] E . g . : J . Dodonow and H . Medox, Ber. Dt. Chem. Ges. 61, 907
(1928); D. D. Cofmann and C . S. Maruel, J. Amer. Chem. SOC.51, 3496
(1929); L. Homer, H. Hoffmann, H. G. Wippel, and G. Hassel, Chem.
Ber. 91, 52 (1958); M . Grayson and P. T. Keough, J. Amer. Chem. SOC.
81, 4803 (1959); 82, 3919 (1960).
[3] H . Schmidbaur and W Tronich, Angew. Chem. 79,412 (1967); Angew. Chem. internat. Edit. 6,448 (1967); Chem. Ber. 101, 595 (1968).
141 B. C. Chang, W E. Conrad, D. B. Denney, D. Z . Denney, R. Edelmann, R . L. Powell, and D. W White, J. Amer. Chem. SOC.93,4004 (1971).
[5] H . Schmidbaur, K : H . Mitschke, and J . Weidlezn, Angew. Chem.
84,165 (1972); Angew. Chem. internat. Edit. fl, 144 (1972).
[6] H . Schmidbaur, K . - H . Mitschke, and J . Weidlein, Chem. Ber. 102,
4136 (1969).
8-
9-
aT9.8
In1
10 -
0 108
@)
2.4
IMe,Si-CI
IMe,Sii
0’09 lMe,SiCH,I
@
2.4
IMe3SiCHZ1
11.8
Nature of the “Silicon P-Effect”
in Allyltrimethylsilane[ll
By Ulrich Weidner and Armin Schweig“’
Allylsilanes, R,Si-CH,CH=CH,,
exhibit unusual spectral properties and a high reactivity towards electrophiles ;
furthermore, like other alkylsilanes having p-functions,
they are readily fragmented with breakage of the Si-C
bond[’]. This “silicon J3-effect” has been interpreted in
terms of two qualitative models, uiz. pd conjugation and
Si-C hyperconjugation[21.In the present communication
[*I
146
U. Weidner and Prof. Dr. A. Schweig
Physikalisch-chemisches Institut der Universitat
355 Marburg, Biegenstrasse 12 (Germany)
CHI= CH-SIPk,
Ill
CH,= CH-CH,-SiMe,
191
Fig. 2. Vertical ionization potentials (numbers above the levels in eV),
relative intensities (numbers under the levels), and assignment of the
bands @ and @@in the spectra of Fig. I.
Angew. Chem. internat. Edit. 1 Val. I 1 (1972)
1 No. 2
a@.
relative intensities of the first two bands @ and
A comparison with other vinylsilanes suggested that the
HOMO in ( I ) is the z MO having a node between the
ethylene z system and the Me,Si sub~tituents[~].
The band
@@is due to ionization of the a M O linking Me,% and C,
and of a second G MO localized in the Me$ group. The
shift of @) by 0.8eV can be regarded as a characteristic
The maximum of the
difference between (2) and
band @remains almost constant in the case of (2). This
band is due to ionization of two MOs localized in the
G skeleton of the Me,SiCH, groups[81.
Since all three properties that characterize the p-effectunusual spectral properties (above all bathochromic UV
shifts), high reactivity towards electrophiles, and ready
fragmentation with breakage of the Si-C bond-are
connected with the high energy of the HOMO and the
participation of the Si-C bond in the z system we can
deduce that the “silicon p-effect” has to be interpreted in
terms of Si-C hyperconjugation.
The reason for the unexpected raising of the HOMO in (2)
is understandable with the aid of Figure 3 which shows the
calculated orbital energies for one conformation of ( I ) and
two conformations of (2). In ( 2 a ) the CH,-Si bond is
perpendicular to the ethylenic z-electron clouds but practically parallel to them in ( 2 b ) ; the two conformations
undergo mutual interconversion on rotation about the
-CH,-CH=
bond. CNDO/2 model calculations were
performed for each of the three conformations with and
without inclusion of d AOs on the silicon atom (spd and sp
basis set respectively). They show that (2 b ) is more stable
[l] Theory and application of photoelectron spectroscopy, Part 4.We are indebted to the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie for their support of this work and to
the Deutsches Rechenzentrum, Darmstadt, for carrying out the calculations.-Part 5 : ref. [7].
[Z] A . kK P . Jarvie, Organometal. Chem. Rev. A 6 , 153 (1970). Hyperconjugation as a significant interaction mechanism between an SI-C
bond and a x system was first suggested as early as 1954: C. E . Eaborn
and S . H . Parker, J. Chem. SOC.1954, 939; ibid. 1956, 4858. For later
relevant work see: C. Eaborn, J. Organometal. Chem. 20, 49 (1969);
W Hanstein, H. J . Berwin, and 7: G . Traylor, J. Amer. Chem. SOC. 92,
829 (1970); C. G . Pitt, J. Organometal. Chem. 23, C 35 (1970); Chem.
Commun. 1971,816.
Received: October 11,1971 [ Z 555 IE]
German version: Angew. Chem. 84,167 (1972)
- 11 8
0-,
-121
‘-
-12 3
I~+SI-CH,I
0 -,
@ ---\
‘
132
\
-13 7
‘-.-138
I Me,Stl
\
‘ -153
\-
-15 2
In1
-154
I
-
/
- 15 8
inI
/
-163
,/
1261
l2ai
lli
Fig. 3. Calculated orbital energies (in eV) for one conformation of trimethylvinylsilane ( I ) and two conformations ( 2 a ) and
(2 b ) of trimethylallylsilane ( 2 ) .
than ( 2 a ) by 5.1 kcal/mol (Sp basis set) or 3.3 kcal/mol
(spd basis set); i. e. the conformation permitting CH,-Si
hyperconjugation is the more stable. It may be seen that
the observed elevation ofthe HOMO can only be explained
by conformation (2b). This elevation arises from a coupiing of the z-type CH,-S~ MO with the ethylenic a system having a node between -CH,-CH=.
According to
these findings, the elevation of the HOMO on going from
(’) can be
unequivocally by si-c hyperconjugation but not by pd conjugation.
~
Angew. Chem. internat. Edit.
Vol. I 1 (1972) N o . 2
[3] J . A . Pople and D. L. Beueridge: Approxmate Molecular Orbital
Theory. McGraw-Hill, New York 1970.
[41 The He I (584 A) PE spectra were recorded with the PS 16 spectrameter Produced by Perkin-Elmer Ltd., Beaconsfield (England).
c5]
U . Weidner and A . Schweig, unpublished results.
[6] Difference measured by mass spectrometry: 10eV. H. Bock and
H.Seidl, J. Organometal. Chem. 13, 87 (1968).
[7] W T h d and A . Schweig, Chem. Phys. Lett. 12,49 (1971).
[8] Note added in proof: A more detailed study of the second band in
the spectra of ( I ) and ( 2 ) has revealed that this band probably should
be attributed to three MOs.
147
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