COMMUNICATIONS TO THE EDITOR
Aug. 20, 1960
Fig. 1.-dX"/dH
Fig. 2.-dX"/dH
4425
vs. H for potassium ketyl of hexamethylacetone in THF.
vs.
H for sodium ketyl of hexamethylacetone in THF.
and suggest structure I as the major ingredient in the ground state of each of the ketyls. 0-
somewhat more complex because of the presence of the isotopes Li6 and Li7. The complexes with sodium and lithium are under further investigation. DEPARTMENT OF CHEMISTRY WASHINGTON UNIVERSITY h-OBORU HIROTA S. I. WEISSMAK SAINTLOUIS,MISSOURI RECEIVED JULY 11, 1960 N.M.R. SPECTRA AND STRUCTURE OF ALUMINUM METHYL CHLORIDES
Sir:
I1 0 -
I
CH,-C=C-C(
CHI)
(111)
Splittings by the methyl carbons and by the unique proton in pentamethylacetone could be accounted for by admixture of conjugated structures such as I1 and 111. (In view of the recent proliferation in names of conjugations we don't know which kind we are reporting.) '4dmixture of two or three per cent. of each of structure I1 and of about 0.5 per cent. of structure I11 could account for the observed splittings. The absence of splittings by the quaternary carbons is not easily explained. Spin polarization of the u bond between them and the central carbon might be expected to produce observable splittings. Probably other structures which produce spin density of opposite sign to that produced by I must be included. When sodium or lithium is used as reducing agent, the splittings by protons and CI3in the ketyls are almost identical with those described in the preceding paragraphs. In addition, splittings by two equivalent alkali metal nuclei per paramagnetic molecule are found. The sodium compound in tetrahydrofuran yields seven evenly spaced lines a t interval 1.58 oe. with intensities 1 : 2 : 3 : 4 : 3 : 2 : 1 (Fig. 2 ) . The spectrum of the lithium ketyl is
Recently Brownstein, et uL,l have published n.m.r. data on Ah(CH3)6, Alz(CH3)4C12 and Alz(CHa)zC14. The interpretation of their results for .412(CH3)4C12and Alz(CHs)zC14is based on the methyl bridge model for these two molecules deduced from Raman Spectra by Van der Kelen and Herman.2 However, for Alz(CH3)& a chlorine bridge structure was deduced from electron diffraction measurements by Brockway and D a ~ i d s o n from , ~ the infrared spectrum by S c h o m b ~ r g ,and ~ from infrared and Raman ~ o r k . ~The , ~ vibrational spectrum of AlZ(CH3)aC14 gives evidence of a centrosymmetrical structure6 but subsequent Raman work did not yet allow of a definite choice between the chlorine bridge model (Czb symmetry) and the methyl bridge model ( D z h symmetry). The n.m.r. spectra of Brownstein, et ul., were (1) S. Brownstein, B. C. Smith, G. Ehrlich and A. W. Laubengayer, THIS J O U H N A L , 83, 1000 (1960). (2) G. P. Van der Kelen and M. A. Herman, Bull. SOC. Chim. Belges, 66,350 (1956). (3) L. 0. Brockway and N. R. Davidson, THISJOURNAL, 68, 3287 (1911) (4) G. Schomburg, "Infrarot spektroskopische Untersuchungen a n aluminum-organischen Verbindungen," Diss., Aachen, 1956. ( 5 ) M. P. Groenewege, 2. physik Chemie, Neue Folge, 18, 147
(1958). ( 6 ) M.P . Groenewege, Revue Universe& des Mines ( L i e g e ) , 9 S6rie. 16,461 (1959).
COMMUNICATIONS TO THE EDITOR
4426
I
I
I
I
1
II
I
A2(CH3)2C14
I
Alz(CH3)a
0.52
- 1. 8 5
0 21
-1 83
-1.05b -2 20"
-1.82" -1.83 -1.85 Alz(CHS)aC12 -1.87 -1.85 -1.30 - 1.8 2 0.93 -1.GO I 0.46 - 1.60 412(CHa)2C14 0 . 19 -1.62 - 1.52O a Concentration in niole/l. of inoiionicr. lJ Bridge CH, Kef. 7. ' Ref. 1. Solugroups. Outer CH3 groups. bility too low. 1.20 0 . GO 0.24
'
I t can be seen from the table that the effect of dilution in cyclopentane solution is within the experimental error. The room temperature peak of A12(CHs)6occurs a t the weighted average of the two low temperature peaks. The effect of temperature is negligible in the case of AU2(CHs)rC12.The low-temperature spectrum of A12(CHa)2C14 could not be observed ( 7 ) S . hIuller and LJ. Ii. Pritchard, THISJ O U R N A L , 82, 218 (1960) ( 8 ) L. hI. Jackman, "Applications of N h l R Spectroscopy in Organic Chemistry," Pergarnon, h-ew York, 1959, p. 5 2 .
Vol. 82
because of the low solubility in cyclopentane at temperatures below 0 '. The compounds were prepared from ill and CH3Clb) according to Soll's procedure. Several conclusions can be drawn : A ~ z ( C I: ~Our ~ )observations ~ and conclusions are in good agreement with those of Muller and Pritchard' and Brownstein, et al.' The bridge CH3 group protons are less shielded than those of the outer CH3 groups. At room temperature a rapid exchange of bridge and outer CHa groups makes the two resonances coalesce into a single peak. Alz(CH3)4C12: The unsplit low temperature resonance can be explained readily in terms of the chlorine bridge model exhibiting four equivalent outer CH3 groups. The shift over about 0.35 p.p.m. to a lower field (see figure) with respect to the outer CH3 group protons in is seen as a consequence of the substitution of CH3 by C1. I t is in good agreement with our unpublished results for Ti(CHs)zClzand TiCH3C13where the proton resonance shifts over 0.30 p.p.m. to a lower field on substitution of CHs by C1. The low temperature spectrum observed does not support the methyl bridge model adhered to in references 1 and 2 . The bridge and outer CHs groups of this model would have to exchange rapidly even a t - 7 5 O . 9 Moreover, the substitution of two CH3-groups by C1 atoms in Al2(CH3)6is expected to shift the weight average resonance to a lower field. A12(CH3)zC14: The low temperature spectrum is expected to be almost identical with the room temperature spectrum. There is little reason to expect any splitting by unequivalent positions in this molecule. The effect of temperature on the resonance of A412(CH3)&12 was seen to be very small. The shift to a lower field over about 0.25 p p m . with respect to A12(CH3)4C12 and over about 0.60 p.p.m. with respect to the outer CHj groups of A12(CH3)6suggests that there are outer CH3 groups in this molecule as well. Finally it is improbable that the bridge CHa group protons of Al2(CH3)8would shift over about 0.5 p.p.m. to a higher field if four CH3 groups should be replaced by C1 atoms. Consequently, our N.m.r. data are thought to support the chlorine bridge hypothesis in both A ~ z (CH3)4C12 and Alz(CHZ)2C14. (9) T h e half width of t h e peak is invariably 1 hertz, even down t c l --86', which is in marked contrast with Alz(CHs)s where t h e broadening sets in B little below room temperature (the half width increases from 1 t o 7 hertz between 22 and - 3 G " ) .
M. P. GROENEWECE CENTRAL LABORATORY STAATSMIJNES IS LIMBL-RC J . SMIDT GELEEX,THESETHERLANDS 11. DE YRIES RECEIVED J U N E 29, 19GO
DIPROPYLCYCLOPROPENONE
Sir: Some time ago we reported' the synthesis of diphenylcyclopropenone, utilizing the reaction between phenylketene acetal and phenyl chlorocarbene, and only shortly thereafter Volpin and his coworkers described' an indepelident synthesis using (1) I
