Preliminary results of two types of experiments at Cornell show that sili­con substituents on benzoic acid be­have as if they possess a negative Hammett σ-constant—that is, they act just as predicted on the basis of simple electronegativity arguments. There­fore, the Cornell chemists reason, there is no need to invoke additional pi bonding to rationalize results.

In the first series of laboratory tests, the team measured intrinsic chemical shifts of p-trimethyl-M-benzoic acid (M equals carbon or silicon) in an inert solvent such as carbon tetrachlo­ride. The second study focused on chemical shifts of the same com­pounds in a hydrogen-bonding inter­action with a Lewis base ( pyridine ).

The chemists note that the NMR spectroscopic method improves the determination of polar effects of sub­stituent groups in ground-state neu­tral molecules. NMR results are free of complications inherent in the use of chemical reactions and yet still par­allel results of chemical studies on similar systems.

Results at Cornell make Dr. Zucker-man skeptical that previous meticu­lous landmark studies on organosili-con acid systems have as straight­forward an interpretation as once thought. Complicating factors include the complexity of acid dissociation in differing solvent systems (evidenced by markedly differing experimental re­sults in the chemical literature) and varying relevancy of reaction param­eters (enthalpy of dissociation, free energy, and equilibrium constants ).

Earlier work at Cornell (with Dr. C. H. Yoder, now at Franklin and Marshall College) on transmission of substituent effects in diphenylimid-azolidines and diphenylsilaimidazoli-dines also indicated possible absence of (p-^d)-7r bonding in the silicon-nitrogen system. Supporting evidence came from measurements of 15N—H coupling constants in 15N-substituted isotopomers of N-trimethylsilyl-, germyl-, and stannylaniline. This work was done with Dr. E. W. Ran­dall (now at Queen Mary College, London).

The response to Dr. Zuckerman's agnostic view of pi bonding in silicon compounds was mixed. Some be­lieve that there's too little evidence to support his contentions yet, but would agree that the topic is worth further investigation. A bit of data supporting the Cornell chemist was announced at the meeting. Com­menting from the floor during the dis­cussion period, Dr. A. G. MacDiarmid of the University of Pennsylvania said that he and Dr. F. Rabel have done computer analyses of the proton magnetic resonance spectrum of the phenyl group in (CH 3 ) 3 Si(C 6 H 5 ) .

The spectrum shows that the res­onance of the para proton, whose po­sition may be largely controlled by resonance effects, has no significant downfield shift. Such a shift might be expected, Dr. McDiarmid says, if sig­nificant (p—»d)-7T bonding between the silicon and the phenyl group were present.

Dr. Zuckerman foresees considera­ble interest in silicon bonding at a symposium on the role of d orbitals in bonding at Oxford University next January. Sponsor of the symposium is the Chemical Society (London).

Rearrangement involves Si move between Ν atoms

ACS NATIONAL MEETING RD Inorganic Chemistry

Certain organosilyl-substituted ethyl-enediamines undergo 1,4-anionic re­arrangement, with silicon moving from one nitrogen atom to another. In other cases, a novel condensation re­action can take place with the elimi­nation of methyllithium, yielding a silicon-substituted imidazolidine. This research carried out at the University of Wisconsin by Dr. Mitsuo Ishikawa, on leave from Kyoto University (Ja­pan) , and Dr. Robert West, extends the 1,2-anionic rearrangement of hy­

drazines discovered earlier by Dr. West and his coworker Dr. Robert E. Bailey.

The first compounds studied by Dr. Ishikawa and Dr. West were N-phenylethylenediamines substituted on each nitrogen atom by an organo-silyl group. For such compounds, Dr. Ishikawa says, rearrangement of the anion is favored thermodynamically because the adjacent phenyl ring de-localizes the negative charge in the rearranged anion.

A typical starting material used by the Wisconsin team was N-phenyl-Ν,Ν' -b is - ( trimethylsilyl ) ethylenedi-amine, prepared from N-phenylethyl-enediamine and trimethylchlorosilane. When this compound is treated with n-butyllithium in ether, followed by pyrrole (a protonating agent), the product is the rearranged isomer, with both organosilyl groups bonded to the same nitrogen atom.

The two isomers were distinguished by nuclear magnetic resonance spec­troscopy. The starting material shows two different methylsilyl proton resonances at 10.01 and 9.8IT; the rearranged product (in which all the silylmethyl groups are equivalent) shows only a single Si—C—H resonance at 9.84r.

Rearrangement to the N-phenyl-N/,N,-bis(trialkylsilyl) isomer takes place at room temperature in ether, with a half-time of about 15 minutes. A similar 1,4 rearrangement is ob-

1,4-Rearrangement and condensation are anionic reactions of silyl ethylenediamine

When a phenyl group is present, rearrangement is found

/CM1-CH2 CHf-CH*

ffokjf Si(CH3)3

But replace phenyl with methyl, condensation takes place

Silicon imidazolidine + CH3U

54 C&EN APRIL 24, 1967

GAS CHROMATOGRAM. Dr. Mitsuo Ishikawa (right) and Dr. Robert West ex­amine gas chromatogram for traces of reaction products. The technique was used to separate silylethylenediamines

served if the two nitrogens are at­tached to an aromatic ring, as in N-phenyl-N,N'-bis ( trimethylsilyl ) -o-phenylenediamine. According to Dr. Ishikawa and Dr. West, other 1,4-anionic rearrangements have been ob­served previously, but none in which substituents moved between nitrogen atoms.

They believe, however, as in hydra­zine rearrangement, that the move­ment of an organosilicon substituent is especially favored because the sili­con atom can use a 3d orbital to form a low-energy, pentacovalent transition state.

When they used methylethylenedi-amine instead of a phenylethylene-diamine as the starting material, re­arrangement did not take place at room temperature. However, at higher temperatures (65° C. in tetra-hydrof uran ), N-metnyl-N,N'-bis- ( tri­methylsilyl ) ethylenediamine under­went a totally unexpected condensa­tion reaction.

In that reaction, a methyl group from one of the silicon atoms was eliminated as methyllithium, and the compound formed silicon imidazoli-dine, a five-membered ring compound. The structure of this new product, Dr. West says, was established by NMR and IR spectroscopy.

It isn't known how general the new condensation reaction is, but it has

also been observed for tris (trimethyl­silyl) ethylenediamine. This com­pound condenses to a previously known compound, l,3-bis(trimethyl­silyl ) -2,2-dimethyl-2-silaimidazolidine.

Dr. Ishikawa and Dr. West point out that this new condensation reac­tion is remarkable in that a C-Li compound is formed from an N-Li compound, even though the N-Li bond is much more stable. The driving force for the condensation reaction, the Wisconsin chemists say, must be the formation of the highly stable N-Si-N-Si structure in the imidazolidine com­pound. The condensation reaction may provide ways of synthesizing novel Si-N-Si polymers, Dr. West says, and such reactions are now un­der study.

Pt complex catalyzes novel hydrogénation Certain complexes of platinum, pal­ladium, and nickel catalyze the hy­drogénation of all but one double bond of methyl linoleate and lino-lenate. The hydrogénations may oc­cur either with elemental hydrogen or with methanol. In work at the Uni­versity of Illinois, Dr. John C. Bailar, Jr., and Dr. Hiroshi Itatani (now at Ube Industries in Japan) found that the hydrogénation of linoleate is pre­ceded by conversion of cis double bonds to trans and by migration of the double bonds to form a con­jugated system. The transition metal complexes may also cause the cis-trans isomerization of methyl oleate.

In a typical experiment, they hydro-genated methyl linoleate with dichloro-b i s ( t r iphenylphosphine ) platinum ( II ) alone and found the reaction yields 4 .1% monoenate and no conjugated diene. Adition of tin (II) chloride, however, increased the yield of monoenate to 14.4% and of con­jugated diene to 43.2% [J. Am. Chem. Soc, 89, 1592 (1967)] .

The discovery and isolation of transition metal complexes containing the hydrido ligand permit the study of their relationship to catalytic hy­drogénation. This catalytic activity is associated with the low-lying un­filled or f orbitals in the metal which can form weak bonds with the hydride ion by accepting electrons from it.

In a solvent mixture of methanol and benzene, platinum complexes of this type catalyze isomerization reac­tions as well as hydrogénation, under either nitrogen or hydrogen pressure. The methanol in the mixture some­times is the hydrogen source. Simi­lar complexes of palladium and nickel also act as catalysts.

The hydrogénations in the Illinois

5ΪΞ co O S ο • in Γ ^ * C O . * =c= g ë1 ί

•S > , LkJ S "ô < Q <U *·* • ! • ttf> ~σ Ο ζ

w w w w w t o i Ε -S r- h ^

™ S S > N I L U ^ | ^

APRIL 24. 1967 C&EN 55