4202
COiUNUNICATIONS TO
THE
EDITOR
Yo1 Y(i
(CH3)2Zn in 15 ml. of pentane. The vessel containing prolyl- L - leucyl- L -valine methyl ester (XIII, oil). the NbCls and pentane was cooled to - i s 3 , arid the Upon removal of benzyl groups from XI11 by hydro(CHJZZn was admitted in small portions because of genation a t 53" over palladium-charcoal, there was obthe exothermic character of the reaction. X precipitained the terminal-blocked octapeptide I, N-formyltate (probably ZnClz) was observed immediately folglycyl-a-L-aspartyl-L-serylg 1y c y l g 1y c y 1- L - p r o 1y 1- Llowing addition of the first portion of (CH3)2Zn. After Ieucyl-L-valine methyl ester, m.p. 192-193', [ a ]-84' ~ ; amino acid ratios in acid hydrolysate g1y3.12asp1.03sero.94-each addition the reaction mixture was warmed nearly to room temperature and agitated. After the final proo.&u0.9.d0.94. addition of (CH3)2Zn, all volatiles were removed a t The optical purity of certain of the peptides syntheroom temperature ; sized was det.ermined using appropriate enzyme systhe (CH3)3NbC1:!was trapped a t tems. The dipeptide V was completely hydrolyzed by -36'. The yield was l2.7Y6based on NbClb. Trileucine a m i n ~ p e p t i d a s e;' ~under similar conditions the methyldichlorotantalum was prepared in an identical manner. All manipulations were carried out in a high corresponding DL-peptide, m.p. 95-97', was only partially hydrolyzed. The tetrapeptide X I I , m.p. vacuum system. 219" dec. (prepared by hydrogenation of X I ) , was conTrimethyldichloroniobium forms golden yellow crysverted completely to its component amino acids by a tals which sublime readily under vacuum a t room temmixture of leucine aminopeptidase and prolidase. l 4 j 1 j perature. However, the compound will darken and Finally, the tetrapeptide VI11 was completely hydrorelease methane when left a t room temperature for lyzed by carboxypeptidase-A. several hours. It appears to be indefinitely stable a t - ~ ) .O The octapeptide I and various intermediate com--,x pounds are being investigated for possible esterase and Triniethyldichlorotantalum forms pale yellow cryspeptidase activity. In addition, compound I serves tals of similar volatility. This compound, however, as an intermediate for more extensive syntheses, inappears to be much more thermally unstable than the cluding its incorporation into macromolecular polyniobium analog. The thermal stability of both conipeptide systems containing other amino acids involved pounds was found to be adversely affected by small in proteolytic enzyme action. quantities of impurities. Both compounds are highly reactive toward water and air. Acknowledgment.-iVe are pleased to acknowledge the support of this work by U. S. Public Health Service The formulas of the new compounds were established Grant No. All-07300 and by the Office of the Surgeon by hydrolyzing freshly prepared samples in ZYLCLIO with General, Department of the Army. Mr. Richard aqueous KOH, measuring the resultant CH4 directly Maine provided competent technical assistance. l y e by means of a Sprengel pump, and determining niobium thank Dr. Elizabeth R . Simoris for participation in the and tantalum gravimetrically as the pentoxides. The enzymic work. chloride was also determined gravimetrically. The CHI was subsequently shown to be quantitatively pure by (1.1) K . Hofmann and H. Yajima, J . A m . Ckem. Soc., 83, 2239 (1961); K . H o f m a n n , H . Yajima, T . - Y . Liu. S . Yanaihara, C. Yanaihara, and J . I,. means of infrared and mass spectroscopy. For each Humes, i b i d . , 84, 4481 (1962). compound, all analytical data were determined inde(15) K . I,, Hill and U'.R . Schmidt, J . Bioi. Chem., 237, 389 (1962). D E P a R T M E X T OF BIOLOGICAL CHEMISTRY H. T. CHEUXC pendently on the same weighed sample. for CH3, 21.39; Nb, HARVARD MEDICAL SCHOOL T. SRINIVASA i M ~ .-1nai. ~ Calcd. ~ ~ (CH3)3XbC12: ~ BOSTOS,MASSACHL-SETTS E. K. BLOUT 44.47; C1, 33.94. Found: CH3, 21.6; Nb, 44.0; c1, RECEIVEDAUGCST24, 1964 3'4.0. .-1nal. Calcd. for (CH3)3TaC12: CH3, 13.19; T a , 60.93: C1, 23.88. Found: CH3, 14.5; T a , 63.0; C1, u-Bonded Alkyl Compounds of Niobium and 23.6. Tantalum. Trimethyldichloroniobium and In addition, samples of the new compounds were Trimethyldichlorotantalum subjected to slow hydrolysis by exposure to the air, and Sir: the residues were analyzed spectrographically. KO Recent developments in the organometallic chemissignificant quantities of zinc or other extraneous metals try of the elements of group V--4 have been confined were found to be present. mainly to arene complexes. T o this date, there have &Ittemptsto obtain the molecular weights by gas been no reports of a-bonded alkyl compounds of these density and vapor pressure depression measurements metals, although many attempts to prepare them have have not been successful because of the instability of been described.2 Accordingly, we wish to report the these compounds under the experimental conditions first successful syntheses of alkyl derivatives of nioused. bium and tantalum. Trimethyldichloroniobium, The mass spectrum of the niobium compound is ( CHa)3NbC12. and trimethyldichlorotantalurn, (CHa)aindicative of (CH3)3NbC12monomer, although parent TaClz. have been prepared by the low temperature peaks (mass 208, 210, 212) were not observable a t an exchange of methyl groups and chlorine between diionizing voltage of 70 e.v. Major fragments are the methylzinc and the pentachlorides of niobium and [(CH:j)2NbC1z]f ions a t masses 193, 193, and 197 (relatantalum, respectively. tive abundances of ,557, 37, and 6yG, respectively). In a typical experiment, 7.43 rnmoles of NbC1: was Proton nuclear magnetic resonance spectra were sublimed in wzcuo and treated with 1S.26 mmoles of obtained for (CH3)3XbClza t - 10' in CCli containing a trace of (CH,),Si. -411 spectra were obtained with a ( I ) This papel- pi-ewnts results of one phase of research carried o u t a t the Jet Propulsion 1.aboratory. California Institute of Technology. under Varian A-60spectrometer. Trimethyldichloroniobium Contract So. SXS7-100,sponsored b y the Sational Aeronautics and Space exhibits a peak (line width 0.6 c.P.s.) 29.8 C.P.S. upfield Administration from (CH3)rSi which is characteristic for protons on a ' , 2 ) F. A . Cntton. C h e m Re?:.,51, 351 ( 1 9 5 5 ) .
Oct. 5 , 1964
COMMUNICATIONS TO THE EDITOR
carbon atom directly bonded to a metal atom. For purposes of comparison, ( C H 3 ) ~ Z ngives a peak (line width 0.6 c.P.s.) 28.6 c.p.s. upfield from (CH3),Si. A sample containing both (CH&NbC12 and (CH3)zZn and ( C H s ) S in CCl, was also examined. Only one peak (line width 0.9 c.P.s.) 29.6 C . P . S . upfield from the internal standard was observed. T h e merging of the two peaks and the slight broadening are suggestive of methyl group exchange between (CH3)zZnand (CH3)3NbC12. N o n.m.r. spectra were obtained for (CH3)3TaC12 because of the rapid decomposition of this material in cc14 at - 1I ) ” . Samples of (CHs)&bC12 and (CH3)3TaC12were pyrolyzed by passage a t pressures
