All other such complexes, whether separated ions181 or con- tact ion pairs are present,"] have nearly pyramidal stannate ions as a result of the construction of o bonds with substan- tial Sn p character and the lone pair on Sn having mainly s character. The geometry of the Sn center in 1 implies a very different use of orbitals than in other stannate~.~']

The title compound 1 is rare among organotin(11) com- pounds in containing three aromatic ligands 7c-bonded to Sn. Polymerically linked (p-$-Cp),SnX units (X = p-F-BF; and THF) have been seen in the structure of {[(q5-Cp),Sn(p- $-Cp)Sn(thf)]+BF,},, produced by the reaction of BF, with [Cp,Sn] in THF (CpSn, ca. 2.29-3.67 A).[1o1 In or- thorhombic [Cp,Pb], (p-$-Cp),($-Cp)Pb units are present within its polymeric zigzag structure" (cf. the well-known bent structure of [Cp,St~][~l). Interestingly [Cp,Pb] forms a 1 : 1 electrolyte in On the basis of our current work, it seems most likely that [CpPb(thf)J+[Cp,Pb]- is present in solution, rather than the suggested [CpPb]+ and discrete cp- ions,[iz. 131

Finally, cryoscopic molecular mass determinations com- bined with variable-temperature 'H NMR spectroscopy show that 1 is involved in a dissociative equilibrium in aro- matic s01vents.l'~~ At room temperature 1 is completely dis- sociated into [($-Cp),Sn]- and [Na . PMDETA]+ ions. As the temperature is reduced (finally to ca. - 80 "C) these ions pair up to give intact 1. These results illustrate that the pad- dle wheel [($-Cp)$n]- ion has a separate existence.

Previous work has concentrated on the reactions of elec- trophiles such as Me1 with [Cp,Sn].['3*151 We are currently investigating the addition reactions (which produce mixed- ligand triorganostannates [Cp,RSn]-) and the substitution reactions (which produce mixed ligand organotin com- pounds [CpSnR]) of various nucleophiles, such as organo- lithium compounds, with [Cp,Sn]. To our knowledge, these are new approaches to triorganostannate complexes and organotin(1i) compounds. We are also investigating the use of [Cp,Sn]- as a new ligand to other main group metals.

Experimental Procedure

1: CpNa (1.25 mL, 2.0 molL-' in THF, 2.5 mmol) was added to a solution of [Cp,Sn] (0.623 g, 2.5 mmol) (preparation ref. [3]) and PMDETA (0.53 mL, 2.5 mmol) in THF (5 mL) at 20 "C under nitrogen. Stirring the reaction mixture (0.5 h) at 20°C gave an orange-red solution. The THF was removed under vacuum. replaced with toluene ( 5 mL), and the solution was filtered to remove a faint precipitate. Storage of the orange-red filtrate at 20 "C (48 h) gave air-sen- sitive. yellow crystals, identified as 1. First-batch yield 20%, m.p. 93-98°C to a yellow oil. IR (solid): J = 3079, 3061 cm-' (s, v, C-H, $-Cp), disappears upon exposure to air. 'H NMR (25 "C. C,D,): b = 6.00 (s, 15H; 7'- and N-7'- CpH; cf. 6 = 5.78 ('J(Sn,H) =15.7 Hz) in [Cp,Sn]), 1.80 (s, 15H; CH,N and (CH,),N), 1.65 (s, 8H; (CH2),). Correct C, H, N analysis.

Received: April 2, 1992 [Z 5274 IE] German version: Angew. Chem. 1992, 104, 1265

CAS Registry numbers: 1, 142981-18-8; CpNa, 4984-82-1; Cp2Sn, 1294-75-3

[l] D. Reed, D. Stalke, D. S. Wright, Angew. Chem. 1991, 103, 1539-1540; Angew Chem. I n t . Ed. Engl. 1991, 30, 1459-1460.

[2] D. R. Armstrong, M. G. Davidson, D. Moncrieff, D. Stalke, D. S. Wright, unpublished results.

[3] a) E. 0. Fischer. H. Grubert, Z . Naturforsch. B 1956, 11,423-424; b) A. Almenuingen, A. Haaland, T. Motzfeld, J. Organomet. Chem. 1967, 7, 97-104.

[41 X-ray structure data: C,,H,,N,NaSn, M = 510.25, monoclinic, space group P2,/c. a = 8.713(2), b = 17.221(3), c = 16.345(3) A, B = 96.61(3)", V = 2436.2(8) A3, Z = 4, fralcd = 1.392 Mg m-3, F(O00) =1056, 1 = 0.71073 A, T=153 K, p(MoK.) =1.081 mm-' . Data were collected on a Siemens-Stoe AED using an oil-coated, rapidly cooled crystal of dimen- sions 0.50 x 0.55 x 0.40 mm by the 2 O/o method (8" 5 2 0 I 60"). Of a total of 10529 collected reflections, 7091 were unique. The structure was

solved by direct methods (SHELXS 92) and refined by full least squares anF2 with all data to R , and wR, values of 0.026 and 0.050. respectively (SHELXL 92); largest difference peak and hole 0.39 and -0.33 e.k3, respectively. All hydrogen atoms were located in the difference Fourier map and their positions were refined freely with common refined U values for chemically equivalent atoms. Further details of the crystal structure investigations are available on request from the Director of the Cambridge Crystallographic Data Centre, University Chemical Laboratory. Lensfield Road, GB-Cambridge CB2 IEW (UK), by quoting the full journal cita- tion.

[S] T. Aoyagi, H. M. M. Shearer, K. Wade, G. Whitehead, J. Orgunornet. Chem. 1979, 175, 21-31.

[6] G . Rabe, H. W. Roesky, D. Stalke. F. Pauer, G. M. Sheldrick, J . Organomet. Chem. 1991, 403, 11 - 3 9.

[7] G. D. Smith, P. E. Fanwick, I . P. Rothwell, hurg. Chem. 1989, 26, 618- 620.

[S] T. Birchall, J. A. Vetrone, J. Chem. SOC. Chem. Commun. 1988, 877-879. [9] MO Calculations are currently being performed on the [(?'-Cp)Jn]- ion

and on models of 1 to probe the nature of the bonding in these. (101 T. S. Dory, J. J. Zuckerman, C. L. Barnes, J. Organomet. Chem. 1985,281,

CI-C7. [ll] a) E. 0. Fischer, H. Grubert, Z . Anorg. Allg. Chem. 1956,286,237-242;

b) C. Panattoni, G. Bombieri, U. Croatto, Acra Crystallogr. 1966, 21, 823-826.

[12] W. Strohmeier, H. Landsfeld, F. Gernet, Z . Electrochem. 1962, 66. 823- 827.

[13] P. Jutzi, Adv. Organomet. Chem. 1986,26, 217-288. [14] Cryoscopic molecular mass measurements (in benzene) show that two

solution species are present over a range of concentrations (n = 0.49 0.02 (0.015 molL-I); 0.51 0.02 (0.030molL-')). 'H NMR spectra of 1 at 25 "C (in C,D,CD,) have, apart from signals due to PMDE- TA, just one sharp signal at b = 5.97 due to discrete [(p'-Cp)3Sn]- ions. No signals below this could be attributed to CpNa. Reducing the temper- ature to ca. -40°C results in the splitting of the $-Cp signal. At ca. - 80 "C these two resonances are fully resolved at a ratio of 2: 1 ; 6 = 6.02 ($-Cp, 'J(Sn, H) = 29.4 Hz), and 6.00 (p-q'-Cp, 'J(Sn, H) = 28.8 Hz).

[15] J. W. Connolly, C. Hoff, Adv. Organomet. Chem. 1981, 19, 123-153.

Opening of an Aza-closo-dodecaborane to an Aza-nido-dodecaborate** By Franc Meyer, Jens Miiller, Peter Puetzold,* and Roland Boese

Aza-closo-dodecaborane NB, ,H , is isoelectronic with di- carba-closo-dodecaborane C,B,,H,, .I1] Upon attack by base the icosahedral framework of C,B,,H,, is broken down into the open frame of dicarba-nido-undecaborate C,B,H;, .['I We wanted to apply this reaction to NB, ,H,,. Its N-bound proton is so acidic, however, that it is removed even by a weak base like NEt, forming [HNEt,][NB,,H,,] 1.['] Thus, before the examination of the attack of a base on the NB,, framework, it was imperative to protect the N atom by alkylation. The salt 1 was treated with the strong methylating agent methyltriflate, and MeNB, ,H,, 2 was ob- tained [Eq. (a)]. The three ' 'B NMR signals of 2, their inten-

1 2

sity ratio of 5 : 5 : 1 , and the characteristic B-H coupling con- stant leave no doubt that NB,,H,, and its methyl derivative are structurally analogous.

[*] Prof. P. Paetzold, F. Meyer, Dr. J. Miiller Institut fur Anorganische Chemie der Technischen Hochschule Templergraben 55, D-W-5100 Aachen (FRG)

Priv.-Doz. R. Boese Institut fur Anorganische Cbemie der Universitat-Gesamthochschule Universitatsstrasse 5-7, D-W-4300 Essen (FRG)

[**I This research was supported by the Deutsche Forschungsgemeinschaft.

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Compound 2 adds methanol in the ratio 1 : 1 and thereby forms the acid H[MeNB, ,HI l(OMe)], which is completely dissociated in dichloromethane solvent in the presence of an excess of methanol. The acid cannot be isolated in a pure form; when solvent is removed under vacuum, the acid de- composes completely to yet-unknown fragments. If the salt [N(PPh,),]Cl is added to a solution of the acid, one obtains product 3 [Eq. (b)], the anion of which is identical to that of the acid as is evident from their identical NMR spectra.

3

The structure of the anion of 3 is derived from the hypo- thetical closo-tridecaborate Bl3H?; Ar3' by removal of the B3 atom. Seven signals are seen in the IIB NMR spectrum with the intensity ratio 1:2:2:2:1:2:1. Thus, three B atoms lie in one mirror plane, namely B2, B12, and B13, and one of these, B2, does not show B-H coupling. The 2D IIB-llB

Me

A anion of 3

NMR spectrum reflects the connectivity of the B,, frame- work; the lack of a cross peak between B2 and B4/B5 is expected for a E B bond bridged by an N atom.141 The 'H{"B} NMR spectrum also consists of seven signals of the B-bound H atoms with the intensity ratio 1:1:2:2:2:1:2; they could be assigned with the help of the 2D "B-'H NMR spectrum. Of special interest are the atoms B8/B9, each bound to a terminal H atom as well as a bridging H atom, and the atom B2, not bound to any H atom.

The unequivocal assignment of the spectroscopic data to the structure of the anion of 3 in solution is in agreement with the results of the X-ray structure analysis (Fig. l).t51

:.: c2 k . .

0

d Fig. 1. Crystal structure of the anion of 3. Selected distances [A], standard deviations in parentheses: N1-B2 1.490(5), N1-B4 1.520(9), NLB5 1.604(10), 82-84 2.004(9), BZ-BS 2.107(10), B2-B6 1.813(8), B2-B7 1.814(8), B4-B6 1.910(10), B4-B9 1.969(9), B4-Bl0 1.852(8), BS-B7 1.863(9), BS-B8 2.053(10), BS-Bll 1.724(8), B6-B7 1.797(10), B6-Bl0 1.782(10), B6-B12 1.788(9), B7-Bl2 1.740(8), B7-Bll 1.774(9), B8-Bll 1.714(10), B8-B9 1.819(11), B9-Bl0 1.766(10), B9-Bl3 1.727(9), B10-Bl2 1.755(11), B10-Bl3 1.820(11), Bll-B32 1.837(11). Bll-B13 1.791(11). B12-Bl3 1.821(6), Nl -Cl 1.464(5); B2-0 1.356(4).

Whereas all 21 of the B-B distances remote from nitrogen fall within the range expected between 1.71 and 1.91 A, the two distances B4-B9 and B5-B8 in the open pentagon of the cluster, as well as the distances B2-B4 and B2ZB5, bridged by the N atom, are all especially long (1.97-2.1 1 A). Accordingly, the triangles B2-B4-N and B2-B5-N deviate from equilateral with angles at the N atom of 83.5 and 85.8 O ,

respectively. The further away the cluster faces are from the N atom, the closer they approach regularity.

The nido structure found for the anion of 3 is exactly that expected. Removal of the B3 atom from the hypothetical closo skeleton A eliminates the unfavorable connectivity k = 6 of the atoms B4 and B5 in the cluster framework, which means sevenfold coordination when the the exo H atoms are also taken into account. Simultaneously, an opti- mal place is made available for the electronegative N atom in position 1 with k = 3. Atoms B8 and B9 attain the connectiv- ity k = 4, and are thus able to accommodate a bridging H atom on the mirror plane of the free anion of 3, whereas all the other B atoms with k = 5 are not likely to form H bridges. Two other compounds set the precedent for the nido structure found here. One is the compound nido- [(Cp*Rh)OBl,H,C1(PMe,Ph)] with the strongly electroneg- ative 0 atom in position 1, the Rh atom surprisingly in position 8 (transition metals usually prefer positions with higher connectivity), and the ligands CI and PMe,Ph in the positions 5 and 9, respectively.[61 In the second example, [(CpCo)Se2B,H,], the Co atom and the two Se atoms occu- py the respective positions 4, 1, and 9, so that the Se atoms have a minimum connectivity and the metal atom a maxi- mum. In addition, the metal atom has the Se atoms as desir- able neighbors.['] The nido compound Et,C,B,H, follows the same structural principle, but in this case the B4-B9 bond is opened so that the electronegative C atoms occupy positions 1 , 4, 8, and 9 with two of these positions having k = 3 and the other two k = 4.18' Other possibilities for ob- taining nido derivatives of A arise from the removal of atom B5, or of B1 with simultaneous opening of the B5-B8 bond.Ig1

The structure of the anion of 3 appears to be thermody- namically very stable, since 3 does not react solvolytically with either water or alcohols. The same anion is formed when 2 is treated with LiMe/tmeda, KOtBu/[l8]crown-6, or KF/[18]crown-6; however, instead of the base MeO-, the respective bases Me-, tBuO-, and F- are bound in the 2 position."'] In the formation of these anions, the bridging H atom apparently comes from the formerly terminal position 2, having been displaced by the base. Methanol may also be considered as a source of the p-H atom. However, in the methanolysis of 2 with CD,OD no deuterium is transferred to the bridging position. This proves that it is the H atom in the 2 position of 2 that is apparently shifted intramolecularly to the bridging position during the methanolysis, and in addition that this H atom does not undergo exchange with either the excess of methanol or the protons of the medium.

The attack of base on closo-MeNB,,H,, proceeds differ- ently to that on closo-C,B,,H,,. In the case reported, in- stead of the strongly basic alkoxide, weakly basic alcohol suffices; the twelve-vertex closo cluster is not broken down into an eleven-vertex nido cluster, but is rather opened to a twelve-vertex nido cluster.

Experimental Procedure 2: Methyltriflate (0.46 mL, 4.19 mmol) was added dropwise to a solution of 1 (1.02 g, 4.14 mmol) in 20 mL of CH,Cl, at -50°C. The reaction mixture was stirred at room temperature for 12h. The solvent was then removed under vacuum and the residue extracted with pentane (3 x 15 mL). The combined

1228 0 VCH Verlugsgesellschufi mbH, W-6940 Weinheim, 1992 OS70-0833192jO909-1228 $3.50+ ,2510 Angew. Chem. Int. Ed. Engl. 1992, 31, No. 9

pentane solutions were concentrated to 10 mL and cooled to -40°C. A color- less solid crystallized (0.28 g, 42%). which sublimed at 2O0C/0.O01 Tom.- NMR spectra measured in C,D, at 25 "C; 'H NMR (499.843 MHz, TMS): 6 = 1.83 (s, Me), 2.16 (H2-H6), 2.47 (H7-Hll), 3.03 (H12), measurement and assignment of the three B-H signals as cross peaks in the 2D IIB-'H NMR spectrum; "B NMR (160.364 MHz, Et,O-BF,): 6 = -11.2 (d, J=147 Hz, 87-Bl1,crosspeaksin the2D "B-"BNMRspectrum with B2-B6and with 812). -5.1 (d, /=183Hz, B2-B6, no cross peak with B12), -0.1 (d, J = 147 Hz, Bl2). 3: To a solution of 2 (39.0 mg, 0.245 mmol) in 2 mL of CH,CI, at room temper- ature was added 0.10 mL of methanol. This mixture was stirred for 30min and then treated with a solution of [N(PPh,),CI] (140 mg, 0.256 mmol) in 3 mL of CH,CI,. Pentane was added to the reaction mixture; 3 precipitated and was filtered, washed with pentane three times, and dried. Analytically pure bis- (triphenylphosphorany1idene)ammonium [2-methoxy-l-methyl-8,9-p-hydro- decahydro-1-aza-nido-dodecaborate] (3) was obtained (175 mg, 98 "A). Single crystals of 3 were obtained from hexane/dichloromethane at -40 "C. - NMR spectra were measured at 25 "C in CDCI, (same frequencies and standard as for 2); 'H{"B} NMR: 6 = - 3.81 (broad, p-H8/9), 0.32 (H13). 1.41 (HlO/ll), 1.52(H4/5),1.85(H6/7),2.15(s,3H,NMe),2.44(H12),3.16(H8/9),3.80(s, 3H. OMe), 7.42-7.68 (30H, Ph), assignment of the exo B-H signals as cross peaks in the 2D "B-IH NMR spectrum; I'B NMR spectrum: 6 = - 34.7 (d, J = 1 3 4 H z , B13), -18,6(d,J=134Hz,B6/7), -13,4(d,J=134Hz, B4/5), - 11.8 (d, J =134 Hz, BlOjll), - 8.2 (d, J = 134 Hz, B12), 1.9 (d, J=147 Hz, B8/9), 9.3 (s, B2), assignment with help of the cross peaks in the 2D I l E

I iB NMR spectrum; "C NMR (125.697 Hz, TMS): 6 = 37.7 (NMe), 53.4 (OMe), 126.9 (d, Jpc =lo8 Hz, @so-C of Ph), 129.6, 132.0, 133.8 (Ph).

Received: April 8, 1992 [ZS288IE] German version: Angew. Chem. 1992, 104, 1221

CAS Registry numbers: 1, 142581-18-8; 2, 142581-19-9; 3.PPN, 142581-21-3; 3 - H 1 , 142611-00-5; Me-, 15194-58-8; rBuO-, 16331-65-0; F-, 16984-48-8.

[l] J. Miiller, J. Runsink, P. Paetzold, Angew. Chem. 1991, 103, 201; Angew.

[2] R. A. Wiesboeck, M. F. Hawthorne, .I Am. Chem. Soc. 1964, 86, 1642-

131 L. D Brown, W. N. Lipscomb, Inorg. Chem. 1977, 16, 2989-2996. [4] J. Muller, P. Paetzold, R. Boese, Hereroutom Chem. 1990, I, 461-465. [5] Crystal data: a =18.181[5], b = 9.015(3), c = 24.401(8), a = p = y = 90",

V = 4000(2) A', Z = 4, pca,c = 1.413 gcm-', Pcu2, (No. 29), Nicolet R3m/V: recording temperature 125K; p(MoK.) =1.6 cm-' ; 4465 inde- pendent reflections with 3<28<45", 3919 of these observed (F0<4o(F)); structure solution and refinement with 398 parameters with SHELXTL- PLUS (version 4.2); R = 0.058, R, = 0.066, ~ r - ' = oz(Fo) + 0.00l5Fo2. All H atoms bound to B atoms were located by difference Fourier synthesis and refined with a common isotropic dislocation factor. The H atoms on C2 are disordered; the H atoms on the cation were treated as riding groups. Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wis- senschaftlich-technische Information mbH, D-W-7514 Eggenstein-Leo- poldshafen 2 (FRG) on quoting the depository number CSD-320454, the names of the authors, and the journal citation.

[6] X. L. R. Fontaine, H. Fowkes, N. N. Greenwood, J. D. Kennedy, M. Thornton-Pett, .I Chem. SOC. Dalton Trans. 1987, 2417-2429.

171 G. D. Friesen, A. Barriola, P. Daluga, P. Ragatz, J. C. Huffmann, L. J. Todd, Inorg. Chem. 1980, 19, 458-462.

[8] T. L. Venable, R. B. Maynard, R. N. Grimes, J. Am. Chem. SOC. 1984,106, 6187-6193.

[9] R. E. Williams in Eleclron Defcienl Boron und Carbon Cluslers, (Eds.: G. A. Olah, K. Wade, R. E. Williams), Wiley, New York, 1991, pp. 11 -93.

Chem. lnt. Ed. Engl. 1991, 30, 115.

1643.

[lo] F. Meyer, Dissertation, Technische Hochschule Aachen, in preparation.

Chain Elongation of Thiodipeptides with Proteases" * By Carlo Unverzagt,* Armin Geyer, and Howl Kessler*

The thioamide bond is a useful element for the structural variation of biologically active peptides to increase their ac-

[*I Dr. C. Unverzagt, Prof. Dr. H. Kessler, Dipl.-Chem. A. Geyer Organisch-Chemisches Institut der Technischen Universitat Miinchen, Lichtenbergstrasse 4, D-W-8046 Garcbing (FRG)

["I This work was supported by the Fonds der Chemischen Industrie, the Deutschen Forschungsgemeinschaft, and the Leonhard-Lorenz-Stiftung.

tivity and selectivity. Thiopeptides are attractive as pepti- domimetics1'1 because of their enhanced stability towards proteases['I and the conformational changes they induce.l3] Their general application has been, however, prevented by insufficient synthetic availability. The OjS exchange in pep- tides by Lawesson's reagentL41 or its variantsI51 may be selec- tive in cyclic pep t ide~[~ , 6bl but its effectiveness is difficult to predict. Only protected dipeptides 1 allow the selective OjS exchange at the amide position (+ 2) in high yields without affecting urethane or ester groups. Deprotection of thiopep- tides 2 generally causes no difficulties.

R O R' 0 I I I I II

Boc-NH- CH - C-NH-CH- C-OMe

R S R' 0 Boc-o-Ala-Y[CS-NHIPhe-OMe 2a I I I I II

Boc-NH-CH-C-NH-CH-C-OMe Boc-LeuY[CS-NHILeu-OMe 2b

= Boc-ASY[CS-NHIAS-OMe Boc-PheYtCS-NHIGiy-OMe 2c 2

Peptide elongation at the C terminus of thiopeptides by means of classical peptide chemistry fails;[61 instead the acti- vated thiopeptide 3 forms a sulfur-containing heterocycle 4. These thiazolones 4 are poor acylating agents and are prone to racemization of the two chiral centers. Hence, only few elongations of thiopeptides have been performed with achi- ral C termini, such as glycine or a-aminobutyric acid.[61

3 4

We report here on the enzymatic C-terminal elongation of thiopeptides 2a, b and the N-terminal elongation of the derivative of 2c deblocked at the N terminus (= 5e). Subtili- sin Carlsberg and subtilisin BPN' accept 2a-c as substrates: in 50 % dimethylformamide (DMF) the corresponding free carboxylic acids are obtained in almost quantitative yield. Under these conditions proteases do not cleave amide bonds.[7981 Proteases can also be used to catalyze the elonga- tion of amino acid esters and peptide esters at their C termi- nus.[81 We have successfully applied this mild synthetic method to sensitive thiopeptides. The subtilisin BPN' catalyzed reaction of thiopeptide methyl ester 2a with leucine amide H-Leu-NH, (5c) yields mainly the hydrolyzed dipeptide acid (60%) but also the desired tripeptide 6c (24%) (see Table 1). Obviously the enzyme activates 2a mild- ly, thereby allowing the elongation to thiopeptide 6c, where- as the undesired side product (thiazolone) is not formed. The 250 MHz 'H NMR spectrum of 6c exhibits a characteristic signal for the thioamide proton at 6 =10.2 and the reso- nances of all three amino acids. It also proves the sterochem- ical uniformity of the peptide bond formation.

The yields in the enzymatic peptide elongation of 2a with H-Leu-NH, (5c) could be increased with chymotrypsin as a biocatalyst (Table 1). Under the reaction conditions the thiodipeptides are completely converted into 6 by hydrolysis

Angew. Chem. Int. Ed. Engl. 1992, 31, No. 9 0 VCH Verlugsgesellschaft mbH, W-6940 Weinheim, 1992 0570-0833/92j0909-1229 $3.50+ ,2510 1229