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Hexaborylbenzene and Tetraborylethene Derivatives.

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111 P. Ahlberg, G. Jonsall. C. Engdahl. Adv. Phvs. Org. Chem. 1982, 19. 223.
[2] a) P. D. Bartlett in Nonclassical Ions. Reprints and Commentaries in Frontiers
in Chenrrrtr.r..W. A. Benjamin. New York, 1965; h) H. G. Richey, Jr., in Curhonium Ion&. VIJ/ 3 (Eds.: G. A. Olah, P. von R. Schleyer). Wiley, New York.
1972. p. 1201. c) K. B. Wiberg, A. B. Hess. Jr.. A. J. Ashe in Curhonijm Ions,
b l . 3 (Ed.: G. A. Olah, P. von R. Schleyer). Wiley. New York, 1972, p. 1295;
d ) H. C . Brown i n The Nunclussicul Ion Prohlem, Plenum Press, New York,
1977. Chap. 5: e) P. Vogel, Curhocuriun Chemistry. Elsevier, Amsterdam. 1985.
p. 350; f ) J. D. Roberts, R. H. Mazur, J Am. Chem. Sue. 1985, 73, 3542;
g) G. A. Olah. V. P. Reddy, G. K. S. Prakash. Chem. Re),. 1992. 92, 69.
[3] G. A. Olah. D. P. Kelly, C. L. Juell, R. D. Porter. J Am. Chem. Soc. 1970. 92,
141 a) G. A. Olah. C. L. Juell, D. P. Kelly, R. D. Porter, J Am. Chem. S i r . 1972.
94, 146; b) J. Staral, I. Ydvari. I. D. Roberts, G. K. S. Prakash, D. J. Donovan,
G. A. Olah, J. A m . Chem. Soc. 1978. 100. 8016.
[5] a) W J. Brittain. M. E. Squillacote, J. D. Roberts, J Am. Chem. Soc. 1984, 106.
7280; b) M. Saunders. K. E. Laidig. M. Wolfsberg. ihid. 1989, 111. 8989.
[6] a) M. Saunders. H.-U. Siehl, J Am. Chem. Soc. 1980. 102. 6868; b) W. J.
Brittain. M. E. Squillacote. J. D. Roberts, ihrd. I%, 106. 7280.
[7] a ) P. C. Myhre. G. G. Webb, C. S. Yannoni. J. A m . Cliem. Soc. 1990,112,8992;
h) M. Saunders. K . E. Laidig, K . B. Wiberg. P. von R Schleyer, rhrd. 1988,110.
7652, and references therein.
[8] T. S. Sorensen. L R. Schmitz. J. Ani. Chem. Sur. 1982. 104. 2600
(91 T S Sorensen. L R. Schmitz, J An?. Chem. Soc. 1982, 104. 2605.
[lo] Alcohol 7 was prepared in 59% yield by the reduction oftriaxdne-2-carboxylic
acid with lithium aluminum hydride. The acid was obtained in 49% yield by the
oxidation of hcetyltriaxane with sodium hypobromite. For the synthesis of
methyl ketone. see: A. Nickon, D. F. Covey, G D. Pandit. J. J. Frank, Tetruhedron Lrrr. 1975, 3681.
[ I t ] 2.10-(puru-[3'.5"]octahedrane)dimethanol (8) was obtained in 74% yield by
the reduction of 1.10-(puru-[3z.5b]octahedrane)dicarhoxylicacid dimethylester
with lithium aluminum hydride. For the synthesis of 2,lO-puru[3'.5"]octahedraiie)dicarboxylic acid dimethylester, see C.-H. Lee. S. Liang, T.
Haumann. R Boese, A. de Meijere. Angew Chern. 1993. 105. 611; Angen
Chwi. Inr. Ed. E q I . 1993, 32, 559.
[12] a ) I Ziegler. C h [ w Rev. 1991. Y i , 651; b) Gaussian 94 (Revision A.1). M. J.
Frisch, G. W. Trucks, H B. Schlegel. P. M. W. Gill, B. G. Johnson, M. A
Rohh. J. R. Cheeseman, T. A. Keith. G. A. Peterson, J. A. Montgomery, K.
Raghavachari. M A. Al. Laham. V. G. Zakrzewski. J. V. Ortiz. J. B. Foresman.
J. Cioslowski. B B. Stefanov. A. Ndnaydkkdra. M. Challacombe, C. Y Peng.
P. Y Ayala. W Chen. M. W. Wong, J. L. Andres. E. S. Replogle. R. Gomperts,
R. L. Martin. D. J. Fox. J. S. Binkley, D. J. Defrees, J. Baker, J. J. P. Stewart. M.
Head-Gordon, C. Gonzalez. J. A. Pople. Gaussian. Inc., Pittsburgh PA, 1995.
[13] Preparation ofcations: In a 5 mm N M R tube. the appropriate alcohol (15 mg)
slurried in S0,CIF (0.2 mL) at - 78' C (dry iceiacetone bath) is treated with a
tenfold excess of precooled SbF, in S0,CIF (0.7 mL) under rapid vortex stirring. Upon dissolution of the alcohol. ionic solutions were obtained.
[14] All absorptions are referred to the CD, signal (6°C = 29.8) of external
[DJacetone in standard capillaries in the same NMR tube.
[IS] (a) M. Schindler. J. Am. Chem. Sol,. 1987, 109. 1020; h) W. Kutzelnigg, U.
Fleischer. M. Schindler. NMR Eu.\ic Princ. Prug., 1991, 23, 165.
[16] P. voii R Schlcyer, D. Lenoir, P. Mison. G . Liang, G. K. S. Prakash, G. A.
Olah. J A m Chcm. Soc. 1980, 102. 683.
[17] T h e Y NMR chemical shifts of the parent hydrocarbon 3-methy[*]nonane is not reported; they were estimated based on the data of
related compounds or calculated based on the IGLO DZ//HF/3-2lG level of
[18] The Greek word distonic (meaning distant) is used to distinguish these dications from gitonic (proximal) dications. For a discussion see G. A. Olah,
Airgeit C/iem. 1993, 105. 805: Angew. Chern. Int. Ed. EngI. 1993. 32, 907.
[19] a ) G . J. Ray. R. J. Kurland, A. K. Colter, Tetruhedrun 1971. 27, 735; b) P. C.
) Spiesecke, W. G . Schneider.
Lauterhur, J. .Am. Chem Sue. 1961.83. 1 8 3 8 ; ~H.
E,i.rruhedron Liar! 1961,468; d) E. W. Lalancette, R. E. Benson, J Am. Chem.
Snc. 1965. XS. 1941: e) G. A. Olah. M. Bollinger. A M. White. ihjd. 1969, 91.
3667: f ) G. A. Olah. D. Mateescu, ihid. 1970, 92, 1430; g) R. Ditchfield, D. P.
Miller. J. A. Pople. C/?em.Phjs. Lett. 1970, 6 , 573; h) R. Ditchfield, D. P.
Miller. J. A. Pople. J Chen:. Phys. 1971,54,4186; i) D. G . Farnum, A d , . P h u .
Org. Chtwr. 1975, if. 123.
[20] Only small changes of up to A6 = 2 were observed in certain peaks in the "C
NMR spectra of the ions over the studied temperature range.
Hirno. Y. Ohuchi. 0. Yonemitsu. J Chem. Sue. Chrin. Commrm. 1982.
[21] (a) KI:
99; b) K.-I. Hirao. H. Takahashi, Y Ohuchi, 0. Yonemitsu, J Chem. Res.
S I W J ~1992.
319; .
Chern. Res. Mmrpurinr 1992. 2601
1221 Ions 16a and 16c are enatiomeric.
Hexaborylbenzene and Tetraborylethene
Andreas Maderna, Hans Pritzkow, and Walter Siebert*
Dedicated to Professor Marianne Baudler
on the occasion of her 75th hirthduy
Functionalized tetraborylethenes and tetra- and hexaborylbenzene derivatives are potential precursors for new boron heterocycles. Tetraborafulvalenes A and benzobis- and benzotris( 1,3-diboroIe) derviatives B and C, respectively, are suitable
for the synthesis of doubly and triply stacked complexes with
0 2 B
12 UGH,
- 6 ti202CBH4
Scheme 1
1,3-diborolyl bridging ligands. Additionally, tetra- and hexaborylbenzenes can be used for synthesizing band and sheet polymers. In this way a graphite analogue (C,B),, in which every
boron atom is surrounded by three C atoms, could be made by
polycondensation of C,(BR,), under elimination of BR,. We
report here on the first synthesis of perborylated C-C multiply
bonded systems by transition metal catalyzed reactions of
bi(l,3,2-benzodioxaborolyl)acetylene (1) .[I1 Cyclotrimerization
of 1 provides 2a, the first example of a hexaborylbenzene derivative. Diboration of 1 with 2,2'-bi(l,3,2-benzodioxaborolyl)borane (3) leads to the tetraborylethene compound 4 a. The
boron-oxygen bonds in 2a and 4 a can be cleaved with methyllithium and trimethylaluminum (Scheme 1).
Angew. Chm7. In1 Ed. Engl. 1996. 35, Nu. 13/14
Prof. Dr. W. Siebert. A. Maderna, Dr. H. Pritzkow
Anorganisch-chemisches Instltut der Universitdt
Im Neuenheimer Feld 270, D-69120 Heidelherg (Germany)
Fax: Int. code +(6221)545609
This work was supported by the Deutsche Forschungsgemeinschaft (SFB 247)
and the Fonds der Chemischen Industrie.
VCH VerlugsgesellschaJtmhH, 0-69451 Wernheim. 1996
0570-0833/96i3513-1501 3 15.00+ .2S!0
The synthesis of substituted arenes by cobalt-catalyzed
trimerization of alkynes is well known and has found broad
applications in organic synthesis.I2I In an analogous manner,
the diborylacetylene 1['] can be cyclotrimerized with (vs-cyclopentadieny1)dicarbonylcobalt in refluxing toluene to give the
hexaborylbenzene 2 a in 67 O h yield. Compound 2 a is only slightly soluble in most solvents; in methanol a slow reaction takes
place leading to the formation of H,C,O,BOMe. Purification of
2a is accomplished b y multiple washings with T H F and
CH,CI, . Subsequent drying provides analytically pure, airstable 2a. The "B N M R spectrum in [D,]toluene at 372K
shows a broad signal at 6 = 33; the chemical shift of the boronbound C atoms in the 13C N M R spectrum (6 = 143) appears in
the range expected for 1,2-diboryIbenzene derivative^.'^]
In analogy to the cleavage of the B - 0 bonds in 2-alkyl-l,3,2benzodioxaborole derivatives with Grignard reagent^,'^] a solution of 2a in toluene reacts with methyllithium in diethyl ether
to form a yellow solution of the methyl derivative 2 b. After
separation of the precipitated C,H,O,Li,(OEt,),,
2 b was identified by GC-MS analysis to be the only product. Surprisingly,
the "B N M R spectrum of the yellow solution shows a broad
signal at 6 = 6, which indicates a donor-acceptor interaction
between the oxygen atoms of dissolved C,H40,Li, and the
boron atoms. Additionally, two signals of lower intensity appear in the highfield region at 6 = - 17.4 and - 20.6, which we
assign to the boranates 2b.(LiMe), and LiBMe,."] Addition of
an excess of pyridine results in an orange-red solution from
which a crystalline, lithium-containing product precipitates.
N M R and mass spectra indicate a compound based on 2b,
dilithio-l,2-dioxabenzene,and pyridine.
The crystal structure analysis indicated the formation of compound 2c (Fig. l ) , in which two oxygen atoms from two
Li,O,C,H, units each bridge two BMe, groups. The boron
Flg. 1. Structure of 2 c in the crystal. Py
atoms of the other two boryl groups, which are para-positioned
with respect to one another, are planar coordinated. The BC,
planes are perpendicular to the central benzene plane. However,
no satisfactory refinement of the structure analysis was possible,
due to the poor quality of the crystal, resulting from disordered
solvent and/or pyridine. The "B N M R spectrum of 2c shows a
broad signal at 6 = 0. N o signals for the BMe, groups were
found in the lowfield region. From this it can be concluded that
only four-coordinate boron atoms are present in solution.
Concentration of the yellow toluene/diethyl ether solution of
2b resulted in the formation of a yellow precipitate. Further
removal of solvent provides a yellow, insoluble, air-sensitive
powder. The "B N M R spectrum of a saturated solution of this
in CH,CI, shows a broad signal at 6 = 85 as well as a signal at
6 = 6, which indicates the presence of 2b. In the EI mass spec1502
Verlagsgesellsrhafr mbH, 0-69451 Wemheim. 1996
trum of the yellow product, the highest mass appears at 317
(M' for 2 b). The reaction of a suspension of 2 a in hexane with
trimethylaluminum also results in 2 b ; removal of the insoluble
components by filtration provides a yellow filtrate whose 'B
N M R (only one signal at 6 = 85) and EI-MS (m/z = 317, M ' )
spectra confirm that this product is formed.
According to Suzuki and MiyauraC6l alkynes react catalytically with 2,2'-bi(4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl)platinum and tetrakis(tripheny1phosphane)platinum to give Z 1,2-diborylethenes. Heating a solution of the diborylacetylene 1
and the diborane(4) 3 in toluene in the presence of bis(triphenylphosphane)(ethene)platinum181 for 24 h results in the
tetraborylethene derivative 4a in 50% yield (see Scheme 1).
Compound 4a is an air-stable solid that is very poorly soluble.
The first C,(BX,), compound (X = F) was obtained by gasphase reaction of carbon with B2F4.1131
The "B N M R spectrum of 4a in [D,]methanol shows a broad signal at 6 = 34 and
a signal of lower intensity at 6 =18 that can be assigned to
C,H,O,BOCD,, which is formed by the methanolysis of 4a.
The 13C N M R signal of the boron-bound C atoms could not be
detected due to the poor solubility of the compound.
Different reactivity is seen upon the stoichiometric addition
of bis(triphenylphosphane)(ethene)platinum. In addition to
traces of 4a, the corresponding substituted 1.1,2,2-tetraborylethene derivative is obtained in 8 %
Its synthesis is probably due to the intermediate formationr1'] of
1,3,2-benzodioxaborole and subsequent double hydroboration
of 1.
The reaction of 4 a with methyllithium already occurs at
-5°C; however, due to the enhanced reactivity of the boryl
groups compared to those in 2 b, the resulting 4 b could not be
identified by GC-MS analysis. Upon addition of pyridine, the
"B N M R spectrum of the red-brown solution shows, just as for
212, a signal at 6 = 0 as well as a signal for LiBMe, (6 = - 19.5).
We assume that a product analogous to 2c is formed from 4b,
Li,O,C,H,, and pyridine. The I3C N M R spectrum, like that of
212, shows a broad signal for the BMe, groups (6 =13) and a
broad signal for the boron-bound, sp2-hybridized carbon atoms
(6 = 158). Attempts to synthesis donor-free 4 b from 4a and
trimethylaluminum provided a yellow solution whose "B
N M R spectrum shows a signal for 4b at 6 = 82 and for
trimethylborane at 6 = 87. The isolation and characterization
of 4b has not yet been successful.
Electronic as well as steric features of 1 are decisive for the
formation of 2a and 4a. The Lewis acidity of the boron atoms
is reduced due to the donor function of the oxygen atoms. The
C - C triple bond is only minimally lengthened''] to allow the
necessary coordination at the metal center during the catalytic
cycle. The planar benzodioxaborolyl substitutents are advantageous in that they are able to rotate with respect to one another
and allow the approach of several alkynes to the reactive metal
center. Transition metal catalyzed cyclotrimerization and diboration of 1 offers a pathway to unsaturated, perborylated compounds, which can be used for the synthesis of new boron heterocycles. These can, in turn, be used for organic synthesis as well
as applications as hydride"'] and fluoride acceptors.
Experimental Procedure
mmol) and (r$cyclopentddienyl)dicdrbonylcobalt (0.042 g.
0.23 mrnol, S mol%) in toluene (25 mL) were heated under reflux for 24 h. Afterwards. the precipitate was isolated by filtration, washed several times with T H F and
CH,CI, and dried in vacuum. Yield: 0.827g (67%). m.p.: >310'C. ' H N M R
(200 MHz. [DJTHF): 6 = 6.4-6.8 (m); 13C NMR (SO MHz. [D,]MeOH)6 =146.2, 120.8. 116.3 (C,H,). 143 (CB, br.); "B N M R (64 MHz, [DJtoluened.
373K). 6 = 33 ( A V ~ = 580Hz); HR-MS (EI). m/; 785.1879 (M' calcd.
785.1862): correct C, H analysis.
Z a : 1 (1.234 g, 4.7
$15.00+ .25/0
Angew. Chem lnt. Ed. EngI. 1996. 35, NO. 13/14
creasing number of posttranslational modifications originating
from enzymatic, ribosomally synthesized precursor proteins has
been detected.[21Thus, the fascinating 43-peptide antibiotic microcin B17, which inhibits DNA-gyrase, contains four oxazole
and four thiazole rings,[3b1which arise by cyclizations and dehydrogenations from di- and tripeptide units containing Gly, Ser,
and Cys residues of the precursor protein (69 residues) followed
by cleavage of a leader peptide (26 residues). A wide variety of
oxazole-, thiazole-, and thiazoline-containing natural products
has also been isolated from marine organismst4"-'] and microorganisms.~4~-m1
The reported syntheses of o x a ~ o l e6][ ~ ~
and/or thiazole['] derivatives include only few examples of
aminoalkyl- or aminoaryloxazolecarboxylic
Substituted five-membered heterocycles are pharmacophores
of many natural and synthetic, bioactive compounds. In particular, the amino- and carboxy-functionalized heterocycles found
in microcin B17 (Scheme 1) are valuable building blocks for
4a: 1 (473 mg, 1.8 mmol), 3 (430 mg, 1.8 mmol), and bis(triphenylphosphane1-
(ethene)platinum (83 mg, 0.12 mmol, 6.2 mol%) in toluene (15 mL) were heated
under reflux for 24 h. Afterwards, the precipitate was isolated by filtration, washed
several times with THF and CH,Cl,, and dried in vacuum. Yield: 455 mg (50.6%))
m.p.: >310"C, 'HNMR (ZOOMHz, [D,]MeOH): 6 =6.5-6.9 (m);I3C NMR
(50 MHz, [DJMeOH): 6 = 146.3,120.9, 116.4(C6H.,), CBnot observed; "BNMR
(64 MHz, [DJMeOH):
6 = 34 ( A v , , ~= 524 Hz), 18 ( A v I j , = 58 Hz,
C,H,02BOCD,); HR-MS (EI): m / z 500.1209 ( M + , calcd '2C,,1H,,'oB,'60,:
500.1217); correct C, H analysis.
2b, 2c: A suspension of 2 a (785 mg, 1 mmol) in toluene (15 mL) and methyllithium
(8 mL of a 1.5 M solution in diethyl ether, 12 mmol) was first stirred for 1.5 h at
- 5 "C and then for 8 h at 25 "C. After removing the insoluble components, a yellow
solution of 2 b was obtained which contained minor amounts of boranates. I I B
NMR (64 MHz): 6 = 6 (AVlj2 = 466 Hz), -17.4 (Av,,, = 87 Hz), -20.6 (AVlj2 =
29 Hz); GC-MS: m / z ("A) 317 (100) [M+], 302 (17.4) [ M + -Me], 41 (92.4)
[BMe,+]. An excess of pyridine was added to the yellow solution. Crystalline 2c
precipitated from the resulting orange-red solution at room temperature. Yield:
296 mg(33%), m.p.: >250°C (decomp), 'HNMR (200 MHz, CDCI,): 6 = - 0.3
to 0.3 (m, BMe), 5.8 -6.9 (m, C,H,), 7.0 (m, Py), 7.5 (m, Py), 8.2 (m, Py); 13CNMR
(50MHz, CDCI,): 6 =13.0 (BMe, br), 114.6, 116.5, 118.2, 120.0, 153.6, 160.0
(C6H,), 123.7, 136.6, 149.6 (Py), 168.0 (CB, br); I l B NMR (64 MHz, CDCI,):
6 = 0.3 ( A V , , =
~ 204 Ht); MS (FD): m / z 982 [ M + - Py - 4Mel.
Received: February 12, 1996 [Z8807IE]
German version: Angew. Chem. 1996, 108, 1664-1666
Keywords: boron compounds
R-cn,-cb 'c-coon
\s-cn/ /
catalysis * diborylacetylenes
[l] H. Schulz, 0. Gabbert, H. Pritzkow, W Siebert, Chem. Ber. 1993, 126, 1593.
[2] K. P. C. Vollhardt, Angew. Chem. 1984, 96, 525; Angew. Chem. Int. Ed. Engl.
1984,23, 539.
[3] D. E. Kaufmann, R. Boese, A. Scheer, Chem. Ber. 1994, 127, 2349.
[4] S . Cabiddu, A. Maccioni, M. Secci, Gazz. Chim. Ital. 1972, 102, 555.
[5] H. Noth, B. Wrackmeyer, Nuclear Magnelic Resonance Spectroscopy of Boron
Compounds, Springer Berlin, 1978.
[6] T. Ishiyama, N. Matsuda, N. Miyaura, A. Suzuki, J. Am. Chem. Soc. 1993, 115,
11018; T. Ishivama, N. Matsuda. M. Murata. F. Ozawa. A. Suzuki. N. Mivaura, Organometalhcs 1996, i5, 713.
[7] C. N. Welch, S G. Shore, Inorg. Chem 1968, 7, 226.
[8] C D Cook, G. S . Jauhal, J Am. Chem. Soc 1968,90, 1464.
191 In the EI mass spectrum, only the molecular peak at m / z = 502 was observed
In the higher mass regon A signal for [M' - H,] (4a+) was not observed,
compare with ref. [lo].
[lo] P. Frankhauser, E Kuhlmann, A. Kramer, H. Pritzkow, W Siebert, Chem. Ber.
1993, 126, 1291.
[Ill C. N. Iverson, M. R. Smith III., J. Am. Chem. Sac. 1995, 117, 4403.
[I21 H. E. Katz, J. Am. Chem. SOC.1985, 107, 1420.
[I31 J. E. Dobson, P. M. Tucker, F. G. A. Stone, R. Schaeffer, J. Chem. SOC.A 1969,
12, 1882.
/ /
\x-cn/ /
Georgi Videnov, Dietmar Kaiser, Christoph Kempter,
and Gunther J u g *
Angew. C h p . Int. Ed. Ehgl. 1996, 35,No. 13/14
\0-cn/ / \s-cn/ /
Synthesis of Naturally Occurring, Conformationally Restricted Oxazole- and ThiazoleContaining Di- and Tripeptide Mimetics""
[*] Prof. Dr. G. Jung, Dr. G. Videnov, Dip1.-Chem. D. Kaiser,
DipLChem. C. Kempter
Institut fur Organische Chemie der Universitit
Auf der Morgenstelle 18, D-72076 Tiibingen (Germany)
Fax: Int. code +(7071)29-6925
[**I This work was supported by the Deutsche Forschungsgemeinschaft (Ju 103/9-1
and 436 EUL-113/68/0).
R-cn2-cd 'c-cd
Residues that restrict the peptide backbone are synthesized in
Nature inter alia by the following modifications: N- and a-alkylations,[ll a,b-didehydroamino acid and sulfide ring formations,[21 and synthesis of aromatic five-membered heterocycle~.[~]
The unusual amino acids of most peptide antibiotics
are incorporated by multienzyme complexes.['] However, an in-
R-cy-cd \c-cd
\s-cn/ / \s-cn/ /
Scheme 1. R = Boc-NH
peptidomimetics, which restrict the w,$-conformational freedom in peptides. As a part of our efforts towards the total
synthesis of microcin B1 7,L31 we developed efficient procedures
for the preparation of Boc- and Fmoc-protected amino acids
derived from oxazole, thiazofe, bisthiazole, oxazolyl-thiazole,
and thiazolyl-oxazole, which can be considered as di- and
tripeptide mimetics. We also discovered a novel, useful procedure for the oxidative conversion of intermediate oxazoline into
corresponding oxazole rings with 1,8-diazabicycl0[5.4.O]undec-
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tetraborylethene, hexaborylbenzene, derivatives
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