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Boron Cations with a Dicoordinate B Atom.

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Table 1. 'H-NMR and "P-NMR data for (3)-(5),
me].
in C6Db[&values, TMS int. ('H) or 85% H,POI ext. (3'P); J and N in H; for assignments see formula sche-
Complex
IPR3
OsCJf,
OsH
'PMe3, 2PMe,
'PMe,
4PRI
7.16 m
-9.80 d x q
J111~qi=23
Jnpt, = 73
-47.42 t
-58.51 m
-58.51 m
Jszp,+=
(3)
14)
@I
(5)
'PMe,, 2PMe,
'PMe,
1.22 vt
1.39 d [a]
1.37 d [a]
N = 6.0
J p H = 5.5
J P H= 6.0
1.14 vt
N = 6.8
Jrii = 6.2
1.17 vt
N = 6.0
JpH
1.09 d
7.32 m
-7.10dxt
-43.93 d
17
J 3 p H = 25
- 9.90 d x q
Jt 2 %p,, = 22
J a p H = 74
J ~ + x p = 15.2
J > . * p ,=
,
1.37 d
= 6.2
1.73 d
Icl
J p H = 5.2
7.23 m
17.1
-48.07 d x d
J s ~ p i p = 15.6
Jtzl3+=
17.7
-58.74 t
15.2
Jt+,p=
-60.01 d x t
15.6
J+*p=
10.2
-42.32 d x t
Js +sp= 17.7
J++= 10.2
J<>pip=
[a] Exact assignment of both signals to the PMe3 groups in the 3- and 4-positions i s not possible. [b] Signals of the C,H,-protons at 6 = 1.88 d x t,J#?,,=3.0,
J+H=4.5. [c] Signal lies under the signal of the PMe2Ph protons.
whether another intermediate species is involved, remains
era1 formulas [R,BLL']X, [RXBLL'IX, [RBL3]X2, and
open; the insertion of the metal must result from this.
[BL4]X3 ( R = H , Hal; L, L'=NH3, NH2R, NR,, pyridine;
The activation of C-H bonds has fundamental signifiX = C1, Br, I). Compounds containing triply coordinated
cance for the comprehension of numerous reactions catacations [X,BL]Y, on the other hand, have so far rarely been
lyzed by metal complexes. The cleavage of a n sp2-C-H
described in the literaturef2I, while dicoordinated cations
bond by a Lewis basic metal was first described by Chatt et
[XBXIO were hitherto unknown131.The latter species forwho obtained the compounds ( d r n ~ e ) ~ R u ( C ~ H ~ ) Hmally contain one B atom with an electron quartet; it
should therefore be very electrophilic and could correand (dmpe),Ru(Cl0H,)H (dmpe = Me2PC2H4PMe2)by reduction of (dmpe)2RuC12with sodium o r potassium in the
spond to a "superelectrophile".
presence of benzene or naphthalene. The compound
It is well known that the electron-deficiency in boron
compounds can be compensated considerably by Ir-back(dmpe)2Ru(C,oH,)H
exists
in
equilibrium
with
bonding. Exploiting this principle we have now succeeded
(dmpe)2R~(C,oH8)f61.
To the best of our knowledge the synthesis of (3) from ( I )
in synthesizing salts whose cations contain boron with
coordination number 2I4l.
or (2) is the first proof that the metal atom of a stable q6arene metal complex, after successive addition of further
Reaction of anhydrous aluminum bromide with a series
donor ligands, can undergo insertion into an sp2-C-H
of 2,2,6,6-tetramethylpiperidinoboron bromides in dichloromethane leads to specific displacement of bromide,
bond without prior dissociation of the (q6-C&) metal
which is trapped as tetrabromoaluminate. By formation of
bond. An extension to the preparation of other aryl(hythis less nucleophilic anion, and owing to the steric and
drido) metal compounds appears to be possible.
electronic shielding of the B atom by the bulky 2,2,6,6-teReceived: January 8, 1981 [Z 854 IE]
tramethylpiperidino-moiety (tmp), dicoordinated boron
German version: Angew. Chem. 93, 826 (1981)
cations are generated.
[I] Basic Metals, Part 32. This work was supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie and by BASF
AG, Ludwigshafen and DEGUSSA, Hanau. Part 31: W. Hofmann. H .
Werner, Chem. Ber., in press.
[2] N . J . Cooper. M. L. H . Green. R. Mahtab, J. Chem. SOC.Dalton Trans.
1979, 1557; M . Berry, K . Elmilt, M. L. H . Green, ibid. 1979. 1950.
131 H . Werner, R . Werner. J. Organomet. Chem. 194. C 7 (1980).
141 H . Werner, R . Werner, Angew. Chem. 90, 721 (1978); Angew. Chem. Int.
Ed. Engl. 17, 683 (1978).
[5] G. Huttner. S . Lange. E. 0. Fischer, Angew. Chem. 83. 579 (1971); Angew. Chem. Int. Ed. Engl. 10. 556 (1971).
[6] J. Chart, J. M. Dauidson. J. Chem. SOC.1965, 843.
6("B)
6(27A1)
Av,,z(~'AI)
M.p. ["C]
Boron Cations with a Dicoordinate B Atom
36.7
80.7
20
1I 4
37.6
80.7
20
90-95
56.0
81
80
59.6
80.7
48
dec.
dec.
By Heinrich Noth and Rudolf Staudigl"'
Cations of boron with the coordination number 4 are
formed on aminolysis of suitable boron halides, o n nucleophilic halide-displacement from amine-borohalide adducts
with bases, and on electrophilic hydride-exchange in amine-boranes with halogen in the presence of basesf']; the
composition of these salts can be represented by the gen[*I Prof. Dr. H. Nhth, Dr. R. Staudigl
Institut fur Anorganische Chemie der Universitat
Meiserstr. I , D-8000 Miinchen 2 (Germany)
794
0 Verlag Chernie GrnbH, 6940 Weinheim, 1981
"Al-NMR spectra confirm exclusive formation of the
AIBry anion, which, compared to A12Br6, is characterized
by its substantially sharper signal; the linewidth of A v ~ , ~
=20 Hz observed in ( I ) and (2) corresponds to an undistorted tetrahedral AIBry. The "B-NMR signals of (1)-(4)
are shifted 6-18 ppm downfield relative to those of the
starting compounds tmpB(Br)Y; their linewidths, which
are greater by a factor of about 5 , are consistent with a linear heteroallene structure (for (1)-(3)). Also consistent
with such a structure are the isotopically split IR bands at
0570-0833/81/0909-0794 S. 02.SO/O
Angew. Chem. I n t Ed. Engr 20 (1981) No. 9
1850-1900 cm-', which we assign to a n antisymmetric
BN2 vibration in ( I ) and (2).
The heteroallene structure of ( I ) was confirmed by X-ray
structure analysis. The projection onto the a / c plane of the
monoclinically crystallizing compound (Fig. 1) shows a
linear NBN framework ( Q NBN 176(3)") and orthogonal
orientation of the C 2 N planes (88"). One of the two BN
distances was found to be 130(4) pm. This is the shortest so
far recorded"].
[31 J. M . Davidson, C.M . French, J. Chem. SOC.1958, 114; D. Armstrong, P.
J. Perkins, ibid. A 1966. 1026.
I41 J. Higashi, A . D . Eastman. R . W . Parry shall report on the characterization of bis(diisopropy1amino)boron tetrachloroaluminate: R . W . Parry,
private communication.
[5] Space group P2,/n, a = 767.8(4), b = 1456(I), c= 1791(1) pm, p = 97.25(4)",
Z = 4 . Refinement to R , =0.104. The rather poor structure determination
is attributed in part to the low quality of the single crystals prepared so
far. Hence a discussion of the bond lengths cannot be meaningful at present. But the crystal structure analysis affords unambiguous information
about the constitution and geometry. In particular it confirms the findings derived from spectroscopic data. Fractional coordinates and parameters of the temperature factors are available on request from the Cambridge Crystallographic Data Center.
[61 A detailed description of our results will appear in Inorg. Chem. together
with the results of Parry et al. 141.
[7] P. 1. Paetzold, A . Richter, T. Thussen. S . Wiirtenberg, Chem. Ber. 112,
3811 (1979).
[S] All compounds (1)-(4/ were characterized by elemental analysis and
NMR spectroscopy ('H, "9, "C, 27A1).
Synthesis of Enantiornerically Pure (R)- and (%-aHydroxyketones and Vicinal Diols; Asymmetric
Nucleophilic Carbarnoylation[**]
Fig. 1. ORTEP diagram of the unit cell of ( I / . projection on the a / c plane,
with omission of the C-skeletal atoms of the tetramethylpiperidino- and dimethylamino-ligands on the boron cation. The shortest Br.. . B distance is
412 pm. Hence there is no interaction between B and Br atoms.
The B atom of the cation of salt (4) can formally be considered as having, at most, an electron sextet. Compared to
( I ) and (2) this compound is thermally less stable. The
crystals rapidly decompose at room temperature; in
CH2C12, decomposition at room temperature is slow
enough to enable a definite NMR spectra to be recorded.
Less sterically demanding nucleophiles such as BrQ (from
[(C,H,),N]Br) react with the new boron cations, e.g. of ( I ) ,
to give the eductF6].
The stabilization of these cations of boron by steric and
electronic factors is obviously more effective than in the
monomeric boron imide C6F5-B=N-tC4H,,
which also
contains a dicoordinated B atomt7]and, in contrast to (1)(4). undergoes dimerization.
Procedure
Diethylamino(2,2,6,6-tetramethylpiperidino)boron(1 + )
tetrabromoaluminate (3): A stirred solution of
tmp(Et2N)Br (2.24 g) in CH2C12(12 mL) is treated portionwise at - 2 0 ° C with 1.92 g of AIBr,. The clear solution is
first warmed to room temperature. Subsequent slow cooling to - 78 "C affords crystalline (3), while rapid cooling
furnishes not quite pure (3) as a colorless oil. Yield 3.7 g
(90%), m.p. 90-95"C, rapid decomposition at 130°C. The
equivalent conductivity of a 1 . 4 1 5 ~solution in CH2Cl, at
20°C was found to be 31.1 SZ-' cm'. (3) is only slightly
sensitive to hydrolysis[*].
Received: August 7, 1980 [ Z 855 IE]
revised: May 18, 1981
German version: Angew. Chem. 93, 830 (1981)
[I] Gmelins Handbuch der Anorganischen Chemie, Supplement to 8th edition, Band 37, Borverbindungen, Teil 10 (1976); references cited therein.
[2] A. T. Balaban. A . Arsene. 1. Baily, A. Barabas. M . Paraschiu, Tetrahedron
Lett. 1965, 3925: H . Noth, P. Fritz. Z. Anorg. Allg. Chem. 322. 297
(1963).
Angew. Chem. In(. Ed. Engl. 20 (1981) No. 9
By Dieter Enders and Hermann Latter"]
Dedicated to Professor Rotf Appel on the occasion of his
60th birthday
The use of masked nucleophilic acylation agents is part
of the repertoire of organic synthesis"]. Reagents which
bring about direct nucleophilic acylation are also increasing in importance[''. While enzymatic variants of the cyanohydrin synthesis and the acyloin addition have to a degree been successfully used[31,generally applicable methods for asymmetric CC-coupling using chiral carbonyl-d I reagents[,] are not yet available15].
The a-hydroxyamides (3) can be obtained in good yield
via asymmetric nucleophilic carbamoylation by addition
of lithium tetramethylpiperidide (Li-TMP) to a mixture of
the ketone ( I ) and the chiral formamide (2) in tetrahydrofuran (THF), ether, and n-pentane (4 :4 : 1) at - 100°C.
The chiral carbamoyllithium compounds of type (4)
( R = C H 3 ) formed as intermediates are highly reactive
and the diastereomeric excess of hydroxyamides (3) is low
(de = 5-20%)f61. The hydroxyamides can, however, be
readily separated into the pure diastereomers (SS)-(3)
and (SR)-(3) by simple column chromatography or, more
rapidly, by preparative high pressure liquid chromatography (HPLC). Subsequent reaction with methyllithium permits the preparation of both enantiomers of the a-hydroxyketones (5) and the vicinal diols (6) in enantiomerically
pure form (see Table 1).
In all cases, methyllithium cleavage of the amides (3)
proceeds to completion; depending on the reaction conditions, either more hydroxyketone (5) o r more diol (6) can
be obtained"]. (5) and (6) are separated by column chromatography.
Since the diols (6) are crystalline, ee-values of >90% can
be achieved after two recrystallizations (n-pentane), even
when the diastereomeric separation step is omitted.
[*I Prof. Dr. D. Enders, DipLChem. H. Lotter
Institut fur Organische Chemie und Biochemie der Universitat
Gerhard-Domagk-StraBe I, D-5300 Bonn I (Germany)
I**] This work (part of the planned dissertation of H . L . ) was supported by
the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie and Degussa, Hanau. Presented, in part, in 1980 (among others,
Chemiedozententagung, Erlangen, ACS-Meeting, Las Vegas and Arbeitsgruppe, Organische Chemie, Bad Nauheim).
0 Verlag Chemie GmbH. 6940 Weinheim, 1981
0570-0833/81/0909-0795 $02.50/0
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