Bernd Schwarze

Reactions between tetraalkyldiboranes(6) and disilazanes — A convenient route to N-silylamino-dialkylboranes

Thermally stable tetraalkyldiboranes(6) such as bis(9-borabicyco[3.3.1]nonane (1) and the 1,2:1,2-bis(tetramethylene)diboranes(6) (2,3) react with disilazanes such as 4 [(Me3Si)2NH] or 5 selectively by cleavage of the NSi bond and formation of the SiH bond. This affords N-silyl derivatives of 9-amino-9-borabicyclo[3.3.1]nonane (7,8) and of 1-amino-boracyclopentane (11–13) in high yield. INEPT-HEED experiments were used to determine coupling constants 1J(29Si15N) in N-silylaminoboranes for the first time. Dimeric 9-amino-9-borabicyclo[3.3.1]nonane was isolated from crude reaction mixtures of 1 and 4, and it was characterized by single crystal X-ray analysis (triclinic, space group P1).

Reactivity of bicyclic N -pyrrolylboranes

Abstract Bicyclic B-alky-N-pyrrolylboranes (1–3) react with alkyl lithium or alkyl Grignard reagents to give the corresponding borates 5 which, in most cases can be protonated to the intramolecular 2 H-pyrrole-borane adducts 4. The molecular structure of 4d was determined by X-Ray structural analysis. The adducts 4 can be deprotonated to the borates 5. Cleavage of the B-N bond in 1a by EtOH to give 7a is reversible, and 1e react with CF3SO3H by protonation of the pyrrole ring. The 2H-pyrrole-borane adducts 4a,d react with CF3SO3H by cleavage of the B-N bond to give trialkylboranes as the 2H-pyrrolium salts 8a,d. Cyclopentadiene reacts with the 2H-pyrrole borane adduct 4d selectively by [4 + 2]cycloaddition to give 10 with endo-configuration. The boranes 1a and 2a react stereoselectively with mono-1-alkynyltin compounds in a 1:1 stoichiometry by an 1,1-organoboration to give the organometallic substituted alkenes 11–13, in which the six-membered ring present in 1a and 2a is retained. In contrast, 3a reacts under the same conditions exclusively by ring enlargement and in a 1:2 stoichiometry. The product from the reaction of 3a with two equivalents of trimethyl(l-propynyl)tin is the 1,3-butadiene derivative 14 which rearranges selectively to its isomer 15 by changing the configuration at both double bonds. The reaction of 3a with two equivalents of bis(trimethylstannyl)ethyne leads selectively to the organometallic substituted allene 16, the result of an irreversible allylic rearrangement of a 1,3-butadiene derivative analogous to 14 or 15. AH products were characterized by 1H, 11B, 13C, 14N and 119Sn NMR spectroscopy.

Molecular structures of tri-N-pyrrolylboranes and the dynamic behaviour of tri-N-indolylboranes in solution

Abstract Various methods for the synthesis of tri- N -azolylboranes, Az 3 B (Az = 1-pyrrolyl ( 1a ), 1-(2,5-dimethyl)pyrrolyl ( 1b ), 1-indolyl ( 2a ), 1-(2-methyl)indolyl ( 2b ), 9-carbazolyl ( 3 )), are compared. The molecular structures of 1a (orthorhombic; space group Pbcn ) and 1b (monoclinic; space group C 2/ c ) were determined by X-ray analyses. There is a slight twist of the N -pyrrolyl groups in 1a against the BN 3 plane (26.7°, 29.3°, 29.3°), and the distinct tri- N -pyrrolylborane molecules are arranged in piles in the crystal lattice, with the boron atoms on top of each other and a staggered conformation of the pyrrolyl rings. The 2,5-dimethylpyrrolyl rings in 1b are strongly twisted against the BN 3 plane (36.7°, 48.5°, 50.8°), and in the lattice the molecules of 1b are arranged in a way to minimize intermolecular interactions. The structural features of 1b are also reflected by solid state 13 C CP/MAS NMR spectra. The molecular structures and the analysis of δ 11 B and δ 13 C data point towards rather weak BN(pp) π interactions in Az 3 B. Thus, the hindered rotation about the BN bonds in tri-1-indolylborane ( 2a ) ( ΔG ≠ (248 K) = 52.0 ± 1 kJ mol −1 ) and in tris[1-(2-methyl)indolyl]borane ( 2b ) ( ΔG ≠ (363 K) = 98.0 ± 1 kJ mol −1 ) must be ascribed mainly to steric hindrance rather than to a significant B double bond character.

Nitridosilicates - A Significant Extension of Silicate Chemistry

A novel synthetic approach is presented which starts from silicon diimide Si(NH)2 and makes available a manifold variety of novel nitridosilicates. Structurally these nitridosilicates represent a significant extension of the family of oxosilicates. With respect to their outstanding chemical and physical stability nitridosilicates might be of particular interest for the development of novel inorganic nitridic materials.

Synthesis of bicyclic N-pyrrolylboranes via hydroboration of 2-vinyl and 2-allylpyrrole

Abstract The hydroboration of 2-vinyl and 2-allylpyrrole with various hydroborating agents [Et 2 BH 2 BEt 2 , (9-BBN) 2 , (CH 2 ) 4 BH 2 B(CH 2 ) 4 , thex(H)BH 2 B(H)thex, Az-BH 2 —THF (Az = pyrrole, 2,5-dimethylpyrrole, indole), Et 2 O-BH 2 Cl, Me 3 Si(H)NB 2 H 5 ] leads in most cases finally to B-substituted bicyclic N-pyrrolylboranes 8–11 , 15–20 . In the case of the reaction with tetraalkyldiboranes(6), stable intramolecular 2-H-pyrrole-borane adducts 6 , 7 , 12–14 are formed first which, in the case of 6 , 12 and 13 , can be converted into the bicyclic N-pyrrolylboranes 8 , 15 and 16 respectively. Although the steric conditions in the bicyclic N-pyrrolylboranes are favourable, 11 B, 13 C and 14 N NMR data do not support any significant π interactions between the boron atoms and the heteroaromatic pyrrole system.

1,1-ORGANOBORATION OF MONO-1-ALKYNYLTIN COMPOUNDS USING DIALKYL(N-AZOLYL) BORANES-STEPWISE RING ENLARGEMENT OF BORON HETEROCYCLES

Abstract Mono-1-alkynyltin compounds 1–4 (Me3Sn-C ≡ CR1; R1 = H (1), Me (2), Ph (3), SnMe3 (4)) react with various dialkyl(N-azolyl)boranes 5–9 (azolyl = pyrrolyl (a), 2,5-dimethylpyrrolyl (b), indolyl (c), carbazolyl (d)) stereospecifically by 1,1-organoboration to give organometallic-substituted alkenes 10–20, 22–25, with the trimethylstannyl and the boryl group in cis-positions at the C=C bond. These reactions proceed via an alkynylborate-like zwitterionic intermediate ZI, and exchange of the azolyl against the 1-alkynyl group may compete (Eq. (1)(b)) with the 1,1-organoboration (Eq. (1)(a)), depending on the reactivity of the boron carbon bonds and the steric requirements. It is also shown that in addition to the products formed in the reactions with 1:1 stoichiometry other products result from the reaction of two equivalents of 1 with one equivalent of the borane. These products may be either 1,3-butadiene derivatives or allenes formed by allylic rearrangement. All products were characterized by 1H, 11B, 13C and 119Sn NMR. The molecular structure of the allene 24a was determined by X-ray structural analysis.

A tricyclic N-pyrrolylborane with an exceptionally stable B–N bond

Abstract Hydroboration of 2,5-diallylpyrrole by using a mixture diethylborane/triethylborane (Et2BH/Et3B) leads to the first tricyclic N-pyrrolylborane 3 which possesses an exceptionally stable B–N bond. In accordance with the pronounced Lewis-acidic character of 3, it is readily converted into adducts (6) or borates (9). The crystal structure of 3 was determined by X-ray analysis (triclinic, space group P1, Z=8) showing a fairly long B–N bond (mean value 144 pm), the first example of a diorgano N-pyrrolyl borane in which the pyrrole ring is not twisted against the C2BN-plane. Nuclear magnetic shielding (11B, 13C, 14N) was calculated using the GIAO procedure. BN(pp)π bonding is discussed on the basis of MO calculations.

Hydroboration of isoprene and 1,4-cyclooctadiene with N-azolylboranes

Abstract The hydroboration of isoprene with N-azolylboranes in THF [az-BH2-THF: az = N-pyrrolyl (1a), N-2,5-dimethylpyrrolyl (1b), N-indolyl (1c), N-carbazolyl (1d)] affords the boracyclopentane derivatives 2a–d. In the case of 2b,c, the reduced species 3b,c with a 2,5-dimethylpyrrolidinyl and a 2,3-dihydroindolyl group respectively attached to the boron atom are also present as side products. The hydroboration of 1,4-cyclooctadiene with 1a–c leads to mixtures of products containing both the 9-borabicyclo[3.3.1]nonane (6a–c) and the 9-borabicyclo[4.2.1]nonane derivative (7a–c). In the case of the reaction of 1d with 1,4-cyclooctadiene, the 9-(N-carbazolyl)-9borabicyclo[4.2.1]nonane 7d is formed selectively. The reaction of the corresponding tetraalkyldiboranes(6) with the azoles gives the boracyclopentane derivatives 2 and 4 in higher purity, and in the case of (H-9-BBN)2, all 9-(N-azolyl)-9-borabicyclo[3.3.1] nonanes 6a–d are obtained as the sole products.

Synthesis of novel heterocycles by the reaction of N,C-dilithio-2-allylpyrrole with electrophiles

Abstract N,C. dilithio-2-allylpyrtole ( 1 ) in THF reacts selectively with various element halides to give Z-isomers, and in the case of element dihalides (Me 2 SiCl 2 , Me 2 SnCl 2 , Cp 2 ZrCl 2 ) or SiCl 4 to give new heterobicycles [E  Si ( 7 ), Sn ( 8 ), Zr ( 12 )] or the spirosilane 6 . Treatment of 1 with triethoxyborane gives the heterobicyclic borate 9 which reacts with Me 3 SiCl to give the heterobicyclic ethoxyborane 10 . The synthetic potiential of 8 is shown by its transformation to the heterobicyclic ethylborane 11 . All compounds were characterized by multinuclear magnetic resonance, including the application of modified (Hahn-echo extended) polarization transfer pulse sequences for measurement of n J ( 29 Si, 15 N), 1 J ( 119 Sn, 15 N) and 2D 13 C/ 1 H heteronuclear shift correlations for the determination of signs of coupling constants n J ( 119 Sn, 13 C) and n+1 J ( 119 Sn, 1 H).

Synthesis of N,C-dilithio-2-allylpyrrole and its solvent-controlled reaction with electrophiles

Abstract The N,C-dilithiated product 1 of 2-allylphyrrole was prepared and studied by 7Li NMR in THF/HMPTA. Compound 1 reacts selectively in THF with electrophiles (H2O, Mel, Me3SiCl) to give the Z-isomers 2a–4a. In diethylether, the analogous reactions lead selectively to the E-isomers 2b–4b.