Three chiral vinyldioxaza­borocanes

The structures of three chiral vinyldioxaza­borocanes are reported, namely (2E)‐ and (2Z)‐6‐benzyl‐2‐buten‐2‐yl‐1,3,6,2‐dioxaza­borocane, C₂₇H₃₀BNO₂, (II) and (III), respectively, and (2Z)‐2‐buten‐2‐yl‐6‐isobutyl‐1,3,6,2‐dioxaza­boro­cane, C₂₄H₃₂BNO₂, (IV). These compounds may be useful in asymmetric reactions. In the structures reported here, the N—B donor–acceptor bond is longer than in any previously reported analogous compounds.     

Synthesis of spiro-fused (C5)-isoxazolino-(C4)-pyrazolones (1-oxa-2,7,8-triazaspiro[4,4]-2,8-dien-6-ones) via 1,3-dipolar cycloaddition and cycloelimination

An efficient and selective method for the synthesis of spiro-fused (C5)-isoxazolino-(C4)-pyrazolones (C) is reported. The process consists of utilizing the Baylis-Hillman reaction-or a quicker, stepwise MAC procedure-to give I followed by 1,3-dipolar cycloaddition and Swern oxidation to give beta-ketoesters H, which were condensed with hydrazine derivatives to provide hydrazones that underwent cycloelimination. These novel spiro-fused (C5)-isoxazolino-(C4)-pyrazolones were confirmed by spectroscopic analysis as well as single-crystal X-ray of 5. We also concluded that all condensations/cycloeliminations, except with hydrazine itself, were more effective with catalysts or higher reaction temperatures. For example, TiCl(4) was an efficient catalyst for hydrazone formation and cycloelimination with methylhydrazine, while phenyl-, benzyl-, and (4-methoxyphenyl)hydrazine reacted effectively without catalyst in refluxing xylene.     

Synthesis and characterization of the non-Kekulé, singlet biradicaloid Ar'Ge(micro-NSiMe(3))(2)GeAr' (Ar' = 2,6-Dipp(2)C(6)H(3), Dipp = 2,6-i-Pr(2)C(6)H(3))

Reaction of Ar'GeGeAr' (1) with an excess of Me3SiN3 gives the non-Kekulé, biradicaloid Ar'Ge(mu-NSiMe3)2GeAr' (3, Ar' = 2,6-Dipp2C6H3, Dipp = 2,6-i-Pr2C6H3) which has a planar Ge2N2Si2 array and pyramidal geometry at the germaniums. DFT calculations for the model MeGe(mu-NSiH3)2GeMe indicate no Ge-Ge bonding and a singlet ground state. The calculated energy difference between the optimized singlet and triplet states is 17.51 kcal/mol.     

Synthesis and structural characterization of [PhE(BNMe2)2]2 (E = P or As): Six-Membered P2B4 and As2B4 Rings with Unusual Structures

The reaction of either Li₂PPh or Li₂AsPh with the diborane(4) derivative B₂(NMe₂)₂Br₂ affords the compounds [PhP(BNMe₂)₂]₂ ( 1 ) or [PhAs(BNMe₂)₂]₂ ( 2 ) in good yield. Both 1 and 2 have cyclic structures featuring non-planar P₂B₄ or As₂B₄ six-membered rings which have chair configurations. Although all four borons in each ring have planar coordination, the two phosphorus or arsenic centers have different degrees of pyramidalization. Bond distances within the rings indicate that the B? B, B? P or B? As bonds are single, whereas the exo-B? N bond lengths are consistent with significant π-bonding. The ring structures of 1 and 2 are in sharp contrast to the related boron-nitrogen species (t-BuN)₂N₄Me₄ which has a nido-N₂B₄ framework. The attempted synthesis of the nitrogen analogue of 1 or 2 by using a similar approach did not result in the isolation of [PhN(BNMe₂)₂]₂, instead the tetramino diborane(4) species [B(NMe₂)NHPh]₂ ( 3 ), which has a structure similar to other tetramine diborane(4) compounds, was isolated.     

Cation-controlled self-assembly of a hexameric europium wheel

The simple asymmetric tetradentate ligand 2,2':6',2' '-terpyridine-6-carboxylic acid leads to the self-assembly of the first europium nanowheel containing europium ions in two different coordination environments. Moreover the self-assembly of bis(terpyridinecarboxylate) europium species to form a hexameric wheel capable of strongly binding LnIII cations is controlled by the ligand/cation ratio.     

Intramolecular Diels-Alder Reactions of Decatrienoates: Remote Stereocontrol and Conformational Activation(1)

The intramolecular Diels-Alder (IMDA) reactions of C(8)-substituted decatrienoates have been studied. The stereospecific formation of 11 via an endo-boat-9 transition state attests to the powerful directing influence of a C(8) substituent in the IMDA of decatrienoate. In addition, the contrasting observations that stereospecific 9 --> 11 occurs at room temperature while the nor-tert-butyl substrate (4a) requires 125 degrees C/5 h reaction conditions and produces a 60:40 mixture of diastereomers provide clear evidence that a bulky C(8) substituent is a powerful conformational activator of the IMDA.