Synthesis, Coupling, and Condensation Reactions of 1,2-Diborylated Benzenes: An Experimental and Quantum-Chemical Study (original) (raw)
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A Convenient Synthesis and a NMR Study of the Diammoniate of Diborane
Chemistry - A European Journal, 2012
Hydrogen storage is defined by the US Department of Energy as a "grand challenge" to the implementation of hydrogen-powered-fuel-cell vehicles. [1] Ammonia borane (AB) is one of the most widely studied compounds for chemical hydrogen storage because of the high gravimetric hydrogen content (19.6 %), the good stability at room temperature, and the general availability. [2] In contrast, the diammoniate of diborane (DADB), an ionic dimer of AB that has the same gravimetric hydrogen-storage capacity as AB and better/lower hydrogen-release temperature, has attracted very little attention [3] as a potential hydrogen-storage material, mostly due to the difficulty in obtaining the pure compound. Although it has been recognized as a key intermediate in the dehydrogenation and oligomerization of AB, [3b] reports on DADB have essentially declined due to issues regarding the synthesis. Furthermore, the instability of the di-A C H T U N G T R E N N U N G ammoniate of diborane in organic solvents at ambient temperature leads to confusion about the purity of the sample. DADB, which was first reported by Stock in 1923 in a reaction between B 2 H 6 and NH 3 , [4] is a colorless, nonvolatile, saltlike compound. Although several structures have been proposed for DADB over the years, it remained a matter of debate until Parrys group
Theoretical Chemistry Accounts, 2009
Ab initio calculations were carried out to investigate the structures, binding energies, bonding, and NMR spin–spin coupling constants of complexes HLB=BLH, for L=CO, NH3, OH2, PH3, SH2, and ClH. Both B–B and B–H bonds lengthen on complex formation relative to singlet HBBH, and except for L=CO, the B–B bonds are double bonds. The order of stability of the trans isomers
A new and convenient synthesis of substituted benzobarrelenes
Tetrahedron Letters, 1985
Bromination of 3-bromo-6,7-benzobicyclo D.Z.flocta-2,6-diene at-50°C gave anti-tribromo adduct (5) in essentially quantitative yield. The double dehydrobromination of (5) was achieved using potassium tert.butoxide to give 2-bromo-benzobarrelene (7). Reaction of (7) with n-BuLi and subsequent quenching with CH3I, C02, and dimethylformamide afforded the corresponding substituted benzobarrelenes in high yield. Benzobarrelene (1) is a molecule of considerable potential mechanistic interest. Zimmerman et al. 2 have reported that benzobarrelene (1) undergoes two types of photochemical reactions, one lea$ing to benzocyclooctatetrane (2) proceeding from the singlet state of (1) via 21~+2~r cycleaddition and the other leading to semibulvalene (3) from the triplet excited state via di-nmethane rearrangement. ca 3 / I co :I-1-2 3 Furthermore, deuterium labelling studies revealed that of the two bonding routes; vinyl-vinyl bridging and vinyl-aryl bridging, the last one is mainly utilized. However, di-v-methane rearrangement was uniquely provided by the vinyl-vinyl bridging. By the introduction of a substituent in a vinyl location the symmetry of benzobarrelene skeleton is destroyed. Therefore, the number of possible initial bonding modes is increased to three 2?r+2T cycloaddition and six di-v-methane rearrangement. On this basis, vinyl substituted benzobarrelenes gain more important by elucidation of the mechanism of the 2~+hcycloaddition reaction and din methane rearrangement.
Reactions of B 2 ( o ‐tolyl) 4 with Boranes: Assembly of the Pentaborane(9), HB[B( o ‐tolyl)(μ‐H)] 4
Angewandte Chemie International Edition, 2021
Reactions of the diborane(4) B 2 (o-tolyl) 4 and monohydridoboranes are shown to give B(o-tolyl) 3 and (otolyl)BR 2 (R 2 = (C 8 H 14) 3, cat 4, pin 5, (C 6 F 5) 2 6) as the major products. The corresponding reaction with BH 3-sources gives complex mixtures, resulting from hydride/aryl exchange, dimerization and borane elimination. This led to the isolation of the first tetra-substituted pentaborane(9) HB[B(o-tolyl)(m-H)] 4 8. The reaction pathways are probed experimentally and by computations.
Angewandte Chemie International Edition, 2013
Vinyl boronates are extremely useful precursors, especially for the formation of new C À C bonds by cross-coupling and conjugate addition reactions. Although alkyne hydroboration is a powerful synthetic route, it is not applicable to the synthesis of trisubstituted vinyl boronates. Thus, alternative regio-and stereospecific methods are needed, particularly for subsequent use in the formation of tetrasubstituted alkenes, as the production of these important biologically active compounds as single isomers by classical methods is challenging. One simple approach to trisubstituted vinyl boronates is the functionalization of internal alkynes by metal-catalyzed 1,2-carboboration and 1,1-carboboration. The introduction of two selectively transformable moieties onto an internal alkyne should enable ready access to tetrasubstituted alkenes by successive cross-coupling reactions. Significant progress has been made in this area, particularly in the dimetalation of internal alkynes to provide two nucleophilic sites of distinct reactivity. The haloboration of internal alkynes is an attractive alternative to dimetalation, as it generates ambivalent synthetic intermediates that contain both a nucleophilic and an electrophilic position. These synthetic intermediates are ideally suited for the diversity-oriented synthesis of tetrasubstituted alkenes. To date, the application of alkyne haloboration with boron trihalides (BX 3 ) has been limited to terminal alkynes, and has proved an effective route to produce trisubstituted alkenes with excellent regio-and stereoselectivity. The haloboration of internal alkynes is unsuccessful with BCl 3 , and it is either slow or produces isomeric mixtures susceptible to BÀC bond cleavage when BBr 3 is used. Recent calculations found that the haloboration of internal alkynes with BCl 3 is endothermic, but as the Lewis acidity of BX 3 increases (Cl < Br < I), haloboration becomes exothermic, and the energy of the key transition state is also reduced. This result suggested that an increase in the electrophilicity at boron beyond that of BX 3 would facilitate the haloboration of internal alkynes.
Angewandte Chemie International Edition, 2008
In the rapidly developing field of terminal transition metal borylene complexes, a fascinating reactivity profile has begun to emerge, which is heavily dominated by reactions involving the partial or total cleavage of the M=B bond. [1] Importantly, terminal Group 6 aminoborylene complexes have been found to transfer the borylene moiety to other transition metals, which has proven invaluable in widening the variety of borylene complexes. Recently we reported the synthesis of aminoborirenes through a similar borylene transfer from Group 6 borylenes to alkynes. In 2007, Aldridge et al. reported the spectacular insertion reaction of carbodiimide substrates into the M=B and the usually unreactive N=B bonds of cationic terminal aminoborylene complexes, completely reorganizing the ligation of the boron atom. Furthermore, terminal borylene complexes have elicited comparisons to carbene complexes owing to their ability to undergo a metathesis reaction with Ph 3 PS. With benzophenone and dicyclohexylcarbodiimide (DCC), intermediate products containing a four-membered ring were obtained, which were formed regioselectively by addition of the more electronegative elements of C = N or C = O to the M = B bond, suggesting an "interrupted" metathesis by [2+2] cycloaddition. As the transfer of a borylene fragment to an alkyne is facile with Group 6 borylene complexes, we were interested in their behavior with olefins, which we presumed would present a greater resistance towards functionalization. In addition to the lower reactivity of the C = C double bond, the simple transfer product (with a saturated three-membered BCC ring) would lack the 2p-electron aromatic stabilization of borirenes. We thus turned our attention to the reactivity of [(OC) 5 Cr=BN(SiMe 3 ) 2 ] (1) towards unactivated olefins, and we present herein the first selective insertion of the borylene moiety into a C À H bond.