Borane hybridization

The triaLkoxy(aryloxy)boranes are typically monomeric, soluble in most organic solvents, and dissolve in water with hydrolysis to form boric acid and the corresponding alcohol and phenol. Although the rate of hydrolysis is usually very fast, it is dependent on the bulk of the alkyl or aryl substituent groups bonded to the boron atom. Secondary and tertiary alkyl esters are generally more stable than the primary alkyl esters. The boron atom in these compounds is in a trigonal coplanar state with bond hybridization. A vacantp orbital exists along the threefold axis perpendicular to the BO plane. 

For c/rwo-boranes and for the larger open-cluster boranes it becomes increasingly difficult to write a simple satisfactory localized orbital structure, and a full MO treatment is required. Intermediate cases, such a.s B5H9, require several resonance hybrids in the localized orbital... 

P-Chirogenic phosphine/sulfide hybrid phosphine-boranes 80 were synthesized from the reaction between (l )-tosylates 79 [94] and sodium thiolate in DMF at ambient temperature as depicted in Scheme 12, or alternatively by a one pot synthesis consisting of the nucleophilic attack of the chirally induced hthium salt of 74 on phenyl disulfide. Both methodologies provided the desired sul-fide/phosphine boranes in excellent yields [10]. 

When some boron atoms in non-classical boranes are exchanged by isolobal C+ units, the multicenter bonding MOs look qualitatively the same, but the contribution of carbon hybrid orbitals is larger than those from boron atoms [compare Figures 3.2-3(b) and (c)]. This polarization is due to the higher electronegativity of carbon versus boron atoms. 

Controlled radical polymerization (CRP) is an attractive tool, because of the resultant controllability of polymerization, and because of it being a versatile method to synthesize of well-defined polymer hybrids. The three main radical polymerization techniques, ATRP, NMP, and RAFT polymerization, have thus been employed. Other techniques, such as the oxidation of borane groups, have also been studied. In general, using CRP techniques, block copolymers can be synthesized from terminally functionalized PO as PO macroinitiator, and block copolymers can be prepared from functionalized PO produced by the copolymerization of olefins with functional monomers. 

Fig. 9 Synthetic route of PE hybrids by living radical polymerization via borane-containing PE...

The boron atom in BH3 is sf hybridized with a vacant p orbital perpendicular to the plane of the three boron-hydrogen bonds. Thus borane and its derivatives are electrophilic (Lewis acidic) and combine readily with electron-rich species. For example, borane interacts with one of the lone pairs on the oxygen atom of tetrahydrofuran as shown below. 

Boranes, carboranes, and metallocarboranes are electron-deficient compounds in which B-H-B three-center two-electron (3c-2e) bridged bonds are found. The B-H-B bond results from the overlap of two boron sp3 hybrid orbitals and the Is orbital of the hydrogen atom, which will be discussed in Chapter 13. 

Borane has the same structure as a carbocation. The boron is sjr hybridized, with trigonal planar geometry, and has an empty p orbital. Although neutral, it is electron deficient because there are only six electrons around the boron. It is a strong Lewis acid. An electron-deficient compound often employs unusual bonding to alleviate somewhat its instability. In the case of borane, two molecules combine to form one molecule of diborane ... 

At first, N-heterocyclic carbenes 20 look bizarre to the organic chemist, since they are organic/inorganic hybrid compounds. However, borazines, sometimes called inorganic benzene , are isoelectronic with benzene and are therefore extraordinarily stable heterocycles. Exchange of a borane... 

This section presents the structure of the boron hybrides and is arranged in accordance with the relationship defined by Wade s Rules and expressed by Williams and Rudolph. Thus for the boranes containing six or more pairs of skeletal bonding electrons, the relationship between the structures of the closo-, nido-, arachno-and hypho-species is described. In cases where the parent borane does not exist, examples from heteroboranes with the correctly predicted structure based on Williams coordination number pattern recognition theory (CNPR) of borane structures will be described 70. Treated separately will be mono- and diborane species and also species with more than 12 boron atoms. Although there have been several reviews on the structures of the boranes in recent years none have used the current approach89. ... 

Compounds of the same three elements with only three bonds are more complicated. Borane, BH3, has only three pairs of bonding electrons. The central boron atom bonds to only three other atoms. Wc can therefore describe it as being sp2 hybridized with an empty p orbital. 

Borane is isoelectronic with the methyl cation, CH3. All the arguments we have just applied to borane also apply to Me+ so it too is sp2 hybridized with a vacant p orbital. This will be very important when we discuss the reactions of carbocations in Chapter 17. 

Now what about ammonia, NH3 Ammonia is not isoelectronic with borane and Me+ As well as three N-H bonds, each with two electrons, the central nitrogen atom also has a lone pair of electrons. We have two choices either we could hybridize the nitrogen atom sp2 and put the lone pair in the p orbital or we could hybridize the nitrogen sp3 and have the lone pair in an sp3 orbital. 

Copoly condensation of Functional Silanes and Siloxanes in Solution Using tris(pentafluorophenyl)borane as a Catalyst in a View to Generate Hybrid Silicones... 

Abstract This paper proposes new ways of preparation of hybrid silicones, i.e. an alternated multiblock seqnence of silicone and alkyl spacers, via a polycondensation process catalyzed by the tris(pentaflnorophenyl)borane, a water-tolerant Lewis acid, between methoxy and hydrogeno fnnctionalized silanes and siloxanes at room temperature and in the open air. The protocol was first developed with model molecules which led to polydimethylsiloxane (PDMS) chains, in order to seize the best experimental conditions. Several factors were studied such as the contents of each reactants, the nature of the solvent or the rate of addition. The best conditions were then adapted to the synthesis of hybrid silicones, condensing alkylated oligo-carbosiloxanes with methoxy or hydrogeno chain-ends and complementary small molecules. A systematic limitation in final molar masses of hybrid silicones was observed and explained by the formation of macrocycles, which cannot redistribnte or condense further while formed. 

Slee, Lin, and Mingos have analyzed the geometries of small silicon clnsters, exploiting a coimection with the deltahedral boranes and carboranes. A silicon atom with an ont-pointing lone pair and three in-pointing hybrids is isolobal to a B-H vertex. However, Si clusters have 4n valence electrons, two fewer than the corresponding boranes, B H . Hence, we shonld not expect the two series of clusters to be isostructural, but it may be helpful to start... 

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