B—H bond

For the ground state of BH3 you would fill the bottom level (B-H bonding), af and e orbitals, with 6 eleetrons. 

Formal charges are based on Lewis structures m which electrons are considered to be shared equally between covalently bonded atoms Actually polarization of N—H bonds m ammonium ion and of B—H bonds m borohydride leads to some transfer of positive and negative charge respectively to the hydrogens... 

There is a pronounced tendency for boron to become bonded to the less substituted car bon of the double bond Thus the hydrogen atoms of diborane add to C 2 of 1 decene and boron to C 1 This is believed to be mainly a steric effect but the regioselectivity of addition does correspond to Markovmkov s rule m the sense that hydrogen is the neg atively polarized atom m a B—H bond and boron the positively polarized one... 

Tetrasubstituted and some hindered trisubstituted alkenes react rapidly only to the monoalkylborane stage. Rarely, when the tetrasubstituted double bond is incorporated in a cycHc stmcture, does hydroboration under normal conditions fail (25—27). However, such double bonds may react under conditions of greater force (25,28—31). Generally, trialkylboranes are stable at normal temperatures, undergoing thermal dissociation at temperatures above 100°C (32—34). In the presence of B—H bonds, trialkylboranes undergo a redistribution reaction (35—38). 

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

In principle MOJ diagrams ean be extended to more dimensions, for example by also including the B-H bond order in the above elimination reaetion, but this is rarely done, not least beeause of the problems of illustrating more than two dimensions. 

In addition to the oxymercuration method, which yields the Markovnikov product, a complementary method that yields the non-Markovnikov product is also useful. Discovered in 1959 by H. C. Brown and cailed hydroboration, the reaction involves addition of a B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2. Oxidation of the organoborane by reaction with basic hydrogen peroxide, H2O2, then gives an alcohol. For example ... 

The structure shown above still has two remaining B— H bonds (look at the BH2 group), and so the reaction can occur again with those B— H bonds, hi other words, one molecule of BH3 can react with three molecules of alkene to give a trialkylborane ... 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

Diborane also has a useful pattern of selectivity. It reduces carboxylic acids to primary alcohols under mild conditions that leave esters unchanged.77 Nitro and cyano groups are relatively unreactive toward diborane. The rapid reaction between carboxylic acids and diborane is the result of formation of a triacyloxyborane intermediate by protonolysis of the B-H bonds. The resulting compound is essentially a mixed anhydride of the carboxylic acid and boric acid in which the carbonyl groups have enhanced reactivity toward borane or acetoxyborane. 

In addition to reactions at the Zn-C bond, the complexes [BpBut]ZnR also exhibit reactivity at the B-H bond. Thus, [BpBu ]ZnR reacts with aldehydes and ketones, (CH20) , MeCHO, and Me2CO, to give HB(OR )(3-Butpz)2 ZnR (R = Me, Et, Pr1), as a result of insertion into the B-H bond. In contrast, the tris(pyrazolyl)hydroborato complexes [TpBut]ZnR are inert towards B-H insertion under similar conditions. 

Initially, two plausible mechanisms were considered, as depicted in Scheme 5.16. The first was a hydroboration route (a), where the B-H bond was added across the olefin from the same face of S-0 and upon aqueous work-up, the resulting C-B bond was replaced with a C-H bond. The tis B-H addition to the olefin led to the cis-stereochemistry of the two adjacent aryl substituents. The reduction of the sulfoxide oxygen occurs in the next step. The alternative mechanism was the borane reduction route (h), which was similar to 1,4-addition of hydride,... 

Evolution did not use this element, only in certain plants is it important as a trace element. The element became well-known because of heat-resistant borosilicate glasses. Boranes are chemically interesting as B-H bonds react very specifically. Perborates are used in laundry detergents (Persil). The hardness of cubic boron nitride approaches that of diamond. Amorphous (brown) boron burns very quickly and gives off much heat and is therefore used in solid-propellant rockets and in igniters in airbags. 

UB and UB H solution-state NMR spectra (see Fig. la) clearly show the different boron environments within the m-carborane unit. The eight peaks in the -10 to 0 ppm range are indicative of the different boron environments in the w-carborane cage, including B—B, B—C, and B—H bonds. The spectrum also contains... 

Fig. 20.11. Substrate quality obtained by comparing basolateral-to-apical with apical-to-basolateral transport of substrates in polarized cell monolayers of MDR1-transfected cell lines [86] plotted versus (A) the log of the air/water partition coefficient, or (B) H-bond energy (arbitrary units, EUh cf. text). Units of the air/ water partition coefficient were [M ]. Compound (concentrations in Ref. [86] in brackets) were clozapine (50 nM) (1) cyclosporin A (2 tM) (2) daunorubicin (3) dexamethasone (2 tM) (4) digoxin (2 pM) (5) domperidone (2 pM) (6) etoposide (7) flunitrazepam (500 nM) (8) haloperidol (50 nM) (9) ivermectin (50 nM) (10) loperamide (2 pM) (11) morphine (2 pM) (12) ondansetron...

The transition state passes over to become an alkylborane. The other B-H bonds of the alkylborane can undergo similar additions, leading finally to a trialkylborane. 

Mannig and Noth reported the first example of rhodium-catalyzed hydroboration to C=C bonds in 1985.4 Catecholborane reacts at room temperature with 5-hexene-2-one at the carbonyl double bond when the reaction was run in the presence of 5mol.% Wilkinson s catalyst [Rh(PPh3)3Cl], addition of the B—H bond across the C=C double bond was observed affording the anti-Markovnikoff ketone as the major product (Scheme 2). Other rhodium complexes showed good catalytic properties ([Rh(COD)Cl2]2, [ Rh(PPh3)2(C O )C 1], where... 

Treatment of ZnMe2 with the thallium salt of a bis(pyrazolyl)hydroborate gave the methylzinc complex [BpZnMe] 104 in which zinc is three coordinate.162 This complex reacted with paraformaldehyde, Scheme 73, with the direct insertion of the CH20 moiety into the B-H bond to produce the methylzinc complex 105 (Figure 51), in which zinc is tetrahedrally surrounded by the methyl group and the newly formed N,N,O-ligand.163... 

In this equation, fH is the ionization potential for H (1312kJ mol-1), fB is the ionization potential for the base B, and EB+ is the energy of the B+-H bond. The term IB is subtracted from fH (the last term is of lesser importance), which leads to the conclusion that the smaller the value for IB, the greater the proton affinity. Because H+ reacts by removing electron density from B, the easier this process is to accomplish, the smaller the value of IB. For the molecules CH4, NH3, H20, and HF, the proton affinities are 527, 841, 686, and 469 kj mol-1, respectively. These values correlate well with the ionization potentials of the molecules, which are in the order NH3 < H20 < CH4 < HF. 

A number of other models were considered and tested (for example, direct B—H bonding). The most significant test was the IR vibrational spectrum, where a sharp absorption band at 1875 cm-1 was found, corresponding to the Si—H stretch mode softened by the proximity of the B-atom. Had the hydrogen been bonded to boron, a sharp absorption band at 2560 cm-1 would have been expected. Also, Johnson (1985) showed that deuteration produced the expected isotopic shift. The most definitive and elegant proof of the correctness of the Si-H-B bonding model was provided by Watkins and coworkers (1990), on the basis of a parametric vibrational interaction between the isotopes D and 10B. 

It was argued that the Raman spectra of the B local mode provide further evidence for the BC configuration of the B—H complex (Stutzmann and Herrero, 1988b Herrero and Stutzmann, 1988a). The B vibration of the complex is not affected by the substitution of D for the lighter H this implies that the B—H bond is weak, consistent with the BC model. 

B4HI0 contains a total of 4x3 + 10x1 = 22 valence electrons or 11 pairs. Ten of these pairs could be allocated to form 10 B—H bonds, leaving but one pair to bond the four B atoms together, clearly an electron deficient situation. 

Figure 3.100 Three-center NBOs and occupancies for H3BC2H4 (7t->-p+ type). Note the high occupancy (0.2567e) of TcBC(7t)+, largely as a result of its strong interaction (estimated second-order stabilization 34.69 kcal mol-1) with the inplane B—H bond, which is thereby weakened and lengthened.

The number (x) of such end groups (or extra B—H bonds) can be combined with the numbers (s) of Tbhb bonds and (/) of tbbb bonds, as well as the number (y) of singly bridged B—B pairs, to give the well-known styx code ... 

When added to the p additional terminal B—H bonds (one per B), the sum of styx values accounts for all two-electron bonds in the molecule, which must therefore correspond to all 3p + q valence electrons of the BPH(/ molecule, if no lone pairs or other bond types are present,... 

---