Boron hydroboration chemistry

Boron Hydrides Boron Inorganic Chemistry Boron Metallacarbaboranes Boron Nitrogen Compounds Boron Polyhedral Carboranes. Hydroboration Catalysis. 

In addition to using boron-hydrides as the reactive reagent in hydroboration chemistry, compounds with B-B bonds have also been utilized successfully. ... 

Asymmetric boronic esters synthesized by hydroboration chemistry have been converted into asymmetric carboxylic acid derivatives having carboxyl carbon in place of the boron atom via homologation with (dichloromethyl)lithium and oxidation [7]. This sequence is relatively inefficient because of the several steps required. If the asymmetric boronic ester is generated via catalytic hydroboration with catecholbo-rane, the homologation process is a more efficient way of introducing the carboxyl carbon [72],... 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

The synthetic method used to accomplish this is an indirect one known as hydroboration-oxidation. It was developed by Professor Herbert C. Brown and his coworkers at Purdue University in the 1950s as part of a broad progran designed to apply boron-containing reagents to organic chemical synthesis. The number of applications is so large (hydroboration-oxidation is just one of them) and the work so novel that Brown was a corecipient of the 1979 Nobel Prize in chemistry. 

This reaction, now termed hydroboration, has opened up the quantitative preparation of organoboranes and these, in turn, have proved to be of outstanding synthetic utility. It was for his development of this field that H. C. Brown (Purdue) was awarded the 1979 Nobel Prize in Chemistry . Hydroboration is regiospecific, the boron showing preferential attachment to the least substituted C atom (anti-Markovnikov). This finds ready interpretation in terms of electronic factors and relative bond polarities (p. 144) steric factors also work in the same direction. The addition is stereospecific cis (syn). Recent extensions of the methodology have encompassed the significant development of generalized chiral syntheses. 

Hydroboration, discovered by Herbert C. Brown of Purdue University (co-winner of the Nobel Prize for Chemistry in 1979), involves an addition of a H-B bond (a boron hydride) to an alkene. 

Hydroboration -oxidation was first reported by H.C. Brown in 1959. For his studies on boron containing organic compounds, Brown shared the 1979 Nobel prize in Chemistry with G. Wittig. 

In the interim period, results have accumulated steadily, in endeavors to address and extend the chemistry beyond the initial perceived limitations. These limitations include the following (a) the effective catalytic syntheses are confined to the reactions utilizing catecholborane (b) the scope of alkenes for which efficient rate, regio- and enantio-selectivity can be achieved is limited, and (c) the standard transformation mandates the oxidation of the initially formed (secondary) boronate ester to a secondary alcohol, albeit with complete retention of configuration [8]. Nonetheless, for noncatalytic hydroboration reactions that lead to the formation of a trialkylborane, a wide range of stereo-specific transformations may be carried out directly from the initial product, and thereby facilitate direct C-N and C-C bond formation [9]. 

The haloboranes BH2C1, BH2Br, BHC12, and BHBr2 are also useful hydroborating reagents.124 These compounds are somewhat more regioselective than borane itself but otherwise show similar reactivity. The most useful aspects of the chemistry of the haloboranes is their application in sequential introduction of substituents at boron. The... 

Hydroboration is the term given to the addition of a boron-hydrogen bond to either the carbon-carbon double bond of an alkene (Equation B 1.1) or the carbon-carbon triple bond of an alkyne (Equation B1.2). The first examples of hydroboration were reported in 1956 from the laboratories of H.C. Brown and since then the reaction has found many important applications in organic chemistry. Indeed, in 1979 Brown was awarded the Nobel chemistry prize for his contributions to hydroboration and related areas of reactivity. 

Several reactions in organometaUic chemistry also appear to contravene the rule, but which can be explained in a somewhat similar way. Hydrometallation [5.45, see (Section 5.1.3.4) page 162], carbometallation, metallo-metallation, and olefin metathesis reactions are all stereospecifically suprafacial [2 + 2] additions to an alkene or alkyne, for which the all-suprafacial pathway is forbidden. Hydroboration, for example, begins with electrophilic attack by the boron atom, but it is not fully stepwise, because electron-donating substituents on the alkene do not speed up the reaction as much as they do when alkenes are attacked by electrophiles. Nevertheless, the reaction is stereospecifically syn—there must be some hydride delivery more or less concerted with the electrophilic attack. The empty p orbital on the boron is the electrophilic site and the s orbital of the hydrogen atom is the nucleophilic site. These orbitals are orthogonal, and so the addition 6.126 is not pericyclic. 

Polyhedral hydroborate anions (commonly referred to as polyhedral borane anions in the current literature) of generic formula have assumed an unusual status in inorganic chemistry within recent years, not only because of their high thermal and hydrolytic stability, but also because of the wide range of geometries which the boron atom skeletons in compounds of the series (n = 6-12) can adoptd Convenient syntheses... 

Summary Precursor-derived quaternary Si-B-C-N ceramics frequently possess an enhanced thermal stability compared to SiC, SisN4 or Si-C-N ceramics. The stability of the materials towards crystallization and/or decomposition is directly coimected to the molecular structure and the elemental composition of the polymeric precursors. This paper highlights recent investigations on the synthesis of boron-modified polysilazanes and polysilylcarbodiimides. Hydroboration of polyvinylsilazanes and dehydrocoupling reactions of boron-modified silanes with ammonia or amines as well as cyanamide are described. It is shown that simple organosilicon chemistry provides a means to efficiently optimize ceramic yields and tune elemental composition as well as thermal properties of the polymer-derived ceramics. 

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