Boron—sulfur bonds hydrogen

The most widely used method for the synthesis of iminoboranes involves the 1,2-addition of boron-element bonds such as boron-hydrogen, boron-halogen, boron-carbon, or boron-sulfur bonds across the C=N bond of nitriles thereby producing variously substituted iminoboranes (Eq. (2)). The formation of iminoboranes as well as the stability of the products depends on the substituent on the nitrile group, the nature of the boron-element bond to be cleaved during the 1,2-addition across the C N bond, and to a lesser extent on the non-reacting boron substituents 26T... 

Of the solvents, aromatic and olefinic hydrocarbons are r-donors ( r-EPD) alcohols, ethers, amines, carboxamides, nitriles, ketones, sulfoxides and N- and P-oxides are n-donors (n-EPD), and haloalkanes are cr-donors (cr-EPD). Boron and antimony triha-lides are acceptor solvents (r-EPA), as are halogens and mixed halogens (c-EPA), and liquid sulfur dioxide (ti-EPA). In principle, all solvents are amphoteric in this respect, i.e. they may act as a donor (nucleophile) and an acceptor (electrophile) simultaneously. For example, water can act as a donor (by means of the oxygen atom) as well as as an acceptor (by forming hydrogen bonds). This is one of the reasons for the exceptional importance of water as a solvent. 

Most of the common elements in organic compounds—C, N, O, and the halogens— follow the octet rule. Hydrogen is a notable exception, because it accommodates only two electrons in bonding. Additional exceptions include boron and beryllium (second-row elements in groups 3A and 2A, respectively), and elements in the third row (particularly phosphorus and sulfur). 

Acid-catalysed addition of alcohols to olefins leads to ethers. It is catalysed by concentrated sulfuric acid,151 hydrochloric acid, acid salts, or aluminum chloride and hydrogen tetrafluoroborate and perchloric acid have also proved valuable,152,153 as has boron trifluoride.154 Addition of trisubstituted ethylenes such as isobutene and isopentene to alcohols is particularly easy it yields te/T-alkyl ethers.151 Disubstituted ethylenes react less easily, requiring higher temperatures and giving poorer yields. Cycloalkenes react only if there is a substituent attached to one of the doubly bonded atoms.155 Primary alcohols add more easily than do secondary ones, and tertiary alcohols rarely add at all. 

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