Boronic tetracoordinated adducts

If there is no other interaction, the reaction proceeds through an acyclic TS and steric factors determine the amount of syn versus anti addition. This is the case with BF3, where the tetracoordinate boron-aldehyde adduct does not offer any free coordination sites for formation of a cyclic TS. Stereoselectivity increases with the steric bulk of the silyl enol ether substituent R1.50... 

The nitrogen chemical shifts of the tetracoordinate boron-nitrogen adducts, R3B - NR3 (Table X, note f)> have been measured. (92) If we compare the data for the corresponding amines (Table VIII) with those for the adducts, high-frequency shifts are observed upon the formation... 

Reetz et al. have published a heterotopic receptor, which in addition to a crown ether moiety for complexation of potassium cations, contains a g-bonded Lewis acidic boron center that is able to form a tetracoordinated adduct with anions and with a selectivity for fluoride anions [ref 31]. Novel artificial polytopic receptors for binding of zwitterionic aminocarboxylates have been also published [ref 32]. 

Catalysts derived from triflic acid decompose at appreciable rates at or above 0 °C, which limits the utility of these reagents in Diels-Alder reactions. Switching to triflimide as the acid source resulted in protonation of 45 to produce 53.16 H NMR from -80 to 23 °C showed the formation of three species including 53 and two diastereomeric tetracoordinated boron species in a ratio of 1 1.2. Additionally, 53 was found to have greater thermal stability and superior catalytic efficiency compared to 46/47 for less reactive dienes. This was illustrated in the Diels-Alder reaction of 54 with 55 to produce adduct 56. 

Amino acids can condense with boronic adds to form 1 1 chelates of type 62 [146]. The tetracoordinate structure of these adducts is very apparent by NMR due to the formation of a new stereocenter at boron. Interestingly, 4-boronophenylalanine (63),... 

Tetracoordinate Boron in Pyridine/Borane Lewis Base Adducts... 

In addition to tricoordinate boron which acts as a strong electron acceptor because of its vacant p-orbital, negatively charged electron-donating tetracoordinate boron in two variations, zwitterionic molecules and as pyridine/borane Lewis base adducts, shows interesting nonlinear optical properties. 

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