Boron trihalides Lewis acidity

Boron-based Lewis acids are often used in organic syntheses. Since the boron atom has an empty / -orbital, many boron compounds can function as Lewis acids. Typical boron Lewis acids are boron trihalides, for which Lewis acidity increases according to the order of fluoride < chloride < bromide < iodide, the reason for this order being the relative abilities of the different halogens to act as 7r-donors to boron. 

Boron trihalides are strong Lewis acids that react with a wide collection of Lewis bases. Many adducts form with donor atoms from Group 15 (N, P, As) or Group 16 (O, S). Metal fluorides transfer F ion to BF3 to give tetrafluoroborate salts LiF + BF3 LiBF4 Tetrafluoroborate anion is an important derivative of BF3 because it is nonreactive. With four <7 bonds, [BF4 ] anion has no tendency to coordinate further ligands. Tetrafluoroborate salts are used in synthesis when a bulky inert anion is necessary. 

The complexes formed with boron trihalides are decomposed to pyridine by boiling water. Complexes with other Lewis acids behave similarly. 

Boron Mechanism. Boron functions as a flame retardant in both the condensed and vapor phases. Under flaming conditions boron and halogens form the corresponding trihalide. Because boron (rihalides are effective Lewis acids, they promote cross-linking, minimizing decomposition of the polymer into volatile flammable gases. These trihalides arc also volatile thus they vaporize into the (lame and release halogen which Ihen functions as a Maine inhibitor. 

The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Thus, for example, in the case of boron trihalides, boron trifluoride coordinates best with fluorides, but not with chlorides, bromides, or iodides. In coordination with Lewis bases such as amines and phosphines, BF3 shows preference to the former (as determined by equilibrium constant measurements).66 The same set of bases behaves differently with the Ag+ ion. The Ag+ ion complexes phosphines much more strongly than amines. In the case of halides (F, CP, Br, and P), fluoride is the most effective base in protic acid solution. However, the order... 

The short B-F distance in BF3 (130 pm compared with 143 pm in BFj") and the large B-F bond energy are suggestive of partial double bond character. BF3 is a weaker Lewis acid than the other boron trihalides, which may indicate that the four boron valence orbitals in BF3 are more fully engaged than in BC13 etc. Molecules of the type R2BNR2 are believed to have substantial p -p bonding ... 

Lewis acids such as boron trihalides will also form stronger bonds with fluoride than with the other halogens, and can be used to abstract fluoride selectively from halogenated compounds. Some examples of Friedel-Crafts alkylations with fluoro-haloalkanes in which only fluoride is displaced are sketched in Scheme 4.14. As shown by the last example, however, hydride migrations can readily occur under such strongly acidic reaction conditions. 

The chemistry of the boron trihalides has been extensively studied. These compounds are strong Lewis acids and form a wide range of simple 1 1 adducts that have served as model compounds for the study of Lewis acid-base interactions. Many reviews of their coordination chemistry have appeared (41, 60, 66,120, 174, 178) and these are summarized in the more recent reviews appearing in the mid-1960s (41,120). 

The four boron trihalides are quite reactive and undergo similar reactions with a variety of other chemicals. Most of these reactions are based on the strong Lewis acidity of the halides. The boron trihalides form complexes with ethers (equation 16) and other Lewis bases based on N, S, P, and As atoms. 

Heavier halogens such as chlorine and bromine provide their boron trihalides with better complexing ability than BF3, but their facile hydrolysis makes BF3 the more effective Lewis acid choice. 

Tris(pentafluorophenyl)borane, a compound first made by Stone et al. in 1963, has in recent years become one of the most widely applied boron reagents, as a highly effective activator for metallocene-based olefin polymerisation catalysts. There are several reasons for this it is a strong Lewis acid, unlike boron trihalides it is resistant to hydrolysis, and it possesses strong B-C and C-F bonds which make it essentially inert to chemical attack for example, it can be recovered unchanged from neat elemental bromine. 

Tri(organoseleno)boranes 35 are prepared from boron trihalides and organic selenolates as stable compounds (Scheme 35) [63]. These selenoboranes have been shown to be useful for the conversion of carbonyl compounds into seleno-acetals 36 [64] and the selective ring opening of epoxides [65]. Recently, it was reported that tri(phenylseleno)borane reacts with cyclic ethers to produce m-hydroxyalkyl phenyl selenides 37 in the presence of a catalytic amount of Lewis acid [43]. 

An i.r. study of the mixed boron trihalide adducts of carbonyl donors (ethyl acetate and benzophenone) shows that the mixed adducts do indeed possess Lewis acidities intermediate between those of the corresponding BX3 systems.267... 

The boron trihalide complexes with a number of phosphine sulphides and selenides can be isolated for the chloride, bromide, or iodide but, consistent with its reduced Lewis acidity, the trifluoride does not react.600 Alkyldithio-phosphonic acids give tin, lead, and mercury derivatives such as Me3SnSP-(S)FEt, Pb[SP(S)FMe]2, and MeHgSP(S)FMe, which are monomeric in solution, and there is n.m.r. evidence for a bidentate phosphonate group in the tin compound.601 Vanadyl chelates of alkoxy-ethyl and alkoxy-phenyldi-thiophosphonates (87) have been synthesized and e.s.r. measurements... 

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