Studies in Stereochemistry. I. Steric Strains as a Factor in the Relative Stability of Some Coördination Compounds of Boron

Abstract

The concept of free rotation about the single bond has been a basic postulate of the stereochemist. This belief is based upon the failure of II Reaction proceeds to right: R = CHS < (CHS)2CH < (CH3)3C.

Acid-base reactions in benzene and other organic solvents: Behavior of bromphthalein magenta with different classes of organic bases

“…Furthermore, recent experiments have shown that in some instances steric factors interfere with the est.imation of r elative basicities [59,60] . For example, the basicities of tertiary aliphatic amines are lower than would be predicted on theoretical grounds [60].…”

Main Group Lewis Acids in Frustrated Lewis Pair Chemistry: Beyond Electrophilic Boranes

“…In 1942, Brown reported that no adduct formation occurred between BMe 3 and 2,6-lutidine. 2 This stood in contrast to the classical donoracceptor adduct formed by the less sterically-encumbered combination of BF 3 and 2,6-lutidine (Scheme 1). Subsequent work by Wittig These observations marked the beginnings of what is now known as "frustrated Lewis pair" (FLP) chemistry, a term coined by Stephan and co-workers 5 to describe combinations of sterically hindered Lewis acidsbase pairs with unquenched basicity and acidity.…”

“…This is consistent with the view that weaker σ-donors enhance the Lewis acidity at boron and thus increase the rate of H 2 cleavage, which is presumed to be the rate-limiting step in the catalytic cycle. The borenium salt [{(ClCNMe) 2 hydrogenation possible at loadings as low as 0.1 mol % at ambient temperature and 102 atm of H 2 .…”

“…Treatment of these complexes (X = Cl, Br) with ammonia-borane effects reduction of the CO 2 moiety affording methanol in yields of approximately 4050% after an aqueous quench (Scheme 10). 63 Additional studies with Mes 3 P(CO 2 )(AlX 3 ) 2 (X = Br, I) revealed that these complexes react further with CO 2 to effect the reduction of exogenous CO 2 to CO along with the generation of Mes 3 P(C(OAlX 2 ) 2 64,65 It is proposed that this reduction proceeds via a rate-limiting associative insertion of exogenous CO 2 into the AlX bond of Al 2 X 6 , to yield the intermediate X 2 AlO(X)C=OAlX 3 . Subsequent nucleophilic attack of the phosphine and exchange chemistry with Mes 3 P(CO 2 )(AlX 3 ) 2 yields the thermodynamic product Mes 3 PC-(OAlX 2 ) 2 OAlX 3 and free CO (Scheme 11).…”

“…This species reacts with CO 2 via an FLP insertion reaction into the strained PN bond, affording C 6 H 4 (NMe)(CO 2 )PFPh 2 (Scheme 28). Similarly, oxidation of (C 6 H 4 (NMe)) 2 PPh to (C 6 H 4 (NMeH)) 2 PF 2 Ph and reaction with t BuLi gave (C 6 H 4 (NMe)) 2 PPh which captures two equivalents of CO 2 to give (C 6 H 4 NMe(CO 2 )) 2 PPh (Scheme 28). 94 These reactions are reminiscent of that of boron-amidinates, 95 suggesting that ring opening affords a transient amido-phosphonium FLP which captures the CO 2 , thus implicating P(V) as the Lewis acid.…”