Classical and Nonclassical Methyleneboranes

Abstract

Methyleneboranes are stable compounds when their reactive C=B bonds are shielded by sterically demanding substituents and the electron deficiency at the dicoordinate boron atom is reduced through delocalization of adjacent electron pairs. In the classical amino-(methy1ene)boranes of Noth and Paetzold, the electron deficiency at the dicoordinate boron atom is relieved through K-z delocalization of a neighboring nitrogen atom's formally nonbonding electron pair, that is, through formation of a classical two-center, two-electron (2c-2e) bond. In nonclassical methyleneboranes, which are the focus of this review, the electrondeficient center at the dicoordinate boron atom forms nonclassical three-center, two-electron (3 c-2e) bonds with neighboring 0 bonds. These methyleneboranes exhibit structural and spectroscopic properties similar to carbenium ions having strong O-K delocalization (hyperconjugation, bridging) and to transition metal complexes having strong agostic interactions.In classical methyleneboranes, as in the amino-substituted vinyl cations (ketiminium ions) 4,18] the electron deficiency at the dicoordinate atom is greatly reduced through 71-K delocalization of the formally nonbonding electron pairs on the neighboring atom (Scheme 1). Typical examples are the 4 \ 9/ \ @ J /c=B-c \ -/c=B=c \ 6 7 Scheme 1. x-rc Delocalization in classical methyleneboranes and related compounds. Telefax: Int. code + (6421)28-5547 0 8 Y = C R2 9 Y=ML, Scheme 2. Nonclassical 3c-2e bonds through interaction ofelectron-deficient centers with neighboring (5 electrons: the a-agostic type.These interactions are called c ( agostic effectst7] if the electron-deficient center is a metal, but if it is a positively charged carbon atom, the name C-X hyperconjugation is used. Pronounced effects in carbocations are termed nonclassical bridging. The expression O-7c delocalization1' is employed as the generic term. Depending upon the extent of the shift of the bonding electrons in the C-X bond towards the electron-deficient center-the X nucleus must follow for electrostatic reasons-a continuum'' 31 in the degree of 3 c-2e bond development can be observed. The following series of formulas show examples of carbocations having no appre-Angrbr. Chem. In! Ed. Engl. 1993. 32, 985-1009 ((? VCH Verlugs~e.~.seIlschuf~ mhH, 0-69451 Weinheim, 1993 0570-0833193/0707-0985 $10.00+ .2510 14* 1 4 15' 15 o e/' n-n .. /x \ Scheme 4. Nonclassical 3c-2e bonds from interaction of electron-deficient centers with neighboring o electrons: the P-agostic type.--C=B f) -C-B \ 2 /c=B-11 1 c,d 20a.b a,c,e Aryl= Dur b,df A w l = Mes Our I l e , f Me,Si-C~C-SrMe, Dur = duryl = 2.3.5.6-tetramethylphenyl; M e s = mesityl = 2,4,6-trtmethylphenyI The [2 + 21 cyclo adduct 22,[3b1 obtained from the reaction of the amino(imin0)borane Me,Si(tBu)N-B=NtBu with 988

Hunting dimers

“…(a) would lead one to expect for the boron dimers some diborane-like structures, but, more interestingly, nonclassical structures of the type that Armin Berndt has so beautifully given us [26]. (b) would point to acidbase reaction-initiated dimerizations.…”

Reactions of Diboratetrahalides(4) with Boriranylideneboranes − Formation, Reactivity, and Structures of Cyclic Tetraborylmethanes and Isomeric Diborylmethyleneborane Derivatives

“…The observed lengthening of the C1ϪB3 bond length in connection with the decreased bond angle and the short C1ϪB1 bond is explained by hyperconjugation of the C1ϪB3 sigma bond with the empty orbital at B1. [8] Crystals of 4a were obtained from hexane, whereas 4c formed crystals in a solution of toluene. Only the structure of 4a is discussed here.…”

“…Recently, the X-ray structure analysis of C[B(OMe) 2 ] 4 revealed, as expected, that it has the classical tetrahedral structure. [7] We present here reactions of the nonclassical boriranylideneboranes 1a,b,c, first prepared by Berndt et al in the early 1980s, [8,9] with tetrahalogenodiboranes (4). In the case of the tert-butyl-substituted compound 1a, the addition of B 2 X 4 (X ϭ Cl, Br) leads to 2a,aЈ, which are the first tetraethylamine, (dimethylamino)trimethylsilane, and lithium pyrrolidinide to give the 1,3-diboretanes 4a,b,c.…”

1,1-Ethylboration of ethynyl(trimethyl)tin. (E)-1-Trimethylstannyl-2-diethylboryl-but-1-ene, isomerization and conversion into hydridoborates

“…On the other hand, this particular conformation around the boryl group of 1a favors hyperconjugative C-B interactions represented in the structure C, where the stannyl group in β-position relative to the positively charged carbon atom exerts a stabilising effect. [15][16][17][18][19][20] Structure C helps to explain the observed fast isomerization of 1a into 2a. Migration of the stannyl group transforms C into D, which is of comparable energy or even more stable because of the additional ethyl group in β-position to the positive charge.…”