π-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements

This review is essentially an update of one entitled “πBonding and The Lone Pair Effect in Multiple Bonds Between Heavier Main Group Elements” which was published more than 10 years ago in this journal.1 The coverage of that survey was focused on the synthesis, structure, and bonding of stable compounds2 of heavier main group elements that correspond to the skeletal drawings reproduced in Tables 1 and 2. A row of numbers is listed at the bottom of each column in these tables. This refers to the number of stable complexes from each class that are currently known. The numbers in parentheses refer to the number of stable species that were known at the time of the previous review. Clearly, many of the compound classes listed have undergone considerable expansion although some remain stubbornly rare. The most significant developments for each class will be discussed in detail under the respective sections. As will be seen, there are also a limited number of multiple bonded heavier main group species that do not fit neatly in the classifications in Tables 1 and 2. However, to keep the review to a manageable length, the limits and exclusions, which parallel those used earlier, are summarized as follows: (i) discussion is mainly confined to compounds where experimental data on stable, isolated species have been obtained, (ii) stable compounds having multiple bonding between heavier main group elements and transition metals are not generally discussed, (iii) compounds in which a multiple bonded heavier main group element is incorporated within a ring are generally not covered, and (iv) hypervalent main group compounds that may incorporate faux multiple bonding are generally excluded. Such compounds are distinguished from those in Tables 1 and 2 in that they apparently require the use of more than four valence bonding orbitals at one or more of the bonded atoms. The remainder of this review is organized in a similar manner to that of the previous one wherein the compounds to be discussed are classified according to those summarized in Tables 1 and 2. The key unifying feature of almost all molecules discussed in this review is that they are generally stabilized by the use of bulky substituents which block associative or various decomposition pathways.3 Since the previous review was published in 1999, several review articles that cover parts of the subject matter have appeared.4

π-Bonding and the Lone Pair Effect in Multiple Bonds Involving Heavier Main Group Elements: Developments in the New Millennium

Bottled! The employment of hexaphenylcarbodiphosphorane as ancillary ligand has allowed the isolation of the otherwise transient dihydrido borenium cation. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis, Structure, and Reactivity of a Dihydrido Borenium Cation

Borylenes as Ligands to Transition Metals

Over the past seven years, complexes of various transition metals with bridged or terminal borylene ligands BR have for the first time been reported and fully characterised. Being closely related to important organometallic compounds, such as carbene, vinylidene, and carbonyl complexes, they have already attracted considerable interest both from experimental and computational points of view. Those studies have provided some detailed information about the nature of the metal−boron bond and about the nature of metal and borylene moieties required for the formation of stable complexes. Various methods for their preparation have been developed and the first studies on their reactivity have demonstrated an interesting chemistry. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2003)

The Chemistry of Borylene Complexes

Die farblosen, flussigen, pyrophoren Alkylalkylidenborane RBCA2 (2c, d: R = Me, tBu; A = SiMe3), bilden sich aus Me(MeO)BCA3 bzw. tBu(F)BCA3 durch Abspaltung von AOMe bei 560°C bzw. von AF bei 490°C. Bei 25°C ist 2d lagerfahig, wahrend sich 2c zu 5 cyclodimerisiert. In 2d, das bei 225 K kristallisiert, findet man bei 120 K ein lineares CBC-Gerust mit BC-Bindungslangen von 154.7 und 136.1 pm. Mit 2,6-Dimethylpyridin bildet 2c das Addukt 6 und mit Benzophenon bzw. dem Iminoboran tBuBNtBu die [2 + 2]-Cycloaddukte 7a und 8; 2d reagiert mit Benzophenon zu 7b.



Alkylaklyideneboranes RBC(Sime3)2: Isolable molecules with a Two-Coordinated Sextet Boron Atom



The colourless, liquid, pyrophoric alkylalkylideneboranes RBCA2 (2c, d: R = Me, tBu; A = SiMe3) are formed from Me(MeO)BCA3 and tBu(F)B-CA3 by elimination of AOMe at 560°C and AF at 490°C, respectively. At ambient temperature 2d can be stored, whereas 2c cyclodimerizes to form 5. 2d crystallizes at 225 K; a structure determination at 120 K reveals the CBC skeleton to be linear with BC bond lengths of 154.7 and 136.1 pm. The adduct 6 and the [2 + 2] cycloadducts 7a, 8 are formed from 2c and 2,6-dimethylpyridine, benzophenone, and the iminoborane tBuBNtBu, respectively. 2d gives the cycloadduct 7b with benzophenone.

Alkylalkylidenborane R BC(SiMe3)2: Isolierbare Moleküle mit zweifach koordiniertem Sextett‐Boratom

The addition of Lewis bases L to the methyl(methylidene)boranes MeB=CA2(A = SiMe3) and MeB=CAA′ (A′ = SiMe2Cl) gives the adducts MeB(L)=CA2 (1a, b; L = trimethylpyridine, PMe3) and MeB(L)=CAA′ (1c, d; L = di- and trimethylpyridine), respectively Alcohols and amines HX are added to the BC double bond to give boranes MeB(X)—CHA2 (8a—c; X = OiPr, OtBu, NiPr2); MeB=CAA′ and HNMe2 react in the ratio 1:2, yielding MeB(X)—CHA(SiMe2X) (2d; X = NMe2) From MeB=CA2 and BH3, the five-membered ring [—CA2—BH—CA2—BMe(Hm)2BMe—] (2e; 2:1) or the six-membered ring [—CA2—BH(Hμ)2BMe—CA2—BH(Hμ)2BMe—] (2f; 1:1) are formed, both containing double hydrogen bridges; the product 2f crystallizes in the space group P1¯ The metallocene trihydrides [Cp2MH3] add to the BC double bond under formation of a double hydrogen bridge to give [Me(A2CH)B(Hμ)2MCp2] (2g, h; M = Nb, Ta) MeB=CA2 can be chloroborated, -stannated, and -phosphated with E—Cl to yield the boryldisilylmethanes MeB(Cl)—CA2—E (2i—l; E = EtClB, tBuClB, Me2ClSn, Cl2P) The alkyloboration and -alumination with E—R leads to the boryldisilylmethanes MeBR—SiA2—E (2m—o; E—R = Me2B—Me, Et2B—Et, Cl2Al—Et) and the bromination to MeB(Br)—CA2Br (2+2) Cycloadditions are achieved, when MeB=CA2 is reacted with unsaturated molecules a=b, yielding four-membered rings [—BMe—CA2—b—a—] [4a—d; a=b = fluorenone, bis(methoxycarbonyl)ethyne (reacting at both of the C=O bonds), phenylisocyanate (reacting at the C=O bond), N-isopropylacetoneimine], or with triple bond systems RC≡Z, yielding four-membered rings [—BMe—CA2—Z=CR—] (4e—g; RC≡Z = PhC≡CPh, AC≡CCl, tBuC≡P) With a series of six molecules with an element-oxygen double bond, a primary (2+2) cycloaddition is followed by a metathetical splitting of the transient four-membered rings 4h—m One of the metathesis products is MeB≡O, which is identified as boroxene (MeBO)3, the other component is an alkene RR′C=CA2 [starting from MeB=CA2 and PhCHO, PhC(Me)O] or an alkene RR′C=CAA′ (starting from MeB=CAA′ and PhCHO, tBuCHO) or the methylidene phosphorane Ph3P=CA2 (starting from MeB=CA2 and Ph3PO) or the dicarbadicobaltatetrahedrane [(CA)2{Co(CO)3}2] {starting from MeB=CA2 and [Co2(CO)8]} The (2+3) cyclodaddition of MeB=CA2 to the azide X2PN3 (X = NiPr2) as 1, 3-dipole gives the five-membered ring [=BMe—CA2—N=N—NX=] (5a) and to RN3 the rings [=BMe—CA=N—NA—NR=] (5′b, c; R = iBu, A; formed from the cycloadducts 5b, c by migration of A); analogously, [=BMe—CA′=N—NA—NA=] (5′d) is formed from MeB=CAA′ and AN3 Finally, the nitrone O—NMe=CHPh and MeB=CA2 or MeB=CAA′ give the corresponding (2+3) cycloadducts 5e, f, respectively All of the products were characterized by their 1H, 11B, and 13C NMR spectra



Reaktionen an der Bor-Kohlenstoff-Doppelbindung von Methyl(methyliden)boranen



Die Addition von Lewisbasen L an die Methyl(methyliden)borane MeB=CA2 (A = SiMe3) und MeB=CAA′ (A′ = SiMe2Cl) fuhrt zu den Addukten MeB(L)=CA2 (1a, b; L = Trimethylpyridin, PMe3) bzw MeB(L)=CAA′ (1c, d; L = Di- und Trimethylpyridin) Alkohole und Amine HX addieren sich an die BC-Doppelbindung unter Bildung von MeB(X)—CHA2 (2a-c; X = OiPr, OtBu, NiPr2); MeB=CAA′ und HNMe2 reagieren im Verhaltnis 1:2 zu MeB(X)—CHA(SiMe2X) (2d; X = NMe2) Aus MeB=CA2 und BH3 bilden sich der funfgliedrige Ring [—CA2—BH—CA2—BMe(Hμ)2BMe—] (2e; 1:2) oder der sechsgliedrige Ring [—CA2—BH(Hμ)2BMe—CA2—BH(Hμ)2BMe—] (2f; 1:1), die beide Wasserstoff-Doppelbrucken enthalten; der Sechsring 2f kristallisiert in der Raumgruppe P1¯ Die Metallocentrihydride [Cp2MH3] addieren sich an die BC-Doppelbindung unter Bildung einer Wasserstoff-Doppelbrucke zu [Me(A2CH)B(Hμ)2MCp2] (2g, h; M = Hf, Ta) MeB=CA2 kann mit E—Cl zu Boryldisilylmethanen MeB(Cl)—CA2—E chloroboriert, -stanniert und -phosphaniert werden (2i—l; E = EtClB, tBuClB, Me2ClSn, Cl2P) Die Alkyloborierung und -aluminierung mit E—R ergibt die Boryldisilylmethane MeB(R)—CA2—E (2m—o; E—R = Me2B—Me, Et2B—Et, Cl2Al—Et) und die Bromierung MeB(Br)—CA2Br (2p) (2+2)Cycloadditionen beobachtet man, wenn MeB=CA2 mit ungesattigten Molekulen a=b zu Vierringen [—BMe—CA2—b—a—] [4a—d; a=b = Fluorenon, Bis(methoxycarbonyl)ethin (an beide C=O-Bindungen wird addiert), Phenylisocyanat (an die C=O-Bindung wird addiert), N-Isopropylacetonimin] oder mit Dreifachbindungssystemen RC≡Z zu Vierringen [—BMe—CA2—Z=CR—] reagiert (4e—g; RC≡Z = PhC≡CPh, AC≡CCl, tBuC≡P) Mit sechs Verbindungen mit einer Element-Sauerstoff-Doppelbindung folgt auf eine primare (2+2)-Cycloaddition eine metathetische Ringspaltung Eines der Metatheseprodukte ist MeB≡O, das als Boroxin (MeBO)3 nachgewiesen wird, das andere ist ein Alken RR′C=CA2 (aus MeB=CA2 und PhCHO, PhMeCO) oder ein Alken RR′C=CAA′ (aus MeB=CAA′ und PhCHO, tBuCHO) oder das Methylidenphosphoran Ph3P=CA2 (aus MeB=CA2 und Ph3PO) oder das Dicarbadicobaltatetrahedran [(CA)2{Co(CO)3}2] {aus MeB=CA2 und [Co2(CO)8]} Die (2+3)-Cycloaddition von MeB=CA2 an das Azid X2PN3 (X = NiPr2) als 1, 3-Dipol ergibt den Funfring [=BMe—CA2—N=N—NX=] (5a) und an das Azid AN3 die Funfringe [=BMe—CA=N—NA—NR=] (5′b, c; R = iBu, A; hervorgegangen aus den Cycloaddukten 5b, c durch Wanderung von A); entsprechend bildet sich [=BMe—CA′=N—NA—NA=] aus MeB=CAA′ und AN3 Schlieslich entstehen aus MeB=CA2 bzw MeBB=CAA′ und dem Nitron O—NMe=CHPh die (2+3)-Cycloaddukte 5e, f Alle Produkte wurden mit Hilfe ihrer 1H-, 11B- und 13C-NMR-Spektren charakterisiert

Reactions at the Boron-Carbon Double Bond of Methyl(methylidene)boranes

A 1.3-Methyl Shift along the Unsaturated BCSi Skeleton



The synthesis of the methoxy[tris(trimethylsilyl)methyl]boranes X-(MeO)B–C(SiMe3)3 (1a-d; X = Cl, Br, Ph, PhCH2) is described. The methylenoboranes Me-B=C(SiMe2Hal) (3a, b), the 2,3-dihydro-1H-1-sila-3-boraindene species 6, and the 1,2,3,4-tetrahydro-1-sila-3-boranaphthalene species 7 are formed by the thermal elimination of MeOSiMe3 from 1a/b or 1c or 1d, respectively. The boranes 3a, b undergo cyclodimerization at the B=C bond at 25 and 69°C, respectively, the diboretanes 4a, b being formed. The oxaboretanes 5a, b are isolated from the reaction of 3a, b with Ph2CO. The thermal elimination of MeOSiMe3 from Me′-(MeO)B–C(SiMe3)3 (1g;; Me′ = CD3) and subsequent addition of Ph2CO gives the oxboretanes 5g′/5g″ in the ratio of 1:6, showing that an equilibrium Me′-B=C(SiMe32 ⇄ Me—B=C(SiMe3(SiMe2Me′) with a statistical distribution of Me′ among seven methyl places is established in the hot tube before Ph2CO finally attacks the double bond. All these results are in accord with the primary formation of the methyleneboranes X-B=C(SiMe3)2 from 1a-d, followed by a 1,3-shift of Me along the unsaturated BCSi skeleton and the subsequent transformation into the isolated products.

Eine 1,3-Methyl-Verschiebung an der ungesättigten BCSi-Kette

The colorless, liq., pyrophoric alkylalkylideneboranes RB:C(SiMe3)2 (I; R = Me, CMe3) are formed from MeOBMeC(SiMe3)3 by elimination of Me3SiOM at 560 Deg and Me3SiF at 490 Deg resp. At ambient temp. I (R = CMe3) can be stored, whereas I (R = Me) cyclodimerized. I (R = CMe3) crystallizes at 225 K; a structure detn. at 120 K reveals the C-B:C skeleton to be linear with BC bond lengths of 154.7 and 136.1 pm. I forms [2 + 2] cycloadducts with 2,6-dimethylpyridine, benzopahenone, and iminoborane Me3CB.tplbond.NCMe3.

Ralf Ziembinski

Papers

Alkylalkylidenborane R BC(SiMe3)2: Isolierbare Moleküle mit zweifach koordiniertem Sextett‐Boratom

Reactions at the Boron-Carbon Double Bond of Methyl(methylidene)boranes

Eine 1,3-Methyl-Verschiebung an der ungesättigten BCSi-Kette

Alkylalkylideneboranes R-B=C(SiMe3)2: isolable molecules with a two-coordinated sextet boron atom.

Alkylalkylideneboranes R-B=C(SiMe3)2: Isolable Molecules with a Two-Coordinated Sextet Boron Atom.