Carbene Complexes with Boron

I would emphasize that the reaction of the amino acid-carbene complex with boron tribromide represents a good possibility of again cleaving the carbenyl protective group under extremely mild conditions at —25°C. 

Carbene Complexes with Boron-Boron Multiple Bonds... 

Carbene Complexes with Boron-Centered Radicals... 

Other Carbene Complexes with Boron and Potential Applications in Catalysis... 

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

A novel class of quinone boronic esters has been prepared via a Dotz annulation of Fischer carbene complexes with alkynylboronates. The origin of the high regioselec-tivity is discussed in terms of steric and electronic effects. Additionally, these compounds undergo Pd-catalyzed coupling reactions to give substituted quinones and hy-droquinones (Scheme 9.49) [102]. 

Unlike boron, the development of heavier Group 13 congener complexes with carbenes has been sluggish. Like boron, however, early examples of carbene complexes with Al, Ga, and In were simple Lewis adducts of the carbene with ER3 species (E = Al, Ga, or In R = alkyl, aryl, halide, or hydride). Recently, Aldridge explored the coordination chemistry of NHC-EHj adducts (104, E = Al or Ga) with [M(CO)4(COD)] (M = Cr, Mo), and was able to prepare the first examples of o-gallane complexes (105, Seheme 5.18). ... 

Although most of the examples of [3S+2C] cycloaddition reactions with carbene complexes are referred to as 1,3-dipolar processes, we should include in this section another kind of non-dipolar transformation dealing with the reaction of pentacarbonyl(methoxymethylcarbene)chromium with a base followed by treatment with an epoxide in the presence of boron trifluoride. This reaction gives cyclic carbene complexes in a process that can be considered a [3S+2C] cycloaddition [44] (Scheme 14). 

Triorganoboron complexes of Af,0-heterocyclic carbenes are also known (36). Thus, triphenylboron-carbene complex 48 can be isolated from the reaction of methanol with an isocyanide adduct of BPh3 (47) in the presence of a catalytic amount of KF. The X-ray crystal structure of 48 demonstrated that the boron atom is tetrahedrally coordinated and that all four B-C bond distances are equal within experimental error. 

Elimination to yield alkenes can be induced thermally or by treatment with acids or bases (for one possible mechanism, see Figure 3.39) [138,206]. Less common thermal demetallations include the thermolysis of arylmethyloxy(phenyl)carbene complexes, which can lead to the formation of aryl-substituted acetophenones [276]. Further, (difluoroboroxy)carbene complexes of molybdenum, which can be prepared by treating molybdenum hexacarbonyl with an organolithium compound and then with boron trifluoride etherate at -60 °C, decompose at room temperature to yield acyl radicals [277]. 

It was also of interest to us how changes in the organic residue of the carbyne ligand influence the stability and the behavior of carbyne complexes. Hence, we treated with boron tribromide a series of pentacarbonyl-[methoxy (aryl) carbene tungsten (0) complexes which were substituted at... 

We could show further that not only methoxy(organo)carbene complexes react with boron trihalides in the above-mentioned sense. For instance, it was found that b aws-(bromo)tctracarbonyl(phenylcarbyne)chromium(0) and -tungsten(0) are also accessible from pentacarbonyl[hydroxy (phenyl)-carbene [[chromium (0) (02) and from pentacarbonyl[methoxycarbonyl-... 

That experimental results cannot always be generalized is shown by the treatment of cis-(bromo) tetracarbonylQiydroxy(methyl) carbene]manga-nese with boron tribromide. This procedure does not lead to the analogous carbyne complex but rather to a product in which the hydrogen atom of the hydroxy group is substituted by a BBr2 residue (94) ... 

Nickel/carbene complexes have also been successfully employed in the Suzuki-Miyaura cross-coupling reaction. One of the first successful applications of this was demonstrated by Blakey and MacMillan, wherein boronic acids were coupled with aryltrimethylammonium salts [52]. It was found that the transformation could be accomplished using 10 mol % Ni(COD)2,... 

A novel class of quinone boronic esters 301465 was synthesized by utilizing a highly regioselective benzannulation of Fischer carbene complexes 300 with 1-alkynylboronates (Equation (88)). 

Boron alkyls are expected to be inactive for coordinated anionic polymerization of olefins because the boron-carbon bond is not sufficiently ionic. The diazomethane polymerization with boron alkyl catalyst reported by Bawn, Ledwith and Matthies (275) is a special case of the growth reaction. A coordination mechanism seems most probable, but it has not been ascertained whether the polymer chain migrates as a car-banion or as a radical. If the complex between diazomethane and boron decomposes into a boron-carbene complex, then the polymer chain could migrate as a carbanion with the driving force provided by the electrophilic carbonium ion ... 

In consideration of conceivable strategies for the more direct construction of these derivatives, nitriles can be regarded as simple starting materials with which the 3+2 cycloaddition of acylcarbenes would, in a formal sense, provide the desired oxazoles. Oxazoles, in fact, have previously been obtained by the reaction of diazocarbonyl compounds with nitriles through the use of boron trifluoride etherate as a Lewis acid promoter. Other methods for attaining oxazoles involve thermal, photochemical, or metal-catalyzed conditions.12 Several recent studies have indicated that many types of rhodium-catalyzed reactions of diazocarbonyl compounds proceed via formation of electrophilic rhodium carbene complexes as key intermediates rather than free carbenes or other types of reactive intermediates.13 If this postulate holds for the reactions described here, then the mechanism outlined in Scheme 2 may be proposed, in which the carbene complex 3 and the adduct 4 are formed as intermediates.14... 

This approach makes the synthesis of neutral zwitterionic transition metal carbene complexes possible, with valuable advantages over cationic complexes [230]. The concept was previously applied to N, P and S ligands [231,232] and has a carbene precursor in Roesler s backbone boron substituted NHC ligands [233] the latter has not been coordinated to transition metals yet. 

Anions formed from group 6 and manganese Fischer carbene complexes undergo aldol condensations with aldehydes and ketones. Allylic carbenes exclusively react in the y position with aldehydes affording dienyl-substituted carbenes. For alkoxy-substituted carbenes, the presence of an excess Lewis acid see Lewis Acids Bases), such as boron trifluoride etherate, titanium tetrachloride, or tin tetrachloride is required for the reaction to proceed in reasonable yield. The initial aldol product can be isolated without elimination (Scheme 12). ... 

The isolation of the initial aldol products from the condensation of the enolates of carbene complexes and carbonyl compounds is possible if the carbonyl compound is pretreated with a Lewis acid. As indicated in equation (9), the scope of the aldol reaction can also be extended to ketones and enolizable aldehydes by this procedure. The condensations with ketones were most successful when boron trifluoride etherate was employed, and for aldehydes, the Lewis acid of choice is titanium tetrachloride. The carbonyl compound is pretreated with a stoichiometric amount of the Lewis acid and to this is added a solution of the anion generated from the caibene complex. An excess of the carbonyl-Lewis acid complex (2-10 equiv.) is employed however, above 2 equiv. only small improvements in the overall yield are realized. 

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