Acyclic carbenes

Acyclic carbenes, with palladium isocyanides, 8, 258 Acyclic diene metathesis polymerization alkynes, 1, 191 characteristics, 1, 138 in ionic liquids, 1, 869... 

The interesting electronic properties of NHC and their azolium precursors can be seen in the H- and C-NMR spectra for the and atoms. Since the W atom of an azolium salt is essentially acidic, the corresponding chemical shift will be observed downfield, typically at 5 = 8-11 ppm (see Figure 1.22). There is a correlation between the proton chemical shift and the ease of deprotonation [50]. The precursor of the acyclic carbene bis-(diisopropylamino)carbene, A, A, A ,A -tetraisopropylformamidinium chloride has a proton chemical shift of 5 = 7.60 ppm [118], significantly upheld of the normal range for azolium salts. 

When the stable acyclic carbene (Pr2 N)2P-C-SiMe3 94 was allowed to react with bis(diisopropylamino)phos-phenium triflate in a CH2Cl2/pentane mixture at 0°C, the 1,2-bisphosphoniacyclopropanide 95 was formed as colorless crystals in 66% yield. From a formal point of view this cation is the cyclic version of a bisphosphinocarbocation 95, which was not observed here. In contrast to the acyclic bisaminocarbocations (amidinium ions), the carbon center in 95 is carbanionic in character (Scheme 33) <2000SCI75, 2002JA2506>. 

There are two volumes of Houben- Weyl on carbenes (Regitz, 1989). The chapters on acyclic carbenes (Zeller and Gugel, 1989), arylcarbenes (Wentrup, 1989), and alkylidenecarbenes (Stang, 1989) are particularly important for the scope of this book. 

The kinetic data in Figure 2.2 indicate that carbene 10, under the conditions of our TRIR experiments, does not form ketene 11, in contrast to the observed reactivity of acyclic carbene 7. Platz and co-workers [94] determined that carbene 7 is separated from ketene 8 by a 3.4 kcal/mol barrier in hexafluorobenzene. It is likely that the higher singlet/triplet gap for carbene 10 relative to that of 7 raises the effective barrier to rearrangement. We find, by monitoring the lifetime of carbene 10 as a function of concentration of diazoester 9, that 10 is effectively quenched by 9 with /tdiazo=(5.0 0.5) x 10 M s in Freon-113. This observation suggests that a major decay route of the carbene under the conditions of our experiment is formation of azine 16. 

Bases on the PA data above, the pKg values for six-membered, and acyclic carbenes may be from 2-6 pKg units higher than for imidazol-2-ylidenes, and this may explain some of the increased difficulties with the generation of these species (see section 2.1.1). 

ABSTRACT. Dicarbonyl(t 5-cyclopentadienyl)carbyne complexes of molybdenum and tungsten prove to be a valuable synthetic tool Reaction with phosphines provides substituted carbyne complexes and leads via an intramolecular CC-coupling to t 1- or Tj -ketenyl complexes respectively. Electrophiles attack the metal carbyne triple bond forming hetero- and acyclic carbene complexes, r 2-acyl compounds, T -ketene complexes and metalla-dithia-bicyclobutane cations. Dithio-carboxylates are formed in reaction of these dicarbonyl(Ti5 cyclo-pentadienyl)carbyne complexes with sulfur or cyclohexene sulfide. 

While the focus of this chapter is on hydroamination with amines, impressive recent advances in the hydroamidation and hydrocarbamation of allenes have been disclosed. Espinet showed that acyclic carbenes can be used as ligands for Au(I) to realize intramolecular hydrocarbamation [240], while Widenhoefer has used the commercially available l,3-bis[(2,6-diisopropylphenyl)imidazole-2-ylidine] (IPr) NHC in combination with cationic Au(I) to realize regioselective intermolecular hydroamination of 1,1-disubstituted allenes with benzyl carbamate to access allylamines with quaternary centers adjacent to N. This same catalyst can also accommodate 1,3-disubstituted allenes and even tetrasubstituted allenes (Table 15.19) [241]. Interestingly, these products are in contrast to the preferred products accessed with related Au(I)-phosphine complexes in combination with aniline substrates (Table 15.18) [239]. Hydroureation has been achieved intramolecularly [242, 243] and will be later discussed (Section 15.3.6). 

The oxacyclopentylidene complexes 101 react with N,N-dimethylformamide dimethyl acetal (DMF-DMA) to form complexes 102 and 103. Related results are seen for acyclic carbene systems. 

A useful metalation approach that has little precedent in cyclic carbene chemistry [18] is the use of 2-chloroamidinium or chloroiminium ions as precursors for acyclic carbene ligands. Fiirstner and coworkers prepared cationic Pd complexes of acyclic diamino-, aminooxy-, aminoarjd, and aminothiocarbenes by oxidative addition of chloroiminium precursors to Pd(PPhs)4 (route d. Scheme 16.1), an approach that was also effective for ADC-Ni complexes [19]. This route permits complexation of sterically nonhindered acyclic carbenes that would not be stable in the free state. Chloroamidinium precursors can be meta-lated without a change in metal oxidation state via lithium-halogen exchange followed by transmetalation (route e). This strategy has been successfully employed with Pd", Rh, and Ir [20]. 

Figure 16.2 Donor abilities of acyclic carbenes in comparison with an NHC.

Scheme 16.3 Decomposition routes of acyclic carbene ligands.

Reports of catalysis with acyclic carbene-metal complexes first appeared in 2005 [19,22a,38] and dealt with Pd- and Ni-catalyzed coupling reactions for which effective NHC-based catalysts were already known [39]. Gradually, the focus has shifted toward identifying reactions that benefit from the distinct steric and electronic properties of acyclic carbenes. In this section, key examples of catalysts that exhibit imusual stabilities, activities, or selectivities resulting from the presence of acyclic carbene ligands are highlighted. Recent review articles provide more detailed compilations of catalytic applications of this ligand class [9]. 

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