Carbenoid coupling

This extremely air-sensitive compound, which is valence isoelectronic to an olefin, has been structurally characterized by X-ray diffraction. It has a short carbon-phosphorus double bond (1.62 A) the phosphorus and carbon atoms adopt a trigonal planar geometry with a dihedral angle of 60° (Fig. 3). This value is significantly larger than that reported for the most crowded olefin.61 Formally, this compound can be viewed as the product of a car-bene-carbenoid coupling between bis(trimethylsilyl)carbene and bis(diiso-propylamino)phosphenium triflate. Note that another route to methylene-phosphonium salt has been reported by Griitzmacher et al.62... 

Other annelation reactions leading to racemic [njhelicenes with n > 6 include carbenoid coupling providing the parent [7]helicene [74],... 

Polycondensation via Carbenoid Coupling Reactions of Bis(dichloromethyl)arenes... 

In connection with nickel-catalysed reductive coupling reactions of dihaloarenes, leading to poly(arylene)s, the copper-catalysed reductive carbenoid coupling reactions involving substituted bis(dichloromethyl)arenes and metals or other reductants should be mentioned. The reductive carbenoid coupling of bis(phenyldichloromethyl)arene with zinc in the presence of Cu-based catalysts is shown below ... 

Jones, G.B. Huber, R.S. Mathews, J.E. Towards enediyne libraries cyclic enediynes via an intramolecular carbenoid coupling protocol. J. Chem. Soc., Chem. Commun. 1995, 1791-1792. 

Cyclopentacorannulene (67) was obtained as a mixture with corannulene (in ratio 7 3) in 10-15% yield from dichloride 66 by flash vacuum pyrolysis at 1,000 °C (Scheme 18) [106]. In contrast, 67 could also be prepared, in 20% yield, by titanium-mediated carbenoid couplings of 68 in solution phase [34]. 

Synthesis of Conjugated Enediynes. LHMDS was used in a carbenoid coupling-elimination strategy to s)uithesize linear and cyclic enediynes from various propargylic halides. The stereoselectivity in the linear series is controlled by substitution at the propargylic center primary propargylic bromides favored the Z-isomer (eq 33), whereas secondary propargylic bromides produced exclusively the L-isomer (eq 34). Conversely, only the Z-isomer was obtained for cyclic enediynes. 

Practical Synthesis of Corannulene by Intramolecular Carbenoid Coupling of Dibromomethyl Croups... 

There are several nonpyrolytic methods of obtaining spherical surfaces. These include titanium-, vanadium-, and nickel-mediated reductive coupling of benzyl and benzylidene bromides, intramolecular carbenoid coupling of dibromomethyl groups, and Pd-catalyzed intramolecular arylation with aryl halides [291]. 

Alternatively, conversion to bromides 45 followed by carbenoid coupling gave 46 (n = 1-5) in good yield on a preparative scale. Though stable at room temperature, photochemical Bergman cycloaromatization gave adducts 48 in moderate yield, presumably via diyls 48 (Table 29.3). ... 

As will be discussed in detail in the next section, when the reduction of the parent compound 6 was conducted at low temperature (—20 °C) with 2 equiv. of Na in tetrahydrofuran (THF) saturated with ethylene, complete salt removal was achieved, and the r 2-ethylene complex 20 was isolated.22 Upon irradiation, the latter released ethylene, behaving as a source of the d2 [W / -Bu -calix[4]-(0)4 ] carbenoid, which coupled to give a new W=W dimer [W=W, 2.582(1) A], isolated as the bis-Bu NC adduct 21. In H NMR, 21 exhibits a C5-symmetric pattern of signals for the calix[4]arene moiety. 

The earlier examples of [2 + 1] cycloaddition of a carbene (or carbenoid) on the double bond of alkylidenecyelopropanes to yield spiropentane derivatives were observed as undesired side reactions in the synthesis of alkylidenecyelopropanes through the addition of a carbene to a substituted allene [161]. In some cases the spiropentane derivative was obtained as the major product [161a, c] especially when a large excess of the carbene reagent was used. For example, when methyl 3,4-pentadienoate (610) was treated with a ten-fold excess of methylene iodide and zinc-copper couple the two products 611 and 612 were isolated in 1 4.5 ratio (Scheme 86) [161a]. 

Allyl chlorides are readily deprotonated a to the halide by strong bases to give allyl carbenoids, which insert into organozirconocene chlorides to afford allyl- or pentadienyl-zirco-nocene chlorides (Scheme 3.23). These allylmetallic species are reactive towards carbonyl compounds, and so an efficient three-component coupling results [50]. 

The deshielding of the resonance in lithium carbenoids is explained by the polarization of the elongated C—X bond and shows that the carbocationic structures Ic and 2c have to be taken into account. The postulated high s character of the C—Li bond is also confirmed by the NMR-spectroscopic investigations insofar as enhanced C- Li coupling constants have been observed in lithium carbenoids . Thus, the effect of the metalation that leads to the formation of lithium carbenoids can be summarized as follows (Scheme 3) . 

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