Carbenes, coupling dimerization

The search for stable diaminocarbenes dates back to the early 1960s and is associated with the name of Wanziick." At that time the preparation of the 1,3-diphenyl imidazolidin-2-ylidenes (Ilia) was first examined (Scheme 8.4). Precursors of Ilia included the dimeric and electron-rich olefin IIIa 2 and the chloroform adduct 3a of the desired carbene. By means of cross-coupling experiments, however, it was shown that IIIa 2 was not in equilibrium with the two carbene units. On the other... 

Interestingly, Arduengo observed that Villa was stable with respect to dimerization in the absence of a Brpnsted or Lewis acid catalyst." Similarly, in the absence of an acid catalyst, dimerization of IXb is extremely slow and is first order in carbene. Therefore, the observed formal dimerization of Villa and IXb,c does not involve the coupling of two carbenes, but the nucleophilic attack of one carbene upon its conjugate acid, followed by proton elimination, as already suggested by Chen and Jordan (Scheme 8.13). It is important to keep in mind that even A,A-dialkylimidazolium ions have pXa values of 24 in DMSO, and based on the calculated proton acidity, Alder estimated the pXa values for acyclic diamino-carbenes to be from 2 to 6 pXa units higher than for imidazol-2-ylidenes. " ... 

In spite of those highly favorable stmctural factors, 16a is very ephemeral. Its lifetime in degassed benzene is 0.5 ps, which is shorter even than that of parent triplet DPC. Product analysis smdies have shown that 16a forms a trimer of dianthrylcarbene (116) as the main product (50-60%) (Scheme 9.37). The trimer is the one formed as a result of a threefold coupling at position 10 of the carbene. This observation suggests that delocalization of the unpaired electrons in 16a leads to their leaking out from the carbene center to position 10, where sufficient spin density builds up for the trimerization to take place. At the same time, the lack of formation of olefin-type dimers through coupling of two units of 16a at their carbene centers indicates that the carbene center itself is indeed well shielded and stabilized. 

Treatment of cyclopropanethiones 437 with triphenylphosphine results in dimerization to form cyclopropenes 438, which isomerize to carhenes 439. The intramolecular coupling of the carbene 439 (R = Ph) affords thieno[3,2- Ithiophene 441, whereas the carbenes 439 (R =Ph, 2-thienyl, R = SBu ) isomerize to bis(allene)s 440, whose... 

C(OMe)C6H4-o-C=CPh (CO)j leads directly to the formation of a chrysene derivative via the formal dimerization of the carbene ligand. A plausible explanation for the formation of the final product involves a doubly alkyne-bridged dinuclear complex, alkyne insertions into metal-carbene bonds, and coupling of the carbene carbons. 

Many types of palladacyclic complexes have been used as precursors to catalysts for a variety of coupling processes146,147. Mixtures of dimeric palladacycles containing bridging halide, acetate or trifluoroacetate ions and a phosphine or carbene ligand have been studied as catalysts for the animation of aryl halides. The isolated phosphine adducts can also be applied in catalysis. 

In a laser flash-photolysis study, 2-phenyladamantene was generated in benzene at room temperature from 3-noradamantyl(phenyl)diazomethane. This strained cycloalkene decays with second-order kinetics to give a dimer, and reacts much faster with O2 and Bu3SnH than with methanol, thus revealing a substantial radical character. Diphenyldiazomethanes possessing stable /er -butylaminoxyl and Ullman s nitronyl nitroxide radicals, e.g. (25), have been prepared by photolysis of the parent diazomethanes. Analysis of ESR fine structures showed that the carbene and radical centres couple ferromagnetically in these molecules, as expected. 

Addition of aryl azides to CpRh(PMe3)(CNMe) results in coupling of the aryl nitrene with the isocyanide to form (110) (Ar = Ph, /7-tol) which can be converted to (111) (R = H, Me), another example of a carbene complex (see also 73, Section 2.4, Carbene Complexes Replacement of COD in Rh2(COD)2(/u-Cl)(/r-PBu 2) by t-butylisocyanide yields Rh2(CNBu04(A.-Cl)(/u,-PBu 2), which on treatment with one equivalent of lithium di(t-butyl)phosphide forms [Rh(CNBu )2(A PBu 2)]2, which has a planar Rh2P2 core and no Rh-Rh bond. In a chloroform solution, the last complex is slowly converted into a mixture of cis and fra/25-[RhCl(CNBuO(/n PBu 2)]2, while oxidation with ferricinium hexafluorophosphate yields [Rh(CNBu )2(/u.-PBu 2)]2 - Both of the oxidized dimers contain Rh-Rh bonds. ... 

---