Alkylidene carbene rearrangement

The formation of ethylene by an alkylidene carbene rearrangement,as observed by Caulton [47], might also be a very significant pathway in Fischer-Tropsch-type reactions under homogeneous conditions. 

Similarly, ethynylation of / -dicarbonyl enolates via the tandem Michael-car-bene rearrangement (MCR) pathway occurs smoothly by the reaction with the parent ethynyl-A3-iodane 128. High migratory aptitude of a-hydrogens of alkylidene carbenes is responsible for this facile ethynylation [199]. 

Most reactions of this category involve the base-induced generation of alkylidene-carbenes (R2C = C ) which undergo an intramolecular 1,5-carbon-hydrogen insertion providing a useful route for the construction of substituted cyclopentenes a competing intramolecular pathway is rearrangement to alkynes. 

Insertion reactions of alkylidene carbenes offer a useful entry to cyclopentene ring systems (4.81). Insertion is most effective with dialkyl-substituted alkylidene carbenes (R = alkyl), since rearrangement of the alkylidene carbene to the alkyne occurs readily when R = H or aryl. A number of methods have been used to access alkylidene carbenes. One of the most convenient uses a ketone and the anion of trimethylsilyl diazomethane. Addition of the anion to the ketone and eUmination gives an intermediate diazoalkene, which loses nitrogen to give the alkylidene carbene. For example, a synthesis of the antibiotic (-)-malyngolide started from the ketone 102 (4.82). The insertion reaction takes place with retention of configuration at the C—H bond. 

It should be noted that structural distortions with short M-HC contacts, very similar to those in 71, 73, and possibly in 74, were previously observed in some Schrock s alkylidene complexes, which was theoretically rationalized in terms of electronic rearrangements of the carbene center. ... 

The formation of reactive carbenes from alkylidene Meldrum s acids has also been observed. Thus pyrolysis of 1-indanylidene Meldrum s acid at 640 °C gave the corresponding carbene which further rearranged to benzofulvene and naphthalene (Scheme 12) <1998JA8315>. Similarly, EVP of 9-fluorenylidene Meldrum s acid at 1100°C provided a mixture of phenanthrene and biphenylene <1996TL6819>. 

Insertion can also be carried out on the C-H bonds of heteroaromatics. Masahiro Murakami of Kyoto University has described (J. Am. Chem. Soc. 2003,125,4720) a Ru catalyst that will effect rearrangement of a silyl alkyne such as 10 into the vinylidene carbene. The intermediate Ru carbene complex is then electrophilic enough to insert into the aromatic C-H bond. The insertion is highly regioselective. The Au and the Ru alkylidene insertions are geometrically complementary, as Ru gives the E-alkcne. 

The currently accepted mechanism-- -- - involves the initial formation of a metal-carbene (alkylidene) complex that reacts with an alkene to form a three-center bound n complex. The three-center bound n complex subsequently rearranges to a metallacyclobutane intermediate (Scheme 6.41). 

Turning to carbene related reactive species, alkylidene carbenoids like 26 (X = halogen, OR, NR2) are particularly valuable for preparative purposes since they can undergo cycloaddition reactions with olefins (to methylenecyclopropanes), isomerizations (to alkynes by the so-called Fritsch-Buttenberg-Wiechell rearrangement), and dimerization (to [3]cumulenes). Although carbenoids have been studied extensively by NMR spectroscopy [23], the first X-ray structural analysis of a stable carbenoid, 27, as a TMEDA 2THF complex has been reported only recently [24]. 

Benzyne (1) and cyclopentylidenecarbene (121) are isoelectronic. The formation of benzyne from 121 in the gas phase was first proposed in 1979, and the ensuing studies have recently been reviewed. The rearrangement of 121 -> 1 was suggested from pyrolysis results with the alkylidene Meldrum s acid 122. Besides high yields of the expected carbon dioxide, acetone and cyclopentadiene, substantial amounts of aromatic products such as biphenylene (27%) and triphenylene (12%) were obtained that were most easily accounted for as a consequence of rearrangement of carbene 121 to benzyne and subsequent di- or trimerization. 

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