Cyclopropylcarbenes

Chromium cyclopropylcarbene complexes react with alkynes to provide cyclopentenone derivatives in a formal [2c+2s+lCo] cycloaddition process (see Sect. 3.2). However, tungsten and molybdenum cyclopropylcarbene complexes... 

The mechanism of cyclopropylcarbene reactions has also been discussed. ... 

The presence of a cyclopropyl moiety in the carbene complexes makes them useful for synthesis. The cyclopropylcarbene complexes 95 undergo a cycloaddition reaction with alkynes to give the cyclopentenones 96 [51]. The reaction course is explained as being metallacyclopentene fragmentation. (Scheme 34)... 

Hypothetical (carbene)gold(i) structures of intermediates and reaction coordinates have been calculated (B3LYP/ 6-31G and LAN2DZ levels) for (H3P)Au+-catalyzed cyclization reactions of terminal enynes. The endocyclic skeletal rearrangement reactions were found to proceed exclusively via cyclopropylcarbene complexes.240... 

It is of interest to compare the cyclopropylhalocarbenes to the parent cyclopropylcarbene (83) and its alkyl derivatives (84-86).110-113... 

Surprising is the absence of evidence for additional stability of 85 over 83. Electron donation from the electron-rich a bonds of the cyclopropyl ring to the carbene s vacant p orbital is widely believed to stabilize cyclopropylcarbenes.4 One would therefore expect 85, with an additional cyclopropyl substituent, to react more slowly than either parent carbene 83 or dimethylcarbene, but all three lifetimes are comparable. The lifetimes of 83-85 need to be redetermined in inert (fluorocarbon) solvents in order to reveal their innate differences. Note, however, that the effect of cyclopropyl substitution is apparent upon comparison of 83 (r 24 ns) to MeCH (r < 0.5 ns).89110... 

There are further complications in the chemistry of cyclopropylcarbenes. Although 84 is widely accepted to undergo a 1,2-C shift ring expansion as its principal intramolecular reaction, reinvestigations indicate that the 1,2-H shift product, cyclopropylethene, is important and, under some circumstances,... 

A caveat in cyclopropylcarbene rearrangements is the possibility that, at least in certain systems, rearrangements may occur in stepwise fashion via diradical intermediates.113 Whether such complications intrude with the simplest cyclopropylcarbenes (83-86) is not yet clear. 

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

Cyclopentenones. Reaction of the cyclopropylcarbene complex 1 with di-phenylacetylene in 1% aqueous dioxane results in the cyclopentenone 2 as the major product. The carbonyl group of 2 is derived from a CO ligand of 1, and C,... 

Cyclopropylcarben (178) aus Cyclopropancarboxaldehyd-tosylhydrazon mit Natriumathylat in aprotischen Losungsraitteln erzeugt, ergibt durch Ringerweiterung iiberwiegend Cyclobuten (179) neben Athylen, Ace-tylen und Butadien (225—228). 

Cyclopropylcarbenes (see Section 2.4 for more on this reaction, and a caution) ring expand to give cyclobutenes in what is likely a concerted reaction. When a vinyl group is attached to the cyclopropane ring in the proper way, new products appear that are best rationalized by diradical intermediate 26 (Scheme 7.16). ... 

A similar story attends the chemistry of cyclopropylcarbenes. In the 1960s it was shown that cyclopropyl methyl diazomethane gives almost exclusively the product of apparent carbon-carbon insertion. " Surely we are now suspicious, and rightly so, as neither photolysis of 62 nor deoxygenation of cyclopropyl methyl ketone reproduces these results. Instead, carbon-hydrogen insertion dominates (Scheme 7.25). Once again, a direct reaction of photoexcited diazo compound appears to be the dominant source of the product of ring expansion (carbon-carbon insertion). 

Another important rearrangement is that of cyclopropylmethyl radicals to the corresponding homoallyl radicals. This is an exceptionally fast reaction (t1/2 10 8) and has been used as a radical clock to determine the rates of other free-radical reactions.95 Cyclopropylcarbene also undergoes rearrangement, leading to cyclobutene.96... 

Cyclopropylcarbene complexes of the type L M=C(XR )R2 (X = O, S R1 = alkyl, aryl R2 = cyclopropyl) having a stabilizing heteroalkyl (XR1) group on the electrophilic carbene ligand (Scheme 3) have found widespread application in organic synthesis. These so-called Fischer carbene complexes are best known via their group 6 transition metal carbonyl complexes (CO)5M=(OR )R2(M = Cr, Mo, W)132. Much less abundant are the Schrock-type cyclopropylcarbene complexes L M=CR R2 where no heteroatom is bound to the carbene carbon atom133. 

While the first transition metal carbene complex was reported in 1964134, the first cyclopropylcarbene complex salt [(CO)5Cr=C(chromium hexacarbonyl, followed by tetramethylammonium bromide135. Subsequent reaction with trimethyloxonium fluoro-borate gave methoxycarbene complex (CO)5Cr=C(OMe)(c-Pr) (equation 62)136. 

The rearrangement of cyclopropylcarbene to cyclobutene was studied theoretically using B3LYP at the 6-311G(d) level.58 The results show that the rearrangement of cyclopropylcarbene to cyclobutene is an electrophilic rearrangement, very different from the traditional mechanism of nucleophilic rearrangement for carbocations. 

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