Carbenes trapping

Two other fluorine-containing diazirines give similar results. Cyanofluorodiazirine (223) and fluoromethoxydiazirine (224) release nitrogen at 100 °C the carbenes formed can be trapped by tetrafluoroethylene to give cyclopropanes (65JHC371). Without a carbene trap alkenes are obtained. 

The group of Nakatani introduced photochemically induced cyclization that takes advantage of a system in which a carbene generator and a carbene trap are combined in the same molecule [37]. Thus, irradiation of compound 5-105 induced a cyclization to give an intermediate carbene 5-106, which underwent an intramolecular trapping by a pericyclic 6jt electrocyclization to afford 5-107 in a very good yield of 95% (Scheme 5.21). 

Additional evidence for the contention that metathesis carbenes are nucleophilic was offered by Gassman in an interesting series of trapping experiments utilizing Michael acceptors as carbene traps (15, 17). Thus, an ethylidene carbene generated from 2-butene was trapped by ethyl acrylate to yield the expected ethylcyclopropyl ester, although yields were quite low. 

Treatment of tetrazolium salts (154) with mercuric acetate gives the bistetrazolium mercury salts (155) which represent essentially a metal-carbene trapping of the tetrazolium ylide species. Replacement of the mercury atom of (155) is readily achieved with halogens to give the 5-halotetrazolium compounds (156) (93CB1149). These electrophilic reactions at the tetrazole C-5 arise from the lability of the 5-CH proton and they show the necessity of generating carbanionic character at C-5 before electrophilic attack can occur on this strongly ir-deficient azole. 

Attempts to trap carbenes 3b and 3d with molecular oxygen, a reaction frequently used to characterize triplet carbenes, were successful at high (>5%) concentrations of O2 in the argon matrix22. In both cases, the corresponding silyl formate was detected, and it was assumed that a carbonyl oxide and a dioxirane are intermediates in this carbene trapping... 

Intramolecularity was the next issue to be probed within the context of alkynyliodonium salt/nucleophile addition reactions.53 1 No prior history was available to guide us, and so the prospects for success remained uncertain. Of primary concern was the potential for iodonium salt/base destructive interactions in competition with the desired N-H deprotonation reaction. A substrate that bore some resemblance to key portions of the agelastatin precursor 33 was prepared (Scheme 6), compound 39. This species duplicated the alkynyliodonium/"amide" pairing of the real system, but it lacked the complex piperazine carbene trap of 33. The tosylimide (pre)nucleophile was proposed as a compromise between what we really wanted (an N-methyl amide) and what would likely work (a tosylamide). Simple treatment of 39 with mild base effected the desired bicyclization to afford the tosylimide product 41 in decent yield. A transition state model 40 for C-H insertion that features an equatorial phenyl unit might rationalize the observed sense of diastereoselectivity. So, at least for 39, no evidence for possible interference by iodonium/base reactions was detected. 

Although a number of ethylenes (tetramethylethylene, dicyanoethylene, maleic anhydride) have been employed as potential carbene traps, the only successful example of such trapping is the formation of [99] when [79a] is irradiated in cyclopentadiene [see reference (13) for a... 

Thermolysis of both F3CSiH3 and F2HCSiH3 in the presence of efficient carbene trapping agents have shown that they decompose predominantly by a-fluorine shift to give fluorosilane and the fluorocarbenes CF2 and CHF, respectively [24]. Secondary reactions at elevated temperature in the absence of any... 

Whilst the existence, energetics and lifetimes of bicyclic intermediates of the type 223 in the mononuclear aromatic series are not formally established , it is certain that annelated analogues are true intermediates in various carbene-carbene rearrangements Thus, cycloheptatrienylidene 226 gives products expected of the arylcarbene 227 when generated in the presence of carbene traps. However, when formed... 

Tetrachloroethene, a much less active carbene trap than other alkenes, forms an adduct with chloro(2-pyridyl)carbene, albeit in very low yield however it does not react with the corresponding 3-pyridylcarbene. T... 

Addition versus rearrangement of chloro(methyl)-, chloro(ethyl)-, benzyl(chloro)-, chlo-ro(methoxymethyl)- and chloro(isopropyl)carbene generated by photolytic or thermal decomposition of 3-alkyl-3-chloro-3//-diazirines has also been investigated.81 However, it is known that chloro(isopropyl)- or ie/7-butylchlorocarbene does not undergo addition to 2,3-dimethyl-but-2-ene, which is used as a carbene trap. Photolysis or thermolysis of 3-arylmethyl-3-chloro-3//-diazirines in the presence of an alkene has also been studied extensively.84 Addition of the ambiphilic benzylchlorocarbene82 to an alkene to give 1-benzyl-1-chlorocyclopropanes is usually accompanied by its rearrangement to co-chlorostyrene (1,2-hydride shift, for details see ref 83). This isomerization was seriously impeded when the decomposition of 3-benzyl-3-chloro-3//-diazirines was performed in the presence of an alkene,85 with 2,3-dimethylbut-2-ene only traces of co-chlorostyrene were observed.81,86 The results of reactions of benzylchlorocar-benes with some alkenes are collected in Table 6.85... 

Synthesis of aziridines.11 Lead tetraacetate oxidation of certain 1,1-disubstituted hydrazones, for example, (1)—(3), in the presence of nucleophilic olefins commonly used as carbene traps, gives aziridines in fair to good yields. Presumably singlet... 

Unlike the dichlorocarbene, in the presence of cyclohexene (a carbene trap), no cyclopropane adduct results. However, a reaction takes place rapidly, even at — 10°C, giving the di-aH compound instead (Ballester and Olivella, 1974). This abnormal result could be due to steric hindrance in the formation of the cyclopropane, and to reaction via the triplet carbene (135). 

The reaction of 17 with McsCCHO or with McsSnCl proceeds at the same rate as reaction with 18. It is therefore postulated that all these reactions proceed via slow dissociation of 17 to Ti(=CH2)Cp2, followed by rapid reaction with PhC=CPh, MesCCHO or MesSnCl. These compounds thus behave as metal carbene traps. 

HFPO (1) is an excellent source of difluorocarbene, superior to hexafluorocyclopro-pane because it decomposes at lower temperatures. It fragments into the carbene and trifluoroacetyl fluoride with a half-life of 169 min at 190°C, as determined by gas-phase NMR. In the absence of a carbene trap, hexafluorocyclopropane and the acyl fluoride are the main products of HFPO decomposition at 200°C. ... 

The formation of ethers 153, 154 and 155 from the corresponding compounds 150a or 150b may be explained by the equilibrium between bitriazolop5UTnes 150a,b and an intermediate diazoalkane that could lose dinitrogen to form a carbene trapped by solvents (ethanol, methanol, water) (Scheme 39). 

The structure of the penam product denounces migration of a phenylthio group, which requires the involvement of dipolar species 320 in the C2-C3 bond formation. This intermediate would arise from the 3-thiacepham sulfonium ylide 319, which is a reasonable product of intramolecular carbene trapping (318 - 319) by the thioether sulfur atom, in the absence of the thioxo sulfur atom. Thus, three different 3-thiacepham dipolar species (313,314,319) formally participate in as many penem ring formation strategies. This is one of the fascinating aspects of P-lactam chemistry. 

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