Deuterium labelling carbenes

The competing ring expansion and cleavage in carbene 30 was confirmed by generating the deuterium labeled carbene 30a by the C atom deoxygenation of alde-... 

The species identified as XA reacts with styrene to give the expected cyclopropane. The rate constant for this reaction is ca 200 times less than the corresponding rate constant for 3BA (Table 6). Also, use of the deuterium-labeled a-methylstyrene reveals that the cyclopropanation occurs with essentially total retention of stereochemistry. Moreover, precisely the same result is obtained when this carbene is formed by triplet sensitization rather than by direct irradiation. These findings also point to a reaction originating from a singlet carbene. 

If cyclopentene would react pair-wise with 2-pentene, only one product would form, namely 2,7-decadiene, and a similar result for cyclodimers etc. of cyclopentene. If somehow, the alkylidene species would be transferred one by one, we would obtain a mixture of 2,7-nonadiene, 2,7-decadiene, and 2,7-undecadiene in a 1 2 1 ratio. The latter turned out to be the case, which led the authors to propose the participation of metal-carbene (metal alkylidene) intermediates [6], Via these intermediates the alkylidene parts of the alkenes are transferred one by one to an alkene. The mechanism is depicted in Figure 16.4. In the first step the reaction of two alkylidene precursors (ethylidene -bottom- and propylidene -top) with cyclopentene is shown. In the second step the orientation of the next 2-pentene determines whether nonadiene, decadiene or undecadiene is formed. It is clear that this leads to a statistical mixture, all rates being exactly equal, which need not be the case. Sometimes the results are indeed not the statistical mixture as some combinations of metal carbene complex and reacting alkene may be preferred, but it is still believed that a metal-carbene mechanism is involved. Deuterium labelling of alkenes by Gmbbs instead of differently substituted alkenes led to the same result as the experiments with the use of 2-pentene [7],... 

On the contrary, a-lithiated epoxides have found wide application in syntheses . The existence of this type of intermediate as well as its carbenoid character became obvious from a transannular reaction of cyclooctene oxide 89 observed by Cope and coworkers. Thus, deuterium-labeling studies revealed that the lithiated epoxide 90 is formed upon treatment of the oxirane 89 with bases like lithium diethylamide. Then, a transannular C—H insertion occurs and the bicyclic carbinol 92 forms after protonation (equation 51). This result can be interpreted as a C—H insertion reaction of the lithium carbenoid 90 itself. On the other hand, this transformation could proceed via the a-alkoxy carbene 91. In both cases, the release of strain due to the opening of the oxirane ring is a significant driving force of the reaction. 

Steinmetz and coworkers carried out mechanistic studies on the far-UV photochemical ring opening of l-silacyclobut-2-ene 80. The intermediates were trapped by alcohols to give 84-87 and by methoxytrimethylsilane to give 88 and 8955. The main reaction is the formation of 1-silabuta-1,3-diene 81, while the formation of silene 2, probably via the carbene 90, is a minor reaction (equation 19). The mechanism suggested was supported by deuterium labelling studies and ab initio calculations. 

When deuterium-labeled 2-methylene-l-silacyclobutane was investigated, scrambling of deuterium between the allylic methylene and the terminal vinyl was observed. This fact has been explained by ring opening to the 1,4-diradical at temperatures below those required for the rearrangement to the carbene <1995JA11695>. 

Visible light irradiation (X > 475 nm) of quinone diazide (21) isolated in Ar matrices at 10 K produces the corresponding carbene quantitatively. Further UV photolysis (X > 360 nm) of this carbene results in formation of a labile species, which is identified as the diradical (22). The identification of (22) was supported by ROSS-BLYP/6-31G(d,P) calculations, and confirmed by deuterium labelling. 

Conclusive evidence for the cation intermediate was obtained by detection of the absorption spectra of several diarylmethyl cations following nanosecond or picosecond laser flash photolysis of their respective diazo, diphenylazi-ridinylimine, or 3H-indazole precursors in acidic media [107-112], Photochemically generated vinyl carbenes were recently shown to similarly protonate by deuterium labeling experiments to give allylic cations that were detected by transient absorption spectroscopy [113],... 

Finally, the high-temperature pyrolysis of benzocyclobutene gives styrene. While this could be the result of cleavage of the stronger cyclobutene bond followed by a hydrogen shift. Chapman and Trahanovsky provided evidence based on carbon and deuterium labeling that reversion to a tolyl carbene and its subsequent rearrangement could be responsible for a substantial amount of product. ... 

A stepwise 1,7-vinyl shift was proposed to account for the reaction after carbene addition to make the cyclopropene. Deuterium labeling studies were consistent with this pathway. Interestingly, upon heating to 145°C the phenyl-substituted 5.2.0 tetraene isomerized to 2-phenylindene in what appears to be a cyclization of the cycloheptatriene moiety followed by opening of the bicyclo[2.1.0]pentene and a 1,5-hydrogen shift (Scheme 10.9). 

A prediction of the rearrangement products a priori was not possible, because their formation depends on too many factors. Table 9 summarizes results on the reactivity of bicyclo[1.1.0]butanes . The Ni°-catalysed rearrangement of bicyclo[ 1.1.0]-butane (Table 9 entry la) proceeds via a metal-carbene complex, as was demonstrated by a deuterium-labelling experiment using 65 as a mechanistic probe (equation 33) . ... 

Cobalt-phosphine and cobalt-carbene catalysts have been developed for the hydroarylation of styrene with (91) via a chelation-assisted C-H bond activation. The regioselectivity can be controlled in favour of the branched (92) or linear product (93) at will by the ligand. Deuterium-labelling studies demonstrated a reversible C-H bond cleavage and alkene insertion steps and reductive elimination as the rate-and regioselectivity-determining step. ... 

The rearrangement products (359) and (360) are obtained on oxymercura-tion of cyclodeca-l,2,5,8-tetraene (361). A bridged intermediate (362) is suggested homoallyl-cyclopropyl rearrangement leads to (363), a metal-complexed carbene which is known to undergo hydride shifts as indicated. This mechanism is supported by deuterium labelling. ... 

In the preceding discussion, it was assumed that elimination products such as 1-octene (110) are formed via transfer of a P-hydrogen atom or a P-proton from the corresponding radical or carbocation, respectively. However, deuterium labeling studies with 1-iodooctane (107a) showed that 17-29% of the 1-octene (110) is formed via a-ehmination to the carbene intermediate 124, depending on the solvent and irradiation conditions (Scheme 4). Similarly, 27-30% of the cyclohexene (116) from 1-iodocyclohexane (114) arises via a-ehmination. However, only 7% of the 1-octene (110) from bromide 107b arises via a-ehmination. ... 

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