Rearrangement carbene

FRITSCH BUTTENBERG - WIECHELL Acetylene Synthesis Alpha elimination from haloethylenes leading via carbene rearrangement to acetylenes... 

The rearrangement of acylcarbenes to ketenes is called the Wolff rearrangement (18-8). A few rearrangements in which carbenes rearrange... 

When the Wolff rearrangement is carried out photochemically, the mechanism is basically the same, but another pathway can intervene. Some of the ketocarbene orieinallv formed can undergo a carbene-carbene rearrangement, through an oxi-... 

The thermal conversion of carbene 53 to styrene could also be observed in Xe matrices. Although stable at 12 K for at least 24 h, increasing amounts of carbene were observed to convert to styrene as the temperature was raised gradually to ca. 60 K. At each intermediate temperature between 12 and 60 K, site clearing behavior was detected, where some amounts of carbene disappeared rapidly, followed by no further decay. At around 60 K, the carbene rearranged with exponential first-order... 

Carbon tunneling in a second singlet chlorocarbene has also been proposed. It has proved impossible to observe noradamantylcarbene 73 spectroscopically, either by solution laser flash photolysis or with matrix isolation at low temperatures. It has been suggested that the carbene rearranges too rapidly, possibly via carbon tunneling, to adamantene (74). 

Tunneling in a Cyciopropene to Triplet Carbene Rearrangement In previous sections, tunneling in the reactions of a number of carbenes to give covalently saturated products has been described. We conclude this section with a unique example where tunneling appears to dominate the converse of this process, cleavage of a cyciopropene to a triplet carbene. 

Intramolecular cyclopropanation of [(7 H-benzocycloheptene-7-yl)diazomethyl]-diphenylphosphinoxide 172 represents the first synthesis of a benzo-annulated octavalene (173)180). Not unexpectedly, carbenic rearrangement products 174 and 175 are also formed. 

The pyridine ylide method depends on the effective trapping of the carbene by pyridine. At high pyridine concentrations every carbene produced by the laser pulse will be trapped as ylide, and the ylide s absorbance (Ay) will saturate or reach a plateau (A °). The magnitude of A ° will vary with carbene structure it will decrease with both the increasing ease of carbene rearrangement and the intervention of RIES during carbene generation.3... 

Equation 1 cannot be used to extract k0 for carbenic rearrangements in the region of A. There, however, a Stem-Volmer analysis can be applied wherein the optical yield of ylide as a function of pyridine concentration is used to obtain ko.4 The optical yield of ylide formation, Ay, is defined in Eq. 3,... 

Hydrogen and 1,2-carbon migrations are the most common singlet carbene rearrangements. They involve simple shifts of substituents from an adjacent carbon to the carbenic center, affording an alkene product Eq. 7. 

One can correct the observed distribution of 1,2-C and 1,2-H products so as to reflect only the carbenic pathway.28 The corrected 1,2-C/1,2-H ratio (4.8), coupled with a pyridine ylide absolute rate constant for overall carbene rearrangement (kc + % = 6.8 x 107 s 1), gives the partitioned rate constants kc = 5.6 x 107 s 1 and ifeH = 1.2 x 107 s-1.28 The dominance of 1,2-C over 1,2-H in this system will be discussed below. 

Clearly, rearrangements do occur in the excited states of diazirine and diazo carbene precursors. Kinetic studies of carbenic rearrangements need to consider the possible intervention of RIES when absolute rate constants are partitioned between competitive rearrangement pathways on the basis of product distributions.28... 

We have seen that 1,2-H migrations in singlet carbenes may be affected by (e.g.) the participation of carbene precursor excited states, QMT, stabilization of the hydride shift transition state by polar solvents, and temperature. Here, we consider our third principal theme, the effect of substituents on the kinetics of carbenic rearrangements. We first examine the influence of bystander and spectator substituents (as defined in Eq. 22) on 1,2-H rearrangements of alkyl, alkylchloro, and alkylacetoxycarbenes. 

SOME ASPECTS OF THE CARBENE BRIDGEHEAD-OLEFIN CARBENE REARRANGEMENT... 

The first example of a carbene bridgehead-olefin carbene rearrangement comes from the laboratory of Eaton.7 Cubylphenyldiazomethane (9), when photolyzed or heated in ethanol, afforded a mixture of ethers 10 and 11. The reaction follows the route shown in Scheme 2. It has been reviewed,1 and the exiting chemistry of the parent system 12 — 13 — 14, elucidated by Jones and... 

Some Aspects of the Carbene Bridgehead-Olefin Carbene Rearrangement 271... 

Summing up these results, it has been shown that the carbene bridgehead olefin-carbene rearrangement is also observed when diazoalkanes are the precursors for the generation of the bicyclo[l.l.l]pent-l-ylcarbenes of type 52. 

Scheme 6.114, a carbene-carbene rearrangement transforms diphenylcarbene to o-phenylphenylcarbene, which is the progenitor of 565. Two phenylbicydo[4.1.0]-hepta-2,4,6-trienes and l-phenyl-l,2,4,6-cycloheptatetraene (562) have to be assumed as further intermediates. The participation of 562 is supported by the structure of the products 563 and 564, which should result from the addition of 562 to diphenylcarbene and the dimerization of 562, respectively. By thermolysis of the sodium salt of 2-phenyltropone tosylhydrazone, 562 was generated directly. At 100 °C in diglyme as solvent, 564 was identified as the only product and at 340°C/4Torr in the gas... 

Since the reactive substructure of cA-2-(l,3-butadienyl)cyclopropylidene is contained in the bicyclic carbene 124, there is a possibility that a carbene-carbene rearrangement occurs together with 1,5-carbon migration. Analysis of the probable reaction pathways allows one to conclude that 1,5-C-migration (124 - 132) in the fixed c -l,3-butadienyl fragment of structure 124 is impossible. The 1,3-carbon migration (124 - 130) which takes place instead is mechanistically analogous to the vihylcyclopropylidene-cyclopentylidene... 

As noted with the reactions between terpenes and dihalocarbenes, mono-insertion adducts at the more electron-rich sites can be isolated from the reaction of non-conju-gated acyclic and cyclic dienes although, depending on the reaction conditions, the bis-adducts may also be formed. Norbomadiene produces both 1,2-endo and 1,2-exo mono-insertion adducts with dichlorocarbene, as well as a 1,4-addition product (Scheme 7.4) [67]. The mono adduct produced from the reaction with dimethylvinylidene carbene rearranges thermally to yield the ring-expanded product (Scheme 7.4) [157] a similar ring-expanded product is produced with cyclo-hexylidene carbene [149]. 

Here again, all these reactions have been widely reviewed and just general trends will be discussed here, with a special focus on the mechanisms. These mechanisms can involve a carbenoid rearrangement or a free carbene rearrangement. The question of the carbenoid or carbene intermediacy has been largely addressed and, in most cases. 

Garcia-Garibay M (2003) Engineering carbene rearrangements in crystals from molecular information to solid-state reactivity. Acc Chem Res 36 491-498... 

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