Rearrangements via carbenes

Very highly constrained bicyclic alkenes rearrange via carbene intermediates to ring-contracted products. Thus, bicyclo[3.3.0]oct-2-ene gave 6-methylenebicyclo[3.2.0]heptane (38), and bicy-clo[2.2.1]hept-2-ene gave 5-methylenebicyclo[2.1.1]hexane (39).113... 

In the last chapter we met diazoalkanes as sources of carbenes in the synthesis of three-membered rings. These same diazoalkanes are useful reagents for rearrangements via carbenes or carbocations. 

On direct irradiation in nonhydroxylic solvents, alkenes typically undergo several competing photoreactions. In addition to ,Z-isomerization, which is also exhibited on triplet sensitization, several processes occur that are usually observed only on direct irradiation rearrangement via carbene intermediates, double bond migration, alkyne formation, and hydrogen atom abstraction. This chapter is concerned with these latter processes. Emphasis is given to developments since the topic was reviewed extensively in 1979. Portions of this material are covered in reviews that have appeared since then. The photobehavior of alkenes in hydroxylic media is discussed in Chapter 9. 

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

Carbenes can rearrange to alkenes in many cases. A 1,2-hydrogen shift leads to an alkene, and this is often competitive with insertion reactions.Benzylchlorocarbene (42) rearranges via a 1,2-hydrogen shift to give the alkene.Similarly, carbene (43) rearranges to alkene (44), and replacement of H on the a-carbon with D showed a... 

Pyrolysis of bis(trimethylsilyl)phenyl methanol 1668 at 500 °C leads, via elimination of trimethylsilanol 4, to the carbene intermediate 1669, which rearranges, via the carbene intermediate 1670, to give l,2-dimethyl-2,3-benzo-l-silacyclopent-2-ene 1671, in 25% yield, or rearranges via olefin 1672 and adds 4 to give the siloxane 1673 in 29% yield and smaller amounts of benzyltrimethylsilane 83 and styrene [43, 44]. Pyrolysis of l,l-bis(trimethylsilyl) cyclohexylalcohol 1674 furnishes, via the carbene intermediate 1675, 90% of olefin 1676 [43, 44] (Scheme 10.20). 

Photoelimination of nitrogen from diazirines, for example, proceeds via carbene intermediates. 3-tert-Butyldiazirine (395) is converted into the cyclopropane 396 and the alkene 397 with the formation of the carbene insertion product being favored from the singlet state.328 3-Cyclopropyl-3-chlorodiazirine (398) has similarly been converted to the carbene 399 which undergoes both rearrangement to l-chlorocyclobutene(400)329 and addition... 

Direct dynamics calculations were carried out with quasiclassical normalmode sampling from a canonical ensemble at 923 K (the experimental reaction temperature). Simulations initiated at the vicinity of TS for rearrangement of carbene 13 to 14 via oxirene 12, and 300 trajectories were obtained at DFT methods. The preliminary results reported in the manuscript showed that preferred formation of 15a over 15b by the ratio of 1.8 7.6 depends on the method used. The results were qualitatively consistent with the value of 2.5 deduced from the experiment. The non-unity ratio likely arises from the situation that two methyl groups in 14 are dynamically unequal on the carbene formation process. 

Aldehyde -> Alkyne Elongation via Carbene and Carbenoid Rearrangements... 

Methyllithium (and likewise BuLi and allyllithium) also adds to the carbene ligand of (CO)5W[C(OMe)Ph]. However, the reaction of the resulting anionic adduct with Si02/pentane at —40°C yields pentacarbonyl(n -olefin)-W complexes, probably via the intermediary formation of the methyl(phenyl)carbene complex and following rearrangement via 1,2-hydrogen shift ... 

All-isomer is therefore regarded as the product of decomposition of the i2 diazo derivative (ii) via a carbene intermediate (12) (see p, 341). In hydroxylic solvents the 12-diazo compound (ii) suffers protonation, probably at C(i2) (c/. p. 340), and the resulting diazonium ion (13) rearranges via the C(i2> carbonium ion with elimination of N2. A possible alternative reaction sequence involving iV-protonation of the diazo compound is also illustrated (14). Present evidence does not permit... 

The carbene generated by photolysis of diazo compound 499 rearranges via phenyl migration to a short-lived phosphene 500, which with aldehydes or ketones gives [2-l-2]-cycloadducts (A -l,2-oxaphosphetanes (501 (80CB3303 81T1039). 

Ratio (ZiE) 75 25. Both products are apparently derived via a singlet carbene acid-catalyzed ring opening gave only the major isomer. Possibly via a di-7i-methane rearrangement via a spiropentane diradical. 

Interaction of a metal carbene with an unshared electron pair of an ether oxygen atom leads to an oxonium ylide which is able to rearrange via a 1,2-carbon shift to a formal C—O insertion product. It seems, however, that this is a rather unfavorable pathway, since reports on this reaction mode are scarce >. One may expect... 

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