Alkenes from carbenes

In some, though not necessarily all, cases loss of H and Cl is thought to be concerted, leading directly to the carbene (cf. p. 50) intermediate (75) formation of the product alkene from (75) then requires migration of H, with its electron pair, from the /J-carbon atom. A 1,1-elimination (Ea) will be indistinguishable kinetically from 1,2-(E2), and evidence for its occurrence rests on isotopic labelling, and on inferential evidence for the formation of carbenes, e.g. (75). 

In pentane, the distribution of 1,3-insertion product 25 to 1,2-Me shift product 26 is 91 9. Upon addition of 2-methyl-1-butene, the yield of 25 smoothly decreases (to 19% with 4 M alkene), but the yield of 26 is unaffected 1 Moreover, correlation of addn/l,3-CH insertion (to 25) for 18 is nicely linear. The simplest interpretation is that 25 comes directly from carbene 18, whereas the 1,2-Me shift product 26 comes from the excited diazirine.27 Interestingly, thermolysis of 24 at 79°C produces 90% of 25 and 10% of 26, but now the yields of both products smoothly decrease in the presence of an alkene. In thermolysis the (electronically) excited diazirine is unavailable, both 25 and 26 stem from the carbene, and their formation is suppressed by the alkene s interception of the carbene. A pyridine ylide kinetic study gave the 1,3-CH insertion rate constant (18 - 25) as 9.3 x 10s s"1.27-47... 

Of all the carbenes considered in this chapter, benzylchlorocarbene (10a) has produced the most debate, from carbene-alkene complexes vs. excited state... 

Miscellaneous. An interesting synthesis of 1,1-difluoro-l-alkenes from ylides and chlorodifluoromethane has been described.47 The ylide acts both as a carbene generator and trapping agent (Scheme 11). 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

For the cleavage of alkenes from a support by metathesis, several strategies can be envisaged. In most of the examples reported to date, ring-closing metathesis of resin-bound dienes has been used to release either a cycloalkene or an acyclic alkene into solution (Figure 3.38, Table 3.44). Further metathesis of the products in solution occurs only to a small extent when the initially released products are internal alkenes, because these normally react more slowly with the catalytically active carbene complex than terminal alkenes. If, however, terminal alkenes are to be prepared, selfmetathesis of the product (to yield ethene and a symmetrically disubstituted ethene) is likely to become a serious side reaction. This side reaction can be suppressed by conducting the metathesis reaction in the presence of ethene [782,783]. 

A variation on Scheme 27 and Equation (19) has been utilized by Keeffe and the author to evaluate p fas of alkyl, aryl and alkoxy carbenes.26 For carboca-tions for which the pKA for loss of a proton from a (3-carbon atom is known, combination of this pKA with the experimental or calculated energy difference between alkene and carbene conjugate bases leads to the pAa for protonation of the carbene, provided it can be assumed that the energy difference between alkene and carbene is insensitive to solvent. Where a pKA for loss of a (3-hydrogen of the carbocation is not accessible, for example, for carbenes lacking a (3-hydrogen, p fR can be used instead. Thus the cycle of Scheme 28 relates a pKa for protonation of the carbene to an experimentally measured... 

The formation of epoxides is a well-investigated synthetic problem and two approaches, either from a double bond system by transfer of oxygen starting from an alkene, or carbene transfer to a carbonyl group, have attracted much interest. The use of the CpFe(CO)2+ fragment was also investigated by Hossain and coworkers with a view to its use for the synthesis of epoxides (Scheme 9.15) [30, 31]. However, the CpFe(CO)2+ fragment not only catalyzes the carbene transfer, but also acts as... 

Many other iron complexes (t -alkyls, t -acyls, t -ynyls, cationic -alkenes, and carbenes) are ultimately erived from either sodium dicarbonyl(cyclopentadienyl)... 

In the early 1950s Bamford and Stevens reported a thermally induced reaction of ketone tosylhydra-zone monosodium salts that gives products derived either from carbene intermediates (aptotic conditions) or carbocations (protic conditions). Both reactions, which are illustrated in Scheme 51, can give alkenes as the major product, although the protic reaction is of marginal synthetic value because mixtures of alkene regioisomers and rearrangement products are normally obtained. On the other hand, the... 

The reactions of P-halophospha-alkenes with carbenes provide a new route to 1-chloro-lH-phosphirines (181). Such compounds are highly reactive, and a study of nucleophilic displacement reactions at phosphorus has been facilitated by preparation of the related P-W(CO)5 complexes.The first pentacoordinate systems derived from phosphirenes, (182), have been formed in the reactions of P-halo- or P-cyano-phosphirenes with tetrachloro-o-quinone. Evidence has been presented for the rearrangement of P-chlorophosphirenium ions (183) to the vinylphosphenium ions (184)... 

Table 1. Arylcyclopropanes from Alkenes and Carbenes, Generated from Arylhalomethane or Aryldihalomethane with Organolithium Compounds... 

The synthesis of 1,1-diiodocyclopropane is carried out via addition of diiodocarbene (car-benoid) to alkenes the carbene, in turn, is generated from iodoform and a base. The process is realized using phase-transfer catalysis,(33% or 50%aqueous sodium hydroxide and a quaternary ammonium salt, typically benzyltriethylammonium chloride, as a catalyst) or in the presence of potassium ter/-butoxide in /ert-butyl alcohol 25,127,128 g... 

Table 37. l-Alkoxy(or -Aryloxy)-l-halocyclopropanes from Carbenes, Generated from Diazirines, and Alkenes... 

Alkyl(or aryl)sulfanylcyclopropanes are prepared by the addition of organosulfanylcarbene (carbenoid) to an alkene. This carbene is, in turn, generated via a-elimination of hydrogen chloride from the corresponding alkyl(aryl)chloromethyl sulfides using a base (Houben-Weyl, Vol.4/3, pp248-252 and Vol.E19b, pp 1690-1691 and 1693-1695). 

Cyclopropanation from alkenes and carbenes with alkyl gem dihalides and Zn-Cu couple (Simmons-Smith) or Et2Zn (Furukawa) EtgAI (Yamamoto) or Sm (Molander) with high diastereoselectivity (see 1st edition). 

Replacement of an unsaturated phenyliodo group. Formation of unsaturated sulfones by a tandem Michael addition-elimination is a highly efficient process that allows the synthesis of (Z)-l,2-bis(benzenesulfonyl)alkenes from (Z)-jS-(benzenesul-fonyl)alkenyliodonium salts. In S-ketoethynyl(phenyl)iodonium salts the electron-withdrawing power of the ketone group is weaker, the Michael addition is followed by carbene formation. Cyclopentenones are formed. A seemingly direct substitution of alkynyl(phenyl)iodonium salts gives alkynyl sulfones efficiently. ... 

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