Carbenes linear additions

Here the alkene and pyridine will compete for the carbene at a constant concentration of pyridine the observed pseudo first order rate constant for ylide formation will increase with increasing alkene concentration. A plot of kobs vs. [alkene] will be linear with a slope of kad, which is the rate constant for the carbene/alkene addition reaction affording cyclopropane 5 (Scheme 1). 

The second intermediate s identity has been debated since the mid-1980s. In 1984, Liu and Tomioka suggested that it was a carbene-alkenc complex (CAC).17 Similar complexes had been previously postulated to rationalize the negative activation energies observed in certain carbene-alkene addition reactions.11,30 A second intermediate is not limited to the CAC, however. In fact any other intermediate, in addition to the carbene, will satisfy the kinetic observations i.e., that a correlation of addn/rearr vs. [alkene] is curved, whereas the double reciprocal plot is linear.31 Proposed second intermediates include the CAC,17 an excited carbene,31 a diazo compound,23 or an excited diazirine.22,26 We will consider the last three proposals collectively below as rearrangements in the excited state (RIES). 

Additional evidence for a second intermediate in supposed carbene reactions comes from numerous studies.17-29 In the earliest experimental approach, the carbene precursor, frequently a diazirine, was photolyzed in the presence of increasing quantities of an alkene, which trapped the carbene with the formation of a cyclopropane (5 in Scheme 1). If carbene 2 were the sole product-forming intermediate, as depicted in Scheme 1, then the ratio of its alkene addition product (5) to its 1,2-H shift rearrangement product (4) would vary linearly with alkene concentration Eq. 9. 

Quantitative studies of the photolysis of 9a or 9c-g in the presence of alkenes provided evidence for the participation of a second reactive intermediate, in addition to carbenes 10a or lOc-g, because the addn/rearr ratio was not linearly dependent on [alkene] cf. Section III.A and Scheme 2. 

The additional reactive intermediate responsible for the curvature was postulated17,33 to be a CAC.30 The mechanism of Scheme 2 was proposed, in which carbene 10a was in equilibrium with the CAC. Thus, styrenes 11a and 12a can be formed by two pathways from the free carbene (kj) and from the CAC (k-). A steady-state kinetic analysis of Scheme 2 affords Eq. 11, which predicts that a correlation of rearr/addn with l/[alkene] should be linear the behavior actually observed by Tomioka and Liu.17,33 The CAC mechanism also accounts for the observation that the lla/12a product ratio depends upon the identity and concentration of the added alkene both k[ and k2, which define the Y-intercept of Eq. 11, depend on the added alkene. The dependence has been observed,19,33-37 albeit with only small variations in the Y-intercepts. 

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... 

The studies of MeCCl refocused attention on benzylchlorocarbene (10a). Earlier studies of 10a, over a temperature range of 0-31°C, afforded linear Arrhenius correlations for the 1,2-H shift, with Ea = 4.5-4.8 kcal/mol and log A 11.2 s-1.36 Additionally, LFP studies of p-CF3 and p-Cl substituted benzylchlorocarbenes (lOf and lOg) in isooctane over a temperature range of —3 to 47°C gave linear Arrhenius correlations with a (4.9 and 4.5 kcal/mol) and log A (10.9 s-1) values comparable to those found for parent carbene 10a.64... 

In addition to the ring opening of cyclopropenes noted above, vinylketene complexes 103 have been prepared by (1) ligand initiated carbonyl insertion of vinyl carbene complexes 104 and (2) benzoylation of ,/3-unsaturalcd acyl ferrates 105 (Scheme 20)114. X-ray diffraction analysis of these vinylketene complexes indicates that the structure may be best represented as a hybrid between an /j4-dicnc type complex (103) and an jj3-allyl r/1 acyl complex (106). The Fe-Cl distance (ca 1.92 A) is shorter than the Fe-C2, Fe-C3, or Fe-C4 distances (ca 2.1-2.2 A)113a-C. In addition, the C—C—O ketene array is not linear (bend angle ca 135°). 

The linear polymerization of Scheme 7.15 represents an unusual case of diazoacetophenone oxidation. For instance, on the action of copper oxide, diazoacetophenone gives ketocarbene, which is involved in typical carbene reactions such as dimerization, addition to olefins, and insertion in the 0-H bonds of alcohols. If the amine cation-radical is used as an oxidant instead of copper oxide, only the polymer is formed. The ketocarbene was not observed despite careful searches (Jones 1981). 

Additional factors which lead to an increased stability of the carbene complexes towards reductive ehmination are the type of NHC ligand and the NA -substitution pattern. The stability of NHC complexes depends strongly on the electronic situation at the carbene center. The oxidative addition of p-tolyl chloride to linear Pd° complexes bearing two unsaturated imidazolin-2-ylidenes (type 5, Fig. 6) or two saturated imidazolidin-2-ylidenes (t3q>e 7, Fig. 6) proceeds readily. The Pd complex with the imidazolin-2-yhdene ligands is stable, while the one with the imida-zolidin-2-ylidene ligands reductively ehminates the C2-arylated imidazolidinium salt [134]. 

The linear cyclobuta[e]cyclopropa[b]naphthalene (114) was synthesized very early by the carbene addition approach, while the synthesis of the angular isomer failed. The most strained of all isolable cycloproparenes, 1,4-dihydrocyclo-propa[b,g]naphthalene (115), was obtained originally by the same approach from the tetrabromide 116. The synthesis was recently improved. An alternative access to 115 via 117 as precursors is also available. The strain energy of 116 is extremely high, and it explodes upon melting (132 °C). ... 

The addition of a carbene to a double bond is easily dealt with by FO theory only the non-linear approach allows a good frontier interaction (Figure 4.9). This result is in agreement with the selection rules quoted earlier the system having four electrons (two coming from the carbene and two from the ethylene) and its n component... 

In addition to the representative [3 + 2] cycloaddition reactions shown in Table I, the delocalized singlet vinyl carbenes have been shown to participate as it2a components of non-linear cheletroplc Dr 2S + it 2g3 cycloadditions to provided cyclopropanes with an observable endo effect,8 and as7r2s components of [it 4s + it 2s], cycloadditions with selected dienes to provide cyclo-... 

The same acidic chloroaluminate ionic liquids have been used as solvent for tungsten aryl oxide complexes for the metathesis of alkenes [24]. Slightly acidic chloroaluminates also dissolve the [Cl2W=NPh(PMe3)3] complex which catalyze ethene oligomerization without the addition of co-catalysts [25]. In a similar way, Ni-catalyzed 1-butene dimerization into linear octenes was carried out in acidic chloroaluminates buffered with small amount of weak bases [26]. Neutral chloroaluminates (l-ethyl-3-methylimidazolium chloride/AlCl3 = 1) were employed to immobilize ruthenium carbene complexes for biphasic ADMET (acyclic diene metathesis) polymerization of an acyclic diene ester [27]. 

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