Alkene complexes cyclopropanation

Evans suggests that the catalyst resting state in this reaction is a 55c Cu alkene complex 58, Scheme 4 (35). Variable temperature NMR studies indicate that the catalyst complexes one equivalent of styrene which, in the presence of excess alkene, undergoes ready alkene exchange at ambient temperature but forms only a mono alkene-copper complex at -53°C. Addition of diazoester fails to provide an observable complex. These workers invoke the metallacyclobutane intermediate 60 via a formal [2 + 2] cycloaddition from copper carbenoid alkene complex 59. Formation of 60 is the stereochemistry-determining event in this reaction. The square-planar S Cu(III) intermediate 60 then undergoes a reductive elimination forming the cyclopropane product and Complex 55c-Cu, which binds another alkene molecule. 

Photolysis ofbenzylchlorodiazirine (3) in the presence of tetramethylethylene (TME) is known to produce ( )- and (Z)-/l-chlorostyrene (4) and the cyclopropane (5). Plots of [5]/[4] vs [TME] are curved, consistent with the existence of two pathways for the formation of the alkenes (4). Benzylchlorocarbene (BnClC ) was generated by laser flash photolysis of the phenanthrene (6) in the presence of TME. In this case, plots of [5]/[4] vs [TME] are linear, mling out the possibility that the second pathway to the alkenes (4) involves reaction of a carbene-alkene complex. Time-resolved IR spectroscopy revealed that diazirine (3) rearranges to the corresponding diazo compound, but this process is too inefficient to account for the curvatures. It is proposed that the second pathway to alkene formation involves the excited state of the diazirine. 

Diastereoselective and enantioselective (see Enantio-selectivity) cyclopropanations of chiral alkenes can be achieved (Scheme 57). Unactivated alkenes usually do not participate in cyclopropanation reactions of Fischer carbenes. However, alkenyl- and heteroaryl-substituted group 6 alkoxy carbene complexes cyclopropanate unactivated alkenes in good yield (Scheme 58). ... 

Chiral cyclopropanes geminally substituted with two EWGs (259) are obtained when alkenes, complexed with Fe carbonyls (257), are treated which sulphur ylides bearing an EWG (equation 78). Reaction of cyanosulphoniummethylide (260) with electron-... 

The transient zirconocene butene complex, 105, has proved to be useful in a number of organic transformations. For example, butene substitution of zirconocene alkene complexes with alkoxy-substituted olefins results in /3-alkoxide elimination to furnish the zirconocene alkoxy compounds (R = Me, 123 R = Bnz, 124) (Scheme 16).50,51 Addition of propargyl alcohols to the zirconocene butene complex, 105, affords homoallylic alcohols. These reactions are of limited utility owing to the lack of stereoselectivity or formation of multiple products. Positioning the alkoxide functional group further down the hydrocarbyl chain allows synthesis of cyclopropanes, though mixtures of the carbocycle and alkene products are obtained in some cases (Scheme 16).52... 

Generation of Alkyl and Cycloalkyl Carbenes - Photolysis or thermolysis of a series of alkylchlorodiazirines (16) (Scheme 7) in the presence of alkenes, such as tetramethylethene, results in 1,2-H shifts, giving the corresponding vinyl chorides (18), in competition with additions of the carbenes (17) to the alkenes, yielding cyclopropanes (19). The mechanism of these reactions is discussed in the light of results obtained from photoacoustic calorimetry, and the ratio of vinyl chloride to cyclopropane seems to depend on the excited states of the carbene precursors and also on carbene-alkene complexes. Similar reactions of related diazirines have been investigated by flash photolysis. 

Path a shows loss of an L-type ligand first (giving complex 56), which allows complexation of the alkene to the metal to yield 57. Rearrangement of 57 to metal-locyclobutane 58 amounts to a formal 2+2 cycloaddition of alkene to compound 56. Intermediate 58 can then undergo RE to give cyclopropane 59, or it may decompose to give 60 and a new alkene 61. Cyclopropanation is stereospecific with respect to the substitution pattern of the alkene, but two stereomeric products (59a and 59b) are possible if two different substituents were originally attached to Cc.irbcnc. 

With 1-phenylpropanone enolate 21 only the iridium complex reacts analogously, while the rhodium complex under these conditions leads to an alkene complex, stemming from terminal carbon attack. At higher temperatures (— 35°C) with short reaction times (0.5 hour) the rhodium complex, too, leads to the expected metallacycloclobutane 22. These results indicate reversible nucleophilic attack to the rhodium complex, while the iridium complex is more stable. Both systems again give the cyclopropane product upon treatment with iodine. ... 

Using the second row of isolobal fragments (40) - (43), one can replace one, two, or all three CH2 units in cyclopropane (51) by isolobal partners to form the molecules listed in (52). Notice that the top row of structures have been drawn as metallacyclopropanes and that these are equivalent (see Section 5.3) to metal-alkene complexes. The essential details of the bonding and their conformational preferences are identical in each molecule. 

The understanding of this catalysis started in 1952, shortly after the concept of carbenes was introduced (see Sect. 8.1). Yates postulated that transition-metal catalysts react with diazo compounds by formation of transient electrophilic metal carbenes, because that complex can be depicted as a metal-stabilized carbocation (8.104). Doyle (1986 a) proposed the catalytic cycle (8-46) for the formation of the carbenoid 8.104 and its reaction with an electron-rich substrate S . The reagent S is, first of all, an alkene in cyclopropanation, but can also belong to other groups of compounds, to be discussed later in this section. 

Our initial explanation of the negative activation energies postulated that the carbene-alkene additions involved the intermediate and reversible formation of loose carbene/alkene change-transfer complexes. The partitioning of these intermediates between cyclopropane formation and reversion to PhCCl and alkene would determine the observed rate constants and their temperature dependence. [95] However, although carbene-arene Jt complexes do appear to modulate the chemistry of some carbenes in solution, carbene-alkene complexes have not been supported by theoretical studies. [96]... 

Oxidative addition involves cleavage of the covalent bonds as described above. In addition, oxidative addition of a broader sense occurs without bond cleavage. For example, tt-complexes of alkenes and alkynes are considered to form complexes by oxidative addition. Two distinct Pd—C bonds are formed, and the resulting alkene complexes are more appropriately described as the palladacyclo-propane 4 and the alkyne complex may be regarded as the palladacyclopropene 5. Thus the coordination of the alkene and all ne results in formal oxidation of Pd. The palladacyclobutane 6 is formed by the oxidative addition of cyclopropane with bond cleavage. 

The thermal decomposition of phenylmethoxycarbene complexes leads predominantly to dimer formation (Fischer et al., 1969). When the decomposition is run in the presence of simple alkenes, no cyclopropanes are formed. Thermolysis of alkylalkoxycarbene complexes leads predominantly... 

Treatment of the carbene complex [Cr C(OMe)Ph (CO)6] with certain alkenes yields cyclopropanes by transfer of the carbene entity. The reaction of diethyl fumarate with (- )-(/ )-methyIphenylpropylphosphine(phenylmethoxycarbene)tetracarbonyl-chromium produces the optically active cyclopropane (19), the formation of which demonstrates that no free carbene is involved in the mechanism for the reaction. ... 

Ru(Tr-pybox)(77 -C2H4)Gl2] 19 " and related pybox" " and 2,6-bis(imino)pyridyl alkene complexes" have attracted attention as catalytic precursors for the asymmetric cyclopropanation of alkenes. 

Facile reaction of a carbon nucleophile with an olefinic bond of COD is the first example of carbon-carbon bond formation by means of Pd. COD forms a stable complex with PdCl2. When this complex 192 is treated with malonate or acetoacetate in ether under heterogeneous conditions at room temperature in the presence of Na2C03, a facile carbopalladation takes place to give the new complex 193, formed by the introduction of malonate to COD. The complex has TT-olefin and cr-Pd bonds. By the treatment of the new complex 193 with a base, the malonate carbanion attacks the cr-Pd—C bond, affording the bicy-clo[6.1,0]-nonane 194. The complex also reacts with another molecule of malonate which attacks the rr-olefin bond to give the bicyclo[3.3.0]octane 195 by a transannulation reaction[l2.191]. The formation of 194 involves the novel cyclopropanation reaction of alkenes by nucleophilic attack of two carbanions. 

There are three main criteria for design of this catalytic system. First, the additive must accelerate the cyclopropanation at a rate which is significantly greater than the background. If the additive is to be used in substoichiometric quantities, then the ratio of catalyzed to uncatalyzed rates must be greater than 50 1 for practical levels of enantio-induction. Second, the additive must create well defined complexes which provide an effective asymmetric environment to distinguish the enantiotopic faces of the alkene. The ability to easily modulate the steric and electronic nature of the additive is an obvious prerequisite. Third, the additive must not bind the adduct or the product too strongly to interfere with turnover. 

The catalytic asymmetric cyclopropanation of an alkene, a reaction which was studied as early as 1966 by Nozaki and Noyori,63 is used in a commercial synthesis of ethyl (+)-(lS)-2,2-dimethylcyclo-propanecarboxylate (18) by the Sumitomo Chemical Company (see Scheme 5).64 In Aratani s Sumitomo Process, ethyl diazoacetate is decomposed in the presence of isobutene (16) and a catalytic amount of the dimeric chiral copper complex 17. Compound 18, produced in 92 % ee, is a key intermediate in Merck s commercial synthesis of cilastatin (19). The latter compound is a reversible... 

Because in metathesis reactions with most catalyst systems a selectivity of nearly 100% is found, a carbene mechanism seems less likely. Banks and Bailey ( ) reported the formation of small quantities of C3-C6-alkenes, cyclopropane, and methylcyclopropane when ethene was passed over Mo(CO)6-A1203, which suggests reactions involving carbene complexes. However, similar results have not been reported elsewhere most probably the products found by Banks and Bailey were formed by side reactions, typical for their particular catalyst system. 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

Scheme 36 Synthesis of donor-acceptor-substituted cyclopropanes 165 and cyclopentenes 166 from complexes 163 and acceptor-substituted alkenes 164 [115,116]...

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