Cyclopropenes reactions

Cyclopropenation reactions are also effectively catalyzed by dirhodium(II) compounds, and high enantiocontrol has been achieved with the Rh2(MEPY)4 catalysts (Scheme 15.3) [47]. A striking example of the catalyst effect on selectivity is found in the behavior of substrate 25 toward Rh2(5S-MEPY)4 and the more reactive Rh2(4S-IBAZ)4 (Eq. 10) [48]. With the less reactive Rh2(5S-MEPY)4 it preferentially undergoes allyhc cyclopropanation with high chemoselectivity and enantiocontrol. With the more reactive Rh2(4S-IBAZ)4 addition to the carbon-carbon triple bond is favored even though this involves construction of a ten-membered ring. 

Cyclopropanation reactions are one set in an array of C-C bond-forming transformations attributable to metal carbenes (Scheme 5.1) and are often mistakenly referred to by the nonspecific term carbenoid. Both cyclopropanation and cyclopropenation reactions, as well as the related aromatic cycloaddition process, occur by addition. Ylide formation is an association transformation, and insertion requires no further definition. All of these reactions occur with diazo compounds, preferably those with at least one attached carbonyl group. Several general reviews of diazo compounds and their reactions have been published recently and serve as valuable references to this rapidly expanding field [7-10]. The book by Doyle, McKervey, and Ye [7] provides an intensive and thorough overview of the field through 19% and part of 1997. 

Chiral rhodium(II) carboxamides are exceptional catalysts for highly enantio-selective intermolecular cyclopropenation reactions (50). With ethyl diazoacetate and a series of alkynes, use of dirhodium(II) tetrakis[methyl 2-pyrrolidone-5-(R)-carboxylate], Rh2(5R-MEPY)4, in catalytic amounts ( 1.0 mol %) results in the formation of ethyl eyelopropene-3-earboxylates (eq 4) with enantiomeric excesses... 

Enantioeontrol in cyclopropenation reactions is obviously highly dependent on the carboxylate substituent of the dirhodium(II) carboxamide ligand and on the carboxylate substituent of the intermediate carbene. High enantioseleetivity is achieved with the use of Rh2(MEPY)4 catalysts and menthyl diazoacetates in reactions with 1-alkynes, and further enhancement in % ee can be anticipated. 

Enantioselective Intermolecular Cyclopropenation Reactions. The use of Rh2(MEPY)4 catalysts for intermolecular cyclopropenation of 1-alkynes results in moderate to high selectivity. With propargyl methyl ether (or acetate), for example, reactions with (—)-menthyl [(+)-(l/ ,25,5/ )-2-isopropyT5-methyl-1-cyclohexyl] diazoacetate catalyzed by Rh2(55 -MEPY)4 produces the corresponding cyclopropene product (eq 3) with 98% diastere-omeric excess (de). ... 

Dirhodium(II) catalysts that possess chiral 2-pyrrolidone-5-carboxylate ester ligands (mepy) are the most effective among those of dirhodium or copper for highly diastereoselective and enantioselective intermolecular cyclopropenation reactions between l-alkynes and diazoesters (eq. (9)). Product yields are moderate, and enantiomeric excesses range from 40 to 98 %. Interestingly, the (R) or (5) catalyst produces the cyclopropene-l-carboxylate respectively with the (/ ) or (5) configuration [26]. 

Reaction with cyclopropenes Reaction of the cyclopropene 1 with Rh2(OAc)4 in (80 ) results in two furans, 2 and 3, in 78% and 3% yield, respectively. Thus the less substituted bond is cleaved selectively. In contrast, treatment of 1 with a Rh(l) catalyst in CH2CI2 at 25° gives only furan 3 in 86% yield. 

An unprecedented cyclopropenation reaction of alkynes catalyzed by ZnCl2 was reported. While Simmons-Smith-type carbenoids failed in the [2 + 1]-cycloaddition with alkynes, the use of enynones as the carbene source enabled the preparation of substituted 2-furyl cyclopropene derivatives with remarkable scope (14OL5780). 

The first example of a transition metal-catalyzed cycloisomerization of cyclopropenes [184,185] into furans was demonstrated by Nefedov (Scheme 8.61) [186]. It was proposed that this rearrangement proceeds via a carbenoid intermediate [187]. Formation of the furan products via the cycloisomerization of potentially involved cyclopropene intermediates in the Rh(II)-catalyzed cyclopropenation reaction of alkynes was later reported by several research groups, including Liebeskind, Davies, and Muller [188-192]. 

Extensive studies involving the use of dirhodium catalysts for cyclopropanation and cyclopropenation reactions have been reported by Doyle [80], Davies [81], and Fox etal. [82] with chiral dirhodium(II) tetracarboxylates and their derivatives being the most common [83]. A large number of chiral dirhodium(II) carboxamidate complexes have been developed, primarily by Doyle et al, [60a] to perform asymmetric cyclopropanation [84] and cydopropenation [85] reactions. 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

MgATP. The numbers in parenthesis represent the number of electrons required for the reaction shown. is cyclopropene A cyclopropane. 

NMR, 3, 542 oxidation, 3, 546 phosphorescence, 3, 543 photoelectron spectra, 3, 542 photolysis, 3, 549 reactions, 3, 543-555 with alkenes, 3, 50 with alkynes, 3, 50 with IH-azepines, 3, 552 with azirines, 3, 554 with cyclobutadiene, 3, 551 with cyclopropenes, 3, 550 with dimethylbicyclopropenyl, 3, 551 with heterocyclic transition metal complexes, 7, 28 29... 

Various carbene-transfer reactions can be used with both electron-rich and electron-poor alkynes to make fluorinated cyclopropenes [9. 13, 79, 80, 81, 82] (Table 4). Haloacetylenes are too thermally unstable for most cycloaddition conditions, and simple fluorinated cyclopropenes are made by other methods [32, 45, 83, 84] (equations 30-32). 

Similarly, partially fluorinated and perfluorinated methylenecyclopropanes [57, 52], cyclopropenes [55, 84, 55], cyclobutenes [75, 56], and bicychc alkenes [57, 55, 59, 90] apparently denve dienophilic reactivity from relief of their ground-state strain during reaction Thus 2,2-difluoromethylenecyclopropane and perfluoromethylenecyclopropane undergo exclusive [244] cycloadditions [57, 52] (equations 72 and 73), whereas (difluoromethylene)cyclopropane undergoes only [24-2] cycloadditions [57]... 

The similarity between the reactions of alkenes and cyclopropanes is further demonstrated by the reactions of electrophilic cyclopropanes and cyclopropenes with enamines. Cyclopropylcyanoester74, when treated with the pyrrolidine enamine of cyclohexanone, undergoes what would be a 1,2 cycloaddition in the analogous alkene case, but is actually a 1,3 cycloaddition here, to form adduct 75 (90). A similar reaction between the... 

These results were justified by assuming that the first photoproduct of the reaction was cyclopropene-3-carbaldehyde. This compound can be obtained via... 

Liquid-phase photolysis of furan atroom temperature occurred in very low yields (1 % conversion), giving a mixture of Diels-Alder adducts deriving from the reaction of cyclopropene-3-carbaldehyde and formylallene with furan (85JOC3034). 

Strained bicyclic compounds can be obtained e.g. when cyclopropenes are used as dipolarophiles. Reaction of 3,3-dimethylcyclopropene 7 with diazomethane 4 gives the heterobicyclic cycloaddition product 8 in 85% yield ... 

Figure 23.4 Intramolecular aldol reaction of 2,5-hexanedione yields 3-methyl-2-cyclopentenone rather than the alternative cyclopropene.

A recent variation of these reactions uses 6/f-l, 3-oxazin-6-ones as the electron-deficient heterodiene in place of the triazine.113114 With cyclopropene at — 35 C oxazinone 45 furnishes the 4//-azepine 46 in excellent yield. Likewise, with 3-methylcyclopropene the 4-methyl derivative 46 (R = Me) is formed. Cycloaddition with 1-methylcyclopropene, however, generates a mixture of 7-tert-butyl, 2-methyl 3-methyl- and 5-methyl-4//-azepine-2,7-dicarboxylate in a 2 1 ratio and a 97 % overall yield. 

The Diels-Alder reaction of cyclopropenes with 1,2,4,5-tetrazines (see Vol.E9c, p 904), a reaction with inverse electron demand, gives isolable 3,4-diazanorcaradienes 1, which are converted into 4H-1,2-diazepines 2 on heating. The transformation involves a symmetry allowed [1,5] sigmatropic shift of one of the bonds of the three-membered ring, a so-called walk rearrangement , followed by valence isomerization.106,107... 

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