Cyclopropanations stepwise

Cyclopropyl chlonde has been prepared by the free radical chlorination of cyclopropane Wnte a stepwise mechanism for this reaction... 

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

While direct photolysis of (46) gave (48), the sensitized photolysis of (46) with acetophenone gave only the cyclopropane product (>93%). These authors favor a stepwise oxa-di-w-methane reaction mechanism (8.69) inasmuch as a concerted reaction should produce the product without a change in multiplicity. That is, the product would have to be formed in the triplet state and not enough energy is available for the product to be formed in an excited state ... 

Syntheses of Carbocyclic Compounds (1.V.2.V )-2 (.V )-Amino(4-metho-xyphenyl)methyl]cyclopropan-l-ol 392 (Scheme 1.45) has been prepared by a stepwise procedure involving a 1,3-dipolar nitrile oxide cycloaddition to allyl alcohol followed by a constmction of the cyclopropa d isoxazole system, and reduction of the bicycle (436). 

Electrochemical reduction of a variety of polyhalogenated alkanes has been undertaken. Stepwise dechlorination of gem-dichloro- and gem-trichloroalkanes occurs. Dichloromethane can be reduced in the presence of an olefin to form a cyclopropane [25], and... 

The literature indicates that cyclopropane formation occurs in a stepwise manner [98-100], with the initial generation of a carbanion and subsequent Sn2 displacement. This was ascertained from experiments involving the reduction of meso- and d,/-2,4-dibromopentane. Each afforded roughly equal amounts of... 

Miscellaneous Iminium Catalyzed Transformations The enantioselective construction of three-membered hetero- or carbocyclic ring systems is an important objective for practitioners of chemical synthesis in academic and industrial settings. To date, important advances have been made in the iminium activation realm, which enable asymmetric entry to a-formyl cyclopropanes and epoxides. In terms of cyclopropane synthesis, a new class of iminium catalyst has been introduced, providing the enantioselective stepwise [2 + 1] union of sulfonium ylides and ot,p-unsaturated aldehydes.As shown in Scheme 11.6a, the zwitterionic hydro-indoline-derived catalyst (19) enables both iminium geometry control and directed electrostatic activation of sulfonium ylides in proximity to the incipient iminium reaction partner. This combination of geometric and stereoelectronic effects has been proposed as being essential for enantio- and diastereocontrol in forming two of the three cyclopropyl bonds. 

Other mechanistic possibilities also exist for the [5-1-2] cycloaddition reaction and cannot be mled out at this point In addition to the stepwise pathways depicted in Scheme 13.8, it is also possible that coordination of the 27r-component to 42 could lead directly to 48 through simultaneous insertion and cleavage. Variations on these events, such as direct insertion into the cyclopropane to form a metaUacyclobutane, are also possible. 

A large number of cyclopropanated derivatives of Cgo in which the bridging atom is an electron rich transition metal (see Fig. 16) such as Pt, Pd, Ni, Ir, W, Mo, and Rh has been reported. Their electrochemical properties have been reviewed [83, 141, 142] and, in general, reductions are Cgo centered and negatively shifted with respect to those of pure Cgo, while oxidations are metal centered. In most cases, however, the first reduction is accompanied by breakage of the carbon-metal bonds and recovery of the pristine [60] fullerene. In multiadduct derivatives, the breakage occurs in a stepwise manner. 

Because of the stepwise formation of the two new carbon-carbon bonds of the cyclopropane product, and because of the intermediacy of the carbanionic adduct (8), the double-bond configuration of the alkene substrate may be lost, depending upon the lifetime of the intermediate relative to carbon-carbon single-bond rotations. Consequently, the stereospecificity of these cyclopropanations varies from one case to another.132... 

Despite the stepwise pathway for these reactions, some cyclopropanations using ylides may be stereo-specific.132 Successful efforts have also been reported for the development of enantioselective cyclopropanations through use of, for example, optically active ylides related to (ll)136 and of alkene substrates bearing chiral auxiliaries. 

The reaction of electron-rich alkenes with diazo compounds containing two electron-withdrawing groups does not always result in cyclopropane formation. Instead, a [3+2] cycloaddition can occur, presumably by means of a stepwise process [1,2]. One report has appeared which demonstrates that the N-amido-prolinate 3 can catalyze a [3+2] cycloaddition between diazocyclohexane-1,3-dione (39) and 2,3-dihydrofuran (40) to form the tricyclic system 41 with very high enantioselectivity [Eq. ( 21)] [8]. 

Stepwise radical mechanisms have been proposed for the apparent [2 + 2] cycloaddition of disilenes with ketones by Baines et a/.140,141 They have found that the reactions of tetramesityldisilene 1 with trara-2-phenylcyclopropane carbaldehyde (208a) and fra/M,fra/M-2-methoxy-3-phcnylcyclopropane carbaldehyde (208b), a mechanistic probe developed by Newcomb et al.,142 undergo characteristic cyclopropane ring-opening as shown in Eq. (99). 

Eq. 52 and 53 demonstrate remarkable characteristics of this [3 + 2]-cycloaddition starting with a pure diastereomer 130, two stereoisomeric cyclopentanes 131 are obtained. This stereorandom outcome is most simply rationalized assuming a stepwise mechanism with a 1,5-zwitterion as an intermediate in the cycloaddition. The vinylcyclopropane 132 only gives five-membered ring products 133 and no cyclo-heptene derivative, which would result from a conceivable [5 + 2]-cycloaddition. Less activated olefins or cyclopropanes do not undergo a similar [3 + 2]-cycloaddition. Due to the specific substitution pattern, the cyclopentane formation from these siloxycyclopropanes is of no preparative value. 

An attempted synthesis of dithioesters 220 by stepwise treatment of enolates from cyclopropanes 97 with carbon disulfide and methyl iodide lead to an unexpected new product without a cyclopropane ring. By spectroscopic means and study of the reactions dihydrothiophene structure 222 has been established 98). 

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