Cyclopropanation complexes

The chiral bimetallic complex 1653 reacts with TMSOTf 20 in the presence of excess styrene, via 1654, to give the cyclopropane complex 1655 in high yield [38]. The chromium can be readily removed from 1655 by treatment with I2 in Et20. Analogously, the complex 1656 reacts with styrene in 90% yield, via 1657, to give MegSiOH 4 and phenylcyclopropane 1658 [39] (Scheme 10.17). 

FIGURE 15. Asymmetric unit in the X-ray crystal structure of [Li-0-C(Me)-(c-Pr)2]6 17a, cyclopropane 17b (Ds ) 17c-f are optimized geometries of Li+ and LiOH cyclopropane complexes (C2 , C2v, C31, C21, respectively) at RB3LYP/6-31H-G (C, H, O), /6-31G (Li) level. Bond distances are given in angstroms. Reprinted with permission from Reference 18. Copyright 1996 American Chemical Society... 

Aurated cyclopropane complexes are prepared by reaction of 1-lithiodicyclopropyl ketone38a and l-lithio-/V,/V-dialkylcyclopropylamides with triphenylphosphinegold complexes (equation 1 l)38b. The crystalline gold complexes provide a rare opportunity to look... 

The carbyne carbon atoms in both [W(CO)2 HB(pz)3)(CSMe)] (449) (266) and [W(CMe)Cp(CO)2] (450) (267) are nucleophilic and react with protons and the methylthio cation, SMe , to form cationic carbene complexes. Whereas the first type of carbene complex is typically electrophilic (257), the latter one, 451, is nucleophilic, and treatment with trifluoroace-tic acid produces a cationic metallathia cyclopropane complex (452) (268). 

Cyclization of metal alkyls with an acceptor group in the y-position affords cyclopropanes. Complexes of various metals can be used and there are many different methods of generating the acceptor group. 

Table I lists some rate constants at room temperature for the chromium(Il) reduction of pentaammine cobalt (III) complexes containing monocarboxylic ligands. The tetraammineglycinate is a complex with a minimum of steric hindrance. The rate of reaction is very similar to that of the acetato complex, indicating that inductomeric effects are of little importance in these redox processes, so that the observed decreases in the reaction rates may be related to the steric hindrance of the ligand. As the bulk of the carbon ring increases the rate decreases, although this decrease is accompanied by an increase in activation entropy (4) (from -38 e,u, for the cyclopropane complex to -23...

Photolysis of cyclopropane complexes [Pt(CH2CH2CH.2)X2(LL)] (X=C1 or Br, LL = bipy or phen) in solution at 25 °C leads to reductive elimination of cyclopropane (85—90%) and propene in first-order reactions. The absence of any significant effect on addition of free phenanthroline or chloride ion suggests a simple mechanism in which absorption of a quantum of light leads to ejection of the organic species in a single-step process. 

Also, the liganded cyclopropane complexes 22 react with SO2 to give the [3-1-2] cycloadducts 23 . 

Cyclopropane complexes of platinum. Treatment of hexachloroplatinic acid, H2HCI6, with cyclopropane affords a brown solid of composition C3H6ChPt [152a, 153]. With pyridine, a white complex is formed whichhas structure 7.25 [153] s reacts in CHCI3 forming the complex 7.26 [154]. 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

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. 

Ethylene occurs naturally in small amounts as a plant hormone Hormones are substances that act as messengers to regulate biological processes Ethylene IS involved in the ripening of many fruits in which it IS formed in a complex series of steps from a com pound containing a cyclopropane ring... 

Reagent-controlled asymmetric cyclopropanation is relatively more difficult using sulfur ylides, although it has been done. It is more often accomplished using chiral aminosulfoxonium ylides. Finally, more complex sulfur ylides (e.g. 64) may result in more elaborate cyclopropane synthesis, as exemplified by the transformation 65 66 ... 

Heterocycles as ligands in complex catalysts for asymmetric cyclopropanation 98T7919. 

Carbenoid complexes with heterocyclic ligands as catalysts in enantioselective cyclopropanation of olefins 97S137. 

Catalytic, enantioselective cyclopropanation enjoys the unique distinction of being the first example of asymmetric catalysis with a transition metal complex. The landmark 1966 report by Nozaki et al. [1] of decomposition of ethyl diazoacetate 3 with a chiral copper (II) salicylamine complex 1 (Scheme 3.1) in the presence of styrene gave birth to a field of endeavor which still today represents one of the major enterprises in chemistry. In view of the enormous growth in the field of asymmetric catalysis over the past four decades, it is somewhat ironic that significant advances in cyclopropanation have only emerged in the past ten years. 

From a historical perspective it is interesting to note that the Nozaki experiment was, in fact, a mechanistic probe to establish the intermediacy of a copper carbe-noid complex rather than an attempt to make enantiopure compounds for synthetic purposes. To achieve synthetically useful selectivities would require an extensive exploration of metals, ligands and reaction conditions along with a deeper understanding of the reaction mechanism. Modern methods for asymmetric cyclopropanation now encompass the use of countless metal complexes [2], but for the most part, the importance of diazoacetates as the carbenoid precursors still dominates the design of new catalytic systems. Highly effective catalysts developed in... 

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. 

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