Chromium cyclopropanation with

Experimental Procedure 2.2.2. Cyclopropanation with a Chromium Carbene Complex Diethyl n-a/i5-3-Methoxy-3-phenylcyclopropane-l,2-dicart>oxylate [325]. 

Table 3.1. Cyclopropanation with stoichiometric amounts of chromium, molybdenum and tungsten carbene complexes.

The reaction of pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with vinyl ethers gives 1-methoxy-l-phenylethene as the main product. If these substrates are allowed to react in the presence of carbon monoxide under pressure, the corresponding cyclopropanes are formed in good yield. 

The reaction of pentacarbonyl[1,3-pentadienyl(methoxycarbene)]chromium(0) with (tert-but-yldimethylsilyloxy)ethene under carbon monoxide pressure affords the expected cyclopropane derivative as the riv-isomer exclusively18. 

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

For cyclopropanation of enol ethers with in situ-generated acyloxycarbene complexes of chromium and in the absence of CO, see reference [10a]... 

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). 

Another unprecedented domino cycloaddition process of a chromium complex, namely a [2+2+l]/[2+l] cycloaddition, was observed by Barluenga and coworkers [316]. These authors treated norbornene 6/4-137 with the Fischer alkynyl Cr car-bene 6/4-138 and obtained, as the main product, not the expected cyclopropane derivative 6/4-139, but compound 6/4-140 (Scheme 6/4.35). 

The substituent effects on the alkene were investigated in the reaction of enyne 12 and chromium carbene complex 2c [8]. In the reaction of enyne -12a having a phenyl group on the alkene with Fischer chromium carbene complex 2c, metathesis product 13a was obtained as a main product along with cyclopropane 14 and cyclobutanone 15 (Eq.4). The reaction of Z-12a with 2c gave only... 

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

Perfluoroalkyl)methyl-substituted electrophilic cyclopropanes are synthesized in excellent yields by a chromium (III) chloride/iron powder-promoted reaction of perfluoroalkyl iodides with allylamalonic ester and its analogues (equation 51)67. Oxidative coupling of... 

Simple alkenes, norbomene and styrene do not undergo cyclopropanation or insertion reactions with cyclopropyl(methoxy)carbene chromium pentacarbonyl complex. However, the conjugated 1-vinylcyclopentene is cyclopropanated under the reaction conditions at the terminal double bond, affording an isomeric mixture (trans cis = 40 60), in 66% yield (equation 72). 

In a mechanism for the [67i+27t]/[67i+27i]/[2o +27t] process, the first two steps involved in this overall transformation were presumed to parallel the pathway observed for the above described [67t+27t], [67t+27t] process. A third cycloaddition of the cyclopropane unit with an additional alkyne component occurred via a [2[Pg.113]

In organic chemistry one surely thinks at once of the construction of cyclopropane derivatives from olefins and carbenes. Indeed, it has been shown that this also is possible with our complexes and with C=C double bonds that are electron-poor and arc either polarized or easily polarizable (77-81). As an example of this, I would like to cite the reaction of penta-carbonyl[methoxy (phenyl) carbene]chromium (0), -molybdenum (0), or -tungsten(0) with ethyl vinyl ether (79). One obtains the corresponding cyclopropane derivatives in this case, however, only when one removes... 

Substituted bicyclo[ . 1.0]alkanes may also be obtained by condensation of secondary amines with 2-haloketones. A variety of nucleophilic reactions can be carried out on the intermediate cyclopropaniminium salt 116251 (Scheme 108). Competing alkene scission and cyclopropanation occurs on reaction of enamines with pentacarbonyl-chromium carbene complexes252 (Scheme 109). N-Silylated allylamines and their derived N-silylated enamines undergo rhodium or copper catalysed cyclopropanation by methyl diazoacetate253 (Scheme 110). 

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