Catalytic and Photochemical Cyclopropanation of Alkenes with Methyl Diazo(trialkylsilyl)acetates:  Steric Effects and Thermodynamic Stabilities of Cyclopropanes

Abstract

Diazo esters 1 (N 2 dC(SiR 3 )COOMe; R ) Me (1a), Et (1b), i-Pr (1c)) have been decomposed in styrene, 1-hexene, and cyclohexene under Cu, Rh, or Ru catalysis as well as by photochemical means with the objective to evaluate the effectiveness and diastereoselectivity of the respective cyclopropanation reaction. With styrene and 1-hexene as substrates, the ability of the catalysts followed the order CuOTf > [Ru 2 (CO) 4 (µ-OAc) 2 ] n ≈ Rh 2 (OAc) 4 , but even CuOTf (copper(I) triflate) did not promote cyclopropanation of 1-hexene with the bulky 1c. Cyclopropanation of cyclohexene with 1a,b succeeded only with the ruthenium catalyst. In all cases, the diastereoisomer having the silyl group anti to the vicinal methyl or phenyl substituent(s) was formed preferentially. In contrast, in the photochemical reactions of 1a,c with styrene and of 1a with 1-hexene the diastereoisomer having the silyl group syn to the vicinal substituent(s) was formed preferentially. The fluoride-induced desilylation of cyclopropanes 2a,c,e was accompanied by a loss of stereochemical integrity. The X-ray crystal structure analysis of the cyclopropane (E)-2f has been determined. The relative thermodynamic stabilities of various 2-R-1-X-cyclopropanecarboxylates (R ) Me, Ph; X ) Me, t-Bu, SiH 3 , SiMe 3 , Si(i-Pr) 3 ) have been calculated by density functional theory methods. These calculations show that for X ) SiMe 3 and R ) Ph the syn-anti energy difference is ca. 0, while for R ) Me the anti isomer is more stable.

Dinuclear ruthenium(I) complexes of the type [Ru2(CO)4L2] with carboxylate or 2-pyridonate ligands: Evaluation as catalysts for olefin cyclopropanation with diazoacetates

“…The color of the ''white polymer'', its solubility in diethyl ether, and an IR absorption at 2153 cm À1 , which is typical for axial carbonyl ligands in complexes of type 2 [15], are not in accord with the properties of 1d prepared by us, and we assume that these authors had prepared the hexacarbonyl complex 2d (L = CO). In fact, we found that heating of Ru 3 (CO) 12 with trifluoroacetic acid at reflux temperature yields 2d (L = CO) which even after prolonged treatment at 180°C/0.001 mbar does not completely release the two axial CO ligands.…”

“…We have prepared butyrato complex 1b analogously by heating of Ru 3 (CO) 12 in an excess of butyric acid (Scheme 2, Eq. (1)).…”

“…(2)). It has been reported [18] that 1d is obtained as a ''white polymer'' from the reaction of Ru 3 (CO) 12 with excess trifluoroacetic acid in toluene at 90°C. This product was characterized by IR spectroscopy only and was directly converted into the bis(triphenylphosphane) adduct 2d (L = PPh 3 ).…”

“…We have identified the dinuclear ruthenium(I,I) acetate complexes [Ru 2 (CO) 4 (l-OAc) 2 ] n , which is a coordination polymer, and the related bis(acetonitrile) complex [Ru 2 (CO) 4 (CH 3 CN) 2 (l-OAc) 2 ] as well suited catalysts for cyclopropanation of olefins with alkyl diazoacetates [10,11], a-silyl-a-diazoacetates [10,12] and aryl-or silyl-diazomethane derivatives [13]. The structurally similar triazenide complexes [Ru 2 (CO) 6 (l-ArNNNAr) 2 ] were also found to catalyze efficiently and effectively the cyclopropanation of nucleophilic terminal alkenes, but were less suited for the cyclopropanation of internal alkenes [14].…”

Sawhorse-type diruthenium tetracarbonyl complexes

“…The diastereoselective syn-cyclopropanation of olefins with diazo acetate [112,113], according to Eq. (37), or with silylated diazo esters [114] is catalyzed by the [Ru 2 (CO) 4 (OOCMe) 2 ] n polymer or the complex Fig. 30.…”