Carbenes, furyl, reactions

W=X is N=N or C=N, never if it is C=C. The ring stability of 3-furyl-carbenes conforms with this rule. Ring opening is again the main reaction in a biradical which is generated by extruding carbon dioxide from a lactone at 675°C and which then collapses to an acetylenic ketone.277... 

This greater resistance of the thienyl system to ring opening is reflected in the fact that this carbene is captured more efficiently (27%) by cyclooctane than is the furyl-carbene (7%). Only dimeric products were obtained from the carbene (375) attached to position 3 of thiophene no ring fragmentation was observed (Scheme 119). The reactions of some other carbenes derived from diazomethylthiophenes have been referred to in Section 3.14.2.2. 

Vinyl-substituted benzo[c]furans can be prepared by reaction of n-alkynylbenzaldehydes with chromium Fischer carbene complexes. Initially a benzo[c]furan chromiumtricarbonyl complex is believed to be formed which is converted into an alkylidenephthalan derivative or can be trapped with electron-deficient dienophiles with excellent ( 3 ti-selectivity (Equation 128) <20000L1267>. More elaborate vinylidene Fischer carbene complexes yield dienyl benzo[c]furans that undergo [8-1-2] cycloaddition with DMAD to furnish furanophane derivatives <2003JA12720>. An equilibrium between 7] -(o-ethynylbenzoyl)rhenium complexes and rhenium benzo[f]furyl carbene complexes has been observed. These species behave like other benzo[< ]furans in the reaction with DMAD <20040M4121>. 

Chiral Fischer-type (menthyloxy)(3-furyl)carbene complexes underwent nucleophilic 1,4-addition with lithium reagents in a regioselective and diastereoselective manner. In particular, a reaction with (Z)-2-phenylethenyllithium provided the (Z)-alkene product shown below in excellent %ee <03CEJ5725>. 

Generation of chloro(2-furyl)carbene and chloro(2-thienyl)carbene from 2-dichloromethyl-furans 1 or 2-diehloromethylthiophenes 2 and a base, followed by addition to an alkene, gave l-chloro-l-(2-furyl)- or l-(2-thienyl)cyclopropanes, respectively "(for earlier work, see Houben-Weyl, Vol. E19b, p 1001). The reactions were carried out using potassium /crt-butoxide as the base with, or without, 18-crown-6, or solid potassium hydroxide and a catalytic amount of benzyltriethylammonium chloride (TEBAC) (solid-liquid variant of a phase-transfer catalytic system ). Under both conditions, yields of cyclopropanes were usually good, yet the phase-transfer catalytic system seems to be more convenient from a practical point of view. Addition of the above described carbenes to alkenes was stereospecific. 

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). 

Here HY is a proton donor and Z is a proton acceptor— in most cases these roles are performed by the amine and/or the solvent. All these reactions have a negative activation energy, and all the experimental observations can be accommodated by the mechanism shown in the Scheme. The same carbene complex, Cr(CO)s C(OMe)Ph, reacts with aminoethanol in two ways, to produce (14) and (15). Both are formed by attack at the carbene-C (14) arises from attack by HN=CHMe (aminoethanol minus the elements of water), and (15) from attack by NHg (aminoethanol minus the elements of acetaldehyde). The compound Cr(CO)6 acetoxy-(2-furyl)carbene reacts with hydrazoic acid to give Cr(CO)5(2-furonitrile). 

In 1999, Trost and Rhee [34] reported the Ru-catalyzed oxidative cyclization of homoallyl alcohol 44 giving rise to y-lactones (Scheme 20). A combination of CpRu(cod)Cl and (2-furyl)3P was found as an effective catalyst and N-hydroxysuccinimide served as a mild oxidant for this transformation. This reaction involves generation of Ru-vinylidene carbene intermediate. 

Two research groups independently extended the electrocyclization system into the stoichiometric formation of pyranylidene complexes (25 and 27), which also proceeds via vinylidene complexes as key intermediates (Scheme 21.10) [16,17], Further reactions of the pyranylidene complexes 25 with a variety of alkenes gave the corresponding naphthalenes 28 in good to high yields. In addition, Miki and others found the catalytic formation of cyclopropanes bearing a furan ring 30 from reactions of ene-yne ketones 29 with a variety of alkenes, where furyl carbene complexes 31 were proposed as key intermediates (Scheme 21.11) [18],... 

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