Tethered vinylcarbenes

Scheme 7-16 Regioselective bis-functionalization by double [3 + 2] cycloaddition of tethered vinylcarbenes to Cgo [98, 99].

The methoxyketene 297, coordinated to Cr carbonyl, is formed from methoxy-carbene easily by insertion of CO under irradiation [90]. An ester is formed by the reaction of ketene with alcohol. The aminocarbene complex 298 was prepared from benzamide and converted to phenylalanine ester 300 under irradiation of sunlight in alcohol via ketene 299 [91]. The eight-membered lactone 304 was prepared in high yield by the reaction of the alkyne 301 having the OH group in a tether with Cr carbene without irradiation. The vinylcarbene 302 is formed at first and converted to the vinylketene intermediate 303 as expected. The keto lactone 304 is formed from 303 by intramolecular reaction with the OH group and hydrolysis [92],... 

Other tethers have been employed in the search for regioselectivity in multiple additions to fullerenes. Some of these methodologies include the use of bis(o-quinodimethanes) connected by a, oo-dioxamethylene tethers [40], tethered nucleophilic vinylcarbenes [41] or azides [42],... 

Coupling of a Fischer carbene complex with an alkene can generate a vinylcarbene intermediate 12 via an insertion-rearrangement reaction, which can then further react with a double bond. For intramolecular reactions of tethered enynes 10, the products formed are bicyclic cyclopropanes 14 intermolecular reactions lead to cycloalkenylcyclopropanes. 

However, yields in the intermolecular cycloaddition reactions of vinylcarbene complexes, formed by intramolecular insertion of an alkynyl tethered metal carbene complex, are higher when molybdenum rather than chromium or tungsten carbene complexes are employed. Mild thermolysis (THF, 65 °C, 1 h) in the presence of ten equivalents of an electronically undemanding alkene directly leads to the 2-alkyl-2-(2-methoxycyclopentenyl)cyclopropanes 31. ... 

Nakamura and co-workers synthesized the oligomethylene-tethered bis(cyclopropenone acetals) 88a-c to generate by thermolysis minute amounts of nucleophilic vinylcarbenes which subsequently underwent two-fold [3+2] cycloadditions to 6-6 bonds of Cgo (Scheme 7-16) [98,99]. Thus, reagent 88a, with a -(CH2)3- tether, afforded the Q-sym-... 

Figure 8.4 (a) Bottom-bound, tethered alkylidene/olefin complex and (b) side-bound, ij -vinylcarbene complex. 

The Dotz benzannulation was employed as a key step in the formation of a metacyclophane natural product, kendomycin (Scheme 21.33) [37]. The intramolecular benzannulation of the vinylcarbene and alkyne, linked with the highly functionalized tether, afforded the corresponding metacyclophane in good yield. 

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