Vinylcarbene, nucleophilic

The chemical behavior of heteroatom-substituted vinylcarbene complexes is similar to that of a,(3-unsaturated carbonyl compounds (Figure 2.17) [206]. It is possible to perform Michael additions [217,230], 1,4-addition of cuprates [151], additions of nucleophilic radicals [231], 1,3-dipolar cycloadditions [232,233], inter-[234-241] or intramolecular [220,242] Diels-Alder reactions, as well as Simmons-Smith- [243], sulfur ylide- [244] or diazomethane-mediated [151] cyclopropanati-ons of the vinylcarbene C-C double bond. The treatment of arylcarbene complexes with organolithium reagents ean lead via conjugate addition to substituted 1,4-cyclohexadien-6-ylidene complexes [245]. 

Because of the strong electron-withdrawing effect of the MICO), fragment, reactions of nucleophiles, dienes or 1,3-dipoles with vinylcarbene or alkynylcarbene complexes are usually faster than with the corresponding a,P-unsaturated esters [242,253,264]. 

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

In principle, the vinylcarbene-to-cyclopropene isomerization is reversible. While this has not been reported for 167 (where ring-opening could produce not only alkenyl(sulfonyl)carbene 166, but also the isomeric l-alkenyl(silyl)carbene), it was found that 1-trimethylsilylcyclopropenone acetal 168 (equation 49) by thermal ring-opening yields the (trimethylsilyl)vinylcarbene 169 besides traces of the isomeric vinylcarbene 170. Both carbenes are obviously nucleophilic since they are able to cyclopropanate the... 

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],... 

Recently, cyclopropane derivatives were produced by a ruthenium-catalyzed cyclopropanation of alkenes using propargylic carboxylates as precursors of vinylcarbenoids [51] (Eq. 38). The key intermediate of this reaction is a vinylcarbene complex generated by nucleophilic attack of the carboxylate to an internal carbon of alkyne activated by the ruthenium complex. Then, a [2+1] cycloaddition between alkenes and carbenoid species affords vinylcyclo-propanes. 

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-... 

Because a number of important natural products contain densely functionalized seven-membered rings, general methods for constructing these ring systems have been actively pursued. In recent years, annulation processes, such as [4+3] annulations between dienes and allyl cations or a nucleophilic vinylcarbene, or [5+2] annulations between heterocyclic betaines and alkenes" have been shown to be particularly versatile. ... 

The alkynyl complex [CpMn(CaCMe)(CO)(PPh3)] reacts with several aldehydes and ketones in the presence of BF3, followed by a MeOH quench, to form cationic vinyl carbynes [CpMn(CO)(PPh3) aC-C(Me)=CRiR2 ]+. The phenyl derivatives react with various nucleophiles (LiAlRj, MgRX, LiCaCBu ) to form either the vinylidenes 159 or the vinylcarbene complexes 160.512... 

An extension of this strategy to sugar nucleophiles such as l,2 3,4-isopropyli-dene-a-D-galactopyranose 226 as trapping reagents affords moderate yields of or-ganometallic 0,C-disaccharides (227, 228) in which two sugar moieties are connected by a metal vinylcarbene spacer (Scheme 11.52). 

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