Carbene from alkynes

Hydroquinone synthesis (regiospecific) from alkynes and carbonyl carbene chromium complexes... 

Scheme 9 Ruthenium carbene complexes from alkynes and application [17]...

Table 2.7. Formation of heteroatom-substituted carbene complexes from alkynes, vinylidene complexes, and alkynyl complexes.

Non-heteroatom-substituted vinylcarbene complexes are readily available from alkynes and Fischer-type carbene complexes. These intermediates can undergo the inter- or intramolecular cyclopropanation reactions of non-activated alkenes. Cyclopropanation of 1,3-butadienes with these intermediates also leads to the formation of cycloheptadienes (Entry 4, Table 2.24). 

Fig. 4.3. Possible mechanisms for the formation of vinylcarbene complexes from alkynes and electrophilic carbene complexes.

Well-established is the formation of hydroquinone and phenol derivatives 273 from alkynes. This reaction is called the Dotz reaction [78,79]. The reaction of carbene complex 271 to give 273 can be expressed by the general scheme 272. 

Gycloreversion to the vinylalkylidene is rarely observed directly, despite being a commonly proposed step in carbene or alkylidene/alkyne reaction cascades. Bridged titanacyclobutene complex 57, however, clearly resists the formation of a strained internal alkylidene from alkyne extrusion, equilibrating instead with vinylalkylidene complex 58, stabilized by the addition of trimethylphosphine (Equation 20) <2003ICA27>. This system provides a very rare simultaneous observation of interconverting metallacyclobutene and vinylalkylidene isomers. 

Additions of other types of carbene to alkynes are not common, though Me2C = C , generated from 2-methylpropenyl triflate and base, does add to alkynes to produce transient methylenecyclopropenes which may be trapped as Diels-Alder adducts 62). 

Apart from the construction of phenanthrenes, carbene complexes have also been used for the synthesis of more extended polycyclic arenes. An unusual dimerization of chromium coordinated ortbo-ethynyl aryl carbenes results in the formation of chrysenes (Scheme 37) [81]. This unusual reaction course is presumably due to the rigid C2 bridge that links the carbene and alkyne moieties, and thus prevents a subsequent intramolecular alkyne insertion into the metal-carbene bond. Instead, a double intermolecular alkyne insertion favored by the weak chromium-alkyne bond is believed to occur forming a central ten-membered ring that may then rearrange to the fused arene system. For example, under typical benzannulation conditions, carbene complex 97 affords an equimolar mixture of chrysene 98a and its monochromium complex 98b. The peri-interactions between the former alkyne substituent (in the 5- and 11-positions) and the aryl hydrogen induce helicity in the chrysene skeleton. 

A variety of annulations leading to ring systems other than benzenes can be isolated from reactions of Fischer carbenes. Cyclopentenone derivatives are formed from a reaction of Z-6-amino-Q ,jS-unsaturated chromium carbenes with alkynes (Scheme 66). In contrast, -B-amino-Q, /3-unsaturated chromium carbenes reacts with alkynes to give cyclopentadienes (Scheme 67). 

The synthesis of furanones (y-lactones) from alkynes and carbon monoxide consititutes a unique variation of the [2 + 2 + 2] pattern. The ring incorporates both carbons from the C=carbon from one CO, and both carbon and oxygen from the other CO. The process is therefcMe a 2 + 2 -i-1 cycloaddition of the form [2TTfw + Zno-o 2ncol- The formal result of such a cycloaddition is a carbene at position 4, the carbon atom of the irco participant (equation 12). 

Dehmlov and coworkers" extended their investigations to a study of the addition of chlorofluorocarbene to highly substituted alkynes. They synthesized the carbene from dichlorofluoromethane and sodium hydroxide under phase-transfer conditions. Although yields are low, chlorofluorocarbene appeared to be more reactive than dichlorocarbene towards alkynes, especially towards alkynes with bulky substituents. 

Apart from the utilization of aryl- and vinyl-diazoacetates that can achieve the moderate to high chemo-, regio-, and enantioselectivity in intermolecular asymmetric C—H bond insertion reactions, Af-sulfonyl-l,2,3-triazole 11 was found to be able to function as an alternative carbene precursor for diverse transformations (Scheme 1.4). One advantage for using the N-sulfonyl-1,2,3-triazole is that it could be easily prepared by the Cu -catalyzed azide-alkyne cycloaddition (CuAAC) reaction, and in some cases, delicately designed reactions can be conducted in a one-pot procedure starting from alkynes and sulfonyl azides. Moreover, since there exists an inherent equilibrium... 

The reaction of heterocumulenes with alkynes sometimes leads to the formation of a carbene, which undergoes dimerization. For example, from alkynes and carbon disulfide under high pressure, tetrathiofulvenes 87 are obtained . 

Cr(CO)3-coordinated hydroquinone from vinylic alkoxy pentacarbonyl chromium carbene complex (Fischer carbene) and alkynes. 

There is relevant material on carbenes in reviews on catalytic methods for metal carbene transformations formation of metal carbon multiple bonds from alkynes, metathesis catalysis. ... 

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