Propargyl carbenoids

Closely related to both allyl carbenoids and the allenyl carbenoids discussed above, propargyl carbenoids 101 are readily generated in situ and insert into zirconacycles to afford species 102 (Scheme 3.27), which are closely related to species 84 derived from allenyl carbenoids [65], Protonation affords a mixture of allene and alkyne products, but the Lewis acid assisted addition of aldehydes is regioselective and affords the homopropargylic alcohol products 103 in high yield. Bicydic zirconacyclopentenes react similarly, but there is little diastereocontrol from the ring junction to the newly formed stereocenters. The r 3-propargyl complexes derived from saturated zirconacycles are inert towards aldehyde addition. 

Scheme 3.27. Insertion of propargyl carbenoids into zirconacycles.

Normant, J. F. Preparation of propargylic carbenoids and reactions with carbonyl compounds. A stereoselective synthesis of propargylic halohydrins and oxiranes. Eur. J. Org. Chem. 2001, 3295-3300. 

Methoxyfurans reacted with ruthenium and platinum carbenoids, derived from tertiary propargyl carboxylates, regioselectively, leading to interesting triene systems, as represented by the example below <06OL1741>. 

Vinylidenecarbene or allenylidene3 (R)2C=C=C has a lance-shaped, unsubstituted and sp-hybridized carbene center and, therefore, will not be easily subject to steric hindrance in its insertion reactions. On this assumption, (2-methyljpropenylidenecarbene or its carbenoid was chosen as a prototype of typical vinylidenecarbenes and its insertion reaction with several different types of alkoxides was investigated by employing two methods (A and B, Scheme 10) for carbene generation.20 The insertion products 20 were obtained almost exclusively except lithium allyloxide (Table 4, entry 10).21 By-products such as propargyl ether and allenyl ether were not formed at all. To be noted here, in... 

Negishi first observed the insertion of the y-halolithium species 75 obtained by deprotonation of propargyl chloride into octylzirconocene chloride protonation of the product afforded the allene 79 (Scheme 3.20) [37]. The overall effect is insertion of an allenyl carbenoid. The a-halolithium equivalent 76 is conveniently generated by addition of two equivalents of base to 2-chloroallyl chloride [52] and affords the same products. The organome-tallic product 77 of allenyl carbenoid insertion is either in equilibrium with the propargyl... 

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. 

Ruthenium and platinum carbenoids, derived from tertiary propargyl carboxylates, also reacted with furans in a similar manner, leading to triene systems (as represented in Scheme 9) <20060L1741>. The initially formed mixture of (2Z,4E) and (2Z,47) isomers 20 and 21, respectively, could be isomerized completely to a single (2E,4E) isomer. 

Allenic sulfides (37) have been obtained with moderate to good enantio-selectivities (up to 81%) in sigmatropic rearrangements of sulfonium ylides generated from propargyl aryl sulfide (35) and aryldiazoacetates (36) as carbenoid source catalyzed by chiral Rh(ll) (38a,b) and Cu(l) (38c) catalysts [34] (Scheme 8). 

The corresponding vinylic carbenoid 6 or metallated a,/S-ethylenic nitrile 7, formed as a unique isomer, reacts in a second step with a stronger electrophile to give the polysubstituted vinylic compounds (Scheme 7-6). This reaction is of synthetic value for obtaining, as required, the two geometrical isomers 8 and 9 starting from the same propargylic ether [8]. 

The tandem cyclization-cycloaddition reaction of 1-diazoacetyl-7,7-dimethylbicyclo[2.2.1]heptan-2-one (1) with dimethyl butynedioate catalyzed by rhodium(II) acetate dimer in benzene at 25 °C, afforded a formal [3 + 2] cycloadduct 2 in 85% yield with complete diastereofacial selectivity48. This reaction is interpreted to proceed via rhodium carbenoids and subsequent transannular cyclization of the electrophilic carbon onto the adjacent oxo group to generate a cyclic carbonyl ylide. followed by 1,3-dipolar cycloaddition. Similar reactions are observed with other dipolar-ophiles, such as propargylic esters and A -phenylmaleimide. Studies dealing with the geometric requirements of dipole formation were undertaken. 

Many metal-catalyzed reactions of propargylic esters proceed as if alkylidene-metal carbenoids are involved. In the Pd-catalyzed reaction such species add to norbomenes/ norbomadienes to give ring expansion products. 

Recently, Chen and coworkers reported an efficient new method to construct a series of densely functionalized 4,5-dihydro-IH-azepine products 41 from the intermolecular reaction of alkyl azides with propargylic esters. In this approach, sequential reaction of vinyl-gold carbenoids 42 with allqrl azides and formation of vinyl imine intermediates 43 may be involved fScheme 4.14). Then a subsequent formal [4+3] cycloaddition with another molecule of vinyl-gold carbenoid 42 afford the desired azepine 41. 

---