Allenyl carbenoid

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... 

Scheme 3.20. Insertion of allenyl carbenoids into organozirconocene chlorides.

Scheme 3.22. Tandem insertion of allenyl carbenoids and electrophiles into zirconacycles.

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. 

The insertion of allenyl carbenoids into Zr-C bonds is an interesting process for the generation of cumulenyl zirconium... 

Compared with five-membered metallacycles, relatively fewer reports are known on the preparative methods and reaction chemistry of six-membered metallacycles. Whitby and coworkers have systematically investigated insertion of carbenoids into five-membered zirconacycles and developed a number of interesting six-membered zirconacycles [17, 26]. Isonitriles, 1-halo-l-lithioalkenes, allenyl carbenoids, allyl carbenoids, propargy carbenoids, benzyl carbenoids, and 1-nitrile-1-lithio epoxides can all insert into zirconacyclopentanes and zirconacyclopentenes to afford various six-membered zirconacycles (Eqs. 23,24). 

Allenyl carbenoids (3-chloro-l-lithioalk-l-ynes) insert into zirconacy-clopentanes and zirconacyclopentenes to afford cyclic rj -allenyl/prop-2-ynyl zirconocene complexes which give cyclized-alcohol products on addition of aldehydes activated with boron trifluoride-diethyl ether (Eq. 52) [56]. 

In conclusion, five-membered metallacycles of titanocene and zirconocene are convenient starting materials for the construction of five-membered car-bocyclic compounds. The formation of five-membered carbocycles can be accomplished by addition reactions (or insertion reactions) of a variety of electrophiles, such as CO, RCN, RNC, bis(trichloromethyl)carbonate, allenyl carbenoids, halogencarbenoids, aldehyde, acyl chlorides, propynoates, and iodopropenoatesthe to the five-membered metallacycles. 

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... 

A more complex cumulenyl carbenoid 80 may be generated in situ from 1,4-dihalobut-2-ynes and two equivalents of base (Scheme 3.21). Insertion into organozirconocene chlorides gives allenyl zirconium species 81, which are regioselectively protonated to afford enyne products 82 [38], The stereochemistry of the alkene in 82 stems from the initial elimination of hydrogen chloride to form 80. 

Propargylic esters 67 have been studied extensively in ttansition-metal catalysis [35]. Recently it has been found that these compounds were effectively converted to useful molecular skeletons by interaction with -ir-acidic metal catalysts. These reactions proceed via initial 3,3-rearrangement, leading to allenyl ester intermediate 68 or 1,2-acyloxy rearrangement to metal carbenoid intermediate 69 (Scheme 27.25). These rearrangement reactions have been extended to the corresponding phosphates and sulfonates. 

---