Allenes carbenoids

Other potential synthetic routes to these unsaturated aziridine derivatives which involve the addition of nitrenes to allenes <75JOC224), carbenes to imines with subsequent hydrolysis <67JA362), and of carbenoid species to ketenimines <76TL1317,79TL559) have been investigated but are collectively of little or no preparative value. 

The reaction between CHCI3 and OH is often carried out under phase-transfer conditions. It has been shown that the reaction between PhCHCl2 and t-BuOK produces a carbenoid, but when the reaction is run in the presence of a crown ether, the free PhCCl is formed instead.Dihalocyclopropanes are very useful compoundsthat can be reduced to cyclopropanes, treated with magnesium or sodium to give allenes (18-3), or converted to a number of other products. 

The earlier examples of [2 + 1] cycloaddition of a carbene (or carbenoid) on the double bond of alkylidenecyelopropanes to yield spiropentane derivatives were observed as undesired side reactions in the synthesis of alkylidenecyelopropanes through the addition of a carbene to a substituted allene [161]. In some cases the spiropentane derivative was obtained as the major product [161a, c] especially when a large excess of the carbene reagent was used. For example, when methyl 3,4-pentadienoate (610) was treated with a ten-fold excess of methylene iodide and zinc-copper couple the two products 611 and 612 were isolated in 1 4.5 ratio (Scheme 86) [161a]. 

The cyclopropanation of a-allenic alcohols 616 gave methylenecyclop-ropanes 617a, b and spiropentanes 618a, b in different proportions depending on the carbenoid reagent used (Scheme 88) [163,4b]. 

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... 

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 discovery of carbene and carbenoid additions to olefins was the major breakthrough that initiated the tapping of this structural resource for synthetic purposes. Even so, designed applications of cyclopropane chemistry in total syntheses remain limited. Most revolve around electrophilic type reactions such as acid induced ring opening or solvolysis of cyclopropyl carbinyl alcohol derivatives. One notable application apart from these electrophilic reactions is the excellent synthesis of allenes from dibromocyclopropanes 2). 

Here an alkynyl sulfoxide 55 is first carbocuprated with an organocopper reagent 56 to provide a vinylcopper intermediate 57, which is then zinc homologated with the primary zinc sp3-carbenoid 58 to yield the allylzinc intermediate 59. This, in a spontaneous syw-/)-climination, gives the corresponding allene 60. This protocol could also be adopted to the preparation of chiral allenes. 

Another feature of carbenoid-type reactivity is the cyclopropanation (reaction c). Again, this reaction does not only take place in vinylidene but also in alkyl carbenoids . On the other hand, the intramolecular shift of a /3-aryl, cyclopropyl or hydrogen substituent, known as the Fritsch-Buttenberg-Wiechell rearrangement, is a typical reaction of a-lithiated vinyl halides (reaction d) . A particular carbenoid-like reaction occurring in a-halo-a-lithiocyclopropanes is the formation of allenes and simultaneous liberation of the corresponding lithium halide (equation 3). ... 

Synthesis of allenes by alkenylation of magnesium aikyiidene carbenoids with a-suifonyi iithium carbanions... 

The electrophilic reaction of magnesium alkylidene carbenoids with other nucleophiles than the original Grignard reagent can also be carried out. For example, treatment of magnesium alkylidene carbenoid 157, derived from 147, with a-sulfonyl lithium carbanion afforded allenes 159 in moderated yields (equation 39/. ... 

The proposed mechanism is as follows First, the a-sulfonyl lithium carbanion attacks the electrophilic carbenoid carbon atom to give the vinyhnagnesium intermediate (158). As the sulfonyl moiety is a good leaving group, a /-elimination takes place to afford the allenes (159). 

The first practical approach applying this strategy consisted of a carbocupration of an alkynyl sulfoxide 47 followed by methylene homologation of 48 with a zinc carbenoid and /3-elimination (equation 20)25. Yields of 1,1-disubstituted allenes 50 are good to high as outlined in Table 4. 

This methodology can be extended to encompass the synthesis of 1,1,3-trisubstituted allenes 51 if substituted organozinc carbenoids are employed as outlined in equation 2125. 

The secondary zinc carbenoid 52 is trapped immediately by the organocopper before it can undergo a rearrangement of the type described in Section I.B.3. This methodology gives 1,1,3-trisubstituted allenes 51 in high yields as shown in Table 525. 

Endocyclic allenes which contain other double bonds lead to rearranged products. An example is the reaction of dibromobicyclononatriene (17) with methyllithium to form initially a carbenoid (18) which... 

When the carbene or carbenoid resulting from a dihalocyclopropane is unable to rearrange to the al-lene due to steric or other factors, insertion or addition reactions characteristic of carbenes take place. Thus dibromonorcarane on reaction with methyllithium gives a bicyclobutane derivative by insertion of the carbene into a 0-C—H bond (equation 57).178 Allene formation is sterically unfavorable in this case. Similarly, dibromotetramethylcyclopropane gives l,2,2-trimethylbicyclo[1.1.0]butane instead of tetra-methylallene (equation 58).179 181 An example involving a tricyclic dibromocyclopropane is given in equation (59).182... 

Bis(phosphoranimine) ligands, chromium complexes, 5, 359 Bis(pinacolato)diboranes activated alkene additions, 10, 731—732 for alkyl group functionalization, 10, 110 alkyne additions, 10, 728 allene additions, 10, 730 carbenoid additions, 10, 733 diazoalkane additions, 10, 733 imine additions, 10, 733 methylenecyclopropane additions, 10, 733 Bisporphyrins, in organometallic synthesis, 1, 71 Bis(pyrazol-l-yl)borane acetyl complexes, with iron, 6, 88 Bis(pyrazolyl)borates, in platinum(II) complexes, 8, 503 Bispyrazolyl-methane rhodium complex, preparation, 7, 185 Bis(pyrazolyl)methanes, in platinum(II) complexes, 8, 503 Bis(3-pyrazolyl)nickel complexes, preparation, 8, 80-81 Bis(2-pyridyl)amines... 

Boron—nitrogen coordination polymers, with ferrocene, 6, 208 Boron nucleophiles, in conjugate additions asymmetric 1,4-additions, 10, 388 mechanisms, 10, 384 to nitroolehns, 10, 388 to a,/3-unsaturated amides, 10, 386 to a,/3-unsaturated esters, 10, 386 to a,/3-unsaturated ketones, 10, 384 Boron-silicon bonds, addition to alkenes, 10, 760 to alkynes, 10, 758 to allenes, 10, 760 to carbenoids, 10, 766 to 1,3-dienes, 10, 762 to isocyanides, 10, 765... 

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). 

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