Regioselectivity allenes

Platinum complexes also exhibit catalytic activity toward the carbonylation of allene with CO and aliphatic thiol can react with allene regioselectively [12]. The PtCl2(PPh3)2-catalyzed carbonylative thiolation of cyclohexylallene with I " Hex-SH) as an aliphatic thiol carbon monoxide by the use of aliphatic thiol ( Hex-CH) proceeds regioselectively at the terminal double bond of the allene. In the case of Pt(PPh3)4-catalyzed reaction, the thiolative carbonylation takes place at both terminal and central double bonds (Scheme 11.9). 

Fluorinated allenes are especially reactive in cycloadditions because of their highly strained double bonds [118, 119] 1,1-Difluoro- and 1-fluoroallene readily undergo both [2+2] and [4+2] cycloadditions [118 124] (equations 50-52) Exten sive studies of stereochemistry and regioselectivity show that cyclobutane forma-... 

Allenynes 160 were also cyclized chemo- and regioselectively to methylen-eyclopentane derivatives 161 and 162 using Rh(acac)(CO)2 as the catalyst and silanes or alkoxysilanes as the reductant (Eq. 32) [96]. The major product resulted from initial insertion of the internal Jt-bond of the allene into the Rh-Si bond. Only 1,1-disubstituted allenes were used for this reaction others may show less selectivity for the internal Jt-bond of the allene. 

Hydride-promoted reactions are also well known, such as the acrylic and vinylacrylic syntheses (examples 7-10, Table VII). Some less-known compounds, which form in the presence of halide ions added to tetracar-bonylnickel, have been described by Foa and Cassar (example 11, Table VII). Reaction of allene to form methacrylates, and of propargyl chloride to give itaconic acid (via butadienoic acid), have been reported (examples 13 and 14, Table VII). 1,5-Hexadiene has been shown to be a very good substrate to obtain cyclic ketones in the presence of hydrogen chloride and tetracarbonylnickel (example 15, Table VII). The latter has also been used to form esters from olefins (example 16, Table VII). In the presence of an organic acid branched esters form regioselectivity (193). 

Allenes add nitrile oxides either to one or two double bonds. For mono- and 1,1-disubstituted allenes, relative activity of the two bonds depends on the nature of substituents. The reaction (Scheme 1.18) of N-propadienylanilines 54 with 3,5-dichloro-2,4,6-trimethylbenzonitrile oxide proceeds site- and regioselectively to give 5-substituted 4-methylene-4,5-dihydroisoxazoles 55, which add a second molecule of nitrile oxide to afford 4,5/-spirobi-(4,5-dihydroisoxazoles) 56. Dihy-droisoxazoles 55 isomerize to 4-(2-aminobenzyl)isoxazoles 57 via a Claisen-type rearrangement (224). 

Vinyl- or arylboronic acids also react with allenes, affording 1,3-dienes or styrene derivatives, respectively.89 This palladium-catalyzed addition proceeds with good regioselectivity and high stereoselectivity in favor of the formation of (ft)-trisubstituted isomer. 

Vinyl Fischer carbenes can be used as three-carbon components in Ni(0)-mediated and Rh(l)-catalyzed [3 + 2 + 21-reactions with alkynes (Schemes 48 and 49)142 and with allenes (Schemes 50 and 51).143 All three of the proposed mechanisms for the [3 + 2 + 2]-cycloadditions involve an initial carbene transfer from chromium to nickel or rhodium (Schemes 49, 52, and 53). As is seen from the products of the two [3 + 2 + 2]-reactions with 1,1-dimethylallene, although the nickel and rhodium carbenes 147G and 147K appear similar, the initial insertion of the allene occurs with opposite regioselectivity. 

On the other hand, platinum-catalyzed silaboration of allenes results in an opposite regiochemical preference. Silaboration of a 1,1-disubstituted allene affords terminal addition product regioselectively in the presence of a platinum catalyst, whereas the internal addition product is obtained with a Pd2(dba)3-PPh3 catalyst (Scheme 49).227... 

A unique system for catalytic silaboration of allenes, in which a catalytic amount of organic halide is used as a crucial additive, has been reported (Equation (86)).232 In the presence of Pd2(dba)3 (5 mol%) with 3-iodo-2-methyl-2-cyclohexen-l-one (10mol%), reactions of terminal allenes with a silylborane afford /3-silylallylboranes in good yields with excellent regioselectivity. It is worth noting that the addition takes place at the terminal C=C bond in contrast to the above-mentioned palladium-catalyzed silaboration. The alkenyl iodide can be replaced with iodine or trimethylsilyl iodide. The key reaction intermediate seems to be silylpalladium(n) iodide, which promotes the insertion of allenes with Si-C bond formation at the central -carbon. 

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. 

Allylzirconation of alkynes with allylzirconocene chloride reagents (obtained by hydrozir-conation of allenes) takes place in the presence of a catalytic amount of methylaluminox-ane (MAO) [67,68]. MAO presumably abstracts chloride to form an allylzirconocene cation, which coordinates to the alkyne triple bond. The subsequent migratory insertion is regioselective, as it is found that the new bond is mainly formed between the a-carbon of the allylzirconium species and the internal carbon of a terminal alkyne (Scheme 8.33). 

The intramolecular competition of two propargylic alkoxy groups, one on each side of the alkyne, is interesting. In a series of substrates related to 94 [180], always the 1,2-disubstituted allene 95 (Scheme 1.41) and not a 1,1,3-trisubstituted allene is formed (see also [225, 226]). The opposite regioselectivity was described in one publication (deprotonation with tBuLi, then protonation), but the allenes described there proved to be labile and quickly converted to other products [227]. 

Introduction of a double bond between the triple bond and the leaving group leads to enyne electrophiles 45, which would give access to vinylallenes 46 if the attack of the nucleophile takes place at the triple bond in an SN2" (1,5) substitution reaction (Scheme 2.16). In addition to the regioselectivity, two types of stereoselectivity also have to be considered in this transformation, i.e. the configuration of the olefinic double bond of the vinylallene and the (relative or absolute) configuration of the allenic chirality axis. 

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