Cyclization allenynes

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. 

Rhodium complexes catalyze hydrosilylation-cyclization of 1,6-allenynes in the presence of (MeO SiH.77 To avoid complex product distributions, the use of substrates possessing fully substituted alkyne and allene termini is imperative. As shown in the cyclization of 1,6-allenyne 62a, the regiochemistry of silane incorporation differs from that observed in the rhodium-catalyzed hydrosilylation-cyclization of 1,6-enynes (see Section 10.10.2.3.2). For allenyne substrates, allene silylation occurs in preference to alkyne silylation (Scheme 40). 

Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

PtCl2 was shown to catalyze a similar Alder-ene transformation, as in the cycloisomerization of allenyne 117 to triene 118 (Equation (76)).78 In the same study, it was noticed that tetrasubstituted allenes cyclized to bicyclic compounds, such as 120 (Scheme 23), under identical PtCl2 conditions, presumably due to A(1,3) strain in intermediate 119. 

Brummond and Shibata independently reported the Rh(i)-catalyzed cycloisomerization of allenynes to cross-conjugated trienes. The rhodium conditions were shown to have broad functional group tolerance. Brummond et al 9 observed rate and selectivity enhancements when they switched to an iridium catalyst (Equation (77)). The rate acceleration observed in the Alder-ene cyclization of aminoester containing allenyne 121 (Equation (78)) was attributed to the Thorpe-Ingold effect.80... 

Distannation of allenynes initially takes place at the allene moiety (Scheme 34).159,161 Upon heating, the produced allylstannane moiety further undergoes palladium-catalyzed intramolecular allylstannation of the alkyne moiety, affording the corresponding cyclized product. 

Reaction of the stannylborane 9 with an allenyne gives a cyclization product, in which the boryl and stannyl groups are introduced to the acetylenic terminus and the allenic central carbon, respectively (Equation (104)).159 Based on the assumption that an unsaturated functionality initially inserts into the Pd-B bond of (boryl)(stannyl)palladium(n) species, it seems likely that the alkyne moiety is more reactive than the allene moiety in this reaction. 

Allenylcopper reagents can be generated from allenyllithium precursors by treatment with stoichiometric amounts of CuBr (Table 9.6) [12]. These intermediates were not characterized, per se, but subsequent reaction with alkenyl iodides led to allenynes in high yield. Thus it is assumed that the reagents are allenic rather than propargylic. The same intermediates afford 2-alkynylsulfmamides on treatment with N-sulfmylaniline (Table 9.7) [13], Cyclization to the N-phenyldihydroisothiazole S -oxides proceeds in nearly quantitative yield on treatment with base. 

Scheme 16.72 Ti-mediated allenyne cyclization followed by condensation with an aldehyde.

Ti-mediated cyclization of an allenynes having a leaving group provides a five-membered ring with cross-conjugated trienes which might be produced by the elimination of an alkoxy group from a titanacycle (Scheme 16.73) [79]. 

It is presumed that the stannyl radical, by addition to the terminal position of the triple bond in allenynes 132, or through bromine abstraction in bromovinyl and haloaryl allenes 134 and 136 gives the vinylic radical intermediates 138 and 142 in the propagation step, followed by cyclization toward the central carbon bond of the... 

Allenynes, reductive cyclization, 10, 516 Allocolchicine, via C-H functionalizations, 10, 162 Allocyathin, via Alder-ene reactions, 10, 592 (+)-Allopumiliotoxin 267A, via cyclization-hydrosilylation,... 

The Pauson-Khand reaction, promoted by (CO)3Mo(DMF)3, has been found to take place under very mild conditions in the absence of any promoter. High yields of the adducts were obtained in the cyclization of a wide variety of functionalized 1,6- and 1,7-enynes. Enynes bearing electron-withdrawing groups at the alkene terminus proved to be particularly good substrates.121 The exclusive formation of cyclopentenones was observed in the molybdenum hexacarbonyl (10 mol%)-catalysed Pauson-Khand reactions of 1,6-allenynes under 1 atm of CO (balloon) in excellent yields.122... 

The formation of these products can be rationalized as shown in equation (46). Thus, upon complexation of Au(I) to the aUcyne in the reacting enyne, an endo cyclization occurs to form a cyclopropyl gold carbene, which can be trapped by a nucleophile. However, the lack of stereospecificity observed in some of these reactions, suggests the formation of open six-membered ring cations as intermediates, which can be intercepted by an O-Boc group.3-Hydroxy-1,5-allenynes react similarly with Pt(II) catalysts. ... 

Very recendy, the cyclizations of allenynes have been described. It has already been pointed ont that gold... 

Oh et al. observed that 1,6-allenynes 131 and 135 underwent cyclization to afford six-membered-ring products 132-134 and 136-138 depending on the catalyst and additives used (Scheme 57) [35]. These products may be formed via the intermediacy of n-allyl palladium intermediate 139, which was formed... 

Representative procedure for the gold-catalyzed hydrative cyclization of allenynes. (E)-(2-Methyl-2-styrylcyclopentyl)(phenyl)methanon ... 

Kim, S. and Lee, H. 2008. Cyclization of allenyne-l,6-diols catalyzed by gold and silver salts An efficient selective synthesis of dihydrofuran and furan derivatives. Adv. Synth. Catal. 350 547-551. 

More recently, the Brummond laboratory reported a series of Rh-catalyzed allenic Alder-ene reactions affording 5- and e-lactams 89 from amide-tethered alleneynes 88 (Scheme 46) (94). Higher temperature was required for the reaction to proceed efficiently. This observation could be attributed to the secondary amide, because the reaction of protected amides proceeded at room temperature to afford cyclized product in less than 1 h. The reaction is somewhat sensitive to terminal alkyne in that product yields for amide-tethered allenyne substrates with terminal alkyne were lower. 

The palladium-catalysed oxidative acyloxylation/carbocyclization of allenynes (78) has been found to produce acyloxylated vinylallenes (82), where a new C-C bond, a new C-0 bond, and a new allene moiety are formed (Scheme 6). The reaction is believed to proceed via the initial Pd-coordination (79), followed by cyclization generating the 7 -complex (82) either via the Pd(II) (80a) or Pd(IV) (80b) complex. The reaction is completed by extrusion of the product (82) and re-oxidation of Pd(0) to Pd(II) by p-benzoquinone (p-BQ). Mechanistic evidence was gathered by isotopic labelling. An aerobic version was also realized using catalytic amounts of p-BQ together with a catalytic amount of cobalt salophen complex. ... 

Mukai et al. have found scission of a cyclopentane ring of allenyne 34 in the rhodium-catalyzed cycloisomerization (Scheme 7.11) [14]. When 34 was treated with a rhodium catalyst, the bicyclo [7.4.0]tridecatriene 37 was formed. Mechanistically, initial coordination of 34 to rhodium(I) would occur between an allenic distal double bond and an alkyne to form the intermediary ir-coordinating complex, which undergoes oxidative cyclization to form the rhodabicyclo[4.3.0]nonadiene intermediate 35. Subsequent P-carbon elimination, presumably assisted by release of the ring strain of the cyclopentane (6.3 kcal mol ), results in the formation of the 10-membered rhodacycle 36. Reductive elimination ensues to give the final product 37. 

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