Allenes allene-diynes

CpCo(CO)2] has been shown to mediate [2 -h 2 -h 2] cycloaddition reactions of allene diynes, where the alkyne terminus contains a OPPh2 moiety. The reaction was found to go with complete regio- chemo- and diastereo-selectivity. 

Cobalt(I)-Mediated [2+2+2] Cyclization of Allene-Diynes A Diastereoselective Approach to 11-aryl Steroid Core. 11-Aryl-Substituted Steroid Systems by Co-catalyzed [2+2+2] Cyclization of Allene-Diynes... 

The strategy proposed by Malacria for the diastereoselective approach to the 11-arylsteroid core comes from the possibility of a one-step synthesis of the ABC system with the desired substituents at the and carbon atoms. Scheme 2.96 shows the retro-synthesis of such compounds [146]. The tetracyclic complex 2.292 is created by intramolecular [2+2+2] cyclization of appropriately substituted allene-diynes 2.293. A manipulation by substituents in compound 2.292 may provide the 11 (3-aryl analogs of corticosteroids 2.291. 

The r 2ns-isomer of allene 2.294 can be used to construct 11-arylstroid core 2.299 with the overall yield of 48% with the simultaneous appearance of substituents at and Note that a different situation occurs in the reaction of the cis isomer of allene-diyne 2.294, where the formal ene-Diels-Alder reaction competes with the [2-E2-I-2] cyclization which leads to yne-triene bicyclic compounds. 

Unsymmetrical nonconjugated allene-diynes with substituted allene groups. The cobalt catalized [2- -2- -2] cycloaddition of these allene-diynes leads to complex tricyclic compounds. The regioselectivity of... 

Other conjugated systems, including trienes, enynes, diynes, and so on, have been studied much less but behave similarly. 1,4 Addition to enynes is an important way of making allenes ... 

When colorless crystals of rac-s-trans-3,8-di-tert-butyl-l,5,6,10-tetraphenyl-deca-3,4,6,7-tetraene-l,9-diyne (123) were heated at 140 °C for 2 h, the ben-zodicylobutadiene derivative (126) was produced as green crystals. As shown in the sequence (Scheme 20), 123 is first isomerized to its s-ds-isomer (124), and intramolecular thermal reaction of the two allene moieties through a [2+2] conrotatory cyclization gives the intermediate 125, which upon further thermal reaction between acetylene moieties gives the final product 126 [19,22].This is another example of the crystal-to-crystal reaction. 

Asymmetric hydrosilylation can be extended to 1,3-diynes for the synthesis of optically active allenes, which are of great importance in organic synthesis, and few synthetic methods are known for their asymmetric synthesis with chiral catalysts. Catalytic asymmetric hydrosilylation of butadiynes provides a possible way to optically allenes, though the selectivity and scope of this reaction are relatively low. A chiral rhodium complex coordinated with (2S,4S)-PPM turned out to be the best catalyst for the asymmetric hydrosilylation of butadiyne to give an allene of 22% ee (Scheme 3-20) [59]. 

We can narrow the difference from 10 kJmol-1 even further once it is remembered that in the comparison of meso-bisallene, 27, and (Z, Z)-diene, 29, there are two extra alkylallene and alkylolefin interactions for which a stabilization of ca 3 kJ mol-1 for the latter was already suggested. Admittedly, comparison with the corresponding 1,5-cyclooctadiyne suggests strain-derived anomalies. From the enthalpy of hydrogenation, and thus derived enthalpy of formation, of this diyne from W. R. Roth, H. Hopf and C. Horn, Chem. Ber., 127, 1781 (1994), we find 1/2S (bis-allene, bis-acetylene) equals ca — 80 kJ mol-1. We deduce that the discrepancy of this last 5 quantity from the others is due to strain in the cyclic diyne. 

In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

In the presence of Pd(PPh3)4, reductive homocoupling of 3-silylpropargyl carbonate 50 proceeded to give a mixture of allenenyne 51 and diyne 52 [65], The highest allene selectivity (51 52 = 95 5) was achieved for the reaction of 50 with R = SiiPr3 and R = Et (Scheme 3.29). 

Scheme 4.67 Asymmetric synthesis of allene 260 by rhodium-catalyzed hydrosilylation of diyne 258.

When the terminal alkynes 96 are treated with the trimethylsilylalkyne 97 in the presence of HfCl4 as a Lewis acid, the silylated vinylallenes 98 are produced in acceptable yields. In an intramolecular variant of this process, 100 was obtained from the diyne 99 [32]. Vinylallenes, incorporated into a cyclic framework and hence of restricted conformational mobility, are of interest for photochemical studies [33] and are among the photoproducts in ring-enlargement reactions of polycyclic allenes [34]. 

Not only electrophilic 1,4-addition, as shown above, but also radical 1,4-addition to conjugated enynes such as selenosulfonation is known to yield acceptor-substituted allenes [118]. Finally, monotitanation of conjugated diynes followed by treatment with benzaldehyde and aqueous workup leads to an ester of penta-2,3,4-tri-enoic acid, which is formally also a product of 1,4-addition [147]. 

Hashmi et al. investigated a number of different transition metals for their ability to catalyze reactions of terminal allenyl ketones of type 96. Whereas with Cu(I) [57, 58] the cycloisomerization known from Rh(I) and Ag(I) was observed (in fact the first observation that copper is also active for cycloisomerizations of allenes), with different sources of Pd(II) the dimer 97 was observed (Scheme 15.25). Under optimized conditions, 97 was the major product. Numerous substituents are tolerated, among them even groups that are known to react also in palladium-catalyzed reactions. Examples of these groups are aryl halides (including iodides ), terminal alkynes, 1,6-diynes, 1,6-enynes and other allenes such as allenylcarbinols. This che-moselectivity might be explained by the mild reaction conditions. 

For a number of other pharmacologically active unsaturated compounds, it is assumed that a reactive allene is formed in situ by an alkyne isomerization [160] or an elimination reaction [161]. The prime example of the formation of such a highly reactive allene through chemical activation of an unsaturated precursor is the ene-diyne antibiotic neocarzinostatin (Scheme 18.57) [162],... 

Complexes M(CH2C CC=CMe)(CO)nCp [325 M = Mo, n = 3 M = Fe, = 2 (Scheme 74)] were obtained from the carbonyl anions and l-chlorohexa-2,4-diyne. Subsequent chemistry involves protonation (HBF4) to cationic allene or diene complexes, or addition of MeOH to give allylic derivatives, which are formed with concomitant insertion of CO. The latter can also be obtained from the cationic species and NaOMe. The allene-iron cation reacts with NHEt2 to form an ynenyl complex. The luminescent complex Re(CO)3(5,5 -Bu 2-bpy) 2 (At-C=CC6H4C CC=CC6H4C=C) has been reported. ... 

The reaction conditions (80 °C) used for the addition of 33 and 34 to the o-quinodimethane 32 are incompatible with the presence of 1,2-bromochlorocy-clopropene (27), thus the potential of this approach was for quite a while not further exploited. However o-quinodimethanes may be synthesized under much milder conditions, and trapped as reactive intermediates with 27. Thus base-induced isomerization of cw-oct-4-ene-2,7-diyne (38) at -78 °C leads to bis-allene (39). Upon warming of 39, rearrangement to o-quinodimethane (40) occurs between -20 and -10 °C this adds smoothly to 27 and furnishes the adduct 41. Conversion of... 

Cook and co-workers took advantage of the molybdenum-based methodology and reported one of the most graceful applications of the allenic PKR. They intended to use PKR to synthesize a theoretically interesting tetracyclic compound 55, which has 14 vr-electrons and is supposed to be aromatic in principle. To this end, they have applied PKR to various alkynyl allenes, and finally employed a diyne-diallene substrate 54 successfully (Equation (27)). Double PKR by employing molybdenum hexacarbonyl in excess and DMSO afforded the requisite intermediate in a high yield. 

Dipropargyl ethers can be cyclized to isobenzofurane derivatives both in palladium or nickel catalysed transformations. In the former case dipent-2-ynyl ether was coupled with allyl tosylate to give the corresponding bicycle, albeit in poor yield.115 The same ring system was obtained in good yield in the nickel catalysed reductive cyclization of the diyne and the allene shown in 3.91... 

The same authors have demonstrated that 1,3-diynes behave in predictable yet distinctive manners compared to simple enynes under electrophilic transition metal-mediated reaction conditions. This characteristic behaviour of 1,3-diynes is presumably caused by the slightly electron-withdrawing nature of the alkynyl substituent, which not only renders preferentially the formation of 5-exotype alkylidenes but also allows for the subsequent [l,3]-metallotropic shift. Several salient features of reactions with this functionality include the following (a) an acetate is more reactive than the tethered alkene as an initiator, generating [l,2]-acetate migrated alkylidene intermediate, whereas an alkene is a better terminator than an acetate/bromide to generate the cyclopropane moiety (b) allene products are not formed at all under current reaction conditions (c) 5-exo/6-endo-type alkylidene formation depends on the heteroatom substituent in the tether (d) facile metallotropic [1,3]-shift of the intermediate alkylidenes occurred whenever possible. 

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