Diynes, tethered

Pyridine compounds 45 can also be produced by the NHC-Ni catalysed cycloaddition between nitriles 43 and diynes 44 (Scheme 5.13) [16]. The SIPr carbene was found to be the best ligand for the nickel complex in this reaction. The reaction required mild reaction conditions and low catalyst loadings, as in the case of cycloaddition of carbon dioxide. In addition to tethered aUcynes (i.e. diynes), pyridines were prepared from a 3-component coupling reaction with 43 and 3-hexyne 23 (Scheme 5.13). The reaction of diynes 44 and nitriles 43 was also catalysed by a combination of [Ni(COD)J, NHC salts and "BuLi, which generates the NHC-Ni catalyst in situ. The pyridines 45 were obtained with comparable... 

Rather complex structures are obtained by a novel chromium(O)-mediated three-component domino [6jt+2jt] cycloaddition described by Rigby and coworkers [315]. Irradiation of a mixture of the chromium complex 6/4-134 and the tethered diyne 6/4-135 with a Pyrex filter at 0 °C gave the polycyclic compounds 6/4-136 in medium to good yield (Scheme 6/4.34). 

The Ti-promoted intramolecular alkyne-alkyne coupling of tethered diynes was extensively developed earlier with TiCp2 derivatives as reagents65 66 (Equation (1) of Scheme 22). Bicyclic titanacyclopentadienes fused to... 

Vollhardt and colleagues have explored the reactions between diynes and enami-nes338-341. The reactions between symmetrically substituted alkynes and alkyne tethered uracil derivatives proceeded in moderate yields, producing adducts with predominant anti configurations342-343. On the other hand, the reactions between diynes and uracil derivatives produced predominantly syn isomers. 

The Ir-catalyzed [2+2+2] cycloaddihon of diynes and monoalkyne advocates a new synthehc approach to silafluorenes [13]. The Ir-PPh3-complex-catalyzed reac-hon of benzene and silicon-tethered 1,6-diynes with disubstituted alkynes gave tetra-subshtuted silafluorenes (Scheme 11.3). The consecutive [2+2+2] cycloaddihon of two types of tehayne provided a ladder-type silafluorene and a spirosilabi-fluorene (Schemes 11.4 and 11.5). 

Transition metal-catalyzed carbocycUzation reactions of tethered diene, enyne, diyne, and vinylallene derivatives represent an important class of transformations in synthetic organic chemistry. This may be attributed to the abihty to significantly increase molecular complexity through the highly selective combination of acyclic components, thereby facilitating the synthesis of complex polycychc products. Recently, rhodium-catalyzed carbocyclization reactions have attracted significant attention due to their immense synthetic versatility and the unique selectivities observed over a range of different transformations. This chapter provides an account of recent developments in rhodium-catalyzed [4-1-2] and [4-i-2-t2] carbocyclization reactions. 

An intermolecular coupling reaction between three organonitrile molecules and a silicon-tethered diyne is reported to give good yields of pyrrolo[3,2-f]pyridine derivatives <2004JA7172>. The reaction is promoted by a low-valent zirconocene species (Equation 13). 

Rhodium carbonyl complexes also catalyze the cascade cyclization/hydrosilylation of 6-dodecene-l,l 1-diynes to form silylated tethered 2,2 -dimethylenebicyclopentanes. For example, reaction of ( )-85 with dimethylphenylsilane catalyzed by Rh(acac)(CO)2 in toluene at 50 °G under GO (1 atm) gave 86a in 55% yield as a single diastereomer (Equation (56)). Rhodium-catalyzed caseade cyclization/hydrosilylation of enediynes was stereospecific, and reaction of (Z)-85 under the conditions noted above gave 86b in 50% yield as a single diastereomer (Equation (57)). Rhodium(i)-catalyzed cascade cyclization/hydrosilylation of 6-dodecene-1,11-diynes was proposed to occur via silyl-metallation of one of the terminal G=G bonds of the enediyne with a silyl-Rh(iii) hydride complex, followed by two sequential intramolecular carbometallations and G-H reductive elimination. ... 

Furthermore, a four-component cycloaddition reaction (7]6-thiepine I,l-dioxide)tricarbonylchromium(0) 52 with tethered diynes under photoactivation afforded pentacyclic adducts formally derived from a sequential [671+271]/ [67l+27t]/[2(T+27i] cycloaddition process <1999OL507>. Photocycloaddition of the complex 52 with excess 1,7-octadiyne 66a or 1,8-nonadiyne 66b (C1CH2CH2C1, hv (Pyrex filter)) afforded the pentacyclic triene sulfones 67a and 67b in 45% and 38% yields, respectively (Equation 4). In contrast, 1,6-heptadiyne 66c afforded only the three-component cycloadduct 68 in 56% yield (Equation 5). 

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. 

Diynes 6, which consist of a trimethylsilyl alkyne tethered to a tertiary propargyl alcohol, undergo ruthenium catalyzed cycloisomerization in aqueous acetone to form the dienone intermediate 7. Concomitant electrocyclization affords 2-trimethylsilyl-2-//-pyrans in high yield (Scheme 3) <2004OL4235>. 

Diynes 8 that contain a heteroatom in the tether can undergo ruthenium catalyzed cycloisomerization, in the presence of only five equivalents of water, to afford 2-silyl 2//-pyrans (Equation 3) <2004OL4235>. 

A nickel(0)-catalyzed cycloaddition of carbon dioxide and symmetrical tethered diynes 649 affords cycloalkyl-fused 27/-pyran-2-ones in high yield (Equation 264) <2002JA15188>. The nickel(0)-catalyzed cycloaddition of carbon dioxide and unsymmetrical tethered diynes 650 leads to the formation of two regioisomeric 27/-pyran-2-ones 651, 652 the major regioisomeric product 651 features the larger alkyne group at C-3 (Equation 265, Table 31) <2004T7431>. 

Table 31 Ratio of products 651 652 formed during the nickei(0)-catalyzed cycloaddition of carbon dioxide and unsymmetrical tethered diynes 650 (Equation 265)...

The treatment of bis(phenyliodonium)diyne triflates, 19 (n = 5,6,8), with sodium p-toluenesulfinate provides access to disulfones possessing spiroannulated or tethered cyclopentene rings (equation 57)86. However, because migration of the / -toluenesulfonyl group in the intermediate carbenes is competitive with insertion, the yields of the bis-cyclopentene compounds are relatively low, and disulfones of general structures 20 and 21 are also obtained. 

The MCI cyclopentene annulations proceed with moderate efficiency and have been utilized for the construction of polycyclic molecules with fused, spiro and tethered ring systems (e.g. equations 118-120)28. With bis(phenyliodonium) diyne salts, biscyclopentene annulations are observed (e.g. equation 121)86. 

Intramolecular [2+2+2] cyclotrimerizations of diynes and triynes possessing heteroatom tethers furnish benzoheterocycles. The cyclization of triynes 88 using the Grubbs catalyst 76 proceeds via cascade metathesis as shown in Eq. (35) to yield a tricyclic product 89 [88]. This novel type of catalytic alkyne cyclotrimerization can be applied to the cycloaddition of 1,6-diynes with monoalkynes [89]. 

The ability of water to attack the ruthenium-carbon double bond suggested that the ruthenacyclopentadiene might add water as depicted in Scheme 1.4. Remarkably, heating a tethered diyne in aqueous acetone to 60 °C in the presence of the trisacetonitrile complex 16 gave a nearly quantitative yield of the hydrated cyclization product as depicted in Equation 1.29 [26], Unsymmetrical diynes showed exquisite regioselectivity wherein the water attacked the least sterically hindered alkyne carbon (Equation 1.30). 

The regioselectivity of the Ni-catalysed cycloaddition of CO2 to asymmetrical tethered diynes 40 is controlled by the relative sizes of the terminal substituents and to a lesser extent by the nature of the catalyst. The bulkier group tends to occupy the 3- rather than the... 

Similarly, tethered conjugated as well as nonconjugated bis-iodonium diyne bistriflates 34, 35 [40], and ditosylates 36 [41], have been reported [Eqs. (17), (18)]. Reaction of the respective bis-tin-alkynes with two equivalents of 7 results in good to excellent yield of the corresponding bis-iodonium diynes 34-36. 

---