Silylcarbocyclization

Table 2. Silylcarbocyclization of propargyl alcohols and propargyl amines with Rh catalysts .

Extensively developed by Ojima and co-workers, SiCaT and carbonylative silylcarbocyclization (CO-SiCaC) represent a rapid entry into polycyclic molecules of interest.271 For instance, the rhodium-catalyzed intramolecular SiCaT of triyne 441 afforded tricyclic compound 442 in high yield, accompanied by a small amount of cycloadduct 443 (Scheme 111).270... 

Under a pressure (20 bar) of carbon monoxide, carbonylative silylcarbocyclization of enyne 92 was examined in the presence of a cationic rhodium-BINAP catalyst (Scheme 31).86 Although the enantioselectivity is low, the five-membered carbocycle functionalized with an alkenylsilane moiety and a formyl group was obtained with high selectivity. 

When submitted to the silylcarbocyclization developed by Ojima et al. (Me2PhSiH, Rh2(CO)i2, CO) enynes 156 afforded a diastereomeric mixture of pyrrolizines 157 in good yield (Scheme 32) <2006TL6312>. 

The binuclear precursor [ lr( 4-Cl)(cod) 2] has been also studied in the carbonyla-tive silylcarbocyclization of enynes occurring according to Equation 14.7 ... 

In the first rhodium-catalyzed carbonylative silylcarbocyclization (CO-SiCaC), which was reported in 1992 [12, 13], silylcyclopentenone 9 was isolated as a minor product in the silylformylation of 1-hexyne 8 (Scheme 7.4). Under optimized conditions using Et3SiH and ( BuNC)4RhCo(CO)4 as the catalyst at 60°C, 9 is formed in 54% yield [13]. A possible mechanism proposed for this intermolecular CO-SiCaC is shown in Scheme 7.4 [13]. In this mechanism, the formation of 9 is proposed to proceed via in-... 

Rhodium-catalyzed three-component domino coupling of 1,6-diyne 41, hydrosilane, and Cgo proceeds to give fullerene adduct 42 in good yield (Eq. 10) [29]. In this case, dienophile Cgo does not interfere with the silylcarbocyclization process and traps the 1,2-bis-exo-methylidenecyclopentane intermediates to furnish the corresponding Cgo-linked carbo- or heterocycle 42. 

Cascade Silylcarbocyclization of Enediynes, Triynes (SiCaT), and Diynals... 

Cascade silylcarbocyclization reactions tiave been developed based on the fact that it is possible to realize successive intramolecular carbocyclizations, as long as the competing reductive elimination is slower than the carbometallation. For example, the reaction of dodec-6-ene-l,ll-diyne 67 with PhMe2SiH catalyzed by Rh(acac)(CO)2 at 50°C under 1 atm CO gives bis(exo-methylenecyclopentyl) 68 in 55% yield [44]. The reaction is stereo-specific that is, (6 )- and (6Z)-dodec-6-ene-l,ll-diynes, ( )-67 and (Z)-67, afford R, R )-68 and (S, R -68 respectively. A possible mechanism for this reaction is outlined in Scheme 7.20. It should be noted that none of the tricyclic product is formed even though a third carbocyclization in the intermediate III.2c is conceptually possible. 

The cascade silylcarbocyclization is applicable to diynals. For example, the reaction of undec-l,6-diyn-ll-al 72 with PhMe2SiH, catalyzed by Rh(acac)(CO)2 affords the corresponding silylated exo-silylmefhylene(hydroxy)bis(cyclopentylidene) 73 (Eq. 20) [48]. 

Cascade silylcarbocyclization of (E)- or (Z)-dodec-6-ene-l,ll-diyne (355) with HSiMe2Ph catalyzed by Rh(acac)(CO)2 gives the bis(exo-methylenecyclopentyl) (356) with complete stereospecificity (equation 144)342. The reaction of dodec-l,6,ll-triyne (357) under the same conditions affords a 3 1 mixture of 358 and 359 in 75% yield... 

Formal silylcarbocyclization of 1,7-diynes such as 360 and 362 catalyzed by Ni(acac)2/DIBAL (DIBAL = /B112AIII) gives the corresponding vicinal exo-dimethylenecyclohexanes (equations 146 and 147)345. 

The Rh-catalysed silylcarbocyclization of the allyl propargyl ether (589) with silane gives 592. The reaction starts by the insertion of the triple bond to the Rh-SiR3 bond... 

The reaction was successfully extended to the hydroformylation of propargyl-type alcohols [154] and propargylamine [155], the silylative cyclocarbonylation of alkynes [156], silylcarbocyclization of alkenynes and diynes [157-160], and other transformations of C=C bonds in the presence of HSiRa and CO (e. g., [161]). A generalized catalytic cycle for the silylformylation of 1-alkynes catalyzed by rhodium-cobalt clusters is illustrated in Scheme 6. 

Ojima et al. reported the rhodium-catalyzed car-bonylative silylcarbocyclization of the 1,6-enynes 76.105 In the presence of the rhodium catalyst the reaction of the 1,6-enynes 76 with the hydrosilane 77 under CO atmosphere gave the heterocycles 78, which had both silylmethylene and formylmethyl groups, in good yields (Scheme 27). 

The characteristic feature in the mechanism of cyclization reactions is the reductive cascade ring closure. The mechanisms of silylcarbocyclizations (SiCaC, SiCaT,f CO-SiCaC, = CO-SiCaB, and CO-SiCaT reductive... 

The first Rh-catalyzed carbonylative silylcarbocyclization (CO-SiCaC) was reported in 1992 in which silylcyclopentenone 531 was isolated as a minor product in the silylformylation of 1-hexyne 530 (Scheme 2-79). 

Scheme 2-79. Rh-catalyzed carbonylative silylcarbocyclization (CO-SiCaC) of 1-hexyne 530.

In 1992, Tamao et al. reported the first example of the Ni-catalyzed silylcarbo-cyclization of diynes. Thereafter, Ojima et al. extensively developed rhodium-catalyzed silylcarbocyclizations of diynes, enynes, triynes, and endijmes. ... 

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