Transition states boat-like

A highly successful route to stereoisomers of substituted 3-cyclohexene-l-carboxylates runs via Ireland-Claisen rearrangements of silyl enolates of oj-vinyl lactones. The rearrangement proceeds stereospeaifically through the only possible boat-like transition state, in which the connecting carbon atoms come close enough (S. Danishefsky, 1980 see also section 4.8.3, M. Nakatsuka, 1990). 

In 1997, Lindstrom and Somfai reported aza-[3,3]-Claisen enolate rearrangements of vinylaziridines (Scheme 2.45) [70]. Treatment of l-acyl-2-vinylaziridines 179 with LHMDS resulted in the stereoselective formation of seven-membered lactams 181, presumably through a boat-like transition state 180. 

The stereochemistry of this reaction is consistent with transition state 2 in which the ethoxycar-bonyl unit adopts an equatorial position. The same result could occur, however, via boat-like transition state 3 with an axial ethoxycarbonyl group47. The reactions of 2-oxopropanoate esters and 9-(2-butenyl)-9-borabicyclo[3.3.1]nonane occur at — 78°C, reflecting the greater reactivity, but stereoselectivity is generally poor except in cases where a very hindered ester is employed3811. 

The preferential conversion of the aluminum enolatc 2 c to diastereomer 3 is consistent with the boat-like transition state A where the enolate has adopted the usual antiperiplanar E geome-try42-50. The aldehyde substituent occupies the lower energy equatorial position of a boat-like transition state which places the bulky dialkylaluminum moiety away from the iron ligands. Possible transition states for the other observed diastereomeric products are also illustrated. 

In the case of an aromatic 7t-nucleophile, a dramatic reversal of diastercoscleetivity is found on going from a phenyl to a cyclohexyl or fert-butyl substituent, which is attributed to a balanee of. syn-axial and A(1,3) interactions, while a boat-like transition state may also play a role149. 

A c/.v-fused chair-chair -like and a Tram-fused boat-boat -like transition state have been suggested to explain the stereochemical outcome from the addition reactions of the (E)- and (Z)-l-(/f>rf-butylsulfinyl)-2-methyl-2-butenes, respectively. 

In the rearrangement of divinylaziridines 289, the participation of a boat-like transition state 290 explained the stereochemical outcome of the reactions to give the azepinones 291 in 73 to 85% yield. The divinylaziridines 289 were synthesized via ex-chiral pool sequences starting from optically active a-amino acids. Table 16, Eq. (26) [55]. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Thus a Cope rearrangement proceeding through a boat-like transition state is the rearrangement of cis 1, 2 divinyl cyclo-propane. 

Rearranging divinylaziridines 95, the passing of a boat-like transition state... 

Surprisingly, the rearrangement of the 4-f-butyldimethylsilyloxy-2-vinyl-pyrrolidines 154 (R =OTBS, R R =H) took another course. The stereochemical outcome had to be rationalized by the passing of a boat-like transition state 156 (bj8) to give the 3,8-trans lactams 157 (Ri=OTBS, entries 15-19, Table 8). The corresponding cis product 158 (Ri=OTBS) resulting from the expected chairlike intermediate 156 (c ) had only once been isolated as a minor compound... 

Catalysed enantioselective aldol additions of latent enolate equivalents have been reviewed and electronic effects of the aldehyde component on such reactions of trichlorosilylenolates of cyclopentanone and cycloheptanone, catalysed by chiral phos-phoramides, have been interpreted in terms of initial aldehyde coordination to the trichlorosilyl enolate and aldolization via a six-membered boat-like transition state. 

The coordination state of the silyl enol ether in the transition state strongly influences the diastereoselectivity (synlanti). If a ligand is sterically demanding, like phosphoramide 33, a boat-like transition state with a pentacoordinated silicate is formed and affords the syn product in the reaction of trichlorosilyl enol ether with benzaldehyde. In contrast, the less hindered ligand 34 gave the anti product through a chair-like transition state with a hexacoordinated silicate (Scheme 25). 

Bredt s rule. In this way, 1-adamantyl p-methoxyacetophenone 86a was forced to yield only cyclobutanols 87a and 88a as photoproducts [281]. Whereas (benzene) solution phase irradiations of 86a yielded a 2.6 ratio of 87a/88a, the solid state photoproduct ratio, 0.5, favors the more sterically hindered cyclobutanol. X-Ray diffraction studies predict a chair-like y-hydrogen abstraction pathway for 86a (in contrast to the boat-like transition states of 82) in which the C=0-Ha distance is 2.67 A. Other abstractable hydrogens (Hb) are at least 0.3 A farther from the carbonyl oxygen (Scheme 44). If i-BR has a conformation which mimics that of the ketone, its least motion pathway favors formation of the more sterically hindered cyclobutanol 88a. 

Asymmetric aldol additions of geometrically defined trichlorosilyl enolates of ketones to aliphatic and aromatic aldehydes have been carried out uncatalysed, and with a chiral phosphoramide as Lewis base promoter.54 Significant differences in rates and diastereoselectivities are interpreted in terms of the changeover from a boat-like transition state, with pentacoordinate siliconate, to a chair-like transition state with hexacoordination. 

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