Boat transition state

It is generally assumed that the boat transition state is higher in,energy than the chair transition state. There have been several studies aimed at determining the energy difference between the two transition states. One study involved 1,1,1,8,8,8 eu/cno-4,5-dimethyl-2,6-octadienes. Different stereoisomeric products would be predicted for the chair and boat transition states ... 

The same reaction utilizing chlorotriisopropoxytitanium gives a lower yield and optical purity of the (Z)-anti product ( + )-4 (yield 33% 64% ee). Utilization of tetraisopropoxytita-nium causes complete racemization16. The reaction of (Z)-l-methylbutenyltitanium with both enantiomers of 2-( er/-butyldimethylsilyloxy)propanal proceeds only very sluggishly with approximately 20% yield99. The results are best explained by the assumption of a (twist)boat transition state. 

As a result of additional 1.3-diaxial interactions, involving the C-l position of the allylmetal moiety in the chair transition state 7, a boat transition state 8 has been proposed, to provide amine 6. 

Lithium and zinc tert-butyl phenylmethyl sulfoxide (1) and A-phenyl imines 2, in which the substituent R is alkenyl or aryl, react at —78 °C over 2 hours with high anti diastereoselection (d.r. >98.5 1.5)6. Shorter reaction times result in poorer yields, due to incomplete reaction. In contrast, the reaction of the sulfoxide anion with benzaldehyde is complete after 5 seconds, but shows poor diastereoselection. When the substituent R1 or R2 of the imine 2 is aliphatic, the substrates exhibit poor chemical reactivity and diastereoselection. The high anti diastereoselection suggests that if a chelated cyclic transition state is involved (E configuration of the imine), then the boat transition state 4 is favored over its chair counterpart 5. 

A mechanistic rationale for the observed cw-selectivity has been proposed based on preorganisation of the Breslow-type intermediate and imine through hydrogen bonding 253, with an aza-benzoin oxy-Cope process proposed. Reaction via a boat transition state delivers the observed cw-stereochemistry of the product (Scheme 12.57). Related work by Nair and co-workers (using enones 42 in place of a,P-unsaturated sulfonylimines 251, see Section 12.2.2) generates P-lactones 43 with fran -ring substituents, while the P-lactam products 252 possess a cw-stereo-chemical relationship. 

In some cyclic system the chair transition is sterically impossible to attain, and the Cope reaction still goes but by a boat transition state. 

Similarly the conversion of cis 1, 2 divinylcyclobutane also involves a boat transition state. 

A very surprising feature of the boat transition state is the distortion of the hydrogen atoms at the center of each allyl moiety out of coplanarity with the C3 unit. If the distortion had been outwards, this could have been attributed to steric repulsion in fact it is inwards. Now the interpretation 31) of this reaction in terms of Evans principle attributes the favouring of 9 to an antibonding interaction in 8 between the 2p AOs of the central carbon atoms in the allyl moieties the distortion of the hydrogen atoms from coplanarity, indicated in Fig. 5b, could reduce this interaction by replacing the two carbon 2p AOs by hybrid AOs with less mutual overlap (cf. 10). 

Given that the boat transition state 8 is unfavourable, it is at first sight surprising that the Cope rearrangements of bullvalene (14), barbaralane (15), and semibullvalene (16) should take place so readily given that the transition states (17) of these reactions are derivatives of 8. We therefore decided 3S-) to calcu-... 

The larger (Z,Z)-l,5-cyclononadiene (169) reacts141 stereoselectively with PhSeCl in AcOH to give the substituted hydrindan 170 (equation 138). In consideration of the anti addition mode of selenenyl reagents to double bonds, the transannular reactions of 169 have been rationalized on the basis of the two reaction intermediates, 171 or 172, which are liable to place the PhSe- and AcO- groups in a cis- 1,4-relationship and trans to the bridgehead hydrogen (equation 139). The preferential formation of 170 has thus been attributed to the fact that the pathway via 172 should involve a boat transition state. 

The basic assumption of the chair-preferred transition state (for tetrahedral metal centers) is clearly tenuous, and diastereomeric boat transition state geometries should not be discounted. For example, the diastereomeric chair and boat transition states for (Z)-enolates are illustrated in Scheme 4, For this enolate geometry it is entirely reasonable to consider that the heat of formation of boat transition state B2 might actually be less than chair transition state C4 for certain combinations of substituents Ri, R2, and R3. For example, boat transition state B2 not only disposes substituents R2 and R3 in a staggered conformation as in chair transition states C3 and C4, but also minimizes Rj R3 eclipsing, which must be significant in chan-transition state C3. The change in kinetic aldol diastereoselection of... 

Reformatsky reaction has covered aspects of this topic (20b). If analogous pericyclic transition states are involved in these condensations, the added stereochemical control element imposed on the condensation by the imine geometry should provide a more well-defined set of transition states than for the analogous aldehyde condensations. The four diastereomeric chair and boat transition states for ( )- and (Z)-enolates with ( )-imines are illustrated in Scheme 15. 

When the alkene moiety is attached to a cyclic structure, the stereochemical outcome can satisfactorily be explained in terms of the chair and boat transition states described above (272,273) (Scheme 6.50). 

Hartree-Fock, local density and MP2 models all yield barrier heights which are slightly larger than those from density functional models, and are outside the experimental range. Additionally, the energies of the twist-boat intermediate and boat transition state (relative to the chair conformer) are also slightly higher. 

Examination of molecular models shows that in most of the examples of the Barton reaction studied so far the quasi-chair form is the most favorable because it allows the maximum overlap of orbitals in the transition state. This, however, is not a sufficiently strong argument to eliminate the chair and boat transition states (95a) and (95b). In fact, there are examples to be discussed later (see Part II) which compel one to consider the chair and boat transition states (95a) and (95b) as well. 

In the boat transition-state there is an antibonding interaction between C-2 and C-5. 

---