S-Z conformation

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

The diazo ketone can exist in two conformations, called s-(E) and s-(Z). Studies have shown that Wolff rearrangement takes place preferentially from the s-(Z) conformation. 

Comparison with published spectra of 16 [24, 23] clearly reveals that only the s-Z conformer is formed. Annealing the matrix at 45 K slowly leads to the thermodynamically more stable conformer s-E-16, which is photochemically stable and especially does not rearrange back to s-Z-16. 

The most reasonable explanation for this is that the photochemical [l,2]silyl shift in 14e yields carbene 13e in its s-E conformation. This carbene either rearranges back to I4e or produces the conformational isomer s-Z-13e (Scheme 7). Since the migrating trimethylsilyl group is located syn to the methyl group, the silyl shift in i-Z-13e is expected to be less feasible than in the s-E conformer, and thus the [1,2]H shift to s-Z-16 can compete. Since the vinyloxysilanes 16 and 8 are both formed in the s-Z conformation it is tempting to assume that a similar mechanism is operating. 

Visible light photolysis of diazo(3-thienyl)methane (25) (Scheme 9) in argon matrices generated a methylenecyclopropene as the s-Z conformer (27), which isomerized to the s-E form (28) on further photon absorption. The intermediate carbene (26) was not observed directly, but it could be trapped by oxygen in O2-doped matrices. A second intermediate in the reaction pathway, l. f-2-thiabicy-clo[3.1.0]hexa-3,5-diene, was also postulated, and a few weak absorptions in the matrix IR spectra may have belonged to this species. 

Here at last some explanation can be offered. The lithium derivative 122 forms a Li-0 bond to the carbonyl group of the ester 123 in its best s-Z conformation and the nitrogen atom adds to the conjugate position Tike a butterfly settling on a leaf 126 to give the lithium enolate 124 in the same conformation 124a. 

In their s-Z conformation, a-oxo-ketenes (e.g., 2) almost exclusively behave as 1-oxa-dienes in inverse demand oxa-Diels-Alder reactions." " In particular, their reactions with preformed imines led to 1,3-oxazin-4-ones (e.g., 4 and 8), a... 

Either direct or triplet-sensitized irradiation of diene ( , )-14 in methanol affords the a s-cyclobutene 15 and the methyl ethers 16 and 17. > Laser flash spectroscopy revealed the formation of a ground-state intermediate having = 360 nm and a lifetime of 0.8 isec at 23°C in methanol. Based on the change of the quantum yields for the formation of ethers 16 and 17 with for a variety of sensitizers, it was concluded that cyclobutene 15 and ether 16 arise from the s-Z-conformation of isomer ( ,Z)-14 and ether 17 from the s- -conformation. In view of large deuterium isotope effects, k /kjy, of 8 1 and 10 2, that were observed for the formation of ethers 16 and 17, respectively, in CH3OD, concerted formation of the CH and CO bonds in the transition states for ether formation was proposed. 

Such a structure implies that there would be a barrier to rotation about the C(2)—C(3) bond and would explain why the s-trans and s-cis conformers lead to different excited states. Another result that can be explained in terms of the two noninterconverting excited states is the dependence of the ratio of [2 + 2] and [2 + 4] addition products on sensitizer energy. The s-Z geometry is suitable for cyclohexene formation, but the s-E is not. The excitation energy for the s-Z state is slightly lower than that for the s-E. With low-energy sensitizers, therefore, the s-Z excited state is formed preferentially, and the ratio of cyclohexene to cyclobutane product increases. ... 

The reaction shows a dependence on the E- or Z-stereochemistry of the enolate. Z-enolates favor anti adducts and E-enolates favor syn adducts. These tendencies can be understood in terms of an eight-membered chelated TS.299 The enone in this TS is in an s-cis conformation. The stereochemistry is influenced by the s-cis/s-trans equilibria. Bulky R4 groups favor the s-cis con former and enhance the stereoselectivity of the reaction. A computational study on the reaction also suggested an eight-membered TS.300... 

Rhodium hydride addition to the enone s-cis conformer through a six-centered transition structure accounts for stereospecific Z(0)-enolate forma-... 

S. Z. Dong and L. A. Paquette, Stereoselective synthesis of conformationally constrained 2 -deoxy-4 -thia beta-anomeric spirocyclic nucleosides featuring either hydroxyl configuration at C5, J. Org. Chem., 70 (2005) 1580-1596. 

One of the most important parameters to determine the reactivity is the distance between the thiocarbonyl sulfur (S) and the alkenyl carbon atom (C6) in Table 1, because a new bond will be formed between these two atoms at the initial step of the photoprocess. The actual distance of the (Z, )-conformation imide la is 3.59 A, which is closely placed and is almost the same as the sum of the van der Waals radii 3.50 A. In the (- ,. ) conformation, the atomic distances are longer than that of la, and are 4.13 A for Ic and 4.32 A for Id. For the second step of cyclization, the atomic distance between the thiocarbonyl carbon (C2) and the alkenyl carbon (C5) is in the range from 3.00 to 3.11 A, which is much smaller than the sum of the van der Waals radii 3.40 A. From this geometrical consideration of the site of the new bond formation, it is expected that the reaction leading to thietane will smoothly progress. 

The E configuration of diene 7 (obtained from 5) was established by its facile reaction with maleic anhydride to form a single Diels-Alder adduct the Z-isomer was believed to be unable to achieve the requisite s-cis conformation and thus to be completely unreactive toward dienophiles281. 

In N-thioacetylindoline, 28% of the Z (C=S-endo) conformer was found (67T4493), significantly less than in the corresponding Af-acetylindoline (95-98%) the bulkier C—S group is no longer better accommodated on the side of the benzene ring, as, instead, in the case of the C=0 bond. 

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