Trans arrangement

Ring systems can also be differentiated into cis/trans arrangements of substituents. 

The use of mesyl chloride for the dehydration of C-11 alcohols has already been mentioned, and mesylates can certainly be intermediates at least in the a-series. The preference for a coplanar trans arrangement is demonstrated by the well-known elimination reactions of tosylates of epimeric 20-alcohols (ref. 185, p. 616), although this does not restrict the usefulness of the reaction, and in some cases (sulfonates of 1 la-alcohols, for example) cw-elimination occurs (ref. 216, p. 293 ref. 224, 225, 226). 

A study of nonsteroidal examples has led to the suggestion that the elimination of vicinal ditosylates involves nucleophilic displacement of one tosy-late by iodide. Reductive elimination then occurs if the geometry is correct otherwise, a second displacement occurs which then gives the required trans arrangement. The reason for the failure of reaction with 2jS (axial) isomers is not clear. 

The transformation of the porphyrin intermediate 4 into a chlorin can be achieved after introduction of a C — C double bond into the 15-propanoate side chain of 4 to yield 5. The cyclization of 5 with participation of the 15-acrylic ester side chain under acidic conditions gives the chlorin 6 which is then transformed in a multistep reaction sequence into chlorophyll a. The driving force of chlorin formation from the porphyrin is believed to be the relief of steric strain at the sterically overcrowded porphyrin periphery which gives the desired trans arrangement of the propanoate side chain and the methyl group in the reduced ring. The total... 

The same nitrotetraphenylporphyrin 14 (M = Cu) undergoes bisalkylation by conjugative addition with malononitrile to give the stereochemically pure chlorin 16 with the thermodynamically favored trans arrangement of the introduced malononitrile residues.22... 

The transmetallation reaction involves the transfer of the organic group from an organometallic species to a Pd(II) species and produces a trails Pd(II) species. Isomerization from the trans arrangement to a cis one is necessary prior to the reductive elimination step. Reductive elimination yields the coupled product and regenerates the transition metal catalyst. Because the reductive elimination is very fast, competing reactions leading to by-products are usually not observed. 

In six coordinate complex ions such as [Co(NH3)4Br2], a similar situation exists, in which the bromine ligands adopt either a cis (1.3, 1.4) or a trans arrangement... 

Consequently, due to preferred cis-cis orientation a dimeric structure is observed for the indium complex and an unprecedented cis-trans arrangement in the thallium structure leads to a polymeric aggregate. Further N-NMR spectroscopic studies show that the aluminum and gallium complexes are stable contact ion pairs even in solution whereas the indium and thallium compounds are solvent-separated ion pairs in THE solution. 

The structures of the TSs have been explored computationally using combined B3LYP-MM methods.150 There are four stereochemically distinct TSs, as shown in Figure 7.4. For the aminoalcohol ligands, the anti-trans arrangement is preferred. Steric factors destabilize the other TSs. The substituents on the ligand determine the facial selectivity of the aldehydes. 

A spectroscopic study of the esters (191) and (192) (R=H,Cl,Br,N02,Me0,Me2N) has shown that the C=C, P=0, and C=0 bonds are coplanar with, in (192), a trans arrangement between the benzene ring and the ethoxycarbonyl group. It then becomes easy to explain the dephosphorylation which occurs when such... 

The existence of a Pd° >Mo" dative bond elongates the quadruple Mo2 bond to some extent. This complex has two Pd° centers that can react with a variety of olefinic compounds, shown schematically in Figure 111 such reactions give Mo2Pd2(pyphos)4(L)2 [L = acrylonitrile, fumar-onitrile, tetracyanoethylene] in moderate yields. Unlike the previous compounds which have a strictly trans arrangement of the P-donor atoms, the last compound has a cis arrangement and thus the four metals are not linear.959... 

Geometrical isomers are possible for complexes having a square-based pyramid structure. For example, the structures for a complex having the structure MLX2Y2 show that cis and trans arrangements are possible for the ligands in the base. 

The first simple sodium phosphide to be structurally characterized was synthesized in 1995 by Raston and co-workers (68). The complex [Na(PHCy)(pmdeta)]2 crystallizes as dimers with a central planar P2Na2 ring and a trans arrangement of the cyclohexyl substituents across the ring [Na-P = 2.884(8), 2.936(7) A Na-P-Na = 86.2(2)°]. 

Bismuth phosphine complexes represent a substantial component of the established phosphine complexes of heavier p-block elements, and an excellent overview has presented an important bonding model for these systems (7). The observed structures are considered as trigonal-pyramidal BiX3 units with three secondary trans bonds. If the acceptor orbitals are the Bi-X relationship between the trans X-Bi-P bond distances. The shortest Bi-P distance [2.7614(2) vs 2.866(3) A] is trans to the longer Bi-Br distance [3.403(1) vs 2.9916(1) A], as the only arrangement that will allow the phosphine ligands to occupy trans... 

C-20 positions. The final cyclization gave the C-3, C-15-trans arrangement of the hydrogen atoms attached to those centers, furnishing the dextrorotatory antipode of 18,19-dihydroantirhine (140). 

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