Double-group transfer

For suprafacial-suprafacial (or antarafacial-antarafacial) double group transfers, the transfer is thermally allowed when p + q = 4n + 2 and photochemically allowed when p + q = 4n. 

For suprafacial-antarafacial double group transfers, the selection rules are reversed. 

FIGURE 6.1 A general representation of synchronous double group transfer reaction. 

The Woodward—Hofmann selection rules for the generalized case are the same as for cycloaddition reactions (1) For supra—supra or an antara—antara double group transfer, the reaction is symmetry allowed under thermal conditions when p + q = 4n -I- 2, and it is photochemically allowed when p + q = 4n. (2) For supra—antara double group transfer, the reaction is symmetry allowed under thermal conditions when p -I- q = 4n, and it is photochemically allowed when p - - q = 4n - - 2. 

T. S. for supra-supra double group transfer, HUckel system, 0 node, 6 electrons, aromatic, A allowed I... 

Double-group transfer This is symmetry allowed in ground state when m+n = 4g+2 and is symmetry allowed in excited state if m+n = 4g mand n are number of Jt-electrons in two reactants andg is an integer (0,1,2...). This rule also applies to antarafacial process on both components and is reversed when process is antarafacial on one component only. 

Following are some important examples of double group transfer. 

Alkene loss via McLafferty rearrangement at the alkoxy group of aliphatic and aromatic carboxylic acid esters competes with yet another reaction path, where two hydrogens instead of one as in the normal McLafferty product are transferred to the charge site. This second pathway leading to alkenyl loss has early been noticed [94] and became known as McLafferty rearrangement with double hydrogen transfer (r2H) ... 

Conjugation of the carbonyl with a double bond transfers the electronic characteristics 8-I-/8— of the carbonyl group along the carbon chain. The alkene would normally be nucleophilic and react with... 

It has been known for some time [see Ref. (176) for earlier work] that if poly(vinyl alcohol), produced by hydrolysis of poly(vinyl acetate) is reacetylated, the PVAc so obtained has a lower MW than the original PVAc prior to hydrolysis, though the MW of the material is not lowered any further by subsequent cycles of hydrolysis and reacetylation. Various explanations had been advanced for this phenomenon Wheeler explained it as a consequence of the presence of branches joined to the main chain through ester linkages which would be broken on hydrolysis and not re-formed on reacetylation. These branches were ascribed to chain transfer reactions with acetate groups, either in the polymer, or in monomer molecule subsequently polymerized at their double bonds. Transfer reactions by attack on hydrogen atoms other than those in... 

Baldini, L., Bracchini, C., Cacciapaglia, R., Casnati, A., Mandolini, L. and Ungaro, R. (2000) Catalysis of acyl group transfer by a double-disiplacement mechanism the cleavage of aryl esters catalyzed by calixcrown-Ba complexes. Chem.-Eur. J., 6, 1322. 

Although this reaction is fully reversible, the relatively high [ATP]/[ADP] ratio in cells normally drives the reaction to the right, with the net formation of NTPs and dNTPs. The enzyme actually catalyzes a two-step phosphoryl transfer, which is a classic case of a double-displacement (Ping-Pong) mechanism (Fig. 13-12 see also Fig. 6-13b). First, phosphoryl group transfer from ATP to an active-site His residue produces a phosphoenzyme... 

The biosynthesis of fatty acids such as palmitate thus requires acetyl-CoA and the input of chemical energy7 in two forms the group transfer potential of ATP and the reducing power of NADPH. The ATP is required to attach C02 to acetyl-CoA to make malonyl-CoA the NADPH is required to reduce the double bonds. We return to the sources of acetyl-CoA and NADPH soon, but first let s consider the structure of the remarkable enzyme complex that catalyzes the synthesis of fatty acids. 

Aldol reactions of both (E)- and (Z)-ketene acetals are highly susceptible to KOBuc catalysis. In the presence of 5 mol% of KOBuc, aldol reactions proceeded to completion within minutes at -78 °C < 1994JA7026>. A double-label crossover experiment, devised to probe the nature of the silicon group transfer in the alkoxide-catalyzed aldol reaction, suggested that free metal enolates are the true reactive species adding to the aldehydes. 

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