Ring formation, relative rates

The photoadditions proceed through 1,4-diradical intermediates. Trapping experiments with hydrogen atom donors indicate that the initial bond formation can take place at either the a- or (3-carbon of the enone. The excited enone has its highest nucleophilic character at the (3-carbon. The initial bond formation occurs at the (3-carbon for electron-poor alkenes but at the a-carbon for electron-rich alkenes.191 Selectivity is low for alkenes without strong donor or acceptor substituents.192 The final product ratio also reflects the rate and efficiency of ring closure relative to fragmentation of the biradical.193... 

Chelate ring formation may be rate-limiting for polydentate (and especially macrocyclic) ligand complexes. Further, the rates of formation of macrocyclic complexes are sometimes somewhat slower than occur for related open-chain polydentate ligand systems. The additional steric constraints in the cyclic ligand case may restrict the mechanistic pathways available relative to the open-chain case and may even alter the location of the rate-determining step. Indeed, the rate-determining step is not necessarily restricted to the formation of the first or second metal-macrocycle bond but may occur later in the coordination sequence. 

Olefin metathesis is a catalytic process whose key step is a reaction between an olefin and a transition metal alkylidene complex, usually M=CHR (Eq. 1) or M= CH2, in a 2+2 fashion to give an unstable intermediate metalacyclobutane ring [1]. All possible reactions of this general type are reversible, possibly nonproductive, and in competition with one another, so the overall result depends heavily on relative rates, and in the case of formation of volatile or insoluble products, displacement of equilibria as those products form. 

The propenyl group can then supply electrons to the cobalt atom by a resonance effect. This basic mechanism may also be applied to explain the marked accelerating effect of the aroyl group. Except in cases where ortho substitution sterically hinders formation of a cyclic transition state substitution on the aromatic ring has relatively little effect on the rate of decomposition. 

It is likely that this factor reflects a fundamental difference in the nature of the nucleophilic centres concerned. This difference is responsible also for the wide differences in the relative rates of formation of three and five-membered rings containing differing hetero-atoms noted by Bird and Stirling (1973). These authors showed that the formation of the episulphonium ion from 2-chloroethyl /7-tolyl sulphide (9) is actually faster than the formation of the corresponding five-membered ring from / -tolyl-S(CH2)4Cl. The EM for the... 

This example illustrates the synthesis of cyclic compounds by intramolecular alkylation reactions. The relative rates of cyclization for ca-haloalkyl malonate esters are 650,000 1 6500 5 for formation of three-, four-, five-, and six-membered rings, respectively.28 (See Section 3.9 of Part A to review the effect of ring size on Sn2 reactions.)... 

In 1959, the coordinated mercaptide ion in the gold(III) complex (4) was found to undergo rapid alkylation with methyl iodide and ethyl bromide (e.g. equation 3).9 The reaction has since been used to great effect particularly in nickel(II) (3-mercaptoamine complexes.10,11 It has been demonstrated by kinetic studies that alkylation occurs without dissociation of the sulfur atom from nickel. The binuclear nickel complex (5) underwent stepwise alkylation with methyl iodide, benzyl bromide and substituted benzyl chlorides in second order reactions (equation 4). Bridging sulfur atoms were unreactive, as would be expected. Relative rate data were consistent with SN2 attack of sulfur at the saturated carbon atoms of the alkyl halide. The mononuclear complex (6) yielded octahedral complexes on alkylation (equation 5), but the reaction was complicated by the independent reversible formation of the trinuclear complex (7). Further reactions of this type have been used to form new chelate rings (see Section 7.4.3.1). 

The work of Herington and Rideal (H4) showed that a good prediction of the relative rates of cyclization of various paraffins could be made based on considerations of the number of ways in which ring closure could be effected, assuming that cyclization involves two-point adsorption in the manner illustrated in Fig. 8. For example, in the dehydro-cyclization of re-hexane, five two-point contacts with the surface are possible, but only two can lead to formation of a six-membered ring. By assuming the cyclization rate constant to be given by... 

Scheme 3 shows the pathway leading to each product. Arrows with loops designate bond rotation preceding ring closure. The proportions reflect the preference for formation of the trans-substituted cyclobutane and the relative rates of bond rotation and of ring closure. Analysis of the data revealed that bond rotation is ten times faster than closure.24... 

Hydroxymethyl-substituted tetrahydrofurans have been prepared with high diastere-oselectivity by reaction of the carbanion derived from 3,4-epoxybutyl phenyl sulfone (g) with aldehydes in the presence of a mixture of lithium and potassium /-buloxidcs (Scheme 8).48 Initial formation of aldol-type adducts is a non-diastereoselective but reversible process thus, subsequent formation of one main diastereoisomer is controlled by the relative rates of cyclization. The configuration of the carbon stereocentre at the oxirane ring is inverted in the course of the 5 2 process, and two new centres are created diastereoselectively (up to 87 13 0 0). 

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