Mono-addition

Another example with porphyrinic dipolar species uses pyridinium salt derivatives as precursors of porphyrinic pyridinium ylides (Scheme 18) <05TL5487>. The procedure involves the reaction of porphyrin 58 with methyl bromoacetate, in refluxing chloroform, to give pyridinium salt 59. The latter, in the presence of K2CO3, reacts with 1,4-benzoquinone to yield only the mono-addition compound 60. Notably, when the reaction was performed in the presence of DBU, bis-addition occurred and the porphyrinic dimer 61 was the only isolated addition product. 

Carbon-centred radicals, generated under photo-thermal conditions from organotellurium compounds, react with a variety of quinones to afford the mono addition products in good yields. ... 

Mechanistic studies of the addition of 27-29 to Cjq [34, 35] support clearly a concerted mechanism with a symmetrical transition state. All three mono-addition products are stable against cycloreversion at least up to 80 °C. Interestingly, the two hexadiene isomers show different reactivity [35]. The trans-trans-2,4-hexadiene 29 reacts smoothly at room temperature whereas the cis-trans-2,4-hexadiene 28 does not react at all at ambient temperature. The ds-trans-isomer 28 cycloadds to C 0 at 80 °C, while isomerization of 28 into 29 occurs. Thus, reaction of 28 leads to a product mixture. 

The first step in this sequence is the binding of a molecule of acetaldehyde ( donor ) to the aldolase to form a Schiff base with the active site lysine followed by addition to CIAA, which acts as the acceptor aldehyde. This reaction delivers the mono-addition product, which then acts as an acceptor again to react with a second molecule of AA, yielding the double addition product which cyclizes spontaneously to the stable lactol 1 (Scheme 6.4). 

A task-specific ionic liquid, [bmIm]OH, has been introduced as a catalyst and as a reaction medium for Michael addition. The addition of open-chain 1,3-dicarbonyls to ,/Tunsaturatcd ketones gave the mono-addition products, whereas a,fi-unsaturated esters and nitriles afforded mainly bis-addition products.162... 

Silver(n) and Silvery hi).—The cationic complex bis-(2,2, 2"-terpyridyl)Ag2+ has been isolated as its peroxydisulphate salt.217 The presence of a d-+d band at 15600 cm-1 is indicative of a six-co-ordinate ion. X-ray photoelectron spectra have provided Ag(3d5/2>3/2) binding energies for this complex and the mono-terpy complex. The influence of pyridine and 2-, 3-, and 4-picoline on bis(diethyl dithio-carbamato)AgH has been examined by e.s.r. spectroscopy.218 The results indicate formation of mono-addition products and the spin hamiltonian parameters for these complexes have been determined. 

C to form the dinuclear (butadienyl)zirconium system 120. Treatment with B(C6F5)3 leads to the formation of a mono-addition product (121), even in the presence of excess borane. Complex 121 (Scheme 40) shows only a marginal ethene polymerization activity. This led to the notion that such formation of dimeric zirconium complexes might represent desactivation pathways in homogeneous Ziegler-Natta catalyst chemistry.125... 

Figure 1.12. (a) The most characteristic patterns of mono-addition to Cgo- The symmetry indi-... 

For reasons of clarity, the following general considerations on chiral addition patterns of C70 will be limited to the most common types of mono-addition... 

Diphenylcarbodiimide reacts with excess thiosemicarbazide 583 to give the mono addition product 584 in high yield.55 ... 

To be synthetically useful, the new organometallic 3 must have a reactivity different from that of 1 in order to avoid the polymerization of the carbometallated substrate [2]. So, the carbometallation ability of 1 must be higher than that of 3, except for an intramolecular carbometallation reaction, in which case the entropy factors favor the mono-addition even if the starting organometallic and the product have similar reactivities. Since an outstanding number of additions of organometallics to carbon-carbon multiple bond have been reported and reviewed [3-5], we will focus this chapter on the more recent advances in this field (from 1991 until the present time), both from our own and from other research groups. 

Because alkeneboronic esters are available from the cis-mono addition of HB(OR)2 to alkynes, both internal and external compounds, a convenient route to isomerically pure... 

One of the reaction steps in a multistep synthesis produces the desired mono-addition product, R, which overreacts to a bis-addition by-product, S in a phase transfer reaction system. The solvent system is water/isopropyl acetate. The reaction is run by feeding reagent B over a several-hour period to a two-phase mixture of the solvents containing the phase transfer catalyst. 

Reaction conditions must be adjusted critically to obtain optimum yields of the mono-addition product. In this connection, the reaction of secondary amines with acrylonitrile is less sensitive, inasmuch as only one addition product is possible. There is as yet no direct evidence of the mechanism of addition of bases to such conjugated unsaturated systems. It is known... 

Fig. 6. One of the four isomers resulting from mono-addition of benzyne to C70 (C(7)-C(8)-adduct), and important resonance structures of C70 illustrating the double bond character of the C(7) -C(8)-bond as well as the benzenoid character of the equatorial hexagons...

In a first report on the mono-addition of benzyne to Cyg [86], the fourth product isomer (34%) besides 43,10 and ( )-42 (Fig. 5) was suggested to result from an unusual transequatorial 1,4-addition mode. The result is a Cy-symmetrical molecule with an inherently chiral functionalization pattern in which the positions C(7) and C(23) (Fig. 6) are bridged. This equator-spanning addition mode was also proposed to occur in CygClig [85,124] (cf. Fig. 11) as well as in multiple benzyne adducts of Cyg [86,125]. 

In a recent report by Meier et al. on the mono-addition of benzyne [121], however, the isolated fourth product (13%) was identified as a different isomer, resulting from addition across the 6-5 bond C(7)-C(8) (44, Fig. 6). The Q-sym-metrical molecule represents the first example of a direct addition to a 6-5 ring fusion and of a 6-5 adduct in which the fusion bond remains intact. This addition mode was explained by the relatively high double bond character of the... 

The [2+2] cycloaddition of ketenes with imines to form P-lactams is perhaps the most studied of ketene reactions, and because of the continued search for new drugs continues to attract close attention. The reaction of methylketene with the bis-imine forms the dx-monoadduct as the only observed mono-addition product (Scheme 7.54). 

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