Addition stereospecific

The enzyme fumarase catalyzes the stereospecific addition of water to fumarate to form L-malate. A standard solution of fumarase, with a concentration of 0.150 tM, gave a rate of reaction of 2.00 tM mim under conditions in which the concentration of the substrate was significantly greater than K. The rate of reaction for a sample, under identical conditions, was found to be 1.15 tM mimh What is the concentration of fumarase in the sample ... 

Photolysis of the parent compound (44) yields singlet methylene, as evidenced by its stereospecific addition to ( )-butene. The cyclopropane is formed together with the characteristic insertion products of methylene (62MI50800, 64PAC(9)527)... 

Stereospecific addition of diazomethane is observed with dimethyl di-methylmaleate (7) and dimethyl dimethylfumarate (10). ... 

Stannanes also add across double bonds offiuonnated olefins in a free radical reaction Trimethylstannane undergoes stereospecific addition to hexafluorocyclo-butene to afford trans 1,2,3,3,4,4 hexafluoro 1 (trimethylstannyl)cyclobutane [5] (equation 1)... 

The cycloaddition of sulfene to bicyclo[2.2.1]heptyl enamines is stereospecific, addition coming from the exo side (156). However, the steric preference of cis and trans isomers relative to the four-membered ring generated does not seem as strong, at least in the case of the addition of chlorosulfene (CICH = SOj) to bicyclic enamines, where a mixture of stereoisomers is obtained (157). 

The intermediacy of zwitterionic species was proposed for the addition of phenylsulfenyl chloride to dimethylaminomethyl-substituted enamines (465,466). They receive support from the observed lack of stereospecific addition of methylsulfonyl chloride to cw-morpholinopropene (although the trans enamine gives only one adduct) (466). 

The stereospecific addition of a trans-trans diene below indicates that the addition is concerted<50) ... 

A singlet carbene was proposed to account for this stereoselectivity. Attempts to produce triplet carbene by collisional deactivation with octafluorocyclo-butane were unsuccessful and stereospecific addition to olefin still occurred. However, nonstereospecific addition to olefins and larger amounts of olefinic (insertion) products result from irradiation of the phenyldiazomethane in a frozen m-butene matrix at — 196°C ... 

The carbenoid then brings about the stereospecific addition of a CH2 group directly to the double bond. 

The transition-metal induced rearrangement of strained cyclopropanes is mostly caused by inserting metal atoms into a three-membered ring, thus producing metallacycles and/or rf- allyl metal complexes. Tipper reported the first example of the metallacycles obtained from [Pt(C2H4)Cl2]2 [3]. The stereospecific addition of cyclopropanes has been investigated from both mechanistic and synthetic view points [4],... 

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

Allylstannanes undergo diasterospecific additions to chiral a-alkoxyaldehydes, as shown in reaction 45295. Stereospecific additions to aldehydes are attained in the presence of... 

An anti stereospecific addition to 1,3-cyclohexadiene (96) has been observed also with benzeneselenenyl trifluoracetate (prepared by treatment of benzeneselenenyl bromide or... 

The dilution technique makes use of the different concentration dependence of a) S-T-intersystem crossing, and b) [1 -f-2]-cyclo-addition of a carbene to an olefin. The decay of the metastable singlet state is monomolecular, while the stereospecific addition is of the first order with respect to the concentration At high dilution with an inert solvent such as hexafluorobenzene or octafluoro-cyclobutane etc., the same cis-/trans-cyclopropane ratio should be obtained with cis- or trans-olefin as the starting compound. 

This technique has been repeatedly utilized. Photolysis of diazomethane in the liquid phase yields methylene in the singlet state, as is shown by the practically stereospecific addition to cis- or trans-butene. Dilution with perfiuoropropane reduces the degree of stereospecifity as well as the amount of C—H-insertion, indicating that triplet methylene is involved A similar effect has been reported for CF3CH which, on dilution with perfluoro-diethylether (in the gas phase), adds in a non-stereospecific manner owing to the presence of the triplet... 

Loss of stereospecifity on dilution with cyclohexane also occurs in the otherwise stereospecific addition of the biscarbene 55 by photolysis of 55 in cis-2-butene i ). 

Iodine is a very good electrophile for effecting intramolecular nucleophilic addition to alkenes, as exemplified by the iodolactonization reaction.49 Reaction of iodine with carboxylic acids having carbon-carbon double bonds placed to permit intramolecular reaction results in formation of iodolactones.50 The reaction shows a preference for formation of five-membered rings over six-membered ones51 and is a strictly anti stereospecific addition when carried out under basic conditions. 

Surprisingly, the critical experiment has been done infrequently over the last one-half of a century The requirements for an experiment that truly speaks to the issue at hand are that one be able to see the results of addition of both spin states of a single carbene, and these requirements rarely have been met. For example, the direct irradiation of methyl diazomalonate leads to the stereospecific addition expected of a singlet carbene, whereas the photosensitized decomposition of the diazo compound leads to formation of the triplet carbene and loss of the stereochemical relationship originally present in the reacting alkene. Rotational equilibration in the intermediate seems to be complete, as it makes no significant difference whether cis or trans alkene is used as starting material (Scheme 7.9). ... 

Product Studios. Alkenes are known as diagnostic reagents for spin state of reacting carbenes. Thus, the reaction of a singlet carbene with an olefin usually results in the formation of a cyclopropane through stereospecific addition to the C—C double bond, while a triplet carbene gives rise to a nonstereospecific addition product (see Section 7). 

What is the cause of these dramatic changes The formation of the cyclopropane (56) is reasonably explained in terms of stereospecific addition of singlet mono-phenylcarbene (55) generated by photolysis of the diazomethane (54), while the... 

Turro69 has reported that several alkyl ketones react with 1,2-dicyano-ethylene (12) by nucleophilic attack of the ketone (rr, n) singlet state to give oxetanes stereospecifically. Addition of diene quenchers which are known to deactivate ketone triplets do not affect the rate of oxetane formation. This... 

Fig. 4. Isotactic, syndiotactic and atactic isomerism in stereospecific addition polymers.

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