Alkenyl carbons

Acetylation of acetals or ketals can be accomplished with acetic anhydride and BF3-etherate. ° The mechanism with acetals or ketals also Involves attack at an alkenyl carbon, since enol ethers are intermediates. Ketones can be formylated in the a position by treatment with CO and a strong base. ... 

Uchida and Irie have reported a photochromic system based on ESIPT to an alkene carbon.82 They observed that vinylnaphthol 121 isomerizes to the ring-closed 123 when irradiated with 334 nm light ( = 0.20, Eq. 1.34). The reaction is photoreversible since irradiation of 123 (at400 nm) regenerates the starting vinylnaphthol. The authors proposed a mechanism in which ESIPT from the naphthol OH to the [3-alkenyl carbon gives intermediate o-quinone methide 122, which undergoes subsequent electrocyclic... 

When the fluorine substituent is located at the 2-position or on any alkyl-substituted alkenyl carbon, it experiences the usual deshielding of 30-40 ppm (Scheme 3.38). Note the interesting variation in the chemical shifts and coupling constants for the 1-fluorocycloalkenes. 

General structure 24 is used throughout to indicate a wide variety of zirconacyclopentanes and zirconacyclopentenes. Generally, these are unsubstituted on alkyl carbons a to zirconium, whereas alkenyl carbons generally have an alkyl, aryl, or trimefhylsilyl substituent a to the zirconium. 

The X-ray crystallographic analysis of 2 -Cu reveals a dimeric structure, where two copper ions are coordinated to two ligand molecules (Fig. 8). Each copper ion is situated in a trigonal planar carbene/alkenyl carbon ligand environment, coordinated to two carbene arms from one chelator and a third carbon from the pendant arm of a second chelator. The average Cu—C bond distance is 1.996 (1)A, consistent with that of other reported Cu(I) carbene complexes (29). 

One of the most important parameters to determine the reactivity is the distance between the thiocarbonyl sulfur (S) and the alkenyl carbon atom (C6) in Table 1, because a new bond will be formed between these two atoms at the initial step of the photoprocess. The actual distance of the (Z,E)-conformation imide la is 3.59 A, which is closely placed and is almost the same as the sum of the van der Waals radii 3.50 A. In the (EyE) conformation, the atomic distances are longer than that of la, and are 4.13 A for Ic and 4.32 A for Id. For the second step of cyclization, the atomic distance between the thiocarbonyl carbon (C2) and the alkenyl carbon (C5) is in the range from 3.00 to 3.11 A, which is much smaller than the sum of the van der Waals radii 3.40 A. From this geometrical consideration of the site of the new bond formation, it is expected that the reaction leading to thietane will smoothly progress. 

From the X-ray structural analysis of the starting thionocarbamate 3b, the distance between the thiocarbonyl sulfur atom and the alkenyl carbon and between the thiocarbonyl carbon and the alkenyl carbon is 4.69 and 3.00 A, respectively. The fact that the reaction proceeded under these restricted conditions, in which the distance of each reacting site is longer than the sum of the van der Waals radii (3.5 A), is accounted for by the fact that the initial reaction occurred in the defect of the crystalline lattice, with the later reaction occurring in increasingly defective regions. Furthermore, two plausible factors are responsible for the relatively low enantiomeric excess of 3b. 

Sakamoto et al. reported the X-ray crystallographic data and the solid-state photoreaction of eight Ar,AT-disubstituted a,p-unsaturated thioamides 23a-h, which involves hydrogen abstraction by the alkenyl carbon atom conjugated with thiocarbonyls (Fig. 9) [44-46]. 

Fig. 13.46). The initial addition to the p-alkenyl carbon gives a imine quinone methide 89. The latter rearomatizes upon addition of another water molecule to give dihydroxyl adduct 90. 

Gyclization/hydrosilylation of enynes catalyzed by rhodium carbonyl complexes tolerated a number of functional groups, including acetate esters, benzyl ethers, acetals, tosylamides, and allyl- and benzylamines (Table 3, entries 6-14). The reaction of diallyl-2-propynylamine is noteworthy as this transformation displayed high selectivity for cyclization of the enyne moiety rather than the diene moiety (Table 3, entry 9). Rhodium-catalyzed enyne cyclization/hydrosilylation tolerated substitution at the alkyne carbon (Table 3, entry 5) and, in some cases, at both the allylic and terminal alkenyl carbon atoms (Equation (7)). 

Cyclization/hydrosilylation catalyzed by Cp 2YCH(SiMe3)2 failed in the case of dienes that possessed substitution on the alkenyl carbon atoms, presumably due to the excessive steric crowding about the yttrium center. In contrast. 

Guided by Marks s report of the samarium-catalyzed hydroboration of alkenes, Molander has developed a samarium-catalyzed protocol for the cyclization/hydroboration of unfunctionalized 1,6-dienes. In an optimized procedure, reaction of 1,5-hexadiene and l,3-dimethyl-l,3-diaza-2-boracyclopentane catalyzed by Gp 2Sm(THF) in toluene at room temperature for 18 h followed by oxidation gave hydroxymethylcyclopentane in 86% yield (Equation (70) R = H, n — ). The transformation was stereoselective, and Sm-catalyzed cyclization/hydroboration of 2-phenyl-1,5-hexadiene followed by oxidation formed /ra/ i--l-hydroxymethyl-2-phenylcyclopentane in 64% yield (Equation (70) R = Ph, n = ). The samarium-catalyzed reactions was also applicable to the synthesis of hydroxymethylcyclohexanes (Equation (70), n=X) but tolerated neither polar functionality nor substitution on the alkenyl carbon atoms. 

An empirical increment system permits prediction of charge distribution in a,/ -unsaturated carbonyl compounds, assuming additivity of electronic effects and neglecting the conformational dependence of carbon-13 chemical shifts [290]. Moreover, carbonyl and alkenyl carbon shifts of a, /3-unsaturatcd ketones may be used to differentiate between planar and twisted conjugated systems, as shown in Table 4.29 [291] and outlined for phenones in Section 3.1.3.8. 

Carbon-13 shifts of cyano groups in nitriles are found between 110 und 125 ppm (Table 4.46) [77 a, 352]. Shift values close to 125 ppm are characteristic of nitriles with a branched alkyl groups (Table 4.46). Similiar to the isoelectronic ethynyl group, the nitrile function shields the oc carbon owing to the anisotropy effect. Alkenyl carbon shifts of... 

The 13C spectrum of crotonaldehyde (CH3 CH=CH CHO Fig. 3.52) provides a good example of the way in which the 13C chemical shift is determined both by the state of hybridisation of the carbon atom and the nature of the substituent. The four carbon atoms have markedly different chemical shifts. The methyl carbon appears at 3 17.1. It is shifted downfield slightly compared to the methyl carbon at the end of a chain of methylene groups as in 3-methylheptane (Fig. 3.41) and hex-l-ene (Fig. 3.51). The two alkenyl carbons appear at <5133.4 and 3 152.9. The effect of conjugation of the carbon-carbon double bond is that the (i-carbon is shifted further downfield. The carbon of the carbonyl group is sp2-hybridised and is directly bonded to an electronegative atom. It is shifted furthest downfield and appears at 3 192.2,... 

Molecules of 43c adopt chiral packing (space group P2 ) and a helical molecular conformation, and crystallize in (E,Z) conformation which is unfavorable for the oxetane formation. The solid-state irradiation of 43c was found to give the oxetane 44c and a 3-lactam derivative 45c. The (3-lactam 45c was revealed to be enantiomerically enriched to 88% ee, whereas the other photoproduct 44c was racemic. The occurrence and the mechanism of transformation of 43c to 45c involve hydrogen abstraction by the alkenyl carbon atom. 

The absolute structure of (-)-(M)-39 and the major isomer (+)-(IS, 4R)-4( was determined by X-ray structural analysis using an anomalous scattering method (Figure 4-a and Figure 4-b). Figure 5 shows the superimposed structure of both absolute structures which was drawn with the overlay program included in CSC Chem3D. The sulfur and the alkenyl carbon atoms are closely placed to make the C-S bond easily, and subsequent cyclization of biradical BR needs the rotation of the radical center like path a to yield (1S,4R)-40. The molecular transformation from (-)-39 to (+)-40 needs... 

The pathway followed by the reaction is depicted in Figure B4.1. Methoxide anion adds to the a-haloalkenylborane generated by hydroboration of the haloalkyne, and induces migration of an alkyl group from the boron atom to the alkenyl carbon atom. The migration displaces halide anion from the alkenyl carbon atom and the centre is inverted. Finally protonolysis of the carbon-boron bond by acetic acid releases the (/f)-alkcne. 

Scheme 14 Asymmetric synthesis involving hydrogen abstraction by alkenyl carbon.

Scheme 19 Asymmetric synthesis via hydrogen abstraction by the alkenyl carbon atom.

AGE474> react with ethane to yield allyl complexes 82 (R = H, COOMe) by insertion into the Ir-C bond. Propene inserts into an Ir-C bond of compound 81 (R = COOMe) in deuterochloroform at room temperature to yield 83, whereas in methylene chloride at elevated temperatures iridabenzene 84 (R = COOMe, R =Et, R = H) is the product. This process may involve isomerizations of propene to propylidene followed by insertion and a-hydride elimination. Compound 81 (R = H) reacts differently and forms iridabenzene 84 (R = H, R = Me) in methylene chloride both at room and moderate temperatures. The process additionally might involve migration of the alkenyl carbon. 

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