The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

Frustrated Lewis pair chemistry: development and perspectives.

Crystal Engineering and Organometallic Architecture.

π-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

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Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

Publisher Summary This chapter discusses the published X-ray investigations of organometallic compounds of Na, K, Rb, and Cs. Organometallic molecules are defined as having metal–carbon contacts or closely related structures. The chapter presents structures from the Cambridge Crystallographic Data Base that might be termed to be “organometallic” in this restricted sense, although representative examples of related compounds are included. Systems that are organic but contain metal heteroatom rather than metal–carbon contacts are illustrated and many structures through 1985 also are reported. The enhanced reactivity of the organometallic compounds of the heavier alkali metals is a desirable asset but may also lead to ready thermal decomposition. Aggregation of organoalkali compounds is essentially electrostatic; two or more M–C bond dipoles interact mutually with one another. With notable exceptions, the structures of organoalkali compounds have not been determined on the pure compounds. The NiAs-like arrangements allow the anions to invert or, in the extreme, to become planar. Ail compounds of the alkynyl type possess layered or three-dimensional structures.

Sodium, Potassium, Rubidium, and Cesium: X-Ray Structurai Analysis of Their Organic Compounds1

Heavy Element π Donation Is Not Less Effective

observed percent I8O incorporation the scrambling rates were calculated by standard methodsi6. Two control experiments in each solvent were performed in order to show that the observed I8O scrambling is not the result of (I) chemical workup or (2) external ion return. In the first experiment 25 mg of the labeled ester was dissolved in the buffered solvent (25 mL) containing 1 equiv of 2,6-lutidinium brosylate, and the reaction mixture was worked up immediately in the same manner. In the second experiment 50 mg of the unlabeled sulfonate ester was dissolved in the buffered solvent (25 mL) containing 1 equiv of \"0 enriched (60.7%) 2,6-lutidinium brosylate and after solvolysis for 1 half-life the solution was worked up in the same manner. In the I3C NMR spectrum of the isolated ester from both experiments, no \"0 was observed to be present at the a-carbon. Oxygen Scrambling Studies. 2. 4-Methyl-3-homoadamntyl Hepta-fluorobutyrate. A 5.3 mM solution of the ether I80 enriched (50.70%) 4-methyl-3-homoadamantyl heptafluorobutyrate in 80E (100 mg/50 mL) containing a I. 1 equiv of 2,6-lutidine was reacted at 25 OC for 9.5 1 h. The reaction flask was then placed in a 0 \"C bath, and the workup was the same as that used in the I8O studies of 2s. The composition of the product mixture was analyzed by IH NMR (300 MHz) in an analogous fashion to that performed in the product studies of the tertiary ester above. In the spectrum the additional methyl doublet of the unreacted tertiary ester occurs at 0.93 ppm. The percentage I80 incorporation at the a-carbon of the two esters and of the solvolysis products was determined from the natural abundance 125-MHz \"C spectrum recorded on a Bruker 500-MHz Fourier transform spectrometer with the conditions for data acquisition being similar to those in the I8O studies of 2a. No \"0 incorporation was observed at the a-carbon of the alcohol and ether solvolysis products. In the unreacted tertiary ester and the rearranged secondary ester the percent present was 43.68% and 41.56% respectively , and their recorded spectra are shown in Figure 3. The proportion of unreacted tertiary ester that was equilibrated is 27.7% [(50.7-43.68)/23.35 X 1001 while that for the secondary ester was 36.0%. The calculation of the rate of I80 equilibration (9.47 X IOd S-I) for the tertiary ester as well as the ratio of return of the originally bonded oxygen relative to the carbonyl oxygen …

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Addition reactions of diarylcarbenium ions to 2-methyl-1-pentene: kinetic method and reaction mechanism

The kinetic method described in the preceding article has been used to determine the rate constants for the reaction of (P-CH30C6H4)(C6HJ)CH+BCII with 30 alkenes and 1,3-dienes in dichloromethane at -80 to -30 \"C. The second-order rate constants, which reflect the rate of attack of @-CH30C6H4)(C6H5)CH+ at these r-systems strongly depend on the electron-releasing ability of the substituents at the developing carbenium center (H,C=C(CH3)R: R = H, 9.39 X lo4; R = vinyl, 15.6; R = CH3, 23.3; R = Ph, 1450; R = cyclopropyl, 5650 L mol-' s-l at -70 \"C). The steric effects of substituents at this position are of minor importance (H2C=C(CH3)R: R = Me, 23.3; R = Et, 18.4; R = i-Pr, 6.08; R = r-Bu, 1.21 L mol-' s-l at -70 \"C). Methyl groups at the electrophilically attacked vinylic position usually lower AH* and AS', while a uniform effect on the rate constants is not found. In the preceding article we have reported that colorless, covalent products are formed when 2-methylI -pentene is added to the colored ionic solutions of several diarylcarbenium tetrachloroborates in CH2CI2,' and we have shown that the rate of disappearance of the diarylcarbenium ions, which can be measured photometrically and conductimetrically, equals the rate of attack of the carbenium ions at the alkenes. We report now on the 1 2 3 relationship between the structure and reactivity of alkenes and 1,3-dienes employing the pmethoxy-substituted benzhydryl cation 1 as the reference electrophile. This is the first report on absolute rate constants of various vinyl derivatives toward carbenium ions since Dorfman's pioneering work on the reactivity of radiolytically generated benzyl and benzhydryl cations toward some unsaturated hydrocarbom2 A smaller set of data on relative reactivities of alkenes has previously been published by u s 3 Experimental Section The kinetic method has been described in the preceding article,l and the results of the individual kinetic experiments are given in the supplementary material. All reaction products obtained from 1.BCIL with 2 were characterized by IH NMR and/or I3C NMR spectroscopy. Spectral and other physical data of the reaction products are documented in ref 4. Reaction Products. The combination of 1.BC14with compounds 2a-dd yields 1:l products in almost quantitative yields, as previously described for analogous reactions of other benzhydryl chloride^.^-^ For un( I ) .May!, H.; Schncider. R.; Schade, C.; Bart], J.; Bederke, R., preceding (2) Wang, J. ; Dorfman, L. M. Mucromolecules 1980, 13, 63. (3) (a) Mayr. H.; Pock, R. Chem. Eer. 1986, 119, 2473. (b) Pock, R.; paper in this issue. Mayr, H. Chem. Eer. 1986, 119. 2497. 0002-7863/90/15 12-4454$02.50/0 symmetrical alkenes, the electrophilic attack of 1 takes place regioselectively to give the most stabilized carbenium ion,6 and the adducts 3 are usually the exclusive or the predominant products. When secondary alkyl cations are involved as intermediates, 1 ,2-hydrogen shifts often cannot be avoided, and in the reactions of 1.BCI4with 2 a 4 , the main products 3a-d are accompanied by some rearranged material. The stereoselectivities of the addition reactions with the E,Z-isomeric 2-butenes 2b,c and I-phenylpropenes 2s,t are comparable to the results reported for the analogous reactions of diphenylmethyl chloride and dip-tolylmethyl chloride.' By analogy with an earlier report,* norbornene 2e reacts with complete Wagner-Meerwein rearrangement to give a 7-(diarylmethyl)-2-~hloronorbornane, and 2-cyclopropylpropene 2dd undergoes the well-known cyclopropylcarbinyl/homoallyl rearrangement with exclusive formation of 1 , l -diaryl-6-chlor0-3-methyl-3-hexene.~~ The ordinary addition products 3 are accompanied by the indans 4 and 5, when 1.BC14is combined with the alkenes 2Fp,s,t. While 4 and 5 are formed in approximately equal amounts in all cases, the ratio 3/(4 + 5) strongly depends upon the reaction conditions as described for the reaction of bis(p-methoxypheny1)carbenium tetrachloroborate with trimethylethylene.' The alkenes 2m and 2p react with exclusive or predominant formation of the indans 4 and 5 (gem-dialkyl effect9). Ph 4 s (4) (a) Schneider, R. Dissertation, Universitit Erlangen-Niirnberg, 1987. (b) Schade, C. Dissertation, Medizinische Universitit zu Liibeck, 1988. (c) Irrgang, B. Dissertation, Medizinische Universitit zu Liibeck, 1990. ( 5 ) Mayr, H.; Striepc, W. J . Org. Chem. 1983, 48, 1159. (6) Unsymmetrical r-systems are attacked at the left sp2 carbon of the formula drawings in Table I. in8 (7) 7767 Pock, R.; Mayr, H.; Rubow, M.; Wilhelm, E. J . Am. Chem. Soc. 1986, . -, . . . . (8) Pock, R.; Klein, H.; Mayr, H. Chem. Eer. 1986, 119, 929. (9) Review: Capon, B.; McManus, S. P. Neighboring Group Participation; Plenum Press: New York, 1976, Vol. I , p 58.

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Christian Schade

Papers

Sodium, Potassium, Rubidium, and Cesium: X-Ray Structurai Analysis of Their Organic Compounds1

Heavy Element π Donation Is Not Less Effective

Addition reactions of diarylcarbenium ions to 2-methyl-1-pentene: kinetic method and reaction mechanism

Kinetics of the reactions of the p-methoxy-substituted benzhydryl cation with various alkenes and 1,3-dienes

n-Butylsodium: The preparation, properties and NMR spectra of a hydrocarbon- and tetrahydrofuran-soluble reagent