Ketenes structure

To select between these two alternative structures it was necessary to synthesize a labeled analog. Three hydrogen atoms of the methyl moiety of the ester group were substituted for deuterium. One of the principal pathways of fragmentation of [M N2]+ ions involves the loss of CH3 radical. Since all R substitutes in diazo ketones 4-1 were also methyls it was important to detect what group exactly is eliminated from the [M N2]+ ion. The spectrum of deuterated sample has confirmed that the methyl radical of the ester moiety leaves the parent ion. As a result the cyclic structure 4-2 was selected as the most probable. The ketene structure 4-3 is hardly able to trigger this process, while for heterocyclic ion 4-2 it is highly favorable (Scheme 5.22). 

A number of poly (thiol esters) have been synthesized by method BC (29). The reaction temperature was increased from room temperature to above 200°C. The products were often colored because of side reactions (replacement of hydroxyl with chlorine, formation of ketene structures, and ester pyrolysis are known side reactions in polyester formation). 

Additional confusion arose by the observation of transient (metastable) intermediates that were assigned to ketocarbene, oxirene, and ketene structures without unambiguous evidence for the structure postulated. 

Flash photolysis allowed even better detection of transient intermediates but the assignment of structures to any metastable intermediate observed in such an experiment is an additional problem This fact is demonstrated very well in an investigation of several naphthoquinonediazides by Tanigaki and Ebbesen (1987, 1989). They assigned oxirene and ketene structures to two successively formed transient species. In other studies (Delaire et al., 1987 Shibata et al., 1988), however, the first transient species was assigned as a ketene and the second the structure 8.99, i. e., the water addition product of the ketene. ... 

The 2-benzylindole-3-carboxylic ester (372) is postulated to form the carbanion on treatment with base, and the product of its addition to DMAD is the hydroxycarbazole (373) (Equation (106)) <92TL1655>. It is possible that the reaction is actually a cycloaddition involving a quinodimethane ketene structure (see Section 2.02.1.8). 

The structure of the unusual betaine (50) has been determined (70JHC895). The bond lengths and angles suggest that a significant contribution to the structure is made by a resonance form (SOb) in which the N(l)—C(5) bond does not exist ( ketene form). 

A wide variety of /3-lactams are available by these routes because of the range of substituents possible in either the ketene or its equivalent substituted acetic acid derivative. Considerable diversity in imine structure is also possible. In addition to simple Schiff bases, imino esters and thioethers, amidines, cyclic imines and conjugated imines such as cinnamy-lidineaniline have found wide application in the synthesis of functionalized /3-lactams. A-Acylhydrazones can be used, but phenylhydrazones and O-alkyloximes do not give /3-lactams. These /3-lactam forming reactions give both cis and /raMS-azetidin-2-ones some control over stereochemistry can, however, be exercised by choice of reactants and conditions. 

Dicyclohexylcarbodiimide is a solid material, the Lewis structure for which resembles that of ketene. The molecule is a widely used catalyst for amide synthesis and other dehydration reactions. 

Analyze and describe the electronic structure of dicyclohexylcarbodiimide in the same way as you did for ketene. 

Step through the sequence of structures representing dissociation oiketene to methylene and carbon monoxide. Plot energy (vertical axis) vs. carbon-carbon bond distance (horizontal axis). Would you describe ketene as a weak complex between singlet methylene and carbon monoxide Explain. (A table of CC and CO bond lengths is found at left.) Is there an energy barrier to the dissociation ... 

To illustrate the value of the mass spectra of the labeled compounds, the peaks at m/e 129 in Figures 7 and 8 will be considered first. These peaks could be from the loss of acetic acid (60 mass units) from m/e 189, or the loss of water (18 mass units) from m/e 189 followed by loss of ketene (42 mass units) structure 15, containing C-1-C-2-C-3 less a rearranged hydrogen atom from C2, is another possibility. The composition of this ion could be important for confirming the presence of a 3-hydroxyl group. 

On the other hand, a metastable-ion peak at m/e 88.1 (calculated, 88.0) is present in the mass spectrum of 11 (Figure 8) for the formation of m/e 129 from m/e 189, by loss of acetic acid. In the mass spectrum of the D20-exchanged analog, m/e 129 partially shifts to m/e 130 and partially stays at m/e 129. Metastable-ion peaks are also present at m/e 154.8 (calculated, 154.7) and m/e 97.3 (calculated, 97.3) for the loss of water from m/e 189 followed by the loss of ketene, to give an ion at m/e 129. Since m/e 171 from the loss of water remains at m/e 171, the loss of water must involve the hydroxyl hydrogens. Scheme 3 is an attempt to summarize this in terms of structures which are entirely... 

Intermediate 7, a viable precursor of intermediate 6, possesses a y,<5-unsaturated ester, the structural prerequisite, or retron, for the ortho ester Claisen transform.5 In the synthetic direction, the convergent union of intermediates 9 and 10 could give mixed-ketene acetal 8 the intermediacy of 8 should be brief, for it should readily... 

A prominent structural feature of 21 and its precursor 22 is the trans C16-C17 trisubstituted double bond. The particular relationship between the ethoxycarbonyl function and the A16 17 double bond in 22 is significant because it satisfies the structural prerequisite for the Johnson ortho ester Claisen rearrangement transform.2130 Mixed ketene acetal 23 thus emerges as the immediate... 

Loss of ketene from bicyclic structures (e.g., for camphor)... 

There is evidence that the reactions can take place by all three mechanisms, depending on the structure of the reactants. A thermal [ 2, + 2s] mechanism is ruled out for most of these substrates by the orbital symmetry rules, but a [ 2s + mechanism is allowed (p. 1072), and there is much evidence that ketenes and certain other linear molecules in which the steric hindrance to such an approach is... 

The basic 5+4 ring structure in (30) is that of a ketene cycloadduct to a cyclopentene, or, if the 1,2-difunctionality (Br and OR) come from a double bond, of adduct (31) (Chapter T 33),... 

CO species in the adducted triiron complexes, the propargylidene-ketene compoimd has e same structure as compound 5 (ii) desilylation by the substitution of the silyl termin species with the ion originating from H2O or the solvent gives the hydrogen-ketene form. 

An interesting carbene, 1-oxobutatrienylidene [25], having cumulated double bonds, has been found by IR spectroscopy in the photolysis (A>230nm) products of matrix-isolated l,2,3,4-pentatetraene-l,5-dione [26] (Maier et al., 1988) (in its turn the unstable dione [26] was generated by thermo- or photo-destruction of compound [27]). The second product was carbon monoxide. The linear structure of the carbene [25] has been suggested on the basis of two intense IR bands at 2222 cm and 1923 cm indicating respectively ketene and allene fragments. 

Oxo-2,5-cyclohexadienylidene [83] was generated in solid argon at 9 K by irradiation of diazo compound [84] with visible light (A>495 nm) (Sander et al., 1988 Bucher and Sander, 1992 Bucher et al., 1992). The IR, UV, and esr spectra of [83] were in accord with a structure having a triplet state with one delocalized electron. In the IR spectrum of the carbene [83] the r (CO) mode was found at 1496 cm which indicates a bond order of the C—O bond considerably less than 2. The low-temperature reaction of carbene [83] with CO generated the keto-ketene [85]. Irradiation (A = 543 10 nm) of [83] led to its transformation into a very labile species, presumed to be [86], which rearranged back to [83] not only under UV or... 

The structure of the adduct formed from triethyl phosphite and diphenyl keten, which has been the subject of some speculation, is now shown to be (51). ... 

Entries 4 and 9 are closely related structures that illustrate the ability to control stereochemistry by choice of the Lewis acid. In Entry 4, the Lewis acid is BF3 and the (3-oxygen is protected as a f-butyldiphenylsilyl derivative. This leads to reaction through an open TS, and the reaction is under steric control, resulting in the 3,4-syn product. In Entry 9, the enolate is formed using di-n-butylboron triflate (1.2 equiv.), which permits the aldehyde to form a chelate. The chelated aldehyde then reacts via an open TS with respect to the silyl ketene acetal, and the 3,4-anti isomer dominates by more than 20 1. 

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