Bent allenes

One of the main benefits of the paper by Bent and Weinhold is a plausible explanation for the n + i rule which does not, at first sight, seem to suffer from the drawbacks of the explanations of Allen and Knight as well as Ostrovsky. However, the recent explanation by Bent and Weinhold comes at a certain cost as will be explained. 

We have previously seen examples of the carbon-like formulas of mononuclear and dinuclear osmium compounds, namely the methane-like tetrahydride (4.50c), ethylene-like H20s=CH2 (4.51c) and H2Os = OsH2 (Table 4.15), acetylenelike HOs = CH (4.54c) and HOs = OsH (Table 4.15), allene-like H2C = Os = CH2 (4.55a), and so forth. While the coordination numbers and Lewis-like formulas are formally analogous, the actual structures of Os and C species may be quite similar (e.g., the Td structures of OsfL and CH4) or dissimilar (e.g the strongly bent Cs structure of H20s = CH2 [Fig. 4.13(c)] versus the planar D2h structure of H2C = CH2). 

DPIBF, the resulting 119 was optically active. This suggests that the progenitor of 119 has the allene structure 117 rather than other conceivable constitutions [80, 81]. Even early quantum-chemical calculations on 117 showed a strongly bent, chiral structure, although the enantiomerization barrier was not correctly estimated [19]. 

In spite of the C-S bonds of 299 being rather long (176and 178 pm), the allene moiety is still strongly bent (C-C-C bond angle 132°), preventing the isolation of the species. In the phosphorus compound 298, this angle has a value of 144° and amounts to even 153° in the diphosphaisobenzene 300 [120]. 

Fig. 15 Syntheses of heterocyclic and all carbon cyclic bent allenes...

The very early reported allene (NMe2)2C=C=C(NMe2)2 [12, 13] can also be considered as a push-push allene and the bent structure is activated upon complex formation as shown in Fig. 34. Reaction with [ClAu(PPh3)] and CU exchange by the weakly coordinating anion SbFg results in the formation of 71 in good yields [20,21]. 

From a preparative point of view the access to new CL2 or CLL compounds and the study of their coordination ability is a challenge for the chemical community. In most cases the carbon(O) atom is stabilized by simple and well known Lewis bases but the uptake as well as the removal of this carbon(O) atom is not trivial. The latter reaction, the transformation of a single carbon atom to other substrates, has not been observed as yet. The access to a variety of stable bisylides or bent allenes often proceeds upon deprotonation of related (HCL2)" cations or (H2C L 2) dications by appropriate strong Lewis bases. 

Alternatively, compound 90 can be described as a strongly bent allene (angle C-C-C 134.8(2)°). The monometalation of 90 to give complex 91 has also been demonstrated. 

Pamela M. Aker, University of Pittsburgh Edward Alexander, San Diego Mesa College Sandra Allen, Indiana State University Susan Bangasser, San Bernardino Valley College Ronald Baumgarten, University of Illinois, Chicago Stacey Bent, New York University Richard Bretz, University of Toledo... 

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