The ketene tBu2C=C=O is prepared from tBu2C=O in three steps (performable as a two‐stage operation) through elimination of HCl from the intermediate product tBu2CCl–CH=O. The acid tBu2CH–CO2H, obtainable in two, three, or four preparative stages from tBu2C=O, adds slowly to the ketene to produce the anhydride (tBu2CH–CO)2O. Elemental lithium together with ClSiMe3 converts tBu2CCl–CH=O into tBu2C=CH–OSiMe3, which is a durable precursor of tBu2CH–CH=O, making this aldehyde easily and cheaply available from tBu2C=O. By exclusion of alternative mechanistic possibilities, the reduction of tBu2CCl–CH=O by tBuMgCl is shown to involve at least one single‐electron transfer, leading to the enolate tBu2C=CH–OMgCl, which can be converted into tBu2CH–CH=O (three steps from tBu2C=O) or into tBu2C=CH–OSiMe3. Hydride transfer from NaBH4 to tBu2CCl–CH=O affords tBu2CCl–CH2OH, the transformations of which provide an entertaining set of SN1‐type reactions. Several other examples of carbenium‐type behavior are encountered in this gem‐tBu2 system; they are attributed to steric congestion, which also impedes bond rotations in the anhydride and in two esters. A convenient route to tBu2CH–C≡N (five steps from tBu2C=O) uses the conversion of tBu2C=CH–OSiMe3 into tBu2CH–CH=NOH. The slow thermal (Z)/(E) equilibration of tBu2CH–NH–CH=O reveals the ranking of ecliptic repulsions as H3C < tBu < tBu2CH.
'H and 13C NMR signals were assigned and CH coupling constants ('J, 'J, '4 determined for a series of a-monoand a,a-disubstituted (1,1,3,3-tetramethyl-2-indanylidene)methanes with the following a-substituents: (mesityl),B, n-propyl, phenyl, rerr-butyl-C(=NH), cyano, (rert-butyl),C(OH), pivaloyl, H,N-CO, PhNH-CO, carboxy, nitro, acetoxy, Me,SiO, Me,Si, PhS, PhSMe+, PhSO, PhSO, , bromo and trimethylstannyl. The ' J couplings with the oleíinic proton span the range 124.3-193.7 Hz. Substituent-induced chemicai shifts (SCS) of most of the nuclei with respect to the a-unsubstituted olefin obey simple additivity in the a,a-disubstituted compounds and are very similar to the SCS values along the C=N double bond in the isoelectronic (1,1,3,3-tetramethyl-2-indanylidene)amines within the error limits. The exceptions concern nuclei in the immediate vicinity of the perturbing substituent. A dominant mechanistic contrihution of electric field effects appears likely for the more distant aromatic part of the indanylidene moiety. The chemical shifts of two (2,2,5,5-tetramethylcyclopentyiidene)methanes are shown to be compatible with the SCS parameters from the indanylidene series.
Short-lived pivaloylmetals, (H(3)C)(3)C-COM, were established as the reactive intermediates arising through thermal heterolytic expulsion of O=CtBu(2) from the overcrowded metal alkoxides tBuC(=O)-C(-OM)tBu(2) (M = MgX, Li, K). In all three cases, this fission step is counteracted by a faster return process, as shown through the trapping of tBu-COM by O=C(tBu)-C(CD(3))(3) with formation of the deuterated starting alkoxides. If generated in the absence of trapping agents, all three tBu-COM species "dimerize" to give the enediolates MO-C(tBu)=C(tBu)-OM along with O=CtBu(2) (2 equiv). A common-component rate depression by surplus O=CtBu(2) proves the existence of some free tBu-COM (separated from O=CtBu(2)); but companion intermediates with the traits of an undissociated complex such as tBu-COM & O=CtBu(2) had to be postulated. The slow fission step generating tBu-COMgX in THF levels the overall rates of dimerization, ketone addition, and deuterium incorporation. Formed by much faster fission steps, both tBu-COLi and tBu-COK add very rapidly to ketones and dimerize somewhat slower (but still fairly fast, as shown through trapping of the emerging O=CtBu(2) by H(3)CLi or PhCH(2)K, respectively). At first sight surprisingly, the rapid fission, return, and dimerization steps combine to very slow overall decay rates of the precursor Li and K alkoxides in the absence of trapping agents: A detailed study revealed that the fast fission step, generating tBu-COLi in THF, is followed by a kinetic partitioning that is heavily biased toward return and against the product-forming dimerization. Both tBu-COLi and tBu-COK form tBu-CH=O with HN(SiMe(3))(3), but only tBu-COK is basic enough for being protonated by the precursor acyloin tBuC(=O)-C(-OH)tBu(2) .
Pivaloin (2), prepared from ethyl pivalate (1) or from dimethyl oxalate (3), reacts with tert-butyllithium by reduction (45% of 10) and addition (45 % of 9), whereas the corresponding Barbier variant takes a different course. Productive transformations of the a,a,P-tri-tert-butyl glycol 9 lead to a,p,p-tritert-butylethanone (17, overall 4 steps), or a,a,P-tri-tert-butyl-ethene (20, 3 or 4 steps), or a,p,P-tri-tert-butyl-P-hydroxyethanone (19, 3 steps, also by Grignard addition to bipivaloyl 4). Steric congestion is assessed by searches for restricted internal rotation.. The alkene 20 and its epoxide 27 are studied with respect to NMR assignments and chemical degradation.We describe short routes to several a,a,P-tri-tert-butyl compounds which are either new or have been previously prepared only by long or inefficient procedures. Some of their properties are reported to show how intramolecular mobility and chemical reactivity may depend on the overcrowding caused by three tert-butyl ("tBu") groups. A. Preparation and teut-Butylation of PivaloinPivaloin (2) is the common source of all material described in this work but is commercially no longer avail-abler2]. For its preparation from ethyl pivalate (1) and sodium metal, the leading references"] are rather laconic and do not mention the considerable amount (ca. 2.50/) of yellow bipivaloyl (4) generated in the process. We preferred to run this acyloin condensation on large scales in to provide for an easier separation of 4r41 and purification of 2 that yielded at least 52% of colourless pivaloin, with the 'H NMR doublet splitting typical"] of the very clean material. Diketone 4 was certainly formed by a protolytic disproportionation of its radical anion that would have either been present as one of the primary products or perhaps resulted from rapid oxidation[6] of the expectedr71 enediolate 5. Trapping of 5 with chlorotrimethylsilane did not appear yield. 4 Na i 25% o be helpful in view of a much lower reported[7]> 52%
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