Isopropylidene carbene

Isopropylidene carbene (Me2C=C ) is generated efficiently from the enol triflate derived from iso-propyl trimethylsilyl ketone using KF/Aliquat [45], but full experimental details are not given. The carbene can be trapped by isocyanides to produce, after solvolysis, -substituted 3-methylbut-2-enoamides in 35-52% yield [46]. 

Cyclohexylidene carbene and cyclopentylidene carbene have been generated by the base-induced decomposition of the appropriate 1 -(A-acety 1-jV-nitroso-aminomcthyl)cycloalkan-l-ol in the presence of Aliquat [47-49] and they have been shown to react in high yield with electron rich alkenes (see Section 7.3). Cyclopropylmethylidene, di(cyclopropyl)methylidene, and isopropylidene carbenes have been generated by an analogous route [49]. 

In some cases involving aliphatic thiocarbonyl ylides, a 1,4-H shift occurs to afford vinyl thioethers. As an example, thiocarbonyl ylides (41), generated by the addition of isopropylidene carbene to aliphatic thioketones, are converted to divinylthioethers (42) (84) (Scheme 5.16). 

We termed a particularly esoteric and intriguing rearrangement the Bicycle Rearrangement for reasons which become obvious. Equation 13 gives one early example473. Superficially, the reaction appears to involve loss of an isopropylidene carbene and its readdition to the exocyclic bouble bond. However, this is not the reaction mechanism as can be seen from the examples in equations 14a and 14b. Here the endo and exo... 

During the synthesis of diphosphiranes from the symmetrical diphosphene by cyclopropanation, whatever the nature of the carbenoids used (diazo derivatives, carbenes or halogenocarbenes), in all cases the formation as byproducts of the phosphaalkenes and the unstable phosphinidene was observed (5-20%). So, for isopropylidene carbene, the formation of the phosphacumulene, more stable than the parent diphosphirane can reach 30% <91TL3687>. 

LaBt man Trifluor-methansulfonsaure-(2-methyl-l-propenylester) mit Azobenzol reagieren, so kommt es unter intermediarer Bildung eines Isopropyliden-carbens (vgl. Bd.E19b,S.96,106ff.) zur Cycloaddition215 ... 

The reaction of tetramethylallene with isopropylidene carbene affords the [2+1] cycloadduct in 12% yield. Likewise, reaction with dimethylvinylidene carbene 89 affords the [2+1] cycloadduct 90 in 35 % yield ... 

Scheme 8.—Proposed Mechanism for Carbene Formation from Photolysis of 5-Deoxy-l,2-0-isopropylidene-/3 L-erythro-pentofuranos-3-ulose (15). Rationalization for the Direction of a-Cleavage.

Alkylidene carbenes (isopropylidene [150] cyclohexylidene [150-152] cyclopentylidene [150] cyclopropylethylidene and dicyclopropylmethylidene carbenes [153] and alkenylalkylidene carbenes (Me,C=C=C [154-159] adamantylvinylidene carbene [160] MeCH=CH.CH=C [161] react in good yield... 

The term carbene will be used as the generic term. Particular structures will be designated using the system of nomenclature in which carbenes are treated as derivatives of the parent species methylene (CH2 ). Thus, for example, 5 will be referred to as dimethylmethylene rather than dimethylcarbene (which derives from the now obsolescent carbinol convention) or isopropylidene (which might lead to confusion with stable molecules in more complex organic structures). 

Isopropyliden-l-methoxycarbonyl-EI9b, 115 (Carben + En) Cyclopropen 2-Methoxycarbonyl-... 

Isopropyliden-l-(2-methyl-propen-yliden)-E19b, 144 (Carben + En) Cyclopropan-(spiro-3)-bicyclo 2.2.0 hexan-<2-spiro>-cyclopropan E17c. 2236 (Cyclobuten + Bi-cyclopropyliden) Cyclopropan--bicyclo 2.2.2 oct-2-en El7b, 1531 (1,3-Cyclo-hexadien + En)... 

Ethoxycarbonyl-2-isopropyliden-l -methyl- E19b, 1101 (Carben + Allen)... 

E19b, 108 (Carben + En) (1-Ethyl-pentyliden)- E17b, 1678 (Phosphoran + H5C2-CO-C4H9) 1-Isopropyliden-tetramethyl- El7b, 1118 (subst. 4,5-H2 — 3H-pyrazol/ hv or A) E19b, 107/113/115 (Carben + En)... 

Bicyclo 5.1.0]octan 8-Isopropyliden-E19b, 112 (Carben 4 En) Cyclobuten 4-(2-Methyl-l-prope-nyl)-1,3,3-trimethyl- E19b, 349 (Ringerweiterung) trails-l,trans-6-Cyclodecadien ... 

Chlor-phenyl)-2,2-dimethyl-l-methylen- E17a, 344 (Ar—CHN2 + 1,2-Dien) 2-(4-Chlor-phenyl)-1 -isopropyliden-E17a, 344 (Ar —CHN2 + 1,2-Dien E19b, 106/113 (Carben + En)... 

Dimethyl-2-isopropyliden-l-(2-methyl-propenyliden)- E19b, 148 (Carben + En)... 

Dimethyl-2-(l, 1 -dimethyl-2-pentenyl)- E17a, 138 [cycl. En + (H5C2)3AI H+] 2-Hexyl-l-isopropyliden- E19b, 105/112 (Carben + En) Nonyliden- E17b, 1485 (—R-SH - En)... 

A similar desire for geminal dimethyl-substituted cyclopropanes resulted in the use of isopropylidene triphenylphosphorane as the carbene source. Reaction with acceptor-substituted alkenes carrying inducing auxiliaries gave intermediates for syntheses of chrysanthemic acid derivatives. Thus, the addition to (A )-butencdioatcs provides 3,3-dimethyl-1,2-cyclopropanedi-earboxylates 3986. The highest diastereoselectivity is observed with the (—)-8-phenylmenthyl derivative which gives the cyclopropane with a d.r. of 91 9. 

The effect of substituents on the temperature dependence of a-carbonyl-carbene reactivity has been examined using carbenes generated by low-temperature photolysis of methyl diazophenylacetate. A correction to the literature on the photoreaction of isopropylidene diazomalonate (98) with 1,3,3-trimethyl-cyclohexane (99) has been reported. The photoproduct, originally thought to be a cyclopropane derivative, has now been shown to be the cyclobutanone (100), the formation of which presumably involves a photo-Wolff rearrangement as illustrated in Scheme 11. Substituent effects observed in the product distribution of diazo-amide photochemistry have been ascribed to conformational factors the jS-lactam, oxindole, and Wolff rearrangement products appear to arise directly from the excited singlet state of the sym-Z form of the diazo-amide itself. 

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