Carbenes deoxygenation

MeO), cyclic phosphonates (50). The formation of (50) is strong evidence for the involvement of carbenes (51). Diethyl alkylphospho-nites (52) deoxygenate aromatic aldehydes upon heating only small amounts of the a-ethoxybenzylphosphinate (53), which is analogous to products obtained from trialkyl phosphites and aromatic aldehydes, are formed. 

Xanthylidene also does not react measurably with 02. The lifetime of XA is the same in 02-saturated cyclohexane as it is in solutions which have been deoxygenated. Bearing in mind that triplet carbenes react with 02 at nearly the diffusion limited rate, if 3XA were in rapid equilibrium with XA, then 02 should shorten the apparent lifetime of the singlet by reacting with the triplet. 

Most electrophilic carbenes, such as 2/f-imidazolylidenes and 3H- and 4i/-triazolylidenes, in nitrobenzene gave rise to deoxygenation processes involving the intermediacy of the ylide 42, which decomposed to ni-trosobenzene and 43 (Scheme 12). However, the azolones 43 are too unstable to be detected or trapped in the reaction conditions. 

N2. The subsequent separation is performed under a nitrogen atmosphere with deoxygenated hexane. Approximately 2 g of carbene complex can be purified at one time on this column. After the portion of the eluant containing the deep-red carbene complex is isolated, this solution is reduced under vacuum to a volume of 10 mL and crystallization is induced by cooling to -15°. Anal. Calcd. for C13H806W C, 35.16 H, 1.82 O, 21.62 W, 41.40. Found C, 35.12 H, 1.67 0, 21.50 W, 41.60. 

Since many of the reactions of C atoms are extremely exothermic, it may be that they proceed without an enthalpic barrier. Thus, selectivities observed in C atom reactions may result from free energy barriers in which entropy considerations are the major factor. In discussions of C atom reactions, we shall see that carbenes are often intermediates. Two ways in which carbenes can be produced in C atom reactions are C—H insertion (Eq. 7) and deoxygenation of carbonyl compounds (Eq. 8). In several cases, the same carbene has been generated by both methods. When this comparison has been made, the reactions will be discussed together even though they represent different aspects of C atom reactivity. 

The competing ring expansion and cleavage in carbene 30 was confirmed by generating the deuterium labeled carbene 30a by the C atom deoxygenation of alde-... 

Figure 10.2. Carbenes that have been produced by the deoxygenation of carbonyl compounds. Numbers below the structures refer to the literature references in the text.

The fact that this same rearrangement occurs in 97a-c produced by deoxygenation at 77 K is strong evidence for energetic carbenes in this system. 

Carbene generation by C atom deoxygenation has been useful in answering questions concerning the intermediacy of free carbenes in certain systems. For example, ferf-butylcarbene (101) from several precursors gives 1,1-dimethylcyclo-propane (102) by C—H insertion, and 2-methyl-2-butene by C—C insertion. However, calculations ([QCISD(T)/6-31+G(2d,p)]// MP2/6-31G(d)) indicate that C—C insertion should not be competitive with C—H insertion in this carbene. 

In agreement with this prediction, deoxygenation of 2,2-dimethylpropanal (103) by C atoms, at 77, 158, and 195 K, yields only the C—H insertion product (Eq. 59). A similar result is observed when free carbene 101 is generated by cheleotropic extrusion from a tricyclopropane. 

Other free carbenes that have been generated in this way and their reactions compared with those from other precursors include bicyclo[2.2.2]octylidene, cyclopropylmethylcarbene, and 1-phenylpropylidine (104). " In the case of 104, the same ratio of (E) to (Z)-l-phenylpropene was produced when the carbene was generated by deoxygenation of the corresponding ketone and by the phenylcar-bene rearrangement (Eq. 60). " Thus, the method represents a viable alternative to... 

In contrast to carbenes, nitrene attacks only the carbon and never the sulfur atom of thiophene the only exception to this is discussed in Section 3.14.3.5. Unsubstituted thieno[3,2-/>]pyrrole and thieno[2,3-6]pyrrole have been prepared by thermolysis of the corresponding vinyl azides (Scheme 68) (81IJC(B)27l). A bis-cyclization of this type has also been reported (73CR(C)(277)1149>. Similarly, in the benzo[Z>jthiophene series the nitrene (252), generated either by thermolysis of the azide or by P(OEt)3 induced deoxygenation of the nitro compound, gives only the condensed indole (253) (79AG(E)900). 

Alkylidene derivatives of phthalic thioanhydride are formed as shown in Scheme 160. Reaction of phthalic thioanhydride with hot triethyl phosphite produces trafts-S -bithioph-thalide (457), probably via the carbene and phosphorane (Scheme 161) (72AHC(14)331>. Support for this mechanism stems from the fact that brief treatment of phthalic thioanhydride with triethyl phosphite in the presence of phthalic anhydride gives (458) in the presence of benzaldehyde the same reaction leads to the benzylidene derivative (456). An alternative mechanism has also been suggested, in which the penultimate step is the formation of an epoxide, which is deoxygenated to yield the product (72AHC(14)331>. 

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